The elbow is a complex hinge joint formed from the end of the humerus in the upper arm and the top of the ulna and radius in the forearm. The elbow allows flexion and extension of the forearm relative to the upper arm, and rotation of the forearm and wrist.
The arch of bone at the end of the humerus is made up of the medial epicondyle on the inside and the lateral epicondyle on the outside of the elbow. The tendons of the forearm muscles attach to these epicondyles.
Inside the bony arch is the capitellum which articulates with the top of the radius and the trochlea around which the olecranon of the ulna bone moves. A layer of smooth articular cartilage covers the ends of thebones forming the elbow joint. The joint capsule has a thin lining of synovium to provide lubrication to the elbow allowing smooth motion.
The bony configuration of the elbow gives it stability that is assisted by an extensive network of ligaments and tendons. The medial (inside) and lateral (outside) collateral ligaments attach and maintain the position of the radius and ulna relative to the epicondyles of the humerus. These ligaments allow various movements of the elbow while resisting dislocation of the joint.
At the front of the elbow, the end of the biceps tendon passes over the joint and attaches to the radius allowing supination (rotation) and flexion of the forearm relative to the humerus.
At the back of the elbow the triceps crosses the joint and controls extension. The olecranon bursa is at the posterior tip and is a sac-like membrane which cushions the tip of the elbow and helps smooth movement of the joint.