Numerical Simulation of the Cervical Spine in a Normal Subject and a Patient with Intervertebral Cage under Various Loadings and in Various Positions

Background: Cervical spine sustains most of thevertebral column injuries, among other injuries, the disc degeneration and damage that lead to replacement of the damaged disc with cage or artificial disc.Methods: The C4 to C6 vertebrae of a normal subject and a person with interbody fusion cage were 3d modelled and then analyzed using Finite element method. The results of maximum stress and strain in cervical spine of the normal subject and patient were compared in three positions: standing, lying with axial rotation of neck and standing with axial rotation of neck.Results: The maximum principal strain and stress in the patient are respectively 10.5% and 14.5% greater than those in normal subject in standing position, howeverin lying position when the head has axial rotation, the maximum principal strain and stress are in the normal subject 6.2% and 16.3% greater than those in patient, respectively. The difference between these results and the results of strain and stress in standing position when the head has axial rotation is very small. This outcome is due to smallness of the stress exerted on cervical spine as a result of the head weight (131-150 Pa).Conclusion: In contrary to the constraint between disc and vertebrae, there is no friction between cage and vertebrae and this leads to maximum stress transfer to the first vertebra above the cage in patient. However, the maximum stress is ultimately less in the patient with fusion cage than the normal subject. Generally, only the neck rotations are the cause of cervical spine injury in normal neck movements

Biomechanical Assessment of Cervical Spine with Artificial Disc during Axial Rotation, Flexion and Extension

Background: The cervical spine is the most vulnerable part of the vertebral column and the rotational movements are the most dangerous movements which may cause damages to cervical spine. A good treatment option for the cervical disc disease is the replacement of a damaged disc with an artificial disc that has shown satisfactory clinical results.Methods: The C4 to C6 vertebrae of a normal subject and a person with an artificial disc between the vertebrae C5 and C6 were 3d modelled and then analyzed using FEM. The results of stress and deformationin both subjects were calculated and compared for three rotational head movements: axial rotation, flexion and extension. A distributed load of 73.6 N was used to simulate the head weight and a moment of 1.8 N.m was used to create all three rotational movements.Results: The maximum Von Mises stress in the normal subject during the axial rotation was respectively 2.2 and 1.8 times greater than the maximum stress during flexion and extension. These numbers were 2.6 and 2.3 in the subject with artificial disc.Following the artificial disc replacement, the cervical spine strength against the extension improved about 2.7%, however, the strength in axial rotation and flexion decreased 6.9% and 24.3%, respectively. The maximum values of deformation in the normal subject during flexion, extension and axial rotation were 2.8, 2.8 and 2 times of the values in the subject with artificial disc during the similar movements.Conclusion: The flexion and extension involve risks of hurting the cervical spine, however, the axial rotation is much more dangerous regarding the damages it may cause especially to the C5/6 intervertebral disc. Numerically, there is a much greater possibility of cervical spine injury during axial rotation