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$Object Oriented Approach to Determination of Biomechanical Parameters of the Hip-Joint V. Stankovski", V. Kralj-Iglie', B. Kersnie*, D. Smrke\ A.$

1 Object Oriented Approach to Determination of Biomechanical Parameters of the Hip-Joint V. Stankovski", V. Kralj-Iglie', B. Kersnie*, D. Smrke\ A. Iglie^ "Clinic oftraumatology, Zaloska 2, SI 1000 Ljubljana, Slovenia vlado. uni-lj. si ^Faculty of Electrical Engineering, Trzaska 25, "Institute of Biophysics, Medical Faculty, Lipieeva 2, Abstract Some recent advances in object oriented programming techniques and in programming languages in particular, the object oriented programming language Java (Sun Microsystems, Inc.) are used to produce a set of objects that comprise the hip-joint geometry (points, axes, profiles, distances, angles), objects that describe the forces and the stress distribution in the hip-joint articular surface (forces, stress), as well as objects that can be manipulated by using Drag and Drop operations in order to simulate changes in the hip-joint geometry (femoral head, femoral neck, greater trochanter and lower part of the femur). Any change in the objects that can be manipulated influences the remaining objects that are currently displayed. Particularly, any change can influence the hip-joint reaction force, and the corresponding contact stress distribution. The changes are estimated automatically by using methods that are attached to the specific objects. Digitized profiles of standard AP radiographs of the hip-joint are used as input to this computer-aided system. A non-homogeneous scaling method that is based on a reference model of the hip musculature, is used to prepare data for each human subject. These data serve as input to a three-dimensional mathematical model of the hip-joint in one-legged stance, which is used to estimate the values of the hip-joint reaction force (R) and its inclination angle (8%). Afterwards, the stress distribution in the hip-joint articular surface is calculated by using a three-dimensional model of the hip-joint articular surface, which takes into account the previously estimated values of R and SR, the femoral head radius (r) and the center-edge angle of Wiberg (8^). The system was applied to 21 male and 79 female radiographs of healthy subjects. The results show that the contact stress in the hip-joint articular surface is significantly greater for females than for males. Because the increased stress in the hip-joint is unfavorable regarding arthrosis development (Hadley et al. [1]), the presented results could partially explain the fact that females have greater incidence of arthrosis in the hip-joint than males. This is related not only to femoral, but also to pelvic geometry.

238 Simulations in Biomedicine IV 1 Introduction The hip-joint is often affected by pathologies which change its geometry like, as an example, the arthrosis of the hip which distorts the spherical

2 238 Simulations in Biomedicine IV 1 Introduction The hip-joint is often affected by pathologies which change its geometry like, as an example, the arthrosis of the hip which distorts the spherical shape of the femoral head and changes the direction and magnitude of the forces and stress distribution acting upon the joint. Since the advent of radiographic examination of bony structure several geometrical parameters have been described to define the geometry of the joint. The use of these parameters in the clinical examination is a well established procedure of diagnostic relevance: for example, the definition of a varus or valgus femoral shape is made according to the value of the angle of inclination (the axis of the femoral neck relative to the axis of the femoral diaphysis). Fischer [2], Pauwels [3] and Bombelli [4] are some of the authors who suggested a refined analysis of the principal forces acting upon the hip-joint. Their work makes the base of the two-dimensional model of the hip-joint in the frontal plane. Together with the development of computer science, there have been several improvements of the two-dimensional model. Iglie et al. [5] introduced a simple three-dimensional mathematical model of the hip-joint in one-legged stance which is used in the present paper. By using this model, it is possible to estimate the magnitude of the hip-joint reaction force (R) and its inclination angle (8%). The three-dimensional co-ordinates of the femoral and the pelvic attachment points of five different muscles and the distance between the two femoral head centres (L) have been taken into account in this model. The stress distribution in the hip-joint articular surface is then estimated by using another model (Iglie et al. [6,7]). The model of stress distribution takes into account the hip-joint reaction force, its inclination angle, the femoral head radius, and the center-edge angle of Wiberg. Estimating a lot of parameters from AP radiographs, anyway, is highly time consuming and because of this reason a widespread use in clinical practice can be significantly impaired. So, study was undertaken to speed-up the procedures by the use of computers to allow examination not only of patients who suffer from hip-joint diseases and require osteotomy or total hip replacement, but also of healthy patients (Iglie et al [8, 9], Jaklie & Pernus [10], Stankovski et al [11])- The computer system Viprecox (Vis et pressus in coxa) is one of the results of our study. The system was written in the object oriented programming language Java (Sun Microsystems, Inc.) and consists of set of objects that comprise the hip-joint geometry, the forces and the stress in the hip-joint, and objects that can be manipulated by using Drag and Drop operations in order to simulate changes in the hip-joint geometry. A brief overview of the system is given in this paper. Our additional aim was to elucidate the fact that female population has greater incidence of arthrosis in the hip-joint than male population. In order to do this the result of Hadley et al. [1] is used, who showed that increased stress

