Prognosis of rheumatoid coxitis.

Geometrical analyses of the anteroposterior X-rays of 81 hips affected by rheumatoid arthritis for 1 to 17 years, have shown that the rate of progression of the inflammatory destructive lesions in the hip, expressed quantitatively in millimeters per year, depends on the local anatomical relations in the coxofemoral joint itself. This relationship is expressed by the angle of penetration of the plane of the acetabular opening (PEC) (angle PEC = angle CCD + angle n - angle PE). The PEC angle was found to be within the range of 85 to 128 degrees, and its mean value for the whole group was 104.6 degrees. In hips affected by rheumatoid arthritis for the same period of time, the average rate of progression of coxitis was twice as high when the PEC angle was up to 104 degrees than when it was 105 or more degrees. This difference is statistically significant. Irrespective of the magnitude of the angle of penetration through the plane of the acetabular opening (PEC), coxitis was found to progress the most rapidly during the first years of hip affection, and to diminish thereafter along a parabolic curve. The knowledge gained in this study makes it possible to predict the rate of progression of coxitis and is a contribution to a better clinical and radiological understanding of rheumatoid arthritis of the hip joint.