An unusual deformity in the hip in juvenile rheumatoid arthritis (JRA) consisting of a small femoral head in a capacious acetabulum has been described as an isolated finding, but no explanation as to the cause of the deformity has been proposed. We have also observed this deformity and the objective of this investigation was to determine the cause.
In 1987, a review of hip disease in JRA was carried out at the Wellesley Hospital. Nine hips in 5 patients were identified as having a mismatch in size between the femoral head and acetabulum. Six of these hips came to surgery during the growth phase.
The 6 hips undergoing surgery between the ages of 16-19 showed an identical picture at the time of surgery. The pattern of pathology consisted of a femoral head devoid of articular cartilage and an acetabulum that showed an intact triradiate cartilage. Each of the 6 hips was treated with a resurfacing arthroplasty.
The femoral head grows in circumference by endochondral ossification of the articular cartilage. If the rheumatoid process destroys that articular cartilage, growth of the femoral head ceases. The intact triradiate cartilage permits continuing acetabular growth and the deformity of a small femoral head in a large capacious acetabulum results.