UNIVERSITY OF BATH
DEPARTMENT OF MECHANICAL ENGINEERING
DESIGN AND DEVELOPMENT OF A MEDICAL
DEVICE TO IMPROVE THE ASSEMBLY...
COPYRIGHT
Attention is drawn to the fact that copyright of this dissertation rests with the
author. This copy of the disse...
ABSTRACT
There has been a significant failure rate in modular total hip replacements
(MTHR) over the past few years, parti...
2
CONTENTS
1. INTRODUCTION 1
1.1 What is a Total Hip Replacement? 1
1.2 What are THRs used to treat? 2
1.3 What is a Modul...
3
4.3.2 Generation of PDS 39
5. DEVELOPMENT & EVALUATION 40
5.1 Concept Generation and Evaluation 40
5.1.1 Initial Product...
4
7.4 Discussion 95
8. CONCLUSIONS 96
10. REFERENCES 98
11. APPENDICES 101
5
NOMENCLATURE
N = Newtons
KN = Kilo Newtons
F = Force
m = Mass
g = acceleration due to gravity (9.81m/s)
m/s = meters per...
LIST OF FIGURES AND TABLES
FIGURES
Figure 1: Illustrated Hip Replacement; Before and After [2]...............................
2
Figure 23: Device for Handling Hip joint Heads [39] Figure 24: Head
holder and impactor [40]...............................
3
Figure 51: The Solenoid Powered Nail Gun [49]...................................................... 57
Figure 52: Electr...
4
Figure 78: Base plate fixed to table with and without Rubber Base ......................... 86
Figure 79: Table of Data ...
1
1. INTRODUCTION
1.1 What is a Total Hip Replacement?
There are two types of Hip Replacement surgery, Hip Resurfacing and...
2
The first modern THRs were designed by John Charnley in the 1960s, which
stemmed from his paper   “Surgery   of   the   ...
3
Figure 3: Illustration of Normal Vs. Arthritic Hip [5]
1.3 What is a Modular Total Hip Replacement?
Modular THRs (MTHR) ...
4
1.4 Why are they Modular?
Modularisation was introduced into the design of THRs in the 1970s [8] to allow
more flexibili...
5
Step A involves making an incision to gain access to the joint area and dislocating
or  “disarticulating”  [11] the femo...
6
stem taper using a mallet and impactor (usually tipped with a softer material than
the head so as not to damage the surf...
7
product recalls. Large diameter (>36mm) MoM bearings are by far the worst
offenders when it comes to early revisions and...
8
2. AIMS AND OBJECTIVES
Aim: To design and develop a medical device to improve the assembly of
head/neck taper junctions ...
9
3. LITERATURE REVIEW
This section of the report contains the findings from the literature review carried out
on the fail...
10
diameter of the bearing. This increase in torque caused wear and fretting corrosion
which again led to the liberation o...
11
greatest frictional moments followed by Metal on UHMWPE and then CoC with the
lowest frictional moments, thus adding to...
12
Now that the severity and reasoning behind the failure MTHRs has been established
the next step is to look at who has i...
13
Figure 7: Example of matched and mismatched taper angles [20]
Figure 7 Part A shows the correct fit with the maximum co...
14
resistant materials plays an important part in the improvement of these designs,
along with the recognition that only a...
15
3.2.2 What influence do Surgeons have on the assembly?
The   “fit   of   the   spigot   head”   is   noted   as   the  ...
16
The impact force is not the only important factor that the surgeon has influence over
during assembly. The axial alignm...
17
Table 1: Data from Pennock et al. (2002) study [29], [10]
This study looks at the effects of varying the magnitude of t...
18
[29]. Pennock et al. (2002) [29] also states the importance of axial alignment when
delivering the impacts, to ensure t...
19
this study can be considered to be a more realistic representation of the average
magnitude of a surgeons impact as the...
20
assembly forces. The average surgeon’s  impact  force  applied  during  assembly in
this study is roughly twice that of...
21
studies. The relevant data acquired from this study is shown in Table 4 [10] as
follows.
Table 4: Data from Rehmer et a...
22
head connections on a modular shoulder prosthesis”. In this study, they use a
surgeon to try to acquire an average asse...
23
head is correctly seated on the neck taper before providing a single impact of no
less than 4KN, which is axially align...
24
4. USER NEEDS & DESIGN REQUIREMENTS
It is now clear from the Literature Review that there is room for improvement in th...
25
Figure 8: Stuart  Pugh’s  Design  Process  Model  [33]
The stage following this will be the development of a Product De...
26
producing a Product Design Specification (PDS), which can then be used to guide
the next stage of the project.
4.1 Fund...
27
minimum force, compared to larger metal taper junctions that may require slightly
more force for optimum assembly.
The ...
28
Figure 9: Orthopaedic Surgeon Survey Introduction
After the introduction to the survey, the first question posed attemp...
29
The next task was to establish the range of femoral head sizes. This was important
to establish so that the device coul...
30
Figure 12: Surgeon Survey, Q3
The next question aims to establish the size of the access area or incision in the
patien...
31
Figure 14: Surgeon Survey, Q5
The next question was not so much based on the establishment of design
requirements for t...
32
question was provided in a format where the participant rates the level of
importance out of 10. This question is shown...
33
However, a brief analysis of the survey has shown that there is a significant amount
of very relevant data available wh...
34
Figure 18: Stuart  Pugh’s  Design Core [33]
Each and every Element’s constituent parts must have a measureable value so...
35
4.3.2 Commercially available designs and patent research
As mentioned previously, the only commercially-available desig...
36
Figure 23: Device for Handling Hip joint Heads [39] Figure 24: Head holder and impactor [40]
Figure 25: Method of apply...
37
this case involved delivering an impact or maintaining an alignment. Rough notes
were taken during the patent search to...
38
4.3.1 Medical device standards review
Medical Device Classifications exist to grade the level of risk that a medical de...
39
for clinical trials, so the standards review is of greater value further down the line in
future clinical design and de...
40
5. DEVELOPMENT & EVALUATION
This chapter of the report looks at the generation, evaluation and development of a
concept...
41
5.1.1 Initial Product Design Specification (PDS)
A full PDS was developed for a prototype device aimed at use in clinic...
42
An initial sketch was made at this point to record any design ideas that had come to
mind so far. This initial sketch i...
43
5.1.3 Radial Thinking
Radial thinking was used to expand on different concepts and help to develop and
record different...
44
Figure 34: Development of Objective C
5.1.4 Visual Concept Analysis
After writing down all the different methods of ach...
45
(A1). Three Prong Flexible Support and Centring Cone
Figure 35: Three Prong Flexible Support and Centring Cone Sketch
(...
46
(B). Holding impactor axially aligned with neck taper axis
*DATUM* Surgeon holding impactor aligned by hand
(B1). Bent ...
47
(B3). Split Cup/Split Mould
Figure 39: Split Cup/Split Mould Sketch
(C). Deliver adjustable impact between 4.5KN and 11...
48
(C2). Slide Hammer - To Charge Spring
Figure 41: Slide Hammer - To Charge Spring Sketch
(C3). Magnets (forced together ...
49
(C4). Electro-Magnets (Solenoid Actuator)
Figure 43: Electro-Magnets Sketch
(C5). Screw Mechanism - No Impact
Figure 44...
50
(C6). Screw Mechanism - To Charge Spring
Figure 45: Screw Mechanism - To Charge Spring Sketch
(C7). Lever - To Charge S...
51
(C8). Pneumatic Piston
Figure 47: Pneumatic Piston Sketch
(D). Isolating impact to head-neck taper junction
*DATUM* Sur...
52
(D2). Mechanism to Hook around the Lips at Base
Figure 49: Mechanism to Hook around the Lips at Base Sketch
5.1.5 Criti...
53
The result of this exercise is the selection of concepts to address each of the four
key performance requirements. This...
54
Table 6: Scoring Table for Sub-System Concepts
55
As the outcome from Table 6 has shown, the following sub-system concepts have
been chosen and are listed as follows.
(A...
56
in the design. This investigation was performed by looking into existing products that
were applying these three poweri...
57
can become quite complex and expensive when designing and manufacturing. This
project is trying to provide the simplest...
58
that may be close to the device when in use. This also, much like the pneumatic
option, this greatly increases the comp...
59
more sustainable and reliable with no dependency on other inputs such as electricity
and compressed air. The surgeon co...
60
5.1.7 Development of Final Powering Concept
Now that the method which would power the device had been chosen, the next ...
61
More can-crusher products were also investigated, and an example of one of the
sketches trying to use this approach is ...
62
Figure 56: Corkscrew Lever System Sketch
This sort of mechanism could be attached onto the end of the device, and the
s...
63
Figure 57: Twisting Adjustment and Release Concept Sketch
Figure 58: Gearbox Style Spring Compression Adjustment Concep...
64
Figure 59: Spring Compression adjustment System Sketch
Trigger/Release Mechanism
A trigger/release mechanism would also...
65
Figure 60: Slotted Trigger Release Mechanism Sketch
Figure 61: Firearm Trigger Concept Sketch
Figure 62: Handle Trigger...
66
Selection of the Concept for Objective C
The final selection of the concept for the device is shown in Figure 63 below....
67
Figure 64: Tubular Casing Surrounding Spring Sketch
5.1.8 Mechanical Feasibility of Chosen Concept
Before progressing f...
68
The leverage force has been simplified and marked out on the Figure 56 which is
shown again below and renamed Figure 66...
69
Using static force balancing analysis, Equation 1 can be derived and is as shown
below.
𝑭𝟐 =
𝑭𝟏×𝑳𝟏
𝑳𝟐
(1)
Substituting ...
70
Substitute in known values;
100 = 𝑚(9.81)
Rearrange and solve for m;
𝑚 = 10.2𝐾𝑔
Where;
Kg = Kilogram
This means that a ...
71
Where;
F = Impact Force
Δt = Impact duration (time over which impact occurs)
m = Mass
Δv = change in velocity
The force...
72
Now that the initial velocity has been found, this can then be used to find the kinetic
energy (KE) at the point of imp...
73
Since both k and x are both unknown at this point, it is decided to use a spring
displacement (x) of 60mm (millimetres)...
74
had to be proven experimentally first. This was to be proven using a testing rig
designed purely for this purpose.
The ...
75
The key functions of the testing rig were to allow the Instron to compress the spring,
to hold the spring in its compre...
76
Figure 68 includes a lever release mechanism, which is actuated by twisting a collar
fixed on the outside of the top cy...
77
The device could be removed from a solid plate base on which a load cell would be
secured, and then mounted using a spe...
78
Figure 71: Lever Design Development
Figure 72: Final Lever Design
79
5.2 Detailed Design
The Test Rig concept could now be developed further on SolidEdge. Stress
calculations and considera...
80
5.2.3 Draft Drawings
Once the final solid models had been completed a full set of draft drawings were
produced for the ...
81
6. TESTING
This section of the report covers the calibration of the load cell and the testing
carried out on the Rig. T...
82
Figure 74: First Calibration Test plot of Voltage Vs. Time
It is clear from Figure 74 that the line sloping downward fr...
83
This Data was then plotted and a trend line added across the average points so that
the slope, and hence the scale fact...
84
6.2.1 Procedure
The testing involved measuring and recording several variables. These variables
are listed as follows.
...
85
Figure 77, as follows, shows the Spring Section of the Device fixed securely in the
mounting points.
Figure 77: Spring ...
86
g-clamps. Figure 78, as follows, shows the base plate mounted to the table with and
without the rubber base and finally...
