The bone cysts of osteoarthritis.

Rounded areas of translucency in the bones forming an osteoarthritic joint are well known to radiologists and to orthopaedic surgeons. They are not given much prominence in pathological accounts : their formation is vaguely ascribed to degeneration in the bone in keeping with the degeneration in the neighbouring cartilage. Their frequency was given by Plewes (1940), from clinical and radiological studies, as thirty-two in 242 cases ; he also drew attention to their situation in areas of greatest bone sclerosis and greatest loss of ‘ ‘ joint space,” and to their frequently paired or “ kissing “ position on either side of the joint line. Collins (1949) described them as incidents in the remodelling of the trabeculae-” a concentric arrangement of healthy trabeculae around an enlarged marrow space filled with ordinary adipose tissue.” The presence of these cysts is often indicated clinically by severe pain in the joint, and in the cases selected for surgical treatment of osteoarthritis the cysts are neither so rare nor so incidental as has been suggested. They have been present in eleven of the last nineteen heads of femur and in one of the last three patellae excised for osteoarthritis and examined in this department. I have found other examples in an osteoarthritic pisiform bone, and in a fractured carpal scaphoid excised ten years after the injury ; and in the course of routine post-mortem examinations of patellae they were found in three of ten consecutive bones, and in one of the only two severely arthritic heads of femur in a series of a hundred ; another was an incidental finding on microscopy of the acromio-clavicular joint of a patient with ankylosing spondylitis. The cysts appear to the naked eye as grey gelatinous rounded spaces from a millimetre up to 2#{149}5 centimetres in diameter; the large cysts, over 1 centimetre across, are found only in the femoral head. The cysts are usually multiple-ten were found in one patella-and surrounded by a zone of dense bone up to half a centimetre thick ; some are multilocular. A good example of a large cyst is shown in Figure 1 , where the greater part of the femoral head is seen to be involved. The smaller cysts (Figs. 3 and 5 to 7) always lie in the bone immediately adjacent to the joint space, and are often pyriform, the pointed end towards the joint. There is always one place where the cyst comes close to the joint cavity, and in a number of specimens it is possible to demonstrate an opening between the two, usually in the form of a linear fissure. It is the purpose of this paper to suggest that these cysts always take origin in such communications and are not due to primary degenerative changes in the bone. PATHOLOGICAL OBSERVATIONS