The brachialis, biceps brachii tendons insert on the ulna and radius, respectively. The triceps inserts on the olecranon and the anconeus is located between the olecranon process and the lateral epicondyle. The common flexor tendon originated on the medial epicondyle and the common extensor tendon originates on the lateral epicondyle. The ulnar nerve traverses through the ulnar groove and enters the cubital tunnel (between the flexor carpi ulnaris and deep to the arcuate ligament). The median nerve traverses medial to the brachial artery and between the heads of the pronator teres. The radial nerve courses distally and laterally beneath the brachioradialis and splits into a deep and superficial branch.
Ultrasonography (US) is an excellent imaging modality for evaluation of elbow joint and soft-tissue disease. The linear high frequency transducers allow for great image resolution. The examination of the elbow joint is most often performed with the patient seated and the elbow placed on an examination table, but the examination can also be done with the patient in supine position. Due to the contours of the elbow, tendons and ligaments are predisposed to anisotropy and careful technique and sufficient amounts of ultrasound gel can be helpful. The transducer can be gently rocked along the long axis of the tendon or ligament in question and if the hypoechogenicity persists, an abnormality is diagnosed. Imaging the contralateral elbow can often be useful to compare the pathologic elbow with the asymptomatic normal one. Dynamic imaging is particularly helpful in assessing the collateral ligaments, subluxation of the ulnar nerve or triceps tendon, and intra articular bodies .
Most diagnostic examinations start with the anterior evaluation. The key structures to be evaluated anteriorly include the anterior joint recess, distal brachialis muscle, distal biceps brachii muscle, and median nerve. The exam should include 5 cm proximal and distal to the joint. This exam can be done with the patient seated or supine, but the hand needs to be supinated with the elbow extended and resting comfortably. The transducer is placed in the short axis to the biceps brachii just superior to the elbow joint [Figure 1]. Many will call this the home position and find the brachial artery, which is superficial and easily seen, and can help with orientation. Deep to the brachial artery is the brachialis muscle and hyaline cartilage separates the brachialis and the humerus. Just lateral to the brachial artery and superficial to the brachialis is the biceps brachii tendon. The brachioradialis is lateral to the brachialis and the oblique fascial layer separates these muscles and contains the superficial and deep branches of the radial artery. The median nerve sits directly medial to the brachial artery and the pronator teres can most likely be seen medial to the nerve depending on field of view. The musculocutaneous nerve can be seen by scanning proximally between the biceps brachii and brachialis .
Alternative techniques can be used if the sonographer is having difficulty. One technique is the medial technique in which the transducer is placed in long axis in the sagittal plane and the beam is angled slightly lateral toward the center of the elbow. The heel-toe maneuver is performed and the probe is moved very slowly until the distal tendon is visualized. Another technique used is placing the transducer transverse to the radius dorsally between the radius and ulna with the hand in pronation to visualize the radial tuberosity insertion. Others have proposed this technique with the wrist in flexion [ Figure 4].
The anterior joint recess needs to be evaluated next. To assess the radiocapitellar joint laterally and ulnotrochlear joint medially, scanning is best performed in the longitudinal axis. A thin layer of hypoechoic hyaline cartilage covers the hyperechoic subchondral bone plate of the articular surface [Figure 5]. Overlying the hypoechoic cartilage is a thin hyperechoic line representing the anterior joint capsule, which averages 2 mm in thickness in the adult population. Within the anterior joint recess, the coronoid fossa and smaller radial fossa are seen as concavities in the distal humerus [Figure 6]. Intracapsular extrasynovial elbow fat pads are found between the hypoechoic synovial lining and hyperechoic linear fibrous capsule within the fossae. They are hyperechoic and triangular and are displaced with distention of the joint.
At the level of the elbow joint, the median nerve is located medial to the brachial artery and can be followed distally, coursing between the humeral and ulnar heads of the pronator teres. This can be a potential source of entrapment. The median nerve has a characteristic speckled appearance in the short axis as a result of its hypoechoic nerve fascicles and intervening hyperechoic epineurium.
For examination of the medial elbow, the patient is asked to lean toward the ipsilateral side with the forearm in forceful external rotation while keeping the elbow extended or slightly flexed, resting on a table [Figure 7]. Coronal planes with the cranial edge of the probe placed over the medial epicondyle reveal the common flexor tendon in its longitudinal axis. The tendon is shorter and larger than the common extensor tendon. The common flexor-pronator mass includes the pronator teres and common flexor tendon.
Deep to the common flexor tendon, the anterior bundle of the medial collateral ligament or ulnar collateral ligament (UCL) can be seen [Figure 8]. The normal joint recess of the elbow extends proximally between the humerus and the ulnar collateral ligament anterior band. The UCL (comprised of the anterior, posterior and oblique bundles) acts as the chief stabilizer of the elbow against valgus stress when it is flexed more than 20°. The anterior band or bundle can be divided into two bands, anterior and posterior, and is functionally the most important in stabilization. It originates from the anteroinferior aspect of the medial epicondyle to insert on the sublime tubercle of the coronoid process of the ulna. This can be dynamically stressed with a valgus force and the joint space can be measured and compared to the opposite side [Figure 9].
