Unicortical Versus Bicortical Proximal Locking Screw for Prevention of Peri-Implant Fracture: A Biomechanical Analysis of an Osteoporotic Distal Radius Model.

PURPOSE

Osteoporotic patients are at risk of peri-implant fractures after distal radius fixation. A unicortical screw in the proximal hole of the plate can theoretically decrease stress riser formation by eliminating the hole in the far bone cortex. This construct has been proposed in orthopedic literature to prevent peri-implant fractures but has not been tested in an osteoporotic distal radius model.

METHODS

Eleven paired cadaver radii were harvested and plated with four-hole titanium volar distal radius plates. No osteotomies were created. The fixation constructs were identical except that group A used a bicortical proximal locking screw and group B used a unicortical proximal locking screw. Bone mineral density was estimated using radiographic measurements. The samples were potted and tested for four-point bending stiffness, torsion stiffness, and load to failure.

RESULTS

Between the bicortical and unicortical screw groups, there was no significant difference in four-point bending stiffness (110.8 vs 106.2 N/mm, apex volar bending; 105.4 vs 107.1 N/mm, apex dorsal bending) or torsional stiffness (430.6 vs 427.6 N-mm/degree, internal rotation; 430.8 vs 429.7 N-mm/degree, external rotation). There was also no significant difference in load to failure with apex dorsal four-point bending (795.3 vs 770.0 N).

CONCLUSIONS

This study shows that a healed osteoporotic distal radius volar plate construct with a proximal unicortical locking screw is not statistically different from a bicortical screw in stiffness or load to failure in apex dorsal bending. Although a unicortical locking screw has been proposed as a mechanism to prevent stress risers at the proximal aspect of the distal radius plate, this study suggests no significant difference when compared with a bicortical locking screw.

CLINICAL RELEVANCE

There is no significant biomechanical advantage to unicortical over bicortical locking screws in the proximal hole of a distal radius plate to prevent diaphyseal peri-implant fractures in osteoporotic patients.