THE EFFECT OF ANTERIOR THORACOLUMBAR PLATE APPLICATION ON THE COMPRESSIVE LOADING OF THE STRUT GRAFT

Study Design. In vitro biomechanical testing was performed using a sim ulated vertebral body bone graft instrumented with strain gauges. Stra ins were recorded from various locations on the graft during axial com pressive loading. Comparisons were made on the strain patterns recorde d before and after application of two different anterior plates. Objec tives. To quantify the changes in axial compression experienced by the strut graft in the presence of an anterior plate. Summary of Backgrou nd Data. The use of anterior instrumentation of augment anterior thora columbar grafting offers the potential advantage of saving additional motion segments while being performed in a single-stage surgery. Sever al biomechanical studies have compared the anteriorly grafted and inst rumented spine to the compressive axial stiffness and torsional rigidi ty of the intact spine. No previous study has addressed the loading pa tterns experienced by the graft before and after plating. Methods. Ant erior spinal plates of two designs (Amset ALPS Anterior Locking Plate System; AMS, Hayward, California; Anterior Thoracolumbar Locking Plate System; Synthes Spine, Paoli, Pennsylvania) were evaluated to determi ne the axial compressive forces experienced by the bone graft before a nd after application of the plates. Bovine spines harvested from 8- to 12-week-old calves were used for testing. All plates were tested in a xial compression to 500 N. Simultaneous recordings were made of the ax ial strains on the simulated bone graft and the load applied to the co nstruct. Comparisons were made on the strain through the graft without any instrumentation (graft alone, or baseline), with the plate applie d, and after removal of the caudal screws (dynamization). Results. Wit h the application of the ALPS plate, the compressive strain through th e graft adjacent to the plate averaged 77% of the graft alone construc t (range, 39% to 158%). After application of the Synthes plate, the co mpressive strain through the graft adjacent to the plate averaged 34% of the graft alone construct (range, -14% to 97%). Once the caudal scr ews were removed, the dynamized construct allowed near-baseline compre ssion through the graft. Conclusions. Although the literature indicate s that the anteriorly instrumented spine may restore the overall spina l structure to near-normal resistance to rotation and flexion forces, the current study demonstrates that a significant amount of compressio n through the graft is lost by its application. This decreased axial l oading further supports the concept of device-related osteopenla obser ved clinically with such devices.