The cubital tunnel can be evaluated with the elbow turned outward so the olecranon process and medial epicondyle can be visualized. The ultrasound is placed in a transverse plane between these for the ulnar nerve evaluation. The ulnar nerve is usually visible in this tunnel and has the typical honeycomb appearance [Figure 10]. When this nerve is seen posterior to the medial epicondyle, it may be surrounded by hypoechoic far and may also be bifid, making the evaluation more challenging. The fascia superficial to the ulnar nerve is the cubital tunnel retinaculum or Osborne fascia). The ulnar nerve can be traced distally between the cubital tunnel between the humeral and ulnar heads of the flexor carpi ulnaris and under the arcuate ligament.
An essential part of the US examination is dynamic assessment of ulnar-nerve stability. Axial images are obtained over the nerve during intermittent elbow flexion and extension [Figure 11]. Under normal conditions, neither subluxation nor anterior displacement of the nerve is seen, even when the elbow is fully flexed. In patients with anterior instability, the nerve shifts out of the tunnel (partially or completely) and into the subcutaneous soft tissues. This shift is accompanied by a snap, which can be felt by the patient as well as by the examiner. US assessment of the medial aspect of the elbow shows the proximal portion of the common flexor muscles and their attachment to the epitrochlea via a short, common tendon [Figure 12].
The lateral elbow evaluation is typically done with the elbow rotated inward and slightly flexed. Structures of interest include the common extensor tendon, lateral collateral ligamentous complex, radiocapitellar joint, annular recess, capitellum, and radial nerve, including the PIN, which is its deep motor branch. The transducer is placed in the coronal plane (long axis) and scanning proximally will show the radial head and capitellum, which is just distal to the lateral epicondyle. With the cranial edge of the transducer on the lateral epicondyle and slightly oblique to the long axis of the upper extremity, the hyperechoic fibrillar pattern of the common extensor tendon origin may be visualized . Care should be taken to include evaluation of the most anterior aspect of the common extensor tendon, where abnormalities most often occur.
Deep to the common extensor tendon is the radial collateral ligament (RCL). The lateral collateral ligamentous complex provides stability to the lateral elbow joint. It consists of the RCL, annular ligament, and LUCL (lateral ulnar collateral ligament) along with an accessory RCL. The RCL is a thick band of compact hyperechoic fibers that originates from the inferior lateral epicondyle and inserts on the radius, where it blends with the fibers of the annular ligament. Although similar in appearance, one can differentiate the common extensor tendon and radial collateral ligament by attachment of the RCL to the annular ligament immediately over the radial head [Figure 13].
The LUCL is the major stabilizer against varus stress and posterolateral rotatory stability . US of the LUCL is challenging due to its posteromedial curvilinear course as it cradles the radial head to insert on the supinator crest of the proximal ulna. This distinct and tightly fibrillated structure is best evaluated with the transducer placed over the proximal RCL and angled posteriorly toward the ulna [Figure 14].
The radial nerve can be evaluated next. The oblique fascial plane between the brachioradialis and brachialis should be located in the transverse plane. The deep and superficial radial nerve branches are round and hypoechoic. These can be followed proximally where they join to form the radial nerve. Distally, the deep branch enters the supinator muscle as the posterior interosseous nerve (PIN) [Figure 15]. At the superficial edge of the supinator muscle, there is a fibrous arch—the arcade of Frohse—where entrapment of the PIN may occur as it dives deep to this fibrous bridge. One thing to note is that the deep branch of the radial nerve often changes shape as it passes beneath the arcade of Frohse. The superficial branch of the radial nerve also travels and can be followed into the forearm.
The key structures to examine posteriorly include the distal triceps muscle and tendon, posterior joint recess, olecranon bursa, and ulnar nerve within the cubital tunnel. Examination of the posterior elbow is best performed with the joint positioned in 90° of flexion, the forearm fully pronated (internally rotated), and the palm resting on a table (“crab” position)  [Figure 16]. It is also performed with the arm at 90 degrees and resting on the stomach with the shoulder adducted.
The transducer is placed in the sagittal plane over the proximal posterior elbow. The bony contours around the humerus should be evaluated along with the hypoechoic hyaline cartilage of the trochlea and capitellum. The olecranon fossa is located at the distal aspect of the humerus and is filled with the hyperechoic posterior elbow fat pad [Figure 17]. This site should be evaluated for joint fluid and intra-articular bodies. The superficial layer of the triceps represents the confluence of the lateral and long heads, whereas the deeper and shorter tendon is the medial head. The olecranon bursa can be evaluated by very lightly placing the probe over the olecranon with copious amounts of gel and evaluating for displacement of fluid.