Introduction

Ultrasound (US) is an ideal imaging modality choice for the evaluation of the elbow joint, as it is able to quickly and inexpensively diagnose abnormalities of tendons, ligaments, and nerves. The superficial location of the anatomic structures and pathologic processes that involve the elbow joint make the elbow US examination advantageous to perform and master. The quality of patient interaction, accessible contralateral imaging, and focused real-time scanning for both diagnostic exams and interventional procedures are less challenging with US compared to magnetic resonance imaging (MRI). Utilizing stress maneuvers and joint motion, dynamic US imaging of the elbow further improves diagnostic accuracy by displaying transient conditions that are not directly visualized with static examinations such as MRI [1•].

Technical Considerations

With the development of advanced high-frequency linear transducers, there is improved visualization of the superficial and small structures of the elbow. High-frequency linear transducers of 12–18 MHz are ideal for the elbow. Whereas most structures of the elbow can be evaluated with the highest frequency transducer available, the distal biceps tendon courses deep to insert on the radial tuberosity and using a lower frequency transducer, such as a 12 MHz, may be necessary. The fibrillary and linear architecture of all tendons and ligaments increases anisotropy. This artifact produces regions of hypoechogenicity when the US beam is less than perpendicular to the fibers. Since these artifactual regions mimic tendinosis and tears of tendons and ligaments, specific scanning techniques should be learned and applied. To avoid this common pitfall, varied amounts of pressure are applied across the transducer surface with subtle movements of the hand and wrist. The transducer can be gently rocked along the long axis (heel–toe maneuver) or toggled back and forth in the short axis of the tendon or ligament in question [1•, 2, 3, 4].

The elbow US exam is performed either with the patient seated and the elbow placed on the examination table or with the patient supine. Most of the time, US examinations of the elbow are focused to target a specific clinical area or structure; however, knowledge of a comprehensive US examination with tips and different approaches to visualize the structures of the elbow can be highly advantageous. Dividing the elbow anatomy into anterior, lateral, medial, and posterior compartments is a practical approach and an efficient scan pattern to implement.

Anterior Elbow

With the patient seated in a chair, the arm is placed on the examination table with the elbow extended and the forearm supinated. Placing the transducer in the transverse orientation just proximal to the elbow crease, the wavy osteochondral surface of the capitellum and trochlea is depicted [2]. This surface produces a “W sign” and is an excellent landmark for orientation [4] (Fig. 1). In the longitudinal plane, the anterior joint recesses of the lateral radiocapitellar and the medial ulnotrochlear joint can be assessed for joint effusion, synovitis, and intraarticular bodies. The structures to evaluate in the anterior compartment of the elbow are the distal biceps brachii tendon, median and radial nerves, and the anterior joint recesses.

Fig. 1
figure 1

Transverse plane proximal to the elbow crease. The wavy osteochondral surface of the capitellum and trochlea (“W sign”) is a landmark (asterisks). The multifasciculated median (double arrowhead) and radial (single arrowhead) nerves are located along the medial and lateral aspects of the elbow, respectively. The muscle directly anterior to the humerus is the brachialis. Superficial to the brachialis is the distal biceps brachii tendon (BcpTen), which is lateral to the brachial artery (ART)

Full size image

Distal Biceps Brachii Tendon

Although distal biceps tendon injuries are rare compared to proximal biceps tendon tears at the superior glenoid tubercle [5, 6], familiarization with the US evaluation of the distal biceps tendon can be a rapid, inexpensive alternative modality to MRI [1•, 6]. The biceps brachii muscle–tendon unit flexes and supinates the forearm at the level of the elbow. The distal biceps tendon has two components that insert onto the radial tuberosity [1•, 5, 6, 7•, 8, 9]. The long head inserts proximally and deep, while the short head inserts distally and more superficially [7•]. The distally inserting, longer tendon of the short head component produces a more favorable tendon lever than the long head component [8]. The distal biceps tendon is located lateral to the brachial artery and the median nerve [1•, 3] (Fig. 1). An extrasynovial paratenon and the bicipitoradial bursa cover the distal biceps tendon. In our experience, the bicipitoradial bursa is typically not well visualized unless it is distended with hemorrhage from a biceps tear or from fluid/synovitis associated with an inflammatory arthropathy.

Because the distal biceps tendon has a deep oblique course and a 90° rotation as it inserts onto the radial tuberosity, knowledge of multiple US scanning techniques increases diagnostic accuracy. The anterior and posterior approaches will be discussed.

