Week 3 Readings PDF

divided into anterior and posterior groups, separated by the humerus and medial and lateral intermuscular septae (Fig. 3.47). The chief action of both groups is at the elbow joint, but some muscles also act at the glenohumeral joint. The superior part of the humerus provides attachments for tendons...

divided into anterior and posterior groups, separated by the humerus and medial and lateral intermuscular septae (Fig. 3.47). The chief action of both groups is at the elbow joint, but some muscles also act at the glenohumeral joint. The superior part of the humerus provides attachments for tendons of the shoulder muscles. FIGURE 3.47 Muscles, neurovascular structures, and compartments of arm. A. In this dissection of the right arm, the veins have been removed, except for the proximal part of the 524 axillary vein. Note the courses of the musculocutaneous, median, and ulnar nerves and the brachial artery along the medial (protected) aspect of the arm. Their courses generally parallel the medial intermuscular septum that separates the anterior and posterior compartments in the distal two thirds of the arm. B. In this transverse section of the right arm, the three heads of the triceps and the radial nerve and its companion vessels (in contact with the humerus) lie in the posterior compartment. C. This transverse MRI demonstrates the features shown in part B; the numbered structures are identified in part B. (Courtesy of Dr. W. Kucharczyk, Professor and Neuroradiologist Senior Scientist, Department of Medical Resonance Imaging, University Health Network, Toronto, Ontario, Canada.) Muscles of Arm Of the four major arm muscles, three flexors (biceps brachii, brachialis, and coracobrachialis) are in the anterior (flexor) compartment, supplied by the musculocutaneous nerve, and one extensor (triceps brachii) is in the posterior compartment, supplied by the radial nerve (Figs. 3.48 and 3.49B–D, F; Table 3.9). A distally placed assistant to the triceps, the anconeus, also lies within the posterior compartment (Fig. 3.49G). The flexor muscles of the anterior compartment are almost twice as strong as the extensors in all positions; consequently, we are better pullers than pushers. It should be noted, however, that the extensors of the elbow are particularly important for raising oneself out of a chair and for wheelchair activity. Therefore, conditioning of the triceps is of particular importance in elderly or disabled persons. 525 FIGURE 3.48 Nerves supplying medial and posterior walls of axilla, and muscles of arm. The pectoralis major and minor muscles are reflected superolaterally, and the lateral and medial cords of the brachial plexus are reflected superomedially. All major vessels and nerves arising from the medial and lateral cords of the brachial plexus (except for the musculocutaneous nerve arising from a segment of the lateral cord) are removed. The posterior cord, formed by the merging of the posterior divisions of all three trunks of the brachial plexus, is demonstrated. It gives rise to five peripheral nerves, four of which supply the muscles of the posterior wall of the axilla and posterior compartments of the upper limb. 526 FIGURE 3.49 Muscles of arm. TABLE 3.9. Muscles of Arm 527 a The spinal cord segmental innervation is indicated (e.g., “C5, C6, C7” means that the nerves supplying the biceps brachii are derived from the fifth and sixth cervical segments of the spinal cord). Numbers in boldface (C6) indicate the main segmental innervation. Damage to one or more of the listed spinal cord segments or to the motor nerve roots arising from them results in paralysis of the muscles concerned. b Some of the lateral part of the brachialis is innervated by a branch of the radial nerve. The arm muscles and their attachments are illustrated in Figure 3.49, and their attachments, innervation, and actions are described in Table 3.9. BICEPS BRACHII As the term biceps brachii indicates, the proximal attachment of this fusiform muscle usually has two heads (bi, two + L. caput, head). The two heads of the biceps arise proximally by tendinous attachments to processes of the scapula, their fleshy bellies uniting just distal to the middle of the arm (Fig. 3.49B). Approximately 10% of people have a third head to the biceps. When present, the third head extends from the superomedial part of the brachialis (with which it is blended), usually lying posterior to the brachial artery. In either case, a single biceps tendon forms distally and attaches primarily to the radius. Although the biceps is located in the anterior compartment of the arm, it has no attachment to the humerus (Figs. 3.47B, C and 3.49A, B). The biceps is a “three-joint muscle,” crossing and capable of effecting movement at the glenohumeral, elbow, and radio-ulnar joints, although it primarily acts at the latter two. Its action and effectiveness are markedly affected by the position of the elbow and forearm. When the elbow is extended, the biceps is a simple flexor of the forearm; however, as elbow flexion approaches 90° and more power is needed against resistance, the biceps is capable of two powerful movements, depending on the position of the forearm. When the elbow is flexed close to 90° and the forearm is supinated, the biceps is most efficient in producing flexion. Alternately, when the forearm is pronated, the biceps is the primary (most 528 powerful) supinator of the forearm. For example, it is used when right-handed people drive a screw into hard wood, and when inserting a corkscrew and pulling the cork from a wine bottle. The biceps barely operates as a flexor when the forearm is pronated, even against resistance. In the semiprone position, it is active only against resistance (Hamill and Knutzen, 2014). Arising from the supraglenoid tubercle of the scapula, and crossing the head of the humerus within the cavity of the glenohumeral joint, the rounded tendon of the long head of the biceps continues to be surrounded by synovial membrane as it descends in the intertubercular sulcus of the humerus. A broad band, the transverse humeral ligament, passes from the lesser to the greater tubercle of the humerus and converts the intertubercular groove into a canal (Fig. 3.49B). The ligament holds the tendon of the long head of the biceps in the groove. Distally, the major attachment of the biceps is to the radial tuberosity via the biceps tendon. However, a triangular membranous band, the bicipital aponeurosis, runs from the biceps tendon across the cubital fossa and merges with the antebrachial (deep) fascia covering the flexor muscles in the medial side of the forearm. It attaches indirectly by means of the fascia to the subcutaneous border of the ulna. The proximal part of the aponeurosis can be easily felt where it passes obliquely over the brachial artery and median nerve (Figs. 3.47A and 3.52A). The aponeurosis affords protection for these and other structures in the cubital fossa. It also helps lessen the pressure of the biceps tendon on the radial tuberosity during pronation and supination of the forearm. To test the biceps brachii, the elbow joint is flexed against resistance when the forearm is supinated. If acting normally, the muscle forms a prominent bulge on the anterior aspect of the arm that is easily palpated. BRACHIALIS The brachialis is a flattened fusiform muscle that lies posterior (deep) to the biceps. Its distal attachment covers the anterior part of the elbow joint (Figs. 3.47, 3.48, and 3.49D; Table 3.9). The brachialis is the main flexor of the forearm. It is the only pure flexor, producing the greatest amount of flexion force. Unlike the biceps, the brachialis flexes the forearm in all positions, being unaffected by pronation or supination. It acts during both slow and quick movements and in the presence or absence of resistance. When the forearm is extended slowly, the brachialis steadies the movement by slowly relaxing, that is, eccentric contraction (e.g., you use it to pick up and put down a teacup 529 carefully). The brachialis always contracts when the elbow is flexed, and it is primarily responsible for sustaining the flexed position. Because of its important and almost constant role, it is regarded as the workhorse of the elbow flexors. To test the brachialis, the forearm is semipronated and flexed against resistance. If acting normally, the contracted muscle can be seen and palpated. CORACOBRACHIALIS The coracobrachialis is an elongated muscle in the superomedial part of the arm. It is a useful landmark for locating other structures in the arm (Figs. 3.47, 3.48, and 3.49C; Table 3.9). For example, the musculocutaneous nerve pierces it, and the distal part of its attachment indicates the location of the nutrient foramen of the humerus. The coracobrachialis helps flex and adduct the arm and stabilize the glenohumeral joint. With the deltoid and long head of the triceps, it serves as a shunt muscle, resisting downward dislocation of the head of the humerus, as when carrying a heavy suitcase. The median nerve and/or the brachial artery may run deep to the coracobrachialis and be compressed by it. TRICEPS BRACHII The triceps brachii is a large fusiform muscle in the posterior compartment of the arm (Figs. 3.47, 3.48, 3.49F, and 3.50; Table 3.9). As its name indicates, the triceps has three heads: long, lateral, and medial. The triceps is the main extensor of the forearm. 