The anterior quadrant contains the biceps muscle and tendon, the radial and median nerves, and the anterior aspects of the radiocapitellar and humeroulnar joints. Scanning is performed with the elbow extended and supinated (Fig. 4.1). The transducer is placed transversely across the distal arm. The pulsating brachial artery is midline and is a major landmark. Lateral to the brachial artery is the distal biceps tendon, and medial to the brachial artery is the median nerve. The biceps aponeurosis (also called the lacertus fibrosus) is a fascial layer that extends from the flexor pronator muscle group to the biceps tendon (Figs. 4.2 and 4.3).

As the transducer slides distally, the anterior aspects of the radiocapitellar and humeroulnar joints come into view. The curvilinear bony surfaces are covered by anechoic articular cartilage. The radial nerve is visible deep to the brachioradialis muscle at this level (Fig. 4.3). As the transducer moves distally, the deep branch of the radial nerve can be identified as it enters the supinator muscle between the superficial and deep muscle heads (Fig. 4.4). Rotating the transducer 90 degrees over either the radial or median nerves demonstrates the hypoechoic fibrillar appearance of the nerve in long axis (Fig. 4.5).

The radial nerve is the terminal branch of the posterior cord of the brachial plexus (C5-8, T1). In the arm it courses posterior to the humerus in the spiral groove, and pierces the intermuscular septum in the distal arm to lie anterior to the lateral condyle of the humerus. The radial nerve supplies the triceps, anconeus, brachioradialis, and the lateral half of the brachialis.

At the level of the elbow joint, the radial nerve divides into a superficial sensory branch, which courses in the forearm along the deep surface of the brachioradialis, and a deep motor branch, which enters the radial tunnel. The radial tunnel extends from the level of the radiocapitellar joint to the proximal aspect of the supinator,⁴ and is bounded by the joint capsule posteriorly, the
brachialis muscle and biceps tendon medially, and the brachioradialis muscle and the extensor carpi radialis brevis and longus muscles laterally.⁵

At the distal aspect of the radial tunnel, the deep branch pierces the supinator muscle anteriorly between the superficial and deep heads of the supinator (Figs. 4.2 and 4.5). The proximal edge of the superficial head of the supinator is called the Arcade of Frohse and is the most common site of radial nerve entrapment (Fig. 4.2). The deep branch of the radial nerve exits the posterior aspect of the supinator as the posterior interosseous nerve (PIN) and enters the posterior compartment of the forearm. At the elbow, the deep branch supplies the extensor carpi radialis brevis and the supinator. In the posterior compartment of the forearm, the deep branch, now called the PIN, supplies the extensor carpi ulnaris, the extensor digitorum communis, the extensor digiti minimi, the abductor pollicis longus, the extensor pollicis longus and brevis, and the extensor indicis proprius.⁴

The median nerve arises from the medial and lateral cords of the brachial plexus (C6-8, T1) and courses
alongside the brachial artery in the anterior compartment of the arm and at the elbow, passing through the antecubital fossa deep to the biceps aponeurosis (see below) and anterior to the brachialis muscle. The nerve lies between the humeral (superficial) and ulnar (deep) heads of the pronator teres muscle at the level of the elbow joint (Fig. 4.2), and enters the anterior compartment of the forearm by passing beneath the fibrous arch of the heads of the flexor digitorum superficialis (FDS)⁶ (Fig. 4.6). At the elbow and proximal aspect of the forearm the median nerve supplies the pronator teres, flexor carpi radialis, palmaris longus, and FDS.⁷ Within the forearm, the nerve courses between the FDS and FDP.

The anterior interosseous nerve (AIN) arises from the median nerve at the level of the humeral head of the pronator teres, and travels along the anterior aspect of the interosseous membrane of the forearm between the flexor pollicis longus and the flexor digitorum profundus (Fig. 4.7). The AIN is purely motor and supplies the pronator quadratus muscle, the flexor pollicis longus muscle, and the FDP muscle for the index and the middle fingers.⁴,⁸

Longitudinal scanning over the radiocapitellar joint shows the joint space and the thin stripe of anechoic articular cartilage of the capitellum (Fig. 4.8). Sliding the transducer medially demonstrates the coronoid fossa of the distal humerus. This is a good place to look for joint fluid and intra-articular loose bodies.

The distal biceps tendon can be challenging to image because it dives deep in the antecubital fossa to insert on the radial tuberosity and because it does not have a straight sagittal orientation. There are several techniques for scanning the distal biceps:

In long-axis the biceps tendon is of uniform caliber and has the typical echogenic, fibrillar appearance of a tendon. It appears round and well-defined in short-axis scans. Long head and short head components of the distal biceps tendon have been described, with the long tendon being deeper and inserting on the more proximal aspect of the radial tuberosity and the short tendon being more superficial and inserting more distally,¹³ but these two components are usually difficult to separate. The bicipitoradial bursa sits saddle-shaped around the
distal biceps tendon to protect it from osseous friction during pronation of the forearm.¹⁴ It is not normally distended.

The lateral quadrant contains the lateral aspect of the radiocapitellar joint, the common extensor tendon, and the lateral collateral ligament complex that consists of the radial collateral ligament, lateral ulnar collateral ligament, and the annular ligament.¹⁵,¹⁶,¹⁷ With the patient’s elbow flexed 80 to 90 degrees and the forearm on the examination table, the transducer is placed longitudinally on the lateral elbow at the level of the radiocapitellar joint using the bony landmarks of the radial head and lateral epicondyle (Fig. 4.19). The common extensor tendon originates from the lateral epicondyle of the humerus and is comprised of the extensor carpi radialis brevis, extensor carpi ulnaris, extensor digitorum communis, and extensor digiti minimi (Fig. 4.20). The common tendon has a long thin echogenic fibrillar appearance (Fig. 4.21).

