2. INTRODUCTION
METALS IN ORTHOPAEDICS USE AND THERE COMPARISON
An ideal implant material should be:
inert
non-toxic to the body
absolutely corrosion proof
inexpensive
great strength
high resistance to fatigue
easily worked
But all properties in a single implant cant be found
Different metals use are 1. Stainless steel
2. Cobalt – chromium alloys
3. Titanium alloys
3. STAINLESS STEEL ALLOY:
Effects of composition of this alloy on implant:
•Chromium produce a protective self regenerating chromium oxide
layer that protect against corrosion.
•Molybdenum decreases the rate of slow passive dissolution of
chromium oxide layer by upto 1000 times
•Nickle imparts further corrosion resistance
Short come of stainless steel alloy:
Though it is strong , stiff , and biocompatible material it has slow
and finite corrosion rate
So long term effects of nickle ions however prevails
So best suited for short term implantation.
4. Titanium alloys
Highly reactive rapidly metals get coated with oxide layer making it
phisiologically inert and resistant to most chemicals
Titanium has elastic modulus of approx. half that of stainless steel
and cobalt chromium alloy
Lower stiffness of bone plate made of titanium reduce severity of
stress shielding and cortical osteoporosis
It is less prone to fatigue failure than stainless steel
Elastic modulus of stainless steel is 12 times EM of cortical bone
Em of titanium is 6 times of cortical bone
Ductility of titanium is lower than stainless steel
So due to this difference surgeon require some adaptation of his feel
while determinig the optimal amount of torque to be
applied to the screw.
5. METHODS OF METAL WORKING AND THEIR EFFECTS ON IMPLANTS
VARIOUS METHODS ARE:
•Forging
•Casting
•Rolling and drawing
•Milling
•Coldworking
•Case hardening
•Maching
•Broaching
•Polishing and passivation
Forging: metal is heated and hammered or squeezed into shape.
It produces an orientation of the grain flow making the metal
strong.
6. Case hardening: metals are treated to cause the outer surface of rod
to be harder than inner core
Advantage is harder outer surface will resist indentation while core is
able to absorb more energy
Most important is
Polishing removes scratches which could act as local stress risers
Passivation produce protective oxide layer
Passivation can be damaged by – cold working
- scratching
- other mechanical trauma
Therefore care is needed in handling implants.
7. WHAT IS IMPLANT FAILURE?
The term implant failure implies that failed implant was
inadequate for the function expected of it.
OR
Clinically , implant failure may be defined as a failure of
implantation procedure to produce satisfactory results.
8. Was the design of the implant
adequate or faulty?
Was the choice of materials
satisfactory with regard to
strength, hardness, corrosion
During after-care of the patient, resistance, and ductility?
were there any mechanical
lapses which might have caused
the implant to fail and which
might have been avoided with Were defects in the implant are
proper precautions? due to errors during fabrication??
Was the clinical condition adverse
Did the surgeon apply the proper Or
mechanical and surgical
principles in implantation of the The surgical judgment in selection of the
device? implant and the conditions under which the
implant was used such that there was a high
probability of failure because of difficulties
in attaining satisfactory mechanical
relationships between the implant and bone
fragments?
9. It is divided into:
1. Surgical •Surgical technique
•Surgical judgement
•Surgically introduced infection
2. Material •Chemistry
•Structural
metallurgy
•Engineering design
3. Idiosyncratic Selective rejection of implant by certain
patients often associated with:
•Pain
•Hypersensitivity reaction
•Implant loosening
•Sinus tract infection
10. 4. Patients compliance
Patients post operative management programme
Significant Re trauma during the consolidation phase of healing
Inadequate postoperative immobilization
5. Other causes : fresh trauma
overweight
early weight bearing before significant union may
lead to loosening or fatigue failure of implants..this more commonly
occur in obese patients then underweight patients.
11. Surgical failure can be due to -
1. Mechanics of fracture fixation
2. Material limitation of devices and implants
3. Mixing of implants
12. Mixing of implants means mix and match implants from different
manufactures in fracture fixation.
