Investigation Of Segment-specific Stability Of Traumatic Subaxial Cervical Spine And Related Reconstruct Strategy

BackgroundStability reconstruction is one of the key points to treat unstable subaxial cervical spine(SCS)injury.Distractive flexion injury(DFI)is one of the three common injuries in SCS.However,the operative level is represented as the potential biomechanical risk factor of contruct failure for DFI.In our previous study,the occurrence rate of construct failure dramatically increases as the operative level shifts to the caudal segments of SCS,especially in the level of C6-7.This phenomenon suggests that the stability of the injured segment is highly dependent on the segments involved,termed as segment-specific.Understanding the underlying biomechanical basis has key role for optimization of surgery strategy for DFI.To our best knowledge,there is no biomechanical study focusing on this phenomenon which needs an ideal biomechanical model which is based on multi-level samples.It is hardly to acquire enough homogeneous samples for traditional experiments.So it is more pratical to study this phenomenon by mathematic model,as finite element model(FE model)in the present study.Since previous FE models only provide poor biofidelity,we developed a FE model of C2-T1 based on No.2 Chinese Visible Human(CVH)dataset and verified its validity.In addition,we tested the stability outcome of common used surgery technique such as anterior plating,posterior pedicle screw-rob fixation and combined operation based on the newly revealed potential biomechaniccal basis.Methods and ResultsChapter 1.Model developmentWe acquired the geometric details from Chinese Visible Human dataset and model ed all structures in tissue level.Current FE model was validated with controlled experiments from healthy young samples of human cadavers and its high biofidelity was testified.During the model development,we found that uncinate process is an important load-bearing structure in the subaxial cervical spine and investigated the biomechanical role of uncovertebral joints.Chapter 2.The biomechanical basis of segment-specific stability in subaxial cervical spine after distractive-flexion injuryFirstly,we reviewed the level distribution of cervical spine injury and found that more the cervical spine injury happens in the caudal segments of subaxial cervical spine.It implies that the caudal segment of subaxial cervical spine is more unstable than the cephalad ones.Secondly,based on the initial cervical osteoligamentous FE model of C2-T1,six additional models were modified to simulate surgical procedure of anterior a pproach for treating single-level DFI,from C2-C3 to C7-T1,and four loading conditions were used to explore the potential mechanism,including no-compression(NC),follower load(FL),axial load(AL)and combined load(CL).Construct stability at the operative level was assessed.Prediction shows that under these loading conditions with the weight of head(AL,CL),segmental stability,measured by range of motion and rate of motion change,decreases when the operative level shifts cephalocaudally,especially at C6-C7 and C7-T1,the stress of screw-bone interface increases cephalocaudally,and in the same operative level,the caudal screws always carries more load than the cephalad ones.All these predicted results are consistent with failure patterns observed in clinical reports.In the contrast,under other loading conditions without the weight of head(NC,FL),no obvious segmental divergence was predicted.Chapter 3.Investigation of the stability provided by surgery in DFI Ⅱ/Ⅲ/ⅣIn this chapter,we defined the stage 2 DFIⅡ as “dysfunction of posterior ligamentous complex with no disc injury”,DFI Ⅲ as “dysfunction of PLC with rapture of the posterior half of disc”,DFI Ⅳ as “all intervertebral connection dysfunctional”.We compared the stability of common surgeries via anterior plating,posterior pedicle screw-rob fixation or combined approach.Predicted results revealed the facts that for the DFI Ⅱ/ Ⅲ,the posterior pedicle screw-rob fixation was rigid enough and the stability provided by the fixation did not decrease as the operative level shifted to the caudal segments,because the supporting function of the anterior column was not fully lost.For the DFI Ⅳ,the stability provided by the posterior operation was not enough to resist the flexion-extension load and decreased cephalocaudally.The combined operation was rigid enough for DFI Ⅳ in all levels.ConclusionThe FE model we developed has high biofidelity and is a useful tool for following study.Uncovertebral joint is important load-carrying structure and detailed uncovertebral joint modeling improves the biofidelity of the model without over-calibration.The underlying mechanism of the segment-specificity of the subaxial cervical spine after DFI is posture related biomechanics,such as the eccentric load of the head and its level arm among different segments,the unsymmetrical muscle force in the flexion-extension movement.All these biomechanical factors contribute to this phenomenon,especially for the distractive flexion injury.For the DFI Ⅱ/Ⅲ,anterior plating is recommended in the cephalad segments and posterior operation for the caudal segments.For the caudal segments in DFI Ⅳ,combined operation is more safe.