The humerus, radius, and ulna join to form the elbow joint. This joint features a number of complex prominences which serve as attachment sites of ligaments and muscles, serving to control the flexibility and adduction of the bones about the elbow. The articulation of the elbow is called a hinge joint, because, like a hinge on a door, the joint only permits flexion in a single plane.
The humerus is a long bone which forms the upper arm. Its head (proximal end) is articulated in a ball-and-socket joint with the scapula (at the glenoid cavity), while the distal end articulates with the bones of the forearm (radius and ulna) to form the elbow joint.
The radius is one of the two long bones (the other is the ulna) which form the lower arm (forearm). The radius articulates at each end with the ends of the ulna, with the humerus at the elbow, and some of the carpal bones at the wrist. When the hand is turned with the palm facing up (called supination), the radius is on the lateral (thumb) side of the forearm. When the hand is placed with the palm down (called pronation), the radius crosses over the ulna in mid-forearm.
The ulna is one of the two long bones (the other is the radius) which form the lower arm (forearm). The ulna articulates at each end with the ends of the radius, with the humerus at the elbow, and some of the carpal bones at the wrist. When the hand is turned with the palm facing up (called supination), the ulna is on the medial (little finger) side of the forearm. When the hand is placed with the palm down (called pronation), the radius crosses over the ulna in mid-forearm.
The lower head of the humerus features an arrangement of prominences and depressions which facilitate its articulation with the radius and ulna. At the lower surface is a rounded area, made of the capitulum and trochlea of the humerus. The olecranon of the ulna can be seen at the back of the elbow, fitting into the depression between these two features. Each side of the head of the humerus features a protrusion, called an epicondyle. When the arm is held beside the body, with the palm facing up, the lateral epicondyle is on the side of the humerus which is away from the body, while the medial epicondyle is next to the torso. Between the ends of the radius and ulna is a small notch (the radial notch) where the two bones join at the end.
The coronoid fossa is a depression located on the lower end of the front
of the humerus. It accepts the coronoid process of the ulna when
the elbow joint is in flexion.