3. Abstract
The knee joint is one of the most complex articulations of the human body. Its stability is the
result of a combined effort from structures such as bones, cartilages and soft-tissues. Due to
several different aspects, the knee joint could be affected by pathologies that bring a person to
feel pain and not behaving with standards movements.
One of the possible solutions when a degenerative condition of the knee occurs is the
replacement of the articulation with a total knee arthoplasty (TKA).
In order to cover the patients’ requests and their different anatomical attitude, several knee
replacement designs are, nowadays, available. Unfortunately, often there is lacking
information about their biomechanical behaviours once after implanted.
For this reason, the biomechanical literature reports certain approaches in order to provide
the improvement of the knowledge about the different solutions available on the market
before that a surgeon needs to select the best prosthesis for a specific patient.
Among the different methods of analysis and in vivo-in vitro tests, there is a specific part
focused on the use of robotic rigs on which experimental evaluations can be performed,
replying and judging the most common movements tasks. Unluckily, these devices are usually
quite expensive, heavy or disagreeable to transport easily from one place to another, besides
specific for only one brand or one type of TKAs.
For these principal reasons, the purpose of this project is the design of a low-cost knee
simulator device, capable of simulating the main daily movements of the articulation. This
robot, in addiction, will be used to analyse movements, forces and torques acting, in order to
observe and compare the different biomechanical behaviours of the TKAs under boundary
conditions.
In the interest of the design and the development such robotic device, several steps must be
necessary to be established:
1) Knee kinematics and kinetics study;
2) CAD design;
3) Static and dynamic study of the model;
4) Optimization of CAD model;
5) Study of materials and costs of device’s parts;
6) Optimization of the device and research on market of motors and controllers;
7) Device construction;
8) Final analysis and data collection;
9) Device testing.
The movements of interest were: walking, stairs motion (climbing and descending phases),
chair motion (sit down-stand up), squat (deep flexion-deep extension).
For each task, this knee simulator device can assure the six degrees of freedom, considering
only the tibio-femoral articulation: flexion and extension rotations, antero-posterior
translations, intra-extra rotations, abduction and adduction rotations, medio-lateral
translations, proximo-distal translations.
4. In order to replicate the requested movements, four motors were considered to be able to
control both forces and kinematics in different directions. Furthermore, the abduction-
adduction, medio-lateral and proximo-distal motions were selected to let free, this could leave
the possibility to adapt the structure at the different prosthesis that will be tested.
According with the scheduled steps and the aim of the project, several prototypes were
developed, evaluated and analysed to find the one that could respect the key points of this
work. Thanks to considerable meetings with specialized staff and the suggestions of
responsible of the experimental laboratory of biomechanics (BEAMS), only the last one among
four prototypes was approved and really developed, making some preliminary tests for
behaviours’ device and prosthesis.
All in all, the project has been completed in regarding of one of the main features to follow:
the budget. After an evaluation of other similar device already existed in the market and
according with the economic resource of BEAMS biomechanics department, a fixed amount of
5.000 € was considered and divided into the two fundamental tasks: the major percentage of
the budget (around the 70%) was directed to the motors and electronic items purchase, the
remaining part (30%) was dedicates at the development of the structure and other
mechanical parts. Considering the total motors cost (2.500 €), the wiring system obtained (50
€) and the purchase of some controls to develops first experimental tests and control
prototype (around 200 €), the percentage budget for this part was correctly planned.
Furthermore, the labour for the structure and the purchase of several mechanical components
was evaluated up to 1100 €. Thus the whole budget was excellent evaluated and divided, with
a great quantity preserved.
To conclude, the knee simulator device was controlled and tested. Thanks to the
implementation of a software that can handle the motors motions, a certain amount of data
was collected and several table generated to drive the device and to allow the simulation of
the desired four daily activities.
Considering its characteristics and its skills, the knee simulator device could be easily
implemented to compare already existing TKAs designs or eventually to develop several tests
to generate new TKA features.
