The supraphysiological demands imposed on the elbow of a throwing athlete result in predictable patterns of injury. This is especially true of baseball pitchers. Knowledge of elbow anatomy, as well as the biomechanics of throwing, assist in making diagnostic and therapeutic decisions and also influence surgical technique when surgery is required. During the late cocking and early acceleration phases of throwing, valgus torque can reach 65 Nm with angular velocities of the forearm reaching 5000°/sec, which is considered the fasted recorded human movment.1 The valgus torque and rapid extension synergistically create 3 major forces placed on the elbow. The first is a tensile stress along the medial aspect of elbow affecting the ulnar collateral ligament (UCL), flexor pronator mass, and medial epicondyle. Secondly, compression forces affect the lateral aspect of the elbow at the radiocapitellar joint. Finally, a shearing stress occurs in the posterior compartment at the posterior medial tip of the olecranon and the olecranon fossa.
These forces generated on the elbow result in predictable pathology. The recurring tensile forces applied on the medial aspect on the elbow can compromise the integrity of the UCL. It is well known that injury to the UCL leads to valgus instability. Individuals with valgus instability who continue to throw may trigger and/or aggravate injury in the posterior and lateral components of the elbow. Lateral compression forces can often reach 500 N, resulting in radiocapitellar overload syndrome, which occurs in combination with medial ligament instability and valgus extension overload.2 Radiocapitellar compression may cause chondral or osteochondral fracture with resulting intra-articular loose bodes. This compression also contributes to the etiology of osteochondritis dissecans (OCD) in skeletally immature athletes. In the posterior elbow, throwing forcefully and repeatedly pushes the olecranon into the olecranon fossa. Shear stress on the medial olecranon tip and fossa, due to combined valgus and extension forces, lead to the development of osteophytes. This collection of injuries in the medial, lateral, and posterior aspects of the elbow is known as “valgus extension overload syndrome” or VEO. Symptoms in VEO can be the result of chondral lesions, loose bodies, and marginal exostosis.3
The aim of this review is to provide understanding regarding both the relevant anatomy and pathomechanics of VEO, key aspects to clinical evaluation, and effective treatment options.
Functional Anatomy
A functional comprehension of elbow anatomy and biomechanics is essential to understanding the constellation of injuries in VEO. The osseous anatomy of the elbow permits a variety of movements. These include flexion-extension and pronation-supination, which are mediated by the ulnohumeral and radiocapitellar articulations. While in full extension, the elbow has a normal valgus carrying angle of 11° to 16°. It is important to know that 50% of the elbow’s stability is attributed to the configuration of the bones.4-6 This is especially true in varus stress while the elbow is in full extension. The soft tissues, including muscle and ligaments such as the UCL, lateral UCL, and radial UCL complexes, provide the remaining elbow stability.4-6
The UCL complex is composed of 3 main segments known as the anterior, posterior, and oblique bundles (transverse ligament). Collectively, these bundles are responsible for providing medial elbow stability. However, each of these bundles contributes to medial elbow stability in its own way. The first and arguably the most important bundle is the anterior bundle; its most important function is providing stability against valgus stress.4,5,7 It is composed of parallel fibers inserting on the medial coronoid process.4,5,7 Furthermore, its eccentric location with respect to the axis of elbow allows it to provide stability throughout the full range of elbow motion.6 The anterior bundle can be further divided into individual anterior and posterior bands that have reciprocal functionality.5,8,9 The anterior band acts as the chief restraint to valgus stress up to 90° of flexion.9 Any flexion beyond 90° renders the anterior band’s role secondary in resisting valgus stress.9 The posterior band’s function in resisting valgus stress is most important between 60° and full flexion, while having a secondary role in lesser degrees of flexion.8,9 Notably, the posterior band is isometric and is more important in the overhead-throwing athlete due to the fact its primary role in resisting valgus stress occurs at higher degrees of flexion.10
The remaining posterior and oblique bundles of the UCL complex have lesser roles in maintaining elbow stability. The posterior bundle of the UCL complex is fan-shaped, originates from the medial epicondyle, and inserts onto the medial margin the semi-lunar notch. It is more slender and frailer than the anterior bundle. This is reflected in its functionality, as it plays a secondary role in elbow stability during elbow flexion beyond 90°.4,5,8 In contrast to the anterior and posterior bundles, the oblique bundle, also known as the transverse ligament, does not cross the elbow joint. It is a thickening of the caudal most aspect of the joint capsule, which extends from the medial olecranon to the inferior medial coronoid process and as a result functions in expanding the greater sigmoid notch.6