Low junctional Hounsfield unit ratio associated with proximal junctional kyphosis after spinal deformity surgeryBone density scan ratio may help predict spinal complication risk after surgery

medRxiv Published April 6, 2026 Study authors: Nagatani, Y.; Segi, N.; Ito, S.; Ouchida, J.; Yamauchi, I.; Ode, Y.; Okada, Y.; Takeichi, Y.; Tachi,… DOI ↗ Editorial oversight: Dr. Lars van Dijk, PhD · Surgical, Procedural & Diagnostic

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Key Takeaway

Consider junctional HU ratio as potential PJK risk marker, but recognize observational limitations.

This retrospective case-control study analyzed 126 patients who underwent corrective fusion surgery for adult spinal deformity, comparing 30 patients who developed proximal junctional kyphosis (PJK) with 96 who did not. After propensity score matching, 56 patients (28 PJK, 28 non-PJK) were included. The study examined Hounsfield unit (HU) values at vertebral levels adjacent to the upper instrumented vertebra (UIV) and calculated a junctional HU ratio (HU at UIV+1 divided by HU at UIV).

Patients who developed PJK had significantly lower HU values at both UIV+2 (117.0 ± 46.6 vs 145.1 ± 45.9, p=0.018) and UIV+1 (105.5 ± 36.2 vs 147.3 ± 44.9, p<0.001). Most notably, the junctional HU ratio was substantially lower in the PJK group (0.88 ± 0.18 vs 1.13 ± 0.25, p<0.001). This ratio demonstrated moderate discriminative ability for PJK with an area under the curve of 0.812. At an optimal cutoff of 0.905, sensitivity was 64.3% and specificity was 89.3%.

Safety and tolerability data were not reported. Key limitations include the retrospective design, unclear optimal HU threshold for clinical use, and the novel nature of this relative HU assessment approach within individuals. The study did not report follow-up duration, funding sources, or conflicts of interest.

While these findings suggest the junctional HU ratio may help improve preoperative risk assessment and UIV selection, this represents an association rather than causation. The retrospective nature and limited sample size after matching (n=56) constrain generalizability. Prospective validation in larger cohorts is needed before considering clinical implementation.

Researchers looked back at medical records for 126 patients who had major spinal surgery to correct a deformity in adults. They wanted to see if a specific measurement from pre-surgery CT scans, called the junctional Hounsfield unit (HU) ratio, was connected to a known complication called proximal junctional kyphosis (PJK). PJK is when the spine above the fused section begins to curve again after surgery.

They compared 30 patients who developed PJK to 96 who did not. After matching patients for similar characteristics, they found the bone density ratio was significantly lower in patients who later developed PJK. The measurement showed a moderately strong ability to distinguish between patients who would and would not get PJK, with a specificity of 89.3% at a certain cutoff.

This study suggests a simple measurement from a standard pre-op CT scan might help surgeons better understand a patient's individual risk before planning surgery. However, this was a small, retrospective look at past cases, which limits how certain we can be. It shows a link, not a cause. More research is needed to confirm if this measurement is a reliable tool that can actually improve surgical outcomes for patients.

What this means for you:

A bone density ratio from CT scans was linked to a spinal surgery complication risk in a small study.

Study Details

Sample sizen = 126

EvidenceLevel 5

PublishedApr 2026

View Original Abstract ↓

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.

Low junctional Hounsfield unit ratio associated with proximal junctional kyphosis after spinal deformity surgeryBone density scan ratio may help predict spinal complication risk after surgery

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Low junctional Hounsfield unit ratio associated with proximal junctional kyphosis after spinal deformity surgeryBone density scan ratio may help predict spinal complication risk after surgery

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