Elbow Anatomy and Biomechanics

Introduction

The elbow consists of three joints: the ulnohumeral joint, radiocapitellar joint, and proximal radioulnar joint (PRUJ). Together, these three joints enable the elbow to flex and extend as a hinge joint as well as rotate about a longitudinal axis for pronation-supination. As such, the elbow is classified as a trochoginglymoid joint.

The distal humerus has a 30-degree anterior tilt relative to the humeral shaft; therefore the anterior cortex of the humeral shaft should intersect the capitellum between the anterior and middle thirds of the capitellum. The trochlea is shaped like a spool of thread; the medial ridge of the trochlea is more prominent than the lateral ridge, and the trochlea extends more distal and posterior than the capitellum. As a result, the joint line is in about 6 degrees of valgus and 5 degrees of internal rotation. ^, This also contributes to the valgus carrying angle of the elbow, which is defined as the angle between the long axis of the humerus and the long axis of the ulna, and ranges between 10 and 15 degrees in males and 15–20 degrees in females. The distal humerus consists of two articulations: medially, the trochlea articulates with the greater sigmoid notch of the proximal ulna and laterally, the capitellum articulates with the radial head.

The ulnohumeral joint acts as a hinge joint. Anteriorly, the proximal aspect of the ulna is known as the coronoid process, which plays a critical role in elbow stability. It provides a highly congruent articulation, and several key soft tissue structures such as the medial collateral ligament (MCL), anterior capsule, and brachialis insert on the coronoid process. Posteriorly, the olecranon limits the extension of the elbow.

The radiocapitellar joint not only acts in flexion and extension but also allows for rotation. Here, the radial head articulates with the capitellum. The concave radial head provides stability at this joint through a concavity-compression method. Rotation occurs via the PRUJ. The PRUJ consists of the articulation between the radial head and the lesser sigmoid notch of the ulna. Approximately 240–280 degrees of the circumference of the radial head are covered with cartilage to allow for pronation and supination. ^, ^, ^,

In the skeletally immature elbow, the ossification centers of the elbow appear in a predictable manner. The mnemonic “CRITOE,” or some variation, is often used to describe the pattern of ossification. The capitellum appears first, followed by the radial head, medial (“internal”) epicondyle, trochlea, olecranon, and lateral (“external”) epicondyle. On imaging, it is particularly important to examine the relationships between the anterior humeral line and capitellum and the radial head and capitellum to ensure appropriate reduction.

Muscular Anatomy

The elbow flexors consist of the biceps, brachialis, and brachioradialis. The biceps tendon attaches to the radial tuberosity, while the brachialis attaches 11 mm distal to the tip of the coronoid.

The triceps attaches to the olecranon and acts to extend the elbow. While the anconeus also contributes to elbow extension, it functions more as a restraint to varus and posterolateral rotatory instability (PLRI).

The forearm flexor-pronator group originates from the medial epicondyle of the elbow and consists of the flexor carpi radialis, palmaris longus, pronator teres, flexor digitorum superficialis (FDS), and flexor carpi ulnaris (FCU). The forearm extensors arise from a common origin of the lateral epicondyle and include the brachioradialis, extensor carpi radialis longus, extensor carpi radialis brevis, extensor carpi ulnaris, extensor digiti minimi, and extensor digitorum communis. The supinator also has one head that arises from the lateral epicondyle, while the other originates from the proximal ulna.

Neurovascular Anatomy

The median nerve arises from the medial and lateral cords of the brachial plexus. It crosses the elbow anteriorly, medial to the brachial artery, and at the level of the elbow joint, it lies deep to the bicipital aponeurosis and superficial to the brachialis. Distally, it continues deep to the pronator teres and then between the two heads of the FDS.

The musculocutaneous nerve arises from the lateral cord of the brachial plexus, travels between the biceps and the brachialis, and becomes the lateral antebrachial cutaneous nerve.

The radial nerve originates from the posterior cord of the brachial plexus, travels along the spiral groove of the humerus, and pierces the lateral intermuscular septum 7.5 cm above the trochlea. It crosses the elbow anteriorly, between the brachialis and brachioradialis. At the level of the joint, it divides into the posterior interosseus nerve, which enters the forearm between the two heads of the supinator, and the superficial radial nerve, which courses deep to the brachioradialis.

The ulnar nerve arises from the medial cord of the brachial plexus. In the arm, it travels medial to the brachial artery and pierces the medial intermuscular septum at the arcade of Struthers to pass into the posterior compartment. It then runs along the back of the medial epicondyle; dives between the two heads of the FCU, giving off the first motor branch to the FCU; and then runs along the anterior surface of the FDP.

The brachial artery also travels in the cubital fossa lateral to the median nerve and divides into the radial and ulnar arteries in the cubital fossa. The radial artery then passes medial to the biceps tendon and runs anterior to the supinator. The ulnar artery passes deep to the deep head of the pronator teres.

