Vivian Grisogono


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Three bones make up the elbow joint, the humerus (arm-bone), radius (outer forearm-bone) and the ulna (inner forearm-bone). The elbow joint consists of two parts, one between the humerus and ulna, the other between the humerus and radius. Technically it is called a compound synovial joint.

The two parts of the elbow joint work together. The elbow is closely linked in structure and function to the superior radioulnar joint, the joint between the top ends of the radius and ulna (outer and inner forearm-bones).

The right elbow, seen from the front. Diagram by Peter Gardiner

Humeroulnar joint formation
The humeroulnar joint works like a hinge. The top end of the ulna is shaped like a hook, called the olecranon, which glides in a special notch between the curved ends (condyles) of the humerus as you bend and straighten the elbow. When the elbow is fully straight, the tip of the hook, the olecranon process, fits into a recess at the back of the humerus called the olecranon fossa, which is lined with fluid-producing synovial membrane. Pressing with your other hand over the back of the elbow, you can feel the top of the olecranon, and the triceps tendon which is attached to it when your elbow is straight and bent. When your elbow is bent, if you press deeply, you can feel the tip of the olecranon process and the olecranon fossa. To the inner side of the olecranon (towards your body) you can feel your “funny bone”, where the ulnar nerve runs down the back of the medial epicondyle (inner side edge) of the humerus.

Humeroradial joint formation
The bones in the humeroradial joint do not fit as closely together as those of the humeroulnar joint. The top, or head of the radius is flattened, while the lower outer end of the humerus, called the capitulum, is rounded and slightly flared outwards. When the elbow is fully bent, the top of the radius fits into a recess called the radial fossa on the humerus. Just below the top of the radius is a small projection called the coronoid process, and this too has its own recess on the humerus, the coronoid fossa, which it fits into when the elbow is fully bent. When the elbow is straight, if you press behind the joint with your other hand you can feel the head of the radius, the joint space and the lower edge of the humerus.

Binding tissues
The elbow joint is protected by a capsule which surrounds it completely, and which is lined with a fluid-producing synovial membrane. At either side of the joint are ligaments which prevent sideways movement, the radial collateral ligament on the outer side, and the ulnar collateral ligament on the inner side. The ulnar collateral ligament is attached to the medial epicondyle on the humerus and the edge of the top of the ulna. The radial collateral ligament is not only attached to the elbow bones, but also the annular ligament, a strong band which encircles the head of the radius and binds it to the top of the ulna.

Although the movements of the elbow are limited to bending and straightening (flexion and extension), there is a slight twisting movement in the ulna during these movements, outwards from the body (into supination) on bending, and inwards (into pronation) on straightening. This twisting movement is imperceptible during normal movements. As the radius is not held tightly against its corresponding bone, its head can be moved passively forwards, backwards and sideways. These movements are called accessory movements. Although they cannot be performed by activating the elbow muscles, they are important for the full freedom of the joint.

The main muscles which act concentrically to bend the elbow against gravity or a resistance are brachialis and biceps brachii, and a third muscle, brachioradialis, will also act if the movement calls for extra strength and speed. Brachioradialis is at its strongest when the forearm is in mid-position, with the thumb up and palm facing the body.  The same muscles pay out eccentrically to control the movement when the elbow is straightened in the direction of gravity or under the influence of a load. Brachialis and brachioradialis connect the front of the arm to the forearm over the elbow, so they are one-joint muscles, while biceps also acts on the shoulder joint, as a two-joint muscle. The muscles which straighten the elbow concentrically against gravity or a resistance are triceps and a small muscle called anconeus, which lie behind the joint. They pay out eccentrically when the elbow is bent in the direction of gravity or under load. Triceps is a two-joint muscle which also acts on the shoulder, while anconeus is a one-joint muscle acting only on the elbow.

The “carrying angle”
The arm and forearm do not form a straight line, because of the way the lower end of the humerus is slightly flared. When the arm is alongside the body with the elbow straight and palm facing forwards, the forearm is set at an angle, so that the hand is little way from the thigh. This is called the “carrying angle”, and is usually just over 160 degrees.

Elbow problems
A fall on to the elbow or the outstretched hand can cause damage to the bones at the elbow, including fracture or dislocation. Soft-tissue injuries include so-called “tennis elbow" or injury to the common extensor tendon on the outside of the elbow, “golfer’s elbow” a similar injury to the common flexor tendon on the joint’s inner side, strain or tear in the joint’s ligaments, and strain or tear to the triceps tendon at the back of the elbow. Another possible problem at the back of the elbow is inflammation in the fat pad which buffers the olecranon fossa. This is usually caused by impingement, when the tip of the olecranon process on the top of the ulna presses too forcefully into the recess. It can happen through sports, and is more likely to happen if the person has very flexible hyperextending elbows, which look almost as though they are “bending backwards” when the elbow is fully straightened.

Specialist care is always advisable for injuries and problems around the elbow. The recovery process should involve progressive rehabilitation exercises to restore strength and mobility to the joint and its structures, as well as co-ordination with the rest of the arm.

You should never try to regain mobility in the elbow forcefully, or rub it hard after an injury. The elbow is especially vulnerable to a problem called myositis ossificans or heterotopic ossification, in which there is calcification (formation of particles of bone) in the muscles around the joint following an injury which has caused internal bleeding (haematoma). This complication is difficult to treat, as the extra bone tends to re-form if it is removed surgically, often causing increasing blockage in the joint, although it is not necessarily painful. To minimize the risk of myositis ossificans, the initial treatment of any elbow injury should be as gentle as possible: pain and swelling should be controlled with cold compresses or ice treatments, and the elbow should be protected from any pain-causing movements, in a removable or fixed cast if necessary. Practitioners who understand the risk of myositis ossificans will take care not to use any forceful movements or testing procedures when they examine a patient's injured elbow.

© Vivian Grisogono 2009. Updated 2014, 2016