The elbow: review of anatomy and common collateral ligament complex pathology using MRI

On the medial side of the elbow joint, the MCL complex is comprised of three bundles: the anterior, posterior, and transverse bundles (Fig. 4a). Twenty-three percent of people have an accessory ulnar collateral ligament, which originates on the posterior joint capsule and inserts onto the transverse bundle [16, 17].

The anterior bundle (A-MCL) arises from the inferior margin of the medial epicondyle and inserts at the sublime tubercle of the ulnar coronoid process (Fig. 5). The A-MCL is composed of a superficial and a deep layer [18]. The superficial layer is a separate structure from the joint capsule and is considered to be associated with deep fibers of the flexor digitorum superficialis tendon [14]. The A-MCL can be separated into two bands, which are taut during different degrees of flexion/extension [19]. The anterior band of the anterior bundle is the most important static stabilizer of the elbow against valgus and internal rotation [20]. The posterior bundle (P-MCL) originates at the posterior aspect of the medial epicondyle of the humerus and attaches to the medial aspect of the olecranon process, forming the floor of the cubital tunnel (Fig. 6). During normal elbow flexion, the flexor carpi ulnaris aponeurosis tenses while the medial collateral ligament relaxes and bulges superficially. These changes result in decreased volume and increased pressure inside the cubital tunnel during flexion. The transverse bundle originates on the proximal medial olecranon and runs distally to insert just distal to the coronoid. Because this ligament originates and inserts on the ulna, it does not provide significant stability [17]. In addition, the ligament is variably present. This combination makes the ligament of relatively low clinical and radiological importance. On MR imaging, the proximal A-MCL has a striated appearance in 87–90% of healthy volunteers, which should not be confused with injury (Fig. 5) [5, 14, 17, 21]. The A-MCL is best visualized on coronal and axial slices. The P-MCL can be traced from origin to insertion on the coronal images and identified as the floor of the cubital tunnel on axial slices [22].

On the lateral side of the elbow joint, the LCL complex is comprised of three primary structures: the radial collateral ligament proper (RCL), the annular ligament (AL), and the lateral ulnar collateral ligament (LUCL) (Fig. 4b). One-third of individuals have an accessory lateral collateral ligament, which runs from the annular ligament to the supinator crest of the ulna [16, 17]. The RCL is a fan-shaped ligament that originates at the lateral epicondyle of the humerus and runs longitudinally underneath the common extensor tendon blending with the anterior annular ligament (Fig. 7). The RCL is best seen on coronal images [16]. The AL wraps around the anterior radial head, originating and inserting on the ulnar sigmoid notch, and is best seen on axial and sagittal images [16, 23] (Fig. 8). The anterior attachment of the AL to the ulna is observed as a single band, but the posterior attachment can be fenestrated normally [24] (Fig. 8c, d). This thick band primarily serves to stabilize the proximal radioulnar joint. The LUCL also originates at the lateral epicondyle of the humerus and partially blends in with the AL as it travels distally to insert on the supinator crest of the ulna [16, 17, 23] (Fig. 7). On MRI, it is incompletely visualized in up to 23% and it has a striated appearance in 78% of healthy volunteers [21]. The LUCL can be seen on coronal and sagittal images (16). In anatomic dissections, the humeral attachment of the LUCL is indistinguishable from that of the RCL because they both originate from the inferior aspect of the lateral epicondyle [25, 26]. The LUCL is considered to be the primary stabilizer of the elbow joint against posterolateral rotatory instability. It prevents the ulna from rotating around its long axis away from the trochlea [27, 28]. The LUCL stabilizes all three articulations of the elbow and contributes to the resistance of the rotatory forces of varus and external rotation. The common extensor tendon origin is the secondary soft-tissue stabilizer of the lateral elbow with a stronger contribution to stability during pronation of the forearm [29].

The main function of the MCL complex is to maintain medial joint stability to valgus stress. The A-MCL is the most important component of the ligamentous complex acting as the primary medial stabilizer of the elbow from 30° to 120° of flexion [22]. The P-MCL becomes a secondary stabilizer of the elbow when the joint is flexed beyond 90° [11].

The most common mechanisms of MCL injury are chronic microtrauma from repetitive valgus stress, as seen in overhead and throwing athletes (baseball, javelin throwing, volleyball, golf, polo, and football), and after a fall on an outstretched hand. In the case of the latter, an acute tear of the MCL may be encountered. MCL rupture frequently occurs with posterior dislocation. Overhead throwing sports can result in medial elbow tension overload, lateral compression, and extension overload (Fig. 13) [1, 16]. The maximum stress on the MCL occurs during the late cocking and acceleration phases of throwing [7, 15]. Injury of the MCL is a common cause of medial elbow pain, and valgus instability in athletes. Repetitive insults to the ligament cause microscopic tears that progress to significant attenuation or frank tearing within its substance. Partial tears can be subtle and are well seen with magnetic resonance arthrography (Fig. 3c) [8, 9]. These patients may have associated findings, including lateral impaction and shearing of the articular surfaces of the capitellum and the radial head (Figs. 14 and 15). Reports have also described this process at the radial aspect of the trochlea.

