Distal humerus refers to the portion of the humerus bone in the upper arm that is farthest from the trunk of the body and nearest to the elbow joint. A term of relative location, "distal" is used in anatomy in opposition to the term "proximal." The distal humerus, then, is the far or lower end of the bone, while the proximal humerus is the end nearest where the arm meets the torso.
Home to several distinct bony structures, the distal humerus offers many sites of attachment for various muscles, tendons, and other fibrous tissues. Also known as the lower extremity of the humerus, this section of the bone has two most prominent features: the lateral and medial epicondyles. The epicondyles are the two familiar rounded bony prominences on either side of the bottom of the bone, with the lateral toward the outside of the arm and the medial on the inside of the arm nearest the torso. As the more prominent of the two, the medial epicondyle can be felt just above and to the front of the elbow, and it serves as a point of origin for the pronator teres muscle in the forearm as well as for the common tendon of several of the forearm flexor muscles. Conversely, the lateral epicondyle is on the opposite side of the elbow and is an attachment site for the tendons of the supinator muscle and some of the forearm extensor muscles.
Between and slightly below the epicondyles on the distal humerus are the articular surfaces of the bone, which are the places where the humerus forms joints with the ulna and radius bones in the forearm. The lateral or outermost of these is the capitulum, which is on the anterior or front side of the humerus and whose rounded surface articulates with a cavity on the radius bone. Another articular surface, this one a small depression found just above the capitulum, is known as the radial fossa, which joins with the radius when the elbow is flexed.
Medial to the capitulum is the trochlea, a cavity found on the anterior, underside, and posterior surfaces of the very bottom of the distal humerus. This is where the semilunar notch of the ulna, the inside of the large hooked structure that resembles a wrench, wraps around the base of the humerus to form the hinging elbow joint. Above the trochlea on both the anterior and posterior sides of the humerus are the fossae, cavities on either side that receive the hooked eminences of the ulna during elbow flexion and extension. During flexion, the coronoid process on the front side of the ulna pushes into a minor depression on the front side of the humerus known as the coronoid fossa. Likewise during elbow extension, a larger triangular depression on the back side of the distal humerus known as the olecranon fossa receives the large olecranon process of the ulna, the pointy projection felt at the elbow joint.