Anterior vertebral screw strain with and without solid interspace support

Study Design. This in vitro biomechanical study examines segmental anterior vertebral screw strain and solid rod construct stiffness with and without the addition of multilevel, threaded cortical bone dowels in a bovine model . Objective. To determine whether strain at the bone-screw interface is highe r at the end levels during physiologic logic range loading, and whether sol id interspace support decreases segmental strain on the implant. Summary of Background Data, Anterior instrumentation provides greater corre ction and preserves distal motion segments. However, nonunion and implant f ailure are observed more frequently than with posterior segmental instrumen tation, and when observed, loss of fixation occurs at the end levels. Methods. Eight calf spines underwent mechanical testing in the following se quence: 1) intact condition, 2) anterior release with anterior solid rod an d bicortical rib grafts, and 3) anterior release with anterior solid rod an d threaded cortical bone dowels (L2-L5). Instrumented vertebral screws were used to assess strain within the vertebral body by the near cortex, wherea s an anterior extensometer spanning the instrumented segments was used to m easure segmental displacements to calculate construct stiffness. The protoc ol included axial compression (-400 N), right lateral bending (4 Nm (Newton meter), away from the implant), and left lateral bending (4 Nm, toward the implant). Statistical analysis included a one-way analysis of variance and a Student-Newman-Keuls post hoc test. A pilot study was performed using fo ur additional specimens loaded for 4000 cycles to in vestigate macroscopic loosening after fatigue loading. Results, In lateral bending toward the implant, the strain was higher at bo th end levels, with no differences between the rib and dowel reconstruction s. The stiffness values were greater than the intact values for both groups . In lateral bending away from the implant, the strain also was higher at b oth end screws, and the dowel group had less strain at these levels than th e rib group. Both groups were stiffer than the intact condition, and the do wel group was stiffer than the rib group. Axial compressive strain also was higher at the end levels, but this difference did not reach statistical si gnificance. The rib group did not reach intact stiffness values, whereas th e dowel group was stiffer than the intact condition, The fatigue study show ed gross loosening at one or both end levels in all cases. Conclusions. Higher strain was observed at the bone-screw interface in both end screws of an anterior solid rod construct during lateral bending, whic h correlates with the clinically observed failure location. This suggests t hat physiologic range loading may predispose to failure at the end levels. Disc spade augmentation with solid implants increased construct stiffness i n all three load paths and decreased strain at the end levels in lateral be nding away from the implant. future implant modifications should achieve be tter fixation at the end screws, and the current modal provides a means to compare different strategies to decrease strain at these levels.