Biomechanical analysis of Perthes' disease using the finite element method: the role of swelling of articular cartilage.

Radiologically, widening of the joint space in the hip affected with Perthes' disease is encountered in the very early stage of the disease when no changes are yet noted in the bone. This suggests that swelling of the articular cartilage may be taking place in the affected joint. Since the intra-articular volume of a ball-and-socket joint with a deep fossa, such as the hip joint, cannot be readily altered, this swelling is expected to exert a marked imbibitional pressure within the bone of the femoral head. We speculated that this biomechanical factor might play an important role in the development of Perthes' disease. A two-dimensional finite-element model with 853 elements was constructed to simulate the hip joint of a child. Stress distribution in epiphyseal bone of the femoral head was computed under the condition of swelling of articular cartilage, and biomechanical effects of the imbibitional pressure on the epiphysis were investigated. On the assumption that high stress concentrated in the bone caused collapse and necrosis, the process of development of Perthes' disease was simulated. The results of simulation using theoretical values were in close agreement with such clinical findings as destruction of the lateral portion of the epiphysis in the early stage, final fragmentation of the epiphyseal bone, and metaphyseal bone destruction.