The joint space in normal hip radiographs.

The hip joint space in standard anteroposterior images of 120 normal subjects was measured with the aid of a special transparent pattern. The normal value was slightly over 4mm, scattered between 3 and 5 mm, except for elderly men in whom values of 6 mm or more may be normal. The lower borderline value does not change with age, nor is there any change in the average joint space with age except in men in whom it increases slightly. A narrow joint space in an elderly patient should not be expected simply because the patient is old. The measuring technique is useful in population studies. In individual cases a simple measurement with a ruler serves the same purpose if the outline of the acetabular roof is well-defined on the images.