The elbow joint consists of three bones: the humerus, the ulna, and the radius.
These bones create three articulations – the ulnohumeral, the radiohumeral, and the proximal radioulnar – that provide flexion, extension, and forearm rotation. The joint provides a link for positioning the hand in three dimensions, acts as a fulcrum for the forearm lever, and also bears some of the load of the upper extremity.
Left elbow joint showing anterior and ulnar collateral ligaments (left) and posterior and external ligaments (right).
The elbow joint is a hinge-type synovial joint. The spool-shaped trochlea of the distal humerus and the trochlear notch of the ulna articulate to form a joint that approximates a hinge, providing the control and stability of flexion and extension. The radiohumeral articulation, comprised of the spheroidal capitulum of the humerus proximally and the slightly concave radial head distally, aids secondarily in flexion and extension. The proximal radioulnar articulation, together with the distal radioulnar joint, is responsible for rotation of the forearm. The articular surfaces are covered with hyaline cartilage. They come in contact when the forearm is midway between supination and pronation and when flexed to ninety degrees.
The configuration of the elbow joint allows for flexion and extension of the forearm. When the elbow is fully extended and the forearm is supinated, the axis of the ulna creates an angle of approximately 170 degrees with the axis of the humerus. The difference between this angle and a straight axis drawn inferiorly from the humerus is known as the carrying angle. The normal carrying angle is 10 to 20 degrees, with slightly greater cubitus valgus observed in females in most studies.
Supination and pronation of the forearm occurs at the proximal radioulnar joint. The radial head rotates within the radial notch of the ulna with soft tissue support provided by the anular ligament. Supination places the palm anteriorly in the anatomical position, or superiorly when the forearm is flexed. Pronation turns the palm posteriorly, or inferiorly when the forearm is flexed. Proximally at the elbow, the axis for supination and pronation exists at the center of the head of the radius. Distally, the radius rotates around the head of the ulna, with the axis passing through the attachment of the triangular ligament complex to the ulna.
Ligaments and Soft Tissues
A fibrous articular capsule encloses the elbow joint. The capsule extends from the margins of the articular surfaces of the capitulum and trochlea on the humerus to the coronoid process anteriorly. Posteriorly, the capsule attaches to the olecranon fossa. The capsule is weaker anteriorly but receives support on each side from collateral ligaments. The capsule is lined by a synovial membrane. Both the fibrous capsule and the synovial membrane extend inferiorly and are continuous with the capsule and synovial lining of the proximal radioulnar joint.
Medially and laterally, the elbow joint is strengthened by collateral ligaments. These ligaments are triangular bands that are actually thickenings of the fibrous capsule. On the lateral side, the fanlike radial collateral ligament extends from the lateral epicondyle of the humerus and blends with the anular ligament of the radioulnar articulation. The ulnar collateral ligament spans the medial side of the elbow. It attaches proximally to the medial epicondyle of the humerus and distally to the coronoid process and the olecranon of the ulna. The ulnar collateral ligament has three separate bands:
- The anterior cord-like band, which is the strongest, is taut when the elbow joint is extended.
- The posterior band, which is fan-shaped and the weakest component, is taut in flexion.
- The thin oblique band functions to deepen the socket for the trochlear aspect of the humerus.
The proximal radioulnar joint possesses specific ligaments for its function as well. The anular ligament encircles the radial head, forming a collar together with the radial notch of the ulna. This ligament surrounds the joint, holding the radial head in place as supination and pronation of the forearm takes place. The deep surface of the anular ligament is lined with a synovial membrane. This membrane continues to the neck of the ulna, forming a sacciform recess. The synovial lining allows the radial head to rotate smoothly within the anular ligament.
Many bursae exist around the elbow. The three olecranon bursae are the intratendinous bursa, the subtendinous bursa, and the subcutaneous olecranon bursa.
- The intratendinous bursa occasionally exists in the tendon of the triceps brachii.
- The subtendinous bursa lies just proximal to the attachment of the triceps tendon, separating the tendon from the olecranon.
- The subcutaneous olecranon bursa lies within the subcutaneous tissues over the olecranon.
Other important bursae function in or around the elbow joint.
- The bicipitoradial bursa lies between the biceps tendon and the anterior part of the radial tuberosity.
- The radioulnar bursa is located between the extensor digitorum, the radiohumeral articulation, and the supinator muscle.
