Jtam.dvi


JOURNAL OF THEORETICAL

AND APPLIED MECHANICS

48, 4, pp. 897-915, Warsaw 2010

CUSTOMISED PROSTHESES OF HUMAN JOINTS AND

ORTHOSIS DEVICES

Krzysztof Kędzior
Marek Pawlikowski
Konstanty Skalski
Warsaw University of Technology, Warszawa, Poland

e-mail: kkedzior@meil.pw.edu.pl; mpawlik1@wip.pw.edu.pl; kskalski@wip.pw.edu.pl

In the paper, a review of achievements of Inter-Faculty Laboratory of
Biomedical Engineering1 (IFLBE) at Warsaw University of Technology
is presented to commemorate Prof. Marek Dietrich who was one of the
pioneers of researches in thefield of biomechanics andbiomedical engine-
ering inPoland.Thus, the paper dealswith two sets of problems that the
members of IFLBE focused their attention on, i.e. prostheses of human
joints and orthosis devices. A detailed process of a custom-made pro-
sthesis manufacturing together with problems related to implantation
will be shown as well as some new constructions of orthoses designed
at Warsaw University of Technology. In the framework of the first pro-
blem, i.e. customised prostheses of human joints, the following joints are
considered: hip joint, lumbar spine, elbow joint, knee joint.

Key words: customised prosthesis, bone adaptation, orthosis device

1. Introduction

Recently, in the so-called developed countries, the diseases and injuries of hip
joint have definitely become a civilization-related problem. The main causes
observed are: aging of societies involving, e.g. osteoporosis, degenerations,ma-
terial wear, etc.; the way of life, especially traffic crashes that are of crucial
importance as well. Also the evolution gave us the vertical position which
causes larger loads acting upon these joints.

1Workers of the Laboratory: Borkowski Paweł, Borkowski Piotr, Dąbrowska
Tkaczyk A., Domański J., Floriańczyk A., Grygoruk R., HaraburdaM., Kędzior K.,
Kuberacki B., Mianowski K., Pawlikowski M., Skalski K. – head of the Laboratory,
Skoworodko J.,Wróblewski G.



898 K. Kędzior et al.

According to theWorldHealthOrganization data about500million people
all over theworld suffer fromchronic diseases of bones and joints.This number
includes about 200millions suffering from osteoporosis. It is estimated that in
2020 about 400 millions will suffer from osteoporosis.

According toEuropeanUniondata in 2010 every 3rdwomanand every 9th
man above 60 will suffer from femur neck fracture. It has also been estimated
that about 1 million operations of alloplasty are performed per year all over
the world, but in that number as many as quarter of million are performed in
the USA. The last two numbers reveal the next problem, a very serious one,
namely these diseases can be treated only in rich countries.

The needs for hip joint replacement in Poland are estimated 30000 a year,
however, only approximately 10000 surgeries a year are performed. This is
because thealloplasty is rather expensive compared to theaverage income.The
patientmustwait sometimes over 2-3 years for Social Security Service. This is
typical situation for many countries being at the same stage of development.

Lumbar spine damages constitute a group of most common diseases the
modern civilisation suffer from.Those damages aremostly related to interver-
tebral discs. The role of the disc is to absorb shocks. It protects the spine
against daily activities. It also protects the spine during straining activities,
such as running, jumping or lifting weights. Themost common disease of the
disc is discopathy when the nucleus herniates toward the spinal cord.

Each year, only in Poland, approx. 200 people suffer frommultifragmental
fractures of the radial head in elbow joint. In the case of a single fracture, the
typical surgical procedure consists in osteosynthesis by means of a surgical
nail which is impossible for multifragmental fractures (Fig.1). Usually such
a damage was, and in most cases still is, treated in a very simple way, i.e.
cutting out the damaged bone part, detoriating substantially the patient’s life
quality.

