Finite Element Modeling Of The Whole Cervical Spine:Influence Of Various Postures Of Cervical Spine On Biomechanics Of The Spine

Objective:The purpose of this study is to develop a detailed realistic three-dimensional, anatomically accurate subject specific finite element model of the whole human cervical spine (C1-C7) using actual data from a healthy volunteer with a set of consecutive CT/MRI images, mimics and Hypermesh software. We then analyze three simulated pre trauma neck postures:neutral, hyperflexion, hyperextension followed by validation with respect to subject’s own capability of hyperflexion and hyperextension. Then we calculate the maximum shear stress distribution in the spinal components under three simulated posture:normal lordotic, hyperflexed and hyperextended posture. Based on this, we want to fully understand whether the abnormal neck posture (hyperflexion/hyperextension) subjects to greater risk of injury or degeneration leading to disc buldge or disc protrusion compared with the normal lordotic posture. This in turn provides theoretical guidance for prevention, clinical diagnosis and treatment for both cervical injury and disease.Methods:A21years old male volunteer with no history of cervical spine injury or disease was selected as normal subject. High resolution,64spirals, CT images of a normal volunteer’s whole cervical spine from C1to C7was scanned in normal lordotic posture along with hyperextended and hyperflexed position using Philips computerized tomography in cross sectional, sagittal and coronal planes. MRI images of the same volunteer’s cervical spine in lordotic posture were obtained using Siemens1.5tesla high field superconducting magnetic resonance in cross-sectional sagittal and coronal planes with T1WI, T2WI images.These CT and MRI image data were saved in sequential DICOM format which are then loaded into Mimics software to create three dimensional (3D) reconstruction model of the cervical spine. The generated3D geometrical surface of the cervical spine was imported into a universal graphics data format (IGES document).Finally with the help of commercially available Hypermesh software3D isotropic hyperelastic nonlinear Finite Element Model (FEM) of intact human C1-C7motion segment was established. The whole cervical spine was then subjected to validation with respect to subject’s capability of hyperflexion and hyperextension fixing the C7vertebra in all6degrees of freedom at the inferior endplate, inferior facets, and inferior part of the spinous process. Finally internal stresses were analyzed and compared with three simulated posture of the validated modelResults:Results show that no stresses are documented in normal lordotic posture. In hyperextended posture of the spine, high stresses concentrations are around lower cervical column, mainly localized in C6-7intervertebral disc. These areas show shear stress that ranges gradually from1.270e-04GPa to the maximum value of1.270e-03GPa with maximum shear stresses in right posterior part of the annulus fiber of5.312e-04GPa. Similarly in the hyperflexed position high concentration of stress are observed in atlanto axial joint mainly localized in Cl vertebra along the anterior arch with the maximum stress of2.507e-04GPa. Result also show that overall stress is greater in extension than flexion and amount of stress is higher in C6-7intervertebral disc in comparison to C1-C2joint.Conclusion:Based on medical MRI/CT images and using commercially available Mimics and Hypermesh software three-dimensional geometrically accurate and materially nonlinear FE model of the whole cervical spine has been successively developed. The generated FE model was then well validated and was successfully used to study the effects of three postures of neck i.e. lordotic, hyperflexion and hyperextension on internal biomechanics of cervical spine. We confirmed that no stress occurs in normal lordotic posture while maximum shear stress is localized in atlanto-axial joint, moreover to anterior arch of Cl vertebra in hyperflexed position and in hyperextended posture maximum shear stress is in C6-7intervertebral disc. Because of the effect of posterior supporting structure of neck including posterior longitudinal ligaments degree of extension is slightly low than flexion, so we have also seen that stress in hyperextension is more than hyperflexion.Thus result shows that the extensive ranges of movement (ROM) in the cervical spine are possible with the articulations between the cervical vertebrae but at the cost of stability. Our research for the first time thus demonstrates the influence of cervical spine curvature on biomechanics of cervical spine. Due to its reproducibility and repeatability characteristic, our model is reasonable and suitable for understanding the clinical biomechanics of the cervical spine both in diseases and injury which further aids in prevention, diagnosis and treatment of both cervical injury and disease including various surgical interventions.