Quantitative analysis of incongruity, contact areas and cartilage thickness in the human hip joint

Joint incongruity and cartilage thickness have been shown to determine the contact stresses and the load partitioning between the solid and fluid phases of articular cartilage. Matrix stresses, which are relevant in the development of osteoarthrosis, can, however, not be determined experimentally but must be calculated using numerical methods. The aim of the present study was to quantify the incongruity and cartilage thickness of the human hip, in order to allow for the construction of morphologically accurate finite element models. Twelve cadaveric specimens (34-86 years), two fresh and ten fixed, were investigated. The loading configuration was based on in vivo measurements of hip joint forces during midstance. The incongruity and contact areas were determined using a polyether casting technique, in the minimally and the fully loaded state. The cartilage thickness was measured at identical coordinate points with an A-mode ultrasonic system. Generally, the contact started at lower loads at the edge of the lunate surface, and the joint space increased towards its central aspects. In some specimens the contact started in the acetabular roof, leaving a joint space of up to 2 mm in the horns of the lunate surface. In others, the initial contact was observed in the anterior and posterior horns of the lunate surface with a joint space width of up to 0.75 mm in the acetabular roof. The size of the contact areas increased from about 20% of the lunate surface to 98% at higher loads. The articular cartilage thickness ranged from 0.7 to 3.6 mm, the maxima being located in the ventral aspects of the femoral head and acetabulum. These quantitative data on joint space width, contact, and cartilage thickness in the human hip joint may be used to construct and validate finite element models which are required to elucidate the mechanical factors involved in osteoarthrosis.