Although non-traumatic entrapment of the posterior interosseous nerve is very uncommon, one cause of this is synovitis of the elbow due to rheumatoid arthritis. Inability to extend the fingers at the metacarpophalangeal joints is the usual presenting feature, and the main differential diagnosis is extensor tendon rupture. Treatment with a corticosteriod injection of the elbow joint has a 43% success rate, whereas early surgical nerve decompression with or without synovectomy results in good or excellent nerve recovery in all cases reported, even in cases of failed joint injection. If treatment is delayed, resulting in chronic denervation, then tendon transfers are an option.