Biomechanical Modeling of Spine Flexibility and Its Relationship to Spinal Range of Motion and Idiopathic Scoliosis

Study design Cross-sectional. Objective To examine the relationships between spine morphology, spine flexibility, and idiopathic scoliosis. Background Girls have a higher incidence of clinically significant scoliosis than boys, along with smaller vertebrae and greater flexibility. Based on biomechanical modeling, we hypothesized that smaller vertebral width relative to intervertebral disc (IVD) height would be associated with both greater lateral flexibility of the spine and with idiopathic scoliosis. Methods Magnetic resonance imaging was used to measure IVD height, vertebral width, and paraspinous musculature in 22 girls with mild and moderate idiopathic scoliosis and 29 girls without scoliosis ages 9–13 years. Clinical measurement of maximum lateral bending was also performed in the girls without scoliosis. A simple biomechanical model was used to estimate bending angle from the ratio of IVD height to vertebral half-width for L1–L4. The average ratio (Ravg) and calculated total bending angle (αtot) for L1–L4 were compared to the clinical measurements of lateral bending flexibility in the control group. These measures were also compared between the scoliosis and control groups. Results There was a significant positive relationship between clinical flexibility and both Ravg (p =.041) and αtot (p =.042) adjusting for skeletal age, height, body mass index, and paraspinous muscle area as covariates. The ratio was significantly higher (Ravg = 0.45 vs. 0.38, p <.0001) and the bending angle was significantly greater (αtot = 107° vs. 89°, p <.0001) for girls with scoliosis compared with controls. Conclusion These results suggest that differences in spine morphology and corresponding changes in spine flexibility may be related to idiopathic scoliosis. If these relationships can be corroborated in larger prospective studies, these easily measured morphologic traits may contribute to a better understanding of the etiology of idiopathic scoliosis and an improved ability to predict scoliosis progression. Level of Evidence Level III. © 2017 Scoliosis Research Society