Anterior: The elbow is extended and the forearm is maximally supinated to bring the radial tuberosity more anterior [6]. In the longitudinal plane, with the brachial artery in view, the probe is moved slightly lateral. Increasing the probe pressure over the distal part of the transducer reduces anisotropy artifact (heel–toe maneuver) (Fig. 2).

Fig. 2
figure 2

Anterior technique for the distal biceps tendon. Longitudinal plane of a normal distal biceps tendon (arrowheads) inserting onto the radial tuberosity (RadTub). The fibrillary, linear echogenicity of the tendon is visualized without areas of thickening or surrounding fluid. Picture inset demonstrates the increasing probe pressure over the distal part of the transducer to reduce anisotropy. RH radial head

Full size image

Posterior or the “cobra” approach: The elbow rests on the examination table in near complete flexion and the hand is fully supinated. The probe is placed in the transverse plane at the level of the radial tuberosity along the posterior or dorsal surface of the forearm (Fig. 3a). When the hand is maximally pronated (cobra appearance), the radial tuberosity will rotate posteriorly and come into view [6] (Fig. 3b).

Fig. 3
figure 3

Posterior or “cobra” technique for the distal biceps tendon. a Transverse plane at the radial tuberosity in supination. The radial tuberosity is pointing away from the transducer. b Transverse plane at the radial tuberosity in pronation. The radial tuberosity (Rad Tub) rotates toward the transducer and the normal distal biceps tendon fibers (between arrows) are visualized

Full size image

Distal biceps tendinosis is from overuse and degeneration of the tendon due to progressive decrease in perfusion, elasticity, and hydration with age [6]. There is hypoechoic thickening of the tendon typically located in the distal 1–2 cm of the distal biceps tendon. Tears demonstrate contour waviness or laxity and some degree of disruption of tendon fibers. This distinction is somewhat arbitrary in that essentially all patients with tendinosis have degeneration and microtearing [1•]. Tears are associated with fluid/hemorrhage in the bicipitoradial bursa. With acute tears, echogenic hemorrhagic clot may obscure visualization of the tendon and can be difficult to distinguish from echogenic tendon fibers [6] (Fig. 4a). With dynamic US imaging with supination–pronation or flexion–extension motions, intact fibrillary fibers can be further distinguished from nonmobile echogenic hemorrhage [2] (Fig. 4b, c).

Fig. 4
figure 4

a Longitudinal plane of a retracted (measurement markers) distal biceps tendon partial tear. Due to the surrounding echogenic hemorrhage (asterisks) it is difficult to distinguish tendon fibers. Utilizing the posterior or “cobra” technique with dynamic motion. b No tendon fibers were present along the distal aspect of the radial tuberosity where the short head inserts. Only fluid and hemorrhage is present (asterisks). c In contrast, at the proximal aspect of the radial tuberosity, intact distinct linear tendon fibers are present (between arrows)

Full size image

Median and Radial Nerves

Due to their superficial location, the median and radial nerves are easy to assess with US. Each nerve has the typical honeycomb appearance with multiple hypoechoic nerve fascicles and intervening hyperechoic epineurium producing the classic multifascicular appearance [10]. Due to their small caliber, dynamically following the nerves on short-axis imaging allows for easier nerve identification compared to static images [2]. Normally, all nerves gradually taper and the cross-sectional area of nerves should decrease throughout their distal course. Comparison to the asymptomatic contralateral side may be helpful.

The median nerve is located medial to the brachial artery, and it innervates the pronator teres, flexor carpi radialis, palmaris longus, and flexor digitorum superficialis muscles. The anterior interosseous nerve (AIN) can be visualized branching off the median nerve, approximately 5–8 cm distal to the lateral epicondyle [2]. The purely motor AIN innervates the flexor pollicis longus, the ulnar half of the flexor digitorum profundus, and the pronator quadratus. Since the AIN is small in caliber, impingement or neuropathy (Kiloh-Nevin syndrome) is difficult to diagnose by imaging alone. Usually associated with atrophy and fatty infiltration of the innervated muscles, these changes are best seen on MRI [2, 11]. The presence of a normal variant supracondylar process along the medial distal humerus and Struthers’ ligament can also cause median nerve impingement [12].

