P17. Biomechanical effects of changes in cervical alignment following single-level ACDF using 3D motion capture

Abstract

<h3>BACKGROUND CONTEXT</h3> Alignment of the cervical spine likely has significant effects on the clinical longevity of the subaxial motion segments. The biomechanical effects of changes in alignment on the cervical spine remain to be understood. Single-level ACDF is a commonly performed procedure and cervical alignment may influence motion transfer to adjacent and noncontiguous subaxial motion segments. <h3>PURPOSE</h3> The purpose of this study is to better understand the role of alignment on unfused cervical motion segments, the present study was designed to evaluate the biomechanical effects on the subaxial cervical spine (C2 to T1) of 1) changes in T1 tilt 2) single-level ACDF and 3) combined effects of ACDF and changes in T1 tilt. <h3>STUDY DESIGN/SETTING</h3> Cadaveric study. <h3>PATIENT SAMPLE</h3> Six cadaveric cervical spines (C2-T1). <h3>OUTCOME MEASURES</h3> Biomechanical metrics. <h3>METHODS</h3> Six cadaveric cervical spines (C2-T1) were screened fluoroscopically and placed with retro-reflective markers. Computed tomography (CT) scans with sagittal/coronal reformats were performed and degeneration was graded. Specimens were tested to 30° flexion and 30° extension in the intact state and following single-level ACDF(C5-C6). Motions of each vertebral body were tracked using a 3D-motion capture system and input into an inverse kinematics model to correlate the 3D reconstruction and motion capture data. A customized testing apparatus was developed to alter the T1 slope in human cervical cadaveric spines to neutral, +15°, and -15°. Generalized estimating equation modeling was used to analyze the main effects as well as the interaction effects of fusion, while we also controlled for the clustered nature of the data. Parameter estimates of the model are reported as mean change in range of motion and standard error, with statistical significance defined as p-value = .05. SAS software was used for all statistical analyses. <h3>RESULTS</h3> In the intact specimen, increase in T1 tilt (0° to +15°) did not result in increased ROM at any motion segment (C2-3, C3-4, C4-C5, C5-6, C6-C7, C7-T1: p=NS). In the intact specimen, decrease in T1 tilt (0° to -15°) resulted in significantly increased ROM at C2-C3 (p < .0001), C3-C4 (p < .0001), C5-C6 (p < .01), C6-C7 (p <.005) and C7-T1 (p < .02), but not at C4-C5 (p=NS). The effects of ACDF on the unfused segments at various cervical alignments was: neutral T1 tilt, ACDF did not result in significantly increased ROM at any level (C2-3, C3-C4, C4-C5, C6-C7, C7-T1: p=NS). At increased T1 Tilt (+15°), motion was significantly increased at C2-C3 (p <.05); however, not at any other segment (p=NS). At decreased T1 tilt (-15°), there was a trend toward increased motion at C7-T1 (p=.06) and no significant increase in motion at any other level (p=NS). The GEE model did not show significant cumulative effects for the interaction of tilt and fusion on range of motion (p=NS). <h3>CONCLUSIONS</h3> Changes in cervical alignment have a biomechanical influence on the observed patterns of motion in both the intact state and following ACDF. In the unfused specimen, increased T1 tilt resulted in no significant hypermobility whereas decreased T1 tilt resulted in significantly increased hypermobility at five motion segments. Following ACDF, hypermobility was transferred to non-contiguous segments, cranially when T1 tilt was increased and trended caudally when T1 tilt was decreased. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.