Biomechanical Stabilities Of Atlas Posterior Screws Under Different Bone Density Via Finite Element Analysis

Objective1.To establish atlas three-dimensional model of normal bone mineral density and osteoporosis by the finite element analysis(FEA)method as a reliable and effective platform,for study of the biomechanical stability of atlas posterior screws.2.To explore the effect of changes in density on biomechanical stability of atlas posterior screws by compare stabilities of C1 pedicle screw(C1PS)and C1 lateral mass screws(C1LMS)using unicortical and bicortical fixation in atlas model.MethodTo reconstructed 3D point cloud model of cervical spine by a male volunteer’s C0-C3 CT data,with professional modeling software to build geometric model.And then we import data to Hypermesh 12.0 to set of model material properties according to the related literature reports parameters.The atlas models of normal BMD and osteoporosis were established.It was verified by comparing the finite element models with literature reports.In the atlas model of normal BMD and osteoporosis,assemble C1 PS and C1 LMS using unicortical and bicortical fixation,and then simulate the screw pullout of the dynamic process in Abaqus6.9 software,recording "bone-screw" interface stress and analysing biomechanical stability.Results1.The atlas finite element model of 61101 elements and 18900 nodes was established(Fig.1-2 and Fig.1-3).By comparing the atlas finite element model of ours with the model of literature reports to verify our model is reasonable.2.In normal BMD,pull-out force of C1 PS bicortical fixation is the best,C1 LMS unicortical fixation is the worst,a difference of about 83%.In Osteoporosis,pull-out force of C1 PS bicortical fixation is the best,C1 LMS unicortical fixation is the worst,a difference of about 199%.Under the condition of osteoporosis,the maximal pull-out force of C1 PS of bicortical and unicortical fixation were reduced by about 40% and 48%,while C1 LMS of unicortical and bicortical fixation were reduced by 63% and 64%.ConclusionDue to the strength of cortical and cancellous bone were decreased in osteoporosis,which has influence on the stability of atlas posterior screws,but the degree of decline is lower of C1 PS than C1 LMS.Because the C1 PS stress mainly concentrated in the posterior arch to the lateral mass,and the difference of pullout force between unicortical fixation and bicortical fixation is small,so the unicortical C1 PS is more reasonable for the patients with osteoporosis.