The Aetiology of Primary Osteoarthritis of the Hip

In most reports on osteoarthritis of the hip approximately half the patients have presented in adult life with no relevant antecedent history. In order to assess, in such cases of so-called primary osteoarthritis, the presence of any minor pre-existing anatomical abnormality, the pelvic radiographs of 200 cases of this type were examined. No case in which the changes could be attributed to symptomatic disorder earlier in life was accepted. In this connection a few cases showing a protrusio acetabuli deformity were arbitrarily regarded as being secondary to inflammatory change of a rheumatoid nature and were therefore excluded. The cases were divided by visual assessment into those arising in the presence of (a) normal anatomy; (b) abnormal anatomy. The variations in the latter group included imperfect acetabular development—acetabular dysplasia—and an abnormal relationship of the femoral head to the femoral neck, comparable to the residual adult deformity of epiphysiolysis in adolescence. This abnormality was designated the tilt deformity. These different types showed somewhat different radiological features. Each hip, together with a control series of 100 hips, was then subjected to geometrical measurement. To assess acetabular dysplasia, the well recognised C E angle of Wiberg was used, but this was supplemented by a new measurement, the acetabular depth, which is considered to be simpler and more reliable when the joint architecture is disturbed by osteoarthritis. The tilt deformity required a new measurement—the femoral head ratio (FHR). The techniques for these measurements are described. These measurements, compared with the control series, were statistically significant (P<0·001) in the case of each of these anatomical abnormalities. Osteoarthritis arising in the presence of normal anatomy, here described as idiopathic, was found in 35 per cent with a female to male preponderance of approximately 4:1. The average age of onset of symptoms was 57·7 years. Osteoarthritis arising in the presence of abnormal anatomy accounted for the remaining 65 per cent. That associated with acetabular dysplasia represented 25·5 per cent of the whole series and also showed a female to male preponderance of approximately 4:1 with a rather earlier average age of onset of symptoms—50·8 years. This type appears to be a forme fruste of congenital dislocation of the hip. The tilt deformity was found in 39·5 per cent with a marked variation in sex incidence, approximately six males being affected for every female. The age of onset of symptoms in this group was 51·5 years. The balance of evidence suggests that this anatomical variation results from minor and unrecognised trauma in adolescence. It is considered that these minor anatomical variations can be recognised from more critical examination of pelvic radiographs and that their presence should indicate joint incongruity and potential osteoarthritic change. Conversely a normal adult pelvic radiograph makes the subsequent development of osteoarthritis of the hip much less likely. Earlier recognition of degenerative change may permit earlier treatment by osteotomy and arrest of the process of deterioration.