Chapter 31. Arm

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Big Picture

The arm (brachium) consists of the humerus, which articulates distally with the forearm (antebrachium) through the elbow complex. The elbow complex consists of three bones: humerus, ulna, and radius. The articulations of these bones result in three separate joints that share a common synovial cavity, enabling the forearm to flex, extend, pronate, and supinate on the humerus.

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Actions of the Elbow Complex

The articulations of the humerus, radius, and ulna in the elbow result in the following actions:

• Flexion and extension (Figure 31-1A)
  • Humeroulnar joint. Articulation between the trochlear notch of the ulna and the trochlea of the humerus.
  • Humeroradial joint. Articulation between the head of the radius and the capitulum of the humerus.
• Pronation and supination (Figure 31-1B and C)
  • Proximal radioulnar joint. Articulation between the head of the radius and the radial notch of the ulna.

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Big Picture

The muscles of the arm are divided by their fascial compartments (anterior and posterior), and may cross one or more joints. Identifying the joints that the muscles cross and the side on which they cross can provide useful insight into the actions of these muscles (Table 31-1).

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Muscles of the Anterior Compartment of the Arm

The muscles in the anterior compartment of the arm are primarily flexors (of the shoulder or elbow or both) because of their anterior orientation (Figure 31-1D). The musculocutaneous nerve (C5–C7) innervates the muscles in the anterior compartment of the arm. However, each muscle does not necessarily receive each spinal nerve level between C5 and C7. The following muscles are located in the anterior compartment of the arm:

• Coracobrachialis muscle. Attaches between the coracoid process of the scapula and the midshaft of the humerus. The coracobrachialis muscle crosses anteriorly to the glenohumeral joint and, therefore, contributes to shoulder flexion. It receives its innervation from the musculocutaneous nerve (C5–C7) and its blood supply via branches of the axillary artery.
• Brachialis muscle. Attaches between the anterior aspect of the humerus and the coronoid process and the tuberosity of the ulna, crossing the anterior elbow joint. The brachialis muscle acts on the ulna (humeroulnar joint), and therefore, it produces flexion of the elbow. As with the other muscles in the anterior compartment of the arm, the musculocutaneous nerve (C5–C6) provides innervation. However, the radial nerve (C7) innervates a small, lateral portion of the muscle. Blood is supplied to the muscle by branches from the brachial artery.
• Biceps brachii muscle. Consists of two heads that attach to the supraglenoid tubercle (long head) and the coracoid process (short head). The biceps brachii muscle converges to insert on the radial tuberosity. The biceps brachii crosses anterior to the glenohumeral joint and the elbow, primarily producing flexion in both joints. Because the distal attachment is to the radius, the biceps brachii will also produce supination due to movement of the radioulnar joints. The biceps brachii receives its innervation from the musculocutaneous nerve (C5–C6) and its blood supply from branches originating from the brachial artery.

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Muscles of the Posterior Compartment of the Arm

The muscles in the posterior compartment of the arm are primarily extensors of the shoulder and elbow because of their posterior orientation (Figure 31-1E). The radial nerve (C6–C8) innervates the muscle in the posterior compartment of the arm.

• Triceps brachii muscle. Consists of three heads. The long head attaches to the infraglenoid tubercle of the scapula, and the medial and lateral heads attach to the posterior humerus. The three heads converge to attach to the olecranon process of the ulna. The long head produces shoulder extension and elbow extension. The other two heads produce elbow extension only. The radial nerve (C6–C8) provides innervation to the muscle, and the profunda brachii and superior ulnar collateral arteries provide most of the blood supply to the triceps brachii muscle.

Biceps tendinopathy is a condition that results most frequently in painful sensations in the anterior region of the shoulder. The long head of the biceps is often irritated by overhead motions and excessive or repetitive lifting, resulting in inflammation of the tendon and the peripheral structures. The pain is most frequently described as occurring between the greater and lesser tubercles of the humerus in the intertubercular groove, where the long head travels. The condition is exacerbated by the actions of the biceps (e.g., shoulder flexion, elbow flexion, and supination) and is frequently confused with pathologies of the rotator cuff.

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Big Picture

Innervation to the anterior and posterior compartments of the arm originates from the lateral and posterior cord, giving rise to the musculocutaneous and radial nerves, respectively. Both nerves are mixed and provide motor and sensory innervation.

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Musculocutaneous Nerve

The musculocutaneous nerve pierces the coracobrachialis muscle, innervating it as it passes, and descends through the arm between the biceps brachii and brachialis muscles, supplying both muscles. The musculocutaneous nerve pierces the deep fascia just distal to the elbow to become the lateral cutaneous nerve of the forearm (Figure 31-2A).

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Radial Nerve

The radial nerve descends posterior to the humerus with the deep artery of the arm (profundus brachii), supplying motor innervation to the triceps brachii (Figure 31-2B). It provides cutaneous innervation via the posterior cutaneous nerve of the arm, the inferior lateral cutaneous nerve of the arm, and the posterior cutaneous nerve of the forearm. The radial nerve pierces the intermuscular septum laterally, anteriorly to the lateral epicondyle between the brachialis and the brachioradialis. It then descends to the posterior compartment of the forearm, providing motor innervation, and to the dorsum of the hand, providing cutaneous innervation.

