Together with the shoulder, the elbow enables us to move and position the hand to a desired location to perform functions that are fundamental to our daily living activities. The elbow is also the intermediate of the upper extremity kinetic chain and provides a foundation for powerful upper extremity motions in various physical occupational and recreational activities. The elbow is a highly complex trocho-ginglymoid joint composed of the ulnohumeral, radiocapitellar, and proximal radioulnar joints. The capsuloligamentous and muscular structures that act across the elbow are responsible for controlled range of motion (ROM) as well as static and dynamic stability of the elbow and forearm. A thorough understanding of elbow anatomy and biomechanics is a key to accurate assessment and treatment of various elbow conditions.

The distal humerus, proximal ulna, and proximal radius form a highly congruent skeletal articulation that contributes to joint stability and determines ROM. The bony anatomy provides approximately 50% of elbow joint stability. The distal humerus articular surface is comprised of two components that articulate with the proximal end of the radius and ulna (Figure 12.1, A and B). The spool-shaped trochlea is the medial articular aspect, and articulates with the greater sigmoid notch of the proximal ulna. The medial ridge of the trochlea projects more distally than the lateral ridge, which results in 6° to 8° of valgus tilt at the ulnohumeral articulation. The hemispherical capitellum is the lateral articular component, and articulates with the radial head. In the sagittal plane, the orientation of the articular surface of the distal humerus is positioned approximately 30° anterior to the long axis of the humerus, which facilitates flexion while limiting the maximum extension of the elbow (Figure 12.2). The center of the rotation of the arc formed by the trochlea and capitellum is in line with the anterior cortex of the distal humerus.

The proximal ulna has two articular components—the greater sigmoid notch and lesser sigmoid (radial) notch—which form highly congruent articulations with the trochlea and radial head (Figure 12.3). The guiding ridge of the greater sigmoid notch articulates with the apex of the trochlea groove. In the sagittal plane, the greater sigmoid notch opens posteriorly by 30° with respect to the long axis of the ulna shaft (Figure 12.4). In flexion, the coronoid process of the proximal ulna locks into the coronoid fossa of the distal humerus, while the radial head fits into the radial fossa. In extension, the tip of the olecranon is engaged in the olecranon fossa increasing the osseous stability of the joint.

The radial head has a concave proximal end, which articulates with the capitellum, and an articular rim, which articulates with the lesser sigmoid notch of the proximal ulna (Figure 12.5). The shape of the radial head is elliptical and is highly variable among individuals. The radial tuberosity marks the distal margin of the radial neck; it is the site of attachment of the distal end of the biceps tendon.

In addition to the highly congruent bony articulations, the capsuloligamentous structures are primary static stabilizers of the elbow. The static soft-tissue stabilizers include the anterior and posterior joint capsule, and the medial and lateral collateral ligament complexes.

The medial collateral ligament (MCL) complex consists of three parts: anterior, posterior, and transverse components
(Figure 12.6). The origin of the complex is at the anteroinferior aspect of the medial epicondyle. The anterior bundle is the most discrete component, and inserts on the sublime tubercle on the anteromedial aspect of the coronoid process. Numerous studies have demonstrated increased valgus joint laxity after sectioning of the anterior bundle of the MCL even with an intact radial head. Clinically, the functional importance of the anterior bundle has been confirmed, and various ligament reconstructions have been developed. The posterior bundle is a thickening of the posterior capsule forming the floor of the cubital tunnel, and is well defined at about 90° of flexion. Although the posterior bundle contributes little to the static stability of the elbow, it does provide a moderate contribution to rotational stability. In addition, contracture of the posterior bundle can limit elbow flexion. The transverse ligament runs between the coronoid and the tip of the olecranon, and consists of horizontally oriented fibers that often cannot be separated from the capsule. The transverse ligament appears to contribute little or nothing to elbow stability.

The lateral collateral ligament (LCL) complex is less discrete than the medial complex, and there is often individual variance. The LCL complex consists of four components: the lateral ulnar collateral ligament (LUCL), the annular ligament, the radial collateral ligament, and the variably present accessory LCL (Figure 12.7). The LUCL originates from the lateral epicondyle, and blends with the fibers of the annular ligament inserting on the tubercle of the supinator crest of the ulna. This ligament is considered to be one of the primary elbow constraints, and provides varus and posterolateral

stability. Disruption of the LUCL has been demonstrated to be an important cause of both acute and recurrent dislocation of the elbow. The annular ligament is a strong band of tissue that wraps around the radial neck, originating and inserting on the anterior and posterior margins of the lesser sigmoid notch. This funnel-shaped ligament stabilizes the proximal radius throughout the range of pronation and supination. The radial collateral ligament originates from the lateral epicondyle and inserts into the annular ligament. This ligament contributes to the stability of the radial head. The accessory LCL originates from the supinator crest and inserts into the annular ligament. It is believed that this ligament contributes to the stability of the annular ligament during varus stress.