Ulnar and Radial Variance in SNAC Wrist
SNAC wrist is associated with negative ulnar variance (ulnar minus), not positive ulnar or radial variance. 1, 2
Key Radiographic Findings
Negative ulnar variance is significantly more common in patients with scaphoid nonunion, with 38.5% of SNAC patients demonstrating ulnar minus wrists compared to 23.1% in controls (p=0.001). 1
The mean ulnar variance in established scaphoid nonunion is -0.26 mm (range: -3 to +2 mm), which is significantly more negative than the control group mean of +0.54 mm (range: -3 to +4 mm). 1
Patients with ulnar variance less than -1 mm have a 4.58-fold increased risk (OR 4.58, CI 1.51-13.89, p<0.007) of developing scaphoid nonunion after acute fracture. 2
Clinical Implications
Negative ulnar variance appears to be a biomechanical risk factor that predisposes to both initial scaphoid fracture and subsequent nonunion development, rather than a consequence of the SNAC process itself. 1, 2
Standard posteroanterior wrist radiographs performed in neutral position and rotation are essential for accurate measurement of ulnar variance, as recommended by the American College of Radiology. 3
The three-view radiographic series (posteroanterior, lateral, and oblique) allows assessment of alignment including ulnar variance, joint spaces, and impaction syndromes. 3
Important Caveat
While SNAC wrists typically have negative ulnar variance, a previously asymptomatic positive ulnar variance can become symptomatic after four-corner fusion due to altered load transmission across the ulnocarpal joint. 4
This represents coincidental ulnocarpal pathology that may require subsequent ulnar shortening osteotomy if ulnar-sided pain develops postoperatively. 4