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The interphalangeal joint of the thumb is formed by the articulation of the head of the proximal phalanx and the base of the distal phalanx of the thumb. There are fourteen phalanges on each hand with each finger possessing a proximal phalanx, a middle phalanx and a distal phalanx, with the exception of the thumb which only has a proximal phalanx and a distal phalanx. Because the thumb only has two phalangeal bones, it only forms one interphalangeal joint, whereas the rest of the fingers form a proximal interphalangeal joint (PIP) and a distal interphalangeal joint (DIP) between the proximal phalanx and the middle phalanx and between the middle phalanx and the distal phalanx respectively.
The interphalangeal joint of the thumb, like the interphalangeal joints of the rest of the fingers, is a hinge joint which means it does not permit lateral movement. But rather, it is a uniaxial joint which allows for a range of motion in only one plane. Movements that occur at the interphalangeal joint of the thumb are flexion and extension.
A fibrous joint capsule at the interphalangeal joint adds to the stability of this joint. The joint capsule is part of a protective envelope that not only protects the joint, but also restricts unwanted movement. Other components of this envelope include the volar plate, also known as the palmar ligaments, and the collateral ligaments.
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