Impact of Surgical Upper Lumbar Changes on Unfused Lower Lumbar Segments in Adolescent Idiopathic Scoliosis

Study Design. Retrospective review. Objective. To determine the impact of upper lumbar lordosis changes in the fused segment on compensatory kyphotic or lordotic changes in the unfused lower lumbar spine in patients with adolescent idiopathic scoliosis (AIS). Summary of Background Data. While the distribution of lordosis and interplay between fused/unfused segments has been studied in adults, less is known about this in AIS. We hypothesize that increased FSLL can result in compensatory kyphosis of the unfused distal segments. Materials and Methods. A retrospective review of Lenke 1/2 patients who underwent posterior spinal fusion (PSF) to L1, L2, or L3 with a minimum follow-up of 2 years. Coronal Cobb angles, thoracic kyphosis, lumbar lordosis, and spino-pelvic parameters (T4PA, L1PA, PT, SS, PI, PI-LL, SVA) were measured. Custom MATLAB scripts were used for 3D segmental lordosis calculations. Statistical analysis, including linear regression analyses and interaction models, assessed the relationship between fused segment lumbar lordosis (FSLL), LIV, and thoracic kyphosis (TK) on lower lumbar compensatory alignment. Results. A total of 158 patients met the inclusion criteria. Changes in FSLL affected segmental lordosis of unfused segments, including loss of distal lordosis. In the L1 LIV group, increased FSLL increased L1–L2 lordosis ( B =0.35, P =0.003). In LIV L2, increased FSLL increased L3–4 lordosis ( B =0.2, P =0.001) and decreased L4–L5 lordosis ( B =−0.23, P =0.012). For LIV L3, increased FSLL caused a reduction in lordosis of L4–5 ( B =−0.14, P =0.026) and L5–S1 ( B =−0.14, P =0.034). Changes in TK also had varying impacts on the unfused segments. The interaction model with LIV levels reveals that the compensation strategy can vary depending on specific fusion levels, although not significant. Overall sagittal alignment was maintained, and PI-LL remained <10°. Preoperative and postoperative T4–L1PA had minimal difference to each other indicating maintained sagittal harmony. Conclusions. In this observational study of segmental changes in lumbar lordosis in AIS, postoperative changes in the fused segments can result in iatrogenic changes in the unfused lower segments to maintain spinal balance. Understanding normal segmental lumbar lordosis distribution is critical in surgical planning (ie, rod contouring) and understanding the health of the unfused segments in the long term.