Fibrous structure and connection surrounding the metacarpophalangeal joint.

The fibrous components of the metacarpophalangeal (MP) joint including the palmar plate, the collateral ligament and the dorsal plate were studied with particular attention paid to the fibrous structure of the fibrous tendon sheath and the deep transverse metacarpal ligament. The tough fibrillar structure around the MP joint, especially the force nucleus, consisted of three types of mixed fibers: the fibrous tendon sheath of the A1 pulley, the deep transverse metacarpal ligament, and the palmar plate. The tendon sheath was located on the ulnar side in the index and middle fingers, on the central position in the ring finger, and on the radial side in the little finger. These fibrous connections among the fingers formed a transverse arch in the hand. The palmar plate of the MP joint was relatively rigid and appears to function as a cushion when flexed. A fold-like protrusion of the synovial layer of the palmar plate of the MP joint had a meniscoid function, which was larger than that of the proximal interphalangeal joint. The capsule of the MP joint was thicker at the dorsal area, forming a dorsal plate, which is a sliding floor of the extensor mechanism and has a meniscoid function for joint congruity. The main lateral stabilizer consisted of collateral ligaments and accessory collateral ligaments anchored to the palmar plate. These structures act together as a "phalangeal cuff", connecting the proximal phalanx to the metacarpal head and stabilizing the MP joint.