The elbow joint is a synovial joint found in the upper limb between the arm and the forearm. It is the point of articulation of three bones: the humerus of the arm and the radius and the ulna of the forearm.
The elbow joint is classified structurally as a synovial joint. It is also classified structurally as a compound joint, as there are two articulations in the joint. Synovial joints, also called diarthroses, are free movable joints. The articular surfaces of the bones at these joints are separated from each other by a layer of hyaline cartilage. Smooth movement at these joints is provided by a highly viscous synovial fluid, which acts as a lubricant. A fibrous capsule encloses the joint, and is lined internally by a synovial membrane. Synovial joints can be further categorized based on function. The elbow joint is functionally a hinge joint, allowing movement in only one plane (uniaxial).
There are three bones that comprise the elbow joint:
- the humerus
- the radius
- the ulna.
These bones give rise to two joints:
- Humeroulnar joint is the joint between the trochlea on the medial aspect of the distal end of the humerus and the trochlear notch on the proximal ulna.
- Humeroradial joint is the joint between the capitulum on the lateral aspect of the distal end of the humerus with the head of the radius.
The humeroulnar and the humeroradial joints are the joints that give the elbow its characteristic hinge like properties. The rounded surfaces of the trochlea and capitulum of the humerus rotate against the concave surfaces of the trochlear notch of the ulna and head of the radius.
At the elbow joint, the proximal ends of the radius and ulna articulate with each other at the proximal radioulnar joint.
This joint, however, is considered to be a separate articulation than those forming the elbow joint itself. The proximal radioulnar joint is the articulation between the circumferential head of the radius and a fibro-osseous ring formed by the radial groove of the ulna and the annular ligament that hold the head of the radius in this groove. The proximal radioulnar joint is functionally a pivot joint, allowing a rotational movement of the radius on the ulna.
Ligaments of the Elbow Joint
There are a collection of ligaments that connect the bones forming the elbow joint to each other, contributing to the stability of the joint. The humeroulnar and the humeroradial joints each have a ligament connecting the two bones involved at the articulation: the ulnar collateral and the radial collateral ligaments.
The ulnar collateral ligament extends from the medial epicondyle of the humerus to the coronoid process of the ulna. It is triangular in shape, and is composed of three parts: an anterior, a posterior and an inferior band.
The radial collateral ligament has a low attachment to the lateral epicondyle of the humerus. The distal fibres blend with the annular ligament that encloses the head of the radius, as well as with the fibres of the supinator and the extensor carpi radialis brevis muscles.
The annular ligament also reinforces the joint by holding the radius and ulna together at their proximal articulation. The quadrate ligament is also present at this joint, and maintains constant tension during pronation and supination movements of the forearm.
Blood Supply and Innervation
The blood supply to the elbow joint is derived from a number of periarticular anastamoses that are formed by the collateral and recurrent branches of the brachial, profunda brachii, radial and ulnar arteries. Proximal to the elbow joint, the brachial artery, the largest in the arm, gives off two branches, a superior and inferior ulnar collateral artery. The profunda brachii gives off a radial collateral and a middle collateral artery. These pass towards the joint contributing to the anastomotic loop supplying the joint.
Distal to the elbow joint, the radial artery gives off the radial recurrent artery, and the ulnar artery gives off the anterior and posterior ulnar recurrent arteries. These arteries ascend towards the elbow joint, anastamosing with the branches from the brachial and profunda brachii arteries in the arm.
As the elbow joint is a hinge joint, movement is in only one plane. The movements at the elbow joint involve movement of the forearm at the elbow joint. Flexion of the forearm at the elbow joint involves decreasing the angle between the forearm and the arm at the elbow joint. Extension involves increasing the angle between the arm and forearm. These movements are performed by two groups of muscles in the arm: the anterior compartment and the posterior compartment of the arm.
Most of the muscles producing flexion are found in the anterior compartment of the arm. There are two muscles in this compartment that produce flexion at the elbow joint:
- Biceps Brachii originates as two heads. The tendon of the long head originates from the supraglenoid tubercle of the scapula. It passes through the joint capsule of the shoulder joint and through the bicipital groove on the anterior surface of the humerus. The short head of the biceps brachii muscle originates from the coracoid process of the scapula. These heads join together to form the biceps brachii muscle belly. The muscle inserts via a single tendon onto the radial tuberosity distal to the elbow joint. In the forearm, there is a continuation of this tendon as a flattened connective tissue sheath, the bicipital aponeurosis. This aponeurosis blends with the deep fascia in the anterior forearm.
