Preoperative Hounsfield Units at the Planned Upper Instrumented Vertebrae May Predict Proximal Junctional Kyphosis in Adult Spinal Deformity

In Brief Study Design. Retrospective cohort study. Objective. To investigate the association between Hounsfield units (HU) measured at the planned upper instrumented vertebra (UIV) and UIV+1 and proximal junctional kyphosis (PJK) in patients with adult spinal deformity (ASD). Summary of Background Data. PJK is a common complication following surgery for ASD and poor bone quality is noted to be one of the risk factors. HUs from standard computed tomography (CT) scans can be used for evaluating regional bone quality. Methods. Sixty-three patients were included from a single institution. The demographic characteristics and radiographic parameters were recorded. Local vertebral HUs at the planned UIV and UIV+ 1 were measured using preoperative CT scans. The patients were divided into three groups: no PJK, non-bony PJK, and bony PJK. The risk factors between the three groups and the correlation between the mean HU and increase in the PJK angle were analyzed. Results. The incidence of PJK was 36.5%. The mean HU was significantly lower in the bony PJK group (HU: 109.0) than in the no PJK group (HU: 168.7, P = 0.038), and the mean HU in the non-bony PJK group (HU: 141.7) was not different compared to the other two groups. There was a significant negative correlation between the mean HU values and the increase in the PJK angles (r = −0.475, P < 0.01). The cutoff value for the mean HU used to predict bony PJK was 120 and a HU value less than 120 was a significant risk factor for bony PJK (OR: 5.74, 95% CI [1.01–32.54], P = 0.04). Conclusions. We noted a significant inverse relationship between the mean HUs at the UIV and UIV+ 1 and increase in the PJK angles postoperatively. In ASD patients, the HUs may be used preoperatively to identify patients with a higher risk of bony PJK. Level of Evidence: 3 Lower Hounsfield unit (HU) values at the planned upper instrumented vertebrae correlated with higher incidence of proximal junctional kyphosis (PJK). A mean HU of less than 120 was a risk factor for postoperative bony PJK. In addition, an increase in HU values correlated with a decrease in the PJK angles.