530 FIGURE 3.50 Muscles of scapular region and posterior region of arm. The lateral head of the triceps brachii is divided and displaced to show the structures traversing the quadrangular 531 space and the radial nerve and profunda brachii artery. The exposed bone of the radial groove, which is devoid of muscular attachment, separates the humeral attachments of the lateral and medial heads of the triceps. (Bony attachments are illustrated in Fig. 3.49E.) Because its long head crosses the glenohumeral joint, the triceps helps stabilize the adducted glenohumeral joint by serving as a shunt muscle, resisting inferior displacement of the head of the humerus. The long head also aids in extension and adduction of the arm, but it is the least active head. The medial head is the workhorse of forearm extension, active at all speeds and in the presence or absence of resistance. The lateral head is the strongest but is it recruited into activity primarily against resistance (Hamill and Knutzen, 2014). Pronation and supination of the forearm do not affect triceps operation. Just proximal to the distal attachment of the triceps is a friction-reducing subtendinous olecranon bursa, between the triceps tendon and the olecranon. To test the triceps (or to determine the level of a radial nerve lesion), the arm is abducted 90° and then the flexed forearm is extended against resistance provided by the examiner. If acting normally, the triceps can be seen and palpated. Its strength should be comparable with the contralateral muscle, given consideration for lateral dominance (right or left handedness). ANCONEUS The anconeus is a small, triangular muscle on the posterolateral aspect of the elbow, usually partially blended (continuous) with the medial head of the triceps muscle (Fig. 3.49G; Table 3.9). The anconeus assists the triceps in extending the forearm and tenses the capsule of the elbow joint, preventing its being pinched during extension. It is also said to exert an abducting force on the ulna during pronation of the forearm. Brachial Artery The brachial artery provides the main arterial supply to the arm and is the continuation of the axillary artery (Fig. 3.51). It begins at the inferior border of 532 the teres major (Figs. 3.47A and 3.51) and ends in the cubital fossa opposite the neck of the radius where, under cover of the bicipital aponeurosis, it divides into the radial and ulnar arteries (Figs. 3.51 and 3.52). FIGURE 3.51 Arterial supply of arm and proximal forearm. Functionally and clinically important peri-articular arterial anastomoses surround the elbow. The resulting collateral circulation allows blood to reach the forearm when flexion of 533 the elbow compromises flow through the terminal part of the brachial artery. FIGURE 3.52 Dissections of cubital fossa. A. Superficial dissection. B. In this deep dissection, part of the biceps is excised and the cubital fossa is opened widely by retracting the forearm extensor muscles laterally and the flexor muscles medially. The radial nerve, which has just left the posterior compartment of the arm by piercing the lateral intermuscular septum, emerges between the brachialis and brachioradialis and divides into a superficial (sensory) and a deep (motor) branch (details are shown in Fig. 3.57A, B). The brachial artery, relatively superficial and palpable throughout its course, lies anterior to the triceps and brachialis. At first, it lies medial to the humerus where its pulsations are palpable in the medial bicipital groove (Fig. 3.47A, B). It then passes anterior to the medial supra-epicondylar ridge and trochlea of the humerus (Figs. 3.51 and 3.53). 534 FIGURE 3.53 Relationship of arteries and nerves of arm to humerus and compartments of arm. The radial nerve and accompanying profunda brachii artery wind posteriorly around, and directly on the surface of, the humerus in the radial groove. The radial nerve and radial collateral artery then pierce the 535 lateral intermuscular septum to enter the anterior compartment. The ulnar nerve pierces the medial intermuscular septum to enter the posterior compartment and then lies in the groove for the ulnar nerve on the posterior aspect of the medial epicondyle of the humerus. The median nerve and brachial artery descend in the arm to the medial side of the cubital fossa, where it is well protected and rarely injured. (Details are shown in Figures 3.50 and 3.57A, B.) As it passes inferolaterally, the brachial artery accompanies the median nerve, which crosses anterior to the artery (Figs. 3.47A and 3.53). During its course through the arm, the brachial artery gives rise to many unnamed muscular branches, and the humeral nutrient artery (Fig. 3.51), which arise from its lateral aspect. The unnamed muscular branches are often omitted from illustrations, but they are evident during dissection. The main named branches of the brachial artery arising from its medial aspect are the profunda brachii artery and the superior and inferior ulnar collateral arteries. The collateral arteries help form the peri-articular arterial anastomoses of the elbow region (Fig. 3.51). Other arteries involved are recurrent branches, sometimes double, from the radial, ulnar, and interosseous arteries, which run superiorly anterior and posterior to the elbow joint. These arteries anastomose with descending articular branches of the deep artery of the arm and the ulnar collateral arteries. PROFUNDA BRACHII ARTERY The profunda brachii artery (deep brachial artery, deep artery of the arm) is the largest branch of the brachial artery and has the most superior origin. The profunda brachii accompanies the radial nerve along the radial groove as it passes posteriorly around the shaft of the humerus (Figs. 3.50 and 3.53). The profunda brachii terminates by dividing into middle and radial collateral arteries, which participate in the peri-articular arterial anastomoses around the elbow (Fig. 3.51). HUMERAL NUTRIENT ARTERY 536 The main humeral nutrient artery arises from the brachial artery around the middle of the arm and enters the nutrient canal on the anteromedial surface of the humerus (Fig. 3.51). The artery runs distally in the canal toward the elbow. Other smaller humeral nutrient arteries also occur. SUPERIOR ULNAR COLLATERAL ARTERY The superior ulnar collateral artery arises from the medial aspect of the brachial artery near the middle of the arm and accompanies the ulnar nerve posterior to the medial epicondyle of the humerus (Figs. 3.47A and 3.51). Here, it anastomoses with the posterior ulnar recurrent and inferior ulnar collateral arteries, participating in the peri-articular arterial anastomoses of the elbow. INFERIOR ULNAR COLLATERAL ARTERY The inferior ulnar collateral artery arises from the brachial artery approximately 5 cm proximal to the elbow crease (Figs. 3.47A, 3.51, and 3.52B). It then passes inferomedially anterior to the medial epicondyle of the humerus and joins the peri-articular arterial anastomoses of the elbow region by anastomosing with the anterior ulnar recurrent artery. Veins of Arm Two sets of veins of the arm, superficial and deep, anastomose freely with each other. The superficial veins are in the subcutaneous tissue, and the deep veins accompany the arteries. Both sets of veins have valves, but they are more numerous in the deep veins than in the superficial veins. SUPERFICIAL VEINS The two main superficial veins of the arm, the cephalic and basilic veins (Figs. 3.47B, C and 3.52A), are described in “Superficial Veins of Upper Limb.” DEEP VEINS Paired deep veins, collectively constituting the brachial vein, accompany the brachial artery (Fig. 3.52A). Their frequent connections encompass the artery, forming an anastomotic network within a common vascular sheath. The 537 pulsations of the brachial artery help move the blood through this venous network. The brachial vein begins at the elbow by union of the accompanying veins of the ulnar and radial arteries and ends by merging with the basilic vein to form the axillary vein (Figs. 3.16 and 3.41). Not uncommonly, the deep veins join to form one brachial vein during part of their course. Nerves of Arm Four main nerves pass through the arm: median, ulnar, musculocutaneous, and radial (Fig. 3.53). Their origins from the brachial plexus, courses in the upper limb, and the structures innervated by them are summarized in Table 3.8. The median and ulnar nerves do not supply branches to the arm. MUSCULOCUTANEOUS NERVE The musculocutaneous nerve begins opposite the inferior border of the pectoralis minor, pierces the coracobrachialis, and continues distally between the biceps and brachialis (Fig. 3.52B). After supplying all three muscles of the anterior compartment of the arm, the musculocutaneous nerve emerges lateral to the biceps as the lateral cutaneous nerve of the forearm (Fig. 3.53). It becomes truly subcutaneous when it pierces the deep fascia proximal to the cubital fossa to course initially with the cephalic vein in the subcutaneous tissue (Fig. 3.52A). After crossing the anterior aspect of the elbow, it continues to supply the skin of the lateral aspect of the forearm. RADIAL NERVE The radial nerve in the arm supplies all the muscles in the posterior compartment of the arm (and forearm). The radial nerve enters the arm posterior to the brachial artery, medial to the humerus, and anterior to the long head of the triceps, where it gives branches to the long and medial heads of the triceps (Fig. 3.48). The radial nerve then descends inferolaterally with the profunda brachii artery and passes around the humeral shaft in the radial groove (Figs. 3.47B, 3.50, and 3.53). The branch of the radial nerve to the lateral head of the triceps arises within the radial groove. When it reaches the lateral border of the humerus, the radial nerve pierces the lateral intermuscular septum and continues inferiorly in the anterior compartment of the arm between the brachialis and the 538 brachioradialis to the level of the lateral epicondyle of the humerus (Fig. 3.52B). Anterior to the lateral epicondyle, the radial nerve divides into deep and superficial branches. The deep branch of the radial nerve is entirely muscular and articular in its distribution. The superficial branch of the radial nerve is entirely cutaneous in its distribution, supplying sensation to the dorsum of the hand and fingers. MEDIAN NERVE The median nerve in the arm runs distally in the arm on the lateral side of the brachial artery until it reaches the middle of the arm, where it crosses to the medial side and contacts the brachialis (Fig. 3.53). The median nerve then descends into the cubital fossa, where it lies deep to the bicipital aponeurosis and median cubital vein (Fig. 3.52). The median nerve has no branches in the axilla or arm, but it does supply articular branches to the elbow joint. ULNAR NERVE The ulnar nerve in the arm passes distally from the axilla anterior to the insertion of the teres major and to the long head of the triceps, on the medial side of the brachial artery (Fig. 3.47). Around the middle of the arm, it pierces the medial intermuscular septum with the superior ulnar collateral artery and descends between the septum and the medial head of the triceps (Fig. 3.53). The ulnar nerve passes posterior to the medial epicondyle and medial to the olecranon to enter the forearm (Fig. 3.46C). Posterior to the medial epicondyle, where the ulnar nerve is referred to in lay terms as the “funny bone.” The ulnar nerve is superficial, easily palpable, and vulnerable to injury. Like the median nerve, the ulnar nerve has no branches in the arm, but it also supplies articular branches to the elbow joint. Cubital Fossa The cubital fossa is apparent superficially as a depression on the anterior aspect of the elbow region (Fig. 3.55A). Deeply, it is a space filled with a variable amount of fat anterior to the most distal part of the humerus and the elbow joint. The three boundaries of the triangular cubital fossa are (Fig. 3.52) 539 1. Superiorly, an imaginary line connecting the medial and lateral epicondyles. 