Three-dimensional evaluation of the association between face and back asymmetry among pre-pubertal subjects

AIM

The aim was to compare the degree of back symmetry in two groups of subjects with and without pathologic facial asymmetry and to assess any possible associations between face and back asymmetry evaluated on three-dimensional surface face and back scans.

MATERIALS AND METHODS

The study design consisted of allocation of 70 subjects (35 females, 35 males) aged 6.4±0.5 years, according to the percentage of whole face symmetry assessed on three-dimensional (3D) facial scans into a 'symmetric' (symG; symmetry ≥70%) and 'asymmetric'(asymG; symmetry <70%). The 3D face and back scans were analysed using colour deviation maps and percentages of symmetry of the whole face and back surfaces as well as their three separate areas: forehead, maxillary and mandibular areas for the face and neck, upper and middle trunk areas for the back, were calculated. Non-parametric statistical tests were used for between-group comparisons (Mann-Whitney U test). Within each group, differences between each face or back area were tested with the Friedman test. Correlations between face and back symmetry were assessed with the Spearman rho coefficient.

RESULTS

The symG exhibited a significantly higher symmetry in each facial area than the asymG. The mandibular area was the least symmetric area of the face within each group, with significantly smaller values than the maxillary area in the symG and significantly smaller values than the forehead and maxillary area in the asymG. The percentage of whole back symmetry did not significantly differ (p>0.05) between the symG ( 82.00% [67.4;88.00]) and asymG (74.3% [66.1;79.6]). The only significant between-group difference was observed for the symmetry of the upper trunk area (p=0.021), with lower symmetry values in the asymG. No significant associations were detected between face and back parameters.

CONCLUSION

The percentages of symmetry in each facial area were significantly higher among subjects without pathologic facial asymmetry. The most asymmetric area of the face, regardless of the degree of whole face symmetry, was its mandibular area. No significant differences were detected within different back areas; however, subjects with asymmetric faces showed significantly smaller symmetry of their upper trunk area.

The role of obesity, biomechanical constitution of the pelvis and contact joint stress in progression of hip osteoarthritis

OBJECTIVE

The aim of our study was to explore whether earlier hip arthroplasty for idiopathic osteoarthritis (OA) might be explained by enlarged contact stress in the hip joint, and to what amount can that be attributed to obesity and biomechanical constitution of the pelvis.

METHOD

Fifty subjects were selected from a list of consecutive recipients of hip endoprosthesis due to idiopathic OA; standard pelvic radiographs made years prior to surgery were the main selection criteria. For 65 hips resultant hip force and peak contact hip stress normalized to the body weight (R/Wb and p(max)/Wb) were determined from the radiographs with the HIPSTRESS method. Body weight and body mass index (BMI) were obtained with an interview. Regression analysis was used to correlate parameters of obesity (body weight, BMI), biomechanical constitution of the pelvis (R/Wb, p(max)/Wb) and mechanical loading within the hip joint (R, p(max)) with age at hip arthroplasty.

RESULTS

Younger age at hip arthroplasty was associated with higher body weight (P=0.009), higher peak contact hip stress normalized to the body weight - p(max)/Wb (P=0.019), higher resultant hip force -R (P=0.027) and larger peak contact hip stress - p(max) (P<0.001), but not with BMI (P=0.121) or R/Wb (P=0.614).

CONCLUSION

Our results suggest that enlarged contact stress (p(max)) plays an important role in rapid progression of hip OA with both obesity (increased body weight) and unfavorable biomechanical constitution of the pelvis (greater p(max)/Wb) contributing.

Higher peak contact hip stress predetermines the side of hip involved in idiopathic osteoarthritis

BACKGROUND

Biomechanical parameters of the hip have been suggested to have an important influence on the development of osteoarthritis. We aimed to find out whether higher stress is generated in a hip that subsequently results in earlier hip arthroplasty compared to the contralateral hip in the same subject.

METHODS

Standard anterior-posterior pelvic radiographs with no or subtle radiological signs of hip osteoarthritis, of 59 female patients, who underwent hip arthroplasty for primary osteoarthritis years later, were selected from the archives. For each subject peak contact hip stress of the hip with earlier arthroplasty and of the contralateral hip (pair of hips), was calculated from the radiographically obtained geometrical parameters with the HIPSTRESS program, which is based on a three-dimensional biomechanical model of the resultant hip force in the one-legged stance and a three-dimensional mathematical model of the contact hip stress distribution. Differences in peak contact hip stress within pairs of hips were determined for subjects with unilateral (22 pairs of hips) and bilateral disease (37 pairs of hips) by using paired-samples T-test.