87
accessed by copying the following file link into the address bar on an internet
browser.
https://www.dropbox.com/s/6o66...
88
Figure 79: Table of Data recorded from Instron
89
The impact data recorded using LabView was extracted from the text file output
using Matlab and then inserted into an e...
90
vibration damping. The rubber can be considered as a representation of the soft
tissue in a patient as it absorbs some ...
91
Figure 82: Extrapolated Load Cell Data, 4KN and 6KN
The extrapolated load cell data for the 4KN and 6KN loads are shown...
92
7. RESULTS AND DISCUSSION
This section of the report covers the analysis and discussion of the Instron and Test
Rig Dat...
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements
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Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements

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MSc Dissertation by Robin Maguire submitted to the University of Bath, September 2013

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Design and Development of Medical Device to Improve Assembly of Head/Neck Taper Junctions in Modular Total Hip Replacements

  1. 1. UNIVERSITY OF BATH DEPARTMENT OF MECHANICAL ENGINEERING DESIGN AND DEVELOPMENT OF A MEDICAL DEVICE TO IMPROVE THE ASSEMBLY OF HEAD/NECK TAPER JUNCTIONS IN MODULAR TOTAL HIP REPLACEMENTS Submitted by Robin Maguire For the Degree of MSc Engineering Design September 2013
  2. 2. COPYRIGHT Attention is drawn to the fact that copyright of this dissertation rests with the author. This copy of the dissertation has been supplied on condition that anyone who consults it is understood to recognise that its copyright rests with its author and that no quotation from this dissertation and no information derived from it may be published without the prior written consent of the author. This dissertation may be available for consultation within the University Library and may be photocopied or loaned to other libraries for the purpose of consultation. CHEATING AND PLAGIARISM “I  certify  that  I  have  read  and  understood  the  entry  in  the  Student  Handbook   on Cheating and Plagiarism and that all material in this assignment is my own work, except where I have indicated with appropriate references. Name: Robin Maguire Student Number: 129399661 Signed: ___________________ Date:_______
  3. 3. ABSTRACT There has been a significant failure rate in modular total hip replacements (MTHR) over the past few years, particularly with the use of large diameter Metal on Metal (MoM) bearings. Various studies have shown that sub-optimal strength of the head-neck taper junction plays an important role in these high failure rates. The purpose of this project is to design and develop a medical device to improve the assembly of this taper junction with an overall aim to reduce the occurrence of early revision surgeries on MTHRs. The device aims to ensure axial alignment of the head and neck tapers before providing an adjustable impact force between 4KN and 6KN to achieve the strongest possible junction assembly, with the target of reducing the incidence of fretting and corrosion at this junction and its associated problems. User-Needs and Design requirements for this device were established thought an in depth investigation of relevant literature. From this investigation a Product Design Specification (PDS) was produced and a final concept generated, based on these requirements. A Testing Rig was then developed and manufactured as a proof-of-concept for the impact delivery system proposed in the final design. Testing and evaluation using this Rig provided useful data emphasising the effect of tip material selection on the impulse force produced by the Testing Rig. This project has resulted in the development of a device design capable of improving the strength of head/neck taper junctions in MTHRs. It has also resulted in the manufacture of a lab testing rig which can be used again in the future to further aid the development of this design.
  4. 4. 2 CONTENTS 1. INTRODUCTION 1 1.1 What is a Total Hip Replacement? 1 1.2 What are THRs used to treat? 2 1.3 What is a Modular Total Hip Replacement? 3 1.4 Why are they Modular? 4 1.5 How are MTHRs implanted? 4 1.6 How are MTHR assembled? 5 1.8 What is the problem effecting MTHR? 6 2. AIMS AND OBJECTIVES 8 3. LITERATURE REVIEW 9 3.1 Understanding the problem and why it is occurring 9 3.2 How to reduce or eliminate the problem 12 3.2.1 What influence do Manufacturers have on the assembly? 12 3.2.2 What influence do Surgeons have on the assembly? 15 4. USER NEEDS & DESIGN REQUIREMENTS 24 4.1 Fundamental Design Requirements 26 4.2 Establishing and Defining User Needs 27 4.3 Product Design Specification 33 4.3.2 Commercially available designs and patent research 35 4.3.1 Medical device standards review 38
  5. 5. 3 4.3.2 Generation of PDS 39 5. DEVELOPMENT & EVALUATION 40 5.1 Concept Generation and Evaluation 40 5.1.1 Initial Product Design Specification (PDS) 41 5.1.2 Discretisation of Design Challenge 42 5.1.3 Radial Thinking 43 5.1.4 Visual Concept Analysis 44 5.1.5 Critical Assessment and Selection 52 5.1.6 Further Investigation of Powering Concept 55 5.1.7 Development of Final Powering Concept 60 5.1.8 Mechanical Feasibility of Chosen Concept 67 5.1.9 Development of Proof-Of-Concept Testing Rig 73 5.2 Detailed Design 79 5.2.1 Solid Modelling 79 5.2.3 Draft Drawings 80 5.2.4 Manufacturing 80 6. TESTING 81 6.1 Calibration of the Load Cell 81 6.2 Rig Testing 83 6.2.1 Procedure 84 6.2.2 Data recorded 87 7. RESULTS AND DISCUSSION 92 7.1 Evaluation of Instron Data 92 7.2 Evaluation of Load Cell Data 93 7.3 Evaluation of Testing Rig 95
  6. 6. 4 7.4 Discussion 95 8. CONCLUSIONS 96 10. REFERENCES 98 11. APPENDICES 101
  7. 7. 5 NOMENCLATURE N = Newtons KN = Kilo Newtons F = Force m = Mass g = acceleration due to gravity (9.81m/s) m/s = meters per second Kg = Kilograms t = Time (in seconds) Δt = Impact duration v = Velocity Δv = change in velocity k = Spring Stiffness (in N/m) ABBREVIATIONS THR = Total Hip Replacement MTHR = Modular Total Hip Replacement MoM = Metal on Metal CoC = Ceramic on Ceramic CoM = Ceramic on Metal UHMWPE = Ultra High Molecular Weight Polyethylene PDS = Product design specification
  8. 8. LIST OF FIGURES AND TABLES FIGURES Figure 1: Illustrated Hip Replacement; Before and After [2]....................................... 1 Figure  2:  Charnley’s  Low  Friction  Arthroplasty  [4] ..................................................... 2 Figure 3: Illustration of Normal Vs. Arthritic Hip [5] .................................................... 3 Figure 4: Exploded View of MTHR Assembly [7] ....................................................... 3 Figure 5: Illustration of MTHR Surgical Procedure [11].............................................. 4 Figure 6: Orthopaedic Mallet and Impactor [12]......................................................... 6 Figure 7: Example of matched and mismatched taper angles [20].......................... 13 Figure  8:  Stuart  Pugh’s  Design  Process  Model  [33]................................................. 25 Figure 9: Orthopaedic Surgeon Survey Introduction................................................ 28 Figure 10: Surgeon Survey, Q1............................................................................... 28 Figure 11: Surgeon Survey, Q2............................................................................... 29 Figure 12: Surgeon Survey, Q3............................................................................... 30 Figure 13: Surgeon Survey, Q4............................................................................... 30 Figure 14: Surgeon Survey, Q5............................................................................... 31 Figure 15: Surgeon Survey, Q6............................................................................... 31 Figure 16: Surgeon Survey, Q7............................................................................... 32 Figure 17: Surgeon Survey, Q8............................................................................... 32 Figure  18:  Stuart  Pugh’s  Design  Core  [33] .............................................................. 34 Figure 19: Controlled Force Hammer [35] Figure 20: Controlled Force impacting device [36]....................................................................................... 35 Figure 21: Handling Device for Hip Implant [37] Figure 22: Hip Joint Prosthesis and Fitting Tool [38]......................................................................................... 35
  9. 9. 2 Figure 23: Device for Handling Hip joint Heads [39] Figure 24: Head holder and impactor [40].................................................................................. 36 Figure 25: Method of applying Femoral head Resurfacing [41] Figure 26: Nail gun Patent 1 [42] ............................................................................................. 36 Figure 27: Nail gun Patent 2 [43] Figure 28: Wire Shelf Driver [44]........................................................................................................ 36 Figure 29: Automatic Centre Punch [44].................................................................. 37 Figure 30: Inserter jaw for knee prosthesis impaction and extraction [45]................ 37 Figure 31: Comparison in grading between EU and USA Device Classification [48] 38 Figure 32: Initial sketch to capture ideas ................................................................. 42 Figure 33: Development of Objective B ................................................................... 43 Figure 34: Development of Objective C................................................................... 44 Figure 35: Three Prong Flexible Support and Centring Cone Sketch ...................... 45 Figure 36: Semi-Cup + Centring Cone Sketch......................................................... 45 Figure 37: Bent Hex-Rod Sketch............................................................................. 46 Figure 38: Bent Rod Circular Profile Sketch ............................................................ 46 Figure 39: Split Cup/Split Mould Sketch .................................................................. 47 Figure 40: Slide Hammer - Direct Impact Sketch..................................................... 47 Figure 41: Slide Hammer - To Charge Spring Sketch.............................................. 48 Figure 42: Magnets Sketch...................................................................................... 48 Figure 43: Electro-Magnets Sketch ......................................................................... 49 Figure 44: Screw Mechanism - No Impact Sketch................................................... 49 Figure 45: Screw Mechanism - To Charge Spring Sketch ....................................... 50 Figure 46: Lever - To Charge Spring Sketch ........................................................... 50 Figure 47: Pneumatic Piston Sketch........................................................................ 51 Figure 48: Rod Inserted In Stem Hole Sketch.......................................................... 51 Figure 49: Mechanism to Hook around the Lips at Base Sketch.............................. 52 Figure 50: Pneumatic Nail Gun Illustration [49]........................................................ 56
  10. 10. 3 Figure 51: The Solenoid Powered Nail Gun [49]...................................................... 57 Figure 52: Electric Powered Nail Gun [49]............................................................... 58 Figure 53: Can-Crushing Device [50] ...................................................................... 59 Figure 54: Adapted Juicer Sketch............................................................................ 60 Figure 55: Adapted Can Crusher Sketch................................................................. 61 Figure 56: Corkscrew Lever System Sketch............................................................ 62 Figure 57: Twisting Adjustment and Release Concept Sketch................................. 63 Figure 58: Gearbox Style Spring Compression Adjustment Concept Sketch ........... 63 Figure 59: Spring Compression adjustment System Sketch .................................... 64 Figure 60: Slotted Trigger Release Mechanism Sketch........................................... 65 Figure 61: Firearm Trigger Concept Sketch............................................................. 65 Figure 62: Handle Trigger System Sketch ............................................................... 65 Figure 63: Final Developed Concept Sketch ........................................................... 66 Figure 64: Tubular Casing Surrounding Spring Sketch............................................ 67 Figure 65: 3 step sketch illustration of corkscrew charging method ......................... 67 Figure 66: Force Balancing Free Body Diagram...................................................... 68 Figure 67: Instron Loading Machine [51] ................................................................. 74 Figure 68: Initial Testing Rig.................................................................................... 75 Figure 69: Simplified Concept for Testing Rig.......................................................... 76 Figure 70: Initial Lever Design................................................................................. 77 Figure 71: Lever Design Development .................................................................... 78 Figure 72: Final Lever Design ................................................................................. 78 Figure 73: Final Assembled SolidEdge 3D Model.................................................... 79 Figure 74: First Calibration Test plot of Voltage Vs. Time........................................ 82 Figure 75: Load Cell Calibration Test Data Plotted on Graph .................................. 83 Figure 76: Custom Mounting points, close up view of top, close up view of bottom . 84 Figure 77: Spring Section in mounting points, close up view of top, close up view of bottom ............................................................................................................. 85
  11. 11. 4 Figure 78: Base plate fixed to table with and without Rubber Base ......................... 86 Figure 79: Table of Data recorded from Instron....................................................... 88 Figure 80: Test 2, Steel, 2KN (with rubber base)..................................................... 89 Figure 81: Extrapolated Load Cell Data, >2KN........................................................ 90 Figure 82: Extrapolated Load Cell Data, 4KN and 6KN ........................................... 91 Figure 83: Images of Failed PVC and Rubber Tips at 4KN Load............................. 92 TABLES Table 1: Data from Pennock et al. (2002) study [29], [10]........................................ 17 Table 2: Data from Lavernia et al. (2009) Study [30], [10]........................................ 18 Table 3: Data from Heiney et al. (2009) Study [31], [10] .......................................... 19 Table 4: Data from Rehmer et al. (2012) Study [32], [10]......................................... 21 Table 5: Initial PDS.................................................................................................. 41 Table 6: Scoring Table for Sub-System Concepts ................................................... 54 Table 7: Data recorded during Load Cell Calibration ............................................... 82
  12. 12. 1 1. INTRODUCTION 1.1 What is a Total Hip Replacement? There are two types of Hip Replacement surgery, Hip Resurfacing and Total Hip Replacement (THR). This project focusses on the THR procedure, which is also known as Total Hip Arthroplasty. THRs are among the most common orthopaedic procedures performed today [1]. The THR procedure involves removing the femoral head (top of the thigh bone) and a layer of bone from in and around the acetabulum (hip socket) and replacing them with artificial materials, thus resulting in an artificial hip joint. The before and after pictures of a hip joint that has undergone a THR is shown in Figure 1 [2] below, with the original diseased hip joint shown on the left and the new replacement joint shown on the right. Figure 1: Illustrated Hip Replacement; Before and After [2]
  13. 13. 2 The first modern THRs were designed by John Charnley in the 1960s, which stemmed from his paper   “Surgery   of   the   Hip   Joint   - present and future developments” [3] published in 1960.  Charnley’s  THR consisted of a high density polyethylene cup that was fixed inside the hip joint socket and a stainless steel component that made up the artificial femoral head and stem which slotted into the patient’s femur. This  “low  friction  arthroplasty” [4] hip design was first implanted in November 1962 and can be seen in Figure 2 [4] below. Figure 2: Charnley’s  Low  Friction  Arthroplasty [4] 1.2 What are THRs used to treat? THRs can be used to treat degenerative arthritis in the hip joint and can also be used to treat femoral neck fractures [1]. The original hip joint shown previously in Figure 1 [2] is arthritic, as you can see the deterioration of the bone at the ball/socket contact point. This is shown in more detail in Figure 3 below [5] where illustrations show the difference between and healthy and arthritic hip joint.
