Radiol Oncol 2001; 35(1): 17-9. 

Computer systems for determination of pressure distribution 
in the hip joint articular surface: validation and results 

Vlado Stankovski and Dragica Smrke 

Department of Surgery, University Medical Centre, Ljubljana, Slovenia 

Background. In this work, we describe the computer systems Viprecox and Active Contours that are used 
in the process of realistic estimation of some biomechanical parameters of the hip joint, including the maximal 
value of the stress in the hip joint p. The computer system Active Contours uses standard antero


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posterior radiographs of the whole pelvis and both hips for its calculations and Viprecox in its kernel uses 
a relatively simple three-dimensional mathematical model of stress distribution in the hip-joint articular surface 
which has been extensively described elsewhere (e.g. Igli. 1996). 
Material and methods. Both state-of-the-art computer systems were tested by analysing the calculated values 
of pfor 81 patients (37 males and 44 females).

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Conclusions. In this way we prove that the described computer systems can be used for the determination 
of the contact stress distribution from standard AP radiographs. 

Key words: hip joint; biomechanics; computer systems; weight bearing 

Introduction 

Higher and unevenly distributed contact 
stress in the hip-joint is a risk factor for the 
development of arthrosis.1,2,3 There are several 
biomechanical parameters that describe 
the distribution and the peak value of the 
stress in the hip joint, such as the gradient of 
the stress at different positions of the articular 
surface or the maximal value of the pressure 
on the articular surface of the hip joint 

Received 5 February 2001 
Accepted 12 February 2001 

Correspondence to: Assist. Prof. Dragica Smrke, 
Ph.D., M.D., Department of Surgery, Zaloäka 2, SI1000 
Ljubljana, Slovenia; Phone: +386 1 543 1419; 
Fax: +386 1 231 6889; E-mail: dsmrke@hotmail.com 

p. The realistic measurement or the estimax


mation of these parameters would be helpful 
in exploring the incidence of degenerative 
joint diseases. 

In this work, we describe the computer 
systems Viprecox and Active Contours that are 
used in the process of a realistic estimation of 
some biomechanical parameters of the hip 
joint including the value of the parameter 

p. Our additional purpose is to test these 
max

state-of-the-art computer systems by analysing 
the calculated values of pfor 81 patients

max 

(37 males and 44 females). 

Materials and methods 

The computer system Active Contours uses 


Stankovski V et al. / Pressure distribution in the hip joint articular surface 

standard antero-posterior radiographs of the 
whole pelvis and the both hips for its calculations 
(Figure1) and Viprecox in its kernel uses 
a relatively simple three-dimensional mathematical 
model of stress distribution in the 
hip-joint articular surface, which has been extensively 
described elsewhere.4 

The system Viprecox was developed as a 
100% pure Java application, which enables 
the compatibility with any platform that uses 
the Java Virtual Machine. In the process of 
design and application development the object-
oriented paradigm was used. The use of 
the object-oriented programming technique 
will allow the system easy maintenance and 
upgrades, but it will also allow a conversion 
to other platforms. 

The system Active Contours uses digitised 
profiles of standard antero-posterior radiographs 
of the pelvis and the both hips in order 
to extract the important data about the 
geometry of the both hips and the pelvis 
(Figure 1). These data are then transformed 
by using a non-homogeneous tailoring procedure5 
in order to prepare the input data for 
the three-dimensional mathematical model of 
stress distribution in the hip-joint articular 
surface.4 For this purpose a reference model 
of the hip-joint musculature is used.6 

As a result of the non-homogeneous tailoring 
procedure certain muscles' origin and in-

Figure 1. The system Active Contours: detection of 
points that lie on the contour of the endoprosthesis 
head. 


sertion points are estimated. The reference 
muscles' origin points6 on the pelvis and the 
reference insertion points on the femur were 
not corrected in the antero-posterior direction 
because of the lack of data. All other coordinates 
are corrected as follows: 