FINDINGS

In the population of subjects with unilateral osteoarthritis, average peak contact hip stress was significantly higher (P = 0.007) in hips with arthroplasty (2.44 kPa/N) than in contralateral hips (2.32 kPa/N). In the population of subjects with bilateral osteoarthritis, average peak contact hip stress was significantly higher (P<0.001) in hips with earlier arthroplasty (2.54 kPa/N) than in contralateral hips (2.35 kPa/N).

INTERPRETATION

Results are consistent with the hypothesis that higher peak contact hip stress results in earlier hip arthroplasty due to faster development of idiopathic osteoarthritis.

Slotted acetabular augmentation for the treatment of residual hip dysplasia in adults: early results of 12 patients

INTRODUCTION

Different pelvic osteotomies and various shelf procedures are used for the operative treatment of hip dysplasia. Slotted acetabular augmentation (SAA) is a well-established technique for the treatment of children and adolescents with hip dysplasia. It has not been widely accepted for treating hip dysplasia in adults although good outcomes have been reported with other augmentation techniques in adults.

MATERIALS AND METHODS

Since 1997, SAA has been used for the prevention of hip arthrosis in 14 dysplastic hips in 12 female patients. The median age at operation was 38.5 (17-42) years; the median follow-up period was 4 (1-8) years. The patients were evaluated on the basis of radiographic, biomechanical and clinical data prior to surgery and at follow-up.

RESULTS

The median centre-edge angle of Wiberg increased from 9 degrees (1-26) before the operation to 43 degrees (31-55) at the latest follow-up (P < 0.001). The median peak stress on the weight-bearing area of the hip, calculated mathematically, was reduced from 14.9 (6.3-28-1) MPa prior to the operation to 4.1 (3-6.1) MPa at the latest follow-up (P < 0.001); the median Harris Hip Score increased from 60 (45-98) points preoperatively to 93 (49-100) points at the follow-up (P < 0.001). There was no difference between the preoperative and follow-up hip joint-space width (P = 0.2).

CONCLUSION

There were no postoperative complications. In our series, the procedure has proved reliable and safe. Its advantages include symptomatic pain relief, adequate acetabular roof coverage and reduced peak stress on the weight bearing area of the hip. It can be used to postpone the development of hip arthrosis in adults with acetabular dysplasia.

High contact hip stress is related to the development of hip pathology with increasing age

BACKGROUND

High contact hip stress is believed to be one of the key biomechanical factors involved in the hip cartilage degeneration and osteoarthritis. Accordingly, with increasing age high contact hip stress is expected to cause elimination of subjects from the population of healthy hips, but its predictive value has not been evaluated so far. The objective of the paper is to investigate whether the exposure of healthy hips to estimated high contact hip stress is related to the development of hip pathology with increasing age.

METHODS

A cross-sectional age- and gender-matched analysis of the peak contact hip stress calculated from pelvic geometry was made in 103 adult subjects with healthy hips. The peak contact hip stress was calculated from anterior-posterior pelvic radiographs of healthy hips by using a mathematical model of the human hip in the static one-legged stance.

FINDINGS

In both female and male population, the average values of the peak contact hip stress normalized to the body weight are significantly higher and the values are also more dispersed in younger subjects when compared to older subjects.

INTERPRETATION

The hip joints which remain healthy in the old age have lower average estimated peak contact hip stress. These results are consistent with the explanation that subjects with high estimated peak contact hip stress are more likely to develop hip disease in the course of life.

Influence of contact hip stress on the outcome of surgical treatment of hips affected by avascular necrosis

INTRODUCTION

Biomechanical analysis is an important tool that could improve the treatment of a diseased hip. However, it is still unclear how the biomechanical status affects the clinical outcome of a certain disease. In this work we studied the long-term effect of contact hip stress on the clinical outcome of hips that were operated on by various intertrochanteric osteotomies due to avascular necrosis of the femoral head. The hypothesis being tested is that the hips with a more favourable postoperative distribution of contact hip stress have a better clinical outcome.

MATERIALS AND METHODS

The study was performed on a population of 30 hips. For each hip, we determined the peak contact hip stress before the operation and immediately after the operation by using a recently developed method based on a three-dimensional mathematical model and the data from standard anteroposterior roentgenographs of both hips and pelvis. The hips were evaluated clinically 9-26 years after the operation and divided into a successful and an unsuccessful group. The average change of the peak stress due to the operation was calculated for each group, and the values were compared by t-test.