Fig. 1. Multifragmental fracture of radial head

Patients who suffer from bone cancer undergo traditional treatments in-
volving chemotherapy, radiotherapy and surgery. Surgery, until very recently,



Customised prostheses on human joints... 899

consisted in cancerous limb amputation.However, the progress reached thanks
to associated treatment pointed out a new role for the oncological surgery and
caused development of so called sparing surgical techniques. Surgeons have
begun performing less maiming amputations and, in the cases where it is po-
ssible, tumour resection together with a part of the bone. This contributed to
the development of new reconstruction techniques. The aim of those techni-
ques is to replace large bone loss applying not only oncological prostheses but
also bone grafts and other reconstructionmethods. The fact that bone cancer
concerns mainly children, who are in the process of growing, makes the pro-
blem even more difficult as the limb with a prosthesis implanted in the place
of diseased bone will be shorter after a certain time. Thus, a sophisticated
prosthesis must be applied, i.e. an expandable prosthesis.

In the world approx. 500 cases of pediatric bone cancer are annually re-
ported.As for themost common types of bone cancer at children, 963 cases of
osteosarcoma and 576 cases of Ewing’s sarcoma were reported worldwide be-
tween 1994 and 1998 for ages 0-19 (Li et al., 2002). Thus, the potential global
market of expandable prosthesis application is approximately 300 patients per
year. Although the number is somewhat small, considering huge improvement
of the patients’ quality of life and, in many cases, possibility to give the pa-
tients hope for longer life if an expandable prosthesis is successfully applied,
the problem of high-quality expandable prosthesis design becomes crucial.

There are approximately 1200 cancer cases being detected yearly by chil-
dren in Poland. The amount covers about 100 bone cancers, out of what ap-
prox. 90 are located in the region of knee joint. Approximately 70 may be
cured by surgeries and prosthesis implementation. In about 25 cases, the ap-
plied prosthesis should be the one of expandable capability due to the growing
process of the patient’s body. The best type of such an expandable prosthesis
are thosenotneeding systematic interventions resulting inadditional surgeries.
A cost of such an anti-interventional expandable prosthesis is approximately
15000¿. The high cost causes that only few such solutions are being used
yearly. This is the reason why we decided to focus on this issue.

As a result of accidents and various diseases, a large number of patients
– including young people – suffer from parapleghia or tetrapleghia (quadri-
pleghia). These people have to spent their lifes in beds or – best of all – in
wheelchairs with no possibility of taking vertical position. This lack influences
badly not only the quality of their life but also shortens it dramaticallymainly
due to disfunctions of digestive and circulatory systems. A kind of a solution
to this problem consists in special orthotic devices.



900 K. Kędzior et al.

2. Hip joint prostheses

Within the scope of hip joint prostheses, we have focused on the customized
ones. The necessity of customised prostheses results from the fact that in
about 5% of cases of hip joint diseases and injuries the common available on
the market prostheses cannot be applied. Since the number of all cases over
the world is very large and still growing these 5% deserves dealing with the
problem. Many researchers all over the world have investigated the problem
(Little et al., 2006; Sridhar et al., 2010). Basing on their results as well as on
our own experience we have developed the procedure presented in Fig.2.

Fig. 2. The process of customised prosthesis design

The figure shows themain stages of themethod: CT scanning, data trans-
fer to the CAD system for implant design and engineering analysis, rapid
prototyping for design examination and surgery planning and finally CNC
machining.
The first step of the procedure consists in geometrical identification of the

damaged or deformed joint, and sometimes even some neighbouring joints,
using the CT technology. It consists in 3D bone shape reconstruction based
on 2D scans – at first glance this task seems to be very simple. However there
are some fundamental difficulties we were confronted with, especially when
dealing with very damaged joints, e.g. caused by osteporosis, sever mechani-
cal damages of bones, etc. This 42 years old patient (Fig.3), a woman, had a
normal right hip joint, however the left one was completely damaged due to



Customised prostheses on human joints... 901

the congenital dislocation of the hip joint which had been never treated. As a
result, the natural joint as well as the femur head were completely damaged
because of wear. That caused also changes in the knee joint orientation. The-
refore, a very difficult decision had to be made at this stage of the method.
In fact, the preoperative planning of surgery started here. We should have
in mind that surgical reconstruction of the damaged hip joint to its normal
shape – like the right-hand one – could involve unacceptable changes in the
knee joint or even necessity for knee joint alloplasty. In the shown case surgical
reconstruction of the hip joint reached only the level allowing the patient to
walk without crutches.

