Is fusion to the stable sagittal vertebra necessary to avoid distal junctional kyphosis in thoracic adolescent idiopathic scoliosis?

Study design: This is a retrospective single-center study.

Purpose: The purpose is to investigate the incidence of distal junctional kyphosis (DJK) when fused proximal to the stable sagittal vertebra (SSV) in adolescent idiopathic scoliosis (AIS) patients undergoing selective thoracic fusion.

Methods: We retrospectively reviewed a consecutive cohort of surgically treated AIS patients with Lenke 1-2 A/B curves between 2011 and 2022 with a minimum of 2 years of follow-up. The SSV was defined as the vertebra bisected by the posterior sacral vertical line on long-standing sagittal radiographs. All patients underwent posterior pedicle screw instrumentation, and the decision of fusion level was at the surgeons' discretion. Distal junctional kyphosis was defined as ≥10° angulation between the lower instrumented vertebra (LIV) and the vertebra below the LIV (LIV + 1). Patients were stratified into Fusion proximal of SSV (Prox-SSV) and fusion including SSV (Incl-SSV). Multivariable backward regression was performed to identify predictors for DJK.

Results: A total of 196 patients were included, with 80 in the Prox-SSV group. The overall DJK rate was 3.6% (7/196), occurring in 6.3% (5/80) in the Prox-SSV group and 1.7% (2/116) in the Incl-SSV group, respectively (p = 0.125). Fusion proximal of SSV did not significantly increase DJK risk (Univariate OR 7.98, 95% CI 0.87-66.6; excluded in multivariable regression). Using SSV for LIV selection would extend the fusion by one level in 63.8%, two in 25.0%, and three in 11.2% of patients.

Conclusion: The overall risk of DJK is small in thoracic curves and fusion proximal to the SSV did not significantly increase the risk of DJK. Standardized use of SSV as LIV would result in a substantial extension of the fusion area with questionable benefits to the patients.