The wrist joint is a complex articulation involving the radius, the ulna, and eight carpal bones. This arrangement allows for the wide range of motion necessary for hand function. The precise length relationship between the distal ends of the radius and the ulna is fundamental to the joint’s biomechanical health. When this alignment is altered, it disrupts the normal distribution of forces through the wrist, potentially causing pain and dysfunction. Measuring the relative lengths of these two bones is an important orthopedic assessment.
Defining Ulnar Variance in the Wrist
Ulnar variance is a standardized measurement quantifying the difference in length between the distal articular surfaces of the ulna and the radius. This measurement is typically taken from a posteroanterior (PA) view X-ray of the wrist, with the forearm in a neutral rotation. The relationship is determined by drawing two lines perpendicular to the long axis of the radius: one tangent to the radial articular surface and one tangent to the distal dome of the ulna.
A difference of 0 to 1 millimeter is classified as neutral ulnar variance, indicating a balanced load distribution. Positive ulnar variance occurs when the ulna extends farther distally than the radius. Conversely, negative ulnar variance exists when the ulna’s articular surface is shorter, or more proximal, than the radius’s articular surface.
Primary Drivers of Negative Ulnar Variance
The causes of negative ulnar variance are typically categorized as either developmental or acquired, all resulting in the ulna being relatively shorter than the radius. Developmental factors include congenital causes where the ulna grows to a shorter length due to genetic predisposition or musculoskeletal disorders. This results in a naturally occurring difference in bone length that may be present throughout a person’s life.
A significant acquired cause is growth plate arrest, particularly in children and adolescents. The distal end of the ulna and the radius each have a physis, or growth plate, that guides longitudinal bone growth. If the distal ulnar physis is injured, often due to trauma, it may prematurely close or slow its growth rate. Since the radial physis continues to develop normally, this differential growth pattern results in a progressive negative ulnar variance over time.
Post-traumatic changes following a severe fracture of the distal radius are another common driver. A fracture that heals with shortening, or a malunion of the radius, effectively pulls the radial articular surface proximally. Even if the ulna remains its original length, the shortened radius makes the ulna appear relatively short in comparison. The resulting negative ulnar variance is thus caused by the structural collapse and shortening of the radius.
Associated Conditions and Clinical Significance
The clinical significance of negative ulnar variance lies in its alteration of biomechanical forces transmitted across the wrist, particularly on the radial side of the carpus. When the ulna is shorter, the normal load distribution (roughly 80% through the radius and 20% through the ulna) shifts. The shortened ulna reduces the force passing through the ulnar side, forcing a higher percentage of the total load onto the radiocarpal joint, specifically the lunate and scaphoid bones.
This increased mechanical stress on the lunate is strongly associated with Kienböck’s disease. This is a rare condition involving the progressive death of the lunate bone tissue due to avascular necrosis (lack of blood supply). The shortened ulna subjects the lunate to greater compressive and shear forces against the distal radius. This repeated, abnormal loading is thought to compromise the lunate’s blood supply, eventually leading to structural breakdown.
The effectiveness of surgical treatments designed to correct negative ulnar variance supports this mechanical hypothesis. Procedures like radial shortening osteotomy physically shorten the radius to neutralize the variance and decompress the lunate bone. By re-establishing a balanced length relationship, these surgeries reduce the excessive force on the lunate, potentially halting the progression of Kienböck’s disease. Negative ulnar variance represents a significant anatomical risk factor that changes the wrist’s functional dynamics.