The elbow is a relatively simple hinge joint which only allows flexion and extension. This makes it quite a stable joint too.
Bones Forming the Elbow Joint
The elbow joint occurs at the junction of three bones, the Humerus (upper arm bone), Ulna (larger of the two forearm bones) and Radius. The Humerus forms the upper part of the joint and widens near the end to form the Medial and Lateral Epicondyles which are the two bony processes you can feel either side of the elbow joint.
The Ulna is situated on the inside of the joint and forms a cup shape which allows articulation with the Humerus. The Radius is the smaller of the two forearm bones and sits on the outside of the joint. The radial head is round and again cup-shaped to allow it to move around the wide base (Capitulum) of the Humerus.
There are actually three joints at the elbow. The first being the hinge joint formed between the Humerus and the Ulna (humeroulnar joint), which allows us to bend and straighten our elbows. The second is the humeroradial joint between the Radius and Humerus which again allows flexion and extension but is also involved in the more complex motion of turning the hand over so the palm faces up/down (Supination/Pronation). The third is a pivot joint formed by the Radius and Ulna (proximal radioulna joint).
Ligaments of The Elbow Joint
There are three main ligaments supporting the elbow joints:
- Medial Collateral Ligament: Sometimes known as the Ulnar Collateral Ligament and consists of two triangular bands, anterior and posterior. Both sections arise from the Medial Epicondyle and pass over the inside of the elbow joint. The anterior portion then attaches to the front part of the top of the Ulna, known as the Coranoid process and the posterior part to the back of the Ulna, or Olecranon process.
- Lateral Collateral Ligament: Sometimes known as the Radial Collateral Ligament and is a short, narrow band which passes from the base of the Lateral Epicondlye to the Annular Ligament.
- Annular Ligament: This is a band of fibres which circle the head of the Radius, maintaining contact between the Radius and Humerus
Muscles of the Elbow Joint
There are a large number of muscles which cross the elbow joint to cause flexion/extension and supination/pronation. The following are the largest and most commonly injured:
- Biceps Brachii: This muscle arises from the Coracoid process and Supragleniod tubercle (both parts of the shoulder blade) and travels down the arm, crosses the elbow joint and inserts on the Radius. Its action is to flex the elbow joint and supinate the forearm.
- Triceps Brachii: Originates from the Scapula and back surface of the Humerus to cross the elbow and attach to the Olecranon process (posterior Ulna). This is the main extensor of the elbow.
- Brachialis: This muscle is the strongest elbow flexor when the palm is pronated. It arises from the lower half of the front of the Humerus and inserts on the Coronoid process (front bony protusion) of the Ulna.
- Brachioradialis: This muscle starts at the outer edge of the lower third of the Humerus, crosses the joint and inserts at the lower end of the Radius. Its job is to flex the elbow and aid pronation and supination.
- Pronator Teres: This muscle is often involved in golfers elbow (Medial Epicondylitis) and its action is to aid flexion of the elbow and pronate the forearm. It originates just above the medial epicondyle and inserts on the outer surface of the Radius.
- Extensor Carpi Radialis Brevis: Most often the muscle involved in tennis elbow (Lateral Epicondylitis), its action is to extend the wrist and aid extension of the elbow. It arises from the lateral epicondyle of the Humerus and inserts on the third Metacarpal of the hand.