Design Of C1 Posterior Arch Plate And Associated Anatomical And Biomechanical Study

Background:Atlantoaxial instability and/or dislocation,which is mainly caused by trauma,inflammation,tumor,and congenital anomalies,is defined as the change of physiological relationship or destruction of motion state of atlantoaxial joint.As common upper cervical disorder,surgical intervention is often proposed by most surgeons because of its tendency to compression of medulla oblongata which may lead to serious outcomes such as disorders of sensation and movement in extremities even to sudden death.Surgical therapies mainly include transoral anterior reduction and bone graft fusion,posterior atlantoaxial reduction and bone graft fusion,posterior occipital cervical fusion and fixation,and a combined anterior-posterior approach.Posterior surgery has been widely accepted in clinical practice for satisfactory surgical exposure,rigid internal fixation and low incidence of complications.With the advancement of internal fixation devices and anatomical as well as biomechanical knowledge,the surgical techniques have developed from the early wire technique,interlaminar apofix technique to C1-C2 transarticular screw(Magerl screw),which was once considered as golden standard for atlantoaxial fixation,and then to screw –rod technique which has been now extensively used.The emergence of each internal fixation instrument has greatly promoted the development of posterior atlantoaxial internal fixation technique and clinical treatment level of atlantoaxial instability.Pedicle screw or lateral mass screw fixation for C1 has been the first choice for posterior atlantoaxial internal fixation for its satisfactory biomechanical stability and high rate of bone graft fusion.But because of the deep location of C1,complicated adjacent structures and anatomical variations,the risk of injury of vertebral artery,C2 nerve root and venous plexus in the process of exposure and screw implantation will lead to bleeding and postoperative numb of occipital-cervical area,even to impossibility for screw placement.In addition,the existing internal fixation method for C1,which is of high demand for surgical instruments and skills,as well as a long learning curve,is not conducive to popularization resulting to some patients undergoing occipitocervical fusion with a severe restriction of neck motion and influence on daily life.Therefore,a new internal fixation technique,not only effectively reducing the risk in atlas screwing but also providing enough stability,which could be a satisfactory supplement of pedicle screw or lateral mass screw of C1 or option of atlantoaxial revision surgery is needed.Althrough there has been one report depicting C1 posterior locking plate,the defects existing in its design and the specifications which completely according to occidental,is not fully applicable to Chinese and relevant anatomical data of Chinese is still blank.Therefore,this study aims to acquire associated anatomical parameters of posterior arch of C1 through computed tomography(CT)and vertebrae specimens and then give preliminary design of C1 posterior arch plate on the basis of the measured data.Then,to explore the stress distribution and three dimensional stability of the posterior arch plate by three-dimensional finite element analysis.Next,to further study the biomechanical stability in vitro biomechanical test,thus provide a reference for the follow-up improvement of design and clinical application.Objective:1.To acquire related anatomical data of C1 posterior arch of healthy adults through CT and specimens,and give a preliminary design of posterior arch plate of C1 according to the acquired data.2.To establish three dimensional finite element models of atlantoaxial instability,simulate posterior arch plate internal fixation and posterior C1-C2 pedicle screw-rod internal fixation,respectively,and then preliminarily explore the stress distribution and three dimensional stability of C1 posterior arch plate.3.To establish vitro models of atlantoaxial instability and then measure range of motion(ROM)between C1-C2 segment.To further evaluate the biomechanical stability of C1 posterior arch plate through biomechanical test aiming to provide a reference for improvement of design and further clinical application.Methods1.CT(simenz)scan data of C1 from 60 healthy cases(male 30,female 30)was randomly sampled through medical image archiving and communication system(PACS)and some associated anatomical structures were measured using built-in measuring tool.30 dry atlas vertebrae specimens with an unknown age and gender were collected and measured with domestic vernier caliper(accuracy,0.01 mm)and angle measurement device(accuracy,0.01°).The parameters included anterior to posterior thickness and superior to inferior height of the posterior ring of C1 at the midline as well as at 5mm,10 mm,15mm lateral to the midline,angle of posterior arch,the semi-distance of vertebral artery groove in inner surface and the semi-distance of vertebral artery groove in outer surface.A preliminary design of posterior arch plate was then given according to the gained data.2.To establish three-dimensional finite element models of atlantoaxial instability using a series of finite element analysis software including Mimics 10.01、Geomagic Studio 2013、SolidWorks 2017 and Abaqus 6.14 on the reference of previous studies.To explore the stress distribution of the posterior arch plate after validating the effectiveness of the established models.Then,to measure and compare the value of ROM within C1-C2 segment in flexion /extension,left/right bending,and left/right axial rotation of different models.Next,to give a preliminary evaluation of the design and stability of the C1 posterior arch plate.3.To establish a series of vitro models including intact group,unstable group,C1 posterior arch plate+C2 pedicle screw-rod fixation group,and C1 pedicle screw+C2 pedicle screw rod fixation group and then validate the effectiveness of the established models.To measure the value of ROM of C1-C2 segment in flexion/extension,left/right bending,and left/right axial rotation of different models.Next,to compare the difference of ROM between different groups and further to explore the biomechanical stability of the posterior arch plate,thereby provide a reference for the follow-up improvement of design and clinical application.Results1.A series of anatomical parameter of posterior arch of C1 was acquired and a preliminary design of C1 posterior arch plate was finished.2.A three-dimensional finite element model of atlantoaxial instability with 62348 nodes and 53346 elements was created which accurately simulates the related material properties.3.With the application of the established three-dimensional finite element models,we succeeded simulating the C1 posterior arch plate fixation and posterior pedicle screw-rod fixation and carrying out the stress distribution test and measurement of the value of ROM of C1-C2 segment in flexion/extension,left/right bending,and left/right axial rotation in different models.4.Three dimensional finite element analysis results showed that the stress of posterior arch plate distributed equally without obvious concentration of stress.The value of ROM measured from unstable models was significantly higher than that from intact group in flexion/extension,left/right bending,and left/right axial rotation.Also,the data of ROM acquired from both posterior arch plate fixation group and posterior pedicle screw-rod fixation group were significantly lower than those of intact group in the above 6 direction.However,there was no significant difference of the ROM value between posterior arch plate fixation group and posterior pedicle screw-rod fixation group.5.A series of vitro atlantoaxial models were established and the ROM values of C1-C2 segment in flexion/extension,left/right bending,and left/right axial rotation of intact group,unstable group,C1 posterior arch plate fixation group,and posterior screw-rod fixation group were measured and the difference between these groups was compared.6.The biomechanical test results suggested that the ROM values of the unstable models were significantly higher than those of intact models in flexion/extension,left/right bending,and left/right axial rotation.Also,the values of ROM measured from both posterior arch plate fixation model and posterior pedicle screw-rod fixation model were significantly lower than those from intact group in corresponding direction of motion.In addition,no significant difference of the ROM values was found between the posterior arch plate fixation group and the posterior pedicle screw-rod fixation group.Conclusions1.Anatomical study suggests the C1 posterior arch plate is applicable to more than 90% of the population which demonstrates the high feasibility of C1 posterior arch plate.2.Three dimensional finite element analysis suggests an equal stress distribution of the C1 posterior arch plate which proves a scientific design of C1 posterior arch plate.3.Biomechanical study suggests a similar three dimensional stability between C1 posterior arch plate fixation and traditional posterior pedicle screw-rod fixation which highlights the C1 posterior arch plate may be a good supplement of posterior atlantoaxial screw-rod fixation techniques.