How Much Do Locked Screws Add to the Fixation of "Hybrid'' Plate Constructs in Osteoporotic Bone?

Background: Hybrid locked plating has become a commonly used technique for treating complex fractures and nonunions, but information is lacking to direct the specific application of this fixation method. The purpose of this study was to determine the effect of the number and location of locked screws on the mechanical properties of hybrid plate constructs in an osteoporotic bone model. Methods: A synthetic commercial composite model of osteoporotic bone with a 5-mm simulated fracture gap was fixed with a 12-hole plate. Seven different constructs (n = 5/construct) were tested including 2 unlocked and 5 hybrid configurations. All constructs used bicortical screws tightened to 4 N.m torque. Cyclic (sinusoidal) testing was performed with a peak torsional load of +/- 8 N.m for 100,000 cycles. Torsional stiffness of each construct was measured in 10,000 cycle increments, and the maximum removal torque of each screw was measured at the conclusion of torsional testing. Results: Stiffness of the constructs at each testing interval was most affected by the number of screws; stiffness increased at least 33% when 4 screws were used on each side of the fracture versus 3 per side. Among the constructs with 4 screws in each fragment, no difference was observed when 1 or 2 unlocked screws were replaced with locked screws on each side of the simulated fracture. In contrast, replacement of 3 unlocked screws with locked screws increased the torsional stiffness of the construct by another 24% (P < 0.001). Compared with baseline (pretesting) values, postcycling screw removal torque was similar for locked screws at all positions (average 50% of peak removal), but removal torque of unlocked screws furthest from the fracture was increased by 274% if they were placed immediately adjacent to a locked screw (P < 0.001). Conclusions: At least 3 bicortical locked screws on each side of a fracture are needed to increase the torsional stiffness in an osteoporotic bone model. Locked screws placed between the fracture and unlocked screws protect the unlocked screws from loosening and may have some clinical utility in improving fatigue life of the construct.