At the level of the elbow crease, the radial nerve is located between the brachialis and brachioradialis muscles and anterior to the capitellum and radial head (Fig. 1). At the level of the proximal elbow, the radial nerve divides into the superficial sensory branch and the deeper posterior interosseous nerve (PIN) which is purely motor. The course of the PIN can be visualized with US as it pierces the superficial edge of the supinator muscle and enters a fibrous arch called the arcade of Frohse. Along this course, entrapment of the PIN may occur, causing weakness in the supinator and extensor muscles of the wrist, hand, and digits. PIN impingement can produce variable degrees of muscle changes, such as edema, atrophy, and fatty infiltration, in the extensor carpi radialis brevis, supinator, extensor digitorum communis, extensor carpi ulnaris, extensor digiti minimi, abductor pollicis longus and brevis, and extensor indicis proprius muscles [1•, 10].

Lateral Elbow

The lateral aspect of the elbow is examined with the elbow in flexion and internally rotated with the thumb pointing up. The transducer is placed longitudinally onto the radial head and capitellum (Fig. 5). The sloped lateral epicondyle and the radial head is the landmark to initiate the examination of the lateral elbow structures which includes the lateral collateral ligament (LCL) complex and the common extensor tendon.

Fig. 5
figure 5

Longitudinal plane of a normal common extensor tendon (asterisks). The RCL (arrowheads) is directly deep to the tendon traversing the lateral elbow joint. LatEpi lateral epicondyle, RH radial head

Full size image

LCL Complex

Injury to the LCL complex can present with recurrent lateral elbow pain and lateral subluxation of the radial head. It consists of the radial collateral ligament (RCL), annular ligament, and the lateral ulnar collateral ligament (LUCL). The RCL is located directly deep to the common extensor tendon and spans the lateral epicondyle and radial head onto the fibers of the annular ligament [2, 13•] (Fig. 5). The annular ligament is a horseshoe-shaped structure surrounding the radial head with anterior and posterior attachments to the proximal ulna. The LUCL originates at the posterior lateral epicondyle blending with the posterior RCL fibers, supports the radial head, and then inserts onto the lateral aspect of the proximal ulna. As an important elbow stabilizer, the LUCL prevents varus stress and posterolateral rotatory instability [1•, 14]. US evaluation of the annular ligament and LUCL are challenging and better evaluated on MRI [2]; however with proper technique, few authors report US visualization of the proximal and distal margins of the LUCL in 94 and 90 % of patients, respectively [15].

Lateral Epicondylitis

Lateral epicondylitis (LE) is the most common cause of elbow pain, affecting 2–3 % of the population and 15 million people in North America [16, 17]. LE is caused by repetitive contraction or overuse of the common extensor tendon leading to microtears, degeneration, and subsequent tendinosis [18]. Although LE was historically thought to be an inflammatory condition, it is now well established by histopathologic studies that tendinosis is related to cellular apoptosis and autophagic cell death [16, 19–21•]. The histologic pattern consists of angiofibrotic hyperplasia, fibroblast proliferation, disorganized collagen, and poorly functioning neovascularity [20•, 21•, 22, 23].

The common extensor tendon is the primary extensor of the wrist and hand and consists of the extensor carpi radialis brevis (ERCB), extensor digitorum communis, extensor digiti minimi, and extensor carpi ulnaris. Although the ECRB contributes to the deep fibers and the extensor digitorum the more superficial portion [1•, 2, 24], the individual contributions of the tendons within the common extensor tendon cannot be visualized [2].

The common extensor tendon US evaluation assesses the severity and extent of tendinosis and tearing. Findings of tendinosis include tendon thickening, hypoechogenicity, calcification, and enthesophyte formation (Fig. 6a). Tearing can occur in severe cases and manifests as an anechoic fluid-filled gap (Fig. 6b). Pathology always involves the ECRB, followed by the extensor digitorum communis [1•, 2, 24]. US imaging plays a major role in image-guided injections and treatments with percutaneous tenotomy or fenestration of the common extensor tendon.

