[The clinical relevance of biomechanical analysis of the hip area]

Some diseases of the hip are due to mechanical causes or are influenced in a positive or negative sense by mechanical factors. In every case, the balance between stressing and biological reaction of the tissues of the locomotor system plays a decisive role. A careful biomechanical analysis should therefore be made at the beginning of any prognostical or therapeutical consideration. This means respective to the hip joint analyses of magnitude and distribution of the articular pressure and of the stressing of the femoral neck. The normal hip joint is characterised by a nearly evenly distributed articular pressure, and this is expressed in a bone condensation of equal thickness in the acetabular roof (sourcil). The consequence of an uneven stress distribution is a triangular shape of the sourcil, increasing either to the lateral or to the medial border of the acetabular roof. The lateral triangle is in general steeper and therefore more dangerous than the medial one. The stress concentration alters first the articular cartilage and then the subchondral bone. The aim of a causal surgical therapy is the decrease and the equal distribution of the articular pressure. It can be attained by increasing the weight-bearing surface of the joint and by centralisation of the stressing force (joint resultant) within this surface. The neck of the normally shaped femur is stressed in the sense of bending, and the magnitude of this stress depends on the neck/shaft angle. In consequence of this result shearing forces at the level of neck fractures, thus preventing the bony reunion. The therapeutical intervention tries either to strengthen the resistance against the shearing force (by nailing) or to eliminate it--as for example in the case of non-union of a neck fracture--(valgisation osteotomy). The reduction of the shearing stress by a valgisation osteotomy (Y-osteotomy, Pauwels) is the decisive factor in the treatment of the congenital coxa vara, a disease, due to a reduced resistance of the tissues of the epiphyseal plate against the normal bending stress. Valgisation osteotomies include the danger of increasing stresses in the hip joint. However, this can be prevented by lateralisation of the greater trochanter.