1217 Quantifying the Importance of Upper Cervical Extension Reserve in Adult Cervical Deformity Surgery and its Impact on Baseline Presentation and Outcomes

INTRODUCTION: Hyperextension of the upper cervical spine is a prominent compensatory mechanism to maintain horizontal gaze and balance in adult cervical deformity (ACD). Relaxation of ER and its impact on postoperative outcomes is not well understood. METHODS: ACD patients undergoing subaxial cervical fusion with 2Y data were included. Upper cervical extension reserve(ER) was defined as: ΔC0-C2 sagittal Cobb angle between neutral and extension. Relaxation of ER was defined as the ER mean in those that met the ideal ACD modifiers. RESULTS: 108 ACD patients met inclusion. Preoperative C0-C2 ER was 8.7°±9.0°, and last follow-up was 10.3°±11.1°. Preoperatively 29% had adequate ER, 60% had improved ER postoperatively, with 50% achieving adequate ER at 2Y. Lower ER correlated with greater cervical deformity (p<.05). Preoperatively, greater ER had lower disability in NDI (p<.001). Controlled analysis found improved ER to have a 7x higher likelihood of NDI MCID (OR 6.94, p=.019). Isolating those with inadequate preoperative ER, found postoperative resolution having 5x odds of good clinical outcomes (p<.05). In those with inadequate ER at baseline, the preoperative C2-C7 of <-18° and TS-CL of >59° for TS-CL was predictive of ER resolution. In those with preoperative C2-C7 >-18°, a T1PA of >13° was predictive of postoperative return of ER (all p<.05). Surgical correction of C2-C7 by >16° from baseline was found to be predictive of ER return, highlighting its compensatory role in cervical deformity. CONCLUSIONS: Increased preoperative utilization of the extension reserve in the upper cervical spine in cervical deformity was associated with worse baseline regional and global alignment while impacting health-related measures. The majority of patients had relaxation of extension reserve postoperatively, however, in those who didn’t, there was a decreased likelihood of achieving good outcomes.