The ulna is one of the two bones in the forearm and is slightly longer and thinner than the other bone, the radius. It supports the radius and gives the forearm a greater range of motion. The ulna is a long bone, which means it is longer than it is wide and has several layers. The exterior layer is compact bone, which surrounds a layer of spongy, cancellous bone. Beyond that layer is the medullary cavity that contains the bone marrow.
Anatomists use several phrases to describe the location of a body part in relation to the body as a whole. Proximal describes a structure closer to the center of the body. The proximal end of the ulna is the end closest to the elbow, and the head of this bone forms part of the elbow joint. The proximal section has two processes and two notches.
Between both ends of the ulna is a long, thin section of bone. This section, the shaft, is triangular, with three borders: the anterior, the posterior, and the interosseous. Between each of the borders is a surface, the names of white reflect their position: anterior, posterior, and medial. Just below the coronoid process is an important bone projection, the ulnar tuberosity. The superior crest is another important bony landmark on the shaft of the ulna.
Distal is the opposite of proximal; the distal end of the ulna is the section closest to the wrist. Though this area of the ulna is important, it lacks the number of bony landmarks that the rest of the ulna has. The only important bony projection on the distal end is the ulnar styloid process, which can be easily seen and felt on the pinky-finger side of the hand.
Anatomically, articulations are areas of the body that allow for movement -- also called joints. The ulna has a few notable articulations. At the proximal end, the trochlear notch of the ulna articulates with the trochlea of the humerus, the long, broad bone in the upper arm. This articulation is a portion of the elbow joint. Also on the proximal end, the radial notch articulates with the head of the radius. On the distal end, the ulna articulates with the ulnar notch on the radius to create the distal radioulnar joint just above the wrist joint.
Though the ulna is thinner and weaker than the radius, it is still capable of performing several important functions for the forearm. Its main function is to support the radius with rotational movements. The elbow joint is exclusively capable of flexion and extension (bending and straightening). By articulating together, the radius and the ulna provide rotational movements necessary for precise functions such as turning handles, using tools, lifting objects, and typing.
Another key function of the ulna is to provide attachment points for the forearm muscles. The vast majority of the muscles attach to one of the surfaces on the ulnar shaft, though some also connect to the various processes.
When an embryo begins to develop, its bones are quite soft. The process of hardening and reshaping bone, ossification, dramatically changes the ulna. In general, the ulna is around 50% thicker than the radius when a child is between four and five months old. Once the bone has finished developing, the ulna is merely half as thick as the radius.
In addition to the many forearm muscles, the ulna also has attachment points for a unique tissue: the interosseous membrane of the forearm. This long, fibrous sheet connects the interosseous borders of the radius and ulna. Though it has attachment sites for muscles, its most notable function is shifting compressive loads onto the ulna from the radius. This protects any single joint from damage due to an excessive load. An action that might cause this shift is a handstand.
Muscles and bones protecting most of nerves in the human body. However, the ulna does not protect the nerve closest to it. The ulnar nerve is the single largest nerve that lacks protection, which causes notable issues when a person injures their wrist, elbow, or ulna. Most people have experienced a prevalent ulnar nerve issue. When a person strikes their “funny bone,” they are actually pinching the ulnar nerve with either the humerus or the ulna.
Because of its location and functions, the ulna is prone to fractures, and the radius is often injured in these cases, as well. A fracture of the ulna alone usually occurs when an object strikes the bone directly. This also means that the shaft is usually the site of the fracture, as opposed to the head. When this happens, muscles will pull the ulna downward, potentially harming the ulnar nerve or disturbing the shape of the arm. Sometimes, the olecranon process is fractured when a person falls onto a flexed elbow. Alternatively, a fall or direct strike can fracture the styloid process near the wrist.
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