Biomechanical Evaluation Of Anterior Multifunctional Cervical Plate System Made Of A New Type Of Titanium-alloy

PrefaceTo date,anterior cervical plate has been widely performed in spinal fractures, spinal tumors,and degenerative cervical diseases.Anterior cervical plate is advocated for the reconstruction of anterior and middle column defects,to establish appropriate anterior spinal load-sharing.The usefulness of the anterior cervical plate is manifold:to maintain the bone graft in place,to promote fusion by providing stability between the bone graft and donor vertebrae,and to keep proper cervical alignment.In this study,we have designed a multifunctional cervical plate(MCP) made by a new titanium-alloy and evaluated the flexibility of the MCP on the cervical spine after simulated static and cyclic physiologic loads.However,with the increasing of anterior cervical plating in spinal surgery,the failure directly related to the instrumentation,such as instrumentation loosening,screw backout,decreased load sharing,and poor compensation for graft subsidence,may happen by the reasons of fatigue and cyclic motion in vivo.Therefore,we have also done fatigue testing and three-dimensional finite element analysis to detecting the designed MCP whether or not to meet the needs of the fusion.Methods24 porcine specimens(C3-C7)(median age 6 months) were collected and randomly divided into four groups with six specimens of each group.Each specimen was loaded in vitro with pure moments of±2.0 Nm in flexion/extension,lateral bending, and axial rotation.Range of motion(ROM) and neutral zone(NZ) were measured in the intact state,with anterior cervical corpectomy in C4-C6 without plating,after addition of either a MCP or C-mark plate(CMP) and after 2000 cycles of axial torsion.The constrained MCP and CMP systems were assembled and tested in accordance with the corpectomy model defined in ASTM F1717-96 for compression testing.The testing standard consisted of the use of an ultra high molecular weight polyethylene (UHMWPE) model simulating a total corpectomy defect and the application of a compressive bending load.The use of UHMWPE blocks removes variability related to the bone density and quality of the vertebral bodies thus providing a reproducible test bed to evaluate the properties of the fixation systems.During construction of the implant assemblies,the manufacturers' recommendations were followed exactly.A pair of stainless steel yokes with pins supported the UHMWPE blocks and permitted free rotation of the blocks in the sagittal plane during deformation of the assembly.All mechanical testing was performed on a servo-hydraulic materials testing machine under software control(INSTRON,8871,England).The static compression testing and fatigue testing were made subsequently.The geometry of the MCP was accomplished in the software Unigraphics.Then the created geometry was imported into MSC software,to accomplish the finite element method analysis.The plastic model and the biomedical titanium alloy were used in the MCP system.The deformable body contacted at the plate-screw interface, and the rigid board was adopted on the depressor.This analysis begins with the discretization of the model.The mesh was made with four nodes tetrahedral elements, obtaining a mesh of 16,409 nodes and 72,198 elements.The nodes of the screws were made coincide with the plate where they are placed.The stress and the strain of the MCP were analysis through the load which the depressors exert to the vertebrae screws.ResultsAll the segments in all loading directions of ROM,no differences were found among MCP fixed state,MCP fatigue state,CMP fixed state and CMP fatigue state.But when the above states compared with the intact and grafted state,significant differences were found(P＜0.01).In flexion and lateral bending of ROM,significant difference was found between the intact and the grafted state(P＜0.01).In the NZ of flexion/extension, no differences were found among MCP fixed state,MCP fatigue,state,CMP fixed state and CMP fatigue state.But when the above states compared with the intact and grafted state,significant differences were found(P＜0.01).In the lateral NZ,significant difference was found between the intact state and other states(P＜0.01),but no significant differences were found among other states.In the rotational NZ,no significant differences were found among all the states.The ultimate bending failure load of the MCP was 180 N.In the application of 63 N under bending loads,the MCP was still not broken when the fatigue test was carried out 10~6 times.The stress and the strain concentration were found in the hole of the MCP in the three-dimensional finite element analysis.DiscussionIn this study,the MCP system was designed and evaluated in a porcine cadaveric anterior cervical corpectomy model.The MCP has some attractive characteristics. Firstly,the material used in our design is a new kind of titanium-alloy (Ti-24Nb-4Zr-7.9Sn),which is non-toxic with lacking noxious metals such as aluminum.The new alloy has a lower modulus of elasticity and is more flexible compared with currently used Ti-6AL-4V alloy.Secondly,the MCP system has the option of constrained,semiconstrained and hybrid constructs.The suitable plate was selected for the specific conditions of the patients.Graft subsidence is common during healing after anterior cervical fusion surgery.The variable-angle screws of the semiconstrained MCP and the hybrid MCP can accommodate with the absorption and/or sinking of bone in a certain range.Semiconstrained plates,in theory,allow continued contact between the graft and the end plate after graft subsidence has occurred,thus improving the chance of obtaining a fusion by maintaining a compressive load on the graft.Thirdly,a set of trabecules holes were designed in the MCP uniformly.The multifunction of the MCP was manifested in several aspects:1) the fixed screw can be set any position of the horizontal hole.2) when the anterior cervical corpectomy fusion was performed,the fixed screw can be used to fix the implanted graft.3) when the anterior cervical discectomy fusion was performed,the fixed screws can be used to fix the vertebrae body.4) when the anterior cervical discectomy and corpectomy was performed simultaneously,the fixed screws can be used to fix the implanted graft and vertebrae body.Fourthly,screw backout is effectively prevented by the locked screws.With grafting alone,the grafted state with anterior cervical corpectomy in C4-C6 without plating did substantially decrease ROM compared with the intact state in the flexion and lateral bending.When the MCP was implanted,the ROM was further decrease compared with the grafted state.The MCP can provide the same stability compared with the CMP,and it also provided enough stability after the fatigue test. Furthermore,the MCP can provide the same stability after the torsional fatigue test compared with the CMP.When there was no load on the plate,there were no differences on MCP and CMP in all the segments in all directions.Also,there was no difference on MCP before and after the fatigue test.The ultimate bending failure load of the MCP was 180 N.In the application of 63 N under bending loads,the MCP was still not broken when the fatigue test was carried out 10~6 times.It was revealed that it could be satisfied the requirement of fusion when graft was implanted.The stress concentration of the MCP in the fatigue test was found in the outside edge of the hole of vertebral screws and the cross section of the fixed screws.It revealed that the original fracture zone was found in the upper surface of the MCP through the electron microscope scanning.The stress and the strain of the MCP were located around the area which fixed screws were implanted,the outside of the vertebrae screws hole and window through the three-dimensional finite element analysis.The concentrated stress and strain region is actually the most possible broken area.The stress and strain concentration area of the MCP in the three-dimensional finite element analysis coincided with the actual area in which the MCP was broken in the fatigue test. ConclusionThree-dimensional stability of the cervical spine was provided by the MCP in the flexibility test.After the torsional fatigue test,the MCP can still provide enough stability.In the application of 63 N under bending loads,the MCP was still not broken when the fatigue test was carried out 10~6 times.It was revealed that it could be satisfied the requirement of fusion when graft was implanted.The stress and the strain concentration area of the MCP in the three-dimensional finite element analysis coincided with the actual area in which the MCP was broken in the fatigue test.