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Medial Cutaneous Nerve of the Arm

The medial cutaneous nerve of the arm branches from the medial cord and, as its name implies, supplies the anteromedial skin of arm.

Radial neuropathy (also known as “Saturday night palsy”) is a condition caused by compression of the radial nerve on the posterior aspect of the humerus, where it spirals around. The compression usually is caused when an intoxicated person falls asleep with the posterior arm being compressed by the edge of a desk, bar, chair, or bench. Although the injury occurs at the posterior humerus, symptoms are identified in the forearm. Symptoms include the inability to extend the wrist and fingers (wrist drop) and loss of sensation to the posterior portion of the hand. In contrast, radial neuropathy occurring in the axillary region due to the improper use of crutches will result in weakness in the triceps muscles because the injury is more proximal.

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Big Picture

The blood supply to the arm is initiated from the axillary artery. The axillary artery becomes the brachial artery after crossing the inferior border of the teres major muscle.

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Brachial Artery

The brachial artery courses through the medial side of the anterior compartment of the arm, supplying the muscles of the anterior compartment (Figure 31-2C). This is accomplished through the following arteries:

• Deep artery of arm (profunda brachii artery, deep brachial artery). Descends, with the radial nerve, posterior to the humerus and through the radial groove to supply the triceps brachii muscle (Figure 31-2D). The deep artery of the arm bifurcates midarm into the radial and middle collateral arteries. Often, an anastomosis forms with the posterior circumflex humeral artery.
  • Radial collateral artery. Courses anteriorly to the lateral epicondyle of the humerus, where it forms an anastomosis with the radial recurrent artery from the forearm.
  • Middle collateral artery. Courses along the posterior humerus, where it forms an anastomosis with the recurrent interosseous artery from the forearm.
• Superior ulnar collateral artery. Courses posteriorly to the medial epicondyle of the humerus, where it forms an anastomosis with the posterior ulnar recurrent artery from the forearm.
• Inferior ulnar collateral artery. Bifurcates around the medial epicondyle of the humerus, where it forms an anastomosis with the anterior recurrent ulnar artery and the middle collateral artery from the forearm.

The brachial artery terminates anteriorly to the elbow, where it bifurcates into the radial and ulnar arteries.

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Veins of the Arm

The veins in the arm consist of a superficial and a deep venous system. The superficial venous system consists of the basilic vein, which is located medially, and the cephalic vein, which is located laterally. The deep venous system may consist of two or three veins that course with each artery. Most veins in the arm drain into the axillary vein.

A humeral fracture is a common fracture of the upper limb and is often caused by a fall. Most humeral fractures heal without surgical intervention; however, depending on the severity of the injury, surgical intervention may be necessary. A patient with a humeral fracture may occasionally develop additional damage to peripheral structures such as the radial nerve. Because of the proximity of the radial nerve to the humerus, the radial nerve may be involved in a displaced humeral fracture, resulting in transient or permanent damage to the radial nerve.

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Big Picture

The elbow consists of articulations between the humerus, radius, and ulna, and provides flexion, extension, supination, and pronation of the forearm.

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Elbow Complex

The elbow is composed of the following joints (Figure 31-3A):

• Humeroulnar joint. Synovial hinge joint producing flexion and extension. The boney articulations include the trochlea of the humerus and the trochlear notch of the ulna.
• Humeroradial joint. Synovial gliding joint that works in conjunction with the humeroulnar joint to produce flexion and extension. Boney articulations include the capitulum of the humerus and the head of the radius.
• Proximal radioulnar joint. Synovial pivot joint that is mechanically linked with the distal radioulnar joint and produces supination and pronation. The boney articulations include the head of the radius and the radial notch of the ulna. The proximal radioulnar joint is interdependent with the distal radioulnar joint and will be discussed in greater detail in Chapter 32.

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Ligamentous and Capsular Support of the Elbow

The following ligaments and the capsule provide support to the elbow joint (Figure 31-3B–D):

• Capsule. The humeroulnar, humeroradial, and proximal radioulnar joints are enclosed in a single capsule. The capsule is loose, which accommodates the high degree of motion; however, the capsule is reinforced with ligaments.
• Ligaments. Most hinge joints in the body, including the humeroulnar joint, contain medial and lateral collateral ligaments to enhance medial and lateral stability.
  • Ulnar collateral (medial collateral). Fibers run from the medial epicondyle of the humerus to the proximal ulna, providing medial stability of the elbow complex.
  • Radial collateral (lateral collateral). Fibers run from the lateral epicondyle to the annular ligament and the olecranon process of the ulna, providing lateral stability of the elbow complex.
  • Annular ligament. The annular ligament is a circular ligament that is attached to the anterior and posterior surfaces of the radial notch of the ulna and forms a ring that encompasses the radial head. The inner surface of the ligament is covered with cartilage and provides stability for the humeroulnar joint while allowing supination and pronation.

Ligaments as well as muscles reinforce the elbow complex. However, a sufficient longitudinal force on the radius may result in the head of the radius being pulled through the annular ligament, an injury known as distraction injury (or more commonly as “nursemaid's elbow”). This injury occurs most frequently in small children and usually when a parent unexpectedly lifts a child by the arm.

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Olecranon Bursa

The most important bursa associated with the elbow complex is the olecranon bursa, which is located between the capsule of the elbow complex and the triceps tendon. The olecranon bursa diminishes friction between the two surfaces as they cross over each other.