Fig. 6
figure 6

Longitudinal plane of the common extensor tendon. a Severe tendinosis of the common extensor tendon with thickening of the superficial tendon, globular areas of hypoechogenicity, and irregular areas of calcification. b Full thickness tear of with an anechoic fluid gap (between arrowheads) in the common extensor tendon near the lateral epicondyle. There is a small joint effusion (asterisk) and the tendon fibers are distally retracted and mildly buckled. LatEpi lateral epicondyle, RH radial head

Full size image

Although steroid injections of the common extensor tendon have been performed, they provide only short term relief of pain and have been shown to weaken the tendon [25]. The reports of multiple studies assessing the efficacy of PRP for the treatment of LE are conflicting [26, 27]. A recent double-blinded randomized control trial demonstrated significant long-term reduced pain and improved function in LE patients treated with PRP when compared to corticosteroid [23]. However, another studying comparing PRP, corticosteroids, and saline injections for chronic LE reported no significant improvement in pain with PRP or steroid when compared to saline injections at 3 months postinjection [21•, 28]. A recent meta-analysis reviewed 23 randomized control trails and 10 prospective cohort studies from the orthopedic literature assessing the effectiveness of PRP for various orthopedic injures. Although there was a small trend favoring PRP, there was no significant effect across the studies [21•, 29]. Variation in protocols, injection technique, and varying platelet concentrations by vendor specific platelet separation techniques are methodological challenges in PRP research [26].

Despite the lack of overwhelming evidence-based data supporting its efficacy, PRP has gained significant popularity among professional and recreational athletes. The market for PRP was valued at 45 million in 2009 and projected to be worth more than 120 million by 2016 [21•].

US-guided percutaneous tenotomy using ultrasonic energy has recently become available with the FDA-approved TX1 needle device (Tenex Health Inc., Lake Forest, CA, USA) [19, 20•, 22]. The Tenex needle system has an outer 18 gauge needle and a rapidly oscillating hollow inner needle that delivers ultrasonic energy to emulsify degenerated tendon tissue, which is removed by a saline aspiration through an inflow–outflow fluid circuit [20•, 22] (Fig. 7).

Fig. 7
figure 7

Longitudinal plane at the lateral epicondyle during US guided percutaneous tenotomy of the common extensor tendon. The rapid oscillations of the TX1 Tenex needle produce the linear “spray” of artifact

Full size image

Early reports of Tenex efficacy for LE have shown some suggestive results. Larger and randomized studies are needed. A study following 20 patients, reported 19 of 20 patients were very or somewhat satisfied with their Tenex treatment for LE. Improvements in pain and function scores were seen as early as 1-week post-procedure and maintained at the 1-year follow-up [22]. Another study reported 19 patients who failed conservative management for chronic LE (greater than 6 months) significantly improved after Tenex. Visual analog scores (VAS) for pain reduced from 6.4 to 2.6 at 6 weeks and 0.7 at 12 months (P < 0.0001). Assessment of pain and function with the Quick DASH demonstrated significant improvement with pretreatment scores equaling 44.1, and 12 months posttreatment scores reduced to 8.6 (P < 0.0001). Also both studies reported no procedural complications [20•].

Sonoelastography is a noninvasive emerging US method to assess the mechanical properties of soft tissue elasticity or deformability [30•]. This is based on the principle that compression of tissue produces a tissue specific displacement, known as strain. Real-time compression-based elastography provides information about tissue elasticity represented as a color-coded map/spectrum ranging from red (softest), orange–yellow–green (intermediate), and to blue (hard) [31]. The elasticity of soft tissues is altered by disorders of tendinosis, neuromuscular disease and wound healing [30•]. The normal common extensor origin is a blue to green elastogram. The typical elastogram finding of LE is softening of the common extensor tendon in the red to yellow range [30•, 31]. Conventional US and sonoelastography was compared with clinical examination in 38 elbows with LE, conventional US had a sensitivity of 95 %, a specificity of 89 %, and an accuracy of 91 %. Sonoelastography findings correlated well with a sensitivity of 100 %, specificity of 89 %, and an accuracy of 94 % [24]. Musculoskeletal sonoelastography appears to have diagnostic potential, and areas of biomechanical alteration can be targeted for image-guided treatment.

Medial Elbow

To evaluate the medial elbow the arm is extended and the forearm is placed in external rotation (Fig. 8). The transducer is placed longitudinal along the medial aspect of the elbow joint, and the medial epicondyle and the ulnotrochlear articulation is visualized. The common flexor tendon and the ulnar collateral ligament (UCL) are evaluated in the medial elbow compartment.