RESULTS

In the successful group the operation caused an average decrease of the peak hip stress of about 10%, while in the unsuccessful group the operation caused an average increase of the peak hip stress of about 4%, the difference between the respective changes of the peak stress due to the operation being statistically significant ( p=0.001).

CONCLUSION

Our results support the hypothesis that the hips with a more favourable postoperative distribution of contact hip stress have a better clinical outcome.

Polyethylene wear in total hip prostheses: the influence of direction of linear wear on volumetric wear determined from radiographic data

PURPOSE

To develop a new mathematical model for calculating the volumetric wear of polyethylene cups from known values of the radius of the prosthesis head, the extent of linear wear and the direction of linear wear determined from standard antero-posterior radiographs.

METHOD

A new mathematical model was developed. The results of this new mathematical model were compared with the results obtained using the standard, frequently used mathematical model, which takes into consideration only the radius of the prosthesis head and the extent of linear wear of the polyethylene cups. The results of both mathematical models were further compared with the results obtained by direct measurement of volumetric wear using the fluid displacement method.

RESULTS

Comparison of the mathematical models shows that the average volumetric wear calculated using the new mathematical model is 8.5% smaller than the average volumetric wear determined by the fluid displacement method, while the average volumetric wear calculated by standard mathematical model is 17.5% higher. The results of the new mathematical model are, thus, notably less biased than those of the standard one.

CONCLUSION

In calculating the volumetric wear from antero-posterior radiographs, not only the radius of the prosthesis head and the extent of the linear wear but also the direction of the latter has to be considered.

Mathematical estimation of stress distribution in normal and dysplastic human hips

By using a mathematical model of the adult human hip in the static one-legged stance position of the body, the forces acting on the hip, peak stress in the hip joint and other relevant radiographic and biomechanical parameters were assessed. The aims were to examine if the peak stress in dysplastic hips is higher than in normal hips and to find out which biomechanical parameters contribute significantly to higher peak stress. The average normalized peak stress in dysplastic hips (7.1 kPa/N) was markedly higher (to approximately 100%) than the average normalized peak stress in normal hips (3.5 kPa/N). The characteristic parameters that contributed to higher peak stress in dysplastic hips included the smaller lateral coverage of the femoral head, the larger interhip distance, the wider pelvis, and the medial position of the greater trochanter. These results are consistent with the hypothesis that stress distribution over weight-bearing surface of the hip joint is the relevant parameter for assessment of the risk for developing coxarthrosis.

The effect of electrostimulation and high load exercises in patients with patellofemoral joint dysfunction. A preliminary report

Traditional conservative treatment for patellar disorders is successful in about 80 percents of cases. We introduced two new conservative treatment protocols for patellar pathology in order to further improve the success rate. The first protocol consisted of high load/low repetition quadriceps femoris training (10 patients) while the second enclosed selective electrostimulation of vastus medialis muscle (7 patients). Results were evaluated clinically and neurophysiologically. High load/low repetition training resulted in significant increase of maximal voluntary contraction of quadriceps muscle (P < 0.001). Significant gain of Activity (P = 0.017) and Kujala scores (P = 0.07) was observed in group with high load/low repetition quadriceps training compared to patients with electrostimulation. There was no significant change in neurophysiological or clinical status between the beginning and the end of treatment with electrostimulation. Our results indicate that high load/low repetition quadriceps femoris training poses an important alternative to traditional conservative treatment protocol for patellar disorders.

Isolation of polyacetal wear particles from periprosthetic tissue of isoelastic femoral stems

We analysed revised Mathys isoelastic polyacetal femoral stems with stainless-steel heads and polyethylene acetabular cups from eight patients in order to differentiate various types of particle of wear debris. Loosening of isoelastic femoral stems is associated with the formation of polyacetal wear particles as well as those of polyethylene and metal. All three types of particle were isolated simultaneously by tissue digestion followed by sucrose gradient centrifugation. Polyacetal particles were either elongated, ranging from 10 to 150 microm in size, or shred-like and up to 100 microm in size. Polyethylene particles were elongated or granules, and were typically submicron or micron-sized. Polyacetal and polyethylene polymer particles were differentiated by the presence of BaSO4, which is added as a radiopaque agent to polyacetal but not to polyethylene. This was easily detectable by back-scattered SEM analysis and verified by energy dispersive x-ray analysis. Two types of foreign-body giant cell (FBGC) were recognised in the histological specimens. Extremely large FBGCs with irregular polygonal particles showing an uneven, spotty birefringence in polarised light were ascribed to polyacetal debris. Smaller FBGCs with slender elongated particles shining uniformly brightly in polarisation were related to polyethylene. Mononucleated histiocytes containing both types of particle were also present. Our findings offer a better understanding of the processes involved in the loosening of polyacetal stems and indicate why the idea of 'isoelasticity' proved to be unsuccessful in clinical practice.

