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Other Terms: Knuckles - big, Metacarpal joint, MP joint
The metacarpo-phalangeal articulations are the joints between the heads of the metacarpal bones and the bases of the proximal phalanges. Although the articulating surfaces are irregularly spherical, the movements of the joints are restricted by ligaments. The articular surfaces of the heads of the metacarpal bones become somewhat cylindrical upon the palmar aspect of the bones, so that the surfaces in contact during extension are spherical, while during flexion they are cylindrical. The joints are consequently a mixture of the hinge and condyloid types. The metacarpo-phalangeal joint of the thumb is an exception; it resembles the interphalangeal articulations and is a true hinge joint. The articular capsules are thin and somewhat relaxed, but they are reinforced in several places. The metacarpophalangeal joint of the thumb usually contains two sesamoid bones, which are embedded in the articular capsule. Sesamoid bones occasionally occur in the metacarpophalangeal joints of the other fingers. The movements of the metacarpo-phalangeal joints of the four fingers consist of anterior-posterior hinge movements by which the phalanges are flexed and extended and abduction and adduction movements as the joints slide over their rounded profile side-to-side. The metacarpo-phalangeal articulation of the thumb is a pure synovial hinge joint.