Comparison of Typical Thoracic Curves and Atypical Thoracic Curves Within the Lenke 1 Classification

Study DesignRetrospective study.ObjectivesTo examine the characteristics of Lenke type 1 curves based on the level of the apical vertebra.Summary of Background DataThe Lenke classification is the most used system for adolescent idiopathic scoliosis, with approximately 50% of the curves falling into the Lenke 1 curve type category.MethodsA total of 611 Lenke 1 curves in a prospectively collected multicenter adolescent idiopathic scoliosis study were analyzed. Minimum follow-up was ≥ 2 years. Curves were subdivided into 3 groups according to their apex: the typical Lenke 1 curve group included apices from T7/8 to T10 (511 patients), the proximal group included apices from T4 to T7 (45 patients), and the distal group included apices from T10/11 to T11/12 (50 patients). Preoperative and postoperative radiographic and clinical outcomes were compared among the 3 groups.ResultsThe proximal and distal groups included significantly more left thoracic curves (proximal: 29%; typical: 1.8%; distal: 19%; p < .01). Flexibility of the main thoracic curve was significantly different among the 3 groups (proximal: 32% ± 17%; typical: 46% ± 18%; distal: 57% ± 18%; p < .001). The distal group included significantly more “A” lumbar modifiers (proximal: 29%; typical: 53%; distal: 96%; p < .01) and had curves characteristics similar to King type 4 curves (L4 tilted to the right: Lenke 1AR). The average lowest instrumented vertebra was significantly lower in the distal group (proximal: T12; typical: L1; distal: L2; p < .01). The proximal group had significantly greater thoracic kyphosis (proximal: 30° ± 18°; typical: 20° ± 13°, distal: 20° ± 10°; p < .001) and more fusion segments (proximal: 10; typical: 9; distal: 9; p < .03).ConclusionsCurves categorized as Lenke 1 curves were less homogeneous than expected. Using only the Lenke type 1 designation to define a study population may introduce unintended bias to the study design.