In acute cases, MRI may show increased T2-w signal within the ligament, discontinuity of the ligament, and soft-tissue edema (Fig. 14). Midsubstance tears of the A-MCL are more common [1]. Distal (Fig. 15) and proximal avulsions (Fig. 16) are found less frequently [5, 7, 15]. Under surface partial tears in the distal insertion of the A-MCL complex have a characteristic aspect on MRI called the “T-sign,” which is produced by extension of fluid or contrast between the distal insertion of the ligament and the sublime tubercle. However, the distal insertion of the A-MCL complex can normally be up to 3 mm distal to the articular cartilage, especially in older patients, simulating the “T-sign” [5, 14]. The presence of other secondary signs of ligament injury such as ligament irregularity or periligamentous edema may be used to differentiate partial lesion from anatomical variant. The strain of the flexor digitorum superficialis frequently accompanies a MCL injury (Figs. 14 and 15). Thickening or acute disruption of the posterior bundle of the MCL may result in ulnar neuropathy [5, 22] (Fig. 17). Up to 40% of throwing athletes with MCL injuries and more than 50% with medial epicondylitis have ulnar neuropathy [22].

In chronic cases, MRI may show thickening, abnormal signal, and discontinuity of the ligament (Fig. 18). The development of heterotopic ossification along the course of the ligament has been described (Fig. 19).

In skeletally immature individuals, a chronic valgus stress causes a repetitive traction on the apophysis by the common flexor tendon and the MCL, causing a medial epicondyle apophysitis, also known as Little leaguer’s elbow [5, 30‐32]. Sometimes, the epicondyle avulses into the joint and can simulate an osseous body. More often, it remains close to the parent bone, presenting on MRI with bone marrow edema and/or a widened gap between the medial epicondyle and the humerus (Fig. 20). Non-union can lead to repeated valgus instability.

The LCL complex resists excessive varus and external rotational stress. Varus stress applied to the elbow may be due to an acute injury, but rarely to repetitive stress, as encountered on the medial side. Tears can involve one or more of the three bundles, but the LUCL is the most important in terms of stability [31]. However, kinematic studies refer to both the LUCL and RCL working in concert to resist valgus stress. If both are injured, it can secondarily lead to subluxation or dislocation of the radiocapitellar joint even with an intact annular ligament, usually in the setting of chronic or repeated injury. LUCL tears usually involve the humeral origin [2, 5]. Failure to recognize LCL complex tears prior to surgical treatment of tennis elbow, particularly the LUCL, will lead to persistent postoperative symptoms.

Injuries of the LCL complex can occur in patients with advanced cases of tennis elbow, who also have tears of the common extensor tendon, and after a fall on the outstretched hand. Among iatrogenic causes of LCL complex disruption, we find overaggressive extensor tendon release for lateral epicondylitis, and radial head excision for comminuted fractures of the radial head [33, 34]. Repeated corticosteroid injections into the common extensor tendon and LCL complex origins might contribute to the weakening and ultimate failure of these structures [34].

The most common pattern of recurrent elbow instability is posterolateral rotatory instability (PLRI) [7]. It represents a spectrum of pathology consisting of three stages depending on the grade of soft-tissue disruption, which extends from lateral to medial, the so-called circle of Hori [5, 7, 17] (Figs. 21 and 22).

The classic clinical presentation of patients with PLRI includes pain as well as a sensation of locking, clicking, or snapping when the arm moves from a flexed to an extended elbow position. Ultimately, the diagnosis of PLRI is based on history and physical examination using provocative maneuvers. The pivot shift test of the elbow is designed to test for PLRI due to insufficiency of the LUCL and the RCL [2]. Diagnosis is often difficult, as the clinical exam can be misleading unless performed under anesthesia. MR imaging can therefore be extremely useful in evaluating for tears of the LUCL in patients presenting with lateral elbow pain or instability. It is best evaluated in coronal oblique, coronal, and axial planes. Associated posterolateral subluxation of the radial head is best appreciated on sagittal images [5]. LUCL tears may appear as an isolated finding in patients with PLRI in stage 1, or they can be detected in association with the rupture of the MCL in stage 3B. MR imaging is also useful in assessment of the cubital tunnel retinaculum, and the ulnar nerve in posterior dislocations.

Incongruity of the elbow can be considered as an indirect sign of instability and can be evaluated with MRI. There is a significant increase in joint incongruity in unstable elbows analyzed in sagittal view through the radial head and in axial view through the motion axis of the distal humerus compared with stable elbow joints. In a sagittal view through the center of the radial head, the radiocapitellar incongruity is the distance between the rotational center of the capitellum (CAP) and a line along the longitudinal axis of the radius through the center of the radial head (R) (Fig. 29b). In an axial view through the motion axis of the distal humerus, the ulnohumeral incongruity is the difference of the lowest and the highest values of four measures extending from the trochlear joint surface to the corresponding joint surface of the olecranon (Fig. 29a) [35]. Posterolateral translation of the radial head of more than 1.2 mm and axial ulnohumeral incongruity of more than 0.7 mm are cutoffs that can be used as screening tools to aid diagnosis of elbow instability [5, 35]. Radiocapitellar incongruity of more than 2 mm (Fig. 29d) and axial ulnohumeral incongruity of more than 1 mm are highly suspicious of elbow instability (Fig. 29c). To exclude the diagnosis of instability, radiocapitellar and axial ulnohumeral incongruity have to be below 0.3 mm [35].

