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.