JOINTS OF THE HAND
The carpometacarpal joint of the thumb is the independent joint between the trapezium and the base of the first metacarpal (see Plate 4-3). The articular surfaces are reciprocally concavoconvex, and a loose but strong articular capsule joins the bones. The biaxial nature of this joint provides for flexion and extension and abduction and adduction, and the looseness of its capsule allows opposition of the thumb that involves a small amount of rotary movement (circumduction).
The carpometacarpal joints of the four fingers participate with the intercarpal and intermetacarpal joints in a common synovial cavity. Dorsal and palmar carpometacarpal ligaments run from the carpals of the second row to the various metacarpals. Short interosseous ligaments are usually present between contiguous angles of the capitate and the hamate and the third and fourth metacarpals.
These joints occur between the adjacent sides of the bases of the four metacarpals of the fingers. Here, also, there are dorsal and palmar ligaments, and interosseous ligaments close off the common synovial cavity by connecting the bones just distal to their articular facets. Only slight gliding movements occur between the metacarpals and between them and the carpals to which they are related. However, the articulation between the hamate and the fifth metacarpal allows that bone to flex appreciably during a tight grasp and also to rotate slightly under the traction of the opponens digiti minimi muscle.
The deep transverse metacarpal ligaments are short and connect the palmar surfaces of the heads of the second, third, fourth, and fifth metacarpals. They are continuous with the palmar interosseous fascia and blend with the palmar ligaments of the metacarpophalangeal joints and the fibrous sheaths of the digits. They limit the spread of the metacarpals, and the tendons of the interosseous and lumbrical muscles pass on either side of them.
These joints are condyloid, and both the rounded head of the metacarpal and the oval concavity of the proximal end of the phalanx have unequal curvatures along their transverse and vertical axes. An articular capsule and collateral and palmar ligaments unite the bones. The articular capsule is rather loose. Dorsally, it is reinforced by the expansion of the digital extensor tendon. The palmar ligament is a dense, fibrocartilaginous plate, which, by means of its firm attachment to the proximal palmar edge of the phalanx, extends and deepens the phalangeal articular surface. It is loosely attached to the neck of the metacarpal; in flexion, it passes under the head of the metacarpal and serves as part of the articular contact of the bones. At its sides, the palmar ligament is continuous with the deep transverse metacarpal ligaments and the collateral ligaments. The collateral ligaments are strong, cordlike bands attached proximally to the tubercle and adjacent pit of the head of the metacarpals and distally to the palmar surface of the side of the phalanx. Their fibers spread fanlike to attach to the palmar ligaments. Movements of flexion and extension, abduction and adduction, and circum- duction are permitted at these joints. With extension is associated abduction, as in fanning the fingers; with flexion is associated adduction, as in making a fist. The metacarpophalangeal joint of the thumb is limited in abduction and adduction; its special freedom of motion derives from its carpometacarpal joint.
Structurally similar to the metacarpophalangeal series, the interphalangeal joints have the same loose capsule, palmar and collateral ligaments, and dorsal reinforcement from the extensor expansion. However, owing to the pulley-like form of their articular surfaces, action here is limited to flexion and extension. Flexion is freer than extension and may reach 115 degrees at the proximal interphalangeal joint. Arteries and nerves serving these joints are twigs of adjacent proper digital branches.
MOVEMENT OF THE HAND
The unique bony and articular anatomy of the hand allow for a myriad of movements, and the cumulative movement of each joint in series increases the total active motion (TAM). By convention, movement toward the palm is described as palmar or volar or anterior and movement toward the back of the hand is described as dorsal or posterior. Movement of the hand toward the thumb side of the arm is described as radial or lateral and toward the small finger as ulnar or medial.