MR imaging findings in acute LUCL injury resemble those described for the MCL: hyperintensity, discontinuity, and surrounding soft-tissue edema on conventional fluid-sensitive MR images (Figs. 23, 24, and 25). Osteochondral lesions and bone contusions may be seen at the posterolateral margin of the capitellum as well as at the radial head, and coronoid process (Figs. 30 and 31). The role of imaging is to provide information regarding the integrity of the overlying articular cartilage, the viability and stability of the separated fragment (Fig. 32), and the presence of associated intra-articular bodies.

The appearance of chronically torn and remodeled LUCL is similar to that described for the MCL, with thickening, abnormally increased signal, and discontinuity as possible findings (Figs. 26, 27, and 28).

The AL can be injured in the setting of trauma and after closed reduction of elbow dislocation (Figs. 2 and 33). A tear of one or more components of the LCL complex is often associated with intra-articular displacement of the annular ligament. In adults, sole injury to the AL is rare. Most often, it is associated with a larger injury to the LUCL complex, including varus elbow stress, elbow dislocation, and PLRI. However, in pediatric population, dislocation and rupture of the AL can be seen in the setting of nursemaid elbow and Monteggia fracture [36]. The AL is best visualized on axial and sagittal MR imaging using standard sequences and field strength. The clinical feature of posterior dislocation and chronic AL injury may be a recurrent painful “click.” Other differential diagnoses are intra-articular loose bodies, a posterolateral plica, an ulnar nerve subluxation, and a snapping triceps syndrome (Fig. 34).

Elbow fractures with ulnohumeral instability tend to occur in five general patterns: radial head fracture with ulnohumeral dislocation, terrible triad, varus posteromedial rotatory instability (VPMRI), olecranon fracture dislocation (OFD), and lateral column fracture of the distal humerus with ulnohumeral dislocation. VPMRI consists of a fracture of the anteromedial coronoid facet and a rupture of the LCL complex. OFD consists of a fracture of the olecranon with subluxation/dislocation of the intact forearm relative to the distal humerus. This is usually accompanied by a radial head fracture. A “terrible triad” consists of a posterior elbow dislocation, radial head fracture, coronoid process fracture, and a rupture of the LCL complex. These patients are at high risk for chronic instability [37].

Displaced radial head and neck (DRHN) fracture is always a complex fracture caused by the combination of a valgus force and pathologic forearm external rotation. They are accompanied by collateral ligament injuries and bony contusion. According to the Charalambous classification [38], type 3D and 4D DRHN fractures tended to have a higher association with MCL rupture compared with type 1D and 2D DRHN fractures, commonly associated with LUCL rupture, although this was not statistically significant [39].

Another important consideration with respect to elbow dislocation is that, as the ring of soft tissues is disrupted posterolaterally to medially, the articular capsule is torn and insufficient. Therefore, fluid in the elbow joint can escape through the capsular tear and a joint effusion, which is an indirect sign of elbow trauma, may not be present.

It is important to recognize that intact postoperative ulnar and lateral collateral ligaments may be thicker and more heterogeneous in signal intensity than native ligaments (Fig. 35). However, retears can be detected by the same criteria of partial or complete ligamentous discontinuity (Fig. 36). Anchoring materials can cause imaging artifacts, although it is usually not necessary to modify the conventional MRI protocol [40].

On the medial side, surgery may be indicated in high-level athletes and manual workers with persistent symptoms of instability and elbow pain after 6 months of adequate conservative treatment. Primary repair techniques generally yield poor results (Fig. 36), and ligament reconstruction with tendinous graft is preferred. There are several techniques for MCL complex reconstruction. The main reconstructive techniques of the MCL complex include the modified Jobe technique, the docking technique, and the interferential screw technique. These techniques use a free graft harvested from the palmaris longus or semitendinosus tendons, which is placed through one or more osseous tunnels in the humerus and ulna oriented in a manner that simulates the anatomy of the A-MCL. The Jobe technique (Fig. 37b) involves the dissection of the origin of the flexor-pronator muscles and transposition of the ulnar nerve. The main complication of this technique is secondary ulnar neuropathy. The modified Jobe technique involves a longitudinal incision of the flexor carpi ulnaris, which reduces the incidence of ulnar neuropathy. The docking technique (Figs. 35 and 37a) is a variant of the modified Jobe technique, with simplification of the humeral bone tunnels [40, 41].

Posterolateral rotatory instability often requires surgical treatment. The LUCL is reconstructed with a free tendon graft, which is fixated to one or more osseous tunnels in the lateral humeral epicondyle proximally and the sublime tubercle of the ulna distally in an orientation that simulates its anatomy (Fig. 38) [40, 42].