Fig. 3. Clinical case; (a) hip joints (X-ray picture) – frontal view; transverse
sections: (b) through hips, (c) through both knees at the condyle

Fig. 4. Solid models of the bone and prosthesis

The CT measurement results create the input data to a common CAD
system which generates the solid model of bones (Fig.4). The first problem
consists in proper identification of the bone contour, especially in the vicinity
of marrow cavity. This obstacle can be overcome by means of the proper Ho-
unsfield coefficient adjustment in the course of contour identification.A special



902 K. Kędzior et al.

software acting as an interface is required.There are packages available on the
market – they are good, but very expensive. For this reason, the researchers
sometimes write their own programs.

The second problem consists in the lack of compatibility between CT and
CAD systems. Using the same CAD system one can design the individual
prosthesis fitted to the marrow cavity of the patient bone. One can also plan
the surgical procedure.So, avery important issueof thedesignprocess consists
in proper designing of the implant stem to ensure the bestmatching with the
marrow cavity, especially in the case of a cementless prosthesis.

During thewhole process of the prosthesis design in theCAD system, con-
sultations with the surgeon whowas to perform the alloplasty operation were
indispensable. The surgeon’s advices and remarks on the shape of the pro-
sthesis stem allowed us to create a set of stem designs fromwhich we were to
select a medically optimal anatomical hip joint prosthesis. In Fig.5, two stem
designs are shown, i.e. the first design, whichwas the initial design of the pro-
sthesis (Fig.5a), and the optimal design (Fig.5b).Themagnified cross sections
presented in Fig.5 allow one to distinguish the differences in the stem shape.
The differences greatly influence the bond between the bone and prosthesis
and, consequently, stability of the implantedprosthesis in themedullary canal.
The criteria of the selection were mainly the degree of medullary canal filling
by the stem and possibility of prosthesis insertion into the femur. The CAD
system is able to generate cross-sections by means of which we could analyse
to what degree the designed prostheses fit the medullary canal. Moreover, we
could also simulate the alloplasty operation by performing virtual femoral he-
ad resection and virtually inserting prosthesis into the bone (Pawlikowski et
al., 2003).

Fig. 5. Two selected designs of the prosthesis: the first one (Prosthesis 1) (a) and
the optimal one (Prosthesis 2) (b)



Customised prostheses on human joints... 903

Another important problem we should solve consists in designing the im-
plant neck in the way ensuring proper cooperation with the artificial cup as
well as proper orientation of the leg, especially the knee joint.

TheFEManalysis is then applied for verification of the strengthproperties
of the bone-implant system. In theFEMsimulations, the phenomenon of bone
adaptation was taken into account. It was simulated by means of kinetics
equation (2.1). Bone adaptation, which is also called bone remodelling, was
consideredas a change of bonedensitywith time. In equation (2.1) signifies the
boneapparentdensity (Weinans et al., 1992), S –bone remodelling stimulator,
S0 – reference bone remodelling stimulator, C – constant, s –width of the so
called ”dead zone” (Fig.6).

dρ
a

dt
=















C[S− (1−s)S0] S¬ (1−s)S0 bone resorption

0 (1−s)S0 <S < (1+s)S0 dead zone

C[S− (1+s)S0] S­ (1+s)S0 bone apposition
(2.1)

Fig. 6. Graphical interpretation of kinetics equation (2.1)

The geometrical models of the femur and prostheses were utilised to ge-
nerate discrete models in the FEM system. The discrete models of the bone-
implant systems consisted of about 7000 elements. The bonewas simulated as
a visco-elastic material. The initial value of bone density was 1.5g/cm3 and
was homogenous in the whole bone volume. The material properties of the
bone in the first time step depended on the initial value of bone density; in
the next time steps the properties changedwith bone density change. The ini-
tial Youngmodulus corresponded to the initial bone density andwas equal to
14000MPa. Poisson’s ratiowas assumed to be equal ν =0.4 andwas constant
duringthewholeprocessof deformation.Theprosthesesweredefinedasanela-



904 K. Kędzior et al.

stic material of the following properties: Young’s modulus E =200000MPa,
Poisson’s ratio ν = 0.3. The equation relating Young’s modulus with bone
density was assumed to be of the form: E =4249ρ3 (Weinans et al., 2000). If
the density is expressed in g/cm3, the Young modulus is calculated in MPa.
The load conditions of the bone-implant systems corresponded to the one-leg
standing position. The reaction force acting on the prosthesis head and the
force of abductor muscles were considered. Perfect bonding between the pro-
stheses and femur was assumed. Selected results of FEM simulations, in the
form of bone density distributions in femur, are shown in Fig.7. The results
indicate that the initial design of the prosthesis would cause significant bone
resorption in the proximal part of the femur. On the other hand, the final
design seems to be the optimal one since the bone density distribution after
implantation is more advantageous.

