BONE MORPHOLOGY OF THE CARPOMETACARPAL JOINT: A QUANTITATIVE COMPARISON OF OSTEOARTHRITIC AND NORMAL PATIENTS

Introduction The first carpometacarpal (CMC) joint is the second most common joint of the hand affected by degenerative osteoarthritis (OA)1. Laxity of ligamentous stabilizers that attach the first metacarpal bone (MC1) and the trapezium bone (TZ), notably the volar anterior oblique ligament (AOL), has been associated with cartilage wear, joint space narrowing, osteophyte formation, and dorsal-radial CMC subluxation2. In addition, the proximal-volar end of the MC1 has a bony prominence known as the palmar lip (PL) that adds conformity to this double-saddle joint, and is thought to be a supplemental dorsal stabilizer. Currently, no study has looked at the changes to the 3D shape and relative positions of these structures with OA.Methods CT scans of patients with clinically diagnosed CMC OA (n=11, mean age 73 [60–97], 8 females) and CT scans of ‘normal’ patients with no documented history of CMC OA (n=11, mean age 37 [20–51], 6 females) were obtained with the hand in a prone position. 3D reconstructions of the MC1 and TZ bones were created, and each assigned a coordinate system3. The long axis of the MC1 and the proximal-distal axis of the TZ were established, and the location where they intersected the CMC articular surface was defined as their articular center points, X and O, respectively (Figure 1).Using the TZ as a fixed reference, we calculated the relative position of X in the dorsal-ventral and radial-ulnar directions. A two sample t-test was performed to compare the normal and OA groups. In addition, the distal position of the PL relative to X was recorded.Results The dorsal position of the MC1 relative to the TZ was significantly greater (p=0.002) in the OA group compared with the normal group, with mean dorsal positions of 7.1 and 3.2mm, respectively (Figure 2).The distal position of the PL relative to X was also significantly greater (p=0.001) in the OA group when compared with the normal group, with mean positions of 5.8 and 1.9mm, respectively (Figure 3).Discussion Dorsal migration of the MC1 in the OA group would suggest a compromised AOL, known to be elongated or absent intraoperatively. Without a sufficient AOL, the PL was positioned more distally in the OA group, as the load on the PL during extension activities could possibly exceed cartilage strength resulting in subchondral bone remodeling and further joint degeneration. We did not observe radial migration of the MC1 bone possibly due to the presence of bony osteophytes that can reduce abduction-adduction function in OA patients4. The relationship discovered between OA and changes to bone morphology and relative bone positions of the CMC joint may provide further insight into the natural progression of this disease.