• Mun Keong Kwan, Sin Ying Lee, Sze Khiong Fam, Yee Wern Evonne Tan, Chun Hong Ngan, Saturveithan Chandirasegaran, Chee Kidd Chiu, and ChanChris Yin WeiCYWDepartment of Orthopaedic Surgery, Faculty of Medici... more ne, National Orthopaedic Centre of Excellence for Research and Learning (NOCERAL), Universiti Malaya, 50603, Kuala Lumpur, Malaysia.. less
• Department of Orthopaedic Surgery, Faculty of Medicine, National Orthopaedic Centre of Excellence for Research and Learning (NOCERAL), Universiti Malaya, 50603, Kuala Lumpur, Malaysia. munkeong42@hotmail.c... more om. less
• Eur Spine J. 2025 Feb 1; 34 (2): 610624610-624.

PurposeTo devise a mathematical model for estimating the intraoperative lowest instrumented vertebra (LIV) tilt angle using preoperative supine left side-bending (LSB) radiographs in adolescent idiopathic scoliosis (AIS) patients with Lenke type 1 and 2 (non-AR curves), and to review its clinical and radiological outcomes.MethodsThe mathematical model for the adjusted LSB LIV tilt angle (α) measured preoperatively, was expressed as the sum of preoperative LSB LIV tilt angle (x) and LIV displacement angle (y) (α = x + y). This model was validated through inter-rater and intra-rater analysis in Part I of the study. The α angle derived was applied to estimate the intraoperative LIV tilt angle. In part II of the study, clinical and radiological outcomes of 50 Lenke type 1 and 2 (non-AR curves) AIS patients operated using the α angle were reviewed. The difference between the intraoperative LIV tilt angle achieved (β) and the preoperative α angle was determined (∆LIV tilt angle = β-α).ResultsThe α angle had excellent inter-rater and intra-rater intraclass correlation coefficients (0.982; 0.907). 42 patients had positive ∆LIV tilt angles whereas 8 patients had negative ∆LIV tilt angles. The overall incidence of distal adding-on (AO) was 10.0% (n = 5/50). Patients with negative ∆LIV tilt angles had a higher incidence of distal AO (n = 4/8, 50.0%) than patients with positive ∆LIV tilt angles (n = 1/42, 2.4%) (p = 0.001).ConclusionAchieving an intraoperative LIV tilt angle (β) greater than or equal to the preoperative α angle derived (β ≥ α) may help avoid the distal AO phenomenon.© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.

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