2. Medially, the mass of flexor muscles of the forearm arising from the common flexor attachment on the medial epicondyle; most specifically, the pronator teres. 3. Laterally, the mass of extensor muscles of the forearm arising from the lateral epicondyle and supra-epicondylar ridge; most specifically, the brachioradialis. The floor of the cubital fossa is formed by the brachialis and supinator muscles of the arm and forearm, respectively. The roof of the cubital fossa is formed by the continuity of brachial and antebrachial (deep) fascia reinforced by the bicipital aponeurosis (Figs. 3.52 and 3.58), subcutaneous tissue, and skin. The contents of the cubital fossa are the (see Figs. 3.52 and 3.57A) Terminal part of the brachial artery and the commencement of its terminal branches, the radial and ulnar arteries. The brachial artery lies between the biceps tendon and the median nerve. (Deep) accompanying veins of the arteries. Biceps brachii tendon. Median nerve. Radial nerve, deep between the muscles forming the lateral boundary of the fossa (the brachioradialis, in particular) and the brachialis, dividing into its superficial and deep branches. The muscles must be retracted to expose the nerve. Superficially, in the subcutaneous tissue overlying the cubital fossa are the median cubital vein, lying anterior to the brachial artery, and the medial and lateral cutaneous nerves of the forearm, related to the basilic and cephalic veins (see Fig. 3.55). Surface Anatomy of Arm and Cubital Fossa The borders of the deltoid are visible when the arm is abducted against resistance. The distal attachment of the deltoid can be palpated on the lateral surface of the humerus (Fig. 3.54A). 540 FIGURE 3.54 Surface anatomy of arm. The long, lateral, and medial heads of the triceps brachii form bulges on the posterior aspect of the arm and are identifiable when the forearm is extended from the flexed position against resistance. The olecranon, to which the triceps tendon attaches distally, is easily palpated. It is separated from the skin by only the olecranon bursa, which accounts for the mobility of the overlying skin. The triceps tendon is easily felt as it descends along the posterior aspect of the arm to the olecranon. The fingers can be pressed inward on each side of the tendon, where the elbow joint is superficial. An abnormal collection of fluid in the elbow 541 joint or in the subtendinous bursa of the triceps brachii is palpable at these sites; the bursa lies deep to the triceps tendon (see Figs. 3.97 and 3.101). The biceps brachii forms a bulge on the anterior aspect of the arm; its belly becomes more prominent when the elbow is flexed and supinated against resistance (Fig. 3.54B). The biceps brachii tendon can be palpated in the cubital fossa, immediately lateral to the midline, especially when the elbow is flexed against resistance. The proximal part of the bicipital aponeurosis can be palpated where it passes obliquely over the brachial artery and median nerve. Medial and lateral bicipital grooves separate the bulges formed by the biceps and triceps and indicate the location of the medial and lateral intermuscular septa (Fig. 3.54C). The cephalic vein runs superiorly in the lateral bicipital groove, and the basilic vein ascends in the medial bicipital groove. Deep to the latter is the main neurovascular bundle of the limb. No part of the shaft of the humerus is subcutaneous; however, it can be palpated with varying distinctness through the muscles surrounding it, especially in many elderly people. The head of the humerus is surrounded by muscles on all sides, except inferiorly; thus, it can be palpated by pushing the fingers well up into the axilla. The arm should be close to the side so the axillary fascia is loose. The humeral head can be palpated when the arm is moved while the inferior angle of the scapula is held in place. The brachial artery may be felt pulsating deep to the medial border of the biceps. The medial and lateral epicondyles of the humerus are subcutaneous and can be easily palpated at the medial and lateral aspects of the elbow. The medial epicondyle is more prominent. In the cubital fossa, the cephalic and basilic veins in the subcutaneous tissue are clearly visible when a tourniquet is applied to the arm, as is the median cubital vein. This vein crosses the bicipital aponeurosis as it runs superomedially connecting the cephalic to the basilic vein (Fig. 3.55). 542 FIGURE 3.55 Surface anatomy of cubital fossa. If the thumb is pressed into the cubital fossa, the muscular masses of the long flexors of the forearm will be felt forming the medial border, the pronator teres most directly. The lateral group of forearm extensors (a soft mass that can be grasped separately), the brachioradialis (most medial) and the long and short extensors of the wrist, can be grasped between the fossa and the lateral epicondyle. The Bottom Line ARM AND CUBITAL FOSSA Arm: The arm forms a column with the humerus at its center. The humerus, along with intermuscular septa in its distal two thirds, divides the arm lengthwise (or more specifically, the space inside the brachial fascia) into anterior or flexor and posterior or extensor compartments. The anterior compartment contains three flexor muscles supplied by the musculocutaneous nerve. The coracobrachialis acts (weakly) at the shoulder, and the biceps and brachialis act at the elbow. The biceps is also the primary supinator of the forearm (when the elbow is flexed). The brachialis is the primary flexor of the forearm. The posterior compartment contains a three-headed extensor muscle, the triceps, which is supplied by the radial nerve. One of the heads (the long head) acts at the shoulder, but mostly the heads work together to extend the elbow. 543  Both compartments of the arm are supplied by the brachial artery, the posterior compartment primarily via its major branch, the profunda brachii artery. The primary neurovascular bundle is located on the medial side of the limb; thus, it is usually protected by the limb it serves. Cubital fossa: The triangular cubital fossa is bound by a line connecting the medial and lateral epicondyles of the humerus, and the pronator teres and brachioradialis muscles arising, respectively, from the epicondyles. The brachialis and supinator form the floor. The biceps tendon descends into the triangle to insert on the radial tuberosity. Medial to the tendon are the median nerve and terminal part of the brachial artery. Lateral to the tendon is the lateral cutaneous nerve of the forearm superficially and—at a deeper level— the terminal part of the radial nerve. In the subcutaneous tissue, most commonly, a median cubital vein runs obliquely across the fossa, connecting the cephalic vein of the forearm and basilic vein of the arm, providing an advantageous site for venipuncture. In about one fifth of the population, a median antebrachial vein bifurcates into median cephalic and median basilic veins, which replace the diagonal median cubital vein. FOREARM The forearm is the distal unit of the articulated strut (extension) of the upper limb. It extends from the elbow to the wrist and contains two bones, the radius and ulna, which are joined by an interosseous membrane (Fig. 3.56A, B, D). Although thin, this fibrous membrane is strong. In addition to firmly tying the forearm bones together while permitting pronation and supination, the interosseous membrane provides the proximal attachment for some deep forearm muscles. The head of the ulna is at the distal end of the forearm, whereas the head of the radius is at its proximal end. The role of forearm movement, occurring at the elbow and radio-ulnar joints, is to assist the shoulder in the application of force and in controlling the placement of the hand in space. 544 FIGURE 3.56 Bones, muscles, and flexor–pronator compartment of forearm. A. Anteroposterior (AP) radiograph of the forearm in pronation. (Courtesy of Dr. J. Heslin, Toronto, Ontario, Canada.) B. Bones of the forearm and radio-ulnar ligaments. C. Dissection showing the superficial muscles of the forearm and the palmar aponeurosis. D. Stepped transverse section demonstrating the compartments of the forearm. E. The 545 flexor digitorum superficialis (FDS) and related structures. The ulnar artery emerges from its oblique course posterior to the FDS to meet and accompany the ulnar nerve. Compartments of Forearm As in the arm, the muscles of similar purpose and innervation are grouped within the same fascial compartments in the forearm. Although the proximal boundary of the forearm per se is defined by the joint plane of the elbow, functionally the forearm includes the distal humerus. For the distal forearm, wrist, and hand to have minimal bulk to maximize their functionality, they are operated by “remote control” by extrinsic muscles having their bulky, fleshy, contractile parts located proximally in the forearm, distant from the site of action. Their long, slender tendons extend distally to the operative site, like long ropes reaching to distant pulleys. Furthermore, because the structures on which the muscles and tendons act (wrist and fingers) have an extensive range of motion, a long range of contraction is needed, requiring that the muscles have long contractile parts as well as a long tendon(s). The forearm proper is not, in fact, long enough to provide the required length and sufficient area for attachment proximally, so the proximal attachments (origins) of the muscles must occur proximal to the elbow—in the arm—and provided by the humerus. Generally, flexors lie anteriorly and extensors posteriorly; however, the anterior and posterior aspects of the distal humerus are occupied by the chief flexors and extensors of the elbow (Fig. 3.57A). To provide the required attachment sites for the flexors and extensors of the wrist and fingers, medial and lateral extensions (epicondyles and supra-epicondylar ridges) have developed from the distal humerus. 