Primary osteoarthritis of the hip joint in Japan.

Diagnostic criteria for primary osteoarthritis of the hip joint in the Japanese population were determined by the roentgenographic measurements of the center-edge (CE) angle, the Sharp angle, and the acetabular roof obliquity in 254 normal hips. Primary osteoarthritis must meet the following conditions: (1) absence of femoral head deformities; (2) a CE angle of Wiberg greater than 19 degrees; (3) a Sharp angle less than 45 degrees; and (4) acetabular roof obliquity less than 15 degrees. These measurements are taken from roentgenograms during the early stage of the disease. Under these criteria, primary osteoarthritis accounted for only 0.65% of 2000 consecutive cases of osteoarthritis. Observation of the natural course of primary osteoarthritis revealed two subtypes. The superolateral type developed from the subset of normal hips with a relatively greater degree of acetabular roof obliquity.