Chapter 9: The arm and elbow
Muscles of arm (table 9-1)
The muscles of the anterior arm are the biceps, coracobrachialis, and brachialis. They are supplied by the musculocutaneous nerve. The triceps is the muscle of the posterior arm, and it is supplied by the radial nerve. The anterior and posterior muscles are separated from each other by lateral and medial intermuscular septa (fig. 9-1).
The biceps brachii arises from the scapula by two heads. The long, or lateral, head, arising from the supraglenoid tubercle, descends within the capsule of the shoulder joint and lies in the intertubercular groove. The short, or medial, head arises from the coracoid process in common with the coracobrachialis. The bicipital tendons give way to muscular bellies that unite and continue as a readily palpable tendon. The insertion is into the tuberosity of the radius (its posterior part; hence the biceps can act as a supinator) and the fascia of the forearm (and ultimately the ulna) by means of the bicipital aponeurosis (see fig. 9-4). The biceps and brachialis are the chief flexors of the forearm. The origin of the brachialis embraces the insertion of the deltoid. The coracobrachialis is generally pierced by the musculocutaneous nerve.
The flexion of the forearm (or the twitch of the muscle without movement) that follows tapping of the tendon of insertion of the biceps is known as the biceps reflex. The reflex center is in segments C5 and 6 of the spinal cord.
The triceps brachii forms the bulk of the posterior arm. Its three heads are arranged in two planes (fig. 9-2): the long and lateral heads occupy a superficial plane, whereas the medial head is deeper. The radial nerve passes between the long and medial heads and then lies on the humerus under cover of the lateral head. The long head separates the triangular from the quadrangular space and the teres major from the teres minor. The triceps is the extensor of the forearm and takes part in pushing, throwing, hammering, and shoveling. A subcutaneous bursa over the olecranon and tendon of the triceps may become thickened ("miner's elbow").
The extension of the forearm (or the twitch without movement) that follows tapping of the tendon of insertion of the triceps is known as the triceps reflex. The reflex center is in segments C6 and 7 of the spinal cord.
Nerves of arm
The muscles of the anterior arm are supplied by the musculocutaneous nerve; the triceps is supplied by the radial nerve (see fig. 9-1).
The musculocutaneous nerve (from the lateral cord) usually pierces the coracobrachialis and descends between the biceps and brachialis (see fig. 9-1) to enter the lateral side of the arm. The musculocutaneous nerve supplies the biceps, coracobrachialis, brachialis, and elbow joint and terminates as the lateral antebrachial cutaneous nerve.
The radial nerve (the continuation of the posterior cord) passes posteriorly with the profunda brachii artery, winds around the humerus (fig. 9-2) under cover of the long and lateral heads of the triceps, and descends anteriorly into the cubital fossa, where it lies in a deep groove between the brachioradialis and the brachialis (figs. 9-3 and 9-4). At, or just inferior to the level of the lateral epicondyle, the radial nerve divides into superficial and deep branches. The nerve has several cutaneous branches (posterior brachial, lower lateral brachial, posterior antebrachial) and muscular twigs to the triceps, anconeus, brachioradialis, extensor carpi radialis longus and usually brevis, and branches to the elbow joint. The superficial branch is the direct continuation of the radial nerve into the forearm. * The deep branch winds laterally around the radius between the layers of the supinator and continues as the posterior interosseous nerve to the muscles of the posterior forearm.
The median and ulnar nerves have no branches in the arm and are in transit to the forearm. The median nerve, which is originally on the lateral side of the brachial artery, crosses anterior to (occasionally behind) that vessel and then descends on its medial aspect. In the cubital fossa (fig. 9-4B), it lies deep to the median cubital vein and the bicipital aponeurosis and gives a branch to the elbow joint. It enters the forearm between the heads of the pronator teres. The ulnar nerve, originally medial to the brachial artery, proceeds posteriorward to the posterior aspect of the medial epicondyle. It enters the forearm between the heads of the flexor carpi ulnaris and under a tendonous arch formed between these heads (the cubital tunnel).
Arteries of arm
At the inferior border of the teres major, i.e., the distal limit of the posterior axillary fold, the axillary becomes the brachial artery (figs. 7-3 and 9-4). The brachial artery lies superficially on the medial side of the arm, at first medial to the humerus and then anterior to the bone. Hence the artery can be compressed against the humerus. The pulsations of the artery can be palpated where it is partly overlapped by the biceps and coracobrachialis. This vessel is used in sphygmomanometry. The brachial artery lies successively on the triceps and brachialis and is crossed (usually anteriorly) by the median nerve. At the elbow the artery lies in the middle of the cubital fossa, between the biceps tendon laterally and the median nerve medially (fig. 9-4B). It is crossed by the bicipital aponeurosis, which separates it from the median cubital vein. Opposite the neck of the radius, it divides into the radial and ulnar arteries, but variations, including high division (in the upper third of the arm), are common.
In addition to muscular and nutrient branches, the brachial artery gives off the profunda brachii artery and one or more ulnar collateral arteries. The profunda brachii crosses posterior to the humerus with the radial nerve and divides into collateral branches on the radial side. These, together with those on the ulnar side, form an extensive anastomosis around the elbow joint, which is completed inferiorly by recurrent branches derived from the radial and ulnar arteries. The existence of the anastomosis is important; the details of the individual branches are not.
