Novel Placement Of Cortical Bone Trajectory Screws In Lumbar Spine:Biomechanics Study By FEM

Study Design A biomechanics study by FEMObjective: A three-dimensional finite element model?3D-FEM?of the lumbar spine L₄-L₅,which is fixed with CBT and TT screw fixation respectively,was established.With that the fixed strength of a single screw and the biomechanics characteristic of the motor unit L₄-L₅ were performed.Materials and Method: Geometry information and the HU value of the screw trajectory was obtained through the CT scan to the fixed lumbar spine L₄-L₅ of a male with osteoporosis.With the information and some accessional software,the 3D-FEM model of the L₄-L₅ motor unit was built using Abaqus software.Several biomechanics index were investigated including the pullout of strength and the fixation stiffness of single screw,and the stress,strain and the free degree of the motor unit,respectively under the CBT and TT conditions.Besides,during the research the influences of cage were also taken into account.Results:?1?The FEM model and the actual model have high geometric similari ty.With the applying of normal pressure and torque,the motion state of flexio n,extension,left & right bending,and left&right rotation can be simulated.?2?Whether or not the cage was placed,the PS and fixation stiffness of the CBT screw are always more large than those of TT screw in the six motor condition s.In the right bending conditions the value of PS is smallest,which is identified as 65 N?no cage?and 144 N?with cage?.In the flexing condition the value of PS is largest,which is identified 195N?no cage?and 227N?with cage?.In the flexing and extending conditions the fixation stiffness are relatively small an d in other motor states the fixation stiffness is relatively large.The PS and fixation stiffness in a cage case is always larger than those of a non-cage case.?3?The maximum stress zone of the motor unit is located on the lateral combinatio n and rob,while the maximum stress zone of the spine is located on the interface between the upper,lower endplate and the intervertebral disc.The characteri stic of the distribution is affected by the motion state.With no cage,basically the maximum stress of the motor unit with CBT screws is larger than that with TT screws,the maximum difference is 75 k Pa.With a cage,the maximum stress of the motor unit with CBT screws is smaller than that with TT screws,the maximum difference is 130 k Pa.The cage can decrease the maximum stress of the motor unit in the CBT condition.?4?The maximum strain of the motor unit is located on the lumbar spine,and the distribution is associated with that of stress.The value of strain in CBT conditions is lower than that in TT case.And the introducing of cage can also reduce the maximum strain of the motor unit.?5?The free degree of the CBT case is lower than TT,the cage can also reduce the free degree of the motor unit.Conclusions: The three-dimension of the lumber spine FEM model,which is built based on the information of the CT scan,is capable for the study of the biological mechanics.The placement of the cage and the change of the motor state make the PS and fixation stiffness of CBT screw vary.The maximum stress of the motion unit is always located on the lateral connection and the rob in all motion state.The maximum stress and strain are always on the interface between the upper,lower endplate and the intervertebral disc.The application of CBT won't change the distribution characteristic of strain and stress of TT.Besides CBT has a positive meaning for controlling the displacement and deformatio n of the motor unit and make the motor unit more stable.Thus the clinical performance of the CBT is better.Similarly,the introduction of cage can reduce the maximum strain and motor of range,and meanwhile the effect of the cage to the stress is related with the type of screws.