Biomechanical Effects of Thoracic facet violation after PSR instrumentation

Introduction: Pedicle screw fixation with rod system (PSR) is one of the most common posterior spine fixation methods on the thoracic spine. However, this procedure is not without complications, particularly facet violation during pedicle screw placement. The reported incidence is as high as 47% during percutaneous pedicle screw placement. Despite this high occurrence, little is known about how facet violation affects the cranial adjacent segment biomechanics after thoracic PSR fusion surgery.

Methods: Seven cadaveric T8-T12 specimens were prepared, T8 and T12 potted and tested in the three conditions. First in intact condition, second instrumented (T10-T12) condition with persevered upper segment facet and followed by third condition instrumented (T10-T12) with violated upper segment facet. Specimens were tested in six directions under pure moment axial loading (5nm). ROM at both the instrumented level and the upper adjacent segment were collected. IVD strains at the upper adjacent segment were collected using DIC. Range of motion (ROM) and IVD strain were normalized to intact upper adjacent-level and compared between the groups using analysis of variance. Statistical significance was set at P< 0.05.

Results: There was no difference in ROM at the upper adjacent segment (T9-T10) between facets preserved and violated groups in all directions (P≥0.3). There was no difference in disc strain at T9-T10 IVD before and after facet violation (P=0.4).

Conclusion : Thoracic facet violation during Pedicle screw placement did not change the upper adjacent segment motion as well as the upper adjacent segment IVD strain.