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Copyright of Annals of the Rheumatic Diseases is the property of BMJ Publishing Group and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Annals of the Rheumatic Diseases is the property of BMJ Publishing Group and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

HL-A antigens were determined in Haida and Bella Coola native Indians, two communities known to have a high prevalence of ankylosing spondylitis. Tests were conducted on those with x-ray evidence of sacro-iliitis and on a sample of the community at large. Sacro-iliitis was found to prevail in approximately 10 per cent of adult Haida males and in over two per cent of Bella Coola adult males. Of 20 Haidas with sacro-iliitis, 17 were HL-A 27 positive. Fifty per cent of the Haida community at large were HL-A 27 positive. Three Bella Coolas known to have sacro-iliitis were all HL-A 27 positive, while 25 per cent of the community sampled at large were HL-A 27 positive. About one in five adult Haida males who were HL-A 27 positive showed evidence of sacro-iliitis, a proportion close to that ascertained in Caucasian communities. It would appear, therefore that the risk of disease in HL-A 27 positive Bella Coola males is considerably lower.

Although congenital subluxation of the hip is an important primary abnormality, upon which OA hip may later supervene, other factors may also be important in individual patients. When osteoarthritis is seen with subluxation, it probably represents congenital subluxation and not displacement of the femoral head secondary to remodelling. Osteoarthritis in these cases is usually superolateral or supermedial in type, with the major impact of the disease in the superior articular cartilage. A similar form of osteoarthritis is seen in cases where leg-length disparity exists, with or without the presence of congenital subluxation, the wear occurring on the side of the long leg. It has been suggested that bio-mechanical considerations support the contention that stress is likely to be increased in the superior area of the joint on the side of a long leg, and wear can be expected to occur here. If either anomaly may provoke the later development of OA hip, it remains to be shown which is more important when both are present in the same patient.

Present knowledge of hip biomechanics supports the contention that the stresses imposed on the hip on the side of the longer leg are greater than normal; those on the short side are comparably reduced. Indirect measurements by various authors have demonstrated greater stress on the hip if the pelvis is adducted, a persistent and chronic condition of the hip joint on the side of a long leg. Furthermore the pressure on the acetabulum will be displaced laterally in these circumstances. The consistent pattern of degeneration in unilateral superolateral O A hip is what would be expected if the consequences of leg-length disparity were as described. Leg-length inequality may be a major contributing factor in the development of unilateral degenerative disease of the hip of this type.A clinical method of estimating leg-length disparity is described. Clinical findings in patients support the hypothesis that hip stress is increased on the side of a longer leg.

A definition and classification of primary OA hip is presented based on radiological criteria. Most cases of unilateral idiopathic OA hip remain unilateral over the years and the majority of these show a distinct radiological pattern with articular cartilage loss in the superolateral area of the joint. In contrast, OA hip with loss of cartilage in the medial area is usually bilateral. A supermedial form appears to be an intermediate type, but is less well defined.It is suggested that superolateral and medial forms of OA hip are fundamentally different in nature. Leg-length disparity, to be discussed in a subsequent section, is related to the superolateral type.

An x-ray method is described for the determination of leg-length inequality in the standing subject, Its precision is compared to that of other methods in current use. The geometrical considerations which are inherent in any radiographic method of leg-length determination are summarized. Sixty-seven patients with OA hip were studied; 62 were considered to be idiopathic and 36 of these idiopathic cases were superolateral in type. Measurements in these 36 patients showed significant leg-length disparity, most of the diseased hips being found on the side of the longer leg. If allowance is made for shortening due to disease, the frequency and bias of the disparity are highly significant. The strong association between OA hip of this type and a long leg suggests a causal relationship.