The elbow is a common site for the insertion and origin of many muscles that act in the upper extremity. Anterior muscles include the flexors of the elbow and the flexor-pronator muscles of the forearm, which originate from the medial epicondlye of the humerus. Muscles that cross the elbow posteriorly include elbow extensors, the supinator, and the wrist and forearm extensors, which originate from the lateral epicondyle of the humerus.
Flexion of the elbow joint is controlled by the brachialis, the biceps brachii, and the brachioradialis.
- The brachialis is the chief flexor of the elbow. Its proximal attachment is on the distal half of the anterior humerus, extending distally to the coronoid process and tuberosity of the ulna.
- The biceps brachii consists of a short head and long head. The short head orginates from the the tip of the coracoid process of the scapula, while the long head begins at the supraglenoid tubercle. The distal attachment is at the tuberosity on the radius and the fascia of the forearm via the bicipital aponeurosis.
- The brachioradialis originates at the proximal two-thirds of the lateral supracondylar ridge of the humerus. Distally, it attaches to the lateral surface the distal radius.
The pronator teres assists when flexion is resisted. This musculocutaneous nerve innervates the muscles responsible for flexion of the elbow, with fibers from C5 and C6 acting as the main segmental innervation.
Extension around the elbow joint is powered by the triceps brachii. The triceps is a large muscle located in the posterior aspect of the arm, and it is comprised of three heads: long, lateral, and medial.
- The long head of the triceps originates at the infraglenoid tubercle of the scapula.
- The lateral head and medial head originate at the posterior surface of the humerus.
- The triceps inserts distally at the proximal end of the olecranon and the fascia of the forearm.
The major component of the triceps involved in extension is the medial head. The triceps receives assistance from the anconeus as well. The radial nerve controls the triceps, with the C7 nerve root primarily responsible for the action of elbow extension.
At the proximal radioulnar joint, supination of the forearm is produced by the supinator and biceps brachii. The biceps brachii performs more work when power is required due to resistance of supination. The extensor pollicus longus and the extensor carpi radialis longus provide some assistance to supination. Pronation is controlled primarily by the pronator quadratus and secondarily by the pronator teres. The flexor carpi radialis along with the palmaris longus and brachioradialis (when forearm is in the midprone position) assist somewhat.
Nerves and Blood Vessels
Nerves and blood vessels are important constituents of the elbow, but for the most part, they are transients, on their way to important tasks in the forearm, wrist, and hand. The elbow is supplied by anastomoses from various vessels.
The brachial artery, adjacent to the median nerve, descends through the anterior compartment of the elbow. Before it reaches the elbow, the brachial artery gives off branches that help form a plexus of vessels around the elbow. The superior and inferior ulnar collateral arteries, the deep brachial artery, and the radial collateral artery supply the elbow from above. At approximately the location of the radial head, the brachial artery bifurcates into the radial and ulnar arteries. Then, the common interosseous artery usually branches off from the ulnar artery. The radial artery travels between two muscles - the brachioradialis and the supinator muscle - then continues to supply the wrist extensors in the forearm. The ulnar artery enters the forearm posterior to the pronator teres. From below the elbow, collaterals from the radial, ulnar and interosseous arteries contribute to the blood supply. These vessels are the radial recurrent artery, the anterior and posterior ulnar recurrent arteries, and the recurrent interosseous artery.
The musculocutaneous, radial, and ulnar nerves contribute to the nervous supply of the elbow joint. The median nerve travels between the humeral and ulnar heads of the pronator teres and continues deep to the flexor digitorum superficialis fibers. The anterior interosseous nerve branches off the median nerves and innervates the pronator quadratus, the flexor pollicus longus, and the flexor digitorum profundus component of the index and long finger. The ulnar nerve passes behind the medial epicondyle of the humerus into the cubital tunnel and then continues into the forearm under the cover of the flexor carpi ulnaris. The radial nerve travels in the posterior compartment of the arm and winds around the humerus to enter the elbow anterior to the lateral epicondyle. It passes by the radial head and continues into the forearm to innervate the extensor muscles of the forearm. The radial nerve pierces the supinator muscle and exits as the posterior interosseous nerve. The sensory innervation of the skin overlying elbow is supplied by the medial, lateral, and inferior lateral cutaneous nerves of the forearm.