The effect of anterior thoracolumbar plate application on the compressive loading of the strut graft

In vitro biomechanical testing was performed using a simulated vertebral body bone graft instrumented with strain gauges. Strains were recorded from various locations on the graft during axial compressive loading. Comparisons were made of the strain patterns recorded before and after application of two different anterior plates.

To quantify the changes in axial compression experienced by the strut graft in the presence of an anterior plate.

The use of anterior instrumentation to augment anterior thoracolumbar grafting offers the potential advantage of saving additional motion segments while being performed in a single-stage surgery. Several biomechanical studies have compared the anteriorly grafted and instrumented spine to the compressive axial stiffness and torsional rigidity of the intact spine. No previous study has addressed the loading patterns experienced by the graft before and after plating.

Anterior 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 determine the axial compressive forces experienced by the bone graft before and after application of the plates. Bovine spines harvested from 8- to 12-week-old calves were used for testing. All plates were tested in axial compression to 500 N. Simultaneous recordings were made of the axial strains on the simulated bone graft and the load applied to the construct. Comparisons were made of the strain through the graft without any instrumentation (graft alone, or baseline), with the plate applied, and after removal of the caudal screws (dynamization).

With the application of the ALPS plate, the compressive strain through the graft adjacent to the plate averaged 77% of the graft alone construct (range, 39% to 158%). After application of the Synthes plate, the compressive strain through the graft adjacent to the plate averaged 34% of the graft alone construct (range, -14% to 97%). Once the caudal screws were removed, the dynamized construct allowed near-baseline compression through the graft.

Although the literature indicates that the anteriorly instrumented spine may restore the overall spinal structure to near-normal resistance to rotation and flexion forces, the current study demonstrates that a significant amount of compression through the graft is lost by its application. This decreased axial loading further supports the concept of device-related osteopenia observed clinically with such devices.