The mixing of implants from different producers can lead to high risk
of corrosion, jamming , broken drills and taps, gaps, loose fits, and
loosening.
So it is therefore good clinical practice to use instruments and implants
from one manufacturer.
13. Material failure 1. Deficiency in engineering design
2. Manufacturing processing
3. Handling in operating room
Clinically , material failure fall into 1. Pure mechanically
2. Pure environmental
3. Conjoint
1. Pure mechanically is due to direct overload including impact or
design.
2. Environmental failure is due to reaction of physiological
environment with the metal , resulting in corrosion which either
weakens the device mechanically elicits a adverse tissue
response neccessiating device removal.
3. Conjoint i.e. mechanical and environmental failures produced
by applied stress in corrosive environment. Conjoint faiure
modes include fretting corrosion and corrosion fatigue.
14. Mechanical failure of implant falls into 3 categories:
1. Plastic failure is one in which implant failed to maintain its original
shape resulting in clinical failure.
2. Brittle failure is effect in the design or metallurgy
3. Fatigue failure is due to repetitive loading on device
therefore when surgeon inserts a implant he must realize that he is
entering a race between fatigue of implant and healing of fracture.
15. Environmental failure is due to corrosion
Corrosion is the gradual degradation of metals by electrochemical attack
Usually orthopaedic implants have inert protective layer to prevent
corrosion
Whenever there is change in pH or oxygen tension in tissue
Damage the oxide layer
Produce corrosion
16. Effects of corrosion :
•Weakens the implanted metal
•Changes the surface of the metal
•Metal ions into the body
Types:
1. Galvanic
2. Crevice
3. Pitting
4. Fretting
5. Stress
6. Intergranular
7. Ion release
17. Infection in orthopedic surgery is a disaster both for the patient and
surgeon.
HOW???
•Increase antibiotic use
•Prolonged hospital stay
•Repeated debridement
•Prolonged rehablitation
•Morbidity and mortality
Although its incidence has been reduced due to modern theatre facilities
and aseptic measures but in developing countries its prevalence is still
high. It is better to prevent infection rather than to treat it.
18. Not united after 4months of surgery After 6 months of surgery
9 months after surgery 18 months after surgery
19. Probable risk factors
•Advanced age ( > 60 yrs )
•Prolonged surgery time
•Smoking
•Co morbidity in patients like DM
•Skin abrasion at fracture site
•Skin at risk
Commonest organisms: Staphylococcus aureus
E coli and proteus
Klebsiella
Pathogenesis:
Infection is related to microorganism which grow in biofilm
Therefore its eradication is difficult
Diagnosed by: Clinical examination
Lab investigation
Histopathology
Microbiology
Imaging like USG, MRI, Bone scan, CT
20. Idiosyncratic failure:
It originates from corrosion products induced
hypersensitisation phenomenon resulting in implant
rejection or loosening.
It is estimated that approximately 6% of the population has existing
hypersensitivities to one or more of the constituents of stainless steel
or cobalt-chromium alloys, suggesting a need for routine
hypersensitivity screening prior to surgery.
Localized attack in the form of fretting (mechanical) or fretting corrosion
(mechanical-environmental) was commonplace at points of metal-on-
metal contact in multi component implants. Only rarely was corrosion of
the bone/plate interface observed.
21. LOCAL TISSUE RESPONSE:
The biologic environment walls off the implanted alloy by interposing a
relatively acellular tissue capsule.
With accelerated implant degradation, however, inflammatory cells,
macrophages, and occasionally foreign body giant cells may be found
adjacent to the device.
Adverse tissue response to the presence of an implant stems from
- toxic nature of the corrosion process
- an individualized sensitivity to certain corrosion products
- biologically accelerated corrosion rate in certain patients.
22. Typically, from device retrieval and analysis studies in humans, a small
number will present with evidence of local infection (pain, inflammation,
edema, fluid accumulation, or draining sinuses) 6months or more after
the original surgery.
