The elbow is a hinge joint made up of the humerus, ulna, and radius.
The unique positioning and interaction of the bones in the joint allow for a small amount of rotation as well as hinge action. This rotation is easily noticed during activities such as hand-to-mouth eating motions.
The primary stability of the elbow is provided by the ulnar collateral ligament, on the medial (inner) side of the elbow.
However, one of the most common injuries to the elbow occurs on the lateral, or outer, side of the elbow—it is called lateral epicondylitis, or tennis elbow.