ObjectiveTo investigate the cl inical characteristics, diagnosis, and treatment of metacarpophalangeal (MCP) joint locking with extension lag. MethodsBetween February 2009 and April 2014, 17 patients (17 fingers) with MCP joint locking with extension lag were treated. The patients included 4 males and 13 females, and the average age was 40.7 years (range, 20-72 years). The index finger was locked in 12 cases and the middle finger in 5 cases. All patients could not fully extend the MCP joint at about 30° flexion without flexion limitation of the interphalangeal joint. The range of motion (ROM) of the MCP joint was (41.2±5.1)°. The visual analogue scale (VAS) score was 2.7±0.5. X-ray and CT scanning showed that there was a bony prominence on radial condyle of the metacarpal head in 15 primary patients, and a hook like osteophyte on ulnar condyle in 2 degenerative patients. All patients were treated with close reduction first, and open reduction was conducted when the manipulation failed. ResultsSuccessful close reduction was achieved in 5 cases, and successful open reduction in 8 cases; 4 cases gave up treatment after failure for close reduction. All patients who achieved successful reduction were followed up 2.3 years on average (range, 6 months to 5 years and 2 months). The ROM of the MCP joint was increased to (80.4±6.6)° at last follow-up, showing significant difference when compared with ROM before reduction (t=-19.46, P=0.00). The VAS score decreased to 0.2±0.4 at last follow-up, also showing significant difference when compared with score before reduction (t=13.44, P=0.00). ConclusionAccessory collateral ligament caught at the bony prominence on the radial condyle of the metacarpal head is the most common cause of the MCP joint locking with extension lag. Close reduction is feasible, but recurrence of locking is possible. Surgical treatment is advised in the event of manipulation failure or recurrent locking.