The radial collateral ligament of the thumb MCP joint runs from the radial side of the first metacarpal head to the radial base of the proximal phalanx, with a proper part to the bone and an accessory part to the palmar plate. It stabilises the radial side of the joint against ulnar-directed forces. It is injured far less often than the ulnar collateral ligament but accounts for a meaningful share of thumb MCP collateral ligament tears.
| Origin | Radial condyle of the first metacarpal head |
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| Insertion | Palmar-radial base of the proximal phalanx of the thumb and the palmar plate |
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Resists ulnar deviation and palmar subluxation of the thumb proximal phalanx at the MCP joint, supporting stable pinch and grasp.
RCL injuries follow forced adduction or ulnar deviation of the thumb, often in ball sports or falls. Patients have radial-sided MCP swelling and tenderness, and laxity is tested by ulnar stress at full extension and 30 degrees of flexion. Because the abductor pollicis brevis aponeurosis is broader than the adductor aponeurosis, a Stener-type interposition is rare, so most partial and many complete tears are managed in a cast or splint. Chronic instability with palmar subluxation of the proximal phalanx may need repair or reconstruction.
Sprain or rupture from forced ulnar deviation of the thumb, causing radial-sided MCP pain, swelling and laxity on ulnar stress testing, usually treated with thumb spica immobilisation.
Persistent laxity after an untreated or failed RCL injury, with palmar and ulnar subluxation of the proximal phalanx, weak pinch and pain, sometimes requiring ligament reconstruction.
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