  14. 14. 3 Figure 3: Illustration of Normal Vs. Arthritic Hip [5] 1.3 What is a Modular Total Hip Replacement? Modular THRs (MTHR) were introduced in the 1970s [6]. The previous leg (femoral) component used in Charnley’s  original  design  was  separated into head and stem components and the previous plastic cup was separated into shell and liner components. An example of such a modern modularised THR design is shown in Figure 4 [7], as follows, using an exploded view of an assembly. Figure 4: Exploded View of MTHR Assembly [7]
  15. 15. 4 1.4 Why are they Modular? Modularisation was introduced into the design of THRs in the 1970s [8] to allow more flexibility in material selection/combination and component sizing to ensure a more individually suited THR for each patient. It also allows surgeons to reduce inventory [9] and simplifies revision surgeries [8]. Several different material choices and combinations are available to surgeons. For the stem and head; Cobalt Chrome or Ceramic Heads can be used on Titanium stems. For the bearing combinations; Metal on Metal (MoM), Ceramic on Ceramic (CoC) and Ceramic on Metal (CoM), and finally Ceramic or Metal heads can also be used on Ultra High Molecular Weight Polyethylene (UHMWPE). [10] 1.5 How are MTHRs implanted? The MTHR procedure is illustrated in four steps in Figure 5 as follows. Figure 5: Illustration of MTHR Surgical Procedure [11]
  16. 16. 5 Step A involves making an incision to gain access to the joint area and dislocating or  “disarticulating”  [11] the femoral head from the acetabulum (or hip socket). Step B involves cutting off the femoral head with a surgical saw. Step C involves reaming out the acetabulum and the femur to prepare them to receive the shell and stem respectively. Step D involves the introduction of the prosthetic components and the final image in the bottom right hand side of the figure shows the fully installed THR. 1.6 How are MTHR assembled? The order in which the components are introduced in this procedure is important to note. The acetabulum shell is first introduced and fixed in place before the stem is inserted into the femur. There are two types of stem designs; cemented, where special cement is inserted into the reamed femur before the insertion of the stem which is then used to permanently fix the stem in the femur; and cement-less, where the surface finish on the stem is designed to encourage bone growth and adhesion to the stem. Once the shell and stem have been introduced, the surgeon can then trial their proposed head and liner size using special trial head and liner components. Once these trial components have been fitted and the head (already fitted to the stem) has been located in the hip socket (liner), the surgeon can then check the fit of the joint in the patient by checking leg length and the range of motion available. Once the surgeon is happy with the proposed size of their head and associated liner the trial versions are removed and the exposed shell cup and stem taper surfaces are cleaned to prepare them for the introduction of the final head and liner components. The liner is inserted and fixed in place before the head is placed on what is called the neck taper of the stem and the head is then impacted onto the
  17. 17. 6 stem taper using a mallet and impactor (usually tipped with a softer material than the head so as not to damage the surface of the head). An example of the sort of mallet and impactor commonly used is shown in Figure 6 [12] below. Figure 6: Orthopaedic Mallet and Impactor [12] The male neck taper on the stem is cone shaped. This taper is designed to match a female taper on the inside of the femoral head component. When the head is impacted onto the neck the taper, the head taper must expand as it is forced down the neck taper by the impact delivered by the surgeon. This increases surface friction and creates hoop stresses that fix the two components firmly in place. 1.8 What is the problem effecting MTHR? However, it is this particular junction which has been the focus of much research and analysis over the past few years and is gaining more and more attention. MTHR have long been associated with earlier than expected revision surgeries and many
  18. 18. 7 product recalls. Large diameter (>36mm) MoM bearings are by far the worst offenders when it comes to early revisions and product recalls. The use of large diameter MoM bearings amplifies an existing issue regarding the strength of the head/neck taper junction more so than other joint material and geometry selections. Large diameter bearings produce an increase in the torque in the joint, as a larger frictional torque is generated since there is a longer lever arm acting between the fulcrum or centre of the joint’s rotation and the surface where the head makes contact with the liner, especially MoM. This increase in force about the junction, leads to increased levels of fretting wear and corrosion, which causes the liberation of prosthesis material and hence early revision surgeries (as the human body has an adverse reaction to the presence of these foreign particles). Fretting corrosion and wear can still occur in all material and geometry combinations but the accelerated and extreme instances found in some of the large diameter MoM bearings, have really highlighted this problem and raised its importance in all MTHR designs. Various studies, which will be discussed in the next section, have shown that the main factor affecting the longevity of MTHRs is the strength of the head/neck taper junction. The main influence on the assembly of this junction is the magnitude of the impact force applied during assembly. The next section of this report lays out the aims and objectives of this project in a clear and concise manner.
  19. 19. 8 2. AIMS AND OBJECTIVES Aim: To design and develop a medical device to improve the assembly of head/neck taper junctions in MTHRs with an overall aim to contribute to the reduction of early revision surgeries for MTHRs Objectives: 1. Review relevant literature to gain greater understanding/scope of problem 2. Establish User Needs and Design Requirements 3. Produce Product Design Specification (PDS) 4. Generate and evaluate design concepts 5. Design and develop a prototype for proof-of-concept 6. Manufacture and Test prototype 7. Evaluate test results and review overall concept
  20. 20. 9 3. LITERATURE REVIEW This section of the report contains the findings from the literature review carried out on the failure of MTHRs due to head/neck taper junctions. The review spread out beyond the borders of this specific issue to ensure an understanding of the bigger picture could be taken into account before focussing on the specific problem itself towards the end of the review. The findings are now presented under two headings, “Understanding  the  Problem and why it is occurring”, followed by “How to reducing or eliminate the  problem”. 3.1 Understanding the problem and why it is occurring Before trying to solve the problem it is essential to take the time to fully understand the background and history of the problem and the reason behind its occurrence. One of the most renowned examples in the failure of large diameter MoM MTHR was the Depuy ASR. Five hundred and five Depuy ASR MTHRs were implanted in total [13]. They were found to have an extremely high failure rate of 48.8% after six years [13]. The design was found to have failed for two reasons, and Heneghan et al. (2012) [14] made an important connection between the ASRs design failings and other existing designs that are still being used today. The first reason for the failure of the ASR was due to the acetablular cup being too shallow. This led to wear at the bearing edges and hence starvation of lubrication in the bearing area which led to increased wear around the edges of the cup, thus creating a self-destructive cycle. The wear causes the liberation of metal particles from the bearing surfaces which cause   “extensive   soft   tissue   necrosis   and   disruption   of   bone” [14]. The second failure reason, which is the most relevant to this project, was due to the increased torque experienced at the head/neck taper junction which was caused by the larger
  21. 21. 10 diameter of the bearing. This increase in torque caused wear and fretting corrosion which again led to the liberation of metal particles and thus patient complications, as with the first failure method. The reason why the second failure method is the most important to this project is because the use of large diameters in MoM bearings is not unique to the ASR design and so plays a role in the failure rates of various other MTHR designs. Both Henghan et al. (2012) [14] and Langton et al. (2011) [13] agree on this point and Langton et al. go on to suggest that bearing diameters of 36mm or greater are most at risk to this failure method. Smith et al. (2102) [15] concluded, following an in depth analysis of National Joint Registry Data covering England and Wales, that MoM bearings are more likely than other bearing material combinations to fail, and also found that their failure rates were increasing proportional to increasing bearing diameter size. Langton et al. (2011) [13] also came to the same conclusion in their study into the failure of the Depuy AST MTHR thus adding further backing to this theory. The UK government, by way of the Medicines and Healthcare Products Regulatory Agency (MHRA), took action on the issue by releasing a Medical Device Alert (MDA/2012/036) [16]. This MDA provided instructions on monitoring patients with MoM Hip Replacements (or Hip Resurfacings), and highlighted the dangers associated with the use of the Depuy ASR models which had been recalled. Smith et al. (2012) suggested in his study on the Joint Registry data that the increased failure rates for Large Diameter MoM bearings could be due to the loosening of the head/neck taper junction caused by the increased torque acting about that junction, which was also suggested by Heneghan et al. (2012) [14], as mentioned earlier. Bishop et al. (2008) [17] carried out a study into the frictional moments in MTHRs and found that MoM bearing combinations produced the
  22. 22. 11 greatest frictional moments followed by Metal on UHMWPE and then CoC with the lowest frictional moments, thus adding to the growing evidence pointing at the failings of MoM bearings. Langton et al. (2011) [13] found that as the trend in increasing bearing diameter grew, there was no increase in the diameter of the neck taper to counter the associated increase in torque. In a different study by Langton et al. (2012) [18] they noted that neck diameters actually decreased as larger and larger bearings sizes became available, thus exacerbating the problem. The reasoning behind reducing the diameter of the neck taper was to increase the range of motion of the prosthesis. One of the biggest studies of MTHR neck/taper junctions was carried out by Goldberg et al. (2002) [19] and looked into various different aspects and failure methods in this junction. One of the recommendations that they put forward following their research was to increase the neck taper diameter with an aim to increasing the stiffness of the neck so as to reduce the fretting and corrosion that they found to be occurring at this junction. To summarise, the importance of the strength of the head/neck taper junction has been highlighted by the recent failing of some designs of MTHRs (example Depuy ASR). These designs have increased the head diameter to reduce the occurrence of dislocations of the head in the hip socket, and reduced the neck taper diameter to increase the range of motion in the joint, but have actually increased the torque about the bearing, which has a loosening effect on the head/neck taper junction. This loosening allows micro-motions in the junction, which then leads to fretting wear and corrosion. The fretting wear and corrosion causes the liberation of taper material particles which are toxic to the human body and produce patient complications that are treated with revision surgeries.