5. Prefazione
L’articolazione del ginocchio è una delle più complesse presenti nel corpo umano. La sua
stabilità è il risultato di un lavoro congiunto tra ossa, cartilagini e tessuti molli. A causa di vari
aspetti, il ginocchio può essere colpito da alcune patologie che causano dolore alla persona e
impediscono di eseguire i movimenti standard.
Una delle possibili soluzioni quando si presentano queste condizioni degenerative
sull’articolazione consiste nel sostituire la sede articolare con una protesi totale di ginocchio
(PTG).
Per soddisfare le varie richieste dei pazienti e per seguire il diverso comportamento
anatomico di ognuno di essi, oggigiorno, esistono differenti tipologie di protesi al ginocchio.
Sfortunatamente però, spesso vi è una mancanza di informazioni sui loro comportamenti
biomeccanici prima di essere impiantanti.
Per questa ragione, alcuni studi scientifici biomedicali riportano vari approcci con il fine di
migliorare le conoscenze che ci sono tra le diverse soluzioni disponibili sul mercato prima che
il chirurgo debba selezionare quale sia la miglior protesi da impiantare in uno specifico
paziente.
Considerando i diversi metodi di analisi e i test in vitro e in vivo, vi è una parte specifica
concentrata sull’uso di dispositivi robotici con cui possano essere svolte delle analisi
sperimentali per riprodurre e giudicare la maggior parte dei principali movimenti del
ginocchio. Generalmente questi dispositivi sono relativamente costosi, pesanti e di dimensioni
che rendono difficile il trasporto da una parte a un’altra. Inoltre questi device sono spesso
specifici per un solo tipo di protesi oppure appartenenti ad una sola azienda biomedica.
Per questa ragione fondamentale, l’obiettivo di questo progetto è basato sulla prototipazione
e sviluppo di un dispositivo robotico a basso costo per la simulazione della cinetica e
cinematica del ginocchio, capace di riprodurre i principali movimenti quotidiani di
quest’articolazione. Questo robot, inoltre, sarà utilizzato per un’analisi dei movimenti, forze e
momenti che agiscono sul ginocchio, in modo tale da poter osservare e comparare i diversi
comportamenti biomeccanici delle varie protesi di ginocchio, secondo determinate condizioni
al contorno.
Per progettare e sviluppare un tale dispositivo robotico, è stato necessario seguire alcuni
passi:
1) Studio della cinematica e cinetica del ginocchio;
2) Progettazione CAD;
3) Studio statico e dinamico del modello CAD;
4) Ottimizzazione del modello CAD;
5) Studio dei materiali e costo delle parti del dispositivo;
6) Ottimizzazione del dispositivo e ricerca in commercio dei motori e controllori;
7) Costruzione del robot;
8) Analisi finale e raccolta dati;
9) Studio finale del dispositivo.
6. L’interesse si è concentrato su quattro movimenti: cammino, salire-scendere le scale, alzarsi
dalla sedia e sedersi, squat (discesa e salita).
Per ognuno di questi, il simulatore robotico deve assicurare i sei gradi di libertà propri
dell’articolazione, considerando esclusivamente la tibio-femorale: flesso-estensione,
traslazione antero-posteriore, intra-extra rotazione, traslazione medio-laterale, abduzione-
adduzione, traslazione prossimo-distale.
Per poter replicare i movimenti e le forze richieste, quattro motori sono stati scelti per
controllare sia le forze che la cinematica nelle differenti direzioni. Nonostante ciò, i movimenti
di abduzione-adduzione, la traslazione medio-laterale e la prossimo-distale sono stati scelti
come liberi, ovvero è stata lasciata la possibilità di adattare la struttura alle diverse tipologie
di protesi che verranno testate.
