Optimal Lowest Instrumented Vertebra for Thoracic Adolescent Idiopathic Scoliosis

Study Design: Retrospective cohort chart review. Objective: To determine the optimal lowest instrumented vertebra (LIV) following posterior segmental spinal instrumented fusion (PSSIF) of thoracic adolescent idiopathic scoliosis (AIS) with LIV at L2 or above. Summary of Background Data: Few studies evaluate the optimal LIV based on rotation or center sacral vertical line (CSVL). Methods: A radiographic assessment of 544 thoracic major AIS patients (average age 14.7 years) with minimum 2 years’ follow-up (average 4.1 years) after PSSIF was performed. The LIV was divided by CSVL: stable vertebra 1 (SV-1) if the CSVL fell between the medial walls of the LIV pedicles; SV-2 if between stable vertebra 1 and 3; and SV-3 if the CSVL did not touch the LIV. LIV was divided by rotation into: neutral vertebra 0 (NV-0) if the LIV was at or distal to the neutral vertebra; NV-1 if one vertebra proximal to the NV; NV-2 if two vertebrae proximal; and NV-3 if three vertebrae proximal to the NV. Results: The prevalence of adding-on (AO) or distal junctional kyphosis (DJK) at ultimate follow-up was 13.6%. Patients with AO or DJK had a higher rate of open triradiate cartilage, LIV not touching the CSVL, and more proximal to the NV (p <.05). Risk factors were SV-3 (39% vs. SV-2 14%, SV-1 9%, p <.05), NV-3 (35% vs. NV-2 9%, NV-1 6%, NV-0 12%, p =.000), open triradiate cartilage (43% vs. closed 13%, p <.05), lumbar C modifier (22% vs. B modifier 8%, A modifier 13%, p <.05), and Risser stage 0 (19% vs. 12% Risser 1-5, p <.05). Conclusion: The prevalence of AO or DJK at ultimate follow-up of PSSIF for AIS with LIV at L2 or above was 13.6%. Risk factors included the CSVL outside the LIV, LIV 3 or more proximal to the NV, open triradiate cartilage, lumbar C modifier, and Risser stage 0. Level of Evidence: Level IV.