The forearm contains two long bones, the radius and the ulna, extending from the elbow to the wrist. The health of the wrist joint depends on a precise, balanced length relationship between these bones. This relative length difference is known as ulnar variance, measured at the distal articular surfaces. Neutral variance means the bones are the same length, while positive ulnar variance occurs when the ulna extends further distally than the radius.
Understanding Ulna Plus Variance
Ulna Plus Variance (UPV), or positive ulnar variance, occurs when the ulna bone is longer than the radius. This length discrepancy disrupts the natural biomechanics of the wrist, which is built to distribute load primarily through the radius. In a wrist with neutral variance, approximately 80% of the force passes through the radius and the remaining 20% through the ulna side.
A longer ulna significantly increases the load transmitted through the ulnar side of the wrist. For instance, a positive variance of just 2 millimeters can shift the ulnar load to about 40% of the total force. This excessive mechanical compression leads directly to a progressive degenerative condition known as Ulnocarpal Impaction Syndrome. The extended ulna repeatedly “impinges” or drives into the structures on the ulnar side of the wrist, particularly the triangular fibrocartilage complex (TFCC).
The TFCC is a complex of cartilage and ligaments that acts as a cushion and stabilizer between the ulna and the wrist bones. Constant, abnormal loading from a long ulna causes wear and tear, thinning, and eventual tearing of the TFCC. Over time, this mechanical abutment can also lead to degenerative changes in the adjacent carpal bones, such as the lunate and triquetrum.
Identifying the Symptoms and Underlying Causes
The excessive pressure and damage caused by UPV lead to symptoms often referred to as Ulnocarpal Impaction Syndrome. The most common complaint is chronic pain localized to the ulnar side of the wrist, beneath the pinky finger. This discomfort is exacerbated by activities involving forceful gripping, twisting of the forearm, or pushing off with the hand.
Patients frequently report a sensation of clicking, snapping, or grinding within the wrist joint, known as crepitus, especially during rotational movements. The pain and mechanical instability can also result in a measurable reduction in grip strength and limited range of motion in the wrist and forearm. Tenderness is often felt when pressure is applied directly over the distal ulna or the area of the damaged TFCC.
Ulna Plus Variance can be either congenital or acquired. Some individuals are born with an ulna that is naturally longer than the radius, representing an anatomical variation. More frequently, the condition is acquired, often following a previous trauma to the forearm.
Acquired Causes
Acquired UPV typically results from conditions that shorten the radius relative to the ulna:
- Malunion of a distal radius fracture, where the radius heals in a shortened position.
- Premature closure of the radial growth plate, often due to an injury sustained during childhood, stopping the radius from reaching its full length.
Managing and Correcting the Condition
Diagnosis begins with a clinical examination and is confirmed through medical imaging, primarily X-rays of the wrist. Standard posteroanterior (PA) X-rays measure static ulnar variance, while a specific “grip view” X-ray assesses dynamic variance during gripping and forearm pronation. An MRI is utilized to evaluate the extent of damage to soft tissues, such as the TFCC and carpal cartilage.
Initial management for mild symptoms involves non-surgical approaches, including rest, nonsteroidal anti-inflammatory drugs (NSAIDs), and physical therapy. Physical therapy focuses on strengthening wrist muscles to improve stability and control, and may include a corticosteroid injection to reduce localized inflammation. These conservative treatments manage pain and symptoms but do not correct the underlying bone length discrepancy.
When non-surgical options fail to provide lasting relief, or for more severe cases of Ulna Plus Variance, surgical correction is often required. The definitive procedure is the Ulnar Shortening Osteotomy (USO). This surgery involves removing a small, precise segment of the ulna bone, typically between 2 to 4 millimeters, to shorten it and rebalance the relationship with the radius. The goal is to achieve a neutral or slightly negative ulnar variance, which effectively decompresses the ulnar side of the wrist and relieves the mechanical impaction.
Following the osteotomy, the two ends of the ulna are fixed together with a plate and screws to allow the bone to heal. The recovery process involves a period of immobilization, often in a splint that prevents forearm rotation for around six weeks, to ensure bone consolidation. This is followed by a comprehensive rehabilitation program involving physical therapy to restore wrist motion, strength, and function.