Role of metacarpophalangeal joint anatomic factors in the distribution of synovitis and bone erosion in early rheumatoid arthritis.

OBJECTIVE: To investigate the role of metacarpophalangeal (MCP) joint anatomic and biomechanical factors in the distribution of synovitis and bone erosion in early rheumatoid arthritis (RA).
METHODS: Thirty-three patients with early RA with clinically diagnosed MCP joint disease and 28 healthy controls were examined by magnetic resonance imaging of the second to fifth MCP joints of the dominant hand. T1 and T2 fat-suppressed coronal sequences were obtained to assess erosion, and dynamic contrast-enhanced images were acquired to assess synovitis in all of the RA patients and in 8 of the controls. Erosions were defined as bone defects with sharp margins observed using T1-weighted imaging in 2 planes, with a cortical break seen in at least 1 plane. The location of erosions was recorded. The volume of synovitis surrounding each MCP joint (divided into 8 regions) was calculated by summation of voxels derived from the maximal enhancement parameters. The synovial volumes adjacent to MCP joint collateral ligaments were determined by correcting synovial volumes for the positions of asymmetrically placed flexor tendons.
RESULTS: In patients with early RA in whom bone erosions were present, there was a propensity for involvement of the radial side of the second (P < 0.0001), third (P = 0.002), and fourth (P = 0.056) MCP joints, but not the fifth. Fifty-two of the 110 erosions (47.3%) occurred adjacent to the radial collateral ligaments of the second, third, and fourth MCP joints. The volume of synovitis was also greater on the radial side of the second (P < 0.0001) and third (P < 0.001) MCP joints. A predilection for synovitis in all of the MCP joints adjacent to the radial collateral ligaments was evident when the positional effects of the flexor tendon were considered. The position of radial collateral ligaments had an effect on erosion formation that was independent of synovitis. A predilection for radial bone damage was also evident in the controls, although lesions were 5-fold less frequent, were generally smaller, and had well-defined margins.
CONCLUSION: This study shows that there is a predilection for both synovitis and bone erosion formation on the radial side of the MCP joints in early RA, and that joint inflammation appears to drive the inherent tendency for bone damage on the radial side of joints. These findings have implications regarding the pathogenesis of joint damage in RA.