Metacarpal Position and Lunate Facet Screw Fixation in Dorsal Wrist Spanning Bridge Plates for Intraarticular Distal Radius Fractures: A Biomechanical Analysis
Evan Guerrero, MD1, Alexander Lauder, MD2, Andrew E Federer, MD3, Richard Glisson, BS4, Marc J. Richard, MD4 and David S. Ruch, MD4, (1)Duke University, Durham, NC, (2)CHI Franciscan St Joseph Medical Center, Tacoma, WA, (3)Duke University Health System, Durham, NC, (4)Duke University Medical Center, Durham, NC
Background: A dorsal wrist spanning plate (DWSP) can be affixed to the second or third metacarpal depending upon distal radius fracture (DRF) morphology and surgeon preference, but comparative biomechanical analyses between these methods are limited. This biomechanical study compared fixation to the second metacarpal versus third metacarpal as measured by lunate facet fragment displacement in an axial loading intraarticular DRF cadaveric model. The construct rigidity of adding an intrafragmentary lunate facet locking screw was also evaluated.
Methods: A sample size of 14 cadaveric matched pairs was calculated to achieve 81.7% power. The 28 limbs were randomized to DWSP fixation to the second metacarpal (Group 1) or third metacarpal (Group 2); each wrist underwent an intraarticular osteotomy simulating an unstable intraarticular DRF. Limbs were tested at axial loads of 50, 100, 150, 200, 250, and 300 N. Lunate facet displacement from the metaphyseal fragment at each load was measured using differential variable reluctance transducers. After initial testing, a locking screw was placed through the plate into the lunate facet fragment in the third metacarpal group (Group 3). Displacement measurements were repeated for all loads. Paired t-tests were performed to assess fragment stability differences between groups.
Results: Average fracture displacement at 300 N was 0.83 mm (range 0-1.65) in Group 1, 0.40 mm (range 0.04-0.78) in Group 2, and 0.21 mm (range 0-0.51) in Group 3. Plating to the third metacarpal more effectively resisted displacement compared to the second metacarpal at all axial loads (P<0.05). Addition of a lunate facet screw further stiffened the construct at loads of 150N or greater (P<0.05).
Conclusions: Plate fixation to the third metacarpal with an intrafragmentary screw was the most effective construct. Mean displacement of the lunate facet fragment at 300 N was less than 1 mm in all groups. All constructs allowed for stable fracture fixation with less than 2 mm of motion in all specimens under axial loads of up to 300N.
Figure 1: Prepared cadaver specimen with dorsal spanning bridge plate affixed to third metacarpal attached to loading jig mounted on servohydraulic testing machine (1a) with DVRT transducers attached (1b)
Figure 2: Graph demonstrating mean displacement, comprised of average dorsal and volar displacement, at sequential axial loads in Group 1 (affixed to second metacarpal), Group 2 (affixed to third metacarpal), and Group 3 (affixed to third metacarpal with addition of a lunate facet locking screw)
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