Typically, culture and sensitivity testing reveals no growth of
microorganisms, thus the designation of "sterile abscess
Removal of the device usually effects prompt relief (24-48 hours), and
very commonly there is obvious tissue discoloration from corrosion.
23. Another well-recognized local effect of fracture-fixation devices is the
osteoporotic remodeling of bone immediately beneath the plate.
Such a response is thought to stem from the load-sharing capacity of the
plate, which acts to bypass forces around the underlying bone.
Accordingly, Wolff's law of dynamic remodeling can in theory not operate to
maintain the appropriate balance of osteoblastic versus osteoclastic activity,
and osteoporosis ensues.
In this regard much emphasis has been directed to reducing the rigidity
(modulus) of the fracture fixation device and its attachment to bone. However,
to date, no suitable low-modulus (or low-rigidity) system is available
commercially.
24.
25. SCREW FAILURE
Conical
1. Countersink
Hemispherical
Conical undersurface should be inserted centered and perpendicular to
the hole in plate
If set to any other angle
Undersurface does not adapt well to the plate hole
Due to which Its wedge sharp create undesirable high forces and
uneven contact which predisposes to corrosion
Both factors weakens the screw
Screw failure
26. 2.Run out:
The screw may break at the run out during insertion if it is incorrectly
centered over the hole or is not perpendicular to the plate. Typically it
breaks with spiral configuration indicating failure under torsional load
3. IT may break
•During insertion if applied torsional
load exceeds its torsional strength
•When pilot whole is too small
•Not tapped in hard bone
•Due to lack of lubrication
•High stress develop in screw when
there is significant resistance to
insertion causing screw to shear at a
cross section and leave a part lodged
in bone
27. IMPLANT FAILURE IN PLATING
Plate failure occurs because of interference with periosteal blood
supply.
Brittle and Plastic failure occur due to -minor loads in small plates
-secondary major trauma in large plates
The most common failure of plate is fatigue failure.
The ends of the plate act as stress riser leading to a fresh fracture
proximal or distal to the original one.
Improper application of plates and poor technique are other causes of
plate failure.
Fatigue failure of plate is inevitable if healing fails to occur
28. Breakage of Fracture
Fixation Plates
Left. When a gap is left on the cortex opposite that to which the plate
is attached, bending of the plate at the fracture site can cause the
plate to fail rapidly in bending.
Right. Compressing the fracture surfaces not only allows the bone
cortices to resist bending loads, but the frictional contact and
interdigitation helps to resist torsion.
29. The application of a plate on the
compressive as opposed to the tensile
side of a bone subjected to bending
causes a gap to open on the opposite
side of the plate during functional
loading.
30. Plate Failure Through a Screw Hole
Placing the plate so that an empty screw hole is located over the fracture will
significantly increase the potential for fatigue fracture of the plate.
A second consideration---
The greater the span or distance of a beam is between its supports, the
lower its stiffness will be, and the more it will deform under load in
bending and torsion. For this reason, screws should be placed as close
together across the fracture site as possible.
31. IMPLANT FAILURE IN
INTERLOCKING NAILING
• Associated with either the insertion of a small diameter nail or use of an
interlocking nail for a very proximal or distal shaft fractures
• Plastic deformation (bending) of the IM rod mainly occurs with nails that
are less than 10 mm in diameter;
minimal nail diameters range 12-14 mm for women & 13-15 mm for men
•Early dynamization, especially of subisthmal fractures, is associated with
increased risk of developing a valgus deformity at the fracture site
•Bending of the nail at the fracture site usually occurs as an early
complication caused by premature wt bearing, lack of adequate support, or
deficient material (nail) strength;
32. • Bent distal screws may result from early wt bearing if the screws are too
close to the fracture site;
• Weak part of the nail is proximal of the 2 distal holes;
- Fractures located with in 5 cm of this hole will be stressed above
endurance limit with ambulation
- These fractures must have delayed wt bearing until callus is
present
33. Femoral Splitting Due to IM Rod Insertion
Mismatch of the curvature between
the IM rod and the medullary canal
results in bending stresses that could
cause splitting of the femur during
insertion
34. IM Rod and Locking Screw Breakage
If the same force acts on IM rods placed in femora with
more proximal (left) or more distal (right) fractures, the
moment arm of the force will be longer in the case of the
more distal fracture, and therefore the moment, acting at
the fracture site, on the implant, will be larger. For the
more distal fracture, the high stress region, close to the
fracture site, is also significantly closer to the distal
locking screw holes, which are significant stress risers.