Seguendo i vari passi organizzativi del progetto e in base allo scopo finale da raggiungere,
sono stati sviluppati, valutati e analizzati differenti prototipi, per riuscire ad arrivare ad uno
definitivo che potesse rispettare i punti chiave prefissati. Grazie a numerosi incontri e
presentazioni del progetto al personale specializzato e in seguito a suggerimenti del
responsabile del laboratorio sperimentale di biomeccanica (BEAMS), solamente l’ultimo
prototipo è risultato essere adatto e quindi approvato per la reale produzione del robot,
svolgendo qualche test iniziale per il comportamento degli impianti protesici.
Nel suo complesso, il progetto è stato completato considerando il budget come una delle
principali caratteristiche da seguire. In seguito ad una valutazione di altri dispositivi simili già
esistenti in commercio e in base alle risorse economiche del dipartimento, una cifra di cinque
mila euro è stata prefissata per il budget e divisa in due parti fondamentali: la percentuale
maggiore (circa il 70%) è stata dedicata all’acquisto dei motori e di alcuni elementi elettronici
mentre la parte rimanente (ovvero il 30%) è stata indirizzata alla manodopera volta alla
costruzione del dispositivo e di altre parti meccaniche. Considerando il costo totale dei motori
(2.500 €), il sistema di cablaggio (50€) e l’acquisto di controllori per lo sviluppo dei primi test
sperimentali, il budget fissato per questa parte è stato dunque correttamente pianificato.
Inoltre, la manodopera per l’ottenimento della struttura unita alla spesa dei materiali e altri
componenti è stata valutata intorno a 1.100 €. Di conseguenza, il budget è stato
efficientemente sfruttato e la spesa complessiva è risultata essere minore, lasciando una
buona quantità inutilizzata.
In conclusione, un dispositivo robotico per la simulazione dei movimenti del ginocchio
ècontrollato e testato. Grazie all’implementazione di un software in grado di poter controllare
il movimento dei vari motori, una certa quantità di dati sono stati raccolti e organizzati in
diverse tabelle per pilotare il dispositivo e permettere al simulazione delle Quattro attività
giornaliere desiderate.
In base alle sue caratteristiche e proprietà, questo dispositivo può facilmente essere utilizzato
nello studio comparativo di PTG già esistenti o eventualmente nello sviluppo di differenti test
per ricavare nuove soluzioni.
7. Table of contents
1) Introduction………………………………………………………………………………………..1
2) State of the art: Knee simulator robots……………………………………………….5
3) Aim of the project………………………………………………………………………………..9
3.1) Design specifications………………………………………………………………………9
3.2) Schedule of the activity…………………………………………………………………11
4) Knee Joint: Kinematics and Kinetics…...…………………………………………….14
4.1) Walking………………………………………………………………………………………..18
4.2) Chair motion………………………………………………………………………………...20
4.3) Stairs motion………………………………………………………………………………..22
4.4) Squat…………………………………………………………………………………………...23
5) First prototype…………………………………………………………………………………..25
5.1) Design & Development…………………………………………………………………25
5.2) Prototype overview……………………………………………………………………...26
5.3) Critical discussion………………………………………………………………………...28
6) Second prototype………………………………………………………………………………29
6.1) Design & Development…………………………………………………………………29
6.2) Prototype overview……………………………………………………………………...30
6.3) Static analysis………………………………………………………………………………32
6.4) Dynamic analysis………………………………………………………………………….38
6.5) Critical discussion………………………………………………………………………...42
7) Third prototype…………………………………………………………………………...……43
7.1) Design & Development………………………………………………………………….43
7.2) Prototype overview……………………………………………………………………...44
7.3) Static and dynamic analysis…………………………………………………………..45
7.4) Critical discussion………………………………………………………………………...46
25. 17
On the others planes, the knee can develop abduction-adduction and proximo-distal
translations on frontal plane and medio-lateral translation and intra-extra rotation on
transversal plane.
