Positive ulnar variance is an anatomical condition affecting the wrist, concerning the two long bones of the forearm. It describes a situation where the ulna, located on the pinky-finger side of the arm, extends further down than the radius, found on the thumb side, at the wrist joint. This length difference is determined through medical imaging, typically an X-ray. While these bones are usually similar in length or the ulna may be slightly shorter, a positive variance indicates the ulna is comparatively longer.
Causes and Associated Anatomy
Positive ulnar variance can develop due to two main reasons: it can be present from birth (congenital) or acquired later in life. Congenital cases arise when the ulna and radius grow unequally during development, resulting in a naturally longer ulna. In acquired cases, the condition often follows an injury, most commonly a fracture of the distal radius, which is the part of the radius bone near the wrist. If this fracture heals in a shortened position, the ulna becomes relatively longer.
The wrist joint involves the radius, ulna, and several small carpal bones. A significant anatomical structure in this region is the Triangular Fibrocartilage Complex (TFCC). The TFCC is a disc-shaped structure located between the ulna and the carpal bones, specifically the lunate and triquetrum. It acts as a cushion and a primary stabilizer for the distal radioulnar joint, which allows forearm rotation.
Symptoms and Related Conditions
Positive ulnar variance can lead to symptoms affecting the ulnar (pinky) side of the wrist. Individuals often experience pain, which may initially occur only during activities involving gripping or twisting motions, but can become constant over time. Clicking, popping, or snapping during wrist movement, particularly rotation, is also common. Reduced grip strength and decreased wrist range of motion, especially when bending or straightening, can also occur.
This anatomical imbalance directly contributes to conditions like ulnocarpal impaction syndrome, where the elongated ulna impacts the TFCC and the adjacent wrist bones, such as the lunate and triquetrum. This chronic pressure can cause the TFCC to thin, stretch, and tear. Degenerative changes, including cartilage wear on the ulna and carpal bones, can also occur due to this continuous friction and compression.
Diagnosis Process
Diagnosis begins with a physical examination. The doctor may press on the ulnar side of the wrist to identify tenderness. Wrist movement is assessed for pain, stability, and limitations. Specific tests, such as the ulnocarpal stress test, which involves applying an axial load to an ulnar-deviated wrist during passive rotation, may reveal pain or instability.
Imaging confirms the diagnosis. A standard X-ray of the wrist is the primary tool to measure the length difference between the ulna and radius. This specific X-ray view is usually taken with the forearm in a neutral rotation, the elbow flexed at 90 degrees, and the shoulder abducted at 90 degrees to ensure accurate measurement. While X-rays show bone lengths, an MRI may also be ordered. MRI provides detailed images of soft tissues, allowing evaluation of the TFCC, assessment for tears, and other related damage not visible on X-ray.
Treatment Approaches
Treatment involves non-surgical and surgical strategies, depending on symptom severity and damage. Non-surgical treatments focus on symptom relief. These include modifying activities that aggravate the wrist, such as forceful gripping or twisting. Wearing a wrist splint or brace can immobilize the joint and allow damaged tissues to heal.
Pain and inflammation can be managed with nonsteroidal anti-inflammatory drugs (NSAIDs). Corticosteroid injections may be administered for localized pain and inflammation. If conservative measures do not provide sufficient relief, surgery may be considered.
The most common surgical procedure is an ulnar shortening osteotomy. During this procedure, a surgeon removes a section of the ulna bone to correct the length discrepancy. After shortening, the two ends of the bone are reconnected and held together with a small metal plate and screws, allowing the bone to heal in its new, corrected length. This surgery aims to decompress the ulnar side of the wrist, reducing impact on the TFCC and carpal bones.