Biomechanical evaluation of hip joint after Salter innominate osteotomy: a long-term follow-up study

The biomechanical state of the hip after a Salter innominate osteotomy was investigated by using the radiographic data of 38 operated and 21 contralateral nonoperated hips from our archives. The centre-edge angle of Wiberg was determined from the radiographs taken shortly after the operation. From the radiographs of the latest follow-up (7-13 years after the operation), we also determined the peak value of contact hip joint stress normalized by the body weight, and the functional angle of the weight-bearing area. A mathematical model was used. We show that the geometrical parameters aside from the centre-edge angle may considerably influence the contact hip stress distribution. We also show that the functional angle of the weight-bearing area is a more relevant parameter than the normalized peak stress if the exact magnification of the images is not known and if there is considerable variation of the image size within the sample. The development of the centre-edge angle of the operated hips and of the contralateral hips was also studied. We found that the centre-edge angle increases on average during the follow-up time in the operated hips as well as in the contralateral nonoperated hips, but the average increase is smaller in the former. It is shown that an unfavorable stress distribution is connected to the decrease of the centre-edge angle over time. Finally, we found a weak positive correlation between the centre-edge angle shortly after the operation and the functional angle of the weight-bearing area at the of the latest follow-up.

Determination of contact hip stress from nomograms based on mathematical model

Nomograms are presented that enable determination of maximal stress on the hip joint weight bearing area if certain geometrical parameters of the hip and pelvis and the body weight are known. The nomograms are calculated by using previously developed mathematical models. It is demonstrated how the maximal stress on the hip joint weight bearing area is determined from the presented nomograms for a hip for which the geometrical parameters were obtained from a standard anteroposterior rentgenograph. This simple and noninvasive method may give insight into the biomechanical status of the hip which should be considered in routine surgical planning and as a part of the routine examination of the patient without the use of any additional tools.

Biochemical aspects of Salter's osteotomy for treatment of acetabular dysplasia

The center-edge (CE) angle was measured on serial antero-posterior radiographs of 44 hips treated by Salter's innominate osteotomy for developmental dysplasia of the hip. Radiographic examination according to Severin was performed and the peak hip joint pressure was calculated using geometrical parameters determined from radiographs at the most recent follow-up examination, 7 to 13 years postoperatively. At the latest follow-up assessment, there was a positive correlation between the postoperative CE angle and radiographic results. Negative correlation was found between the postoperative CE angle and peak hip joint pressure at the latest follow up. In conclusion, Salter's osteotomy performed to treat developmental dysplasia of the hip should involve radical acetabular correction resulting in maximum increase of the CE angle.

Evaluation of biomechanical status of dysplastic human hips

The evaluation of the biomechanical status of human hip joint was performed. Required parameters were assessed from standard antero-posterior rentgenographs and the value of contact stress on the weight bearing area is calculated. We analysed 45 dysplastic human hips and compared measured values to 81 healthy hips. The peak contact stress on the weight bearing area is considerably and statistically significantly higher in dysplastic hips compared to healthy hips. The Wiberg centre-edge angle is statistically significantly smaller in group of dysplastic hips compared to healthy hips.

Different operative treatment of slipped capital femoral epiphysis: a comparative study of biomechanical status of the hip

The biomechanical status of the hips subject to two different operative treatments was studied. The biomechanical status was determined by the peak stress on the weight bearing area using the 3-d mathematical model. It was found that the procedure according to Imhäuser yields considerably and statistically significantly higher peak stress than the procedure according to Dunn-Fish.

Peak contact stress in human hip during gait

The contact stress in a human hip is not uniform and it changes with different body positions. The changing location of the peak contact stress during gait may indicate the predilection sites for further development of osteoarthritis in the hip. On the basis of laboratory measurements and by using mathematical models of forces and stresses in human hip we determined the points of the peak contact stress in successive phases of gait. Results show that the peak stress points are mostly located in the posterior-medial portion of the weight bearing area, which corresponds well to the clinical observations. It is also shown that in the pathological conditions of hip dysplasia the peak contact stress trajectory is located more laterally and anteriorly.

