Junctional Hounsfield unit ratio: understanding patient-specific vertebral bone strength for proximal junctional kyphosis risk assessment in adult spinal deformity surgery

Study design A retrospective case control study Objective To predict proximal junctional kyphosis (PJK) risk by normalizing individual vertebral bone strength using the ratio of vertebral Hounsfield unit (HU) values around the upper instrumented vertebrae (UIV). Summary of background data PJK poses a significant challenge in treating patients after adult spinal deformity (ASD) surgery. While the vertebral body HU value is associated with PJK risk, the optimal threshold remains unclear, and a relative assessment of HU values within individuals has not been conducted. Methods Data on patients who underwent corrective fusion of the middle to lower thoracic region of the pelvis for ASD were assessed. The 126 patients were categorized into PJK and non-PJK groups. We compared the patients' backgrounds, vertebral body HU, and junctional HU ratio, defined as the HU value of UIV+1 divided by the HU value of UIV (HUUIV+1/HUUIV). The UIV+2/UIV+1 HU ratio was calculated similarly. Results The PJK and non-PJK groups included 30 and 96 patients, respectively. After propensity score matching, 28 patients from each group were analyzed. HU values at UIV+2 and UIV+1 (117.0 {+/-} 46.6 vs 145.1 {+/-} 45.9, p=0.018, and 105.5 {+/-} 36.2 vs 147.3 {+/-} 44.9, p<0.001, respectively) were lower in the PJK group. Junctional HU ratio was significantly lower in the PJK group (0.88 {+/-} 0.18 vs 1.13 {+/-} 0.25, p<0.001), and receiver operating characteristic analysis showed that the junctional HU ratio had the highest discriminative ability (area under the curve 0.812). At the optimal cutoff value (HU ratio of 0.905), the sensitivity and specificity for PJK were 64.3% and 89.3%, respectively. Conclusions A low junctional HU ratio was strongly associated with PJK after ASD surgery. This parameter reflects the bone strength mismatch at the proximal junction and may help improve preoperative risk assessment and UIV selection.