Fig. 7. Bone density distribution in femur for Prosthesis 1 and Prosthesis 2

In order to design an anatomical prosthesis of extended durability, it is
essential to conduct the design process of the prosthesis as itwas shownabove,
i.e. to begin with CT data acquisition, then to model the femur and design
a custom-made prosthesis in a CAD system and finally to verify the new-
designed prosthesis numerically in a FEM system. The factor of including the
bone remodelling phenomenon in the process is of significant importance. It is
obvious that during the design process consultations with a surgeon must be
carried out.



Customised prostheses on human joints... 905

At thefinal stage of theprocess the implantwas coveredfirstwithapowde-
red pure titanium and a selected part of it was covered with hydroxyapatite
using the plasma spraying technology (Skalski et al., 2001). However, the cu-
stom design procedure of the implant forces the necessity for custom design
surgical tools, e.g. the rasp. It rises the cost of surgery. The rasp is made of
a cheaper material, however due to technological reasons, i.e. file cutting and
polishing, it is also very expensive (Fig.8).

Fig. 8. Instrumentation for customized prosthesis implantation

Several operations were performed using the developed procedure. The
patients have been observed for several years proving that themethod can be
used in clinical practice.

Fig. 9. Customised prosthesis of hip joint acetabulum

Exactly the samemethodology of designingandmanufacturing canbeused
in the case of pelvis bone reconstruction (Skalski et al., 2006). It applies to the
case when the level of bone damagemakes it impossible to implant a standard



906 K. Kędzior et al.

artificial cap. Such a case can be seen in Fig.9. Following the procedure, a
new joint was designed using the CAD system. The damaged part of pelvis
bone was replaced with a special customized acetabular cage which allows
for fixing an artificial joint cup. The surgery set was made also using a CNC
machine. It contains also a test cage used at the first step of operation to
prepare the matching area like the rasp in the previous case (Fig.10). In this
case however, the extra material was added using a minced bone taken from
bone bank. Finally, we arrive at the crucial moment of the whole process, i.e.
themoment of truth when during the surgery the applicability of the implant
is proved. In this case, aswell as in the otherswedealtwith so far, the artificial
joint was designed andmanufactured properly (Fig.11).

Fig. 10. Customised prosthesis of hip joint acetabulum

Fig. 11. X-ray picture of an implanted hip joint

The surgical procedure however does not end the treatment procedure. A
long lasting rehabilitation process supported by periodic biomechanical ana-
lysis should be performed as well (Fig. 12).



Customised prostheses on human joints... 907

Fig. 12. Biomechanical analysis of the surgery effect on Vicon system

3. Lumbar spine

Lumbar spine damages constitute a group of most common diseases the mo-
dern civilisation suffers from. Since computer advanced methods and techno-
logies are now available, modern bioengineering can attempt at solving these
problems. Following the procedure presented before, the first stage consists in
reconstruction of the vertebra system using the Computer Tomography and
the CAD system (Fig.13).

Fig. 13. Geometrical model of the lumbar segment of spine

We have proposed a three-element prototype which substitutes for a ma-
jority of functions of the natural disc, i.e. rotation and bending (Fig.14). The
most important factor affecting the designing process of the artificial disc con-



908 K. Kędzior et al.

Fig. 14. Prosthesis of intervertebral disc

sists in ensuring that the stress distribution, after implementation, remains the
same or almost the same as in the case of the natural disc. It can be achieved
using FEMmodelling (Fig.15). It should be noted that the polyethylene pad
provides also certain compressibility (Borkowski et al., 2004). A few prototy-
pes have been constructed which are now under testing on the experimental
stand shown inFig.16.Themeasurement results obtained so far are promising
(Dietrich et al., 2005).