546 FIGURE 3.57 Cross sections demonstrating relationships at 547 cubital fossa, proximal forearm, and wrist. A. At the level of the cubital fossa, the flexors and extensor of the elbow occupy the anterior and posterior aspects of the humerus. Lateral and medial extensions (epicondyles and supra-epicondylar ridges) of the humerus provide proximal attachment (origin) for the forearm flexors and extensors. B. Consequently, in the proximal forearm, the “anterior” flexor–pronator compartment actually lies anteromedially, and the “posterior” extensor–supinator compartment lies posterolaterally. The radial artery (laterally) and the sharp, subcutaneous posterior border of the ulna (medially) are palpable features separating the anterior and posterior compartments. No motor nerves cross either demarcation, making them useful for surgical approaches. Ext. digit., extensor digitorum; ECU, extensor carpi ulnaris; FCR, flexor carpi radialis; FCU, flexor carpi ulnaris; FDP, flexor digitorum profundus; FDS, flexor digitorum superficialis; FPL, flexor pollicis longus; PL, palmaris longus; PT, pronator teres. C.At the level of the wrist, nine tendons from three muscles (and one nerve) of the anterior compartment of the forearm traverse the carpal tunnel; eight of the tendons share a common synovial flexor sheath. The medial epicondyle and supra-epicondylar ridge provide attachment for the forearm flexors, and the lateral formations provide attachment for the forearm extensors. Thus, rather than lying strictly anteriorly and posteriorly, the proximal parts of the “anterior” (flexor–pronator) compartment of the forearm lie anteromedially, and the “posterior” (extensor–supinator) compartment lies posterolaterally (Figs. 3.56D, 3.57B, and 3.61C). Spiraling gradually over the length of the forearm, the compartments become truly anterior and posterior in position in the distal forearm and wrist. These fascial compartments, containing the muscles in functional groups, are demarcated by the subcutaneous border of the ulna posteriorly (in the proximal forearm) and then medially (distal forearm) and by the radial artery anteriorly 548 and then laterally. These structures are palpable (the artery by its pulsations) throughout the forearm. Because neither boundary is crossed by motor nerves, they also provide sites for surgical incision. The flexors and pronators of the forearm are in the anterior compartment and are served mainly by the median nerve; the one and a half exceptions are innervated by the ulnar nerve. The extensors and supinators of the forearm are in the posterior compartment and are all served by the radial nerve (directly or by its deep branch). The fascial compartments of the limbs generally end at the joints; therefore, fluids and infections in compartments are usually contained and cannot readily spread to other compartments. The anterior compartment is exceptional in this regard because it communicates with the central compartment of the palm through the carpal tunnel (Fig. 3.57C; see also Fig. B3.32). Muscles of Forearm There are 17 muscles crossing the elbow joint, some of which act on the elbow joint exclusively, whereas others act at the wrist and fingers. In the proximal part of the forearm, the muscles form fleshy masses extending inferiorly from the medial and lateral epicondyles of the humerus (Figs. 3.56C and 3.57A). The tendons of these muscles pass through the distal part of the forearm and continue into the wrist, hand, and fingers (Figs. 3.56C, E and 3.57C). The flexor muscles of the anterior compartment have approximately twice the bulk and strength of the extensor muscles of the posterior compartment. FLEXOR–PRONATOR MUSCLES OF FOREARM The flexor muscles of the forearm are in the anterior (flexor–pronator) compartment of the forearm and are separated from the extensor muscles of the forearm by the radius and ulna (Fig. 3.57B) and, in the distal two thirds of the forearm, by the interosseous membrane that connects them (Fig. 3.56B, D). The tendons of most flexor muscles are located on the anterior surface of the wrist and are held in place by the palmar carpal ligament and the flexor retinaculum (transverse carpal ligament), thickenings of the antebrachial fascia (Figs. 3.56C and 3.58). 549 550 FIGURE 3.58 Fascia of distal upper limb and superficial muscles of forearm. The flexor–pronator muscles are arranged in three layers or groups (Fig. 3.59; Table 3.10): FIGURE 3.59 Flexor muscles of forearm. TABLE 3.10. Muscles of Anterior Compartment of Forearm 551 a The spinal cord segmental innervation is indicated (e.g., “C6, C7” means that the nerves supplying the pronator teres are derived from the sixth and seventh cervical segments of the spinal cord). Numbers in boldface (C7) indicate the main segmental innervation. Damage to one or more of the listed spinal cord segments or to the motor nerve roots arising from them results in paralysis of the muscles concerned. 1. A superficial layer or group of four muscles (pronator teres, flexor carpi radialis, palmaris longus, and flexor carpi ulnaris). These muscles are all attached proximally by a common flexor tendon to the medial epicondyle of the humerus, the common flexor attachment. 2. An intermediate layer, consisting of one muscle (flexor digitorum superficialis). 3. A deep layer or group of three muscles (flexor digitorum profundus, flexor pollicis longus, and pronator quadratus). 552 The five superficial and intermediate muscles cross the elbow joint; the three deep muscles do not. With the exception of the pronator quadratus, the more distally placed a muscle’s distal attachment lies, the more distally and deeply placed is its proximal attachment. All muscles in the anterior (flexor–pronator) compartment of the forearm are supplied by the median and/or ulnar nerves (most by the median; only one and a half exceptions are supplied by the ulnar). Functionally, the brachioradialis is a flexor of the forearm, but it is located in the posterior (posterolateral) or extensor compartment and is thus supplied by the radial nerve. Therefore, the brachioradialis is a major exception to the rule that (1) the radial nerve supplies only extensor muscles and (2) that all flexors lie in the anterior (flexor) compartment. The long flexors of the digits (flexor digitorum superficialis and flexor digitorum profundus) also flex the metacarpophalangeal and wrist joints. The flexor digitorum profundus flexes the digits in slow action. This action is reinforced by the flexor digitorum superficialis when speed and flexion against resistance are required. When the wrist is flexed at the same time that the metacarpophalangeal and interphalangeal joints are flexed, the long flexor muscles of the fingers are operating over a shortened distance between attachments, and the action resulting from their contraction is consequently weaker. Extending the wrist increases their operating distance, and thus, their contraction is more efficient in producing a strong grip (see Fig. 3.73A). Tendons of the long flexors of the digits pass through the distal part of the forearm, wrist, and palm and continue to the medial four fingers. The flexor digitorum superficialis flexes the middle phalanges, and the flexor digitorum profundus flexes the middle and distal phalanges. The muscles of the anterior compartment of the forearm are illustrated in Figure 3.59 and their attachments, innervation, and main actions are listed by layers in Table 3.10. The following discussion provides additional details, beginning with the muscles of the superficial and intermediate layers. Pronator Teres. The pronator teres, a fusiform muscle, is the most lateral of the superficial forearm flexors. Its lateral border forms the medial boundary of the cubital fossa. To test the pronator teres, the person’s forearm is flexed at the elbow and 553 pronated from the supine position against resistance provided by the examiner. If acting normally, the muscle is prominent and can be palpated at the medial margin of the cubital fossa. Flexor Carpi Radialis. The flexor carpi radialis (FCR) is a long fusiform muscle located medial to the pronator teres. In the middle of the forearm, its fleshy belly is replaced by a long, flattened tendon that becomes cord-like as it approaches the wrist. The FCR produces flexion (when acting with the flexor carpi ulnaris) and abduction of the wrist (when acting with the extensors carpi radialis longus and brevis). When acting alone, the FCR produces a combination of flexion and abduction simultaneously at the wrist so that the hand moves anterolaterally. To reach its distal attachment, the FCR tendon passes through a canal in the lateral part of the flexor retinaculum and through a vertical groove in the trapezium in its own synovial tendinous sheath of the flexor carpi radialis (Fig. 3.57C). The FCR tendon is a good guide to the radial artery, which lies just lateral to it (Fig. 3.56C). To test the flexor carpi radialis, the person is asked to flex the wrist against resistance. If acting normally, its tendon can be easily seen and palpated. Palmaris Longus. The palmaris longus, a small fusiform muscle, is absent on one or both sides (usually the left) in approximately 14% of people, but its actions are not missed. It has a short belly and a long, cord-like tendon that passes superficial to the flexor retinaculum and attaches to it and the apex of the palmar aponeurosis (Figs. 3.56C and 3.58). The palmaris longus tendon is a useful guide to the median nerve at the wrist. The tendon lies deep and slightly medial to this nerve before it passes deep to the flexor retinaculum. To test the palmaris longus, the wrist is flexed and the pads of the little finger and thumb are tightly pinched together. If present and acting normally, the tendon can be easily seen and palpated. Flexor Carpi Ulnaris. The flexor carpi ulnaris (FCU) is the most medial of the superficial flexor 554 muscles. The FCU simultaneously flexes and adducts the hand at the wrist if acting alone. It flexes the wrist when it acts with the FCR and adducts it when acting with the extensor carpi ulnaris. The ulnar nerve enters the