Bones and Joints
The elbow is the junction of the distal humerus and the two bones of the forearm, the radius and ulna (Fig. 9).
Unlike the knee, at which the femur makes contact with only one of the two distal bones (the tibia but not the fibula), in the elbow joint, the humerus makes contact with bone at the ulna (a joint built for elbow flexion and extension) and the radius (a joint that allows forearm rotation). The distal humerus flares medially and laterally, forming two condyles. The condyles are covered with articular cartilage and serve as the contact points for the joint. The smaller prominences just proximal to the joint are the epicondyles. These are not cov- ered with cartilage but rather serve as the points of origin for many forearm muscles. The medial epicondyle is the site of attachment of the flexor-pronator muscle group of the forearm as well as the ulnar collateral ligament. The lateral epicondyle is the origin of the extensor muscles of the forearm.
The elbow is made up of two joints. The first, the humeroulnar joint, is hinge-like and allows flexion and extension. This joint is formed by the trochlea on the medial surface of the distal humerus and the proximal ulna. The ulna has a coronoid process anteriorly and an olecranon process posteriorly; these grasp the humeral trochlea front and back. Lateral to the trochlea is the second joint in the elbow, mating the capitellum of the humerus to the radial head. The radial head is discoid and the capitellum is hemispherical, allowing pronation and supination of the forearm. Since the radius is intimately bound to the ulna, a so-called proximal radioulnar joint is formed.
Ligaments and Soft Tissues
The bony congruity between the trochlea and the ulna provides significant stability to the elbow. This joint is nonetheless supplement- ed by the ligaments and capsule (Fig. 10).
The ulnar (or medial) collateral ligament is the primary stabilizer of the medial side of the elbow. This ligament is particularly important in the throwing athlete, who places significant forces on the elbow when cocking the arm. The radial (or lateral) collateral ligament supports the lateral side of the elbow.
The joint between the radial head and the capitellum lacks bony stability. This joint is therefore stabilized by the annular ligament. This structure, as its name implies, is a ring that surrounds the radial neck just distal to the radial head. It attaches the radius to the ulna—but since the ulna is firmly attached to the humerus, it also stabilizes the radius to the humerus.
Most of the muscles that cross the elbow joint originate from one of the two epi- condyles. Muscles on the dorsum of the fore- arm originate from the lateral epicondyle, whereas those on the volar surface originate from the medial epicondyle. The volar mus- cles are primarily flexors (Fig. 11)
and the dorsal muscles are extensors (Fig. 12).
There are three muscles that cross the elbow but originate proximal to the epicondyle. The bi- ceps originates from two points on the scapula and proximal humerus and courses on the anterior surface of the arm. Its tendon crosses the anterior aspect of the elbow joint and inserts on the proximal radius. As such, it both flexes the elbow and supinates the forearm. The brachialis muscle originates from the anterior humerus and inserts on the proximal ulna, providing elbow flexion. The triceps brachii originates from both the scapula and the posterior humerus, courses posterior to the elbow joint, and inserts on the olecranon process of the ulna, providing elbow extension.
The medial epicondyle is the origin of the “superficial” flexor muscle group. This group includes the pronator teres, the flexor carpi radialis, the palmaris longus, the flexor carpi ulnaris, and the flexor digitorum superficialis, all of which provide some stability to the medial elbow. The pronator teres allows for forearm pronation and elbow flexion. The others flex the wrist, with the exception of the flexor digitorum superficialis, whose action is on the proximal interphalangeal (fin- ger) joint. The median nerve innervates all of the superficial flexor muscles, except for the flexor carpi ulnaris, which is supplied by the ulnar nerve. The “deep” flexors of the forearm originate in the medial forearm, most of them from the ulna. This group includes the flexor digitorum profundus, the flexor pollicis longus, and the pronator quadratus, all of which are supplied by the anterior interosseous branch of the median nerve, except for the ulnar half of the flexor digitorum profundus, which is supplied by the ulnar nerve.
From the lateral epicondyle arise the ex- tensors of the wrist and hand. The superficial group includes the brachioradialis, the extensor carpi radialis longus, and the extensor carpi radialis brevis. These three muscles are known as the mobile wad of three—a name that makes sense if you pinch and shake them. The brachioradialis attaches to the distal radius and flexes the elbow; the extensor carpi radialis longus and the extensor carpi radialis brevis insert on the index and middle metacarpals, respectively, and extend the wrist. All receive innervation from the radial nerve. The lateral epicondyle is the point of origin for three other superfi- cial extensors: the extensor digitorum, the extensor digiti minimi, and the extensor carpi ulnaris. The extensor carpi ulnaris attaches to the fifth metacarpal and extends the wrist. (The insertions of the finger extensors are somewhat complex and are discussed in the hand section below.) The posterior inter- osseous nerve, a branch of the radial nerve, innervates these three superficial extensor muscles as well as the deep extensors below them. The deep extensors include the supina- tor, three muscles to the thumb (the abductor pollicis longus, the extensor pollicis longus, and the extensor pollicis brevis), and the ex- tensor indicis proprius.
Nerves and Blood Vessels
The median nerve crosses the anterior elbow, superficial to the brachialis muscle and medial to the brachial artery. The median nerve supplies the superficial flexors and travels through the carpal tunnel into the hand. In the forearm, the median nerve gives off the anterior interosseous nerve, which supplies all of the deep flexors, except the ulnar half of the flexor digitorum profundus. The ulnar nerve crosses the elbow joint just posterior to the medial epicondyle, a common site of nerve compression and irritation. When the medial elbow is hit, the superficial placement of the ulnar nerve causes it to absorb some of that energy, resulting in paresthesias (colloquially termed “hitting the funny bone”).
In the forearm, the ulnar nerve supplies the flexor carpi ulnaris and the ulnar half of the flexor digitorum profundus. Its main targets are in the hand, however. The ulnar nerve reaches the hand by way of Guyon’s canal at the wrist. It supplies the hypothenar muscles, most of the intrinsics, and the adductor pollicis. The radial nerve crosses the elbow anterior to the lateral epicondyle, where it gives off the posterior interosseous nerve. The radial nerve itself supplies the mobile wad of three. The posterior interosseous branch of the radial nerve feeds all extensor muscles distal to the mobile wad of three.
The brachial artery travels on the anterior surface of the humerus. It crosses the elbow anteriorly, where, at the level of the neck of the radius, it bifurcates into the ulnar and radial arteries. It also gives off recurrent branches that course back toward the arm both medially and laterally. The radial artery runs on the dorsum of the forearm under the brachioradialis muscle and terminates in the deep palmar arch in the hand. The ulnar artery runs on the volar surface under the su- perficial flexor muscles and terminates in the hand as the superficial palmar arch.