Cubital fossa (fig. 9-4)
The brachioradialis laterally and the pronator teres medially form a V-shaped interval known as the cubital fossa. These muscles, which belong to the forearm, arise from the lateral and medial supracondylar ridges of the humerus, respectively, and descend to the radius. The floor of the fossa is formed by the brachialis and by the supinator laterally. The contents include the biceps tendon, brachial artery, and median nerve, from lateral to medial. The artery usually divides at the apex of the fossa. The radial nerve is in a deep groove between the brachioradialis and brachialis. The fossa is roofed by fascia, strengthened by the bicipital aponeurosis, and crossed by the superficial veins. The median cubital vein is frequently used for intravenous injections and blood transfusions. Its close relationship to the underlying brachial artery and median nerve should be kept in mind.
Joints.
Elbow joint.
The humerus, radius, and ulna form a hinge joint, situated 2 or 3 cm inferior to the epicondyles (figs. 6-16, 9-5, and 9-6). The capitulum of the humerus articulates with the upper aspect of the head of the radius (humeroradial joint), and the trochlea of the humerus articulates with the trochlear notch of the ulna (humero-ulnar joint). These two parts of the elbow joint are continuous with each other and share a common cavity with the proximal radio-ulnar joint. Effusions of the elbow joint generally occur posteriorly, as do dislocations, and it is from this aspect that the joint is most easily approached surgically.
The capsule is weak ion the anterior and posterior sides but is strengthened on each side by ligaments. The radial collateral ligament extends fan wise from the lateral epicondyle to the annular ligament. The ulnar collateral ligament runs from the medial epicondyle to the coronoid process and the olecranon. The elbow joint is supplied by adjacent nerves.
Proximal Radio-ulnar Joint.
The circumference of the head of the radius fits into the radial notch of the ulna to form a pivot joint. It is surrounded by the strong anular ligament, which is attached to the anterior and posterior margins of the notch. In children, the head of the radius may be subluxated through the anular ligament by a sudden jerk on the limb. Some fibers extend from the lower margin of the notch to the neck of the radius (quadrate ligament). The synovial membrane is continuous with that of the elbow joint.
Movements at Elbow and Proximal Radioulnar Joints.
The elbow is a hinge joint; hence movement is limited to flexion and extension. Owing to the shape of the medial part of the trochlea, however, the supinated forearm makes a "carrying angle" with the arm during extension.
The term "supination" is used for the position of the forearm and hand when the palm faces anterior, as in the anatomical position. The term "pronation" is used when the palm faces posterior. These terms are used also for the movements that bring about these positions. Supination and pronation are considerably more complicated than a mere lateral and medial rotation. The axis of movement extends from the middle of the head of the radius to the lower end of the ulna. Although the head of the radius merely rotates within the annular ligament, its inferior end describes an arc around the lower end of the ulna and carries the hand with it. In pronation, the shafts of the radius and ulna cross each other (the main reason for selecting supination to be the anatomical position). The movements are usually accompanied by rotation of the humerus (minimal when the elbow is flexed), and the ulna does not remain fixed. Supination has been thought to be generally stronger than pronation, and the threads of screws are arranged to take advantage of this (for right-handed people).
Flexion is controlled by segments C5 and 6 of the spinal cord, pronation and supination by C6, and extension by C7 and 8. The flexors of the forearm are the brachialis, biceps, and brachioradialis. The extensor is the triceps, particularly the medial head. The pronators are the pronator quadratus and the pronator teres. The supinators are the supinator and the biceps.
QUESTIONS
9-1 What are the two main compartments of the arm?
9-2 What are the chief actions of the biceps brachii?
9-3 Which spinal segments are associated with (a) flexion and (b) extension at the elbow?
9-4 Where is the ulnar nerve in contact with the humerus?
9-5 Between which muscles, or heads of muscles, do the radial, median, and ulnar nerves enter the forearm?
9-6 Which important structures lie medial to the biceps tendon in the cubital fossa?
9-7 Under which conditions is subluxation of the head of the radius found?
9-8 Look up the origin of the word cubital.
Figure legends
Figure 9-1 Horizontal section through the middle of the arm. In A, the nerves and vessels are identified. B, shows the flexor muscles (supplied by the musculocutaneous nerve) anterior and the extensors (supplied by the radial nerve) posterior. B.B., biceps brachii; Br., brachialis. The three heads of the triceps are indicated.
Figure 9-2 A, The triangular and quadrangular spaces, separated by the long head of the triceps. The subscapular and posterior circumflex humeral arteries, respectively, pass through these spaces, and the axillary nerve traverses the quadrangular space. The spine of the scapula has been cut. B and C show the superficial and deep planes of the triceps.
Figure 9-3 Horizontal sections through the upper and lower parts of the arm.
Figure 9-4 The cubital fossa. A, The superficial nerves and veins. B, The contents of the cubital fossa.
Figure 9-5 Schematic sagittal section through the humero-ulnar part of the elbow joint. The width of the joint cavity is exaggerated.
Figure 9-6
ligaments of right elbow joint.
* There is a small and functionally
insignificant branch to the
brachialis muscle as well.