The elbow is a complex joint formed by the articulation of three bones the humerus, radius, and ulna. The elbow joint functions to bend and straighten the arm and helps with lifting or moving objects.
Bones and Joints of the Elbow
The elbow joint is formed at the junction of three bones:
- The humerus (upper arm bone) forms the upper portion of the joint. The lower end of the humerus divides into two bony protrusions known as the medial and lateral epicondyles, which can be felt on either side of the elbow joint.
- The ulna is the larger bone of the forearm located on the inner and posterior surface of the joint. It articulates with the humerus.
- The radius is the smaller bone of the forearm situated on the outer surface of the joint. The head of the radius is circular and shallow, which allows movement with the humerus. The articulation between the ulna and radius helps the forearm to rotate.
The elbow consists of three joints, namely:
- The ulnohumeral joint is formed between the humerus and ulna and allows flexion and extension of the arm.
- The radiocapitellar joint is formed between the radius and part of the humerus called the capitellum and allows movements like flexion, extension, supination, and pronation.
- The proximal radioulnar joint is formed between the ulna and radius bones and allows rotation of the forearm.
Articular cartilage lines the joint surfaces of the humerus, radius, and ulna. It is a thin, tough, flexible and slippery surface that acts as a shock absorber and cushion to reduce friction between the bones. The cartilage is lubricated with synovial fluid, which further enables the smooth movement of the bones.
Muscles of the Elbow Joint
There are several muscles extending across the elbow joint that help in various movements. These include the following:
- Biceps brachii: Upper arm muscle, enabling flexion of the arm
- Triceps brachii: Muscle in the back of the upper arm that extends the elbow
- Brachialis: Upper arm muscle beneath the biceps, which flexes the elbow towards the body
- Brachioradialis: Forearm muscle that flexes, straightens and pulls the arm at the elbow
- Pronator teres: Muscle that extends from the humerus across the elbow towards the radius and helps to turn the palm facing down
- Extensor carpi radialis brevis: Forearm muscle that starts at the lateral elbow and helps extend the wrist
- Extensor digitorum: Forearm muscle that helps in extend the fingers
Ligaments and Tendons of the Elbow
The elbow joint is supported by ligaments and tendons, which provide stability to the joint.
Ligaments are a group of firm tissues that connect bones to other bones. The most important ligaments of the elbow joint are the:
- Medial or ulnar collateral ligament: a triangular complex of tissue on the inner side of the elbow joint
- Lateral or radial collateral ligament: A band of tissue on the outer side of the elbow joint
- Annular ligament: Group of fibers that surround the radial head and hold the radius in place during movement of the forearm
Together, the medial and lateral ligaments hold the humerus and ulna tightly in place during movement of the arm.
The ligaments around a joint combine to form a joint capsule that contains synovial fluid.
Any injury to these ligaments can lead to instability of the elbow joint.
Tendons are bands of connective tissue fibers that connect muscle to bone. The various tendons that surround the elbow joint include:
- Biceps tendon: attaches the biceps muscle to the radius, allowing the elbow to bend
- Triceps tendon: attaches the triceps muscle to the ulna, allowing the elbow to straighten
Nerves of the Elbow
The main nerves around the elbow joint are the ulnar, radial and median nerves. These nerves transfer signals from the brain to the muscles that aid in elbow, wrist and hand movements. They also carry sensory signals such as touch, pain, and temperature back to the brain.
Any injury or damage to these nerves may cause pain, weakness or joint instability.
Blood Vessels Supplying the Elbow
Arteries are blood vessels that carry oxygen-pure blood from the heart to the rest of the body. The main artery of the elbow is the brachial artery that travels across the inside of the elbow and divides into two small branches below the elbow to form the ulnar and the radial arteries.