Scapho-lunate distance and cortical ring sign.

The scapho-lunate distance was measured on posteroanterior radiographs of 100 normal wrists. The scapho-lunate distance measured a mean of 3.7 mm +/- SD 0.6 (range from 2.5 to 5.0 mm). The mean for 44 male wrists was 4.0 mm, and for 56 female wrists it was 3.6 mm. These results indicate that a scapho-lunate distance of up to 5 mm is not necessarily indicative of carpal instability. None of these normal wrists had a scaphoid "cortical ring" sign.