Anatomical Study On Cervical Cortical Bone Trajectory And The Accuracy Of Drill Template For Cervical Cortical Bone Screw

BackgroundCervical pedicle screw(CPS)fixation is one of many methods of treatment of cervical diseases as an effective means.It is widely used in traumatic,degenerative,orthopedic,tumor,etc.Regarding the trajectory,the current PDTs usually use the center line(CL)of the pedicle as their ideal trajectory,where the screw is mainly engaged with cancellous bone in the pedicle and vertebral body.In this situation,it was reported that screw loosening might occur,leading to a loss of correction and nonunion,particularly in patients with poor bone quality.Therefore,the cortical bone trajectory(CBT)was advocated.Biomechanical studies further demonstrated that the CBT technique achieves a screw purchase and strength greater than the traditional CL trajectory.However,no study has been conducted using CBT in the cervical vertebrae.Generally,pedicle fixation is considered risky because it is proximal to vital structures such as the vertebral arteries,the spinal cord,and nerve roots.Therefore,accurate screw insertion is a key to successful clinical application of CPS.Patient-specific drill templates(PDTs)produced by 3-dimensional printing(3DP)were then introduced to assist with cervical transpedicular insertion with good effectiveness and accuracy.However,the materials used in 3DP are mainly photosensitive resin,which is not biocompatible and cannot be sterilized by high temperatures.A bio-safe metal PDT can manufactured by computer numerical control(CNC)and its high accuracy have been verified for posterior thoracic pedicle insertions.Therefore,the aims of this study were to explore the parameters of the cortical bone trajectory for cervical vertebra,and to evaluat the efficacies and accuracies of the patient-specific drill templates in cervical transpedicular insertion via CBT.Method1.Eighty sets adult healthy cervical specimens were randomly selected,and the scanned date were imported to the MIMICS 19.0 software.The measured parameters in C3～C7 vertebrae were outer pedicle width(OP W)of vertebral body,outer pedicle height(OPW)of vertebral body,medial cortical shell thickness(MCT)of the pedicle,and the cortical bone screw parameters of CPS and cervical anterior transpedicular screw fixation(ATPS).Then the statistical analysis were made.2.Preoperative CT images of 20 cadaveric cervical vertebras(C3-C7)were obtained.After image processing,20 patient-specific drill templates were randomly assigned to be constructed via 2 CBTs(CBTO or CBT0.7)and manufactured by 2 methods(computer numerical control or 3-dimensional printing).Then,their security and accuracy were evaluated in facilitating cervical pedicle screw insertion.3.Preoperative CT images of 20 cadaveric cervical vertebras(C3-C7)were obtained.After image processing,20 patient-specific drill templates were randomly assigned to be constructed via 2 CBTs(CBTO or CBTO.7)and manufactured by 2 methods(computer numerical control or 3-dimensional printing).Then,their security and accuracy were evaluated in facilitating cervical anterior transpedicular screw insertion.Result:1.There are significant difference of OPH and MCT between two sides of each segment(P<0.05).The OPH and OPW of each segment for men were grater than those for women(P<0.05).The parameters of the cortical bone trajectory for CPS was:the entry point for C3～C7 was 1-3.5 mm beside operated pedicle in the median sagittal plane and 1-7 mm from the upper end plate,the mean TPA from C3 to C7 were 39.13°,41.00°,40.91°,37.28°,and 31.84° respectively,the mean SPA from C3 to C7 were 90.85°,97.23°,108.97°,111.60°,and 104.83° respectively.The parameters of the cortical bone trajectory for ATPS was:the entry point for C3-C7 was below the vertebral articular process of upper vertebra and 1-3mm from the lateral mass,the mean TPA from C3 to C7 were 48.91°,50.23°,50.21°,46.78°,and 44.69° respectively,the mean SPA from C3 to C7 were 74.86°,76.94°,78.04°,74.58°,and 75.57° respectively.2.CBT for ATPS:Absolute deviations at the entry point and midpoint of the pedicle had no significant differences on the transverse and sagittal planes(P>0.05).However,3 screw positions were in grade 3 in CBT0,while the others were in grade 1.3.CBT for CPS:Absolute deviations at the entry point and midpoint of the pedicle had no significant differences on the transverse and sagittal planes(P>0.05).All the screw positions were safe and have no critical pedicle breachs.Conclusion1.The cortical bone trajectory is a theoretically feasible option for ATPS and CPS,and the parameters inserting screw are established.2.CBT0.7 appears to be a safe and feasible trajectory for cervical anterior transpedicular insertion.CBT0 appears to be a safe and feasible trajectory for cervical pedicle screw insertion.3.CNC-PDTs via cortical bone trajectory are viable in facilitating cervical anterior transpedicular insertion and cervical pedicle screw insertion with good feasibility and accuracy.It is a new alternative method to lower cervical pedicle insertion.