The ulna is one of the two long bones in the forearm, positioned on the side of the little finger when the arm is in the anatomical position. It extends from the elbow to the wrist, providing significant stability to the forearm. The ulna’s shape, broader near the elbow and narrower towards the wrist, reflects its different roles in articulating with other upper limb bones. This bone is involved in movements like arm bending and straightening, and forearm and hand rotation.
The Ulna’s Connection to the Humerus
The ulna articulates with the humerus at the humeroulnar joint, forming a significant part of the elbow. This hinge joint allows for controlled flexion and extension movements of the forearm. The trochlear notch, a hook-like depression on the proximal ulna, fits precisely around the trochlea at the distal end of the humerus.
The ulna also features two prominent bony projections that limit the range of motion. The olecranon, forming the point of the elbow, fits into the olecranon fossa of the humerus during full extension, preventing hyperextension. Conversely, the coronoid process of the ulna projects forward and fits into the coronoid fossa of the humerus during full flexion. Ligaments, such as the ulnar collateral ligament, reinforce this joint, extending from the humerus to the coronoid and olecranon processes of the ulna.
The Ulna’s Connection to the Radius
The ulna articulates with the radius at two locations: proximally near the elbow and distally near the wrist. These articulations enable the forearm’s rotational movements. At the proximal radioulnar joint, the radial head articulates with the radial notch on the ulna. This pivot joint allows the radius to rotate around the ulna.
The annular ligament, a strong fibrous band, encircles the radial head, holding it against the ulna while allowing rotation. This enables pronation and supination, movements that turn the palm downward and upward. Distally, the ulna connects with the radius at the distal radioulnar joint. Here, the ulnar head articulates with the ulnar notch on the distal end of the radius.
This distal articulation is also a pivot joint. During these movements, the radius crosses over the ulna, with the ulnar head rotating within the ulnar notch of the radius. The interosseous membrane, a sheet of connective tissue running the length of both bones, connects the ulna and radius, providing stability and helping to transfer forces between them.
The Ulna’s Connection to the Wrist
The ulna does not directly articulate with the carpal bones of the wrist. Instead, an articular disc, known as the triangular fibrocartilage complex (TFCC), bridges the space between the ulna’s head and the wrist bones. This disc is positioned between the inferior surface of the ulnar head and the proximal row of carpal bones, specifically the triquetrum and lunate. The TFCC cushions the joint and distributes forces.
The TFCC stabilizes the distal radioulnar joint and allows smooth movement during forearm rotation. It binds the radius and ulna together at their distal ends, maintaining their relationship during pronation and supination. The ulnar styloid process, a small projection from the distal end of the ulna, provides attachment points for ligaments that stabilize the wrist region.