A determination of distal ulnar length in comparison to distal radial length. Radial length normally exceeds the ulna. If the ulna projects beyond the distal edge of the radius it is referred to as Positive Ulnar Variance.
Negative ulnar variance refers to an ulna which is shorter than the radius.
Positive ulnar variance has been implicated in a number of ulnar sided wrist complaints, including TFCC tears, ulnar impaction syndrome and Keinbock’s disease.
There are 3 techniques, perpendicular and lateral techniques are described below.
Drawing a line, Line A, parallel to the axis of the radius on an AP radiograph. Draw a perpendicular line, Line B, from this radial line through the ulnar edge of the radial corner and across the wrist. Then draw a line, Line C, across the distal edge of the ulnar plateau (not styloid), parallel to line B. Measure the distance between the two lines. If Line C is distal to Line B, you have positive ulnar variance. See Figure 1.
Figure 1 – Positive Ulnar variance (this is a mild case)
A second method of reading the distal ulnar head distance between the radial head line can also be done.
Both techniques are rater dependent as well as dependent on the positioning of the wrist when the radiograph was taken.
Parker AS, Nguyen M, Minard CG, Guffey D, Willis MH, Reichel LM. Measurement of ulnar variance from the lateral radiograph: a comparison of techniques. J Hand Surg Am. 2014 Jun;39(6):1114-21.
Dr. Neil Dilworth (June 12, 2014)