Fig. 8
figure 8

Longitudinal plane of a normal common flexor tendon (asterisks). The UCL (arrowheads) is directly deep to the tendon traversing the medial elbow joint. Med Epi medial epicondyle

Full size image

Medial Epicondylitis

The tendons of the flexor carpi radialis, palmaris longus, flexor carpi ulnaris, and flexor digitorum superficialis form the common flexor tendon. The common flexor tendon is involved in flexion of the wrist/hand and supports the UCL in resisting valgus stress [2]. Medial epicondylitis or golfer’s elbow describes tendinosis or tearing of the common flexor tendon. Due to the proximity of the ulnar nerve, medial epicondylitis can be associated with ulnar neuritis with patients developing radicular symptoms to the 4th and 5th digits. Similar to its lateral counterpart, the US appearance of medial epicondylitis is associated with tendon hypoechogenicity, calcification, and enthesophyte formation (Fig. 9) with anechoic fluid noted in more severe cases when tearing is present. US has been shown to have high sensitivity and specificity for the diagnosis of medial epicondylitis, 95 and 92 %, respectively [32]. Thus it can serve as a less costly imaging alternative to MRI for confirming a clinically suspected diagnosis.

Fig. 9
figure 9

Longitudinal plane of severe tendinosis of the common flexor tendon (arrowheads). Calcifications are visualized at the enthesis onto the medial epicondyle (Med Epi) surrounded by thickened and hyperechoic tendon

Full size image

UCL

The UCL complex is composed of the anterior, posterior, and transverse bands. The anterior band of the UCL is the most important dynamic stabilizer during valgus extension of the elbow. The anterior band of the UCL originates from the anteroinferior aspect of the medial epicondyle and inserts onto the sublime tubercle of the coronoid process of the ulna. It has a fan-shaped hyperechoic fibrillar appearance, where the humeral attachment is thicker/wider with progressive tapering to the sublime tubercle [33]. Overhead throwing activities predispose participants to injure the UCL complex. Another technique to evaluate the medial elbow and the UCL is to have the patient supine on the examination table with the arm abducted, elbow flexed 90 degrees, and the forearm supinated (Fig. 10). Dynamic US can be used to assess both the integrity of the UCL and widening of the medial joint space with valgus stress. The normal ligament has been reported to be identifiable in 100 % of normal subjects with good interobserver variability [34]. Ultrasound has also been utilized to monitor the integrity of the UCL following reconstruction (Tommy John surgery). The intact UCL graft may be more hyperechoic and thicker than the native ligament; however, it should be contiguous throughout its course. Slight medial laxity with valgus stress may be normal [33]. Ultrasound has been utilized to identify adaptive changes in the UCL in high level pitchers in several recent studies, with the most common change being progressive thickening of the ligament [35•, 36].

Fig. 10
figure 10

Another technique to evaluate the medial elbow structures—the ulnar nerve and common flexor tendon. The patient is supine on the examination table with the arm abducted, elbow flexed and the forearm/palm supinated (facing the patient)

Full size image

Posterior Elbow

The posterior elbow US examination evaluates the triceps muscle and tendon, olecranon bursa, and ulnar nerve. Examination of the posterior elbow is best evaluated with the palm of the hand flat on the examination table and the elbow flexed at 90° (Fig. 11). Because the ulnar nerve is also in the medial elbow, another position to evaluate the ulnar nerve is with the patient supine and the arm abducted, elbow flexed, and forearm internally rotated [1•] (Fig. 10).

Fig. 11
figure 11

Longitudinal plane of a normal triceps tendon inserting onto the olecranon process. The superficial longer fibrillary tendon is comprised of the lateral and long heads of the triceps (arrowheads). The medial head of the triceps tendon is located directly below with a very small tendon component (asterisk)

Full size image

The triceps tendon is composed of three heads: medial, lateral, and long. The distal triceps tendon inserts as a superficial thick linear and echogenic tendon of the lateral and long heads. Deep to this tendon is the medial head insertion onto the olecranon which is mostly muscular [2, 37]. The triceps tendon is evaluated in both the transverse and longitudinal planes from the myotendinous junction to the olecranon process. The distal superficial triceps tendon inserts approximately 1 cm distal to the apex of the olecranon process [1•, 4] The fibers near the insertion course slightly oblique and may appear hypoechoic due to anisotropy artifact which can be eliminated by the heel-toe maneuver.