Fig. 15. FEMmodel of the lumbar segment (Borkowski et al., 2004)

4. Elbow joint

Each year – only in Poland – about 200 people suffer from multifragmental
fractures of the radial head. In the case of a single fracture, a typical surgi-
cal procedure consists in osteosynthesis by means of a surgical nail which is
impossible for multifragmental fractures. Usually such a damage was, and in



Customised prostheses on human joints... 909

Fig. 16. Experimental stand for testing intervertebral disc prostheses

most cases still is, treated in a very simple way, i.e. cutting out the damaged
bone part, deteriorating substantially the patient’s life quality. We have de-
cided therefore to design a new type of prosthesis. Upon application of our
procedure, i.e. from CT to manufacturing including reconstruction of geome-
trical features of bone structures,we have obtained the prosthesis better fitted
to the real joint. The new design comprises two or three elements (Fig.17). It
should be noted that the design is provided with an additional spherical joint
not presented in nature,which allows for better adaptation of the prosthesis to
the anatomy of the elbow joint (Pomianowski et al., 2003). This feature is vi-
sible in the picture which presents the result of one of over hundred successful
radial head prosthesis implantations performed so far (Fig.18).

Fig. 17. New designs of the radial head prosthesis

We emphasize also the necessity for supplying the surgeon with special
surgical instruments allowing formakingmore reliable intro-operative connec-
tions of prosthesis elements (Fig.18). The promising results or clinical and



910 K. Kędzior et al.

technical tests allowed for industrial implementation of the design. The pro-
sthesis is available on themarket.

Fig. 18. Surgical instrumentation and an implanted radial head prosthesis

5. Knee joint

There are approximately 1200 cancer cases being detected yearly by children
in Poland. The amount covers about 100 bone cancers, out of what about 90
are located in the region of the knee joint. Approximately 70 may be cured
by surgeries and prosthesis implementation. In about 25 cases the prosthesis
used should be the one of expandable capability due to the growing process of
the patient’s body. The best type of such expandable prostheses are those no
needing systematic interventions resulting in additional surgeries. The cost of
such anti-interventional expandable prostheses is considerable. The high cost
causes that only few such solutions are being used yearly. This is the reason
why we decided to focus on this issue.

One of the constructions of non-invasive prosthesis invented by our gro-
up consists of a screw gear which is propelled by an electrical driving unit
(Fig.19). The prosthesis comprises three tubes connected with each other by



Customised prostheses on human joints... 911

means of a thread: an external tube inwhich the driving unit is located, inter-
nal tube where the power screw is placed and intermediate tube. The driving
unit consists of an electrical engine that is activated by means of electroma-
gnetic coil, which during the elongation process, is placed on the skin. The
electromagnetic field drives the motoreducer and the screw gear of the pro-
sthesis is powered.Duringtheprosthesis expansion, the internal tube ismoving
forward fromthe external one.The internal tube is drivenbymeans of themo-
toreducer which puts the power screw into motion. The motoreducer located
inside the prosthesis is powered from outside the body by means of a special
control unit. The exciter and impulse generator create an electromagnetic si-
gnalwhich ismodulated, amplifiedand then transmitted to the receiver placed
inside the prosthesis. The signal is next demodulated and transmitted to the
control system of the power unit (motoreducer). This way the prosthesis can
be extended to a desired length. During the process of the prosthesis design
it was assumed that the reactive force of the muscles during extension was
equal to 100N. Themaximum possible elongation of the prosthesis is 45mm.
It was also assumed that the friction coefficient between the screw and nut
was 0.08, thread efficiency equalled 0.33 (Borkowski et al., 2003). The artifi-
cial knee joint is to restore functionality of the natural knee joint extracted
from the body. The stems of the prosthesis are coated with pure titanium and
hydroxyapatite.

Fig. 19. Solid model of the non-invasive expandable prosthesis



912 K. Kędzior et al.

The screw gear makes it possible to precisely control the elongation of
the prosthesis. In addition to this, the expansion process can be frequently
repeated without any discomfort to the patient.

