The Finite Element Analysis Of Pedicle Screw Tunnel On The Strength Of The Vertebral Body

Objectives: To evaluate the impact of nail tunnel on the biomechanical stability of the vertebral bodies after pedicle screws removed by using finite element,and to provide suggestions for the choice of pedicle screw.Methods: Chosing 20 patients who had been done pedicle screw fixation in Orthopedic Surgery of the first affiliated hospital of nanchang university due to vertebral fractures,and pedicle screws were removed were included in the study.The CT values of cortical bone,cancellous bone and the wall of the nail tunnels of the vertebral bodies were measured,and were compared with each other.Select one case of normal adult volunteer,The lumbar vertebrae of the volunteer were scanned in thin layer by 64 slice spiral CT(0.75 mm per layer).The scanned data is imported into the Mimics 16.0 in DICOM format for dimensional reconstruction.Then Hypermesh13.0 software was used to mesh the cortical bone and cancellous bone of vertebral body.Get L1 meshed model(the model of cortical bone and cancellous bone is independent of two parts,not including the intervertebral disc and ligament tissue),Mimics 16.0 software was used to construct a unilateral,bilateral and different diameter(4.5mm,5.5mm,6.5mm)nail tunnels model according to the principle of pedicle screw insertion in the proper position of L1 vertebral body.Finally,Single normal vertebral body model,unilateral and bilateral nail tunnels model were established respectively.Different vertical pressure was forced on the model after constrainting boundary,and the distribution of force on different models were recorded.Results: The CT value of the wall of nail tunnel was 375.077±51.869,which was no statistically difference with the value of cortical bone(403.308±52.733)(P=0.149);The model of three dimensional tetrahedral mesh finite element of normal vertebra was established with good geometric similarity and realistic appearance.And the utility of it was verified.In each model,the cortical bone was the first one to reach the limit stress value.When the cortical bone reached the maximum stress,the ultimate loads forced on the bilateral nail tunnel vertebra,unilateral nail tunnel vertebra and normal vertebra were 3.97 Mpa,3.83 Mpa and 3.78 Mpa,respectively.However,when the cancellous bone reached the maximum stress,the force trend was contrary to it.When the wall of nail tunnel reached maximum value,the ultimate loads forced on the unilateral and bilateral nail tunnel model were 22.83 Mpa and 23.02Mpa;The stress forced on the unilateral and bilateral nail tunnel model were mainly located on the posterior part;With the increasing of nail tunnel diameter,the nail tunnel model could bear greater force.Conclusion: 1、the pedicle screw make the bone of the wall of nail tunnel obviously similar to the cortical bone.2、The anti-loading capacity of the withing nail tunnel vertebral body model is better than the normal vertebral body model.The anti-loading capacity of the bilateral nail tunnel model is better than the unilateral nail tunnel model,therefore,in the clinical setting bilateral pedicle screw as more as possible.3、With the increasing of the diameter of tunnel,the model could bear greater force.