Bones of the Upper Limb: Ulna

The ulna has two key projections at its proximal end: the olecranon process, a hook-like bony tip that articulates with the olecranon fossa of the humerus, and the coronoid process on the anterior side that articulates with the coronoid fossa. Together, these projections form a deep trochlear notch that fits the trochlea of the humerus, creating a hinge joint essential for elbow flexion and extension.

During arm extension, the olecranon fossa of the humerus holds the olecranon process of the ulna, preventing the forearm bones from sliding beyond a certain anatomical point. This mechanical stop limits the range of extension and protects the elbow joint from hyperextension, maintaining proper joint stability and function.

The radial notch is a narrow depression lateral to the coronoid process that helps attach the radius and forms the proximal radioulnar joint. This joint is essential for supination and pronation of the forearm, allowing rotational movements that enable the hand to turn palm-up or palm-down.

The ulna and radius are parallel forearm bones joined at their proximal and distal ends by radioulnar joints and connected by a flat, flexible interosseous membrane across their length. This membrane allows the bones to move relative to each other while maintaining structural integrity during forearm rotation and other movements.

The ulnar head, a disc-shaped structure at the distal end of the ulna, joins the wrist bones and is separated from them by triangular fibrocartilage. The styloid process, a short bony projection from the posterior side of the ulnar head, attaches triangular fibrocartilage and radioulnar ligaments that stabilize the distal radioulnar joint and support the carpal bones.

Several muscles attach to the ulna, including the pronator teres, flexor carpi ulnaris, flexor digitorum superficialis, flexor digitorum profundus, and pronator quadratus. These muscles facilitate elbow and wrist joint movements such as flexion, extension, pronation, and supination, enabling coordinated upper limb function.

During forearm flexion, the coronoid process slides into the humeral coronoid fossa, forming a V-shaped angle between the forearm and arm. This articulation, combined with the olecranon and trochlear notch, creates a stable hinge joint that allows controlled elbow flexion while the coronoid process serves as an insertion point for the brachialis muscle.