Anatomy of the upper limbChapter 1

A fine network of prominent skin folds known as the papillary ridges. The skin papillary ridges serve two important functions: first a tactile function (sense of touch) and second a mechanical function. A moist palm due to secretion of sweat together with these papillary ridges creates a friction coefficient that prevents slipping of objects during prehension. Tension skin lines, also known as Langer lines. It is postulated that along these lines, skin incisions would result in the least amount of scarring when the skin is closed by primary intention.

Underneath the skin is a layer of a fatty tissue, which is divided into fat lobules by fibrous septa extending from the skin to the deep fascia. This creates areas of prominent fat pads that are most noticeable over the hypothenar muscles, thenar muscles, the metacarpal arch, and over the fingers in between joint creases. The fat cushions/pads protect the hands against external forces and allow the hands to conform nicely around objects of various shapes, thus improving the act of prehension.

Superficial to the TCL passes the palmar cutaneous branch of the median nerve to provide cutaneous innervation to the central area of the palm. During carpal tunnel surgery, the palmar cutaneous branch is rarely seen; however, if encountered it is best to preserve it to avoid the development of a painful neuroma.

The retinacular aspect of the flexor sheath consists of a series of transverse annular and cruciate pulleys (fibrous bands) that overlay the flexor tendons and synovial sheaths, and extend from the palmar surface of the MCP joint to the DIP joint. The dorsal aspect of this tunnel is formed by the deep transverse metacarpal ligament, palmar plates of MCP and interphalangeal (IP) joints, and the palmar surfaces of the proximal and middle phalanx. There are five annular pulleys (A1–A5) and three cruciate pulleys (C1–C3) in the index to small fingers. However, in the thumb there are two annular pulleys (A1 and A2) and one oblique pulley. The main function of these retinacular pulleys is to prevent the forward displacement (bowstringing) of flexor tendons during fingers flexion. Keeping tendons close to the volar surface of the fingers concentrates the force of the flexor tendons during finger flexion, thus optimizing flexion movement and enhancing handgrip. Appreciation of the anatomy and biomechanics of the finger pulley system is important during flexor tendons repair and reconstruction procedures. The pulley system of the fingers and vinacular blood supply of the flexor tendons are depicted in Figure 1.9a and b.

8Upon emergence from the extensor retinaculum, the extensor tendons diverge to their respective fingers. Studies have found several variations in the anatomical arrangement of extensor tendons over the dorsum of the hand. Nevertheless, the most common pattern of extensor tendons on the dorsum of the hand is as follows: a single tendon of the extensor digitorum communis (EDC) to the index and long fingers, a double EDC tendon to the ring finger (a slip of ring finger tendon often powers the little finger), and an absent EDC tendon to the little finger. Additionally, a single extensor indicis proprius (EIP) tendon that lies ulnar and deep to the EDC of the index finger and a double extensor digiti minimi/quinti (EDM/EDQ) tendon that has a double insertion on the small finger. Because the index and the small finger are powered by additional (EIP and EDM) tendons, these two fingers can be extended independently from the other fingers. For example, pointing toward an object using the index finger while the rest of the fingers are in flexion. This posture demonstrates an extension of the index finger by the EIP tendon only. Even so, the EDC is the main extensor of the index to small finger. At a level near the MCP joints, the tendons of the extensor communis are joined by fibrous interconnections called tendon juncturae tendinum (Figure 1.10). 9It is because of these fibrous interconnections the long and especially the ring fingers have limited independent extension. However, the role of these connections becomes evident when an extensor tendon is completely divided proximal to the level of the juncturae. Despite a complete division of a tendon, the neighboring extensor tendon can achieve extension of the MCP joint that corresponds to the divided tendon, by powering the distal end of the severed tendon via the juncturae.

