A Finite Element Analysis Of Subaxial Alignment Following Different Occipitocervical Fusion Angle And Segments

BackgroundOccipitocervical fusion and internal fixation is a classical surgical method for the treatment of craniocervical junction instability and malformation.The technology has been widely used in the treatment of occipitocervical instability which was caused by degeneration,trauma,congenital malformations and inflammation,etc.Occipitocervical fusion and internal fixation has been proved very effective in recovery and reconstruction of the stability of craniocervical junction region,improvement of the bone graft fusion rate,and maintaining reduction,but the subaxial accelerated degeneration and malalignment even subluxation following occipitocervical fusion and internal fixation are found in some patients.These complications which maybe result from the abnormal distribution of the biomechanical properties due to the occipitocervical fusion angle beyond the physiologic range and extensive segments fusion,can lead to neck pain,neurological dysfunction and poor living quality of patients.Some patients with severe complications even need reoperation.So the optimal occipitocervical fusion angle and fusion segments which play important roles in ensuring a well long-term prognosis should be taken into consideration to avoid the complications of the subaxial accelerated degeneration and malalignment.ObjectiveWe explore the ideal occipitocervical fusion angle and segments by comparative analysis of test data including the activities of the various segments of the lower cervical spine through the establishment of full three-dimensional finite element model of the cervical spine in different occipitocervical fusion angle and segments.Methods1.We constructed three-dimensional finite element model from the skull base including external occipital protuberance to the first thoracic vertebra.We compared the test data of the model activity in flexion,extension,lateral flexion,rotation condition with the reference,and verified the validity of the model by applying CT data in healthy volunteer.2.One kind of the cervical three-dimensional finite model which included the skull base was loaded on C2 bilateral pedicle screws with midline occipital plate internal fixation system,another was loaded on C2 bilateral pedicle screws and C3 bilateral lateral mass screws with midline occipital plate internal fixation system,and three kinds of cervical three-dimensional finite model with different occipitocervical fusion angle(flexion 10 °,normal flexion angle,extension 10 °)were created by changing the posterior occipital-cervical angle(POCA).The range of motion and von mises stress of the lower cervical spine were analyzed.Results1.The Three-dimensional finite element model of us contains 660876 elements and 208631 nodes.It has proved that our model is good for finite element analysis of cervical spine by compared with former references.2.Fixed to the same occipitocervical fusion segment,occipitocervical fusion angle which is flexion 10 ° or stretch 10 ° from the neutral position of craniovertebral junction can lead the lower cervical spine to increase its corresponding activity and bear undue stress load in order to compensate for the range of motion(ROM)of the overall cervical spine and maintain the sagittal balance of cervical sequence,finally,the abnormal curvature and accelerated degeneration of lower cervical spine will be caused.3.Fixed on the same occipitocervical fusion angle,the more segments of cervical spine is fixed,the less range of motion(ROM)of the lower cervical spine can reach.In order to compensate for the range of motion(ROM)of the overall cervical spine,the lower cervical spine need more torque to drive and bear undue stress load which can result in the accelerated degeneration of the lower cervical spine.ConclusionsOccipitocervical fusion angle beyond the normal physiological range and more occipitocervical fusion segments can cause the acceleration of the cervical vertebral degeneration and secondary abnormal curvature of the lower cervical spine.According to the results of experimental studies,we recommend to choose the short-segment fusion(fix to C2)and the optimal occipitocervical fusion angle should be close to the neutral position of craniovertebral junction when the posterior occipital-cervical angle(POCA)is about 110°.