forearm by passing between the humeral and ulnar heads of its proximal attachment (Fig. 3.56C). This muscle is exceptional among muscles of the anterior compartment, being fully innervated by the ulnar nerve. The tendon of the FCU is a guide to the ulnar nerve and artery, which are on its lateral side at the wrist (Fig. 3.56C, E). To test the flexor carpi ulnaris, the person puts the posterior aspect of the forearm and hand on a flat table and is then asked to flex the wrist against resistance while the examiner palpates the muscle and its tendon. Flexor Digitorum Superficialis. The flexor digitorum superficialis (FDS) is sometimes considered one of the superficial muscles of the forearm, which attach to the common flexor origin and therefore cross the elbow (Table 3.10). When considered this way, it is the largest superficial muscle in the forearm. However, the FDS actually forms an intermediate layer between the superficial and deep groups of forearm muscles (Figs. 3.56C and 3.57B). The median nerve and ulnar artery enter the forearm by passing between its humero-ulnar and radial heads (Fig. 3.59A, C). Near the wrist, the FDS gives rise to four tendons, which pass deep to the flexor retinaculum through the carpal tunnel to the fingers. The four tendons are enclosed (along with the four tendons of the flexor digitorum profundus) in a synovial common flexor sheath (Fig. 3.57C). The FDS flexes the middle phalanges of the medial four fingers at the proximal interphalangeal joints. In continued action, the FDS also flexes the proximal phalanges at the metacarpophalangeal joints and the wrist joint. The FDS is capable of flexing each finger it serves independently. To test the flexor digitorum superficialis, one finger is flexed at the proximal interphalangeal joint against resistance and the other three fingers are held in an extended position to inactivate the flexor digitorum profundus. The fascial plane between the intermediate and deep layers of muscles makes up the primary neurovascular plane of the anterior (flexor–pronator) compartment; the main neurovascular bundles exclusive to this compartment course within it. The following three muscles form the deep layer of forearm flexor muscles. 555 Flexor Digitorum Profundus. The flexor digitorum profundus (FDP) is the only muscle that can flex the distal interphalangeal joints of the fingers (Fig. 3.59A, E). This thick muscle “clothes” the anterior aspect of the ulna. The FDP flexes the distal phalanges of the medial four fingers after the FDS has flexed their middle phalanges (i.e., it curls the fingers and assists with flexion of the hand, making a fist). Each tendon is capable of flexing two interphalangeal joints, the metacarpophalangeal joint and the wrist joint. The FDP divides into four parts, which end in four tendons that pass posterior to the FDS tendons and the flexor retinaculum within the common flexor sheath (Fig. 3.57C). The part of the muscle going to the index finger usually separates from the rest of the muscle relatively early in the distal part of the forearm and is capable of independent contraction. Each tendon enters the fibrous sheath of its digit, posterior to the FDS tendons. Unlike the FDS, the FDP can flex only the index finger independently; thus, the fingers can be independently flexed at the proximal but not the distal interphalangeal joints. To test the flexor digitorum profundus, the proximal interphalangeal joint is held in the extended position while the person attempts to flex the distal interphalangeal joint. The integrity of the median nerve in the proximal forearm can be tested by performing this test using the index finger, and that of the ulnar nerve can be assessed by using the little finger. Flexor Pollicis Longus. The flexor pollicis longus (FPL), the long flexor of the thumb (L. pollex, thumb), lies lateral to the FDP, where it clothes the anterior aspect of the radius distal to the attachment of the supinator (Figs. 3.56C, E and 3.59A, D; Table 3.10). The flat FPL tendon passes deep to the flexor retinaculum, enveloped in its own synovial tendinous sheath of the flexor pollicis longus on the lateral side of the common flexor sheath (Fig. 3.57C). The FPL primarily flexes the distal phalanx of the thumb at the interphalangeal joint and, secondarily, the proximal phalanx and 1st metacarpal at the metacarpophalangeal and carpometacarpal joints, respectively. The FPL is the only muscle that flexes the interphalangeal joint of the thumb. It also may assist in flexion of the wrist joint. To test the flexor pollicis longus, the proximal phalanx of the thumb is held and the distal phalanx is flexed against resistance. 556 Pronator Quadratus. The pronator quadratus (PQ), as its name indicates, is quadrangular and pronates the forearm (Fig. 3.59E). It cannot be palpated or observed, except in dissections, because it is the deepest muscle in the anterior aspect of the forearm. Sometimes it is considered to constitute a fourth muscle layer. The PQ clothes the distal fourth of the radius and ulna and the interosseous membrane between them (Fig. 3.59A, E; Table 3.10). The PQ is the only muscle that attaches only to the ulna at one end and only to the radius at the other end. The PQ is the prime mover for pronation. The muscle initiates pronation and is assisted by the PT when more speed and power are needed. The pronator quadratus also helps the interosseous membrane hold the radius and ulna together, particularly when upward thrusts are transmitted through the wrist (e.g., during a fall on the hand). EXTENSOR MUSCLES OF FOREARM The muscles of the posterior compartment of the forearm are illustrated in Figure 3.60, and their attachments, innervation, and main actions are provided by layer in Table 3.11. The following discussion provides additional details. FIGURE 3.60 Extensor muscles of forearm. TABLE 3.11. Muscles of Posterior Compartment of Forearm 557 a The spinal cord segmental innervation is indicated (e.g., “C7, C8” means that the nerves supplying the extensor carpi radialis brevis are derived from the seventh and eighth cervical segments of the spinal cord). Numbers in boldface (C7) indicate the main segmental innervation. Damage to one or more of the listed spinal cord segments or to the motor nerve roots arising from them results in paralysis of the muscles concerned. The extensor muscles are in the posterior (extensor–supinator) compartment of the forearm, and all of them are innervated by branches of the radial nerve (Fig. 3.57B). These muscles can be organized physiologically into three functional groups: 1. Muscles that extend and abduct or adduct the hand at the wrist joint (extensor carpi radialis longus, extensor carpi radialis brevis, and extensor carpi ulnaris). 558 2. Muscles that extend the medial four fingers (extensor digitorum, extensor indicis, and extensor digiti minimi). 3. Muscles that extend or abduct the thumb (abductor pollicis longus, extensor pollicis brevis, and extensor pollicis longus). The extensor tendons are held in place in the wrist region by the extensor retinaculum, which prevents bowstringing of the tendons (protruding beyond the contour of the bent limb, like the string of an archer’s bow) when the hand is extended at the wrist joint. As the tendons pass over the dorsum of the wrist, they are provided with synovial tendon sheaths that reduce friction for the extensor tendons as they traverse the osseofibrous tunnels formed by the attachment of the extensor retinaculum to the distal radius and ulna (Fig. 3.61). The extensor muscles of the forearm are organized anatomically into superficial and deep layers (Fig. 3.57B). 559 FIGURE 3.61 Extensor–supinator compartment of right forearm. A. Superficial layer of extensor muscles. The distal extensor tendons have been removed from the dorsum of the hand without disturbing the arteries because they lie on the skeletal plane. The fascia on the posterior aspect of the distal- most forearm is thickened to form the extensor retinaculum, 560 which is anchored on its deep aspect to the radius and ulna. B. Deep layer of extensor muscles is shown. Three outcropping muscles of the thumb (star) emerge from between the extensor carpi radialis brevis and extensor digitorum: abductor pollicis longus, extensor pollicis brevis, and extensor pollicis longus. The furrow from which the three muscles emerge has been opened proximally to the lateral epicondyle, exposing the supinator muscle. C. This transverse section of the forearm shows the superficial and deep layers of muscles in the posterior compartment (pink), supplied by the radial nerve, and the anterior compartment (gold), supplied by the ulnar and median nerves. Four of the superficial extensors (extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, and extensor carpi ulnaris) are attached proximally by a common extensor tendon to the lateral epicondyle (Figs. 3.60A and 3.61A, B; Table 3.11). The proximal attachment of the other two muscles in the superficial group (brachioradialis and extensor carpi radialis longus) is to the lateral supra-epicondylar ridge of the humerus and adjacent lateral intermuscular septum (Fig. 3.60A, B). The four flat tendons of the extensor digitorum pass deep to the extensor retinaculum to the medial four fingers (Fig. 3.62). The common tendons of the index and little fingers are joined on their medial sides near the knuckles by the respective tendons of the extensor indicis and extensor digiti minimi (extensors of the index and little fingers, respectively). 561 FIGURE 3.62 Synovial sheaths and tendons on distal forearm and dorsum of hand. A. Observe that the six synovial tendon sheaths (purple) occupy six osseofibrous tunnels formed by attachments of the extensor retinaculum to the ulna and especially the radius, which give passage to 12 tendons of nine extensor muscles. The tendon of the extensor digitorum to the little finger is shared between the ring finger and continues to the little finger via an intertendinous connection. It then receives additional fibers from the tendon of the extensor digiti minimi. Such variations are common. Numbers refer to the labeled osseofibrous tunnels shown in part B. B. This slightly oblique transverse section of the distal end of the forearm shows the extensor tendons traversing the six osseofibrous tunnels deep to the extensor retinaculum. 