Biomechanical Research Of Posterior Asymmetric Fixation Of Atlantoaxial Joint

Objectives：1、 To creat the whole instability cervical cadaver model and evaluate thebiomechanical stability of five different posterior fixation of atlantoaxial in vitrobiomechanical methods;2. Establish and validate3-D finite element model includes cervical ligaments,intervertebral discs, facet joint capsule and other structures of whole cervical.Load5different atlantoaxial posterior fixation and compare the stability and stress of the5different fixation,the impact on the lower cervical vertebrae.Materials and Methods1. Six human specimens (C0-T1) were selected and establish intact and atlantoaxialdestabilized model which were compared with the range of motion (ROM) at the level ofC1-C2in the direction of flexion extension lateral bending and rotation was measured inthe intact, destabilized and different fixation condition. Six specimens were implanted withfive different fixations: right of C1lateral mass screw\C2pedicle screw and left of C1hook\C2pedicle screw fixation (LPS+HPS);right of C1lateral mass screw\C2pediclescrew and left C1-2transarticular screw fixation (LPS+TA); right of C1lateral massscrew\C2pedicle screw and left of C1hook\C1-2transarticular screwfixation(LPA+HTA); right of C1lateral mass screw\C2laminar screw and left of C1hook\C2laminar screw fixation(LILS+HILS); Bilateral C1lateral mass screw\C2pedicle screwfixation(LPS).2. Construct the3-D finite element model of whole cervical spine(C0-C7) whichcontains of the cervical ligament, intercalated disc, capsula articularis and5differentfixation,by using Using three-dimensional finite element software and validate.Load thefive different fixation to the finite element model and compare the range of motion at thelevel of C1-C2and the impact on the lower cervical vertebrae in the direction of flexionextension lateral bending and rotation.The five fixation is: right of C1lateral massscrew\C2pedicle screw and left of C1hook\C2pedicle screw fixation (LPS+HPS);rightof C1lateral mass screw\C2pedicle screw and left C1-2transarticular screw fixation(LPS+TA); right of C1lateral mass screw\C2pedicle screw and left of C1hook\C1-2 transarticular screw fixation(LPA+HTA); right of C1lateral mass screw\C2laminar screwand left of C1hook\C2laminar screw fixation(LILS+HILS); Bilateral C1hook\C2pedicle screw fixation(HPS).Results1..Destabilized model on each specimen showed significant atlantoaxialinstability.But all specimens which were loaded the five different fixation showed betterstability and the ROM of C1-C2was significantly lower than the destabilized model insix direction.LPS+TA group and LPS+HTA group showed excellent stability than thecontrol group of LPS,even though the result is no significant difference.The ROMs ofC1-C2in the LPS+HPS group were lower than the control group of LPS specifically inthe direction of left bending.The stability of C1-C2of LILS+HILS group is the worst inall groups.2. The3-D finite element model of whole cervical spine(C0-C7) which contains ofthe cervical ligament, intercalated disc, capsula articularis and5different fixation wereconstructed by using Using three-dimensional finite element software and validated.Allcondition which were loaded the five different fixation showed better stability and theROM of C1-C2was significantly lower than the intact model in the direction of flexionextension lateral bending and rotation. LPS+HTA group was the best of the stability inall condition,followed by LPS+TA group,and the group with hooks showed poorer.Thedifference is particularly evident in the direction of bending but no significant differencein the six directions.The results in finite element research is basically consistent with thevitro biomechanical experiments.The transarticular screw fixation in LPS+HTA groupand LPS+TA group was significantly restrict the activities of atlantoaxial and make agreater stress concentration. The right titanium rod of LPS+HPS group has a moresignificant stress concentration in the direction of right/left bending.The stressconcentration of HPS group is mainly in the bilateral tails of C2pedical screw,but notobvious in all activities of LILS+HILS group. After the fusion of C1-C2,compensatorymovement，the ROMs，stress of intercalated disc,contact pressure of facet joint ofadjacent segments will increase.Conclusions1. The Harms technologies combined with C1-2transarticular screws（LPS+TA group）and laminar hook (LPS+HTA group) fixation provided the best biomechanicalstability and can be applied as the most ideal asymmetric posterior fixation of C1-C2fusion.The group (right of C1lateral mass screw\C2pedicle screw and left of C1hook\C2pedicle screw fixation–LPS+HPS group) can be a substitute technology whenunilateral C1lateral mass screws placement is difficult. The group (right of C1lateralmass screw\C2laminar screw and left of C1hook\C2laminar screwfixation--LILS+HILS) can provide better biomechanical stability than the intact anddestabilized group.This group maybe a appropriate choice in some conditions.2. The3-D finite element model of whole cervical spine(C0-C7) which contains of thecervical ligament, intercalated disc, capsula articularis and5different fixation wereconstructed by using Using three-dimensional finite element software and validated.Comparing the stability of5different atlantoaxial fixation conditions in flexion, extension,left/right bending, left/right rotation,LPS+TA group and LPS+HTA group weresignificantly better than HPS group and have a high overall stability of all techniques.Thetwo groups both have an uniform stability on the bilateral sides.The stability of LPS+HPSgroup was significantly better than the intact group but significant difference in eachside.The stability of LILS+HILS group is the worst in all groups.All results accord withthe results of in vitro biomechanical experiment. C1-2transarticular screws in LPS+TAgroup and LPS+HTA group significantly restrict the activities of atlantoaxil resulted in agreater stress concentration.The point of the maximum stress maybe break off easily inclinic.The right posterior titanium rod in LPS+HPS group and the Spiketail of thebilateral C2pedical screws in HPS group were also easily break off for the samereason.The maximum stress of fixation is not obvious in all direction in LILS+HILSgroup may cause the risk of prolapse and loosening of fixation in clinic.The fivedifferent fixation technology can impact two segments on inferior cervical vertebrase,upto the atlantooccipital joint and down to the C3、C4。The impact of the five differentfixation on the intervertebral disc and facet joint of adjacent segments is no significantlyincreased than intact model.The compensatory movement of adjacent segments generatedby the fusion of C1-C2may be one cause of degeneration of adjacent setments or theresult of the slowly accumulated low amplitude stress.The adjacent segments should notproduce degenerative changes or damage immediately after the fusion of atlantoaxialjoint.