Fracture Compression and Construct Stability with Variable Pitch Locking Screws in Cadaveric Specimens

Category: Trauma; Basic Sciences/Biologics Introduction/Purpose: A variable pitch locking screw is intended to provide interfragmentary compression combined with fixed angle stability of locking plate constructs. The objective of this study was to compare variable pitch locking screws with standard locking screws in bicortical fixation scenarios in cadaver bone, by assessing: (1) interfragmentary compression and plate-bone compression; and (2) construct biomechanical stability. Methods: Nine matched pairs of fresh-frozen cadaveric specimens were used to compare variable pitch locking screws and standard locking screws. First, interfragmentary compression and plate-bone compression associated with insertion of single bicortical screws at increasing levels of torque were measured in distal tibiae having a simulated longitudinal fracture. Second, fibulae were osteotomized to create a stable fracture pattern, fixed with five-screw plated constructs, and tested in either cyclic axial and torsional loading or four-point bending. Results: Interfragmentary and plate-bone compression forces were significantly higher with variable pitch locking screws compared to standard locking screws (p =0.002 & p=0.028, respectively). The amount of compression was variable across specimens. In cyclic loading of fibula constructs, no significant differences were detected in construct axial displacement or angular displacement. In four-point bending, no differences were detected in maximum force or bending stiffness. Conclusion: Variable pitch locking screws can produce high interfragmentary compression between cortices, along with moderate plate-bone compression. In a stable bicortical fixation scenario under external loading, construct stability was similar to normal locking screws.