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Effects of Volar Tilt, Wrist Extension, and Plate Position on Contact Between Flexor Pollicis Longus Tendon and Volar Plate
Caroline Nicole Wolfe, MD; Kagan Ozer, MD; Andy F Zhu, MD; Geoffrey Burns, MS
University of Michigan, Ann Arbor, MI

Introduction
Flexor pollicis longus (FPL) rupture is a known complication after volar plate fixation. Plates positioned closer to the watershed line have been shown to have a higher incidence of FPL rupture. However, the effect of wrist extension and volar tilt has not been investigated with an in vivo model. Our objective was to evaluate the effect of distal radius malreduction, plate position and wrist extension on contact between FPL and volar plate. We hypothesized that following volar plate application; loss of native volar tilt and distal plate position will increase contact between the FPL tendon and the volar locking plate at lower degrees of wrist extension.

Materials & Methods A volar locking plate (Acu-Loc Distal Radius Plate System standard; Acumed, Beaverton, OR) was applied on 6 fresh frozen cadavers. The FPL tendon and the plate were wrapped with conductive wire and circuit conductivity was monitored at various degrees of wrist extension. A lateral wrist radiograph was obtained upon circuit closure, indicating tendon-plate contact. Baseline measurements were taken prior to an osteotomy at 3 different plate positions, Soong grade 0, 1, and 2. A dorsal extra-articular osteotomy was made and WristJack external fixator (Agee WristJack; Hand Biomechanics Lab, Inc, Sacramento, CA) was applied to enable reproducible degrees of malreduction at +5, 0, -5, -10, -15 and -20. Degree of contact was measured in all malreduction angles at 3 plate positions using lateral radiographs of the wrist. Data were analyzed using two-way repeated measures ANOVA (alpha=0.05) followed by post-hoc paired t-tests.

Results
Summary of all results are depicted on FPL/plate contact chart (Figure 1). Loss of volar tilt and plate positioning were two independent risk factors determining contact between plate and tendon. Significantly less wrist extension was required for a full contact in malreduced wrists (p<0.001) as well as in distally placed volar plates (p =0.02) (Graph 1). Plates placed in Soong grade 2 had the highest range of contact (p < 0.001).

Conclusion
In addition to plate position, fracture malreduction and degree of wrist extension help determine contact between FPL and plate. This study demonstrates contact, which does not predict the risk of rupture. FPL/plate contact chart generated in this study may be used to assess the risk of rupture at the clinical setting.




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