Essential anatomy of the upper extremity

The surgeon planning reconstructive surgery on the upper extremity must be aware not only of the complex anatomy of the hand and arm but also of the physiologic interplay of balanced muscular functions under the influence of complex central nervous coordination, as well as the maintenance of physiologic viability by the central and peripheral circulatory and lymphatic systems.

A thorough understanding of the fundamentals of hand and upper extremity anatomy is paramount.

Dorsal skin of the hand is thin and pliable and anchored to the deep investing fascia only by loose areolar tissue.

Palmar skin has a thick dermal layer and a heavily cornified epithelial surface that is held tightly to the thick fibrous palmar fascia by diffusely distributed vertical fibers.

The skin of the palm is laden with a high concentration of specialized sensory end organs and sweat glands.

Palmar skin creases can be used to identify and locate underlying joints and structures to help plan precise skin incision placement.

Kaplan's cardinal line is an important landmark for critical internal structures within the hand ( Fig. 22.1 ; Box 22.1 ).

To avoid contracture in the palm, Littler outlined imaginary diamond-shaped skin surfaces where a longitudinal scar should be avoided. These diamond surfaces can be visualized by noting each joint axis and the kissing surfaces of the palmar skin in full flexion ( Fig. 22.2A,B ) .

The palmar fascia consists of resistant fibrous tissue arranged in longitudinal, transverse, oblique, and vertical fibers ( Fig. 22.3 ) .

Longitudinal fibers:

• Concentrate at the proximal origin of the palmar fascia at the wrist.

• Originate from the palmaris longus when present (80–85% of patients).

• Make up the fibrous flexor sheath of the digits.

• Attach to the volar plate and intermetacarpal ligaments at the level of the metacarpal heads.

Transverse fibers:

• Concentrated in the midpalm and the web spaces.

• Make up the transverse palmar ligament.

• Act as pulleys for the flexor tendons proximal to the level of the digital pulleys.

Vertical fibers:

• Superficial to the longitudinal and transverse fibers, vertical fibers travel to the palm skin dermis ( Fig. 22.4 ) .

  

• Deep to the palmar fascia, they coalesce into septae and form eight individual compartments for the flexor tendons to each digit and the neurovascular bundles together with the lumbrical muscles ( Fig. 22.5 ) .

  

In the fingers, there are two important bands of fascia, which help contain and protect the ulnar and radial digital arteries and nerves: Grayson ligaments and Cleland ligaments.

• Grayson ligaments are volar to the neurovascular bundles and are quite flimsy.

• Cleland ligaments are dorsal to the neurovascular bundles and are much stouter ( Fig. 22.6 ) .

  

The hand skeleton is divisible into four elements:

• i.

  The fixed unit of the hand: the second and third metacarpals and the distal carpal row.

• ii.

  The thumb and its metacarpal: displays a wide range of motion at the carpometacarpal (CMC) joint allowed by the joint, ligaments, and insertion of five intrinsic and four extrinsic muscles.

• iii.

  The index digit: independence of action within the range of motion allowed by its joints, ligaments, and the action of three intrinsic and four extrinsic muscles.

• iv.

  The third, fourth, and fifth digits with the fourth and fifth metacarpals: function as a stabilizing vise to grasp objects for manipulation by the thumb and index finger, or in concert with the other hand units in powerful grasp ( Fig. 22.7 ) .

  

The distal row of carpal bones forms a solid architectural arch with the capitate bone as a keystone. Together with the second and third metacarpals, they form the fixed unit of the hand.

As a stable foundation, this unit creates a supporting base for the three other mobile units: the first metacarpal, the fourth metacarpal, and the fifth metacarpal.

The first metacarpal moves through a wide range of motion as a result of loose capsular ligaments and the shallow saddle articulation between it and the trapezium.

Motion is stabilized by the capsular ligaments, including the volar beak ligament, and by its attachment to the fixed hand axis through the adductor pollicis, the first dorsal interosseous, and the fascia and skin of the first web space.

The mobile fourth and fifth metacarpal heads move dorsally and palmarly in relation to the central hand axis by limited mobility at their CMC joints.

These metacarpal heads are tethered to the central metacarpals by the intermetacarpal ligaments [unite adjacent metacarpophalangeal (MCP) volar plates].

The third MCP joint acts as the anatomic center of the hand. With the fingers fully abducted, the tips form radii of equal length from this point. The same radius projected proximally falls at the wrist joint ( Fig. 22.8 ) .

The most important single motor operating the central hand beam at the wrist level is the extensor carpi radialis brevis (ECRB), which works against gravity, positioning the pronated hand into extension.

The wrist joint has a multi-articulated architecture that creates a potentially wide range of motion in flexion, extension, radial deviation, ulnar deviation, and circumduction ( Fig. 22.9 ) .

The distal radioulnar joint (DRUJ) allows pronation and supination of the hand as the radius rotates around the head of the ulna ( Fig. 22.10 ) .

The proximal row of carpal bones (scaphoid, lunate, triquetrum, pisiform) articulates with the distal radius and ulna, providing the ability to flex and extend the hand and perform radial and ulnar deviation.

All four of the bones in the distal carpal row present articular surfaces for junction with the metacarpals.

The principal articulation of the carpus is with the distal surface of the radius at the radiocarpal joint.

With an articular surface that slopes in several planes, fractures of the distal radius frequently result in a loss of the normal dorsal-to-palmar tilt of the articular surface, leading to a change in the biomechanical properties of the wrist joint and degenerative arthritis.

The relationship of the length of the radius to the length of the ulna is fairly constant in individuals and is termed ulnar variance.

Normal ulnar variance: the distal ulna completes the curve of the articular surface of the radius.

Ulnar negative variance: the end of the ulna falls short of this curvature.

• Higher incidence of Kienbock disease (avascular necrosis of the lunate).

Ulnar positive variance: the ulna extends distal to this imaginary extension.

• If variance greater than 2–3 mm, associated with ulnar impaction ( Fig. 22.11 ) .

  

Gilula lines: denote normal extracarpal and intracarpal architecture. Any disruption of these lines is a sign of carpal abnormality.

Greater arc: a line that follows the proximal articular contours of the proximal row of carpal bones circumscribing a smooth arc ( Fig. 22.12 ) .

Lesser arc: a line between the proximal and distal row of carpal bones circumscribing another smooth arc.

The scaphoid and lunate form the primary articulation with the distal radius.

The scaphoid also articulates with the distal carpal row through attachments to the trapezium and trapezoid.

The triquetrum articulates with the lunate in the proximal row and with the hamate across the midcarpal joint.

The pisiform is essentially a floating bone, unimportant for carpal stability.