Virtual Modeling of Postoperative Alignment Following Adult Spinal Deformity (ASD) Surgery Helps Predict associations between Compensatory Spinopelvic Alignment Changes, Overcorrection and Proximal Junctional Kyphosis (PJK)

STUDY DESIGN.: Retrospective review of a prospective multicenter database. OBJECTIVE.: To develop a method to analyze sagittal alignment, free of PJKʼs influence, and then compare PJK to non-PJK patients using this method. SUMMARY OF BACKGROUND DATA.: Proximal Junctional Kyphosis (PJK) following Adult Spinal Deformity (ASD) surgery remains problematic as it alters sagittal alignment. This study proposes a novel virtual modeling technique that attempts to eliminate the confounding effects of PJK on postoperative spinal alignment. METHODS.: A virtual spinal modeling technique was developed on a retrospective ASD cohort of patients with multilevel spinal fusions to the pelvis with at least 2 year post-operative follow-up. The virtual post-op alignment (VIRTUAL) was created from the post-op alignment of the instrumented segments and the pre-op alignment of the unfused segments. VIRTUAL was validated by comparisons to actual 2-year post-op alignment (REAL) in NOPJK patients. Patients were then divided into two groups: PJK and NOPJK based on the presence/absence of PJK at 2 years post-op. PJK and NOPJK patients were compared using VIRTUAL and REAL. RESULTS.: 458 patients (78F, mean 57.9y) were analyzed. The validation of VIRTUAL versus REAL demonstrated correlation coefficients above 0.7 for all measures except SVA (r?=?0.604). At 2-years, REAL alignment in PJK patients demonstrated a smaller PI-LL and a larger thoracic kyphosis than NOPJK patients, but similar SVA, TPA, and PT. An analysis of VIRTUAL demonstrated that PJK patients had a smaller PI-LL, PT, SVA, and TPA than NOPJK patients (p?<?0.05). CONCLUSION.: This technique demonstrated strong correlations with actual postoperative alignment. Comparisons between REAL and VIRTUAL alignments revealed that postoperative PJK may develop partially as a compensatory mechanism to the over-correction of sagittal deformities. Future research will evaluate the appropriate thresholds for deformity correction according to age and ASD severity.Level of Evidence: 3