A fall on an outstretched hand (forceful loading during triceps contraction) or indirect impact are typical mechanisms of injury for a distal triceps tear [38]. Although these injuries are uncommon, steroid use, renal disease, and chronic olecranon bursitis are some of the factors predisposing to triceps injury [1•, 38]. US plays a role in the accurate detection and evaluation of the extent of the tear and the degree of retraction, ultimately allowing for more focused management [2, 39] (Fig. 12). Disruption of the distal triceps tendon fibers, fluid gap, retraction, and surrounding fluid indicates full thickness tendon tear. Partial tendon tears typically involve retraction of the more superficial fibers and an associated olecranon enthesophyte avulsion [37]. Partial tears are managed nonsurgically, whereas surgical intervention is reserved for complete tears or incomplete tears with associated loss of strength [40].

Fig. 12
figure 12

Longitudinal plane of a full-thickness tear of the triceps tendon. The fibers of the lateral and long head are torn and retracted (between the arrowheads). An anechoic fluid gap is present (white asterisks). The deeper medial head is intact (black asterisk). Fluid/hemorrhage is also present in the olecranon bursa (arrows)

Full size image

The olecranon bursa is located superficial to the olecranon process and distal triceps tendon and is best interrogated with elbow extension [1•]. To evaluate the superficial olecranon bursa, gentle probe pressure and application of sufficient US gel are helpful to visualize even a small amount of bursal fluid or synovitis [2]. Olecranon bursitis typically appears as a hypoechoic collection along the posterior aspect of the olecranon with potential findings of synovitis and associated hyperemia. Hyperechogenicity and heterogeneity within a collection in the olecranon bursa suggests gout as a potential etiology for the bursitis [41]

Ulnar Nerve/Cubital Tunnel Syndrome

At the posteromedial elbow, the ulnar nerve is located in a fibro-osseous tunnel. This tunnel has a floor formed by the posterior band of the UCL and a roof formed by the Osborne ligament, which connects the olecranon process of the ulna to the medial epicondyle of the humerus. The true cubital tunnel is located approximately 1 cm distal to this fibro-osseous tunnel [2]. The cubital tunnel can be a site of ulnar nerve entrapment as it resides between the two heads of the flexor carpi ulnaris muscle. When the elbow flexes and extends, the cubital tunnel changes shape and volume (Fig. 13). During elbow motion, a 55 % decrease in nerve cross-sectional area and a six-fold increase in interstitial pressure of the cubital tunnel occurs [2, 42], and repetitive motion may contribute to the development of cubital tunnel syndrome. Enlargement of the ulnar nerve >9 mm2 maximal cross-sectional area, is 94 % sensitivity and 88 % specificity for ulnar neuritis. A separate study comparing US to nerve conduction studies found that US accuracy was lower but approached that of nerve conduction studies, with an accuracy rate of 85 % for nerve conduction studies and 77 % for US [43•].

Fig. 13
figure 13

Transverse plane of the multifasiculated ulnar nerve (within circle) in the true cubital tunnel, which is located 1 cm distal to the medial epicondylar prominence and between the two heads of the flexor carpi ulnaris

Full size image

If the Osborne ligament is lax or absent, the ulnar nerve is prone to instability, and patients may complain of medial painful snapping elbow with motion [44]. Dynamic US of the ulnar nerve evaluates for nerve subluxation. With the transducer in the transverse plane between the olecranon process and fixed at the medial epicondyle, the patient is asked to flex the elbow (Fig. 14a, b). This movement evaluates for abnormal translation of the ulnar nerve over the medial epicondyle. Commonly seen in athletes with hypertrophic arm muscles, another etiology of elbow snapping is snapping triceps syndrome from subluxation of the muscle over the medial epicondyle (specifically the medial head of the triceps). Snapping triceps syndrome should be assessed with ulnar nerve evaluation. If a snapping triceps is not diagnosed and treated, snapping and ulnar nerve irritation may persist [2].

Fig. 14
figure 14

Transverse plane of the posteromedial elbow with ulnar nerve dislocation. a In extension, the ulnar nerve (encircled) is located normally posterior to the medial epicondyle within the fibro-osseous tunnel. b With dynamic flexion, the ulnar nerve (encircled) dislocates and travels abnormally anterior to the medial epicondyle, which is associated with a snap or palpable “clunk”. The ulnar nerve is enlarged and flattened due to mass effect from the medial epicondyle

Full size image

Conclusion

By understanding the anatomy and learning scanning techniques, US of the elbow is an excellent modality for the diagnosis of elbow joint pathology. The clinical value of US is the ability to perform real-time imaging, dynamic stress imaging, and image-guided injections/treatments.