Design And Biomechanical Study Of The Novel Posterior Atlantoaxial Fixator Composed Of Bilateral Transarticular Screws And C1 Laminar Hooks

BackgroundThe upper cervical spine includes the occipitoatlantal and the atlantoaxial joint complexes.Anatomic characteristics include the absence of intervertebral discs,the absence of ligament flava,and the distinct shape of C1 and C2.The highly specialized anatomy of this region is designed to provide seemingly paradoxical attribute:loose enough to allow nearly 60%of the cervical spine axial rotation,sufficiently tight to protect the delicate structures of the spinal cord and vertebral arteries,and strong enough to resist muscle forces.Motion is coupled between the two joints which provides a relatively large range of motion between the head and the torso.Motion in the sagittal plane is the primary function at the occipitoatlantal junction and reported on average as being between 13 degrees and 25 degrees.The atlantoaxial complex is composed of two facet joints and the unique atlantodental articulation.Rotation in the atlantoaxial complex represents 50%of the entire cervical spine rotation.Articulation is provided by the synovial joints.The versatile function of the upper cervical spine necessitates numerous synovial joints,and this-together with the complexity of the region-makes it vulnerable.The instability of atlantoaxial complex can result from trauma,malformation, malignancy,or inflammatory diseases such as rheumatoid arthritis.The atlantoaxial fusion is a good choice to treat C1-C2 instability.There is no doubt that the initial strength of the construct is greatest at the time of the surgery and subsequently is reduced in the process of the host response until a final fusion is achieved.Atlantoaxial fusion can be accomplished with anterior approach or posterior approach.Most surgeons prefer posterior approach because of its easy exposure and fixation;for the other hand,the muscle bulk in the neck is notably more massive posteriorly and is related to the tendency of the head to be poisoned in such a way that a center of gravity is positioned anterior to the vertical support provided by the spinal column.Thus it can be seen that atlantoaxial fusions are best served with a dorsal fusion mass.Osseous fusion is due to many reasons including age,health,and the quality of bone, stability,the method of bone grafting,and so on.The most important of which is stability and method of bone grafting.The surgical intervention of atlantoaxial instability developed from one-point fixation,two-point,until to three-point fixation;from two dimensional fixation to three dimensional fixation,rigid internal fixation can reduce the time of adjunctive postoperative immobilization such as a halo vest,avoids complications of external orthosis.Recently studies have showed that the highest stability is provided by the three-point fixations of the atlantoaxial complex.Two main aims of internal stabilization are immediate postoperative stability and long-term stability.The immediate postoperative stability contributes to the duration and acceptability of the postoperative treatment.The long-term stability usually is achieved by bone fusion,for which a rigid internal fixation is considered a main factor for occurrence.The atlantoaxial fixation method includes:Gallie and Brooks technique,Apofix or Halifax lamina clamp,bilateral C1-C2 transarticular screw fixation,direct polyaxial screw fixation to the lateral masses of C1 or the pedicle of C2.To maximize stability,the transarticular screw fixation has to be combined either a Gallie or a Brooks fusion.More recently,Brooks combined with bilateral transarticular screw fixation has become the gold standard for achieving atlantoaxial arthrodesis.But this increase the risk of neural injury caused by the passage of sublaminar wires.A new fixation technique for C1-C2 arthrodesis is presented by NI-bin et al.It consists of a hook construct for the posterior arch of C1 that is rigidly attached to C1-C2 transarticular screws to form an instrument that combines anterior and posterior fixation in the same construct.This bilateral atlantoaxial transarticular screws and atlas laminar hooks technique has the advantage over C1-C2 transarticular screw and Halifax which allows reliable stabilization against motion in six degrees of freedom:anterior-posterior slip as well as extension- flexion,lateral slip as well as lateral bending,and axial translation as well as rotation.Furthermore,bone graft can be compressed tightly between the posterior arch of C1 and lamina of C2 by rod between the hook and screw.The novel technique provides excellent atlantoaxial stability that is equivalent to the Magerl-Brooks construct, but it can avoid risky to place cerclage wire under the C1-C2 lamina.The NI-bin technique obtained satisfied treatment effectiveness in clinical application. But the now available instruments have some problems and are not suitable for Chinese population.For example,the now available laminar hooks are made for thoracic laminar originally and not suitable for atlas posterior arch,the hook and the screw are very difficult to connect,et al.A novel posterior atlantoaxial fixator composed of bilateral transarticular screws and C1 laminar hooks needs to be designed to meet the technique and anatomy property of Chinese population.The study measured linear and angular parameters of atlas and axis in Chinese population and provide morphological basisi for the fixator design.A novel posterior atlantoaxial fixator composed of bilateral transarticular screws and C1 laminar hooks was designed and manufactured.An in vitro biomechanical study using cadaveric model were performed to investigate the novel posterior atlantoaxial fixator composed of bilateral transarticular screws and C1 laminar hooks by comparing with other three different contemporary posterior atlantoxial fixations.Finite element