| ObjectivesFor atlantoaxial instability,rigid internal fixation is the most efftive treatment approach.As to now too many fixations are available.However,atlantoxial posterior scre-rod internal fixation,as a three-point fixation,provided the best biomechanical stability and the highest fusion rates.The rod in current rod-screw fixation is standard cylinder–shaped which will be bent severly before placed to the U-shaped groove of screws to obtain the best reduction effect of atlas and axis.However,the bent rod will rotate within the U-shaped groove of screws easily during screwing down the nut.then additional tools must be applied to hold the rod and the surgeon need to adjust the direction of the rod many times to make the prebent direction of rod matched with the curve of cervical spine.During this procedure,the spinal cord damage will occur when the assistant does not hold the instrument rightly.In order to simplify this procedure and enhance the safety of treatment,we have designed one new automatic anti-rotation rod for screw-rod fixation system of atlantoaxial posterior approach.And we have analyzed its biomechanical properties via finite element modeling of atlatoaxial and then tested its biomechanical stability based on cadaveric cervical spine experiments compared with standard rod-screw fixation system.So we can provide comprehensively theoretical support for its clinical application.The main research purposes as follows:1.Establish a finite element model of atlantoaxial complex with the new automatic antirotation screw-rod fixation,and analyzed its biomechanical properties.2.To compare the biomechanical stability difference between the new and standard screw-rod fixation based on cadaveric cervical spine experiments.3.The clinical application of the new screw-rod fixation system.Materials and methodsPart oneOne female volunteer with normal cervical spine was selected.And 125 pieces sequential original CT data photos harvested and saved as Dicom format.Then 3D softwares,such as Mminics 10.01,Geomagic Studio 2013,and SolidWorks 2012,were applied to establish,modify,and create a finite element model(FEM)of atlantoaxial complex with posterior screw-rod fixation.The FEM model was analyzed with Abaqus 6.14 and the mechanical properties of it was obtained.The values of Von Mises Stress were measurable data and recorded as mean+/-SD,then the data were analyzed by SPSS21.0 statistical software,comparison between new and standard rod in screw-rod fixation system was conducted with independent sample t-test.P=0.05.Part two6 fresh cadaveric cervical spine specimens were haverested to establish the atlantoaxial instablility models.And the specimens were tested in the following order:group N1(Normal cervical spine models)with all ligament and joint capsule attached),group N2(Atlantoaxial instability models),group A(Standard rod+screw fixation),group B(New automatic antirotation rod+screw fixation).The 3D biomechanical stability of C0-C2 segments was measured.A,B were compared between the two groups and compared with group N1 and N2 respectively.Part three6 patients with atlantoaxial instability have went posterior new screw-rod fixation after signed operation agreement.X-ray films,CT and MRI of cervical spine have been checked before operation to clarify the dislocation of atlas and axis,fracture of odontoid,and the location and severity of compression spinal cord,to determine which approach will be the option.Also,X-ray checking was done during operation to check the condition of the new screw-rod fixation system.X-ray films and CT of cervical spine were reviewed 1 week and 3 months after operation to check the condition of fixation,reduction of atlas and axis,reduction and union of fracture.Results Finite element modeling investigationThe mean values of Von Mises Stress of the new screw-rod fixation is higher than standard one,there has statistical significance bwteen the two fixations(P<0.05).However,all the maximum Von Mises Stress of the two fixation system indicated that the yield and ultimate strengths of the titanium rod are in the range of(less than)795-827 MPa and 860-896 MPa.Biomechanical tests resultsThe biomechanical tests for the two different internal fixation based on cadaveric cervical spine as follows:In flextion direction,two fixation instruments had no statistical significance(P>0.05),were better than the controlled group N1 and N2(P<0.05).In extension direction,two fixation instruments had no statistical significance(P>0.05),were better than the controlled group N1 and N2(P<0.05).In left lateral bending direction,two fixation instruments had no statistical significance(P>0.05),were better than the controlled group N1 and N2(P<0.05).In right lateral bending direction,two fixation instruments had no statistical significance(P>0.05),were better than the controlled group N1 and N2(P<0.05).In left-hand rotation direction,two fixation instruments had no statistical significance(P>0.05),were better than the controlled group N1 and N2(P<0.05).In right-hand rotation direction,two fixation instruments had no statistical significance(P>0.05),were better than the controlled group N1 and N2(P<0.05).Clinical application of new screw-rod fixation6patients,including 5 cases of fixation with bone graft fusion and 1 case without bone graft fusion,have underwent the posterior fixation with the new screw-rod instruments opertation successfully..The average operative time was 92 mins(60-145mins),and the average estimated blood loss was 45ml(30-75ml).No veterbral artery injuries and spinal cord were found during the operation.Screws,rods of this new fixation instrument were placed successfully at one go,no additional tools required to hold and adjust the direction of the rods.Postoperative X-ray films,CT and MRI results showed a satisfied reduction,bone union and decompression.Conclusions1.The design of new automatic anti-rotation screw-rod fixation instrument has satisfied biomechanical properties.2.The new screw-rod fixation system has similar biomechanical stability compared to the current standard system,can provide sufficient biomechanical stability for reduction and graft bone union.3.The new fixation instrument is a safe,effective device,which can not only simplify the operation procedure,but also has a satisfactory preliminary clinical effects and prosperous application prospect. |
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