  23. 23. 12 Now that the severity and reasoning behind the failure MTHRs has been established the next step is to look at who has influence or control over the reduction or elimination of the problem. 3.2 How to reduce or eliminate the problem This part of the chapter looks at who has control over or influence on the key factors that contribute towards the strength of the head/neck taper junction. This part finishes with an in depth analysis of four particularly relevant studies with an aim to establishing the optimum conditions and provisions for assembling a head/neck taper junction. 3.2.1 What influence do Manufacturers have on the assembly? Studies by Langton et al. (2011) [13], Langton et al. (2012) [18] and Goldberg et al. (2002) [19] have all stressed the importance of the neck taper diameter in ensuring a strong head/neck taper junction. It must be large enough to provide the neck stiffness required to support the joint and prevent loosening under high torques. Manufacturers have control over this dimension, since they are the ones designing the products. Now that they have been made aware of this factor by the aforementioned studies they can develop their future designs with this in mind. Others factors that can influence the strength of this junction are the tolerances applied during manufacture. For example, it is very important that the head and neck taper angles are as closely matched as possible for an optimal fit. This is illustrated in Figure 7 [20] as follows.
  24. 24. 13 Figure 7: Example of matched and mismatched taper angles [20] Figure 7 Part A shows the correct fit with the maximum contact area between the head and neck tapers. Figure 7 Part B on the right shows a poor fit where there is a significant taper angle mismatch leaving low contact area creating stress concentrations and room for micro-motion (also described as toggling) which leads to fretting corrosion and wear. Goldberg et al. (2002) [19] stressed the importance of angular mismatch and conicity of the tapers when trying to reduce fretting wear and corrosion at the head/neck taper junction. They also mention the importance of a good  quality  surface  finish  or  “roughness”  and  its  effects  on  the  strength  of  the  taper   junction. Fessler and Fricker (1989) [21] established a connection between the presence of high hoop stresses in alumina heads and only small levels of taper angle mismatch. Aside from hoop stress concentrations angular mismatch also facilitates micro-motion since the taper is not rigidly supporting itself along its length, as it is only held over a small collar of area (where the stress is concentrated). Shareef and Levine (1995) [22] confirmed a link between taper angle mismatch (including general manufacturing tolerances) and micro-motion found in taper junctions, in their study into this phenomenon. Scharmm et al. (2000) [9] also made the valid point that the accuracy of machining and the development of better wear-
  25. 25. 14 resistant materials plays an important part in the improvement of these designs, along with the recognition that only a very small mismatch is required to begin a cycle of fretting corrosion and wear. In a study which involved measuring the forces required to disassemble three different model of MTHRs, which had been retrieved from patients undergoing revision surgeries, Lieberman et al. (1994) [23] made an interesting discovery. One of the models required a much greater force than the other two to be disassembled and was the only model type of the three examined not to show any signs of corrosion after a 78 month period. These particular MTHRs had a different assembly history from the other two model types in that they had been assembled by the manufacturer and were supplied to the surgeon in a preassembled condition. These MTHRs had been shrink fitted with a sealant, applied during this assembly. Lieberman et al. (1994) [23] believe the greater junction strength and resistance to corrosion was due to improved manufacturers tolerances and the use of the sealant during assembly. However, based on the studies in the next part (3.2.2) of this report, it is suggested that another influence had made the difference in the assembly. The depth to which the neck would have reached inside the head taper would have been increased due to the shrink fitting process and no doubt would have made the junction stronger and hence more difficult to disassemble. In normal assembly conditions with a surgeon assembling the head and neck tapers at room temperature, the previous fit could only be replicated with an axially aligned impact force of optimum magnitude. As mentioned previously the assembly of MTHRs is carried out in-vivo by a surgeon using a mallet and impactor. The next section addresses the influence that the surgeons can have on the strength of the head/neck taper junction.
  26. 26. 15 3.2.2 What influence do Surgeons have on the assembly? The   “fit   of   the   spigot   head”   is   noted   as   the   “most   important   source   of   error”   in   Fessler   and   Fricker’s   (1989)   [21] study   into   the   “Stresses   in   Alumina   Universal   Heads   of   Femoral   Prosthesis”.   Bobyn   et   al.   (1994) [24] would agree with their statement since they found a reduction in taper surface contact area and an increase in wear and fretting corrosion in two Modular Femoral Prosthesis, after assembling them both using one fifth of the manufacturers recommended assembly force and exposing them to the sort of cyclic loading that they would experience in- vivo. Thus the impact load applied has a significant influence on the fit or the assembly of the head on the neck. A study carried out by Goldberg and Gilbert (2003) [25] entitled “In   vitro   corrosion   testing   of   hip   tapers”   concluded that the “proper  seating  of  the  head  onto  the  neck”  increases  the  forces  required  to  cause   micro-motion, and hence wear and fretting corrosion. A study by Mroczkowski et al. (2006) [26] mimicked much of the Goldberg and Gilbert’s (2003) study but added a sub-study looking at varying the impact applied during assembly and found that, out of head/neck tapers assembled in air and water and using either hand press assembly or a 6.7KN impact, the tapers assembled in air using 6.7KN showed the best resistance to fretting corrosion and wear under cyclic loading. This study again adds to the proof of the importance of the impact magnitude when trying to prolong the life of MTHRs in-vivo. However, as much of the previous studies would lead to the assumption that the greater the impact the better the assembly strength, this is not the case because if too great a force is used during assembly it can actually damage the taper interface and cause potentially catastrophic damage to the femoral head as well as other issues with the interface between the stem and the femur.
  27. 27. 16 The impact force is not the only important factor that the surgeon has influence over during assembly. The axial alignment when placing the head on the neck taper prior to impact and the axial alignment of the impact delivered is also extremely important. Both Callaway et al. (1995) [27] and Pansard et al. (2012) [28] traced back the failure of a number of MTHRs to incorrect fitting of the head on the neck taper by examining retrieved MTHRs removed during revision surgeries. They both found that their retrieved Hip Replacements had failed due to extreme corrosion caused by incorrect fitting of the head during original assembly. Due to varying manufacturing tolerances between different brands, it is also strongly recommended not to mix different manufacturers components as this can result in poorly fitted parts that can reduce the life of the prosthesis. Four key papers are now discussed with a focus on the effects of the impact/s applied during the assembly of the head/neck taper junction with an aim to establishing the optimum impact magnitude and number of impacts so that this information can then be used to guide the design of the device that this is being developed for this project. The relevant information has been extracted from each of the four studies in an effort to simplify them for ease of comparison and add clarity to the investigation. The implant quantities, head diameter, assembly tools, impact forces, number of impacts and the pull-off forces (forces to pull the assemblies apart) have been used as the categories by which to analyse these studies. The studies will appear in chronological order, starting with Pennock et al. (2002) [29] study  entitled  “Morse- type tapers: factors that may influence taper strength during  total  hip  arthroplasty”. The relevant data from this study has been populated in the standard table for this investigation and is shown in Table 1 [10] as follows.
