286 Bone Quality as Measured by Hounsfield Units More Accurately Predicts Proximal Junctional Kyphosis Than Vertebral Bone Quality Following Long-segment Thoracolumbar Fusion

INTRODUCTION: Both Hounsfield units (HU) on computed tomography and Vertebral Bone Quality (VBQ) score on MRI have been advanced as means of assessing underlying bone quality and predicting mechanical complications amongst spine patients. METHODS: Patients = 50 with =12-month follow-up following long-segment thoracolumbar fusion were included. Bone quality around the upper instrumented vertebrae (UIV) was measured using HU on preoperative CT and VBQ on preoperative MRI. Spinopelvic parameters were also measured. PJK was defined as a change of =10° in the Cobb angle between the cephalad endplate of UIV+2 and caudal endplate of UIV. Univariable analysis to identify predictors of the occurrence of PJK and survival analyses with Kaplan-Meier method and Cox regression were performed to identify predictors of time to PJK. Sensitivity analyses showed thresholds of HU 2.7 to be most predictive for PJK. RESULTS: Seventy-six patients (66.0 ± 7.0 yr; 27.6% male) were identified, with 15 suffering PJK. On univariable analyses PJK was predicted by high postoperative pelvic tilt (p = 0.038), postoperative T1-pelvic angle (p=0.041), and postoperative PI-LL mismatch (p = 0.028). On multivariable survival analysis, bone quality, as assessed by average HU of the UIV and UIV+1 was the only significant predictor of time to PJK (OR=3.053; 95%CI 1.032, 9.032; p = 0.044). VBQ measured using the UIV, UIV+1, UIV+2, and UIV-1 vertebrae approached, but did not reach significance (2.913; 0.797, 10.646; 0.106). CONCLUSIONS: In larger cohorts, VBQ may prove to be a significant predictor of PJK following long-segment thoracolumbar fusion. However, it has less predictive power than Hounsfield units on CT, suggesting preoperative workup for patients undergoing long-segment thoracolumbar fusion may benefit from CT versus MRI alone to identify those at increased risk of PJK.