562 Brachioradialis. The brachioradialis, a fusiform muscle, lies superficially on the anterolateral surface of the forearm (Figs. 3.58 and 3.61A). It forms the lateral border of the cubital fossa (Fig. 3.56C). As mentioned previously, the brachioradialis is exceptional among muscles of the posterior (extensor) compartment in that it has rotated to the anterior aspect of the humerus and thus flexes the forearm at the elbow. It is especially active during quick movements or in the presence of resistance during flexion of the forearm (e.g., when a weight is lifted), acting as a shunt muscle resisting subluxation of the head of the radius. The brachioradialis and the supinator are the only muscles of the compartment that do not cross and therefore are incapable of acting at the wrist. As it descends, the brachioradialis overlies the radial nerve and artery where they lie together on the supinator, pronator teres tendon, FDS, and FPL. The distal part of the tendon is covered by the abductors pollicis longus and brevis as they pass to the thumb (Fig. 3.61B). To test the brachioradialis, the elbow joint is flexed against resistance with the forearm in the midprone position. If the brachioradialis is acting normally, the muscle can be seen and palpated. Extensor Carpi Radialis Longus. The extensor carpi radialis longus (ECRL), a fusiform muscle, is partly overlapped by the brachioradialis, with which it often blends (Fig. 3.61). As it passes distally, posterior to the brachioradialis, its tendon is crossed by the abductor pollicis brevis and extensor pollicis brevis. The ECRL is indispensable when clenching the fist. To test the extensor carpi radialis longus, the wrist is extended and abducted with the forearm pronated. If acting normally, the muscle can be palpated inferoposterior to the lateral side of the elbow. Its tendon can be palpated proximal to the wrist. Extensor Carpi Radialis Brevis. The extensor carpi radialis brevis (ECRB), as its name indicates, is a shorter muscle than the ECRL because it arises distally in the limb, yet it attaches adjacent to the ECRL in the hand (but to the base of the 3rd metacarpal rather than the 2nd). As it passes distally, it is covered by the ECRL. The ECRB and 563 ECRL pass under the extensor retinaculum together within the tendinous sheath of the extensor carpi radiales (Fig. 3.62). The two muscles act together to various degrees, usually as synergists to other muscles. When the two muscles act by themselves, they abduct the hand as they extend it. Acting with the extensor carpi ulnaris, they extend the hand (the brevis is more involved in this action). Acting with the FCR, they produce pure abduction. Their synergistic action with the extensor carpi ulnaris is important in steadying the wrist during tight flexion of the medial four digits (clenching the fist), a function in which the longus is more active. Extensor Digitorum. The extensor digitorum, the principal extensor of the medial four digits, occupies much of the posterior surface of the forearm (Figs. 3.60 and 3.61A). Proximally, its four tendons join the tendon of the extensor indicis to pass deep to the extensor retinaculum through the tendinous sheath of the extensor digitorum and extensor indicis (common extensor synovial sheath) (Fig. 3.62A, B). On the dorsum of the hand, the tendons spread out as they run toward the digits. Adjacent tendons are linked proximal to the knuckles (metacarpophalangeal joints) by three oblique intertendinous connections that restrict independent extension of the four medial digits (especially the ring finger). Consequently, normally none of these digits can remain fully flexed as the other ones are fully extended. Commonly, the fourth tendon is fused initially with the tendon to the ring finger and reaches the little finger by an intertendinous connection. On the distal ends of the metacarpals and along the phalanges of the four medial digits, the four tendons flatten to form extensor expansions (Fig. 3.63). Each extensor digital expansion (dorsal expansion or hood) is a triangular, tendinous aponeurosis that wraps around the dorsum and sides of a head of the metacarpal and proximal phalanx. The visor-like “hood” formed by the extensor expansion over the head of the metacarpal, holding the extensor tendon in the middle of the digit, is anchored on each side to the palmar ligament (a reinforced portion of the fibrous layer of the joint capsule of the metacarpophalangeal joints) (Fig. 3.63A, C). 564 FIGURE 3.63 Dorsal digital (extensor) apparatus of 3rd digit. The metacarpal bone and all three phalanges are shown in parts A, B, D, and E; only the phalanges are shown in part C. A. Note the extensor digitorum tendon trifurcating (expanding) into three bands: two lateral bands that unite over the middle phalanx to insert into the base of the distal phalanx, and one median band that inserts into the base of the middle phalanx. B. Part of the tendon of the interosseous muscles attaches to the base of the 565 proximal phalanx; the other part contributes to the extensor expansion, attaching primarily to the lateral bands, but also fans out into an aponeurosis. Some of the aponeurotic fibers fuse with the median band, and other fibers arch over it to blend with the aponeurosis arising from the other side. On the radial side of each digit, a lumbrical muscle attaches to the radial lateral band. The dorsal hood consists of a broad band of transversely oriented fibers attached anteriorly to the palmar ligaments of the metacarpophalangeal (MP) joints that encircle the metacarpal head and MP joint, blending with the extensor expansion to keep the apparatus centered over the dorsal aspect of the digit. C. Distally, retinacular ligaments extending from the fibrous digital sheath to the lateral bands also help keep the apparatus centered and coordinate movements at the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints. D. Contraction of the extensor digitorum alone results in extension at all joints (including the MP joint in the absence of action by the interossei and lumbricals). E. Because of the relationship of the tendons and the lateral bands to the rotational centers of the joints (red dots in parts D and E), simultaneous contraction of the interossei and lumbricals produces flexion at the MP joint but extension at the PIP and DIP joints (the so-called Z-movement). In forming the extensor expansion, each extensor digitorum tendon divides into a median band, which passes to the base of the middle phalanx, and two lateral bands, which pass to the base of the distal phalanx (Fig. 3.63D, E). The tendons of the interosseous and lumbrical muscles of the hand join the lateral bands of the extensor expansion (Fig. 3.63). The retinacular ligament is a delicate fibrous band that runs from the proximal phalanx and fibrous digital sheath obliquely across the middle phalanx and two interphalangeal joints (Fig. 3.63C). It joins the extensor expansion to the distal phalanx. During flexion of the distal interphalangeal joint, the retinacular ligament becomes taut and pulls the proximal joint into flexion. Similarly, on 566 extending the proximal joint, the distal joint is pulled by the retinacular ligament into nearly complete extension. The extensor digitorum acts primarily to extend the proximal phalanges, and through its collateral reinforcements, it secondarily extends the middle and distal phalanges as well. After exerting its traction on the digits, or in the presence of resistance to digital extension, it helps extend the hand at the wrist joint. To test the extensor digitorum, the forearm is pronated and the fingers are extended. The person attempts to keep the digits extended at the metacarpophalangeal joints as the examiner exerts pressure on the proximal phalanges by attempting to flex them. If acting normally, the extensor digitorum can be palpated in the forearm, and its tendons can be seen and palpated on the dorsum of the hand. Extensor Digiti Minimi. The extensor digiti minimi (EDM), a fusiform slip of muscle, is a partially detached part of the extensor digitorum (Figs. 3.60B, 3.61A, B, and 3.62). The tendon of this extensor of the little finger runs through a separate compartment of the extensor retinaculum, posterior to the distal radio-ulnar joint, within the tendinous sheath of the extensor digiti minimi. The tendon then divides into two slips; the lateral one is joined to the tendon of the extensor digitorum, with all three tendons attaching to the dorsal digital expansion of the little finger. After exerting its traction primarily on the 5th digit, it contributes to extension of the hand. To test the extensor digiti minimi, the little finger is extended against resistance while holding digits 2–4 flexed at the metacarpophalangeal joints. Extensor Carpi Ulnaris. The extensor carpi ulnaris (ECU), a long fusiform muscle located on the medial border of the forearm, has two heads: a humeral head from the common extensor tendon and an ulnar head that arises by a common aponeurosis attached to the posterior border of the ulna and shared by the FCU, FDP, and deep fascia of the forearm. Distally, its tendon runs in a groove between the ulnar head and its styloid process, through a separate compartment of the extensor retinaculum within the tendinous sheath of the extensor carpi ulnaris. Acting with the ECRL and ECRB, it extends the hand; acting with the FCU, it adducts the hand. 567 Like the ECRL, it is indispensable when clenching the fist. To test the extensor carpi ulnaris, the forearm is pronated and the fingers are extended. The extended wrist is then adducted against resistance. If acting normally, the muscle can be seen and palpated in the proximal part of the forearm and its tendon can be felt proximal to the head of the ulna. Supinator. The supinator lies deep in the cubital fossa and, along with the brachialis, forms its floor (Figs. 3.60A, C, 3.61B, and 3.64). Spiraling medially and distally from its continuous, osseofibrous origin, this sheet-like muscle envelops the neck and proximal part of the shaft of the radius. The deep branch of the radial nerve passes between its muscle fibers, separating them into superficial and deep parts, as it passes from the cubital fossa to the posterior part of the arm. As it exits the muscle and joins the posterior interosseous artery, it may be referred to as the posterior interosseous nerve. 