Chapter 9: The Wrist and Hand Complex

INTRODUCTION

The human hand may well surpass all body parts but the brain as a topic of universal interest. The human hand has been characterized as a symbol of power,¹ an extension of intellect,² and the seat of the will.³ The symbiotic relation of the mind and hand is exemplified by sociologists' claim that while the brain is responsible for the design of civilization, the hand is responsible for its formation. The hand cannot function without the brain to control it; likewise, the encapsulated brain needs the hand as a tool of expression. The entire upper limb is subservient to the hand. Any loss of function in the upper limb, regardless of the segment, ultimately translates into diminished function of its most distal joints. The significance of this potential loss has led to the detailed study of the finely balanced intricacies of the normal upper limb and hand.

THE WRIST COMPLEX

The wrist (carpus) consists of two compound joints: the radiocarpal and the midcarpal joints, referred to collectively as the wrist complex (Fig. 9–1A, B). Each joint proximal to the wrist complex serves to broaden the placement of the hand in space and to increase the degrees of freedom available to the hand. The shoulder serves as a dynamic base of support; the elbow allows the hand to approach or extend away from the body; and the forearm adjusts the approach of the hand to an object. The carpus, unlike the more proximal joints, serves placement of the hand in space to only a minor degree. The major contribution of the wrist complex is to control length-tension relationships in the multiarticular hand muscles and to allow fine adjustment of grip.^4,5 The wrist muscles appear to be designed for balance and control rather than for maximizing torque production.⁶ The adjustments in the length-tension relationship of the extrinsic hand muscles that occur at the wrist cannot be replaced by compensatory movements of the shoulder, elbow, or forearm (radioulnar joint). The wrist has been called the most complex joint of the body, from both an anatomical and physiological perspective.⁷ The intricacy and variability of the interarticular and intra-articular relationships within the wrist complex are such that the wrist has received a large amount of attention with agreement on relatively few points. Two points on which there appears to be consensus are (1) that the structure and biomechanics of the wrist, as well as of the hand, vary tremendously from person to person; and (2) that even subtle variations can produce differences in the way a given function occurs. The intent of this chapter, therefore, is less to provide details on what is "normal" and more to describe the wrist complex and hand in such a way that general structure is clear and a conceptual framework is developed within which normal function and pathology can be understood.