Perhaps the most complicated part of the extensor tendon anatomy is the composition of the extensor mechanism of the finger (Figure 1.12). On the dorsum of the proximal phalanx, the EDC trifurcates into a central tendon and two lateral slips. The central tendon becomes the central slip and inserts onto the base of the middle phalanx, while the two lateral slips join the lateral bands of the intrinsic muscles (interossei and lumbricals) to become the conjoined lateral bands on the radial and ulnar sides of the fingers, respectively. The conjoined lateral bands then unite to form a terminal tendon that inserts on the dorsum of the distal phalanx of each finger. At the MCP joints, the extensor tendons are encircled by the sagittal bands, which centralize the tendons over the dorsum of MCP joints. The sagittal bands arise from the palmar plate and span both sides of the MCP joint to insert with oblique and transverse fibers into the joint’s collateral ligaments and the extensor tendons. When the central extensor tendon contracts, it moves proximally and lifts up the proximal phalanx through the insertion of the sagittal bands onto the palmar plate of the MCP joint (Figure 1.13). Along this extensive network of tendons, there are three ligaments on the dorsum of the fingers that interconnect components of the extensor mechanism, and each serves a specific function:

The lumbricals arise from the FDP tendons in the palm and insert into the radial lateral band of the extensor expansion of the index to small fingers by passing deep to the deep transverse metacarpal ligament. The lumbrical muscles are weak flexors of the MCP joints; however, as the lumbricals arise from the flexor tendons and insert into the extensor tendons, they have a unique function of coordinating the fine flexion extension movement of the fingers.

The delicate balance between the intrinsic and extrinsic muscles of the hand may be disrupted by pathologic conditions leading to the development of various deformities. For example, paralysis of the intrinsic muscles as a result of ulnar nerve palsy may lead to varying degrees of finger clawing, depending on the level of nerve injury (i.e. high vs. low ulnar nerve palsy). Finger clawing (aka. intrinsic minus deformity) is characterized by hyperextension of the MCP joint and flexion of the IP joints. This deformity is a result of weakness of intrinsic muscle function, which subsequently results in an unopposed extension of the MCP joints by the long extensors and flexion of the IP joints by the long flexors. In contrast, intrinsic muscle tightness/contracture, which occurs with conditions such as rheumatoid arthritis, is often associated with intrinsic plus deformity, characterized by flexion of the MCP joints and extension of the IP joints (see chapter on Stiff Digit for more details).

The wrist is fairly a complex geometrical structure that is composed of eight carpal bones articulating among each other and also with the distal radius and ulna proximally and distally with the metacarpal bones. The carpal bones are arranged into two rows: the proximal carpal row that consists of the scaphoid, lunate, triquetrum, and pisiform bones and a distal row that is formed by the trapezium, trapezoid, capitate and hamate bones. Between the two carpal rows is the midcarpal joint that permits flexion and extension movement. The articular surface of the distal radius has two bony depressions: the scaphoid fossa and lunate fossa that accommodate the scaphoid and lunate bones, respectively and are known as the radiocarpal joint. On the ulnar aspect of the distal radius, the sigmoid notch articulates with the distal ulna head to form the DRUJ. The DRUJ permits pronosupination of the forearm. There is a discrepancy between the circumference of the sigmoid notch of the radius and that of the ulnar head. This mismatch results in joint incongruity, subsequently resulting in translation and rotation of the ulna during pronation and supination. The ulna translates dorsally in pronation and volarly in supination. However, the DRUJ is stabilized by several structures often referred to as the static and dynamic constraints. The static constraints include the triangular fibrocartilage complex (TFCC), the dorsal radioulnar ligament and volar radioulnar ligament, the ulnar collateral ligament, and the joint capsule. The dynamic muscle stabilizers include the extensor carpi ulnaris (ECU) tendon and the pronator quadratus (PQ) muscle.

