Wedge-Shaped Deformity of the First Sacral Vertebra Associated with Adolescent Idiopathic Scoliosis: A Comparison of Cases with and without Scoliosis

Introduction: Scoliosis is the three-dimensional (3D) deformity of the spine. Scoliosis curvatures, such as the lower lumbar curve and the angle of the upper endplate of the sacrum observable on radiographs, are associated with postoperative outcomes; however, the relationship between postoperative outcomes and sacral morphology remains unknown. This study aimed to investigate sacral morphology in patients with adolescent idiopathic scoliosis (AIS) and to clarify its relationship with wedge-shaped deformity of the first sacral vertebra and radiographic parameters.Methods: This study included 94 patients who underwent fusion surgery for AIS (scoliosis group). As the control group, 25 patients without scoliosis (<10(degrees)) under 50 years of age were also investigated. S1 wedging angle (S1WA) using 3D Computed tomography (CT) and Cobb angle, L4 tilt, and sacral slanting using radiography were measured. The relationship between S1WA and other radiographic parameters was analyzed using correlation coefficients. Differences in sacral morphology between the Lenke lumbar modifier types A and C were also investigated.Results: S1WA was significantly larger in the scoliosis group than the control group (scoliosis: 1.7 degrees +/- 2.5 degrees, control: 0.1 degrees +/- 1.5 degrees, p = 0.002). Furthermore, the number of patients with S1WA >3 degrees or >5 degrees was significantly higher in the scoliosis group (>3 degrees: 33%, 8%, p = 0.012; >5 degrees: 16%, 0%, p = 0.039). S1WA correlated with sacral slanting (r = 0.45, p < 0.001) and L4 tilt (r = 0.35, p < 0.001) and was significantly greater with Lenke lumbar modifier C than A (2.4 degrees +/- 2.6 degrees, 0.8 degrees +/- 2.0 degrees; p < 0.001).Conclusions: The S1 vertebra was deformed and wedge-shaped in AIS, especially in cases with a large lumbar curve. Additionally, S1WA is associated with sacral slanting and L4 tilt on radiography in AIS.