Three- instead of two-dimensional evaluation of key parameters alters the choice of the lowest instrumented vertebra in Lenke 1 and 2 AIS patients.

Purpose: Treatment of AIS, a three-dimensional spinal (3D) deformity, is guided by a two-dimensional (2D) evaluation. Novel 3D approaches that address the 2D limitations have not been adopted in AIS care due to their lengthy and complex 3D reconstruction procedures. This study aims to introduce a simple 3D method that translates the 2D key parameters (Stable vertebra (SV), Lenke lumbar modifier, Neutral vertebra (NV)) into 3D and to quantitively compare these 3D corrected parameters to the 2D assessment.

Methods: The key parameters of 79 surgically treated Lenke 1 and 2 patients were measured in 2D by two experienced spine surgeons. Next, these key parameters were measured in 3D by indicating relevant landmarks on biplanar radiographs and using the 'true' 3D CSVL which was perpendicular to the pelvic plane. Differences between the 2D and 3D analysis were examined.

Results: A 2D-3D mismatch was identified in 33/79 patients (41.8%) for at least one of the key parameters. More specifically, a 2D-3D mismatch was identified in 35.4% of patients for the Sag SV, 22.5% of patients for the SV and 17.7% of patients for the lumbar modifier. No differences in L4 tilt and NV rotation were found.

Conclusion: The findings highlight that a 3D evaluation alters the choice of the LIV in Lenke 1 and 2 AIS patients. Although, the true impact of this more precise 3D measurement on preventing poor radiographic outcome needs further investigation, the results are a first step toward establishing a basis for 3D assessments in daily practice.