A Study To Evaluate The Feasibility And Bone Inductive Effects Of PEEK Rods Used In Cervical Fusion

Background: With the growing population aging and increased numbers of desk-bound workers,the morbidity of cervical spondylosis is gradually ascending.Decompression surgery is necessary for those who are unresponsive to conservative treatment or in severe symptoms.Pedicle screw fixation system is widely used in posterior cervical surgery.Traditional posterior spinal internal fixations which are mainly based on titanium alloy were defined as rigid fixation.It can provide sufficient primary stability and limited painful motion.However,stress shielding induced by high modulus of elasticity of titanium alloy(110 GPa)will cause variety of complications,such as adjacent segment degeneration(ASD),pseudoarthrosis,and nails or rods broken.Polyetheretherketone(PEEK)possesses desirable biocompatibility,low modulus of elasticity(3.2 GPa)and radiotransparency.Accordig to biomechanical research,PEEK rods which were defined as semi-rigid fixation can produce sufficient stability and keep physiological loading distribution.PEEK rods have been applied to thoracic and lumbar vertebrae to reduce stress shielding,increase the incidence of bony fusion and decrease the incidence of ASD.There is no report about using PEEK rods in cervical spinal fusion.The diameter of rods used in posterior cervical fixation is 3.2mm usually.There is no reports concern with the feasibility to use PEEK rods with diameter of 3.2mm in cervical vertebra.Meanwhile,there is no clinical evidence to confirm the hypothesis that PEEK rods have the potential to increase the incidence of bony fusion.Objective: Sheep cervical vertebra specimens were used to evaluate the feasibility and biomechanical properties of PEEK rods based on biomechanical experiment.Further research based on animal experiment was conducted to assess the feasibility and bone inductive effects of PEEK rods.Methods: 1.in vitro biomechanical experiment: 6 sheep cervical C3-C4 vertebra were adopted.CT scanning was conducted in all specimens.Then dates were analyzed by Mimics software and anatomical data of sheep cervical were obtained according to 3D reconstruction model.Screw passage were designed based on anatomical data.Polymethyl methacrylate(PMMA)was used to fix the two ends of the specimens with muscle eliminated and ligament,intervertebral disk and bone preserved completely.Axial pressure range from 0 N to150 N was exerted by testing apparatus and axial displacement and microstrain of vertebral body was recorded during the whole experimental process.Pure moment of 2.5 N2 M was exerted to specimens to achieve flexion-extension and left-right lateral bending with the help of self-made fixture and maximal angular displacement was recorded.Zygapophysial joint were destroyed to obtain destabilized specimens after intact specimens were tested.PEEK rods or titanium alloy rods were used randomly to fix the destabilized specimens.Biomechanical tests describe above were conducted to destabilized groups,PEEK rods groups and titanium alloy rods groups.The loads were cycled 3 times for each group and the final cycle date was used for analysis in order to eliminate the negative effects of firmo viscosity.The comparison between groups was performed using the repeated measures Anova,and the pairwise comparison was performed using the LSD method.2.Animal experimental: Twelve mature female Small Tail Han sheep with a mean age of 1.5±0.4years old and a mean body weight of 45±4.5kg were selected and randomly divided into the test group,which received PEEK rods and control group which received titanium alloy rods.Each sheep underwent internal fixation with cervical pedicle screws and anterior cervical arthrodesis.Animals were euthanized at 12 and 24 weeks post-operatively and the C3-C4 vertebra were harvested for X-ray,micro-CT and histology evaluation.Two independent samples t test were used for comparison between groups.Results: 1.in vitro biomechanical experiment:(1)Axial compression experiment showed that stiffness was higher in PEEK rods groups and titanium alloy rods groups compare to intact groups(p<0.05).Stiffness of PEEK rods groups was lower than titanium alloy rods groups(p<0.05).(2)Microstrain in titanium alloy rods groups were lower than PEEK rods groups and intact groups(p<0.05).There is no distinct difference between PEEK rods groups and intact groups.(3)Both PEEK rods groups and titanium alloy rods groups have lower range of motion compare to intact groups in flexion-extension and leftright lateral bending(p<0.05).PEEK rod groups have greater ROM in left-right lateral bending compare to titanium alloy rods(p<0.05).2.Animal experimental:(1)There is one sheep died during operation because of invasion of spinal cord and have been replaced by the other one.The rest of animals survived in the whole study process without any complications observed,such as infection,cage migration or breakage of screws/ rods.X ray scan which has been taken immediately after operation show that the rods and screws are in proper position.(2)According to gross inspection,there is no inflammatory reaction around rods and cage.No screw unloosing and rods braking can be observed.(3)At postoperative 12 weeks,obvious radiolucency can be observed in the X-ray images in titanium alloy rods groups.There is only one specimen achieved fusion in titanium alloy rods groups.However,there are three specimens achieved fusion in PEEK rods groups.At postoperative 24 weeks,complete fusion were achieved in both groups with no radiolucent region can be seen in the X-ray images.(4)According to the result of micro CT,bone infiltration can be observed in both groups at 12 weeks postoperatively.Bone volume fraction(BVF)in PEEK rods groups is higher compare to titanium alloy rods groups(BV/TV:20.26±4.36% VS 14.48±3.49%,,p<0.05).BVF is increase over time.At 24 weeks postoperatively,such difference still exist but is not significant(27.95±4.82 %、VS 26.53±4.55%).(4)Rapid bone ingrowth was observed in both groups without evidence of inflammatory according to histomorphometric analysis.At 12 weeks postoperatively,pores of cages were infiltrated by bone matrix with cartilaginous and fibrous tissue take the rest of the space.A higher mineralized bone fraction(MBF)and can be observed in PEEK rods groups(MBF:21.01±3.48% VS 16.73±2.95%).The same trend can be seen in PEEK rods groups concern to bone apposition(BA:16.22±3.24% VS 11.67±3.63%).At 24 weeks postoperatively,bone infiltration progressed consistently with cartilaginous and fibrous tissue reduced continuously in both groups.A similar degree of osteointegration and bone ingrowth was observed between two groups at 24 weeks postoperatively.Conclusions: PEEK rods with a diameter of 3.2 mm can provide sufficient stability for destabilized sheep cervical vertebra.Fixation by PEEK rods can increase axial load of anterior vertebral body compare to titanium alloy rods.The feasibility of applying PEEK rods in cervical vertebra has been proved by animal experiment.Besides,PEEK rods achieve rapider bone fusion and obtained better bone integration quality at early postoperative period.