Interphalangeal

Interphalangeal refers to joints situated between phalanges, the small bones of the fingers and toes. In the human hand, there are two interphalangeal joints per finger (except the thumb): the proximal interphalangeal joint (PIP) between the proximal and middle phalanges, and the distal interphalangeal joint (DIP) between the middle and distal phalanges. The great toe has a single interphalangeal joint (IP) between its proximal and distal phalanges; digits 2–5 possess both a PIP and a DIP.

Anatomy and structure: Interphalangeal joints are synovial hinge joints that primarily allow flexion and extension. The articular surfaces are typically the head of one phalanx and the base of the adjacent phalanx. A fibrous joint capsule surrounds each joint, reinforced laterally by collateral ligaments. A volar (palmar) plate lies on the palmar aspect to help prevent hyperextension and provide stability. Flexor and extensor tendons crossing the joints contribute to movement, with their sheaths and surrounding structures aiding lubrication and control. The joints receive blood supply from digital arteries and nerves from digital nerves.

Function: The interphalangeal joints enable bending of the fingers and toes, supporting grasp, precision manipulation, and weight-bearing during gait. They possess primarily uniaxial movement, with flexion and extension as the main motions, and only limited rotation or lateral movement.

Clinical relevance: These joints are subject to degenerative and inflammatory conditions, such as osteoarthritis and rheumatoid arthritis. In the fingers, Heberden’s nodes affect DIP joints and Bouchard’s nodes affect PIP joints. Injuries include mallet finger (DIP extensor avulsion) and boutonniere deformity (PIP disruption), among others, which can alter alignment and function of the digits.