Predicting a safe screw length for volar plate fixation of distal radius fractures: lunate depth as a marker for distal radius depth.

Purpose: We hypothesized that the lunate depth as measured on plain lateral radiographs can be used to predict distal radius depth radially and ulnarly and serve as a useful reference for intraoperative screw placement in volar plate fixation of distal radius fractures.

Methods: Plain radiographs and magnetic resonance imaging (MRI) of the wrists of 30 patients were reviewed. The lunate depth and the maximal depth of the distal radius were determined from plain lateral radiographs. Depth of the distal radius, measured in quartiles, was determined from axial MRI images, and the lunate depth was obtained from sagittal MRI images. The depth of the distal radius in each quartile was then calculated related to the lunate depth.

Results: The mean depth of the lunate on plain radiographs and MRI was 17.5 mm and 17.4 mm, respectively. The depth of the distal radius from ulnar to radial was 18.4 mm, 20.2 mm, 19.4 mm, and 15.1 mm for the 1st through 4th quartiles, respectively. The depth of the distal radius is the least radially (4th quartile), with a mean 87% of the lunate depth, and greatest in the 2nd quartile, with a mean 116% of the lunate depth.

Conclusions: The depth of the lunate as measured on plain radiographs can be used as a marker for drilling and placement of safe screw lengths during volar plate fixation of distal radius fractures. We recommend that surgeons use the lunate depth as an estimate for the length of their longest screw when fixing distal radius fractures with volar plate techniques to avoid extensor tendon irritation and rupture.

Type Of Study/level Of Evidence: Therapeutic III.