3 Simulations in Biomedicine IV 239 in the hip-joint articular surface is unfavourable regarding arthrosis development. In the discussion, the results are also related to the results of Kersnie el al [12], who observed differences between the male and the female pelvic and femoral geometry, which influence the resultant hip-joint force (R) and the corresponding contact stress in the hip-joint articular surface (Iglie et al [8,9]). 2 The system ViprecoxO: objects and methods 2.1 Configuration The configuration needed to run the system Viprecox consists of an IBM PC compatible computer with Pentium processor and 16 MB of RAM on which a multi-threading operating system is installed (e.g. MS Windows 95 or MS Windows NT ). Due to the fact that the system Viprecox has been entirely written in the platform independent programming language Java, it can actually run on any multi-threading operating system on which the Java Virtual Machine is installed (Ramakrishnan [13]). Similar packages that performed two-dimensional analysis of the hip-joint were built by Tranquilli & Merolli [14] (ANCA) and by Jaklie & Pernus [10] (HIJOMO). The Viprecox package is a continuation of the work of Jaklie & Pernus. The both systems were able to measure several different biomechanical parameters of the hip-joint, such as points, distances and angles. In addition, the package ANCA was also able to determine the hip-joint reaction force according to the model of Fischer [2], Pauwels [3] & Bombelli [4]. Both packages were written in the C programming language. ANCA worked on a DEC 11/23 minicomputer and HIJOMO on an IBM PC computer. 2.2 The object oriented paradigm The object-oriented programming technique allows us to build systems that behave in a strictly specified way. The objects can be composed of objects, which themselves can be composed of objects, and so on The final aim is to improve the complexity, the design, theflexibility,the fastness, the simplicity and the reliability of the computer programs (Martin & Odell [15]). Because of these reasons, from 1995 the programming language Java started to gain on importance (Ramakrishnan [13]). Not only that Java is object-oriented and platform independent, but it also has the advantage that it is the language of the Internet. An interpreter runs Java applications using run-time support (the Java run-time environment is provided with the Netscape and other Internet browsers). As a result, the latest version of the system Viprecox is available on Internet.

4 240 Simulations in Biomedicine IV 2.3 Description The user can work with Viprecox (see Fig. 1) either on Internet by using his favorite Internet browser or on his local machine in an user-friendly environment. There is a pull-down menu situated at the top of the application, and two additional control panels: the Select panel at the top, and the Status panel at the bottom of the page. In the center of the page there are two animation windows: the West animator on the left-hand side and the East animator on the right-hand side of the page. stress distribution Figure 1. Stress distribution in the hip-joint articular surface for the currently selected patient determined by the system Viprecox from an AP radiograph. The main menu offers the following options: File, Edit, Graphics, Simulation, Status, Report and Help. The File option allows the user to Open(Save) a new patient or a group of patients to be examined. The Edit option allows the user to create groups of patients according to the user's criteria, to delete patients from the current group, or to move, and copy patients from one group to another. The Graphics option decides which data will be displayed on the animation screens. The user

Simulations in Biomedicine IV 241 can choose any of the options: Points, Axes, Profiles, Distances, Angles, Forces or Stress, and in each of these, he can set specific details to be displayed.

5 Simulations in Biomedicine IV 241 can choose any of the options: Points, Axes, Profiles, Distances, Angles, Forces or Stress, and in each of these, he can set specific details to be displayed. Simulation is probably the most important option in the application. In this option the user is allowed to simulate any of the following operations of the hipjoint: Chiary, Valgus, Varus, Greater Trochanter Shift and Femoral Rotation. It is also possible to Update or to Forget the results of the simulation. The Status menu gives the user a possibility to view the values of all different parameters that are kept about the patient. The Report option (Patient, Group, Special) gives various reports about the currently observed patient or group. Guidance throughout the application is available by using the Help menu. From the Select panel the currently displayed patient or group of patients can be selected, as well as the left or the right hip-joint on each of the animation windows. The Status panel explains what is currently happening in the animation windows. 2.4 The objects The West and the East animators allow the use of Drag and Drop operations in order to manipulate the following objects on the screen, the femoral head, the femoral neck, the greater trochanter, and the lower part of the femur. In addition, there are objects that can be shown on both animators, but can not be manipulated. These objects present various points, axes, profiles, distances, angles, forces and the stress on the hip-joint. At present, only the projection of these objects in the frontal plane is shown, although the incorporated models are three-dimensional. These objects will be extended in the future to allow more functionality. All objects are updated after any change in the input conditions of the implemented models. As a result of the Drag and Drop operations it is immediately possible to view the changes in the displayed biomechanical parameters (e.g. a change of the position of the femoral head results in a change of the stress distribution). 2.5 Model of stress distribution in the hip-joint articular surface In the present state-of-the-art computer system, we incorporated threedimensional mathematical models in order to estimate the forces acting in the hip and the corresponding stress distribution in the hip-joint articular surface. These models are presented in detail elsewhere (Iglie et al. [5, 6], Ipavec et al [7]). Following is only a brief review. The system Viprecox takes as input geometrical data which is acquired from standard AP radiographs by a digitizer. In order to prepare data as input to the previously mentioned models there is a need to estimate the three-dimensional co-ordinates of the femoral and the pelvic attachment points of the following muscles: gluteus medius, giuteus minimus, piriformis, tensor fasciae latae and rectus femoris and the distance

