Finite Element Analysis Of Occipitocervical Fixation By The Anterior Occiput-to-axis Locking Plate System

Occipitocervical instability,easy to cause the cervical cord compression,happens unrarely in the clinical.For this kind of patient,operation is often necessary to remove the compression and stabilize the occipitocervical structures.Due to complex anatomy and important functions,occipitocervical fixation procedures have always presented a challenge to spine surgeons.In the vast majority of cases,fixation and arthrodesis between the occiput and the axis is achieved through a posterior approach by means of applying autogenous bone graft and posterior wires,rod,hooks,plates,or screws.There exist unique clinical situations where,by virtue of congenital or iatrogenic absence of the bony elements of the upper cervical spine in which these common posterior fixation techniques may not be possible.Anterior occiput-to-axis screws fixtion technique have been described to achieve occipitocervical fixtion.In flexion and extension, this technique was not as effective as a posterior plate system.The screw fixtion require a complete vertebral axis,not apply to patients with fractures of axis.On this basis,we invented a anterior occiput-to-axis locking plate system.To analyze the biomechanical properties of this instrument by the method of 3-D finite element analysis and provide theoretical data for clinical application.Part one Development of the unstable occipital condyle-C2 model with finite element analysis Objective:To establish and validate a 3-D finite element model of the unstable occipital condyle(Co)-C2.Method:Sectional images of the C0-C2 were obtained from CT scans of a healthy male adult.The 3-D geometric model was constructed by the combination of software Mimics10.0 and Freeform5.0. The osseous finite element model was developed with Ansys10.0.The structure of ligament were established supplementarily based on the osseous model. The unstable model was established by deleting unit structure based on the validated normal model.Proper material properties and unit type for finite element mesh division.Results:The finite element model of the unstable C0-C2 approaches to real biomechanical model.The whole model consisted of 84690 nodes and 54841 elements.Conclusions:The finite element model of the unstable C0-C2 can simulate the biomechanical condition and facililate the further ralated study.Part two Development of finite element models of two anterior craniocervical fixation techniquesObjective:To establish and validate 3-D finite element models of two anterior craniocervical fixation techniques.Method:The finite element models of anterior occiput-to-axis locking plate system and anterior occiput-to-axis screws were developed with Mimics10.0 and Ansys10.0. two anterior craniocervical fixation devices were respectively performed on unstable C0-C2 model from test one. Proper material properties and unit type for finite element mesh division.Results:1. The finite element models of anterior occiput-to-axis locking plate system and anterior occiput-to-axis screws composed of 19071 nodes 9877 elements and 10168 nodes 5332 elements, respectively.2.The finite element models of anterior occiput-to-axis locking plate system fixtion and anterior occiput-to-axis screws fixtion approach to real biomechanical model. Both anterior craniocervical fixation models consisted of 103761 nodes 64718 elements and 94858 nodes 60173 elements, respectively.Conclusions:Both element models of anterior craniocervical fixation were built clearly and distinctly,with good geometric similarity.Moreover,there models could be used for further biomechanical research.Part three Three-dimensional finite element biomechanical analysis of the anterior occiput-to-axis locking plate systemObjective:To analyze the biomechanical properties of the anterior occiput-to-axis locking plate system by the method of 3-D finite element analysis and provide theoretical data for clinical application.Method:The inferior surface of two postoperative models from test two were fully constrained, physiological load were exerted on the base of skulls in flexion,extension,lateral bending and axial rotation,respectively.The VionMisees stress and displacement of the two kinds of internel fixation systems were evaluated and analyzed.Results:When anterior occiput-to-axis locking plate system was underwent the flexion load, the peak diaplacement was 1.373×10-4M, and the maximum stress value was 0.539×108 Pa.When this instrument was underwent the extension load, the peak diaplacement was 2.003×10-4M, and the maximum stress value was 1.055×108 Pa.At the lateral bending load, the peak diaplacement was 0.427×10-4M, and the maximum stress value was 0.121×108 Pa.At the axial rotation load, the peak diaplacement was 0.438×10-4M, and the maximum stress value was 0.310×108Pa. The peak displacement in transarticular screws of the anterior occiput-to-axis locking plate system were less than anterior occiput-to-axis screws under load of different directions. The level of decreased in flexion,extension,lateral bending and axial rotation about 15.5%,12.5%,14.4%and23.7%,respectively.When anterior occiput-to-axis screws was underwent the flexion load, the peak diaplacement was 1.625×10-4M, and the maximum stress value was 0.561×108 Pa. When this instrument was underwent the extension load, the peak diaplacement was 2.288×10-4M, and the maximum stress value was 1.086×108 Pa.At the lateral bending load, the peak diaplacement was 0.499×10-4M, and the maximum stress value was 0. 134×108 Pa.At the axial rotation load, the peak diaplacement was 0.574×104M, and the maximum stress value was 0.334×108 Pa. The largest stress force in transarticular screws of the anterior occiput-to-axis locking plate system were less than anterior occiput-to-axis screws under load of different directions.The level of decreased in flexion,extension,lateral bending and axial rotation about 3.9%, 2.9%,9.7%and7.2%,respectively.The peak stress of two instruments occurred at the screws through the atlantoaxial joint zone under load of different directions.Conclusions:The stress distribution and stability after the operation of the anterior occiput-to-axis locking plate system were better than anterior occiput-to-axis screws,which provides an alternative method of occipitocervical fixation.