A New Occipital Internal Fixation System Design、biomechanical Analysis And Preliminary Clinical Research

BACKGROUND:The diagnosis and treatment with the instability of the occipital cervical junction is always one of the difficulties in spine surgery.Trauma,inflammation,tumor and congenital malformations are one of the important reasons for the instability of the anatomical site.The more serious the occipitocervical junction instability can cause atlantoaxial dislocation,even basilar invagination etc.So as to oppress the brain stem and spinal cord,causing numbness,weakness and paralysis.By this case,it is often necessary to intervene through surgery.The purpose of the procedure is to restore the normal occipital cervical sequence and effective spinal canal volume.Release of the brainstem or spinal cord compression to promote recovery of spinal cord function and to reconstruct the stability of the junction of the cranio-cervical through effective fixation and bone grafting.As the children’s skeletal development is not yet mature,Conventional adult internal fixation system can not be used as usual,children patients with cranio-cervical junction diseases were always treated by wire fixation、cranio-cervical pedicle screw fixation or transarticular screw fixation through posterior approach.Because of its soft bone,steel wire fixation is easy to cause bone fracture,fixation unfirm,fixation loosening,thus affecting the surgical effect.Post-operation always need to use Halo external fixation until complete bony fusion.Halo external fixed frame wear is very painful.Patients usually difficult to self-care and cooperation.The incidence of complications such as screw track infection is high,Patients and their families are often difficult to accept.At present,special for children with cervical spine internal fixation system products are very scarce in the market.Spine surgeons who admissions to the patients often feel at a loss what to do.In recent years,the U.S-Johnson and other international brands of spinal instrumentation have developed a small number of children cervical fixation products.However,has not yet entered the domestic market.According to foreign information,Johnson children occipital-cervical fixation system is include 3.5mm pedicle screws and occipital plate with 2-3 screws.This system was only reported to use for older than 7 years white children,for domestic children is not known yet.In view of the present situation at home and abroad,Some clinicians who do operation on these children can only use adult instruments for substitute.That can cause difficulties and inconvenience during operation.The surgeons who have ever used domestic adult internal fixation system for instead do the operation for the children with cranio-cervical diseases.However,They often deeply aware of the limitations and shortcomings with the current research and development of medical devices for the children’s cervical junction.For example,the following problems are often found in clinical surgery:① For the elder one who have better development can be implemented atlantoaxial pedicle screw fixation with 3.5 mm screws.But for children under 5 years old are not appropriate,because there is no other choice,The operation can only be used to penetrate cortical bone by the long screws or use other methods such as laminar screw fixation.②The occipital bone of the children whose age under 7 were significantly thinner than the adult.For adult,if fixed the midline of occipital,the length of screw can be up to 10 mm,2 or 3 screws can be obtained very strong fixation.But for the children who under 7,the maximum thickness of the midline is sometimes less than 5mm,only 2 or 3 screws fixation is difficult to obtain sufficient mechanical strength.Thus,we have to improve design:to increase anchor numbers and expanded fixed fusion area.In order to solve this problem of children’s fixation,how to develop a suitable fixation system and meet the surgical needs of this special part of the population is extremely urgency.At present,despite the rapid development of the domestic medical instrument industry,many of the previously imported spinal surgical instruments are now mostly manufactured domestically.However,the research and development of the instrument for this special population is stand still.China is one of the big country in the world,The population of Children is reached 2-3 billion.Although the incidence of the cranio-cervical junction disease is only 0.01～0.02%,the total population is reach 40～60 thousand.This is a huge number also.However,Guangdong is a large population province,The number of children with this disease is not a minority.According to the characteristics of domestical children’s bone,do research and development of special instrument for cervical spine surgery and provide better surgical equipment for children is not only not only has important social benefits,but also can create enormous economic value.OBJECTIVE:Atlantoaxial diseases which caused by occipito-cervical junction trauma,fracture dislocation and congenital malformations are often need to be carried out the atlantoaxial internal fixation or occipito-cervical fusion surgery.However,There are no corresponding research and development in the relevant instrument for these children.The spine surgeon had to choose unsuitable instrument for the surgery,The results caused a series of adverse conditions,such as the difficulty of clinical operation,the increase of