Anatomical Imaging Measurement Of Craniocervical Malformations And Device Design For Transoral Lateral Atlantoaxial Arthrodesis

Research purposesCraniocervical malformations are considered rare diseases, which lead to the medulla oblongata and spinal cord compression, severe limb dyskinesia and even life-threatening problems. Surgical treatment is effective, but has high risks and challenge. At present, the surgical treatment with transoral release and posterior fixation is used at home and abroad for the patients with basilar invagination who had failed reduction of the odontoid invagination with cervical traction. The surgery requires two stages with two incisions and could result in separation in front of atlantoaxial articulation after operation. Because of the lack of the anterior support for atlantoaxial articulation, the posterior fixation alone could result in failure. According to the mechanics, fusion at atlantoaxial articulation is more reasonable than occipital cervical or inter-laminal fusion. We designed a fixation device for transoral lateral atlantoaxial arthrodesis on the basis of imaging anatomical parameters. The new fixation device combines the advantages of plate and cage. The transroal release and fixation is completed with one step and one incision by using this new fixation device. This new fixation device could avoid the risk of posterior surgery, reduce operative time and surgical trauma, and improve the fusion rate and long-term effect.At present, no paper in this subject had been reported in both domestic and foreign literaturesMaterials and methods1 Imaging anatomy measurementAfter collecting the imaging data of 50 patients with clinical craniocervical malformation, we obtained pictures through SCT cutting and three-dimensional reconstruction and measured the related parameters of anatomical atlas and axis. The seven relevant parameters are following:the transverse diameter of atlantoaxial lateral mass articulation, the sagittal diameter of atlantoaxial lateral mass articulation, odontoid height above the McGregor line, the length from the midpoint of the front edge of atlantoaxial lateral mass articulation to atlas block entry point, the length from the midpoint of the front edge of atlantoaxial lateral mass articulation to axis block entry point, the height of odontoid process above the McGregor line, the outside obliques angle of atlantoaxial lateral mass articulation, the angle formed by atlas block entry point,axis block entry point and the midpoint of the front edge of atlantoaxial lateral mass articulation.2 Fixation and fusion device design for atlantoaxial lateral mass articulationWe design atlantoaxial lateral mass articulation fixation and fusion device that is able to keep reduction, fixation and fusion on the basis of imaging anatomical parameters. We called it a new internal fixation device for short.3 Biomechanical studySeven human cervical specimens were fixed with the new internal fixation device we designed and atlas lateral mass screws and axis pedicle screw\rod system in sequence. Through biomechanical testing we recorded the axial displacement under the load of 30N,50N,70N respectively.Results1 Imaging anatomyThe transverse diameter of atlantoaxial lateral mass articulation fixation, the sagittal diameter of atlantoaxial lateral mass articulation fixation, odontoid height above the McGregor line, the length from the midpoint of the front edge of atlantoaxial lateral mass articulation fixation to atlas block entry point, the length from the midpoint of the front edge of atlantoaxial lateral mass articulation fixation to axis block entry point, the height of odontoid process above the McGregor line, the outside obliques angle of atlantoaxial lateral mass articulation fixation, the angle formed by atlas block entry point,axis block entry point and the midpoint of the front edge of atlantoaxial lateral mass articulation fixation were 16.47±3.02 (11.0～22.3) mm,17.61±3.50 (8.4～28.1) mm,9.016±1.64(5.6～14.3) mm,12.38±1.33 (6.8～14.2) mm,12.91±2.71 (7.1～18.0) mm,25.15±6.32 (6.5-44.1)°,118.19±12.63 (77.8～140.2)°, respectively. All data were analyzed by SPSS17.0 software. Through paired t test, there is a significant difference between right and left side in the angle formed by atlas block entry point,axis block entry point and the midpoint of the front edge of atlantoaxial lateral mass articulation fixation (P<0.05), There were no significant difference in the other 6 parameters(P>0.05). Through single-sample t test, Atlantoaxial lateral mass of the articular surface diameters were compared with the reference value of statistical parameters from the normal anatomy of the relevant parameters in literatures. There were significant difference in the parameters of the transverse diameter of atlantoaxial lateral mass articulation fixation, the sagittal diameter of atlantoaxial lateral mass articulation fixation, the outside obliques angle of atlantoaxial lateral mass articulation fixation and the height of odontoid process above the McGregor line (P<0.05)。2 Structural features of the fixation and fusion deviceThe new internal fixation device is composed of symmetrical combination with two symmetrical plates and one cage. The plate has a certain angle, which make it easily fixed in a safe and effective place. The cage is a hollow cylinder with a slope. There is spike-like protrusion in the upper and lower ends of cage to prevent from slipping.3 Biomechanical studyUnder the same normal physiological load, axial displacement of the new internal fixation device is smaller than that of posterior fixation surgery (atlas lateral mass screws and axis pedicle screw\rod system). Through paired t test, There was significant difference between two fixation methods (P<0.05)Conclusion(1) The anatomy of craniocervical malformations is complex and there are significant difference between the right and left sides, so the new fixation and fusion device should be designed with individuation accordingly.(2) The biomechanical stability of the new fixation and fusion device we design is reliable. It may be used for the surgical treatment of craniocervical malformations in the future with the advantages of maintaining reduction, reasonable fusion and convenience.