The cysts of osteoarthritis of the hip; a radiological and pathological study.

Bone cysts are a well recognised radiological feature of advanced osteoarthritis (Plewes 1940, Brailsford 1952). They are seen frequently in relation to the hipjoint in both acetabulum aild femur, but it is perhaps not always appreciated how numerous and extensive they may be. The present study attempts to correlate the radiological and pathological features of the cysts occurring in the head of the femur, and to elucidate their pathogenesis. A general account of the pathology of osteoarthritis of the femoral head, with a brief discussion of the cysts, was presented by Harrison, Schajowicz and Trueta (1953). The pathology of the c ’sts was described in detail by Landells ( I 953) , who suggested that they were caused by the intrusion of svnovial fluid under pressure. Landells believed that the cancellous trabeculae yield under the pressure transmitted through the fluid, and that the cysts are formed in enlarged marrow spaces. Accordingly, he interpreted the cysts as ‘ ‘ virtually incomplete fractures ‘ ‘ of the trabecular system, and attributed the development of dense bone around them to ‘ ‘ bodily displacement ‘ ‘ of trabeculae by fluid and to new bone formation in response to strain. The hypothesis that subarticular cysts are excavations produced by synovial fluid is not new. It was propounded by Freund (1940) to explain the formation of cysts in rheumatoid arthritis and haemophilic arthrosis. In Freund’s opinion synovial fluid would be forced into the suharticular bone through defects in the surface by the sustained rise in intra-articular pressure resulting from an effusion into the joint. But Cruikshank, Macleod and Shearer (1954) reported the development of cysts in rheumatoid arthritis without clinical evidence of a significant increase of synovial fluid in the affected joints. In our opinion satisfactory proof that the defects in the bone in osteoarthritis are caused by synovial fluid has not been established. The evidence to be presented is believed to prove that such lesions are foci of bone necrosis caused by violent impact between the opposing surfaces of the joint in the absence of the protection of healthy articular cartilage. Such injury is not unlikely, and it would explain the development of “ kissing “ lesions, as described by Plewes (1940), on opposing surfaces of the joint. Our studies lend support to the view that cavities filled with synovial fluid are often, but by no means invariably, found in the bony defects. We differ from Landells, however, in believing that bone destruction precedes the entrance of synovial fluid, and suggest that the cavities filled with fluid are formed in the course of repair as the dead hone is removed. The use of the word ‘ ‘ cyst ‘ ‘ is liable to give rise to confusion because radiologists and pathologists employ it in such different ways. To the radiologist a “ cyst “ is simply a circumscribed area of translucency in hone; to the pathologist the term connotes an encapsulation of gas or fluid in any tissue. Since many, but by no means all, of the “cysts” detected radiologically in osteoarthritis do contain encysted fluid, the two usages of the word