Relevant Anatomy, Biomechanics, and Diagnostics
The elbow is a complex hinge composed of three independent joints: the ulnotrochlear, the radiocapitellar, and the proximal radioulnar joint. The normal valgus carrying angle of the elbow is between 11 and 16 degrees, such that the forearms can clear the hips during the gait cycle. Stability of the elbow joint through range of motion and during athletic activity depends on the integrity of these bony articulations as well as ligamentous and other soft-tissue stabilizers.[24,25] Roughly 50% of elbow stability comes from osseous constraints while the remaining 50% comes from soft-tissue structures around the elbow. The anterior bundle of the UCL serves as the primary restraint to valgus moment force at the elbow. Other soft-tissue contributors to that stability include the flexor-pronator mass and the joint capsule. The UCL is itself composed of three bundles – the anterior, transverse, and posterior bundles (Figure 1). The anterior bundle originates on the anteroinferior aspect medial epicondyle and inserts on the sublime tubercle and UCL ridge, which extends medial and distal from the sublime tubercle. The anterior bundle has two bands – the anterior and posterior, which have a synergistic relationship with regard to tension and relaxation, depending on the degree of elbow flexion.
The kinematics of throwing have been well described. Translational and rotational moment forces throughout the phases of the throwing cycle build from the lower body up through the core and torso culminating in the upper extremity – ultimately placing extraordinary forces on the elbow. Structures about the medial elbow contend with the greatest biomechanical forces during the late cocking and early acceleration phases of the throwing cycle. Torque loads about the elbow while throwing a fastball have been shown to be in the range of 55 to 63 Nm in the high-level athlete.[26,27] The force transmitted through the medial elbow in these instances approaches the ultimate tensile strength of the UCL.[24–26,28] The flexor-pronator mass is an important dynamic stabilizer of the elbow and shields the UCL from a portion of these forces during the throwing cycle. This explains the propensity for injury of the UCL that occurs while pitching with muscular fatigue.[20,22] Adaptive physiologic changes in the biomechanical characteristics of the UCL in the throwing arm have been described with use of ultrasound. This includes UCL thickening and increased laxity which are theorized to allow the ligament to withstand greater valgus stress. Increased laxity is thought to allow the MCL to withstand greater amounts of physiologic deformation during throwing without failure. A recent report from Chalmers et al. saw similar trends in UCL thickness and laxity in professional pitchers with multivariate analysis. They followed 185 baseball pitchers from the start of one season to the next recording UCL thickness and laxity at 30 degrees of flexion at three time points. They noted that during the season the UCL thickens (P=0.002) and becomes more lax to valgus stress (P=0.002). During the season, they noted that the incompetence of the UCL alters the biomechanics of the posteromedial elbow by decreasing articular contact area (articular congruency), increasing contact pressure and, over time, increasing valgus laxity.
As with any injury in a throwing athlete, the physician must perform a comprehensive history and physical examination of the elbow, though the authors encourage evaluation of the patient's entire kinetic chain, including shoulder, hips, and core. Key history includes duration of symptoms, at what point in the pitching cycle the symptoms occur, and any prior treatments. The physician should note that not all medial elbow pain is related to the ulnar collateral ligament, and the examination should rule out other sources of pain including epicondylitis, ulnar neuritis, and flexor/pronator strain or tear. Physical exam begins with a complete neurovascular exam of the extremity as well as assessment of elbow and shoulder range of motion because the presence of GIRD and total motion deficits can lead to repeat injury if left unaddressed. The presence or absence of the palmaris longus tendon should be recorded, as this is a common graft source for reconstruction. The physician should identify any areas of tenderness to palpation and whether this corresponds to the ligament itself (proximal or distal), the flexor mass, posterior joint, etc. The presence of ulnar nerve symptoms on physical examination findings is critical because these could influence the decision to decompress or transpose the nerve during surgical intervention.
The most sensitive and specific exam maneuvers for diagnosis of UCL tear include the milking maneuver and moving valgus stress test (author's preferred provocative exam), the latter of which demonstrates a sensitivity of 100% and specificity of 75%. Delineating the nature of the tear prior to visualization in the operating room can guide treatment. A 2017 study examined the relationship between UCL tear location on MRI and likelihood to fail nonoperative treatment in 32 professional pitchers. Those with distal tears (compared to proximal) were more than 12 times more likely to fail conservative treatment and require surgery. Magnetic resonance arthrography (MRA) has proven more effective at diagnosing and elucidating the details of a tear as compared to traditional MRI. MRA was 92% sensitive and 100% specific compared with 57% and 100% for traditional MRI in a study by Magee. Ultrasound (US) can also be useful in determining the competency of the UCL. Ciccotti et al. examined the degree of ulnohumeral joint space opening on dynamic stress US of the elbow. They found that even partial UCL tears demonstrated a statistically significant increase in medial gapping on valgus stress. Despite the accuracy of diagnostic imaging in this instance, surgeons should always be prepared to perform either repair or reconstruction based on intraoperative judgment of tissue quality and nature of the injury.
Curr Orthop Pract. 2022;33(4):315-319. © 2022 Lippincott Williams & Wilkins