Elbow Biomechanics

While full elbow range of motion is from 0 degrees (some individuals may be capable of up to 15 degrees of hyperextension) to 140 degrees of flexion with a 180-degree arc of pronation/supination, the functional arc of motion of the elbow is from 30 to 130 degrees of flexion, as well as 50 degrees of pronation and 50 degrees of supination. The ulnohumeral joint bears roughly 40% of the axial load across the elbow, while the radiocapitellar joint bears the other 60%. However, this distribution can vary depending on the degree of flexion. Forces across the radiocapitellar joint are greatest in 0–30 degrees of flexion and in pronation. During activities of daily living, joint reaction forces vary from a maximum of 350 N with activities, such as eating and dressing, to 2000 N with maximal isometric flexion. During push-ups, the elbow bears 45% of body weight.

The stabilizers of the elbow can be divided into (1) primary and secondary static stabilizers and (2) dynamic stabilizers. Primary stabilizers are those that lead to laxity when they are released. Injury or release of a secondary stabilizer alone does not lead to instability, but increased laxity occurs when a secondary stabilizer is damaged in addition to a compromised primary stabilizer.

Primary Stabilizers

Primary static stabilizers of the elbow include the bony anatomy (i.e., ulnohumeral joint), the MCL, and the lateral collateral ligament (LCL) complex. The articular anatomy provides 50% or more of the stability to valgus and varus stress, most notably at the extremes of flexion and extension when the coronoid and olecranon engage into the coronoid and olecranon fossae, respectively. ^, ^, ^, At mid-flexion, stabilization from ligamentous structures predominates. Prior studies have shown that with a loss of 50% of coronoid height, the elbow develops posterior instability, which is greater in positions of flexion, as well as varus laxity, particularly at lower flexion angles, because of the loss of soft tissue attachments.

The elbow MCL, or ulnar collateral ligament, originates from the medial epicondyle posterior to the axis of rotation and acts as a restraint against valgus stress and posteromedial rotatory instability. The elbow MCL consists of an anterior bundle, a posterior bundle, and oblique or transverse fibers between the two. The anterior bundle inserts on the sublime tubercle of the coronoid process and can be further divided into anterior, central, and posterior bands. While there is no true isometric point for the MCL, the central band originates closest to the axis of rotation of the ulnohumeral joint, and so it is nearly isometric throughout the elbow’s arc of motion. The anterior band acts as the primary restraint to valgus stress in flexion up to 90 degrees; at 120 degrees, the anterior and posterior bands of the anterior bundle act as coprimary restraints. The anterior band also resists internal rotation. Overall, the effect of anterior bundle deficiency is greatest in 70–90 degrees of flexion. ^, The posterior bundle of the MCL inserts on the medial olecranon and provides restraint to valgus stress when the elbow is in greater than 90 degrees of flexion. ^, The posterior bundle also forms the floor of the cubital tunnel.

The LCL complex protects against PLRI and contributes about 10% of overall restraint to varus instability. It consists of the radial collateral ligament (RCL), lateral ulnar collateral ligament (LUCL), accessory collateral ligament, and annular ligament. The LUCL and RCL originate from the lateral epicondyle at the isometric point. The LUCL inserts on the supinator crest of the ulna and is the primary ligamentous stabilizer to varus stress throughout the elbow’s arc of motion. The annular ligament encircles the radial head and attaches at the lesser sigmoid notch, and the RCL attaches to the annular ligament to stabilize the radial head. Studies have suggested that if the annular ligament is intact, stability is maintained even if the LUCL and RCL are transected.

Secondary Stabilizers

Secondary static stabilizers include the radiocapitellar joint, joint capsule, and flexor and extensor origins. The radial head is the second most important restraint to valgus stress following the anterior bundle of the MCL. This is most notable from 0 to 30 degrees of flexion. While some studies have shown that the radial head provides approximately 30% of valgus stability, Morrey et al. demonstrated that as long as the MCL is intact, loss of the radial head does not lead to clinical instability.

The capsule provides its greatest contribution to stability at maximum extension. It can be maximally distended at about 70–80 degrees of flexion, and intra-articular volumes up to 25–35 mL can be achieved. ^,

Dynamic Stabilizers

The dynamic stabilizers of the elbow include the muscles that cross the joint, such as the biceps, brachialis, triceps, and anconeus. These muscles provide stability through compressive forces. In general, the flexor mass resists valgus forces and the extensors resist varus forces. In particular, the anconeus provides restraint against varus and PLRI. In fact, muscle tension after release of the MCL can restore near-normal patterns of motion.

Summary

The elbow is a complex joint involving multiple different articulations with a variety of osseous and soft tissue support structures. A fundamental knowledge of elbow anatomy and biomechanics is critical to understand how sports-related injuries impact the structure and function of the elbow joint.