www.nature.com/scientificreports OPEN The infuence of the rib cage on the static and dynamic stability responses of the scoliotic spine Shaowei Jia1,2, Liying Lin3, Hufei Yang2, Jie Fan2, Shunxin Zhang2 & Li Han3* The thoracic cage plays an important role in maintaining the stability of the thoracolumbar spine. In this study, the infuence of a rib cage on static and dynamic responses in normal and scoliotic spines was investigated. Four spinal fnite element (FE) models (T1–S), representing a normal spine with rib cage (N1), normal spine without rib cage (N2), a scoliotic spine with rib cage (S1) and a scoliotic spine without rib cage (S2), were established based on computed tomography (CT) images, and static, modal, and steady-state analyses were conducted. In S2, the Von Mises stress (VMS) was clearly decreased compared to S1 for four bending loadings. N2 and N1 showed a similar VMS to each other, and there was a signifcant increase in axial compression in N2 and S2 compared to N1 and S1, respectively. The U magnitude values of N2 and S2 were higher than in N1 and S1 for fve loadings, respectively. The resonant frequencies of N2 and S2 were lower than those in N1 and S1, respectively. In steady-state analysis, maximum amplitudes of vibration for N2 and S2 were signifcantly larger than N1 and S1, respectively. This study has revealed that the rib cage improves spinal stability in vibrating environments and contributes to stability in scoliotic spines under static and dynamic loadings. Scoliosis, a three-dimensional deformity, prevents healthy development. It results in body deformation, afects growth and development in adolescents, and quality of life in adults. When scoliosis patients are exposed to a moving environment, the asymmetry of the distribution of the load on each lateral convex segment may result in them experiencing higher stresses and strains, exacerbating deformation and leading to more serious damage 1,2. A number of studies have reported that subjects with scoliosis exhibit a higher risk of lower back pain (LBP) than healthy individuals3,4. Long-term whole-body-vibration (WBV) contributes to LBP and aggravates the deforma- tion already experienced in scoliosis patients5. Te stress and strain generated in a scoliotic spine subjected to a dynamic load are two to threefold greater than a static load of the same magnitude6,7. Many researchers have studied the structural characteristics, tendency to strain, and stress distribution of normal and scoliotic lumbar, and scoliotic thoracic spines without the rib cage when subjected to external static (or dynamic) loads8, but they have generally neglected the role of the rib cage (the cage structure comprising the ribs, sternum and costal cartilage) when investigating spinal stability. Tere are a few studies which have examined the mechanical impact of the rib cage on thoracic stability9,10. Watkins11 and Mannen12 used cadaveric specimens to study the efect of the rib cage on stability and stifness of a normal spine exposed to static loading. Because the dynamic characteristics of normal and scoliotic spines are critical for understanding the efects of WBV on spines and their stability, the responses of the rib cage to the stability of scoliotic and normal spines also need to be examined. Finite element analysis (FEA) is an efective method for quantifying the biomechanical characteristics of human organs, and the spine in particular. Te biomechanical characteristics of lumbar facet joints were studied using the FEA method 13. Since many researchers have established FE models to simulate spinal biomechanical characteristics and predict potential clinical risks, a spinal FE model was established to investigate the dynamic responses of scoliotic spines under axial cyclic loads 4. A previous study which investigated the vibration responses of three kinds of scoliotic lumbar spine using the FEA method 14 showed that vibration was strongly harmful to the lumbar spine, especially to scoliosis segments. Tese studies have indicated that FEA methods can be used to evaluate the biomechanical responses of the spine, including static and dynamic responses under diferent 1Key Laboratory for Biomechanics and Mechanobiology of Ministry of Education, Beijing Advanced Innovation Center for Biomedical Engineering, School of Biological Science and Medical Engineering, Beihang University, Beijing, China. 2School of Mechanical Engineering, Hebei University of Technology, Tianjin, China. 