Bone Quality as Measured by Hounsfield Units More Accurately Predicts Proximal Junctional Kyphosis than Vertebral Bone Quality Following Long-Segment Thoracolumbar Fusion.

OBJECTIVE: To compare the prognostic power of Hounsfield units and VBQ score for predicting proximal junctional kyphosis (PJK) following long-segment thoracolumbar fusion to the upper thoracic spine (T1-T6) METHODS: Vertebral bone quality around the upper instrumented vertebrae (UIV) was measured using HU on preoperative CT and VBQ on preoperative MRI. Spinopelvic parameters were also categorized according to the SRS-Schwab classification. 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 (defined as ≥10° change in Cobb angle of UIV+2 and UIV). Sensitivity analyses showed thresholds of HU<164 and VBQ>2.7 to be most predictive for PJK.

RESULTS: Seventy-six patients (mean age 66.0±7.0yr; 27.6% male) were identified, of whom 15 suffered PJK. Significant predictors of PJK were high postoperative pelvic tilt (p=0.038), high postoperative T1-pelvic angle (p=0.041), and high postoperative PI-LL mismatch (p=0.028). On survival analyses, bone quality, as assessed by the average Hounsfield units 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).

CONCLUSION: In larger cohorts, VBQ may prove to be a significant predictor of PJK following long-segment thoracolumbar fusion. However, Hounsfield units on CT have greater predictive power, suggesting preoperative workup for long-segment thoracolumbar fusion benefits from CT versus MRI alone to identify those at increased risk of PJK.