  28. 28. 17 Table 1: Data from Pennock et al. (2002) study [29], [10] This study looks at the effects of varying the magnitude of the impact force, the order in which the different impact forces are applied and the total number of impacts delivered during assembly and their effect on the resulting junction strength (determined by pull-off tests). This study also looked at the effects of wet and dry taper surfaces on junction strength, but the wetted samples were not included in this table as Pennock et al. (2002) established that wet taper surfaces reduced the strength of the junction. However, since the wet tapers were not used, this halved the size of the data that could be used in Table 1 [10] and hence limited the amount of data available for use and thus limited the credibility of the data used in the study. It is also worth noting that the impact forces used were based around an average force that was found by measuring the impact force applied by a single surgeon over 11 impacts and came to an average of “2075N”   (N   for   newtons). It can be assumed   then   that   the   “Medium”  force   magnitude   listed   in   this  study   is  2075N   or   very   close   to   it,   though   it   is   impossible   to   say   what   magnitude   the   “Light”   and   “Heavy”  impacts  are. From the pull-off values shown in Table 1 [10] it can be assumed that the highest impact provides approximately 95% of the strength to the junction with further lighter impacts still adding a small contribution (approximately 5%) to the overall strength
  29. 29. 18 [29]. Pennock et al. (2002) [29] also states the importance of axial alignment when delivering the impacts, to ensure that all of the force is transmitted during the impaction. One of the findings taken from their study (especially when the wet tapers were taken into consideration) was that they noted an increase in junction strength with increasing impact magnitude [29]. The next study was carried out by Lavernia et al. (2009) [30] and looked mainly at the effects of blood and fat contamination on the taper surfaces and the effect they had on the junction’s   strength. However,   as   they   used   “control”   or   dry   tapers   for   comparison the data recorded for these was of benefit to this investigation and has been recorded in Table 2 [10] below. Table 2: Data from Lavernia et al. (2009) Study [30], [10] Since this data only covers the control for the main experiment, much like the previous study, it does not have the largest sample size and this is recognised as a limitation for the purpose of this investigation. It is also worth mentioning that the four specimens used were assembled and disassembled 5 times each to establish an average disassembly, and it is apparent that the repeated assembly and disassembly of these specimens lowers the strength of the junction and can askew the average disassembly forces to some degree. The impaction magnitude used in
  30. 30. 19 this study can be considered to be a more realistic representation of the average magnitude of a surgeons impact as they recorded the impact forces applied by 8 different surgeons as opposed to the previous study by Pennock et al. (2002) [29] that only used 11 impacts by a single surgeon. The result is a 27% (approx.) decrease of the force used by Pennock et al (2002). It is worth noting that this study does not vary the impact magnitude or the number of impacts applied, but does give a good representation of the average pull-off force for the prescribed magnitude with a single impaction and provides another average value for surgeon impaction magnitude, which will both be of use later when comparing this study to those that follow on in this part of the chapter. The overall study showed that a clean and dry taper provides the optimum assembly condition to facilitate maximum junction strength. The next study was carried out by Heiney et al (2009) [31] and had a much larger sample size of useable data for the population of Table 3 [10] as seen below. Table 3: Data from Heiney et al. (2009) Study [31], [10] The average impact force applied  by  surgeon’s  was  also measured for this study and improved upon the two previous study mentioned, as this time they took values from 10 surgeons to create the average value to base their range of varied impact
  31. 31. 20 assembly forces. The average surgeon’s  impact  force  applied  during  assembly in this study is roughly twice that of either of the two previous studies showing a large range of magnitudes arising across the different surgeons used to create the averages in each study. Heiney et al. (2009) [31] found there to be a difference in junction strength between using one impaction and two impactions but found no difference when applying more than two impactions. This finding compliments one of the findings from the study by Pennock et al. (2002) [29], in that the first impaction provides the majority of the junction strength with subsequent impacts providing a small but additional increase. Heiney et al. (2009) [31] also found that the junction strength increased along with the impact magnitude thus adding weight to this original finding by Pennock et al (2002) [29]. It is unfortunate that neither Pennock et al (2002) [29] nor Heiney et al. (2009) [31] provided the impact forces in newton values instead of written descriptions such as light, medium and heavy, so that the studies could be compared in more detail. Following the completion of their study Heiney et al. (2009) [31] recommended “at   least two firm,  axially  aligned  blows” to achieve optimum junction strength, but they also warned that  impacts  of  an  “excessively  high  magnitude”  can  lead  to  “femoral   fractures around cement-less  stems”, as the impact force may travel down into the patient during assembly and could damage the bone stem interface. The next study was carried out by Rehmer et al. (2012) [32] and was the largest and most relevant to this project. They looked at the effect that different material combinations, impact magnitudes and the number of impactions can have on the taper junction strength. One immediate benefit is that they provided the assembly impact forces in newtons and not in written descriptive terms as in the previous
  32. 32. 21 studies. The relevant data acquired from this study is shown in Table 4 [10] as follows. Table 4: Data from Rehmer et al. (2012) Study [32], [10] Using pull-off and twist-off disassembly tests, Rehmer et al. (2012) [32] found that a single impact of a minimum of 4KN (kilo-newtons) was required to ensure optimum taper junction strength. They also discovered, just like Heiney et al. (2009) and Pennock et al. (2002), that there was little or no benefit from more than one impaction and even went as far as suggesting that more than one impact could actually loosen the taper junction or reduce the strength provided by a previous impact. Rehmer et al. (2012) go on to state that if one single impact of a minimum of 4KN was set as a recommendation by manufacturers for the assembly of the femoral prosthesis, then this would place the responsibility on the surgeon to ensure “suitable   taper  fixation,   by   firm   and   careful   impaction”.  They   state   that   this would greatly improve the strength of the taper junction in modular femoral prosthesis, but unfortunately it is clear that this is currently not possible. This is proven in a similar study by Loch et al. (1994) [8] entitled  “Axial  Pull-Off Strength of Dry and Wet Taper
  33. 33. 22 head connections on a modular shoulder prosthesis”. In this study, they use a surgeon to try to acquire an average assembly impact force for which they can design a drop rig. The drop rig is then used to assemble their specimens with a constant impact force but under different conditions (i.e. dry or wet) prior to disassembly testing. The surgeon assembled 6 shoulder taper junctions using a mallet and impactor (the same as is used in a modular femoral hip assembly), and Loch et al. (1994) [8] then measured the force required to pull the joints apart. They repeated this process with the surgeon and the same six specimens 16 times to acquire their average pull-off value, which they then used to set a drop rig to assemble the test specimens to replicate this average pull-off value (under control conditions). It is acknowledged the average pull-off values may have been affected by the reduction in strength that can be experienced when repeatedly assembling and disassembling the same specimens. The pull-off forces, from the surgeon’s   assemblies, ranged from 958N all the way to 4893N which is a very wide range. This data on its own would not be enough evidence given the reduced taper junction strength associated with repeated assembly. However, looking back on each of the four key studies featured previously it is clear to see that surgeons are not providing the same magnitudes of impact, and even if they were, the axial alignment of the impact cannot be guaranteed. So, even if the surgeons could apply an impact force of no less than 4KN, this could still be diminished if the impact was not axially aligned with the head and neck tapers, or if the head taper was not seated correctly on the neck taper prior to impaction. Even though it has not been recorded in the studies featured here, it seems there is nothing stopping an impaction exceeding 12KN and creating the potential for internal damage to the patient. This prompts the suggestion that there is need for a device that can help surgeons ensure that the
  34. 34. 23 head is correctly seated on the neck taper before providing a single impact of no less than 4KN, which is axially aligned with the taper axis. The impact should also not exceed 6KN to ensure that it does not stray into the region where it could cause internal damage to the patient or damage to the tapers, as mentioned previously. The findings from this review will be used to guide the design of the device proposed in this project, and to aid the assembly of MTHRs. The next step in the process is to establish the user needs and hence design requirements for such a device.
  35. 35. 24 4. USER NEEDS & DESIGN REQUIREMENTS It is now clear from the Literature Review that there is room for improvement in the assembly of head/neck taper junctions in MTHRs. This “room for improvement” grows and becomes a serious problem when considered in the use of large diameter bearings, particularly MoM bearings. Even if manufacturers applied perfect tolerances, surfaces finishes and optimum neck taper diameters it is still essential to correctly assemble this taper junction to benefit from these improvements. It is clear from the wide ranging impact forces applied by different surgeons that it is unfair to expect them to be able to repeatedly provide the very specific forces and alignments required for the optimum assembly of MTHRs using the current tools at their disposal (mallet and impactor). Therefore the development of a new device is completely justified. It is also worth mentioning at this point in the report that the Design Process Structure being followed in this project is based  loosely  around  Stuart  Pugh’s  Total   Design Approach (1991) [33].  This  section  of  the  report  is  mimicking  the  “Market”   stage in Pugh’s  model,  as  shown  in  Figure  8  [33] as follows.
  36. 36. 25 Figure 8: Stuart  Pugh’s  Design  Process  Model  [33] The stage following this will be the development of a Product Design Specification (PDS) which is referred to in Figure 8 as  “Specification”.  The  “Concept design”  and   “Detail design” stages feature in the next chapter of this report. This section of the project involves extracting and clearly defining the design requirements from the Literature Review. It also includes researching and investigating other design requirements for the proposed device, with an aim to
  37. 37. 26 producing a Product Design Specification (PDS), which can then be used to guide the next stage of the project. 4.1 Fundamental Design Requirements These design requirements have been extracted directly from the findings in the literature review and form the foundation and basis for the entire design, i.e. the design must achieve all of these requirements to be successful. The fundamental design requirements are listed as follows. 1. Ensure axially aligned seating of head on neck taper axis prior to impaction 2. Impact must be delivered in axial alignment with neck taper axis 3. Deliver impact force of between 4KN and 6KN, adjustable to 0.5KN 4. Must try to isolate majority of impact to head/neck taper junction The first requirement is very much self-explanatory and is to try to prevent the failures recorded by Callaway et al. (1995) [27] and Pansard et al. (2012) [28] as mentioned previously in the Literature Review. The second requirement is essential to ensure the efficient transfer of the impact force into the junction assembly. The third requirement has also been added from the Literature Review as the device must now be adjustable to 0.5KN (or 500N). This is to account for the different manufacturing tolerances and the effect that they will have on the efficient transmission of the impact force into the taper junction. The adjustability will also be useful when using different materials, such as ceramic heads, that may require the
  38. 38. 27 minimum force, compared to larger metal taper junctions that may require slightly more force for optimum assembly. The fourth requirement involves trying to concentrate the impact to the taper junction and not down the stem where it could cause damage to the stem/femur interface. It is also intended to ensure the efficient transfer of impact energy into the junction and not to have it wasted through dissipation into the surrounding region. 4.2 Establishing and Defining User Needs Since the fundamental design requirements had now been established, the next step was to look beyond these fundamentals to establish other design requirements. It is extremely important to involve the end user in the design of anything to ensure that it meets their specific needs, so a survey was used to try to gather design information that would help to ensure that the device would not miss out on any important design features. After looking through several different methods of gathering these requirements through a user-centred-approach [34], such as ethnography or contextual inquiry, and consideration of the time and finance available for this project, a survey directed at orthopaedic surgeons (the end users) became the most suitable option. The survey consisted of 8 questions, with a combination of both text response and multiple-choice answers. The questions posed in this survey are now presented as follows. The beginning of the survey contained a very brief introduction into the background of the survey and its aim. This is shown in Figure 9 as follows.
  39. 39. 28 Figure 9: Orthopaedic Surgeon Survey Introduction After the introduction to the survey, the first question posed attempted to gain an understanding of the size of the range of different hip implants that were being used and whether or not they were cemented or cement-less. This would influence whether or not the device would be designed solely for use with a very popular model. If a wide variance was uncovered, it would lead to designing a more flexible device that could work with all different types of implants, or at least a large range of them. The specific wording chosen for this question can be seen in Figure 10 as follows. Figure 10: Surgeon Survey, Q1
  40. 40. 29 The next task was to establish the range of femoral head sizes. This was important to establish so that the device could be designed to facilitate the most common head sizes. This also fulfils the purpose of establishing a general impression of the current use of large diameter (>36mm) MoM bearings, given their associated problems previously mentioned in the literature review. The wording and layout of this question is shown in Figure 11 below. Figure 11: Surgeon Survey, Q2 Since it is important to understand the environment and orientation that the device would be used in, the next question attempts to establish which surgical approaches are used and which are the most common. The different approaches can determine the patient’s position, i.e. lying face up, face down or on their side. This can have an effect on the angle that the device may have to be used at, and impacts the ergonomics of the design. This question is shown in figure 12 below.
  41. 41. 30 Figure 12: Surgeon Survey, Q3 The next question aims to establish the size of the access area or incision in the patient that the device must fit and function inside. The average size is 10cm, so this question looks to see if many surgeons work under or above this incision size. This question is shown in Figure 13 below. Figure 13: Surgeon Survey, Q4 No instrumentation for the assembly has been discovered in previous investigations, apart from the mallet and impactor. However, it is still worth raising the subject with the surgeons, in case any custom-made or other such instrumentation is already being used. This question is shown in Figure 14 as follows.
  42. 42. 31 Figure 14: Surgeon Survey, Q5 The next question was not so much based on the establishment of design requirements for the device, but more so at gathering data to compare with the four studies listed at the end of the literature review. It was acknowledged that the responses could not be looked upon too strongly, as the information provided by the surgeons is opinion-based and thus is quite subjective. This question is shown in Figure 15 below. Figure 15: Surgeon Survey, Q6 An important opinion to gauge is the perceived importance of the isolation of the impact to the taper junction so as not to damage the stem/femur interface. This
  43. 43. 32 question was provided in a format where the participant rates the level of importance out of 10. This question is shown in Figure 16 below. Figure 16: Surgeon Survey, Q7 The   final   question   allowed   the   surgeon’s   to propose any features that they felt should be included in the design of the device. The intention of this question was to give the user an opportunity to directly propose things that were of importance to them so that the design would have some sort of user-centred-design approach. This question is shown in Figure 17 below. Figure 17: Surgeon Survey, Q8 The survey was created and distributed among the members of the Royal British Orthopaedics Association, following the completion of the Literature Review. The survey received a very good response, as 109 surgeons participated. However, survey participation only started towards the end of the project. Therefore the survey responses could not be considered in the design of the device during this project.
