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We determined contact stress on the articular surface of the hip joint in a group of patients who underwent operative treatment for severe slipped capital femoral epiphysis. Two different procedures were considered: the modified osteotomy of Dunn-Fish and the osteotomy of Imhäuser. In order to determine the stress distribution, a three-dimensional mathematical model was used taking into account the geometrical parameters of the pelvis and hip, which were measured from standard antero-posterior radiographs. We found that the Dunn-Fish procedure produced lower peak stress than the Imhäuser procedure.Résumé Nous avons determiné la pression sur la surface articulaire de la hanche dans un groupe de patients operés pour épiphysiolyse femorale supérieure. Deux opérations ont été considereés: l'opération de Dunn-Fish modifieé et l'ostéotomie d'Imhäuser. Pour déterminer la distribution des pressions, un modèle mathématique tridimensionel a été employé, tenant compte des paramétres géometriques du bassin et de la hanche, obtenus à partir des clichés antéro-postérieurs. Nous avons trouvé que dans les hanches opérées par la procedure de DunnFish modifiée les pics de pression sont inférieurs à ceux dans les hanches opérées par la technique d'Imhäuser.

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A mathematical model was developed to study the effect of the Chiari osteotomy on the distribution of contact hip stress over the weight-bearing area. It was shown that Chiari osteotomy can increase the weight-bearing area directly (on the lateral side), owing to the additional area formed by the ala ossis ilii segment, and indirectly (on the medial side), owing to the shift of the stress pole in the medial direction. As a consequence, the contact hip stress is reduced after Chiari osteotomy. The indirect effect is important and often larger than the direct one. Using the proposed mathematical model and standard anteroposterior roentgenographs from archives, the average peak stress on the weight-bearing area, normalised with respect to the body weight (pmax/ W(B)), was determined before and after Chiari osteotomy (8,310m(-2) and 4,480 m(-2), respectively) on a population of 29 dysplastic hips. The difference was statistically significant (p < 0.005). Based on the results presented, it can be concluded that the hip joint contact stress in dysplastic hips considerably decreases after Chiari osteotomy, indicating a favourable biomechanical effect of this operation.