Elbow joint is a synovial joint of the hinge variety between the lower end of the humerus and the upper ends of radius and ulna. It communicates with the proximal radioulnar joint, in contrast to the wrist, which does not communicate with the distal radioulnar joint.
The lower end of the humerus shows the prominent conjunction of capitulum and trochlea. The capitulum, for the head of the radius, is a portion of a sphere. It projects forward, and also downwards, where its lower border lies at the distal extremity of the humerus. In contrast the trochlea, which lies medial, is a grooved surface that extends around the lower end of the humerus to the posterior surface of the bone. The groove of the trochlea is limited medially by a sharp and prominent ridge and laterally by a lower and blunter ridge that blends with the articular surface of the capitulum. Thus is produced a tilt on the lower end of the humerus that accounts in part for the carrying angle of the elbow. Fossae immediately above the capitulum and trochlea receive the head of the radius and coronoid process of the ulna in full flexion; posteriorly a deep fossa receives the olecranon in full extension.
The upper surface of the cylindrical head of the radius is spherically concave to fit the capitulum. The upper end of the ulna shows the deep trochlear notch. A curved ridge joins the prominences of coronoid process and olecranon; the ridge fits the groove in the trochlea of the humerus. The obliquity of the shaft of the ulna to this ridge accounts for most of the carrying angle at the elbow. There are commonly two separate articular surfaces in the trochlear notch, one on the olecranon and the other on the coronoid process.
Ligaments of the elbow joint:
The capsule is attached to the humerus at the margins of the lower rounded ends of the articular surfaces of capitulum and trochlea, but in front and behind it is carried up over the bone above the coronoid and olecranon fossae Distally, the capsule is attached to the trochlear notch of the ulna at the edge of the articular cartilage, and to the annular ligament of the proximal radioulnar joint. It is not attached to the radius.
The capsule and annular ligament are lined with synovial membrane, which is attached to the articular margins of all three bones. The synovial membrane thus floors in the coronoid and olecranon fossae on the lower end of the humerus, and bridges the gap between the radial notch of the ulna and the neck of the radius. The quadrate ligament prevents herniation of the synovial membrane between the anterior and posterior free edges of the annular ligament.
The ulnar collateral (medial) ligament of the elbow joint is triangular and consists of three bands. The anterior band is the strongest. It passes from the medial epicondyle of the humerus to a small tubercle (unnamed but previously called the sublime tubercle) on the medial border of the coronoid process. The posterior band joins the sublime tubercle and the medial border of the olecranon. A middle band connects these two and lies more deeply; it lodges the ulnar nerve on its way from the arm to the forearm. The radial collateral (lateral) ligament is a single flattened band attached to the humerus below the common extensor origin; it fuses with the annular ligament of the head of the radius. The anterior and posterior ligaments are merely thickened parts of the capsule. The annular ligament is attached to the margins of the radial notch of the ulna, and clasps the head and neck of the radius in the proximal radioulnar joint. It has no attachment to the radius, which remains free to rotate in the annular ligament.
Nerve supply of elbow joint:
The joint is supplied by the musculocutaneous, median, ulnar and radial nerves (Hilton’s law).
Movements of elbow joint:
The only appreciable movement possible at the elbow joint is the simple hinge movement of flexion and extension. From the straight (extended) position the range of flexion is about 140°. This movement does not take place in the line of the humerus, for the axis of the hinge lies obliquely. The extended ulna makes with the humerus an angle of about 170°. This so-called ‘carrying- angle’ fits the elbow into the waist when the arm is at the side, and it is significant that the obliquity of the ulna is more pronounced in women than in men. However, the line of upper arm and forearm becomes straightened out when the forearm is in the usual working position of almost full pronation. A pathological increase in this ‘valgus’ angle (e.g. from a fractured lateral epicondyle or damaged epiphysis) may gradually stretch the ulnar nerve behind the epicondyle and cause an ulnar nerve palsy. During pronation-supination of the forearm there is some rocking movement of the ulna on the trochlea.