The hip joint as a conchoid shape.

The hip joint is not an exact ball and socket joint. In a meridian section, the mean deviation from a conchoid shape is quite small, so that this shape might better describe the joint's shape. This conclusion was reached by measuring the cartilaginous and osseous shapes of eight normal hip joints (multiorgan donors with average of age 33 yr, range 19-46 yr) using a CNC coordinate measuring machine (CMM). On two additional hip joints, only the osseous shape was determined. A rotational axis was first determined by finding parallels of latitude at the femoral head and acetabulum. At the meridian sections, the best-fitting circle or conchoid was determined from the scanned measuring points, using least-squares regression. Two perpendicular meridians were then measured for each sample and used to evaluate the three-dimensional shape. The medium squared deviation showed a better fit for a conchoid shape compared to a sphere for all samples tested. Furthermore, the equation of the conchoid for the femoral head (r = a + b cos phi) and that for the acetabulum (r' = a' + b' cos phi) were related in that a = b' and b = a' within mean deviation factors of 4%. Their special shape makes the joint less likely to sublux compared to a ball and socket joint. In addition, the rolling and gliding mechanisms between the two shapes may result in less wear.