Joint space in radiologically normal knees.

Joint space loss is a characteristic feature of rheumatoid arthritis and osteoarthritis. It cannot be fully evaluated, however, without knowledge of the normal variability of joint space size. We have measured joint space size using digital image analysis in a population of radiologically normal individuals. Anteroposterior films of the knee were studied from 685 consecutive patients attending Casualty with unexplained knee pain or following trauma, but with no clinical or radiological evidence of arthritis. Results show that in a radiologically normal population, men have larger joint spaces than women and there is a steady decline in joint space size with age. We found no significant difference in joint space size between weight bearing and non-weight bearing women. There was also no difference in patients presenting with pain and those presenting following an injury. Normal joint space size was not related to height, weight or body mass index in a subgroup of 213 patients. We suggest that patients lose joint space with increasing age and eventually reach a 'pain threshold' at which symptoms of osteoarthritis occur. This explains the increase in joint symptoms in those who begin with smaller joint spaces; that is in women and in the elderly.