6 242 Simulations in Biomedicine IV between the two femoral head centres (L) for each individual. Reference threedimensional co-ordinates of the femoral and the pelvic attachment points and the distance between the two femoral head centres (L^f) were obtained from the study conducted by Dostal & Andrews [16] as well as the pelvic breadth (B^f) These data were then used for scaling purposes. Dostal & Andrews emphasised the possible problems when using homogeneous scaling methods to tailor their anatomical data to particular human subjects. Viprecox performs a nonhomogeneous scaling method, which is described elsewhere (Stankovski el al It should be noted that the gluteus muscles have a relatively great attachment areas on the pelvis and are therefore both divided into three groups relative to the frontal plane of the body: anterior, middle and posterior. The study of Dostal & Andrews [16] provides reference co-ordinates for the attachment areas of these muscles. Viprecox also automatically measures the distance L After the non-homogeneous tailoring procedure the prepared data are presented to the mathematical model of the hip-joint. In the one-legged stance the forces acting on the pelvis are the hip-joint reaction force, the weight of the body except for the loaded leg and the effective muscle force which is the vector sum of forces of different muscles which are active particularly in this body position. The resultant hip-joint force and its inclination angle are obtained by solving a system of two equations regarding the equilibrium of the forces and of the torques acting on the pelvis (Iglie et al [5]). In calculating the radial stress distribution in the hip-joint articular surface, the femoral head is represented by a sphere, while the acetabulum is represented by a fraction of a spherical shell. An articular surface is defined as the sphere with the mean of the radii of the femoral head sphere and of the acetabular shell. The weight bearing area of the hip-joint is taken to be a portion of the articular surface and is determined by the hip geometry. On the lateral side, the boundary is obtained by intersecting the articular surface sphere with a plane inclined for the center-edge angle of Wiberg with respect to the sagittal plane of the body. It is assumed that the radial stress is proportional to the cosine of the angle between the observed point on the weight bearing area and the point of maximal stress on the articular sphere (Brinckmann et al. [17]). The distribution of the radial stress on the articular surface is obtained by solving the three components of the vector equation requiring that the radial stress integrated over the entire weight bearing area yields the resultant hip-joint force (Iglie et al [6,7]). The estimated maximal value of stress in the hip-joint articular surface was then analysed by using descriptive statistical methods. The female and the male population were compared by comparing the average values of the maximal stress. The probability P describing statistical significance of the difference between the average values of the maximal stress in males and females was determined by using the Mann- Whitney test.

7 3 Results Simulations in Biomedicine IV 243 For the purpose of analysis, standard AP radiographs of the hip were taken from the archive of the Department of Orthopaedic Surgery in Ljubljana from 1985 on. Only radiographs of the subjects for which the pelvic and the femoral shapes were found to be normal were taken into account in order to observe only differences between healthy males and females. Thus the final analysis was performed for 21 adult male and 79 adult female healthy human subjects. The results show important differences between the male and the female population regarding the radial stress distribution in the hip-joint articular surface. The histograms presented in Fig. 2 show the maximal value of stress in the hip-joint articular surface (p x) normalised by the body weight (We) for male and for female population. The value of pmax/we is significantly greater in females (2653,53) than in males (2276,89) (P < ) Pmax/WB [1/m*] Figure 2. Histograms of the normalised maximal value of stress in the hip-joint articular surface (pmax/wg) for male and for female population. 4 Conclusions The object-oriented computer system Viprecox was built for the purpose of analysis of various biomechanical parameters of the hip-joint including the stress distribution in the hip-joint articular surface. The system was applied to AP radiographs of male and female healthy subjects. The results show that the contact stress in the hip-joint articular surface is significantly greater in females than in males.

