Additional Risk Factors for Adding-On After Selective Thoracic Fusion in Adolescent Idiopathic Scoliosis: Implication of Lowest Instrumented Vertebra Angle and Lumbosacral Takeoff

Study Design Retrospective chart and radiographic review. Objective The purpose of this study was to identify any additional risk factors adding-on after selective thoracic fusion in adolescent idiopathic scoliosis (AIS). Summary of Background Data Previous studies have shown that the proper selection of lowest instrumented vertebra (LIV) is critical in preventing adding-on (AD) phenomenon after selective thoracic fusion (STF). However, even after selecting the correct LIV, AD can still occur. Methods This is a retrospective review of 380 consecutive AIS patients who underwent corrective surgery by a single surgeon. LIV was chosen as stable vertebra (SV) or SV–1 if intersected by the center sacral vertical line (CSVL). One hundred thirty-five patients were identified who had complete radiographic images stored in PACS. After Lenke 5, anterior or combined surgeries, TL fusion, and hybrid construct were excluded, two groups were identified out of 78 patients who received STF with all pedicle screw construct: Adding-on (AD) and Control. AD was defined as > 5° angle of the superior endplate of LIV–1 to the inferior endplate of LIV+1 between postoperation and the final follow-up. We analyzed the values for the AD group and compared them with those for the control group to identify risk factors not previously defined. Results Patients with other complication and follow-up less than 2 years were excluded, and there remained 11 patients in AD (14%), and 8 in the control group. There was no significant difference between the 2 groups in terms of demographic data, LIV selection, and curve types. In AD, there was significant curve progression within fused segments. Even though lumbosacral takeoff (LSTO) was not significantly different, (LIV-H)+LSTO and LIV-H (LIV horizontal) showed significant difference. Conclusion After excluding the effect of known risk factors for AD, LIV-H angle, and LIV-H mismatch with LSTO were found to be the additional risk factors for AD. In addition to consideration of other risk factors such as LIV selection or Risser stage, these risk factors should be considered.