  44. 44. 33 However, a brief analysis of the survey has shown that there is a significant amount of very relevant data available which would be of great value to the future development of the device. A summary of the survey response is contained in Appendix 1. The original plan had been to gather the findings from the literature review and the feedback from the survey and allow this to contribute to the PDS, but as mentioned above this was not possible so for this reason none of the feedback from the survey influenced the PDS. The PDS will now be discussed in more detail in the next section. 4.3 Product Design Specification The purpose of a PDS is to provide the designer with a list of design requirements which can be regularly referred back to, so as to ensure that the design requirements are being satisfied at each stage in the design process.  Stuart  Pugh’s   “Total  Design   Approach”  had   a   significant   influence  of   the   way   in   which   the  PDS   was generated for this project. Pugh’s  Product  Design Elements that made up the Design core of his PDS document are shown in Figure 18 below. This provides a good example of what sort of information goes into a full PDS.
  45. 45. 34 Figure 18: Stuart  Pugh’s  Design Core [33] Each and every Element’s constituent parts must have a measureable value so that it can be clearly seen as to whether or not the design has satisfied the PDS. However, given the time and detail involved in an industrial level PDS where such detail is a requirement, this project used a slightly more simplified version. This version which contains a listing of the different design attributes and considerations that must be taken into account for the design to be successful in a clinical environment. One of the first steps in generating requirements, and also in aiding with concept generation in the later stages of the project, is to look into existing designs and patent research. This allows the examination of features from competitors or similar designs, and ensures that such a feature is not missed in the design of this device.
  46. 46. 35 4.3.2 Commercially available designs and patent research As mentioned previously, the only commercially-available designs for the assembly of the head/neck taper junctions are the orthopaedic hammer and impactor methods. An in-depth patent search was carried out to establish what other like- minded or similar and applicable designs already existed. Samples of some of the more interesting designs are shown as follows. Some of these have made an influence on the design of the device rig as can be seen later on in the Design and Development stage. Figure 19: Controlled Force Hammer [35] Figure 20: Controlled Force impacting device [36] Figure 21: Handling Device for Hip Implant [37] Figure 22: Hip Joint Prosthesis and Fitting Tool [38]
  47. 47. 36 Figure 23: Device for Handling Hip joint Heads [39] Figure 24: Head holder and impactor [40] Figure 25: Method of applying Femoral head Resurfacing [41] Figure 26: Nail gun Patent 1 [42] Figure 27: Nail gun Patent 2 [43] Figure 28: Wire Shelf Driver [44] As can be seen from Figure 25 (resurfacing), and from Figure 26 to 27 (nail guns), the patent search extended out to different devices with a similar purpose, which in
  48. 48. 37 this case involved delivering an impact or maintaining an alignment. Rough notes were taken during the patent search to keep track of any good ideas, which could then be applied directly or manipulated to fit into the device proposed in this project. Figure 29 and Figure 30, shown below; display two more applicable technologies that could be of use for the concept generation stage later in the project. Figure 29: Automatic Centre Punch [44] Figure 30: Inserter jaw for knee prosthesis impaction and extraction [45]
  49. 49. 38 4.3.1 Medical device standards review Medical Device Classifications exist to grade the level of risk that a medical device poses to a patient or user; the higher the grading, the more stringent the regulations imposed on the development and manufacture of the device. There are different classification systems for both the EU (EU/ISO [46]) and USA (FAA/ISO [47]), and as the risk or grading increases, so too do the design regulations imposed by the standardisation bodies to meet their audit requirements. The two grading streams are illustrated side by side in an extract from medical Device Design by P.J. Ogrodnik [48], as shown in Figure 31 below. Figure 31: Comparison in grading between EU and USA Device Classification [48] The device proposed in this project received the lowest grading in both systems (grade I), meaning it has the lowest risk, as it does not remain inside the patient and is in the same class as other common surgical tools, such as bone drill bits. This means, as is indicated in Figure 31, that the device design is mainly self-regulated. Several specific EU and US standards have been established that relate to the device and are featured and referenced in the PDS, which is found in Appendix 1, and is discussed in the next step. The purpose of the standard review was to establish any significant design constraints that would affect the device. However, as the device is only in an early prototyping stage in this project, it will not be ready
  50. 50. 39 for clinical trials, so the standards review is of greater value further down the line in future clinical design and development of this device. 4.3.2 Generation of PDS The PDS pooled all of the design requirements acquired through the project so far, and so took information from the Literature Review, the Surgeons’ Survey (although left open, pending response from participants), the Standards Review, and the patent and existing design research. The PDS is a working document, and can be added to and edited as future work progresses on the development of the device proposed in this project. The most up-to-date version of the PDS is included in Appendix 2 [10].
  51. 51. 40 5. DEVELOPMENT & EVALUATION This chapter of the report looks at the generation, evaluation and development of a concept for the device proposed in this project. It then examines the concept generation, development, detailed design and manufacture of a proof-of-concept Testing Rig, to verify the functionality of the overall device concept established in the first phase. 5.1 Concept Generation and Evaluation Since the PDS created in the previous chapter was quite detailed and in-depth, not all of the points covered will be relevant at this stage in the design. It is for this reason that the PDS was condensed down into its more critical attributes. This created a less constrained environment to work in when generating creative concepts. The new condensed PDS will be referred to as the “Initial PDS” from this point on. Shown below is a summary of the steps taken in this concept generation and evaluation phase. Step 1: Creation of Initial Product Design Specification (PDS) Step 2: Discretisation of design challenge Step 3: Radial Thinking Step 4: Visual Concept Analysis Step 5: Critical Assessment and Selection Step 6: Further Investigation of Powering Method Step 7: Development of Final Powering Concept Step 8: Mechanical Feasibility of Chosen Design Step 9: Development of proof-of-concept Testing Rig
  52. 52. 41 5.1.1 Initial Product Design Specification (PDS) A full PDS was developed for a prototype device aimed at use in clinical trials but for the purpose of this project, which will only be tested in laboratory conditions, the original PDS was condensed down. This was carried out in order to focus on the fundamental design requirements and to allow more creative freedom for concept development. The PDS used for the project at this stage is shown below. 1. Performance 1.1 Must hold head taper axially aligned with neck taper axis 1.2 Must deliver impact axially aligned with neck taper axis 1.3 Must deliver adjustable impact from 4KN - 6KN to +/- 0.5KN 1.4 Must isolate impact to head-neck taper junction 2. Customer 2.1 Must be able to use with varying head size 2.2 Must be able to use with varying stem size 2.3 Must be able to position itself inside cavity created by incision 2.4 Easy to disassemble for cleaning and sterilization 3. User Friendly 3.1 Weight, preference to be lightweight 3.2 Easy to use, quick, low task complexity 4. Manufacture 4.1 Easy to Manufacture 5. Cost and Materials 5.1 Meet requirements above at minimum cost 5.2 Meet requirements above at minimum material use Table 5: Initial PDS
  53. 53. 42 An initial sketch was made at this point to record any design ideas that had come to mind so far. This initial sketch is shown in Figure 32 as follows. Figure 32: Initial sketch to capture ideas 5.1.2 Discretisation of Design Challenge Even though the PDS was condensed down, the design challenge was still quite complex. For simplification, this design challenge was split into four sub-systems so that the concept generation could focus on the four key performance requirements listed in the Initial PDS. These four key objectives are labelled with letters for clarity during concept generation. The assignment of these letters is shown below. (A). Hold head taper axially aligned with neck taper axis (B). Hold impactor axially aligned with neck taper axis (C). Deliver adjustable impact between 4KN and 6KN to +/- 0.5KN (D). Isolate impact to head-neck taper junction
  54. 54. 43 5.1.3 Radial Thinking Radial thinking was used to expand on different concepts and help to develop and record different concept ideas. Each of the four key objectives, A to D, were listed in a bubble in the middle of a blank page and ideas stemmed outwards from this starting point. Two examples of this exercise are shown as follows. Figures 33 shows the development of Objective B, and Figure 34 shows the development of Objective C. Figure 33: Development of Objective B
  55. 55. 44 Figure 34: Development of Objective C 5.1.4 Visual Concept Analysis After writing down all the different methods of achieving the objectives, the next step was to roughly sketch them out so that they could be reasoned out visually and analysed (keeping the Initial PDS in mind). For comparison to the method currently used by the surgeons, a datum has been included at the beginning of each section (where one exists). The rough sketches of the concepts are grouped within the four key objective headings as follows. (A). Holding head taper axially aligned with neck taper axis *DATUM* Surgeon press fitting the head on the neck prior to impact
  56. 56. 45 (A1). Three Prong Flexible Support and Centring Cone Figure 35: Three Prong Flexible Support and Centring Cone Sketch (A2). Semi-Cup + Centring Cone Figure 36: Semi-Cup + Centring Cone Sketch
  57. 57. 46 (B). Holding impactor axially aligned with neck taper axis *DATUM* Surgeon holding impactor aligned by hand (B1). Bent Hex-Rod Figure 37: Bent Hex-Rod Sketch (B2). Bent Rod Circular Profile Figure 38: Bent Rod Circular Profile Sketch
  58. 58. 47 (B3). Split Cup/Split Mould Figure 39: Split Cup/Split Mould Sketch (C). Deliver adjustable impact between 4.5KN and 11.5KN within +/- 0.5KN *DATUM* Surgeon providing a hammer blow from a standard hammer (C1). Slide Hammer - Direct Impact Figure 40: Slide Hammer - Direct Impact Sketch
  59. 59. 48 (C2). Slide Hammer - To Charge Spring Figure 41: Slide Hammer - To Charge Spring Sketch (C3). Magnets (forced together when opposing poles) Figure 42: Magnets Sketch
  60. 60. 49 (C4). Electro-Magnets (Solenoid Actuator) Figure 43: Electro-Magnets Sketch (C5). Screw Mechanism - No Impact Figure 44: Screw Mechanism - No Impact Sketch
  61. 61. 50 (C6). Screw Mechanism - To Charge Spring Figure 45: Screw Mechanism - To Charge Spring Sketch (C7). Lever - To Charge Spring Figure 46: Lever - To Charge Spring Sketch
  62. 62. 51 (C8). Pneumatic Piston Figure 47: Pneumatic Piston Sketch (D). Isolating impact to head-neck taper junction *DATUM* Surgeons hand can grip the stem during impaction to prevent the impact from being transferred to the patient (D1). Rod Inserted In Stem Hole Figure 48: Rod Inserted In Stem Hole Sketch
  63. 63. 52 (D2). Mechanism to Hook around the Lips at Base Figure 49: Mechanism to Hook around the Lips at Base Sketch 5.1.5 Critical Assessment and Selection Each of the subsystems has been scored against their key design requirements and the relevant Initial PDS requirements from 0 to 10, ascending in increments of two. They are scored by how well they satisfy each of the requirements, as explained below. 0 = “not  at  all”, 2 = “a  little”, 4 = “below  average”, 6 = “above  average”, 8 = “very  good” 10  is  “perfect”
  64. 64. 53 The result of this exercise is the selection of concepts to address each of the four key performance requirements. This exercise was carried out using an excel spread sheet and is shown on the following page in Table 6. The highest scoring concept from each of the four sections was highlighted in yellow.