35. Because the distal end of the femur flares
rapidly, the length of the locking screw
required to cross lock the rod can be quite
variable. If the screw is not well supported by
trabecular bone but mainly by cortex, then its
stiffness and strength decrease with the third
power of its length between cortices. If the
screw length doubles, the deformation of the
screw under the same load increases by a
factor of eight.
36. Loosening of External Fixator Pins
A proposed mechanism for loosening external fixation
pins involves under- or oversizing the diameter of the
pin relative to the bone hole.
A. If the pin and bone hole are the same diameter,
micromotion can occur with bone resorption.
B. If the pin is more than 0.3 mm smaller in diameter
than the hole in bone, microfracture may occur during
insertion.
C. If the bone hole diameter is about 0.1 mm smaller
than the pin diameter, the bone is prestressed but does
not fracture, micromotion is eliminated, and pin stability
is maintained
37. To produce more rigidity in construction of an external fixator, the basic
principles that should be considered are that for pin-and-rod-type sidebars;
stiffness increases with the fourth power of the cross-sectional area (the moment
of inertia, and decreases with the third power of their span or unsupported length
. This explains why it is beneficial to decrease sidebar to bone distance, increase
pin diameter, place pins as close together across the fracture site as possible, and
use larger-diameter or multiple sidebars in frame construction
39. ASEPTIC LOOSENING :
The most important cause of aseptic loosening
is an inflammatory reaction to particles of wear
debris.
Abrasive, adhesive, and fatigue wear of
polyethylene, metal and bone cement produces
debris particles that induce bone resorption and
implant loosening.
Particles can cause linear, geographic, or
erosive patterns of bone resorption (osteolysis),
the distributions of which are influenced by the
implant design.
Micromotion of implants that did not achieve adequate
initial fixation is another important mechanism of
loosening.
40. Fatigue failure at the bone/cement and bone/implant interface may cause
aseptic loosening, and may be especially important for implants with
relatively smooth surfaces.
Stress shielding can influence local bone density, but is rarely an isolated
cause of implant loosening.
Infection causes failure of about 1–5% of cases of primary
arthroplasty.
Clues to the presence of infection include clinical signs, a periosteal
reaction, a positive culture of aspirated joint fluid, and acute
inflammation identified in tissue around the implant.
41. Now, surgeon encounters evidence of failure of an appliance by
•Breakage
•Tissue reaction
•Or suspect failure
What he will do
He will plan to remove the implant and plan for another operative
procedure
BUT
Most important now is surgeon has to investigate and analyze what
causes the failure
1 . The details of the condition for which the device was originally
inserted, including dates, place of operation, operative procedure,
and so forth.
2. The details of postoperative treatment and, in particular, any
episode, such
as premature weight-bearing or undue loading, which directly
preceded the failure.
42. During removal of implant surgeon should record his operative findings
carefully, and, in particular, the orientation of the device or of its fragments
with respect to grossly visible tissue reaction-discoloration, granulation
tissue, hemorrhage, or pus formation.
He should then obtain enough material for biopsy amid label it as to origin
amid orientation with respect to the device. Only with this information can
the pathologist interpret the findings in a pertinent fashion.
If there is a suspicion of infection superimposed on a tissue
reaction to time device, bacteriological cultures of suspicious
material are mandatory.
