Biomechanical Evaluations Of Integrated C1 Laminar Hooks For C1-C2 Fusion

BackgroundTrauma,tumor,deformity,and inflammation of the upper cervical spine can cause atlantoaxial instability,and atlantoaxial fixation should be performed for the affected region.Previously,wiring techniques and laminar clamps were mainly used for fixation,the main wiring methods being the Brooks and Gallie techniques.Halifax clamps were also used for single-point fixation.At present,C1-2 transarticular screws or short-segment screws fixation is mainly used for atlantoaxial posterior stabilization surgeries.The past biomechanical tests and clinical researches have demonstrated that these fixation techniques can enhance the stability of atlantoaxial articulation and improve the bone graft fusion.It's universally acknowledged that the bilateral transarticular screws combined with posterior wiring fixation and bone grafts is the gold standard of atlantoaxial fixation at present.However,there is risk to damage spinal cord while passing sublaminar wire under C1 arch.Thereby,according to atlantoaxial anatomical features,a integrated C1 laminar hook was designed,which is linked to C1-2 transarticular screws or C2 pedicle screws that it is more convenient and safer for the surgery.There is still no in vitro biomechanical study to evaluate the biomechanical feasibility of this method.Spine has a complex structure,and in vitro human cadaveric biomechanics study plays an important role in the study of spine biomechanics study.However,it's not easy to keep the specimens of human cadaveric,and it's also difficult to get.Because of the biomechanics of individuals difference itself,the accuracy of experimental result can be influenced greatly.With the development of computer technology and the improvement of infinite element research method,three-dimensional finite element can not only simulate the bony structure of spine,but also simulate the surrounding ligaments and structures of spine.It can not only simulate a complete spine model,but also regression and trauma spine models to have biomechanics study.Various internal fixations can be loaded upon spine models.The biomechanics characteristics of internal fixation can be evaluated to offer help to the design of internal fixation.Three-dimensional finite element method can be used to have mathematical simulation on spine structure and loading condition,and understand the impact on the whole spine structure by changing running parameters arbitrarily.At present,the action and stress of spine which are close to the normal condition of human body can be simulated,and the result is more reliable,which is an important tool used to study spine biomechanics.Based on the study of in vitro biomechanics,this task is to have three-dimensional finite element models on the upper cervical spine of normal human beings,verifying the availability.Besides,the biomechanics stability of integrated C1 laminar hook combining with C1-2 transarticular screw or C2 pedicle screw internal fixation entrapment bone graft is studied by the method of three-dimensional finite element,including internal fixation in dynamic process,stress distribution of bone grafting,and the mechanism of biomechanics is demonstrated,which offers theoretical direction to clinic treatment.Objective1.To estimate the biomechanical stability of a integrated C1 laminar hook combined with transarticular screws or C2 pedicle screws in comparison with established techniques.2.Three-dimensional finite element model of upper cervical spine was built on normal human beings,and the availability was verified.Besides,internal fixation in dynamic process and the stress distribution of bone graft block were studied by the method of three-dimensional finite element.The mechanism of biomechanics is demonstrated,which offers new ideas to the design of internal fixation instrument and theoretical direction to improvement.Methods1.7 fresh-frozen human cadaveric cervical spines(C0–C3)were selected to have pure moments loading and the range of motion(ROM)of C1-C2 was measured in flexionextension,lateral bending,and axial rotation.The specimens were tested in six conditions:(1)intact(Intact),(2)II-type odontoid fracture(Destabilized),(3)C1 lateral mass screws combined with C2 pedicle screws fixation and bone graft(C1+C2),(4)C1-2 transarticular screws combined with Gallie fixation and bone graft(TA+G),(5)novel integrated C1 laminar hooks combined with C2 pedicle screws fixation and bone graft(P+H)and(6)novel integrated C1 laminar hooks combined with C1-2 transarticular screws fixation and bone graft(TA+H).2.Spiral CT was used to have cervical spine thin-slice scan on healthy and young male volunteers and extract CT scanning data to make modeling,by which upper