Since the aim is to observe the prosthesis’ behaviours, this project’s study has focused the
attention on four main daily motions:
Ø Walking;
Ø Stairs motion (Climbing and descending phases);
Ø Chair motion (Sit down-Stand up);
Ø Squat (Deep flexion-deep extension).
For each of these, it was done a research about all the information available to find the ranges
of values for the forces and the torques that are applied to the joint and, furthermore, how
these ranges are dispenses during the previous movements.
Following this research, several data were taken from Orthoload site, a free public database.
(The loads acting in human joints were measured in the Julius Wolff Institute of the Charité in
Berlin directly in patients by using instrumented implants. Measurements during many
routine and sportive activities were taken in hip, knee, shoulder and spinal implants.
OrthoLoad supplies numerical load data and videos, which contain load-time diagrams and
synchronous images of the subject’s activities).[13]
Besides, these data were collected considering that z is the axis of the implant stem, x is the
axis for medial translation and y is the anterior/posterior translation axis.
Additional information were achieved thank to database of the BEAMS Department, helpful to
make a comparison with the other data and to be a reference for subsequent studies or tests.
45. 37
Ø 5,80 ∙ 10!!
mm (maximum value) for the parts with rectangular section.
Fig. 42: Capture of SW Ftool: Linear deflection in the parts with rectangular section.
The values obtained are in the order of the micrometres, thus as mite as to be retain
meaningless for influencing the behaviour of the structure dimensioned.
On y-z plane, the structure is represented only as a beam with a single hinge:
Fig. 43: The structure view on y-z plane. P is the weight and F is axial forces from the lower part.
The equations are:
!! = −! + ! = 960,76 !
!! = 0 !
!! = !ℎ + !" = 32,26 !"
This case is not realistic because the weight is all concentrated at the end of beam but it was
study to be able to simulate, as usual, the worst possible situation.
Mm is the maximum motor torques that the device must have to allow the movement.
ℎ = 185 !!
! = 25 !!
59. 51
Ø Axial load and antirior/postirior translation:
The choice for simulating these two parameters was immediately focused on the
linear actuators type.
They are systems that create motion in a straight line, in contrast to the circular
motion of a conventional electric motor. Generically, it is possible to divided linear
actuators into 6 types:
1) Mechanical actuators: they typically operate by conversion of rotary motion into
linear motion. Conversion is commonly made via a few simple types of
mechanism: screw (it translates turning motion into linear motion), wheel, axle
and cam.
2) Hydraulic actuators: also named as hydraulic cylinders, typically involve a
hollow cylinder having a piston inserted in it. An unbalanced pressure applied
to the piston generates force that can move an external object.
3) Pneumatic actuators: they are similar to hydraulic actuators except they use
compressed gas to generate force instead of a liquid. They work similarly to a
piston in which air is pumped inside a chamber and pushed out of the other side
of the chamber. Air actuators are not necessarily used for heavy-duty machinery
and instances where large amounts of weight are present.
4) Piezoelectric actuators: The piezoelectric effect is a property of certain
materials in which the application of a voltage to the material causes it to
expand. Very high voltages correspond to only tiny expansions. As a result,
piezoelectric actuators can achieve extremely fine positioning resolution, but
also have a very short range of motion.
5) Electro-mechanical actuators: Electro-mechanical actuators are similar to
mechanical actuators except that the control knob or handle is replaced with an
electric motor. Rotary motion of the motor is converted to linear displacement.
There are many designs of modern linear actuators and every company that
manufactures them tends to have a proprietary method.
6) Telescoping linear actuator: this type is a specialized linear actuators group
used where space restrictions exist. Their range of motion is many times greater
than the unexpended length of the actuating member. A common form is made
of concentric tubes of approximately equal length that extend and retract like
sleeves, one inside the other, such as the telescopic cylinder.
Analysing the list of these actuator, with their characteristics, the stroke and the
dynamic load that they can sustain, the only one that seemed to be respectful of the