  65. 65. 54 Table 6: Scoring Table for Sub-System Concepts
  66. 66. 55 As the outcome from Table 6 has shown, the following sub-system concepts have been chosen and are listed as follows. (A1) 3 Prong Flexible Support + Centring Cone (B1) Bent Hex-Rod (C8) Pneumatic Piston (D1) Rod Inserted In Stem Hole Since  the  main  function  of  the  device  is  key  requirement  C  (“Must deliver adjustable impact from 4KN to 6KN, to +/- 0.5KN”) and the second and third ranked ideas in this  category,  which  were  “Electro-Magnets”  (C4)  and  “Lever  to  charge  spring”  (C7),   also scored relatively high, all of the top three designs in this category will be looked into in more detail before finally settling on a single concept. It is also worth mentioning that all combinations of selected successful concepts listed in the paragraph above function collectively, and do not   work   against   each   other’s   individual aims in the overall device design. 5.1.6 Further Investigation of Powering Concept As mentioned previously, the top three design concepts to achieve Objective C are as follows: 1. Pneumatic Piston 2. Electro Magnets 3. Lever to Charge Spring Due to the importance and influence of correctly achieving objective C, each of the top three designs were explored in more detail to ensure their engineering feasibly
  67. 67. 56 in the design. This investigation was performed by looking into existing products that were applying these three powering methods, and checking the suitability of each method for the device in this project. These are examined as follows, starting with the Pneumatic approach. Pneumatic Piston One of the best products to look at to examine the functionality of the different methods of powering a device that provides an impact is the Nail Gun. A pneumatic Nail Gun is shown in Figure 50 [49], below. Figure 50: Pneumatic Nail Gun Illustration [49] Figure 50 illustrates the basic process by which a pneumatic system works and it is clear that it could be used in the device for this project. Pressurised air is provided in the operating theatre up to 6 bar and pneumatic nail guns can run from 4 bar up to 22 bar for very heavy duty work. It is a clean and sterile method of operation. However, it would reduce the simplicity of the design as the use of pressurised air
  68. 68. 57 can become quite complex and expensive when designing and manufacturing. This project is trying to provide the simplest possible powering method, so for this reason the pneumatic approach does not seem to be the best fit. Electro magnets Electro magnets, or specifically in this case electro solenoids, can be used to electrically initiate magnetic fields, which can propel objects to create an impact. This type of system is explained once again using a nail gun example in Figure 51 [49] below. Figure 51: The Solenoid Powered Nail Gun [49] Again, it is clear that this sort of technology could be used to power the device for this project. Electricity is available in the operating room to power other electric medical devices, and battery packs can also be used to power such a system. However, the use of electro solenoids means that there will be electromagnetic fields generated which can interfere with other medical devices and patient implants
  69. 69. 58 that may be close to the device when in use. This also, much like the pneumatic option, this greatly increases the complexity in the design and means that the device will have to abide by more constraining standards during the design process. This will add difficulty and complexity to the future work on a clinical device, and thus rules out this technology as a possible option. Lever to Charge Spring The final option for powering the device is a charged spring, which is compressed using mechanical advantage, such as a lever system. An electrically powered mechanical spring system is shown in Figure 52 below, again using an example of an electric powered nail gun. Figure 52: Electric Powered Nail Gun [49] This sort of powering method would be the simplest to design, and could be adjusted to remove the need for an electric power system. By using a pure mechanical system, it would simplify the design even more and make the device
  70. 70. 59 more sustainable and reliable with no dependency on other inputs such as electricity and compressed air. The surgeon could use their energy to provide the work required to compress/charge the spring could be made easier with a leverage system. An example of the employment of this sort of powering mechanism is demonstrated in a simple can-crushing device as shown in Figure 53 below. Figure 53: Can-Crushing Device [50] Final Selection of Powering Method After reviewing the different strengths and weaknesses of employing each of the three different powering methods, as discussed previously, the option that seemed the most appropriate to this design was the spring compressed/charged by a mechanical advantage or levering system. This concept was then developed further in the next step of this process.
  71. 71. 60 5.1.7 Development of Final Powering Concept Now that the method which would power the device had been chosen, the next step was to develop the design in more detail to prove that it could actually work. There were three main design objectives that had to be met for the device to be able to function. The first was to confirm the final leverage method to compress the spring. The second was to come up with a way in which the device could deliver an adjustable impact (which had not been focussed on previously). Finally, the third objective was to design a trigger/release mechanism to actuate or initiate the impact. Mechanical Leverage System This stage involved more sketching and research, but this time just focussed on leveraging systems. Items such as hand-operated juicers, in which they compress the fruit to extract juice, were seen as applicable to the design of the device. A rough sketch showing the integration of such a system into the spring compression system is shown in Figure 54 below. Figure 54: Adapted Juicer Sketch
  72. 72. 61 More can-crusher products were also investigated, and an example of one of the sketches trying to use this approach is shown in Figure 55 below. Figure 55: Adapted Can Crusher Sketch These designs all seemed as though they would need quite a long handle to produce an adequate amount of enough force to compress a spring stiff enough to provide the impact energy needed for the device. One way of shortening the handles was to use two handles simultaneously. This is the sort of leverage system that is used on a typical wine bottle corkscrew. A sketch utilising the application of this design in the concept for this device is shown in Figure 56 as follows.
  73. 73. 62 Figure 56: Corkscrew Lever System Sketch This sort of mechanism could be attached onto the end of the device, and the surgeon could use both hands to push the levers down to the sides of the device. This would compress a spring that can be held in place by a locking mechanism and released by a trigger when ready for use. Adjustability The spring would need to be adjustable to between 4KN and 6KN as mentioned previously, and so a mechanism would need to be designed to allow such flexibility over   the   device’s   output. Several spring adjustment designs are shown in the Figures 57, 58 and 59 as follows.
  74. 74. 63 Figure 57: Twisting Adjustment and Release Concept Sketch Figure 58: Gearbox Style Spring Compression Adjustment Concept Sketch
  75. 75. 64 Figure 59: Spring Compression adjustment System Sketch Trigger/Release Mechanism A trigger/release mechanism would also be required so that the spring could be compressed away from the patient, locked in the compressed position, and then brought to the patient for use so that it could be discharged when the surgeon had the device in place. Several trigger mechanism concepts are presented in Figure 60, 61 and 62 as follows.
  76. 76. 65 Figure 60: Slotted Trigger Release Mechanism Sketch Figure 61: Firearm Trigger Concept Sketch Figure 62: Handle Trigger System Sketch
  77. 77. 66 Selection of the Concept for Objective C The final selection of the concept for the device is shown in Figure 63 below. This incorporates the corkscrew method of leverage seen in Figure 56 previously, positioned out of sight to the left of the lower sketch within Figure 63 below. It also incorporates an adjustable force mechanism behind the spring and a trigger mechanism as shown. Figure 63: Final Developed Concept Sketch To give an indication of the three dimensional shape of the device, Figure 64 as follows, illustrates a conceptual representation of the tubular casing which surrounds the spring.
  78. 78. 67 Figure 64: Tubular Casing Surrounding Spring Sketch 5.1.8 Mechanical Feasibility of Chosen Concept Before progressing further, the leverage mechanism must be validated theoretically to ensure that it could realistically function and be used by a surgeon. A rough sketch of the device moving through the charging motion is shown in Figure 65 below from left to right. Figure 65: 3 step sketch illustration of corkscrew charging method
  79. 79. 68 The leverage force has been simplified and marked out on the Figure 56 which is shown again below and renamed Figure 66 for clarity. Figure 66: Force Balancing Free Body Diagram Analysing the diagram in its static state, it can be assumed that all forces acting on the spring and lever are balanced, as it is not in motion and the spring is loaded in compression. L1 is the distance from the centre of the gear wheel to the point at which the gear teeth mesh on the ridged shaft. L2 is the distance from the centre of the cog wheel to the end of the handle. F1 (take as 6KN) is the spring force acting at this point in the direction of impact. F2 is the resultant force of the spring acting on the handle. Taking L2 as 0.3m (300mm) and L1 as 0.01m (10mm) for a trial basis, F2 can be found and it can be established whether or not it is humanly possible to compress and hence charge the device.
  80. 80. 69 Using static force balancing analysis, Equation 1 can be derived and is as shown below. 𝑭𝟐 = 𝑭𝟏×𝑳𝟏 𝑳𝟐 (1) Substituting in the values; 𝐹2 = (6 × 10 ) × 0.01 0.3 Solving for F2; 𝐹2 = 200𝑁 Since there are two levers used on the device this force can be divided by 2, so; 𝐹2 = 100𝑁 To put this in terms that are easy to comprehend, the answer is put in terms of a weight hanging in the opposing direction to F2 (as seen in Figure 66). The weight can be found using Equation 2 shown as follows; 𝑭 = 𝒎𝒈 (2) Where; g = acceleration due to gravity (constant = 9.81m/s2 )
  81. 81. 70 Substitute in known values; 100 = 𝑚(9.81) Rearrange and solve for m; 𝑚 = 10.2𝐾𝑔 Where; Kg = Kilogram This means that a 10.2Kg weight could be hung on one side, and it can be visualised that the same force could be mirrored onto the other handle to statically hold the whole device in place. Since this is has been sized for 6KN, it is the toughest possible scenario. This is thus deemed an acceptable method as the handles can be made longer and other design attributes can be manipulated to reduce this value. So far, the mechanism has proven itself enough at this stage of the design to continue in the process. The next step was to size a spring capable of providing an impact magnitude of up to 6KN. Using the impact impulse equation (Equation 3) below; 𝑰𝒎𝒑𝒖𝒍𝒔𝒆 = 𝑭. ∆𝒕 = 𝒎. ∆𝒗 (3)
  82. 82. 71 Where; F = Impact Force Δt = Impact duration (time over which impact occurs) m = Mass Δv = change in velocity The force used in this equation is 6KN, since it is the highest end of the scale. The mass used for this calculation was based on the average mass of an orthopaedic mallet, which was found to be approx. 0.4Kg (kilograms). Since the impact duration is unknown, a previously-recorded impulse value was taken from a PhD student at the University of Bath who had carried out a study on impacts and had acquired this data for 2KN, 4KN and 6KN impact forces. It should be noted that the tip material used during these tests is unknown, and this may have a large effect on the testing results. The impulse recorded for 6KN was 11.1Ns (newtons per second). The change in velocity (Δv) is equal to the initial velocity (u) minus the final velocity (v), i.e. Δv = u – v. The final velocity is zero since the mass comes to a complete stop. These new values can be subbed into Equation 3 as follows. 11.1 = 0.4(𝑈 − 0) Rearranging and solving for U; 𝑈 = 27.75  𝑚/𝑠 Where m/s = metres per second
  83. 83. 72 Now that the initial velocity has been found, this can then be used to find the kinetic energy (KE) at the point of impact. This can be found using Equation 4. 𝑲𝑬 = 𝟏 𝟐 𝒎𝒗 𝟐 (4) Substituting in the known values; 𝐾𝐸 = 1 2 (0.4)(27.75) Solving for KE; 𝐾𝐸 = 154.0125  𝐽𝑜𝑢𝑙𝑒𝑠 If it is assumed that there are no losses, and the KE at the point of impact is equal to the Potential Energy (PE) stored in the spring after it has been compressed but before it has been released, then the KE found in the previous step can be used as the PE in Equation 5 as follows. 𝑷𝑬 = 𝟏 𝟐 𝒌𝒙 𝟐 (5) Where k = spring stiffness x = spring displacement
  84. 84. 73 Since both k and x are both unknown at this point, it is decided to use a spring displacement (x) of 60mm (millimetres) or 0.06m (meters). This has been chosen as this displacement should leave enough room for sensitive adjustments to be made to the spring later in the design process. Substituting these values into Equation 5; 154.0125 = 1 2 𝑘(0.06) Rearranging and solving for k; 𝑘 = 85562.5𝑁/𝑚 Where N/m = Newton per metre Using this k value as a guide, a spring could then be sized from a components catalogue. The spring had to have a minimum k value of 85562.5 N/m but also had to have a compression displacement range of roughly 60mm to ensure the adjustability of the spring for different impact force magnitudes. 5.1.9 Development of Proof-Of-Concept Testing Rig Before any further development could continue on the concept developed at this point in the project, the basic idea of using a spring to deliver the assembly impacts