6. Orthosis devices

As a result of accidents and various diseases, a large number of patients – inc-
luding young people – suffer fromparapleghia or tetrapleghia (quadripleghia).
These people have spent their lives in beds or – best of all – in wheelchairs
with no possibility of taking vertical position. This lack influences badly not
only the quality of their life but also shortens it dramatically mainly due to
disfunctions of digestive and circulatory systems.
Akindof a solution to thisproblemconsists in special orthotic devices.The

”Parapodium PW” (Fig.20) is designed for paraplegics (Olędzki and Szym-
czak, 1999). This device allows the patient with still active upper extremities
to take thevertical position,walk aroundwithoutanyadditional drive, andwi-
thout any assistance of other people and undertake basic everyday life actions,
including visits to toilet. The same smaller device was designed for children
(Fig.20). Over 500 pieces of the device have been manufactured and sold up
till now.

Fig. 20. ”ParapodiumPW” for paraplegics

In the much worse condition are people with four extremities paralyzed.
The device for assisting them needs therefore an additional drive, e.g. electric
motor. The ”Tetrapodium PW” (Fig.21) allows such people to take vertical
position and undertake basic every-day life actions (Kuberacki et al., 2010). It
should be noted that this device, like the previous one, is perfectly safe. The



Customised prostheses on human joints... 913

stability of the device is ensured by design as if keeps the mass center of the
body-devicw within the base contour.

Fig. 21. ”TetrapodiumPW” for quatriplegics

7. Final remarks

Patients undergo sometimes such severe pathological changes that a customi-
sedprosthesis shouldbedesigned and implanted.As a custom-madeprosthesis
fits only to one particular patient its cost is greater than a standard one. Ho-
wever, the high cost is compensated by the fact that such a prosthesis fulfils
its function for a much longer time period than a standard prosthesis does.
The number of patients who require customdesign prostheses and otrhosis

devices will increase due to the process of world population ageing (main
factor – cancer). At present, this procedure is very expensive. However, the
application of computerised techniques (CT, CAD, CAM) may considerably
reduce thecost provided that thewholeprocess (includingdesign,manufacture
and surgery) will be performed in specialised centres.
Theanatomical shapeof customisedprosthesis stemsmakes the load trans-

fer from the upper part of the body through prosthesis to the bone inwhich it
is inserted more natural. This reduces consequently the range of undesirable
bone behaviour, i.e. bone resorption.
Customised prostheses can be also applied in revision arthroplasty. In the

case of a standard prosthesis failure due to, for instance, its loosening, a
custom-made prosthesis can be implanted. This way the joint functionality
is restored.



914 K. Kędzior et al.

In order to design a customised prosthesis of extended durability it is
essential to conduct the design process of the prosthesis as it was shown in
the paper, i.e. to begin with CT data acquisition, then tomodel the joint and
design a custom-made prosthesis in a CAD system and finally to verify the
new-designed prosthesis numerically in aFEMsystem.The factor of including
the bone remodelling phenomenon in the process is of significant importance.
It has to be emphasised that this approach proved to be essential in the area
of custom-made prosthesis design. It is obvious that during the design process
consultations with a surgeon must be carried out. Thus, the modern process
of customised prosthesis design is interdisciplinary and makes it possible to
designandmanufacture aprosthesis of higherdurabilityandbio-compatibility.

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Indywidualne endoprotezy stawów człowieka i urządzenia ortotyczne

Streszczenie

Praca zawiera przegląd osiągnięć Międzywydziałowego Laboratorium Inżynierii
Biomedycznej (LIB) PolitechnikiWarszawskiej, przedstawiony dla uczczenia pamięci
prof. Marka Dietricha, który był jednym z pionierów badań w zakresie biomechaniki
i inżynierii biomedycznej w Polsce. W szczególności zaprezentowano badania w za-
kresie endoprotez stawów człowieka i urządzeń ortotycznych. Przedstawiono proces
produkcji endoprotez indywidualnych oraz problemy związane z ich wszczepianiem,
jak również nowe konstrukcje ortoz zaprojektowanych na PolitechniceWarszawskiej.
W zakresie zaś endoprotez stawów człowieka przedstawiono nowe konstrukcje endo-
protez: stawu biodrowego, kręgosłupa, stawu łokciowego i kolanowego.

Manuscript received April 21, 2010; accepted for print June 25, 2010