An interphalangeal joint is any of the joints between the phalangeal bones, which are the bones in the fingers and toes. A hinge joint capable of flexing and extending — or bending and straightening — the digits, this articulation is a type of synovial or movable joint. There are two in each of the four fingers and toes, and only one apiece in the thumb and big toe. The interphalangeal joint nearest the hand and foot is known as the proximal, or near, joint, and the one nearest the tips of the fingers and toes is known as the distal, or far, joint.
As a hinge or ginglymoid joint, an interphalangeal joint makes up an articular capsule inside of which is a fluid-containing cavity to lubricate the joint, cartilage to cushion against bone-on-bone rubbing, and a synovial membrane to protect the contents of the joint. Outside of the articular capsule are ligaments connecting the adjacent phalanx bones to each other. To either side of the joint are ligaments known as collateral ligaments, which run parallel to the digit, provide lateral stability to the joint, and hold the bones in place.
On the joint’s palmar surface in the hand and plantar surface in the foot is what is referred to as the volar ligament, which is a thin, flat ligament that prevents the finger or toe from hyperextending. Likewise, the tendon of the muscle in the hand or foot that flexes the finger or toe is found on this side. Crossing the dorsal or back surface of the digit is the extensor tendon, which is the tendon of the muscle in the hand or foot that straightens the digit.
In the hand, the proximal interphalangeal joint is identifiable almost midway down each of the four fingers where the large knuckle is found, whereas the distal joint is the smaller one an inch or so below the fingertips. The proximal joint exhibits a larger range of flexion and extension than the distal joint, with about 100 degrees of movement possible in the proximal joint and 80 degrees of movement possible in the distal joint. Flexion of both joints in the fingers is initiated by two muscles of the anterior forearm, the flexor digitorum profundus and flexor digitorum superficialis. In the thumb, the flexor pollicis longus flexes the single interphalangeal joint. Extension of these joints is initiated by the extensor digitorum muscle in the posterior forearm and, in the case of the thumb, the extensor pollicis longus.
Because the interphalangeal joints in the foot are analogous to those in the hand, there is also a proximal and distal interphalangeal joint in each of the smaller toes and a single interphalangeal joint in the big toe. These are situated much closer together than those in the hand and exhibit a smaller range of motion, with the proximal joint similarly capable of a greater degree of flexion and extension than the distal joint. Flexion of these joints in the four smaller toes is activated by the flexor digitorum longus muscle in the posterior lower leg, with the flexor hallucis longus curling the big toe. Extension of the interphalangeal joints in the toes is activated by the extensor digitorum longus in the anterior lower leg, with the extensor hallucis longus straightening the big toe.