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 (ΔTK) 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 ΔTK was calculated as standing TK- supine TK. Prone ΔTK was calculated as standing TK - prone TK. Receiver-operating characteristic (ROC) curves were analyzed to determine the thresholds of supine ΔTK and prone ΔTK for PJK occurrence.

RESULTS:
PJK was observed in 64 of 144 (44%) patients 2 years postoperatively. Prone and supine ΔTKs were significantly larger in patients with PJK. A significant positive correlation between prone ΔTK and supine ΔTK 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 ΔTKs. The cutoff values of prone and supine ΔTKs for PJK risk were determined using ROC curve analysis.

CONCLUSION:
Because of their significantly high risk for PJK, in patients with ASD and prone ΔTK >11.5° and supine ΔTK >18.5°, the upper-thoracic spine should be considered for UIV.Level of Evidence: 3.





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