568 FIGURE 3.64 Relationship of radial nerve to brachialis and supinator muscles. In the cubital fossa, lateral to the brachialis, the radial nerve divides into deep (motor) and superficial (sensory) branches. The deep branch penetrates the supinator muscle and emerges in the posterior compartment of the forearm as the posterior interosseous nerve. It joins the artery of the same name to run in the plane between the superficial and the deep extensors of the forearm. 569 The supinator is the prime mover for slow, unopposed supination, especially when the forearm is extended. The biceps brachii also supinates the forearm and is the prime mover during rapid and forceful supination against resistance when the forearm is flexed (e.g., when a right-handed person drives a screw). The deep extensors of the forearm act on the thumb (abductor pollicis longus, extensor pollicis longus, and extensor pollicis brevis) and the index finger (extensor indicis) (Figs. 3.60 to 3.62; Table 3.11). The three muscles acting on the thumb are deep to the superficial extensors and “crop out” (emerge) from the furrow in the lateral part of the forearm that divides the extensors. Because of this characteristic, they are sometimes referred to as outcropping muscles of the thumb (Fig. 3.61A). Abductor Pollicis Longus. The abductor pollicis longus (APL) has a long, fusiform belly that lies just distal to the supinator (Fig. 3.60) and is closely related to the extensor pollicis brevis. Its tendon, and sometimes its belly, is commonly split into two parts, one of which may attach to the trapezium instead of the usual site at the base of the 1st metacarpal. The APL acts with the abductor pollicis brevis during abduction of the thumb and with the extensor pollicis muscles during extension of this digit. Although deeply situated, the APL emerges at the wrist as one of the outcropping muscles. Its tendon passes deep to the extensor retinaculum with the tendon of the extensor pollicis brevis in the common synovial tendinous sheath of the abductor pollicis longus and extensor pollicis brevis. To test the abductor pollicis longus, the thumb is abducted against resistance at the metacarpophalangeal joint. If acting normally, its tendon can be seen and palpated at the lateral side of the anatomical snuff box and on the lateral side of the adjacent extensor pollicis brevis tendon. Extensor Pollicis Brevis. The belly of the extensor pollicis brevis (EPB), the fusiform short extensor of the thumb, lies distal to the APL and is partly covered by it. Its tendon lies parallel and immediately medial to that of the APL but extends farther, reaching the base of the proximal phalanx (Fig. 3.62). In continued action after acting to flex the proximal phalanx of the thumb, or acting when that joint is fixed by its antagonists, it helps extend the 1st metacarpal and extend and abduct the hand. When the thumb is fully extended, a hollow, called the anatomical snuff box, can 570 be seen on the radial aspect of the wrist (Fig. 3.65). FIGURE 3.65 Anatomical snuff box. A. When the thumb is extended, a triangular hollow appears between the tendon of the extensor pollicis longus (EPL) medially and the tendons of the extensor pollicis brevis (EPB) and abductor pollicis longus (APL) laterally. B. The floor of the snuff box, formed by the scaphoid and trapezium bones, is crossed by the radial artery as it passes diagonally from the anterior surface of the radius to the dorsal surface of the hand. 571 To test the extensor pollicis brevis, the thumb is extended against resistance at the metacarpophalangeal joint. If the EPB is acting normally, the tendon of the muscle can be seen and palpated at the lateral side of the anatomical snuff box and on the medial side of the adjacent APL tendon (Figs. 3.61 and 3.62). Extensor Pollicis Longus. The extensor pollicis longus (EPL) is larger and its tendon is longer than that of the EPB. The tendon passes under the extensor retinaculum in its own tunnel (Fig. 3.60), within the tendinous sheath of the extensor pollicis longus, medial to the dorsal tubercle of the radius. It uses the tubercle as a trochlea (pulley) to change its line of pull as it proceeds to the base of the distal phalanx of the thumb. The gap created between the long extensor tendons of the thumb is the anatomical snuff box (Fig. 3.65). In addition to its main actions (Table 3.11), the EPL also adducts the extended thumb and rotates it laterally. To test the extensor pollicis longus, the thumb is extended against resistance at the interphalangeal joint. If the EPL is acting normally, the tendon of the muscle can be seen and palpated on the medial side of the anatomical snuff box. The tendons of the APL and EPB bound the anatomical snuff box anteriorly, and the tendon of the EPL bounds it posteriorly (Figs. 3.61, 3.62, and 3.65). The snuff box is visible when the thumb is fully extended; this draws the tendons up and produces a triangular hollow between them. Observe that the radial artery lies in the floor of the snuff box. radial styloid process can be palpated proximally and the base of the 1st metacarpal can be palpated distally in the snuff box. scaphoid and trapezium can be felt in the floor of the snuff box between the radial styloid process and the 1st metacarpal (see the clinical box “Fracture of Scaphoid” and Fig. 3.71). Extensor Indicis. The extensor indicis has a narrow, elongated belly that lies medial to and alongside that of the EPL (Figs. 3.61B and 3.62). This muscle confers independence to the index finger in that the extensor indicis may act alone or together with the extensor digitorum to extend the index finger at the proximal interphalangeal joint, as in pointing. It also helps extend the hand. 572 Arteries of Forearm The main arteries of the forearm are the ulnar and radial arteries, which usually arise opposite the neck of the radius in the inferior part of the cubital fossa as terminal branches of the brachial artery (Fig. 3.66). The named arteries of the forearm are illustrated in Figure 3.67, and their origins and courses are described in Table 3.12. The following discussion provides additional details. 573 574 FIGURE 3.66 Flexor digitorum superficialis and related vasculature. Three muscles of the superficial layer (pronator teres, flexor carpi radialis, and palmaris longus) have been removed, leaving only their attaching ends. The fourth muscle of the layer (the flexor carpi ulnaris) has been retracted medially. The tendinous humeral attachment of the FDS to the medial epicondyle is thick. The linear attachment to the radius, immediately distal to the radial attachments of the supinator and pronator teres, is thin (Table 3.10). The ulnar artery and median nerve pass between the humeral and the radial heads of the FDS. The artery descends obliquely deep to the FDS to join the ulnar nerve, which descends vertically near the medial border of the FDS (exposed here by splitting a fusion of the FDS and FCU). A (proximal) probe is elevating the FDS tendons (and median nerve and persisting median artery). A second (distal) probe is elevating all the remaining structures that cross the wrist (radiocarpal) joint anteriorly. FIGURE 3.67 Arteries of forearm. 575 TABLE 3.12. Arteries of Forearm and Wrist ULNAR ARTERY Pulsations of the ulnar artery can be palpated on the lateral side of the FCU tendon, where it lies anterior to the ulnar head. The ulnar nerve is on the medial side of the ulnar artery. Branches of the ulnar artery arising in the forearm participate in the peri-articular anastomoses of the elbow (Fig. 3.67, palmar view) and supply muscles of the medial and central forearm, the common flexor sheath, and the ulnar and median nerves: The anterior and posterior ulnar recurrent arteries anastomose with the inferior and superior ulnar collateral arteries, respectively, thereby participating in the periarticular arterial anastomoses of the elbow. The anterior and posterior arteries may be present as anterior and posterior branches of a (common) ulnar recurrent artery. The common interosseous artery, a short branch of the ulnar artery, arises in the distal part of the cubital fossa and divides almost immediately into 576  anterior and posterior interosseous arteries. The anterior interosseous artery passes distally, running directly on the anterior aspect of the interosseous membrane with the anterior interosseous nerve, whereas the posterior interosseous artery courses between the superficial and the deep layers of the extensor muscles in the company of the posterior interosseous nerve. The relatively small posterior interosseous artery is the principal artery serving the structures of the middle third of the posterior compartment. Thus, it is mostly exhausted in the distal forearm and is replaced by the anterior interosseous artery, which pierces the interosseous membrane near the proximal border of the pronator quadratus. Unnamed muscular branches of the ulnar artery supply muscles on the medial side of the forearm, mainly those in the flexor–pronator group. RADIAL ARTERY The pulsations of the radial artery can be felt throughout the forearm, making it useful as an anterolateral demarcation of the flexor and extensor compartments of the forearm. When the brachioradialis is pulled laterally, the entire length of the artery is visible (Figs. 3.66 and 3.67; Table 3.12). The radial artery lies on muscle until it reaches the distal part of the forearm. Here it lies on the anterior surface of the radius and is only covered by skin and fascia, making this an ideal location for checking the radial pulse. The course of the radial artery in the forearm is represented by a line joining the midpoint of the cubital fossa to a point just medial to the radial styloid process. The radial artery leaves the forearm by winding around the lateral aspect of the wrist and crosses the floor of the anatomical snuff box (Figs. 3.65 and 3.66). The radial recurrent artery participates in the peri-articular arterial anastomoses around the elbow by anastomosing with the radial collateral artery, a branch of the profunda brachii artery. The palmar and dorsal carpal branches of the radial artery participate in the peri-articular arterial anastomosis around the wrist by anastomosing with the corresponding branches of the ulnar artery and terminal branches of the anterior and posterior interosseous arteries, forming the palmar and