Salter osteotomy for treatment of acetabular dysplasia in developmental dysplasia of the hip in patients under 10 years

Serial radiographs of 44 hips in 39 patients undergoing Salter innominate osteotomy for the treatment of dysplastic acetabulum owing to developmental dysplasia of the hip were reviewed. The hips were also evaluated clinically at the last follow-up examination, 7 years to 13 years postoperatively. At 7 years to 13 years postoperative, excellent or good clinical results were assessed in 43 hips (98%), and excellent or good radiographic results in 32 hips (73%). In patients with a postoperative center edge (CE) angle > 24 degrees, the CE angle remained significantly greater throughout the follow-up period compared with patients with a postoperative CE angle < or = 24 degrees. A positive correlation was found between the degree of operative CE angle correction and radiographic findings 7 years to 13 years postoperatively.

Weight bearing area during gait in normal and dysplastic hips

The size and the shape of the weight bearing area in adult human hips depend on the forces acting in the hip and therefore change during the body motion. In this work the size and the shape of the weight bearing area in several phases of gait are estimated. The forces acting in the hip were determined through laboratory measurements and analyzed by mathematical models. The dysplastic hips are distinguished from the normal ones by a smaller center-edge angle of Wiberg while the time course of the forces acting in the hip is assumed to be the same in both cases. It is shown how radial articular stress is distributed over the weight bearing area in both cases. In normal human hips the weight bearing area occupies a rather large portion of the acetabulum-femoral head contact area while in dysplastic hips stress distribution is unfavourably concentrated in a smaller weight bearing area.

[Anterior cruciate ligament function: a non-invasive quantitative technique]

PURPOSE OF THE STUDY

The purpose of the study was to develop a new non-invasive and quantitative method for evaluation of the anterior cruciate ligament function.

PATIENTS AND METHODS

The new method was tested in 24 asymptomatic subjects and in 58 patients (39 males and 19 females) with arthroscopically proven unilateral rupture of the anterior cruciate ligament. Anterior displacement of the tibia with the knee in 20 degrees flexion was achieved by using a redesigned device, constructed originally for stress-radiography, and detected by a 3-D optical motion analysis system.

RESULTS

In the normal group, the average difference (d) in anterior displacement of the tibia between the left and the right side under a displacement force of 250N was 0.56 mm (range 0.1 mm to 1.9 mm) or 17 p. 100 (range 2 p. 100 to 64 p. 100), while in patients the average difference (d) in anterior displacement of the tibia between the injured and uninjured side under a displacement force of 250N was 4.02 mm (range 0.6 mm to 9.4 mm) or 133.6 p; 100 (range 26 p. 100 to 451 p. 100). By the criteria for false negative results, i.e. the maximum value of the parameter d in normal subjects (1.9 mm and 64 p. 100), 10 p. 100 of the patients (6 patients) fell into the normal group. According to the criteria of false positive results, i.e. the minimum value of the parameter d in patients (0.6 mm and 26 p. 100), 17 p. 100 of the normal subjects (4 normal subjects) fell into the group of patients. The achieved level of differentiation is in the similar range as the most frequently used present methods.

DISCUSSION

The main advantage of the newly developed noninvasive optical measurement technique is that the direction of force and the amplitude and site of application are accurately defined and documented, and that dislocation movement of the tibia towards the femur during the application of a growing force is continuously monitored with the 3-D optical system and that it allows for several consecutive assessments based on statistical data.

Increased muscle stiffness after anterior cruciate ligament reconstruction--memory on injury?

Residual impairment of knee function was tested in 25 patients 6 to 12 months after successful unilateral reconstruction of the anterior cruciate ligament. The control group comprised 25 healthy individuals. Jumping onto one foot from a height of 20 cm was taken as a prototype of similar everyday activities. We measured maximum flexion of the knee during the landing phase of the jump, and the degree of knee flexion at the time of maximum deceleration of the knee against gravity during jumping using the three-dimensional optical motion analysis system-ELITE. In the group of patients, measured parametres were significantly smaller on the operated side than on the unaffected side. These two measured values were significantly smaller on both legs in group of patients as compared to the values in the control group. The results of our study indicate that functional disturbance persisted for 6 to 12 months after anterior cruciate reconstruction. This may be due to the changed muscle activation pattern of the knee causing increased stiffness of knee muscles. It seems that re-programming of the central nervous system occurred in order to protect the injured lower extremities from another injury.

