| Objective:The optimal screw path for unilateral double iliac screw placement in combined spinal and pelvic fixation was designed by using computer-aided techniques such as three-dimensional CT reconstruction and reverse engineering,and verified by finite element analysis.To provide theoretical and technical support for individualized placement of unilateral double iliac screw in spine and pelvic fixation.Methods: A healthy adult volunteer's waist 2-pelvis was scanned by thin-slice CT.The imaging data were imported into mimics 16.0 software in Dicom format for three-dimensional reconstruction,and then the three-dimensional finite element model of the waist 2-pelvis was established in SOLIDWORK 2015 software.According to the orientation of iliac screw placement in previous literatures,the reference axis is determined,and the iliac part of the spine-pelvis model is divided into several layers longitudinally by automatic segmentation and manual intervention.The geometric boundary of the iliac safe zone is determined on each layer of the sectional plane,and the two-dimensional image coordinates of the geometric boundary points are extracted,and the maximum circle that can be accommodated within the safe boundary is fitted.Record the coordinates of these centers,and then use the least square method to fit the space straight line closest to these centers.The straight line is regarded as the central axis of the optimal iliac screw path in geometry,along which the geometric screw path of the iliac screw can be generated.The geometric screw path of the second iliac screw can be generated by the same method,and the position,maximum diameter and length of the two screw paths can be confirmed.The "free screw path" was created by means of duplication,translation and rotation tools to simulate the traditional unilateral double iliac screw placement in clinical operation.According to the size and mechanical parameters of the implant,the model of the implant was drawn in Solidworks 2015 software,and the model of iliolumbar joint fixation was assembled according to the designed nail track.According to the previous finite element analysis,the structures of the implant,the cortical bone,cancellous bone,intervertebral disc,fibrous ring,nucleus pulposus,ligament and articular cartilage of the lumbar spine and pelvis were assigned and meshed.The displacement,stress and screw stress of the spine-pelvis model were observed under the conditions of flexion,extension,lateral flexion and rotation,respectively.The feasibility of using geometric screw path to insert double iliac screw was verified,and the optimal screw path for single-sided double iliac screw placement was compared and determined.Results: A three-dimensional finite element model of lumbar 2-pelvis in normal adult males was successfully established.Based on the model,the geometric screw path and free screw path models of unilateral double iliac screw in spine-pelvis fixation were established by using the geometric screw path method.The finite element analysis and verification of the two screw path models were carried out.Among them,"geometric nail path" model has nodes: 709411,tetrahedral element: 41 6673,and "free nail path" model has nodes: 72 5525,tetrahedral element: 41 6793.Conclusion: Geometric screw path technique can be successfully applied in the design and establishment of the screw path of unilateral double iliac screw.The established "geometric screw path" iliac screw has a more accurate insertion path and a longer insertion depth than the traditional iliac screw.Under the conditions of extension and rotation,the "geometric screw path" iliac screw has higher fatigue resistance,rotation resistance and overall stability than the traditional iliac screw,and the stress distribution between the unilateral iliac screw and the bilateral iliac screw is more balanced.It is a good way to insert the unilateral iliac screw. |
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