Biomechanical Evaluation Of Atlantoaxial Internal Fixation System Incorporated With A Combination Of U-shaped Rod And Titanium Cable

OBJECTIVE1.To design a new atlantoaxial internal fixation system incorporated a combination of U-shaped rod and titanium cable based on traditional screw-rod technique,to evaluate the safety and the biomechanical stability via radiological measurement and using cadaveric cervical spines.METHODS1.The study involved 50 patients(25 of each sex)who underwent computed tomographic(CT)scanning and three-dimensional(3D)reconstruction involving atlantoaxial complex between June 2016 and August 2016,the dimensions of the atlas pedicles,including the height and the width were quantified using Philips IntelliSpace Portal software.The trajectory was designed for each C1 pedicle on CT images.Optimal medial angle,optimal entry point and the length along each trajectory were measured.2.Cervical areas between C0 through C3 were obtained from 6 cadaveric spines.With the use of a machine,the specimen was loaded three times with the torque of 1.5 Nm and held for 10 seconds,then the range of motion in the models of flexion-extension,lateral bending and axial rotation was collected and tested from the third cycle in the following sequence:(1)Intact(M1),(2)Destabilization with Gallie fixation(M3),(3)Bilateral-cable fixation(M4),(4)Unilateral-cable fixation(M5),(5)Bilateral PS(pedicle screw)fixation(M6),and(6)Destabilization(M2).RESULTS1.No statistically significant difference was found between the left and right sides of the data on the measurement of atlas pedicle screw(P>0.05),while there was a statistically significant difference between male and female on several data(P <0.05).No statistical difference was found in data of the medial angle between male and female(P> 0.05).The proportion of the pedicle height <4.0 mm was 23%(23/100),and the proportion of the pedicle height <3.5 mm was 13.0%(13/100).The vertical distance between the entry point and the saggital plane was 19.93 ± 1.32 mm in male,18.01 ± 1.24 mm in female,the medial angle of the screw trajectory was 7.07 ± 2.19 ° in male and 7.20 ± 1.65 ° in female,length of screw trajectory was 28.52 ± 1.70 mm in male,and 27.74 ± 1.96 mm in female.2.The ROM of destabilization group was significantly greater than that of the intact groups(P<0.05).All instrumented groups were significantly less in the ROM than that of the intact and the destabilization groups,and stiffer accordingly.The ROM of Unilateral-cable group was less than that of other instrumented group,especially showed significant differences in the condition of lateral bending and axial rotation(P<0.05).CONCLUSION1.The safety of the atlas pedicle screw fixation depends on the external height of the pedicle and the medullary cavity height inferior to the vertebral artery groove.The preoperative evaluation of the atlas pedicle by CT scans and three-dimensional reconstruction is of paramount importance.For patients with an external wall height <3.5mm,the insertion of the screw should be cautiously,and for the patients without the medullary cavity,the pedicle screw was forbidden,then an alternative was needed.2.The atlantoaxial internal fixation system incorporated with a combination of U-shaped rod and titanium cable is simple,safe and stable,therefore it can be used as an alternative or a salvage of the atlantoaxial pedicle screw fixation to effectively treat atlantoaxial dislocation,especially irreducible dislocation.