Session IX - Basic Science

Relative Strength of Conventional and Locking Plate Fixation in Osteoporotic Bone

Daniel C. Fitzpatrick, MD¹; Michael Bruehl, BS²; Larry Ehmke, MS⁴;

Keith A. Mayo, MD³; Michael Bottlang, PhD⁴; (all authors a-Synthes USA)

¹Orthopedic Healthcare Northwest, Eugene, Oregon, USA

²University of Applied Sciences, Gissen Friedberg, Germany;

³Tacoma Trauma Center, Tacoma, Washington, USA;

⁴Legacy Biomechanics Laboratory, Portland, Oregon, USA

Purpose: Locking plates have been proposed as a solution to the marginal fixation obtained with nonlocked plating of osteoporotic fractures. This study was designed to investigate the stability of unicortical and bicortical locking plate constructs relative to the gold standard of conventional, nonlocked plates in an osteoporotic fracture model. We hypothesized that locked plate constructs were at least as mechanically stable as conventional nonlocked constructs.

Methods: Surrogate foam specimens were custom manufactured to simulate osteoporotic femoral diaphyseal bone. Each specimen was fixed with a 10-hole locking compression plate with screws in the 1, 3, 5 position on one side of a 1-cm fracture gap. Conventional nonlocking 4.5-mm cortical screws were used in one group of specimens. A second group was filled with 5.0-mm locked unicortical screws. A third group was plated with 5.0-mm bicortical locking screws. Finally, an additional three sets were plated with one bicortical locking screw in the place of a unicortical locked screw in either the 1, 3, or 5 hole. Each group was then loaded in axial compression, torsion, and 4-point-bending. The initial stiffness and loosening load (defined as a 10% loss in initial stiffness) were calculated.

Results: Axial stiffness did not significantly differ between the conventional plates and any of the locked configurations. There was a significant increase in the loosening load for all of the locked configurations relative to the conventional plate during compressive loading. In torsion, the bicortical locked plates showed significantly increased initial stiffness and loosening loads relative to the conventional plate, while the all-unicortical configuration showed significantly lower initial stiffness and loosening loads. Adding a single bicortical locked screw to the otherwise all-unicortical configuration significantly improved the initial stiffness and loosening
loads. No difference was noted in initial bending stiffness among the groups. All specimens fractured in bending rather than loosening.

Conclusion/Significance: Bicortical locking screws provide initial fixation equivalent to that of conventional plates with a much improved resistance to loosening. Unicortical screws are especially weak in torsion and should be avoided in situations in which high torsional loads are expected. The addition of a single locked bicortical screw to an otherwise all-unicortical construct significantly improves construct strength.