risk and postoperative failure incidence.In this study,we developed a special internal fixation system for the children with cranio-cervical junction diseases and solved the surgical problems of this special group.The invention of this system has important significance to improve the operation method and perfect the curative effect for these diseases of special population.METHODS:1.Design a new type of internal fixation system with three dimensional software and verify the mechanical validity of this system with finite element analysis.Design concept:A novel occipitocervical internal fixation system,named the Aliform Occiputal-Cervical Plate(AOP),is uniquely designed to mimic the leaves of a clover.The use of screws within its lower notches produces relatively little soft tissue damage during surgery.In order to improve the strength of fixation,We intend to increase the occipital plate rivet point to improve its anti pullout force and with Sufficient initial mechanical strength.Mechanical validation:CT(computer tomography)data from the occiput-C3 region,was obtained from a healthy male volunteer.The C0-3 data were stored in DICOM format and were imported into the Mimics 10.01 software program.CT(computer tomography)greyscale was used to differentiate bone and soft tissue.Noise was removed from the model,and the model was paved and smoothed using Geomagic Studio 2013.Then,the geometric model was imported into Solidworks 2012,which was used to evaluate and design the intact FE model that included the two occipitocervical internal fixation systems.ANSYS 13.0 software was used to produce a three-dimensional finite element model for element setup,material properties defi-nition and FE analysis.The same boundary and loading conditions were applied to the two models.Finally,the mechanical properties of two internal fixation systems were analyzed by FEM data.2.Six cadavers of occiput-C2 were fixed using AOP and Summit fixation system to evaluate their three-dimensional stabilityChosen six cadavers,whose three dimensional motion range of the occipito-cervical was measured on a three dimensional motion experiment machine.Then,the specimens were tested in two groups:one was fixed by AOP system,the other was fixed by Summit system.Finally,The 3D biomechanical stability of atlantoaxial segments were measured and compared.3.The clinical effect was observed in one child applied with the AOP systemA set of occipito-cervical fixation device with low-profiles and multi-anchor points was designed according to pediatric anatomic characteristics.This AOP system was applied for a 2-year-old child with atlantoaxial dislocation.Evaluate its clinical effect by follow up.RESULT:1.Finite element analysis of two sets of internal fixation system:The maximum stress was under the loads of extension,which was less than the yield strength of titanium alloy.The maximum stress of the occipital in the 3-screw group and 7-screw group increased by 16.5%、15.0%,while the 11-screw group decreased by 15.6%,compared with the Summit group.The peak values of screw stress decreased gradually with increased number of screws in the new occipital internal fixation system,which was even less than in the Summit system.The peak values of displacement of occipital were under thle loads of extension,which was decreased by 10.0%,11.4%,11.8%,respectively,compared with the Summit group.The maximum displacement of occipital showed a decreasing trend with increased number of screws.2.Biomechanical analysis of fresh cadaver specimens with two sets of internal fixation system:During the test with this two internal fixation system,The maximum motion was under the loads of extension.There was no statistical difference between two fixation methods for any motions.Summit system fixation was significantly more stable than the 3-screw group and 7-screw group.However,3-screw group and 7-screw group didn’t show fixation failure.3.The equipment was co-designed by 3D software and manufactured in collaboration with Xiamen Double Medical Instrument Co.,Ltd.This system is mainly composed of an aliform plate,pedicle screws and titanium rods.That was applied for a 2-year-old child with atlantoaxial dislocation.Postoperative results showed that atlantoaxial reduction was obtained,spinal cord compression was relieved and fixation placement position was satisfactory.The patient was followed-up for 1 year,during the follow-up,spinal function was improved significantly,and normal walking function was restored.CONCLUSION:1.In this study,two occipito-cervical internal fixation devices were evaluated,and both systems provided sufficient strength and instant stability during occipito-cervical fixation.The use of screws within its lower notches produces relatively little soft tissue damage during surgery.The newly developed occipital internal fixation device provides the option to select screw rivet points and offers clinicians an attractive alternative to the SUMMIT system.2.Both internal fixation systems can provide reliable mechanical strength.But with the thin occipital bone and immature children,choosing a new system can not only reduce the risk of penetrating the meninges of brain,but also increase the postoperative mechanical stability.3.Novel Aliform-style occipitocervical fixation system could improve the occipital fixation rigidity,and is suitable for underage children who have to receive upper cervical spine surgery.This instrument deserved to apply in surgical treatment.