Corpus: Elbow joint

The elbow joint is a compound joint (Articulatio composita) where the humerus articulates with the radius and ulna. Three sub-joints can be distinguished:

• humeroulnar joint
• humeroradial joint
• proximal radioulnar joint

Note: Some authors consider the proximal radioulnar joint (PRUJ) and the distal radioulnar joint (DRUJ) as independent of the elbow joint. Considering the anatomical unity of the three mentioned sub-joints, the proximal radioulnar joint is regarded as part of the elbow joint in this context.

The three sub-joints of the elbow joint are enclosed by a common joint capsule, forming a functional unit.

2.1. Humeroulnar joint

In the humeroulnar joint, the trochlea of the humerus articulates with the trochlear notch of the ulna. Functionally, it is a hinge joint allowing flexion and extension of the forearm in relation to the upper arm.

2.2. Humeroradial joint

The articular surfaces of the humeroradial joint are the capitulum of the humerus and the radial head's articular fovea. Despite having the structural features of a ball-and-socket joint, the interosseous membrane antebrachii fixes the radius to the ulna, limiting the joint to two degrees of freedom (supination/pronation, flexion/extension).

2.3. Proximal radioulnar joint

The proximal radioulnar joint is a pivot joint, specifically a pivot hinge. The circumferentia articularis radii articulates with the radial notch of the ulna and the cartilage-covered ligamentum anulare radii.

The joint capsule of the elbow joint encompasses all sub-joints widely. On the humerus, the capsule is attached ventrally above the articular surfaces and encloses dorsally the olecranon fossa. On the ulna, the joint capsule is closely attached to the transition between cartilage and bone. Additional attachments exist at the olecranon and coronoid process. On the radius, the capsule is extensive (recessus sacciformis) and extends into the neck region of the bone. During forearm extension, the dorsal side, and during flexion, the ventral side of the joint capsule folds. Adipose bodies formed in the joint fill the spaces created by the bones' movement. The capsule itself is tensioned by muscle fibers from the triceps dorsally and the brachialis ventrally, protecting it from being pinched between the movable joint surfaces.

The elbow joint is guided laterally by robust collateral ligaments. The ulnar colleteral ligament (medially located) extends from the medial epicondyle of the humerus to the medial side of the trochlear notch of the ulna and the coronoid process of the ulna. This ligament prevents valgus stress on the elbow joint. The radial collateral ligament originates from the lateral epicondyle of the humerus, it merges with the annular ligament of radius. It is often fused with muscle fibers of the superficial extensors. The annular ligament of radius is considered as part of the joint capsule, it surrounds the head of the radius from the ulnar side, functionally securing the radial head. However, due to the cartilage-covered inner surface, it allows unrestricted rotation of the radius relative to the ulna. Some sources also mention a quadrate ligament, spanning between the radial notch of the ulna and the neck of the radius.

Another part of the ligamentous apparatus is the membrana interossea antebrachii, forming between the interosseous margins of the radius and ulna. In the proximal section, it is reinforced by the oblique cord, representing an oblique fiber tract whose direction opposes that of the fibers of the interosseous membrane of the forearm. The oblique cord runs from the ulna distolaterally, just below the tuberosity of the radius. It limits supination between the ulna and radius.

In the vicinity of the elbow joint, bursae occur variably in response to increased mechanical stress, typically with no connection to the joint cavity. Chronic overuse may lead to inflammation in the bursae between the biceps tendon and the tuberosity of the radius and between the olecranon and the skin.

In the elbow joint, flexion and extension of the forearm relative to the upper arm and pronation and supination of the radius relative to the ulna are possible. The following movement ranges are based on the neutral-zero position.

6.1. Flexion and Extension

Flexion/extension occurs in the humeroradial and humeroulnar joints. Flexion can be performed up to 150° (angle between forearm and upper arm). Extension beyond the neutral position is not consistently present. Children and women may exhibit an overstretched capability of approximately 10°.

6.2. Pronation and Supination

Pronation/supination occurs in the proximal radioulnar joint in collaboration with the distal radioulnar joint. Both movements are possible up to an angle of 80-90°. During radial rotation, the radial head moves in the humeroradial joint. In pronation, there is a crossed position of the radius and ulna when viewed from the front, while in supination, there is a parallel position.

Aseptic bone necrosis, overload syndromes, and traumas such as Monteggia fractures, radial head fractures, unhappy triad, olecranon bursitis, tennis elbow, and Panner's disease may occur in the elbow joint area. Diagnosis is usually clinical and through imaging (e.g., X-ray of the elbow).