The ulnar styloid is a small, pointed bony projection at the end of the ulna, one of the two long bones in the forearm. It is located on the little-finger side of the wrist. This structure plays a part in wrist stability and movement.
Anatomical Overview
The ulnar styloid process is situated on the distal end of the ulna, the forearm bone extending from the elbow to the wrist on the little-finger side. It projects distally from the posteromedial aspect of the ulna’s head, pointing downwards and towards the body’s midline from the back and inner side. This conical bony projection is easily felt on the back of the wrist, just above the wrist crease.
Compared to the styloid process of the radius, the ulnar styloid is smaller and does not extend as far distally. It typically ranges from 2 to 6 millimeters in length. A shallow groove exists between the ulnar styloid process and the head of the ulna, where the tendon of the extensor carpi ulnaris muscle is located.
Functional Role
The ulnar styloid serves as an attachment point for various ligaments and structures that stabilize the wrist joint. It connects to the ulnar collateral ligament of the wrist, which helps prevent excessive radial deviation (movement of the hand towards the thumb). Radioulnar ligaments also attach to the base of the ulnar styloid, contributing to the stability of the distal radioulnar joint (DRUJ).
This bony prominence is also an attachment site for the triangular fibrocartilage complex (TFCC). The TFCC is a network of cartilage and ligaments that cushions the wrist and allows for smooth rotation of the forearm. Its connection to the ulnar styloid helps stabilize the ulnar side of the wrist, particularly during movements like pronation and supination (turning the palm up and down) and gripping actions.
Common Conditions Affecting the Ulnar Styloid
The ulnar styloid, despite its small size, can be affected by several conditions, often leading to pain and limited wrist function.
Ulnar Styloid Fractures
One common issue is an ulnar styloid fracture, a break in this bony projection. These fractures frequently occur alongside distal radius fractures, which are breaks in the larger forearm bone. In some cases, a fall onto an outstretched hand can cause both injuries simultaneously.
Symptoms typically include immediate pain on the little-finger side of the wrist, along with swelling, tenderness, and bruising. While isolated ulnar styloid fractures are less common, they can still result from direct trauma or forceful twisting of the wrist. The impact of these fractures on wrist stability depends on the type of fracture, especially if it involves the base of the styloid or disrupts the distal radioulnar joint.
Tendinitis
Tendinitis, or inflammation of tendons, can also affect the area around the ulnar styloid. The extensor carpi ulnaris (ECU) tendon, which runs in a groove near the styloid, can become inflamed due to overuse or repetitive wrist movements. This causes pain on the little-finger side of the wrist, and can lead to pain when extending the wrist or moving it towards the little finger.
Triangular Fibrocartilage Complex (TFCC) Issues
Issues with the triangular fibrocartilage complex (TFCC) are another source of problems related to the ulnar styloid. TFCC tears, which can be traumatic (from an injury) or degenerative (from wear and tear over time), often cause pain on the ulnar side of the wrist, especially with twisting or gripping motions. Because the TFCC attaches to the ulnar styloid, injuries to this complex can sometimes involve an avulsion fracture of the styloid, where a piece of the bone is pulled away by the attached ligament. Ulnar styloid impaction syndrome can also occur if the ulna, and thus its styloid, is proportionally longer than the radius, leading to painful contact and wear on the TFCC and wrist bones.