The thumb interphalangeal joint has radial and ulnar collateral ligaments running from the condyles of the proximal phalanx to the lateral base of the distal phalanx. The IP collateral ligaments are shorter and less robust than the MCP collateral ligaments and are the primary stabilisers against lateral deviation at the IP joint level. They are taut throughout the range of IP joint flexion due to the bicondylar nature of the proximal phalanx head.
Provide lateral stability to the thumb IP joint throughout flexion and extension, resist radial and ulnar deviation forces, and complement the volar plate in preventing hyperextension.
Thumb IP collateral ligament injuries occur from radial or ulnar stress at the joint, most commonly in occupational or sports trauma. Unlike the MCP UCL (gamekeeper injury), IP collateral injuries of the thumb are less common and rarely require surgical intervention. Stress X-rays in flexion assess ligament integrity. Splinting in slight flexion for 3-4 weeks manages most injuries. Avulsion fractures from the collateral origin at the proximal phalanx condyle require reduction and fixation when displaced.
Lateral stress to the thumb at the IP joint level sprains the collateral ligament, producing localised pain and swelling at the IP joint line with stress tenderness, managed by buddy-taping or a thermoplastic splint for 3-4 weeks, with surgical referral only for complete tears with significant instability.