At the ulnocarpal joint, the distal ulna is covered by triangular fibrocartilage, which is together with the ulnocarpal ligaments and the sheath of the ECU, forms theTFCC. The TFCC arises from the lunate and distal end of radius and inserts into the base of the ulnar styloid process. The peripheral part of the TFCC is well vascularized, whereas the center of the TFCC is an avascular load bearing part that articulates with the distal ulna and triquetrum. The intricate assembly of the carpal bones is supported by a number of volar and dorsal ligaments. The ligaments of the wrist are divided into extrinsic and intrinsic ligaments as described by Taleisnik. Extrinsic ligaments connect the distal radius and ulna to the carpal bones, and intrinsic ligaments connect carpal bones together, as they have their origins and insertions within the carpal bones.

On the dorsum of the hand, the posterior interosseous artery and the perforating branches of the anterior interosseous artery (branches of the common interosseous artery that arise from the ulnar artery at the level of the radial tuberosity proximal in the forearm) form the dorsal carpal arch. The 2nd, 3rd, 4th, and 5th dorsal metacarpal arteries branch from the dorsal carpal arch (Figure 1.18) and are the source of the blood supply of many of the local flaps performed on the dorsum of the hand (dorsal metacarpal artery system). The dorsal metacarpal arteries bifurcate to give off the dorsal digital arteries, which communicate with the palmar digital arteries in the fingers.

Distal in the palm, the common digital arteries bifurcate into the proper palmar digital arteries. The proper digital arteries run dorsal to their corresponding radial and ulnar digital nerves, respectively (Figure 1.19) and anastomose by perforating branches with the dorsal digital arteries. The venous drainage of the hand corresponds to these arteries; however, a superficial network of superficial veins is obvious on the dorsum of the hand that confluences to form the cephalic and basilic veins on the radial and ulnar sides of the dorsum of the hand, respectively. Finally, one must realize that there are large anatomical variations in the blood supply and venous drainage of the hand.

The radial nerve arises from the posterior cord (C5-T1) of the brachial plexus. In the arm, the radial nerve runs in the spiral groove of the humerus and supplies the triceps and anconeus muscles. The radial nerve then pierces the lateral intermuscular septum of the arm to run in the anterior compartment of the arm. The radial nerve enters the forearm anterior to the lateral epicondyle of the humerus and divides into two branches: a deep branch called the posterior interosseous nerve (PIN) that is a motor branch and supplies most of the extensors of the forearm below the elbow and a superficial sensory branch. The superficial radial nerve is a sensory branch and provides cutaneous innervation over the anatomical snuffbox and the 1st dorsal webspace of the hand (Figure 1.21a and b).

The median nerve is formed by contributions of the medial and lateral cords of the brachial plexus (C6-T1). The median nerve runs in the anterior compartment of the arm and through the cubital fossa (a triangular space on the volar side of the elbow joint) medial to the brachial artery. The median nerve enters the forearm by passing between the two heads of the pronator teres (PT) muscle (see anatomy of the forearm below). In the forearm, the median nerve gives off two branches: the anterior interosseous nerve (a motor branch) and a sensory palmar cutaneous branch. The median nerve then continues its course and passes through the carpal tunnel to supply the muscles and skin of the hand. The sensory distribution of the median nerve in the hand involves the palmar side of the radial three and a half fingers (thumb, index, long fingers, and radial half of the ring finger) as well as the skin over the dorsum of the distal phalanges, including the nail bed.

The ulnar nerve arises from the medial cord of the brachial plexus (C7-T1). The ulnar nerve initially runs in the anterior compartment before piercing the medial intermuscular septum to enter the posterior compartment of the arm. The ulnar nerve then enters the forearm behind the medial epicondyle by passing through the cubital tunnel (potential site of nerve compression). In the forearm, the ulnar nerve runs underneath the FCU alongside the ulnar artery. The ulnar nerve gives motor innervation to the medial half of the FDP and the FCU muscles. Along its course, the nerve is accompanied by the ulnar artery and both enter the hand by passing through the Guyon canal. Before entering the hand, the ulnar nerve gives off a dorsal and a palmar cutaneous branch that supplies the skin of the ulnar one and a half fingers (small and ulnar half of the ring finger). In the hand, the ulnar nerve supplies the intrinsic muscles of the hand as mentioned previously.

