Posterior interosseous nerve entrapment in rheumatoid arthritis.

Posterior interosseous nerve (PIN) entrapment is a rare complication of rheumatoid arthritis (RA) which, together with extensor tendon rupture and metacarpophangeal joint dislocation, should be considered in the differential diagnosis of inability to extend the fingers. The inability to extend the thumb in PIN entrapment is a useful distinguishing clue on physical examination, and nerve conduction studies confirm the diagnosis. Elbow joint swelling and compression of the PIN at the arcade of Frohse are the main reasons for PIN entrapment in RA. Intraarticular steroid injections and surgical intervention resolve symptoms of PIN entrapment in RA. In our case, the addition of methotrexate, which induces a rapid antiinflammatory effect, resulted in resolution of weakness with complete recovery in the extensor muscles of the fingers.