8 244 Simulations in Biomedicine IV Because increased contact stress in the hip-joint accelerates the development of arthrosis (Hadley et al. [1]), we propose that females have unfavourable pelvic and femoral geometry compared to the males. Such characteristic difference in the geometry may be the greater pelvic width in females than in males (Kersnie et al. [12]). The unfavourable pelvic and femoral geometry could be one of the reasons for higher incidence of arthrosis development in female population, especially in combination with increased body weight (Vingard [18], Leger [19]). This assumption is in agreement with some observations which indicate that ethiologic factors associated with hip arthrosis may differ for males and females (Tepper & Hochberg [20]). Acknowledgement This work was supported by a grant from the Ministry of Science of the Republic of Slovenia: indications for Implantation of Bimodular Hip Endoprothesis«. All rights on the system Viprecox are reserved by V. Stankovski, A. Iglie & V. Kralj-Iglie. References 1. Hadley, N.A., Brown, T.D. & Weinstein, S.L. The effects of contact pressure elevations and aseptic necrosis on the long-term clinical outcome of congenital hip dislocation, J. Orthop. Res., 1990, 64, Fischer, O Der Gang des Menschen, Hirzel, Leipzig, Pauwels, F. Biomechanics of the Normal and Diseased Hip, Springer- Verlag, New York, Bombelli, R. Osteoarthritis of the Hip, Springer-Verlag, New York, Iglie, A., Srakar, F, Antolie, V., Kralj-Iglie, V. & Batagelj, V. Mathematical analysis of Chiari osteotomy, Ada Orthop. lugosl., 1990, 20, Iglie, A., Srakar, F, Kralj-Iglie, V. & Antolie, V. The influence of pelvic shape on the stress distribution on the articular surface of the human hip joint, Zdrav. Vest., 1994, 63,

9 Simulations in Biomedicine IV Ipavec, M, Kralj-Iglie, V. & Iglie, A. Stress in the hip joint articular surface during gait, Eng. Modelling, 1995, 1-2, Iglie, A., Srakar, F & Antolie, V. Influence of the pelvic shape on the biomechanical status of the hip, Clin. Biomech., 1993, 8, Iglie, A., Kralj-Iglie, V., Antolie, V., Srakar, F. & Stanie, U Effect of the periacetabular osteotomy on the stress on the human hip joint articular surface, IEEE Trans. Rehab. Engr., 1993, 1, Jaklie, A. & Pernus, F Morphometric analysis of the AP pelvic and hip radiographs (ed B Zajc & F Solina), p , Proceedings of the 3rd Slovenian Electrotechnical and Computer Science Conference, Ljubljana, Slovenia, Stankovski, V., Iglie, A., Kralj-Iglie, V & Kersnie, B The hip-joint resultant force in healthy male and female population: a comparative study, Ada Chirurgiae Orthopaedicae et Traumatologiae Cechosl, 1996, 63, Kersnie, B, Iglie, A., Kralj-Iglie, V. & Srakar, F Increased incidence of arthrosis in female population could be related to femoral and pelvic shape, Arch. oforth. and Trauma Surg., 1997 (in press). 13. Ramakrishnan, S. Server-side Java script, Dr. Dobb's Sourcebook, 1996, November/December, Tranquilli, L.P., Merolli A Analysis of hip-joint geometry and its modification after a computer simulated osteotomy, pp , MIE 87 Proceedings, 3, Edi Press, Martin, J & Odell, J. The Object-Oriented Revolution, Savant Institute, Carnforth, Dostal, W.F. & Andrews, J.G. A Three-dimensional biomechanical model of hip musculature, J. Biomechanics, 1981, 14, Brinckmann, P., Frobin, W & Hierholzer, E Stress on the articular surface of the hip-joint in healthy adults and persons with idiopathic osteoarthrosis of the hip-joint, J. Biomechanics, 1981, 14, Vingard, E Overweight predisposes to coxarthrosis. Body-mass index studied in 239 males with hip arthroplasty, Act. Orthop. Scand., 1991, 62 (2),

10 246 Simulations in Biomedicine IV 19. Leger, O. L'epiphysiolyse femorale superieure en Polynesia Francaise. Une douleur de hanche aux concequences lourdes, Medicine Tropicale, 1992, 52 (1), Tepper, S. & Hochberg, M.C. Factors associated with hip osteoarthritis: data from thefirstnational health and nutrition examination survey, Am. J. Epidemiol., 1993, 138,

The shape of acetabular cartilage optimizes hip contact

J. Anat. (2005) 207, pp85 91 The shape of acetabular cartilage optimizes hip contact Blackwell Publishing, Ltd. stress distribution Matej Daniel, 1 Ale7 IgliC 2 and Veronika Kralj-IgliC 3 1 Laboratory