The radial collateral ligament complex is composed of the radial collateral ligament, the lateral ulnar collateral ligament, and the annular ligament¹⁵,¹⁶,¹⁷ (Fig. 4.22). The radial collateral ligament lies deep to the common extensor tendon, extending from the undersurface of the lateral epicondyle to blend with the fibers of the annular ligament. It is often difficult to distinguish the radial collateral ligament as a distinct structure from the overlying common extensor tendon (Fig. 4.21).

Short axis scanning over the radial head demonstrates the thin annular ligament. Oblique scanning is necessary to visualize the lateral ulnar collateral ligament, which extends from the undersurface of the lateral epicondyle, passes posterior to the radial head and neck, thus acting as a supporting sling, and inserts on the supinator crest of the ulna.¹⁵,¹⁶

The posterior quadrant contains the triceps tendon, olecranon fossa, and ulnar nerve, and it can be scanned in several ways:

The triceps tendon is a short broad structure with an echogenic fibrillar appearance composed of contributions of the long and lateral heads,¹⁸ and inserts on the superficial aspect of the olecranon process The medial head of the triceps muscle has a separate insertion on the olecranon process, deep to the tendon (Fig. 4.26). With the elbow flexed, the olecranon fossa of the humerus is exposed and can be viewed both in short and long axis (Fig. 4.27).

In short axis, slide the transducer medially, posterior to the medial epicondyle to visualize the ulnar nerve. The ulnar nerve arises from the medial cord of the brachial plexus (C8, T1) and courses along the medial side of the brachial artery in the anterior compartment of the proximal arm. It then passes posterior to the medial epicondyle of the humerus in the cubital tunnel, a fibro-osseous channel formed by the olecranon process
laterally, the posterior cortex of the medial epicondyle medially, the elbow joint capsule and posterior bundle of the medial collateral ligament (MCL) anteriorly, and the Ligament of Osborne (the cubital retinaculum) posteriorly (Fig. 4.28). The nerve exits the cubital tunnel to enter the medial aspect of the forearm between the superficial and deep heads of the flexor carpi ulnaris muscle. At the elbow, the ulnar nerve supplies the flexor carpi ulnaris and the medial half of the FDP.

In the cubital tunnel, the ulnar nerve appears round and hypoechoic, surrounded by echogenic fat in transverse scans, and narrow, linear, and hypoechoic in longitudinal scans. Nerve stability is assessed while scanning transversely during flexion and extension of the elbow.

The medial quadrant contains the common flexor tendon and MCL and is scanned with the elbow extended and the forearm supinated (Fig. 4.29). The common flexor tendon originates from the medial epicondyle of the humerus. It is composed of the flexor-pronator group of muscles: pronator teres, flexor carpi radialis, palmaris longus, and flexor carpi ulnaris (Fig. 4.30).
The common flexor tendon is shorter and broader than the common extensor tendon, and has an echogenic fibrillar appearance as it attaches on the medial epicondyle (Fig. 4.31). It is well demonstrated by placing the transducer longitudinally over the medial joint line.

Deep to the common flexor tendon is the anterior bundle of the MCL. The MCL is also composed of a posterior bundle and a transverse bundle, but the anterior bundle is the primary restraint to valgus stress, is the bundle that is usually of clinical concern, and is most visible at imaging (Fig. 4.32). The anterior bundle has a fan shape, with a broad origin on the undersurface of the medial condyle and a thin insertion on the sublime tubercle of the coronoid process of the ulna (Fig. 4.31).

Lateral epicondylitis (“tennis elbow”) is the term used for pain over the lateral epicondyle and proximal lateral forearm during wrist extension and supination. It is an overuse injury typically affecting the extensor carpi radialis brevis component of the common extensor tendon,¹⁹ and got the name “tennis elbow” because of its association with poor backhand technique in amateur players,²⁰ but most cases are not due to tennis, and it may be encountered in the lead elbow of professional golfers²¹ and activities that require repetitive extension and supination of the wrist.¹⁹ Lateral epicondylitis and radial nerve neuropathy may have similar clinical presentations, and imaging plays an important role in distinguishing the two conditions.²²

Medial epicondylitis is the term used for pain over the medial epicondyle with wrist and finger flexion and wrist pronation. It is less common than lateral epicondylitis. Medial epicondylitis is also called “golfer’s elbow”, affecting the trail arm as a result of poor swing technique in amateur golfers,²¹ but most cases are not due to golf. Weight lifting, bowling, and throwing are all associated with medial epicondylitis due to the valgus stress that accompanies these activities.²³ The pronator teres and flexor carpi radialis components are predominantly affected.²⁴ Patients with medial epicondylitis may also have ulnar neuritis due to tensile and valgus compressive loads that produce medial epicondylitis.

The term “epicondylitis” is a misnomer, since histologically there is no acute inflammatory process.²⁵,²⁶,²⁷ Rather, it is a degenerative process due to repetitive microtrauma of the common extensor or flexor tendons, with micro tearing of the fibers leading to mucoid degeneration, angiofibroblastic proliferation, and eventual macro tearing if the offending activity continues.

Sonographically, the process looks similar regardless of the side affected. In the early stages the tendon may be hypoechoic and/or thickened (Fig. 4.33).