20. Biomechanical effect of facet joint violation on adjacent segment motion and disc strain following single level lumbar fusion with transpedicular screw and rod instrumentation

Abstract

<h3>BACKGROUND CONTEXT</h3> Facet joint violation (FV) is a complication occurring while placing pedicle screws at the upper-most level using an entry point and trajectory too close to the superior facet joint. The incidence of FV has been reported up to be as high as 50%. However, little is known about how FV affects superior adjacent level spinal stability and especially intervertebral disc (IVD) strain after lumbar fusion. <h3>PURPOSE</h3> To investigate the biomechanical effects of FV in terms of mobility and disc surface strains at the upper level adjacent to L4-5 pedicle screw-rod fixation (PSR) in cadaveric specimens. The disc strain was studied with a new optical technique of soft tissue strain analysis (3D digital image correlation [DIC] technique). <h3>STUDY DESIGN/SETTING</h3> Biomechanic testing in human cadaveric spine specimens. <h3>PATIENT SAMPLE</h3> A total of 14 human cadaveric L3-S1 specimens. <h3>OUTCOME MEASURES</h3> 1. Upper adjacent segment range of motion. 2. Disc surface strain/stress. <h3>METHODS</h3> A total of 14 human cadaveric L3-S1 specimens were divided into two equal groups. L4-5 pedicle-rod fixation was performed in a different fashion in each group ie, facet joint preservation (FP) and facet joint violation (FV). The violation of the facet joint was defined by denuding the facet joint capsule and removing of the external cortical surface of the superior articular process with a 3 mm Leksell-rongeur. The entry point of these pedicle screws was located at the junction between the inferolateral aspect of the facet joint. All specimens were tested multi-directionally under pure moment loading (7.5Nm). DIC was performed with cameras positioned laterally (left side) to capture the change in principal disc strains (Pmax and Pmin) at the upper adjacent level (L3-4) under peak load. Optical motion tracking was simultaneously performed on the right side. The IVD region was divided into four similar sized quarters and included upper and lower endplates. Analysis of variance of upper adjacent level range of motion (ROM) and disc strain, normalized to intact, was performed between the groups. Statistical significant was set at P≤0.05. <h3>RESULTS</h3> Normalized ROM was significantly greater with FV than FP at upper adjacent level (L3-4) during right axial rotation (1.22 vs 0.99, 23% difference, p=0.047). Normalized IVD Pmax strains at the upper adjacent level (L3-4) were significantly greater with FV than FP in the anterior quadrant during right axial rotation (1.80 vs 0.83 (ratio to intact), 97% difference, p=0.003). However, there were no significant differences in ROM and disc Pmax or Pmin strains during flexion, extension and lateral bending. <h3>CONCLUSIONS</h3> Facet violations at the uppermost level of fusion construct during pedicle screw-rod fixation were associated with adjacent level torsional instability and the increase of IVD strain. This study suggested that the preservation of uppermost facet joint of fusion segment was helpful for alleviating adjacent segment mobility and stress concentrations of the IVD above the fusion segment. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.