- Brachialis originates from the distal half of the anterior surface of the humerus, as well as from the intermuscular septa on either side of the anterior compartment. It is located deep to the biceps brachii muscle. It forms a singular tendon that inserts onto the tuberosity of the ulna.
Both the biceps brachii and brachialis muscles are innervated by the Musculocutaneous nerve.
While the biceps brachii and the brachialis muscles are the main flexors of the elbow joint, the brachioradialis muscle is also involved in flexion of the forearm at this joint. Brachioradialis originates for the lateral aspect of the distal humerus above the lateral epicondyle. It inserts onto the lateral aspect of the distal radius. Although this muscle is primarily in the forearm, it crosses the elbow joint so therefore it acts on the elbow joint. It is innervated by the radial nerve.
Extension of the forearm at the elbow joint is the increase of the angle at the elbow to bring the forearm back to the anatomical position from a flexed position. There is one muscle involved in extension, the triceps brachii muscle. It is the only muscle in the posterior compartment of the arm.
Triceps Brachii originates as three heads. The long head originates from the infraglenoid tubercle of the scapula, the lateral head originates from the lateral aspect of the humerus above the radial groove, and the medial head originates from the medial aspect of the humerus below the level of the radial groove. The three heads converge on a single tendon that inserts onto the olecranon of the ulna. It is supplied by the radial nerve, which passes down through the arm in the radial groove between the lateral and medial heads of the muscle.
While flexion and extension are the only movements that can occur at the elbow joint itself, movement is also afforded at the proximal radioulnar joint, which contributes to the elbow joint. Movements at this joint are called pronation and supination. These are rotational movements that occur when the distal end of the radius moves over the distal end of the ulna by rotating the radius in the pivot joint formed by the circular head of the radius, the radial groove of the ulna and the annular ligament.
Pronation and supination are easily visualised when the elbow is flexed at 90°. Supination is where the palm of the hand is facing upwards; pronation is rotation of the forearm so that the palm is facing downwards. In the anatomical position, the forearm is in the supine position. Pronation in the anatomical position is movement of the forearm so that the palm is facing posteriorly.
Common injuries to the elbow joint include fractures of the bony structures contributing to the joint. Care must be taken when diagnosing a fracture of the elbow joint with respect to the age of the patient. This is because secondary ossification centres in children and adolescents can easily be mistaken for a fracture on a radiograph. Therefore, it is vital that a physician know the age of the child when examining their radiograph. Some of the areas of secondary ossification are as follows:
- Capitulum (1 year)
- Radial head and medial epicondyle (5 years)
- Trochlea (11 years)
- Olecranon (12 years)
A supracondylar fracture is a fracture to the humerus above the level of the humeral condyles. This injury most commonly occurs in children. In such injuries, the distal bone fragment can be pulled posteriorly by the triceps muscle. This can cause bowstringing of the brachial arteries by stretching them, which can have adverse effects.
Fracture of the head of the radius is a common fracture of the elbow joint. It is often caused by a fall on an outstretched hand, and can have severe implications including loss of full extension of the forearm at the elbow joint.
Epicondylitis & Arthritis
Epicondylitis is inflammation of the soft tissues surrounding the epicondyles of the humerus. It typically occurs due to overuse of the flexor and extensor muscles of the forearm. Pain is localised around the epicondylar region. Tennis players typically get epicondylitis on the lateral epicondyle (common extensor origin), whereas golfers usually have it on the medial epicondyle (common flexor origin).
Arthritis can occur at the elbow joint, and is usually more severe in the dominant limb of the patient.
Anterior to the elbow joint is a transitional zone between the arm and the forearm called the cubital fossa. Located in the subcutaneous tissue above the cubital fossa is a very superficial vein: the median cubital vein. This is a short vein connecting two longer superficial veins draining the upper limb, the cephalic and basilic veins, together. The medial cubital vein is one of the most common sites for venipuncture, which is collecting blood samples in the upper limb.