The reference origin points on the pelvis 
for all muscles were corrected in the mediolateral 
direction by using the ratio C/Cref, 
where C is the distance between the sagital 
plane passing through the femoral head centre 
and the most lateral point on the pelvis. 
The system Viprecox automatically measures 
this point. Cref is the corresponding distance 
in the model of Dostal and Andrews6 
Cref=(Bref-Lref)/2. The insertion points on the 
femur are divided into three groups: 1. m. 
gluteus medius and m. gluteus minimus, 2. 
the lateral point of m. piriformis and 3. the inferior 
points of m. tensor fasciae latae and m. 
rectus femoris. The system Viprecox automatically 
measures the Cartesian coordinates of 
the insertion points of the gluteus muscles in 
the frontal plane from the digitised anteroposterior 
radiographs. The insertion point of 

m. piriformis lies laterally and was therefore 
corrected only its medio-lateral coordinate by 
using the ratio between the already corrected 
medio-lateral coordinates of the gluteus muscles 
and the corresponding reference values. 
The distance L between the two centres of rotation 
of both hips was taken as automatically 
measured by the system Viprecox. 
After such non-homogeneous tailoring 
procedure the prepared data are presented to 
the mathematical model of the hip-joint.4 In 
this way the maximal pressure on the hip 
joint articular surface pis calculated.

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Results and conclusions 

Figure 2 shows the calculated hip joint contact 
stress for a patient with implanted hip 
endoprosthesis (for the other healthy side). In 
order to establish the clinical relevance of the 

Radiol Oncol 2001; 35(1): 17-9. 


Stankovski V et al. / Pressure distribution in the hip joint articular surface 

Figure 2. An example of distribution of the stress on 
the hip joint articular surface graphically presented by 
the system Viprecox. 


parameter pthe computer system should

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be applied to various populations of patients 
where the correlation between the clinical 
status and the hip stress may be studied. 
Recently, these systems have been used in order 
to determine the peak contact stress in 
the articular surface of the hip joint from 
standard AP radiographs for 37 male and 44 
female healthy hips of patients subject to 
trauma of the other hip (Figure 2). It was 
shown that the peak contact stress is considerably 
higher (cca 20%) (p<0.00005) in the female 
population (1.99 MPa) than in the male 
population (1.63 MPa). These results are in 
favour of the hypothesis that the increased 
hip joint contact stress in the female population 
could contribute to the greater incidence 
of arthrosis in the female population relative 
to the male population. 

To conclude, the described computer systems 
Active Contours and Viprecox can be used 
for the determination of the contact stress 
distribution from standard AP radiographs. 
The systems can be applied in the clinical 
practice to predict an optimal stress distribution 
in different operative interventions in 
the hip and to analyse the short and long 
term outcome of the treatment of various conditions 
of the hip. 

References 

1. 
Hadley NA, Brown TD, Weinstein SL. The effects 
of contact pressure elevations and aseptic necrosis 
on the long-term clinical outcome of congenital 
hip dislocation. J Orthop Res 1990; 8: 504-13. 
2. 
Maxian TA, Brown TD, Weinstein SL. Chronic 
stress tolerance levels for human articular cartilage: 
two nonuniform contact models applied to 
long term follow-up of CDH. J Biomech 1995; 28: 
159-66. 
3. 
Brand RA. Hip osteotomies. A biomechanical consideration. 
J Am Acad Orthop Surg 1997; 5: 282-91. 
4. 
Igli. A. Matemati.no modeliranje operativnih posegov 
v kolku. Univerza v Ljubljani: doktorska disertacija; 
1996. 
5. 
Stankovski V, Igli. A, Kralj-Igli. V, Kersni. B. The 
hip-joint resultant force in healthy male and female 
population: a comparative study. Acta Chir 
Orthop Trauma .ech 1996; 63: 211-3. 
6. 
Dostal WF, Andrews JG. A three-dimensional biomechanical 
model of the hip musculature. J 
Biomech 1981; 14: 149-56. 
Radiol Oncol 2001; 35(1): 17-9.