Positive ulnar variance (PUV) is a wrist condition where the ulna (the forearm bone on the pinky-finger side) is longer than the radius (the forearm bone on the thumb side) at the wrist joint. This relative difference in bone length causes the ulna to extend further toward the small wrist bones, leading to chronic pain and instability in the wrist. When the ulna is relatively longer, it increases the force transmitted through the ulnar side of the wrist, predisposing the joint to wear and tear. This mechanical overloading is often associated with a painful condition known as ulnar impaction syndrome. The condition’s severity is measured by the length difference between the two bones, which correlates with the potential for long-term complications.
Understanding the Anatomy of Positive Ulnar Variance
The forearm is composed of the two long bones, the radius and the ulna, which meet at the wrist to form the distal radioulnar joint. Normally, the articular surfaces of the ulna and the radius are nearly level (neutral variance), or the ulna may be slightly shorter than the radius. Positive ulnar variance occurs when the ulna’s joint surface projects farther down than the radius’s joint surface. The ulna articulates with the carpus through a soft tissue structure called the triangular fibrocartilage complex (TFCC).
When the ulna is comparatively longer, it causes increased pressure and direct abutment against the TFCC and the small carpal bones, particularly the lunate and triquetrum. The TFCC acts as a cushion and stabilizer for the distal radioulnar joint. Prolonged, excessive force from PUV leads to thinning and degeneration of this structure. The increase in ulnar length by even a few millimeters can substantially raise the proportion of the total wrist load borne by the ulna.
Common Causes and Related Conditions
Positive ulnar variance stems from several distinct underlying processes, categorized as acquired or congenital. The most frequent acquired cause is the malunion of a distal radius fracture, such as from a fall on an outstretched hand. When the radius heals in a shortened position, it effectively makes the ulna relatively longer, disturbing the normal relationship between the two bones.
Other cases of PUV are developmental, resulting from unequal growth rates of the forearm bones. This includes conditions like Madelung deformity or growth plate injuries that prematurely stop the growth of the radius while the ulna continues to grow. Distinguishing between primary ulna overgrowth and secondary radius shortening is important, as the specific cause influences treatment choice. Regardless of the origin, PUV is strongly associated with ulnar impaction syndrome, the degenerative process caused by overloading the wrist’s ulnar side.
Recognizing the Symptoms
Patients with positive ulnar variance often report chronic, deep-seated pain localized to the ulnar side of the wrist. This discomfort is typically exacerbated by activities involving firm gripping, forearm rotation, or heavy loading. The pain often starts intermittently with exertion but can progress to become persistent.
A common complaint is a distinct clicking, popping, or catching sensation within the wrist, especially during forearm rotation (e.g., turning a doorknob). This mechanical feeling is often due to degenerative changes or tears in the TFCC resulting from chronic impaction. Over time, the pain and joint pathology can limit the range of motion, particularly forearm rotation, and decrease grip strength.
Treatment Pathways
The management of positive ulnar variance and the associated ulnar impaction syndrome begins with conservative, non-surgical approaches. Initial treatment involves a period of rest and modifying activities that aggravate ulnar-sided wrist pain, such as avoiding forceful gripping or pronation. Non-steroidal anti-inflammatory drugs (NSAIDs) may be used to manage pain and inflammation in the short term.
Physical therapy helps stabilize the wrist joint, and splinting or bracing restricts painful movements. If these measures fail after several months, or if the variance is substantial, surgical intervention is considered to decompress the wrist joint. The primary goal of surgery is to shorten the ulna relative to the radius to alleviate excessive pressure on the TFCC and carpal bones.
Two main surgical procedures correct the length discrepancy:
Ulnar Shortening Osteotomy (USO)
This is an extra-articular procedure involving the removal of a measured segment of bone from the ulna, usually in the mid-shaft. The two ends are then fixed with a plate and screws to allow for bone healing. This method is typically reserved for cases requiring shortening of 3 millimeters or more.
Wafer Procedure
This procedure involves shaving or resecting a small portion of the distal ulna. It can be performed arthroscopically using small incisions and a camera. The Wafer Procedure is often preferred for smaller positive variances, generally less than 3 millimeters, especially when a central TFCC tear is also present. This procedure avoids the risk of nonunion and hardware complications associated with the osteotomy, but its ability to decompress the wrist is more limited.