Is ulnar variance suitable for a parameter of Colles' fracture pre-operatively?

To evaluate ulnar variance (UV) as a parameter for Colles' fracture as a measure of the true length of the distal radius pre-operatively, we measured UV pre-operatively in 20 patients with Colles' fractures being treated surgically using the method of perpendiculars. Because the distal fragment is by definition dorsi-flexed, the dorsal edge of the most distal part of the radius is seen proximally and the volar edge of it is seen distally on true postero-anterior x-ray of the wrist.

We measured three different UVs in x-rays. UVd is the distance from the dorsal edge of the distal radius to the distal end of the ulnar head. UVv is the distance from the volar edge of the distal radius to the distal end of the ulnar head. We calculated UVc using lateral x-ray of the wrist at the midpoint of the lunate fossa to describe the true length of the distal radius. We compared UVd with UVv or UVc in 20 patients as well as in 11 patients who had dorsal tilt over 15 degrees.

UVv was -3.8 +/- 2.5 mm (average+/-SD). UVd was 2.2 +/- 2.4 mm. UVc was 1.7 +/- 2.2 mm. Statistically, there was a significant difference between UVv and UVd (p < 0.0001). There was no statistically significant difference between UVd and UVc. However, in 11 cases with dorsal tilt over 15 degrees, UVd was 2.2 +/- 2.8 mm and UVc was 1.1 +/- 2.7 mm. There was a statistically significant difference between both groups (p = 0.003).

The length of the distal radius can be described by measuring UVd pre-operatively in Colles' fracture patient in general. However, the true length of the distal radius can not be described by measuring UVd pre-operatively in cases with marked dorsi-flexion of the distal fragment.