3School of Medical Imaging, Tianjin Medical University, Tianjin, China. *email: [email protected] SCIENTIFIC REPORTS | (2020) 10:16916 | https://doi.org/10.1038/s41598-020-73881-9 1 Vol.:(0123456789) www.nature.com/scientificreports/ loadings. However, vibration characteristics and the efects of the rib cage on stability are still not clear in either normal or scoliotic spines. Terefore, the aim of this study was to comprehensively study the vibration charac- teristics and the efects of the rib cage on the stability in normal and scoliotic spines under static and dynamic WBV loadings. In this study, spinal FE models, including a normal spine with rib cage (N1), a normal spine without rib cage (N2), a scoliotic spine with rib cage (S1) and a scoliotic spine without rib cage (S2) were established based on CT images. Static analysis of the four FE models was performed to observe the distribution of stress and strain on spines. Modal analysis of the four FE models was conducted, and the resonant frequencies and modes of vibration around 3–8 Hz were obtained. Steady-state analysis of the four FE models was performed to compare the diferences in dynamic WBV characteristics of the normal and scoliotic FE models with and without a rib cage, and to study the efects of the rib cage on the dynamic stability of the scoliotic spine. Tese results will help to raise awareness of the importance of the rib cage in stabilization in clinical practice, and will help scoliosis patients to understand their spine’s characteristics and prevent further deterioration. Materials and methods Data source. A typical Lenke 1BN scoliosis subject (male, 25 years old, 71.2 kg, 165 cm, BMI = 26.1 kg/cm2, Cobb angle: 67° for thoracic spine and 46° for lumbar spine; no other musculoskeletal diseases) and a healthy subject (male, 27 years old, 69.5 kg, 173 cm, BMI = 23.2 kg/cm2, no musculoskeletal diseases) were recruited for this study. Written informed consent was obtained from both participants and research was approved by the Ethics Commission of Tianjin Medical University with an ethical report. All methods used in this study were performed in accordance with the relevant guidelines and regulations in the Declaration of Helsinki. A 64-slice spiral CT (Siemens, Germany) was used to scan the scoliosis patient and the healthy volunteer from the upper portion of T1 to the end of the sacrum in Beijing Union Hospital. Te scanning parameters were as fol- lows: 120 kV, 211.20 mAs, 0.625 mm scanning thickness, 0.75 mm × 0.75 mm in-plane resolution and 512 × 512 matrix. A total of 867 images in DICOM format were obtained for each scan. Authorization was obtained to use this data. Establishment of spinal FE models. Tree-dimensional models of both parts of the vertebrae were estab- lished using Mimics 16.0 sofware (Materialize Inc., Belgium, https ://www.mater ialis e.com/en/medic al/mimic s-innov ation -suite /mimic s) based on CT images. Boolean calculation was performed to divide the smoothed model of the vertebrae into the posterior part and cortical bone with a 1 mm thickness. Tese intervertebral discs include an annulus, nucleus pulposus (which takes up one-third of the discs and is located at the posterior end), and upper and lower endplates which were 1 mm thick. A detailed disc is shown in Fig. 1. A thoracic structure was created with a sternum, ribs, costal cartilage, and rib joints (although limited by CT data sharpness, rib transverse joints were omitted). In this study, all the parts were meshed in Mimics, each edge of the element size was set to about 1 mm, and all vertebrae and discs were C3D4 elements. Te material properties were homogeneous and isotropic, as according to published literature14–17. Te types and material properties of the fnite element models are shown in Table 1. Te principal ligaments connecting the thoracic vertebrae include the supraspinous ligament (SSL) in the thoracolumbar segment, interspinous ligament (ISL), anterior longitudinal ligament (ALL), posterior longitudinal ligament (PLL), intertransverse ligament (ITL) and ligamentum favum (LF). In this study, linear tension springs were used to simulate ligaments. Te defnition of stifness (Spring stifness = Elastic modu- lus × Section-area/Mean length) and the material properties of the ligaments were based on accepted data in the published literature18. Te N1, N2, S1, and S2 FE models are shown in Fig. 1. Boundary and loading conditions. Tie constraints which prevented sliding displacement were estab- lished between the all adjacent parts in the FE models of spines with and without rib cages. In addition, all free- dom degrees of the two sides near the sacroiliac plane of the sacrum were set entirely constrained according to the anatomical characteristics of the spinal structure. Loadings on the FE models difered according to the con- ditions shown below. Te four FE spinal models were established using Abaqus 6.14 (Dassault SIMULIA Inc., France, https ://www.3ds.com/produ cts-servi ces/simul ia/produ cts/abaqu s/) fnite element analysis sofware, as shown in Fig. 1. For static analysis, given the complexity of human body movement during daily life and travel, spine move- ments were simplifed to postures such as lef and right lateral bending, fexion, extension, and axial compression. According to the mass of a head, a 100 N force was added from the posterior to the anterior on T1 for fexion, from the anterior to the posterior on T1 for extension, from right to the lef on T1 for lef lateral bending, from the lef to the right on T1 for right lateral bending, and applied from the top to the bottom on T1 for vertical compression10.