cervical spine(C0-C3)three-dimensional finite element models were obtained.According to the fixed form of in vitro biomechanics study,instable models were made and different internal fixations were loaded to measure stress value and analyze the distribution of stress and estimate the biomechanical stability of a novel integrated C1 laminar hook combined with transarticular screws or C2 pedicle screws.Results1.As for Intact and Destabilized,fixation constructs can improve atlantoaxial stability obviously.For the flexion-extension,lateral bending,axial rotation,ROM of P+H is the maximum in the four fixation models.When in flexion and extension,right and left lateral bending,the difference of ROM value of Harms,TA+G,P+H and TA+H has no significant statistical significance(P>0.05).When in axial rotation,There was statistically significant difference between P+H and other three fixation models(P<0.05).2.According to the CT of volunteers,the three-dimensional finite element models of complete upper cervical spine C0-C3 were built successfully,and the availability was verified.According to the relevant parameters of actual internal fixation,various threedimensional finite element models of internal fixation were built and six models was established:(1)intact(Intact),(2)II-type odontoid fracture(Destabilized),(3)C1 lateral mass screws combined with C2 pedicle screws fixation and bone graft(C1+C2),(4)C1-2 transarticular screws combined with Gallie fixation and bone graft(TA+G),(5)novel integrated C1 laminar hooks combined with C2 pedicle screws fixation and bone graft(P+H)and(6)novel integrated C1 laminar hooks combined with C1-2 transarticular screws fixation and bone graft(TA+H).3.The corresponding ROM(range of motion)was calculated by the method of displacement nephogram.The result showed that ROM value of right and left lateral flexion,and right and left rotation were TA+G < TA+H < C1+C2 < P+H < Intact < Destabilized,which were consistent with the result of biomechanics.4.The internal fixation material and bone grafting stress were studied through stress nephogram that when TA+G and TA+H were fixed,stress nephogram showed that TA screw stress was mainly concentrated on the passing C1-2 joint of screw.Besides,when it's in rear protraction,stress increases,and small when it's in lateral flexion and rotation.The stress of pedicle screw fixation is mainly concentrated on the interface between screw and bone.Besides,stress is large when it's in right and left lateral bending,and right and left rotation,and stress decreases when it's in flexion and extension.The stress of integrated C1 laminar hook was mainly concentrated on the joint between integrated C1 laminar hook and C1-2 transarticular screw or C2 pedicle screw.C1 laminar hook and rod integration were in unstressed concentration.When it's in flexion and extension,right and left lateral flexion,right and left rotation,the stress of TA+G and Harms fixed instruments is small,and the stress of TA+H and P+H is big.Besides,in TA+Gallie,TA took the minimum stress,and compared with the bearing stress of other internal fixation,Gallie took the maximum stress.The main bearing stress of bone block is in the contact part between bone block and atlantoaxial.When P+H was fixed,bone block took the maximum bearing stress when in flexion and extension,right and left lateral flexion,right and left rotation.For different internal fixation models in rear protraction,the stress of bone grafting block increased compared with anteflexion,which was consistent with reality.Conclusions1.Novel integrated C1 laminar hook fits atlantoaxial anatomical structure well,and it's more convenient and safer when it's connected with C1-2 transarticular screws or C2 pedicle screws.When it's combined with C1-2 transarticular screw and bone graft,biomechanical stability which is similar to C1-2 transarticular screws combined with Gallie fixation and bone graft.The novel integrated C1 laminar hook combined with C2 pedicle screws and bone graft is poor to control rotary motion,which can be regarded as an alternative technique that meets the requirements of which is not suit for C1-2 transarticular screws fixation.2.There is no significant stress concentration in Novel integrated C1 laminar hooks,and internal fixation crack can be avoided effectively.However,the stress of TA screw is big,and attention should be paid to the potential crack of TA screw.Integrated C1 laminar hook can restrict the motion of bone grafting block of entrapment,which makes it efficient to avoid bone block displacement and detachment.When bone grafting block is in P+H model,the rear protraction of all the fixed models increases compared with bone grafting stress in anteflexion,which can better promote bone graft fusion.