Increased incidence of arthrosis in women could be related to femoral and pelvic shape

In this study various femoral and pelvic geometrical parameters important for hip joint contact stress were determined. The parameters were measured from standard anteroposterior radiographs of healthy adult subjects and analysed by using descriptive statistical procedures. Women proved to have a significantly smaller femoral head radius and larger distance between the inner acetabular rims than men, both features which lead to an increase of contact stress in the hip joint articular surface. Since too high, long-lasting contact stress is unfavourable regarding the development of arthrosis, we propose that these differences in the femoral and pelvic geometry could be one of the reasons for the increased incidence of arthrosis in women.

Determination of the femoral and pelvic geometrical parameters that are important for the hip joint contact stress: differences between female and male

The difference between male and female femoral and pelvic geometry was studied by considering some geometrical parameters such as interhip distance, inclination of the crista iliaca, diameter of the femoral head and centre-edge angle of Wiberg. The values of these parameters were determined for 79 healthy female and 21 healthy male subjects. Standard anterior-posterior radiographs were used and processed by the computer-aided system. The results show some important sex differences in femoral and pelvic geometrical parameters which determine the hip joint contact stress.

Relative abductor strength after varus and valgus osteotomy of the proximal femur

Relative abductor strength (RAS) is defined as the difference between the available (maximum) abductor force and the required abductor force i.e. the force needed to maintain the mechanical equilibrium of the pelvis. In this work the dependence of RAS on the centrum-collum-diaphyseal angle and on the position of the greater trochanter was studied using a 3-D mathematical model of the human hip in the monopodal stance. Simulation of the varus and valgus osteotomy was done for the normal, 3 cm distal and 5 cm proximal position of the greater trochanter before the operation. It was shown that after valgi-sation a significant reduction in RAS is to be expected. On the other hand, varisation osteotomy is not associated with a significant change in RAS. The changes in RAS after valgisation for normal and distal position of the greater trochanter occur within the scope of the negative Trendelenburg sign. A negative RAS and, consequently, a positive Trendelenburg's sign occur only in extreme valgisation with a proximally situated greater trochanter. Reduction in RAS may reduce the capability of the hip muscles during strenuous physical activity and may thus participate to earlier muscle fatigue. The importance of special rehabilitation programme after valgisation osteotomy is stressed. Key words: hip biomechanics, abductor muscles, varus and valgus osteotomies, proximal femur.

Biomechanical study of various greater trochanter positions

A mathematical model was used in order to evaluate the mechanical situation after lateral, medial, distal and proximal displacements of the greater trochanter. It was calculated that lateral displacement may considerably reduce the hip joint contact force, while medial displacement greatly increases it. The influence of proximalization and distalization is much less pronounced. It was further shown that, regarding the postoperative relative hip abductor muscle strength, lateral and distal displacements of the greater trochanter are favourable, while proximal and medial displacements are not.

Reducing the Stress in the Articular Surface of the Hip Joint after Shifting the Upper Part of the Body towards the Painful Hip

In this work the change of stress distribution in the hip joint articular surface after shifting of the upper part of the body towards the painful hip due to pain reflex is estimated theoretically by using a three dimensional mathematical model. It is shown that after displacement of the upper part of the body towards the painful hip the stress in the hip joint can be in case of poor femoral head coverage and small inclination of the pelvis very little decreased in spite of the fact that the resultant hip joint force is reduced considerably. Key words: stress in the hip joint, body shitting.

Biomechanical analysis of various operative hip joint rotation center shifts

A mathematical model was used to evaluate the mechanical situation after various operative shifts of the hip joint rotation center. It was concluded that while performing different pelvic osteotomies and the total hip replacement the hip joint rotation center should be shifted as far medially as is technically possible to reduce the magnitude of the hip joint contact force. By contrast, lateralization of the hip joint rotation center strongly increases the magnitude of the hip joint contact force; therefore this should be avoided whenever possible. The superior shift of the hip joint rotation center decreases the strength of the patient's hip abductor muscles and must therefore also be avoided. The inferior shift of the hip joint rotation center is favorable because it increases the strength of the patient's hip abductor muscles.

Computer simulation of periacetabular osteotomy

A simple static three-dimensional mathematical model of an adult hip in one-legged stance was used to evaluate the mechanical situation after periacetabular osteotomy. We found that the hip joint rotation center shifted as a consequence of the osteotomy. This may have considerable effects on the hip joint resultant force and therefore also on the pressure on the femoral head, which could cause the development of arthrosis.