method grown greatly since it was first used for biomechanics study more than 30 years before.It's an invaluable tool that can supplement experimental research in understanding the clinical biomechanics of the upper cervical spine. Furthermore,because of its reproducibility and repeatability,a fully validated model can be used to analyze detailed parametric studies of novel instrumentation,allowing for the avoidance of otherwise costly experimentation.The destabilized three-dimensional finite element model of the upper cervical spine (C0-C3) established by Doctor Ren is available.The novel posterior atlantoaxial fixator composed of bilateral transarticular screws and C1 laminar hooks were solid modeled, using commercially available design software,imported to the destabilized model and meshed.The instrumented models were run in flexion,extension,and lateral bending and axial rotation for evaluating the biomechanical properties and the stress of transarticular screw and hook.Objectives1.To measure linear and angular parameters of atlas and axis in Chinese population and provide morphological basisi for the design of the novel posterior atlantoaxial fixator. To design a novel posterior atlantoaxial fixator composed of bilateral transarticular screws and C1 laminar hooks.2.To investigate the acute stability afforded by the novel posterior atlantoaxial fixator composed of bilateral transarticular screws and C1 laminar hooks by comparing with other three different contemporary posterior atlantoxial fixations.3.To investigate the stress distributions of the destabilized models with the novel posterior atlantoaxial fixator composed of bilateral transarticular screws and C1 laminar hooks under quasistatic loading in flexion,extension,lateral bending and rotation. Materials and Methods1.Thirty-five sets of normal adult dry atlas and axis vertebral samples and 40 sets of normal adult CT images of the upper cervical spine were measured.The parameters include posterior atlantal arch width,height,length,inside semi-diameter,outside semi-diameter,the transarticular screw path length,the transarticular screw path angle in the transverse plane and in the sagittal plane.A novel posterior atlantoaxial fixator composed of bilateral transarticular screws and C1 laminar hooks was designed.2.In this study,6 fresh-frozen human candaveric cervical spines with occiput(C0-C5) were used.Osteoligamentous specimens were tested in their intact condition and destabilizetion via transverse-alar-apical ligament disruptions and were analyzed by the three dimensional motion testing system.The reconstructions tested were TA+G(the transarticular screw-wiring fixations),TA+H(the transarticular screw-C1 hooks fixations), C2+H(the C2 Pedicle screws-C1 hooks fixations),C1+C2(C1-C2 screw and rod fixations).3.The novel posterior atlantoaxial fixator composed of bilateral transarticular screws and C1 laminar hooks were solid modeled,using commercially available design software, imported to the destabilized three-dimensional finite element model of the upper cervical spine(C0-C3) and meshed.The instrumented models were run in flexion,extension,and lateral bending and axial rotation for evaluating the biomechanical properties and the stress of transarticular screw and hook.The study also evaluated the change of stress of transarticular screws after the atlas laminar hooks were imported.Results1.There was no statistical significance between the left and right side data.The outside semi-diameter was(20.49±3.22)mm.The inside semi-diameter was (11.74±2.47)mm.The image measurements show the posterior atlantal arch width was (7.4±1.3)mm.The transarticular screw path length was(39.6±3.4)mm.The transarticular screw path angle in the transverse plane was(52.0±4.4)~0.The transarticular screw path angle in the sagittal plane was(9.1±4.0)~0.2.The data indicate that destabilization via transverse-alar-apical ligament disruptions significantly increase C1-C2 motion.The ROM values of TA+H were lower than other fixations in all directions.The p values were less than 0.05 for TA+H versus TA+G in extension and for TA+H versus C1+C2 in axial rotation.The ROM values of C2+H were higher than other fixations in al directions.There were no p values less than 0.05 for C2+H versus TA+G in flexion,extension and lateral bending.But the P values were less than 0.05 for C2+H versus TA+H,TA+G and C1+C2 in axial rotation.3.The finite element model predicted that the maximum von Mises stress was in the region where screws penetrated the atlantoaxial facet joints,and the second was located at the junction of the rod and screw.The stress of C1 hook increased in flexion and lateral bending.The stress of transarticular screws decreased in flexion/extension and lateral bending after the atlas laminar hooks were imported.Conclusions1.Exposure range during operation is limited in order to avoid damage of vertebral artery.The transarticular screw should be(39.6±3.4)mm long and be inserted(52.0±4.4)~0 cephalad and(9.1±4.0)~0 lateral.The axis pedicle varied largely and therapy should be individualized treatment.2.The novel posterior atlantoaxial fixator composed of bilateral transarticular screws and C1 laminar hooks provides the best biomechanical stability.Although the the fixator composed of bilateral C2 Pedicle screws and C1 hooks is not as stable as traditional three-point fixtions,it provides the similar stability with other fixations in flexion, extension and lateral bending.3.The atlas laminar hook add a posterior fixation point,and at the same time which can compress bone graft between the C1 posterior arch and C2 lamina.The atlas laminar hooks fixation can reduce the stress of the transarticular screws.The novel posterior atlantoaxial fixator composed of bilateral transarticular screws and C1 laminar hooks is consistent with the three-dimensional fixation theory and provides excellent atlantoaxial stability.