The nerves of the upper limb travel a long distance from the upper arm to the hand, thus they are prone to mechanical compression at several anatomic locations that leads to several clinical sequelae that are discussed in detail in later chapters of this book.

On the radial border of the forearm lies a long muscle known as the brachioradialis muscle, which arises from the lateral supracondylar ridge of the humerus and inserts into the lateral side of the distal radius. The brachioradialis is the main flexor of the elbow joint and overlays the radial side of both the dorsal/extensor and volar/flexor compartments of the forearm. The muscle belly of the brachioradialis is noticeable when the elbow is in flexion against resistance while the forearm is in a neutral position.

On the dorsum of the forearm, we can find that the muscle bellies of the extensor compartment are arranged in layers and parallel to each other, this is also true when we will look later at the muscles of the flexor compartment of the forearm. The muscles of the superficial layer of the extensor forearm arise from a common extensor origin, which is the lateral epicondyle of the humerus. From radial to ulnar, the muscles are arranged as follows: extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), EDC, EDM/EDQ, and the ECU. With the exception of the supinator muscle, which inserts proximally on the dorsum of the forearm, the extensor muscles of the forearm pass underneath the extensor retinaculum as described previously to insert into the hand. A clinical caveat: most of the muscles of the extensor compartment of the forearm are supplied by the PIN except two muscles, the ECRL and brachioradialis, which are supplied by the radial nerve at a level above the elbow joint. Thus, with radial nerve palsy at the forearm, such as in PIN syndrome, the synergistic action of the extensor muscles is lost and wrist extension is achieved mainly by the ECRL, which results in extension and radial deviation of the wrist joint. A clinical distinction between high or low radial nerve palsy can 14be made based on the knowledge of this anatomy. In high radial nerve palsy, there is a wrist and finger drop deformity due to the paralysis of all of the muscles of the extensor compartment, whereas in low radial nerve palsy (i.e. below the elbow joint such as in PIN syndrome) there is only a finger drop deformity as the ECRL muscle is spared.

In the next layer of the extensor compartment of the forearm, the following muscles lie from radial to ulnar; the APL, EPB, EPL, and the EIP. The APL, EPB, and the EPL form the medial and lateral borders of the anatomical snuffbox, respectively, which is located over the radial side of the distal radius and carpal bones. The EPL tendon courses at an acute angle around a bony prominence on the dorsum of distal radius called the Lister tubercle (see surface marking above) to reach the dorsum of the thumb. This sharp angulation around the bony protuberance creates tension on the EPL tendon and may precipitate tendon attrition rupture. The layers of the extensor muscles of the forearm are shown in Figure 1.22.

On the flexor side of the forearm, the muscles are arranged from superficial to deep into three layers, with the FCR, the PL, and the FCU being the most superficial layer of the volar forearm. More proximal on the volar side of the forearm is the PT muscle. The PT, FCR, PL, FCU, and FDS all have a common flexor origin arising from the medial epicondyle of the humerus. The next layer is the FDS muscle belly as well as the FPL muscle. Underneath the FDS muscle lies the median nerve after emerging from between the two heads of the PT muscle. The median nerve (and its motor branch, AIN) supplies all of the muscles of the forearm except the FCU and medial part of the FDP (ring and small fingers) that is innervated by the ulnar nerve. In the distal forearm, the median nerve continues as described previously deep to the PL and ulnar to the tendon of the FCR to enter the carpal tunnel after giving off the palmar cutaneous branch of the palm approximately 5 cm proximal to the distal wrist crease. The ulnar nerve and ulnar artery lie deep and ulnar to the FCU tendon and superficial to the muscle belly of the FDP to enter the hand above the TCL by passing through the Guyon canal. In the deepest layer of the forearm, tendons of the FDP and the PQ muscle are found. The PQ muscle, together with the PT, pronates the forearm (Figure 1.23).