The ulna (Latin ulna) is a bone of the forearm that runs parallel to the radius. Its body is diamond-shaped and consists of two end pieces, with the more rigid end piece forming much of the elbow joint and the smaller one connected to the wrist.
What characterizes the ulna?
Overall, the forearm consists of two bones: Elle and radius. Both are connected to each other with the help of fibrous strands. The ulna is located on the little finger side and is weaker than the radius. It consists of an ulnar shaft or body, the ulnar head, and the proximal and distal end pieces, respectively.
Anatomy and structure
Anatomical diagram during outward rotation and inward rotation of the forearm. Click to enlarge. The end of the ulna is located about midbody and terminates in a bony spur that is beak-like and broad, also called the olecranon. The posterior surface is smooth, triangular in shape, and surrounded by a bursa. The bursa (or olecranon) is the dorsal (back) part of the ulna and forms the bony prominence located at the back between the humerus and forearm. The bone here is very close to the skin surface and is barely protected by fatty tissue. Therefore, the bursa is located here as a protection against external overloads or impacts. At the front edge of the bone spur there is a depression that serves as the attachment of the joint capsule. The elbow process projects forward in the shape of a hook. When the elbow joint is extended, it engages the bone spur pit, which is located on the humerus. On the medial edge of the bone spur is the ulnar head, and on the lateral edge is the elbow muscle. In front, the bone spur is smooth and is covered by the articular cartilage, which forms part of the articular surface. The middle section of the ulna is called the ulnar shaft or body. Together with the radius, the ulna forms a unit, because both bones are coupled in different ways. On the one hand, they have an articulated connection to each other, and on the other hand, a ligamentous bond is stretched between them, creating a rim. Due to its sharp edge, it can also be felt through the skin. Although the ulna is diamond-shaped, different surfaces can be delineated. The anterior surface is the so-called bony surface, between the posterior and anterior edges lies the surface directed towards the center, and the posterior surface serves as the origin surface for the ligamentous adhesion. The lower end of the ulna is slightly widened and is called the ulnar head. Above the wrist lies the stylar process and laterally in front is the articular surface circumference. Regarding the length ratio of the ulna and radius, there are three variants. The most common case is that the ulna and radius are of equal length. If the ulna is shorter, this is called ulna-minus; if it is longer, it is called ulna-plus.
Function and tasks
Together with the humerus, the ulna forms the elbow joint; in addition, with the stylar process, a small part of the wrist joint is formed. The elbow joint is a hinge joint and connects the upper arm and forearm. The rotational joint between the ulna and radius participates in the rotational movement of the hand and forearm. The radius is attached to the ulna in a connective tissue ring, and within this ring the rotational movement of the forearm also occurs. The counterpart to this is found in the wrist, where the ulna can rotate on the radius. In everyday life, the rotational joint is subjected to a great deal of stress, which is why a ligament between the ulna and the radius – known as the triangular fibrocartilaginous complex (TFCC) – provides more stability, or rather good joint guidance. Part of this complex is the ulnocarpal disc, which spans the ulnar head. It acts like a buffer and demarcates the ulnar head from the triangular bone and the lunate bone.
Diseases and complaints
Cartilage damage to the ulnar head or tears to the discus triangularis can occur, especially with heavy rotational loads or during sports. The pain then occurs primarily on the small-finger side of the wrist and often intensifies during rotation with additional loading, for example, when opening a fastener. Inflammation often occurs in the bursa of the elbow joint. If this cannot be contained, the bursa must be surgically removed. Arthrosis of the elbow joint is rather rare.This occurs in patients who already have a rheumatic disease or in patients who subject the elbow joint to high physical stress. So-called tennis elbow is also a common condition. In this case, the tendon branch of the forearm extensor muscles is inflamed. The cause is usually a combination of wear and tear and overuse. Tennis players are particularly affected, but so are people who perform very monotonous movements, such as handling tools. Pain occurs mainly above the bony prominence when the affected person tries to push the wrist upwards. Sometimes a feeling of weakness in the wrist is also noticeable, making it difficult to grip. This is in contrast to golfer’s elbow, in which the tendon insertion of the forearm flexor muscles is inflamed. The pain here radiates into the forearm or upper arm, and sometimes there is also a stabbing pressure pain at the base of the bone. Swelling may also occur, and bending the hand or closing the fist also causes the patient pain. The strength in the hand and finger muscles is reduced, making it very difficult to grip. Occupational groups that are exposed to constant mechanical overuse are particularly affected here.
