200 PART III THE UPPER EXTREMITY


bases of the proximal phalanges are biconcave to form articulations with the metacarpals.


Hand Arches


Extensor carpi ulnaris


Extensor carpi radialis longus


Extensor carpi radialis brevis


Extensor digitorum


Extensor digiti minimi


Extensor retinaculum


Extensor indicis


Extensor pollicis brevis (cut)


Extensor pollicis longus


The carpals and metacarpals of the hand are arranged into one longitudinal and two transverse arches. The proximal transverse arch is a relatively rigid arch formed by the distal carpus with the capitate as the keystone of the arch. The distal transverse arch passes through all the metacarpal heads with the third metacarpal as its key- stone. This arch is more mobile than the proximal arch. The longitudinal arch of the hand is formed by the long axis of the four digits and the proximal carpus, with the second and third metacarpals being the central pillars of the arch (Fig. 10.12). The mobility of the thumb and the fourth and fi fth digits around the second and third digits allow the hand to cup an object or the palm to fl atten out. The intrinsic muscles of the hand support these hand arches. Paralysis of these muscles, bone injury, or joint deformity secondary to disease can lead to collapse of these arches and severe disability of hand function.


JOINT STRUCTURES


The hand joints are designed for functional use in various degrees of fl exion. The hand functions in fl exion as it holds large objects in the palm or small items between the fi ngers. The multiple joints in the hand complex allow for these complex actions.


Carpometacarpal Joints


Figure 10.9 Posterior view of the primary wrist extensors: extensor carpi radialis longus, extensor carpi radialis brevis, and extensor carpi ulnaris. Secondary wrist extensors that act on the digits are also shown.


BONY STRUCTURES


The hand is made up of the bones distal to the carpals of the wrist and includes the fi ve metacarpals and the 14 phalanges of the digits or fi ngers of the hand (Fig. 10.11). The thumb is considered the fi rst digit, the index fi nger is the second digit, the middle fi nger is the third digit, the ring fi nger is the fourth digit, and the “pinky fi nger” is the fi fth digit. The fi fth digit is the smallest in the hand. The metacarpals articulate proximally with one or more carpal bones and with each adjacent metacarpal. The shaft of each metacarpal is slightly concave on the palmar surface and extends distally to form the head of each metacarpal.


The thumb is composed of two phalanges, the proxi- mal and the distal. Digits 2 through 5 have three phalan- ges each, the proximal, middle, and distal phalanges. The


The bases of the second through fi fth metacarpals articu- late with the distal row of carpals to form the carpo- metacarpal (CMC) joints. The third CMC joint is the most stable of these joints and forms the central pillar of the hand. The CMC joints on either side are more mobile and allow the hand to fold around the third CMC joint. This mobility gives the palmar surface of the hand its concave shape to fi t around cylindrical objects and enhances the hand’s ability to grasp objects. Each of the synovial CMC joints has articular joint capsules that are reinforced by several small dorsal and palmar CMC and intermetacarpal ligaments (Fig. 10.13).


The CMC of the thumb is formed by the articula- tion between the trapezium and the fi rst metacarpal (Fig. 10.14). The joint is classifi ed as a saddle joint, as each of the two joint surfaces is convex in one direction and concave in the other. This complex design allows the thumb to have 2° of freedom: fl exion and extension and abduction and adduction. The joint also permits some axial rotation. A combination of movements allows the thumb to oppose the other fi ngers, enhancing hand dex- terity. The loose capsule of the thumb CMC joint accom- modates the movement of the joint and is supported by collateral and oblique ligaments.


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