Preoperative Radiographic Evaluation of Thoracic Flexibility and Compensation for Adult Spinal Deformity Surgery. How to Select Optimal Upper Instrumented Vertebra to Prevent Proximal Junctional Kyphosis

Study Design. Retrospective study of a cohort of consecutive patients. Objective. The aim of this study was to clarify the usefulness and value of the difference in thoracic kyphosis (Delta TK) angle in various positions by imaging the patient standing, prone, and supine to evaluate TK flexibility and compensation, and to establish optimal radiography to determine an appropriate thoracic level for upper instrumented vertebra (UIV) because a strategy to select an appropriate level to avoid proximal junctional kyphosis (PJK) remains elusive. Summary of Background Data. Postoperative reciprocal progression of TK is a significant risk factor for PJK. However, how to predict and prevent postoperative reciprocal progression of TK remains unclear. We hypothesized that preoperative evaluation of both TK flexibility and compensation is essential to predict PJK and determine the UIV level. Methods. We included 144 consecutive patients with adult spinal deformity (ASD), >= 2 years' follow-up, and UIV Th9-11 in this retrospective cohort study. TK was measured from images with patients standing, prone, and supine. Supine Delta TK was calculated as standing TK- supine TK. Prone Delta TK was calculated as standing TK - prone TK. Receiver-operating characteristic (ROC) curves were analyzed to determine the thresholds of supine Delta TK and prone Delta TK for PJK occurrence. Results. PJK was observed in 64 of 144 (44%) patients 2 years postoperatively. Prone and supine Delta TKs were significantly larger in patients with PJK. A significant positive correlation between prone Delta TK and supine Delta TK was observed. When data from patients with and without PJK were plotted separately, a significantly higher proportion of patients with PJK had large prone and supine Delta TKs. The cutoff values of prone and supine Delta TKs for PJK risk were determined using ROC curve analysis. Conclusion. Because of their significantly high risk for PJK, in patients with ASD and prone Delta TK >11.5 degrees and supine Delta TK >18.5 degrees, the upper-thoracic spine should be considered for UIV.