What is the distal interphalangeal joint made up of?

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The distal interphalangeal joint (the coffin joint) is a three-bone arrangement in the horse. The joint’s surfaces involve the distal articular area of the middle phalanx and the proximal articular area of the distal phalanx, but the distal sesamoid bone (navicular) sits within the joint region and articulates with the distal phalanx. This navicular bone acts as a pulley for the deep digital flexor tendon and helps distribute load across the joint, so it is considered part of the joint complex. For that reason, the distal interphalangeal joint is made up of the distal phalanx, the middle phalanx, and the distal sesamoid.