CONTACT AREAS IN THE THUMB CARPOMETACARPAL JOINT

The thumb carpometacarpal joint is a common site of osteoarthritis. It has been hypothesized that peaks of localized stress on the dorsoradi al or volar-ulnar regions, or both, of the articular surfaces of the t rapezium and metacarpal lead to erosion of cartilage and may be respon sible for the progression of the disease. The objective of this study was to determine the contact areas in this joint under the functional position of lateral (key) pinch and in the extremes of range of motion of the joint. These contact areas were assessed relative to the obser ved sites of cartilage thinning. Eight hands from cadavers of women an d five from cadavers of men were tested in vitro with the thumb under a 25 N load in the lateral pinch position, and under small muscle load s (0-5 N) with the thumb in flexion, extension, abduction, adduction, and neutral positions. Contact areas of articular surfaces of the thum b carpometacarpal joint were determined for these positions using a st ereophotogrammetric technique. The lateral pinch position produced con tact areas predominantly on the central, volar, and volar-ulnar region s of the trapezium and the metacarpal. In three specimens, contact are as were distinctly separated between the dorsoradial and volar-ulnar r egions, and in one specimen, from a man, contact occurred exclusively on the dorsoradial region of the trapezium. Using stereophotogrammetry , maps of cartilage thickness also were determined for a subset of nin e specimens. The volar-ulnar, ulnar, and dorsoradial regions of the tr apezium were the most common sites of thin, cartilage, and these may b e sites of cartilage wear. The results of this study indicate that the lateral pinch position produced stresses in the same regions where ca rtilage thinning was observed; this lends support to the hypothesis th at high stresses can lead to osteoarthritis in this joint.