The Relationship of CT Hounsfield Units at UIV+1 and UIV+2 Relative to UIV Predicts Proximal Junctional Kyphosis in Patients With Long Spinal Constructs

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STUDY DESIGN: Retrospective review. OBJECTIVE: To determine CT Hounsfield units (CTHU) thresholds at the upper instrumented vertebra (UIV), UIV+1, and UIV+2 that preclude adult spinal deformity (ASD) patients to develop proximal junctional kyphosis (PJK). SUMMARY OF BACKGROUND DATA: Low CTHU at the UIV and UIV+1 have been shown to increase risk of PJK, pseudarthrosis, and pedicle screw loosening. In contrast, higher CTHU values have been associated with greater fusion potential. For this study, we studied the relationship of CTHU at the UIV, UIV+1, UIV+2 in ASD patients with long fusion constructs and sacropelvic fixation. MATERIALS AND METHODS: One hundred ninety-one ASD patients with a UIV at L2 or cephalad and pelvic fixation were identified with minimum two-year follow up. Patients with UIV distal to L2, prior anterior fusion at UIV, or UIV at C2 were excluded. CTHU were measured at the UIV, UIV+1, and UIV+2 of each patient. Statistical analysis was performed with significance set to P <0.05. RESULTS: Forty patients (20.9%) developed radiographic PJK, of which 19 went on to have revision surgery. PJK was more common at lower thoracic or thoracolumbar UIVs, as compared with upper thoracic and cervical UIVs ( P =0.0048). After multivariable logistic regression, CTHU below 160 at the UIV+1 and UIV+2 was found to be a significant predictor of PJK (OR=8.10, P =0.0002 and OR=4.60, P =0.106, respectively). Cox proportional hazards regression analysis also identified CTHU at UIV+1 and UIV+2 as significant predictors of PJK (HR=6.52, P =0.0002 and HR=3.59, P =0.0131, respectively). Kaplan-Meier survival analysis demonstrates that patients with CTHU ≥160 at UIV+1 and UIV+2 demonstrated significantly greater PJK free survival. Lastly, a subanalysis found that patients with CTHU at UIV+1 ≥ UIV, were significantly less likely to develop PJK ( P =0.0035). Among patients with CTHU at the UIV <160, if the CTHU at UIV+1 increased, these patients were less likely to develop PJK. CONCLUSIONS: CTHU under 160 was a risk factor for developing PJK in ASD patients undergoing deformity correction with long fusion constructs. CTHU at UIV+1 ≥ UIV were significantly less likely to develop PJK, even among those with CTHU < 160 at the UIV. These findings highlight the critical role of regional bone health in mitigating PJK risk.