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Interphalangeal Joint (Thumb)

articulatio interphalangea pollicis

The thumb IP joint is a simple hinge joint with a single degree of freedom allowing the thumb tip to flex and extend for pinching activities. Flexor pollicis longus inserts on the palmar base and extensor pollicis longus on the dorsal base of the distal phalanx. Its collateral ligaments and volar plate provide side-to-side and hyperextension stability. Gamekeeper thumb involves the MCP UCL rather than the IP joint.

Anatomical Data

Structure

Bones	Head of the thumb proximal phalanx Base of the thumb distal phalanx
Ligaments	Radial and ulnar collateral ligaments Volar plate

Biomechanics

Function & Actions

Movements	Flexion 80 degrees Hyperextension variable (0 to 30 degrees in hypermobile individuals)

Clinical Relevance

Clinical Notes

Interphalangeal joint dislocation of the thumb, usually dorsal from hyperextension, is reduced by traction and gentle flexion, followed by splinting that blocks extension while the volar plate heals. Seymour-type injuries in children involve the distal phalanx physis with an open nail bed injury and must be treated as open fractures requiring irrigation and fixation. IP joint arthritis, common in rheumatoid arthritis, is managed by splinting and injection, and arthrodesis in slight flexion gives durable pain relief and stable pinch.

Pathology

Common Injuries & Conditions

Thumb IP Dislocation

Dorsal IP dislocation from hyperextension requiring closed reduction and splinting for 2 to 3 weeks.

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