A patient with rheumatoid arthritis develops ulnar drift and volar subluxation of the metacarpophalangeal joints of the fingers. The underlying pathomechanism is:

• A Extensor tendon central slip rupture
• B Lumbrical muscle fibrosis
• C Flexor tendon adhesions in Zone I
• D Synovitis causing radial collateral ligament laxity with unopposed ulnar pull of intrinsics ✓

Explanation

In rheumatoid arthritis, synovitis of the MCP joints causes progressive stretching and attenuation of the radial collateral ligaments and volar plate. The intrinsic muscles (interossei and lumbricals), which insert ulnarly on the extensor mechanism, exert an unbalanced ulnar deviating and volar subluxing force on the proximal phalanges. The extensor tendons are displaced ulnarly over the metacarpal heads, further worsening the deformity. Central slip rupture causes boutonniere deformity, not ulnar drift.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.