Which deformity is most characteristic of rheumatoid arthritis at the metacarpophalangeal joints?

• A. Boutonniere deformity
• B. Swan neck deformity
• C. Ulnar drift
• D. Z deformity

Answer: (C)

Chronic inflammation from rheumatoid arthritis at the metacarpophalangeal joints causes destruction of the joint capsule and supporting ligaments, leading to imbalance of the extensor and intrinsic muscles and a tendency for the fingers to deviate toward the ulna. This ulnar drift, with subluxation of the MCP joints, is the most characteristic deformity seen at these joints in RA.

Other patterns, like Boutonnière deformity (PIP flexion with DIP extension) and Swan neck (PIP hyperextension with DIP flexion), involve mainly the PIP/DIP joints rather than the MCP. The Z deformity refers to a thumb deformity involving the MCP/CMC joints, not the fingers' MCP alignment.