dorsal carpal arches. The unnamed muscular branches of the radial artery supply muscles in the adjacent (anterolateral) aspects of both the flexor and extensor compartments 577  because the radial artery runs along (and demarcates) the anterolateral boundary between the compartments. Veins of Forearm In the forearm, as in the arm, there are superficial and deep veins. The superficial veins ascend in the subcutaneous tissue. The deep veins accompany the deep arteries of the forearm. SUPERFICIAL VEINS The pattern, common variations, and clinical significance of the superficial veins of the upper limb were discussed earlier in this chapter. DEEP VEINS Deep veins accompanying arteries are plentiful in the forearm (Fig. 3.68). These accompanying veins (L. venae comitantes) arise from the anastomosing deep venous palmar arch in the hand. From the lateral side of the arch, paired radial veins arise and accompany the radial artery. From the medial side, paired ulnar veins arise and accompany the ulnar artery. The veins accompanying each artery anastomose freely with each other. The radial and ulnar veins drain the forearm but carry relatively little blood from the hand. 578 FIGURE 3.68 Deep venous drainage of upper limb. The deep veins ascend in the forearm along the sides of the corresponding arteries, receiving tributaries from veins leaving the muscles with which they are 579 related. Deep veins communicate with the superficial veins. The deep interosseous veins, which accompany the interosseous arteries, unite with the accompanying veins of the radial and ulnar arteries. In the cubital fossa, the deep veins are connected to the median cubital vein, a superficial vein (Fig. 3.55B). These deep cubital veins also unite with the accompanying veins of the brachial artery. Nerves of Forearm The nerves of the forearm are the median, ulnar, and radial. The median nerve is the principal nerve of the anterior (flexor–pronator) compartment of the forearm (Figs. 3.57B and 3.69A). Although the radial nerve appears in the cubital region, it soon enters the posterior (extensor–supinator) compartment of the forearm. Besides the cutaneous branches, there are only two nerves of the anterior aspect of the forearm: the median and ulnar nerves. The named nerves of the forearm are illustrated in Figure 3.69, and their origins and courses are described in Table 3.13. The following sections provide additional details and discuss unnamed branches. 580 FIGURE 3.69 Nerves of forearm. Abd. poll. longus, abductor pollicis longus; ext. indicis, extensor indicis; ext. poll. brevis, extensor pollicis brevis; ext. poll. longus, extensor pollicis longus. TABLE 3.13. Nerves of Forearm MEDIAN NERVE IN FOREARM The median nerve is the principal nerve of the anterior compartment of the forearm (Figs. 3.69A and 3.70; Table 3.13). It supplies muscular branches directly to the muscles of the superficial and intermediate layers of forearm flexors (except the FCU) and deep muscles (except for the medial [ulnar] half of the FDP) via its branch, the anterior interosseous nerve. 581 FIGURE 3.70 Neurovascular structures in anterior aspect of forearm and wrist. A. At the elbow, the brachial artery lies between the biceps tendon and the median nerve. It bifurcates into the radial and ulnar arteries. In the forearm, the radial artery courses between the extensor and the flexor muscle groups. B. Deep dissection of the distal part of the forearm and proximal part of the hand showing the course of the arteries and nerves. The median nerve has no branches in the arm other than small twigs to the brachial artery. Its major branch in the forearm is the anterior interosseous nerve (Fig. 3.69A; Table 3.13). In addition, the following unnamed branches of the median nerve arise in the forearm: Articular branches. These pass to the elbow joint as the median nerve passes it. Muscular branches. The nerve to the pronator teres usually arises at the elbow and enters the lateral border of the muscle. A broad bundle of nerves 582  pierces the superficial flexor group of muscles and innervates the FCR, palmaris longus, and FDS. Anterior interosseous nerve. This branch runs distally on the interosseous membrane with the anterior interosseous branch of the ulnar artery. After supplying the deep forearm flexors (except the ulnar part of the FDP, which sends tendons to 4th and 5th fingers), it passes deep to and supplies the pronator quadratus. It then ends by sending articular branches to the wrist joint. Palmar cutaneous branch of the median nerve. This branch arises in the forearm, just proximal to the flexor retinaculum, but is distributed to skin of the central part of the palm. ULNAR NERVE IN FOREARM Like the median nerve, the ulnar nerve does not give rise to branches during its passage through the arm. In the forearm, it supplies only one and a half muscles, the FCU (as it enters the forearm by passing between its two heads of proximal attachment) and the ulnar part of the FDP, which sends tendons to the 4th and 5th digits (Fig. 3.69B; Table 3.13). The ulnar nerve and artery emerge from beneath the FCU tendon and become superficial just proximal to the wrist. They pass superficial to the flexor retinaculum and enter the hand by passing through a groove between the pisiform and the hook of the hamate. A band of fibrous tissue from the flexor retinaculum bridges the groove to form the small ulnar canal (Guyon canal) (Fig. 3.70B). The branches of the ulnar nerve arising in the forearm include unnamed muscular and articular branches, and cutaneous branches that pass to the hand: Articular branches pass to the elbow joint while the nerve is between the olecranon and the medial epicondyle. Muscular branches supply the FCU and the medial half of the FDP. The palmar and dorsal cutaneous branches arise from the ulnar nerve in the forearm, but their sensory fibers are distributed to the skin of the hand. RADIAL NERVE IN FOREARM Unlike the medial and ulnar nerves, the radial nerve serves motor and sensory functions in both the arm and forearm (but only sensory functions in the hand). However, its sensory and motor fibers are distributed in the forearm by two 583 separate branches, the superficial (sensory or cutaneous) and deep radial/posterior interosseous nerve (motor) (Fig. 3.69C, D; Table 3.13). It divides into these terminal branches as it appears in the cubital fossa, anterior to the lateral epicondyle of the humerus, between the brachialis and brachioradialis (Fig. 3.64). The two branches immediately part company, the deep branch winding laterally around the radius, piercing the supinator en route to the posterior compartment. The posterior cutaneous nerve of the forearm arises from the radial nerve in the posterior compartment of the arm, as it runs along the radial groove of the humerus. Thus, it reaches the forearm independent of the radial nerve, descending in the subcutaneous tissue of the posterior aspect of the forearm to the wrist, supplying the skin (Fig. 3.69D). The superficial branch of the radial nerve is also a cutaneous nerve, but it gives rise to articular branches as well. It is distributed to skin on the dorsum of the hand and to a number of joints in the hand, branching soon after it emerges from the overlying brachioradialis and crosses the roof of the anatomical snuff box (Fig. 3.65). The deep branch of the radial nerve, after it pierces the supinator, runs in the fascial plane between superficial and deep extensor muscles in close proximity to the posterior interosseous artery. This part of the nerve is usually referred to as the posterior interosseous nerve (Figs. 3.64 and 3.69C). It supplies motor innervation to all the muscles with fleshy bellies located entirely in the posterior compartment of the forearm (distal to the lateral epicondyle of the humerus). LATERAL AND MEDIAL CUTANEOUS NERVES OF FOREARM The lateral cutaneous nerve of the forearm (lateral antebrachial cutaneous nerve) is the continuation of the musculocutaneous nerve after its motor branches have all been given off to the muscles of the anterior compartment of the arm. The medial cutaneous nerve of the forearm (medial antebrachial cutaneous nerve) is an independent branch of the medial cord of the brachial plexus. With the posterior cutaneous nerve of the forearm from the radial nerve, each supplying the area of skin indicated by its name, these three nerves provide all the cutaneous innervation of the forearm (Fig. 3.69D). There is no “anterior cutaneous nerve of the forearm.” (Memory device: This is similar to the 584 brachial plexus, which has lateral, medial, and posterior cords but no anterior cord.) Although the arteries, veins, and nerves of the forearm have been considered separately, it is important to place them into their anatomical context. Except for the superficial veins, which often course independently in the subcutaneous tissue, these neurovascular structures usually exist as components of neurovascular bundles. These bundles are composed of arteries, veins (in the limbs, usually in the form of accompanying veins), and nerves as well as lymphatic vessels, which are usually surrounded by a neurovascular sheath of varying density. Surface Anatomy of Forearm Three bony landmarks are easily palpated at the elbow: the medial and lateral epicondyles of the humerus and the olecranon of the ulna (Fig. 3.71). In the hollow located posterolaterally when the forearm is extended, the head of the radius can be palpated distal to the lateral epicondyle. Supinate and pronate your forearm and feel the movement of the radial head. The posterior border of the ulna is subcutaneous and can be palpated distally from the olecranon along the entire length of the bone. This landmark demarcates the posteromedial boundary separating the flexor–pronator (anterior) and extensor–supinator (posterior) compartments of the forearm. 585 FIGURE 3.71 Surface anatomy of posterior forearm. The cubital fossa, the triangular hollow area on the anterior surface of the elbow, is bounded medially by the prominence formed by the flexor–pronator group of muscles that are attached to the medial epicondyle. To estimate the position of these muscles, put your thumb posterior to your medial epicondyle, and then place your fingers on your forearm as shown in Figure 3.72A. The black dot on the dorsum of the hand indicates the position of the medial epicondyle. 586