413 Selection of Upper Instrumented Vertebra in Adult Spinal Deformity: Risk Calculator and Recommendations Based on Proximal Junctional Kyphosis

INTRODUCTION: The surgical correction of adult spinal deformity(ASD) presents a complex and multifaceted challenge, further intensified by the need for revision surgery. Determination of the upper instrumented vertebra can often be challenging. METHODS: We included operative ASD patients with a minimum of a 2-year follow-up undergoing fusion from at least L1 and proximal to the sacrum. Patients without PJK were isolated to determine predictive thresholds based on patient and surgical factors. Variable importance was determined utilizing random forest analysis to determine the weighting of variables with multivariable logistic regression. RESULTS: 334 patients met inclusion. (Age 63±10, 77% F, BMI 27.6±5.1 kg/m2, frailty 3.5±1.5, CCI 1.9±1.7). The model for predicting PJK was significant for osteoporosis, LL, TK, TLPA, with posterior UIV and IBD UIV (p<.05). Baseline UIV slope of >42.4 had a higher rate of PJK postoperatively (63% vs 27%, p<.001). Evaluating factor importance for the selection of UIV determined UIV slope to have the greatest weight, with T1PA, PJK prophylaxis, PI-LL, frailty, osteoporosis, and CCI following in those who didn’t have PJK. For those with UIV slope <12.7, selection of upper thoracic UIV was contingent on T1PA being <7 (p=0.018). Patients with UIV slope >27 and T1PA >30 were likely to have UIV in the upper thoracic (T4 mean) in those who didn’t develop PJK. Whereas, those with a UIV slope between 12.7 to 30 with T1PA >30 were less likely to develop PJK with a lower thoracic UIV (p<.001). CONCLUSIONS: The selection of UIV was strongly correlated to UIV slope and T1PA for avoidance of proximal junctional kyphosis. Frailty and lumbar lordosis were important contributors to the model for the selection of optimal UIV.