  85. 85. 74 had to be proven experimentally first. This was to be proven using a testing rig designed purely for this purpose. The first step in this process was to size a spring. The k value found in the last section and the displacement of approx. 60mm was used to source a spring from Lee Springs. The specifications for this spring are shown in Appendix 3. It was also decided to use three magnitudes of impact force during testing. This was done so that a line could be graphed through the three averaged points on an impact impulse vs. spring compression displacement chart to show the predictability and repeatability of the spring method. It could also be used as an opportunity to explore the effects of different tip material on the impactor, and what effect they have on the process. To simplify the design, an Instron Impact Loading machine (30KN max load), as shown in Figure 67 below, would be used to compress the spring to the required compression displacements. This would make the design stage simpler and faster to design and manufacture. Figure 67: Instron Loading Machine [51]
  86. 86. 75 The key functions of the testing rig were to allow the Instron to compress the spring, to hold the spring in its compressed state and then to allow the spring to be released over a load cell in order to measure the impact force administered. One of the early sketches in the concept development stage for this rig is shown in Figure 68 below. Figure 68: Initial Testing Rig
  87. 87. 76 Figure 68 includes a lever release mechanism, which is actuated by twisting a collar fixed on the outside of the top cylinder (illustrated in the top left of the Figure). It also features a threaded shaft with a threaded stopper disc that can be adjusted to allow varied spring compression displacements. The design was refined further to try to reduce its complexity, so as to save time during the detailed design phase and manufacture. Figure 69, below, shows the refined version of the concept for the testing rig. Figure 69: Simplified Concept for Testing Rig
  88. 88. 77 The device could be removed from a solid plate base on which a load cell would be secured, and then mounted using a special jig. This would be done so as to allow the Instron to push down the body of the device with the impactor tip placed on a fixed spigot so that the spring could be compressed inside. Various trigger mechanisms were generated, with the final idea being a side-acting lever. This lever would then fit into one of three specifically laid out slots that were designed to allow the spring to be held in a compressed state under 6KN, 4KN and just over 2KN of load in the Instron. Figure 70 below, shows the first concept, followed by Figure 71, which shows the next development, and then finally Figure 72 showing the simplified lever release mechanism. Figure 70: Initial Lever Design
  89. 89. 78 Figure 71: Lever Design Development Figure 72: Final Lever Design
  90. 90. 79 5.2 Detailed Design The Test Rig concept could now be developed further on SolidEdge. Stress calculations and considerations could also be made for the manufacture of the device, and could be followed by, the provision of draft drawings to the Machine Shop and the manufacture of the testing rig. 5.2.1 Solid Modelling The design had three specific compression displacement slots, slotted into an internal impactor rod (visible in Figure 73 below protruding from the top of the device), which was propelled by the release of the compressed spring. An isometric view of the finished model is shown in Figure 73 below, with fasteners removed for clarity. The assembled model is shown at the 6KN load setting. Figure 73: Final Assembled SolidEdge 3D Model
  91. 91. 80 5.2.3 Draft Drawings Once the final solid models had been completed a full set of draft drawings were produced for the design. These drawing are listed in Appendix 4 and can also be found towards the end of the report. 5.2.4 Manufacturing The SolidEdge solid modelling program allowed the interaction between the parts to become visible, and the manufacturing methods could be taken into consideration. The Testing Rig was manufactured in a machine shop based in the University of Bath and so the development of the design on SolidEdge was reviewed in stages with the Machine Shop Technician that would be manufacturing the Rig. This helped to simplify the manufacturing stage, as the design was customised to make use of the most easily-available materials that were currently in stock. The device was manufactured from steel, with exception of the bushings, which were made from PVC, and the interchangeable material tips which were made of PVC, Mild Steel, Nylon and Rubber.
  92. 92. 81 6. TESTING This section of the report covers the calibration of the load cell and the testing carried out on the Rig. The Results from this testing will be discussed in the next Chapter. 6.1 Calibration of the Load Cell The first step was to calibrate the Load Cell, which would be used to measure the impact loading during the Rig testing. This was performed to ensure that the load cell was fully functional, and also to establish a scale factor so that the output, when Rig testing, could be provided in newton instead of in volts, which is what the load cell measures. An Instron loading machine (max loading of 30KN) was programmed to descend onto the Load-Cell in steps of 2KN up to 8KN. The Load-Cell was attached to computer through an Analogue to Digital Card which was linked to a load measurement program designed for use on LabView. This allowed the load cell voltage recordings, which were being recorded at a sample rate of 10,000 samples per second, to be written and stored in a text-file on the hard drive of this computer. The loading test was performed three times to gain an average value across the tests for each load, which would allow for any errors or outliers. After the testing was completed text-files were then imported into Matlab and the voltage was plotted against time. Figure 74 as follows, shows the graph for the first test.
  93. 93. 82 Figure 74: First Calibration Test plot of Voltage Vs. Time It is clear from Figure 74 that the line sloping downward from right to left has four small steps, which represent 2KN, 4KN, 6KN and 8KN Loads. These loads are represented on the graph in volts. The voltage at each of these steps was recorded and inserted into a spread sheet on excel. Table 7 below shows the data recorded from the three tests and shows the average voltage achieved at each load. Table 7: Data recorded during Load Cell Calibration
  94. 94. 83 This Data was then plotted and a trend line added across the average points so that the slope, and hence the scale factor, could be established. This chart is shown in Figure 75 below. Figure 75: Load Cell Calibration Test Data Plotted on Graph The close proximity of the trend line to the points on Figure 75 showed that the load cell was fully functional and the change in voltage for each load was proportional. It is clear to see from Figure 75 above that the scale factor is -2678.5. This scale factor was then programmed into LabView prior to testing so that the text-file readings were produced in newtons as opposed to volts. 6.2 Rig Testing After the Load Cell was calibrated, verified as fully functional and the scale factor was established, the Rig Testing could begin. y = -2678.5x + 13.413 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 -4.0000 -3.0000 -2.0000 -1.0000 0.0000 Load Vs. Voltage Load Vs. Voltage Linear (Load Vs. Voltage)
  95. 95. 84 6.2.1 Procedure The testing involved measuring and recording several variables. These variables are listed as follows. 1. Final displacement of spring during spring compression on Instron 2. Final load recorded during spring compression on Instron 3. Impact data recorded via LabView Variable 1 and 2 were read directly from the computer monitor hooked up to the Instron. The LabView data was analysed later using Matlab. The Testing procedure involved several steps. The first step involved slotting the required material tip into the hole at the tip of the impactor rod and then holding it in place with the grub screw. The next step was to mount the Spring Section of the Testing Rig onto the test fixtures, which had been designed and manufactured as a part of this project. They held the rig in a safe and secure position during the compression of the spring on the Instron. Figure 76 below shows the two fixtures mounted on the top and bottom of the Instron attachment points. Figure 76: Custom Mounting points, close up view of top, close up view of bottom
  96. 96. 85 Figure 77, as follows, shows the Spring Section of the Device fixed securely in the mounting points. Figure 77: Spring Section in mounting points, close up view of top, close up view of bottom ger The top fixture was then lowered to the point where the device was securely in position. The spring section was then compressed in very small increments until spring compression was recognised visually by watching the top of the impactor rod rising up out of the casing. Once this point was reached the Instron displacement and load measurements were set to zero and the spring was slowly compressed until the release lever could fit all the way into the assigned slot in the impactor rod (for that particular test). Once the lever had been moved into place in the slot, the displacement and load were then recorded on an excel spread sheet. The next step was to raise the top fixture to its original position, insert the safety pin which held the release lever in place, remove the spring section which was then mounted onto the base plate which has the load cell fixed to it with two bolts. The first round of material testing at just over 2KN was carried out without a rubber base between the base plate and the table, to which the base plate was held in place with
  97. 97. 86 g-clamps. Figure 78, as follows, shows the base plate mounted to the table with and without the rubber base and finally with the spring section sitting top of the base structure prior to the assembly nuts being screwed attached. Figure 78: Base plate fixed to table with and without Rubber Base The Spring Section of the Device was then fixed in place with nuts above and below its lower flange (eight nuts below, and four above the flange, in total). The LabView program was then initialised, the offset reset (for each test) and the sampling started. The safety pin was then removed from the lever and a copper faced mallet was used to aid the quick release of the lever by impacting the end of the release lever to  ensure  a  swift  removal  of  the  lever  from  the  impactor  rod’s  path. This procedure was repeated for each individual impact test. To provide a walkthrough of one such test a 3 minute video of one impact test was recorded with a walkthrough narrative and posted to a DropBox online account. This video can be
  98. 98. 87 accessed by copying the following file link into the address bar on an internet browser. https://www.dropbox.com/s/6o668aoik0xtirz/video-2013-08-21-12-46-46.mp4 6.2.2 Data recorded The materials were changed over after each test. The material testing order was steel, nylon, PVC rubber and then back to steel again, which started off the next round of testing. This allowed the rubber time to return to its original shape and elasticity, as it temporarily deformed following impaction. Figure 79, as follows, displays the displacement and load data which was recorded during the Rig testing on an excel spread sheet. This figure also contains notes that were taken to record changes in material tip samples following the failure of a material tip during a test.
  99. 99. 88 Figure 79: Table of Data recorded from Instron
  100. 100. 89 The impact data recorded using LabView was extracted from the text file output using Matlab and then inserted into an excel spread sheet for analysis. The parameters analysed for each impact test were as follows. 1. Peak Force recorded during impact in Newtons 2. Duration of Max Peak in Seconds 3. Duration of impact These were extracted from graphs generated on Matlab and then fed into the excel spread sheet. Figure 80, below illustrates the 3 parameters recorded from these graphs. Figure 80: Test 2, Steel, 2KN (with rubber base) The extrapolated load cell data for the 2KN loads are shown in Figure 81 as follows. Only one round of data was taken without the rubber base present, to see effects of
  101. 101. 90 vibration damping. The rubber can be considered as a representation of the soft tissue in a patient as it absorbs some of the impact force from the Testing Rig. However, it must be noted that an error occurred and the steel test wrote over the Nylon test, hence why they both have the same data in the single round of testing without the rubber base. The analysis of this data is covered in the next section of the report. Figure 81: Extrapolated Load Cell Data, >2KN The impulse data was calculated by multiplying the “Peak Force” by the “Duration of Impact”, using Equation 3 as stated previously in the report.
  102. 102. 91 Figure 82: Extrapolated Load Cell Data, 4KN and 6KN The extrapolated load cell data for the 4KN and 6KN loads are shown in Figure 82 as follows.
  103. 103. 92 7. RESULTS AND DISCUSSION This section of the report covers the analysis and discussion of the Instron and Test Rig Data acquired in the previous chapter and also an evaluation of the Testing Rig. 7.1 Evaluation of Instron Data During the testing phase, some notes were made on the spread sheet where the Instron data was being recorded. For example, as can be noted from the comments on Figure 79 shown previously, both PVC and Rubber failed on the first round of testing at 4KN of loading on the Instron. New replacement tips for both materials failed after Round 2 (their first impacts) at 4KN loading. Figure 83 below shows images of the PVC and Rubber Tip materials which were destroyed on Round 1 of the 4KN Testing. Figure 83: Images of Failed PVC and Rubber Tips at 4KN Load

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