The shape of the acetabular cartilage surface and its role in hip joint contact stress

The acetabular cartilage is normally represented as a spherical shape in orthopedic clinic and related researches. The aim of the study was to present a new mathematic representation with better fit to the acetabular cartilage surface and to investigate the role of its shape on the hip joint contact stress. Combined with reverse engineering technique, surface-fitting algorithms and mathematical curve surface theory, we found that the shape of the acetabular cartilage surface was not conventionally spherical but rotational ellipsoidal by statistically analysis of 25 specimens. To investigate the role of the non-spherical acetabular cartilage shape in the hip joint biomechanics, three finite element models involving the original shape of acetabular cartilage shape, the acetabular cartilage shape replaced by the rotational ellipsoid and the sphere respectively, were constructed using a healthy volunteer´s CT image data. The simulation result showed that, the shape of natural healthy acetabular cartilage surface contributed to the optimal contact stress distribution of hip joint. The replaced fitting spherical surface led to an increased contact stress of hip joint and uneven distributed patterns of contact stress, whereas the replaced fitting rotational ellipsoid surface was comparatively more consistent with the natural results than sphere. The results provide a new better-fit mathematic presentation of the acetabular cartilage surface which can be used in the hip joint simulation study, and demonstrate the important role of the non-spherical acetabular cartilage shape in the hip joint biomechanics.