The Thrower’s Elbow

Introduction

Throwing places unique demands on the elbow, resulting in predictable injury patterns. In the late cocking and early acceleration phases of throwing, valgus torques are estimated to reach 64 N-m and angular velocities of 5000 degrees per second as the elbow extends from 110 to 20 degrees of flexion. Appreciation of the elbow anatomy and biomechanics relevant to throwing assists the clinician in diagnostic and therapeutic decision making in addition to surgical technique. The combination of valgus torque and rapid extension generates three major forces on the elbow—tensile stress along the medial aspect (ulnar collateral ligament [UCL], flexor pronator mass, medial epicondyle), compression force in the lateral aspect (radiocapitellar joint), and shearing stress in the posterior compartment (posteromedial tip of the olecranon and olecranon fossa).

On the medial side of the elbow, the repetitive tensile forces during throwing challenge the ultimate strength of the UCL, creating the well-known injury risk to the ligament. Patients who develop valgus instability and continue to throw may initiate and exacerbate pathology in the posterior and lateral aspects of the elbow. In the lateral compartment, compression forces reaching as high as 500 N may result in injuries that have been referred to as radiocapitellar overload syndrome. The syndrome often occurs in conjunction with medial ligament instability and valgus extension overload. Persistent repetitive radiocapitellar compression may eventually result in chondral or osteochondral fracture and the production of intraarticular loose bodies. In skeletally immature athletes, this is in part the proposed cause of capitellar osteochondritis dissecans (OCD). In the posterior compartment, throwing repeatedly drives the olecranon forcefully into the olecranon fossa. The combined valgus and extension forces create a shear stress across the medial aspect of the olecranon tip and fossa and can lead to the development of osteophytes ( Fig. 66.1 ). This constellation of injuries is referred to as valgus extension overload syndrome and can generate symptomatic chondral lesions, loose bodies, and marginal exostosis.

Pathomechanics

The mechanics of high-velocity throwing generate distinctive forces in the elbow that must be resisted by articular, ligamentous, and muscular constraints. The ulnohumeral articulation provides stability in the extremes of motion, from 0 to 20 degrees of flexion and beyond 120 degrees of flexion, whereas static and dynamic soft tissue restraints are responsible for the intervening 100 degrees arc of motion. As the elbow approaches terminal extension during arm deceleration, the combined valgus force and angular moment are dissipated as the posteromedial olecranon contacts the trochlea and olecranon fossa. The resultant shear and compressive forces can be amplified by poor control of dynamic muscular constraints, and traumatic abutment in the posterior compartment ensues. The repetitive compressive and shear forces induce reactive bone formation, resulting in osteophytes localized to the posteromedial tip of the olecranon. Corresponding so-called kissing lesions of chondromalacia may exist in the olecranon fossa and posteromedial trochlea in addition to loose bodies.

The relationship of posteromedial impingement (PMI) of the olecranon and valgus instability has been the focus of several clinical and biomechanical studies. In a radiographic examination of 135 asymptomatic professional pitchers, Conway has identified olecranon tip exostosis in 24% of lateral radiographs and found that 21% of pitchers had 1.0 mm or more of increased relative valgus laxity on stress radiographs. For players with exostosis formation, 34% had relative valgus laxity of 1.0 mm or greater, compared to only 16% of players without exostosis formation. These data supported a plausible relationship between posteromedial impingement and valgus laxity. In a subsequent biomechanical study, Ahmad et al. investigated the effect of partial- and full-thickness UCL injuries on the contact forces of the posterior elbow. Pressure-sensitive film was placed in the posteromedial compartment of the cadaver specimens, which were then subjected to physiologic valgus stresses. UCL insufficiency was found to alter the contact area and pressure between the posteromedial trochlea and olecranon, helping explain the development of posteromedial osteophytes.

Other biomechanical studies have investigated the reciprocal relationship of the posteromedial olecranon acting as a stabilizing buttress to medial tensile forces. It has been suggested that aggressive bone removal when treating PMI increases valgus instability and subsequent strain on the UCL, which may result in UCL injury following olecranon resection. Kamineni et al. used an electromagnetic tracking device in cadaver elbows to investigate the strain in the anterior bundle of the UCL as a function of increasing applied torque and posteromedial resections of the olecranon. A nonuniform change in strain at 3 mm of resection during flexion and valgus testing suggested that removal of the posteromedial olecranon beyond 3 mm jeopardized UCL injury. A subsequent cadaveric study examined the kinematic effects of increasing valgus and varus torques and sequential posteromedial olecranon resections. Although no single critical amount of olecranon resection was identified, valgus angulation of the elbow increased with all resection levels.

In summary, the basic science literature supports the posteromedial articulation of the elbow as an important stabilizer to valgus stress. The implications for the professional throwing athlete are important, and it is recommended that only pathologic osteophytes be removed from the olecranon during resection, and normal bone should be preserved.