The Finite Element Analysis For Indications Selection In The Interlaminar Endoscopic Surgery Based On The Principle Of "Emphasis On Both Tendons And Bones"

Backgrounds:Lumbar disc herniation(LDH)was a certain kind of common diseases for middle age and elderly people and the risk of LDH was increasing with a larger number of aging population.Previously published studies repeatedly proved that the deterioration of biomechancial environment was the most important reason for the generation of lumbar degenerative diseases(LDD).Recently,with surgical techniques development and the progression of surgical instruments,minimally invasive spine surgery has been become a development tendency in the treatment of LDD for its advantages as small incisions,less trauma and quick recovery.As a kind of new minimally invasive spine surgical method,the percutaneous endoscopic interlaminar discectomy(PEID)have been widely used in the treatment of several kinds lumbar disc herniation.For the endoscopic anatomic structures and surgical approach was similar to the traditional posterior open lumbar surgery,the PEID can be seen as the preferred in the training of novice endoscopic spine surgeons.The rinciple of"Emphasis on Both Tendons and Bones" was the classical principle of orthopaedics of Chinese medicine,and there was a credible relation between the "damage of bone structures" and "injury of tendons".And published literatures indicated that in the interlaminar endoscopic surgery(IES),a larger grade of osteotomy could enlarge the of surgical field and decrease the risk of inadequate decompression and damage of nerve roots.Nevertheless,osteotomy,especially facetectomy have been proven to be a risk factor for the deterioration of local biomechanical environment and resulting postoperative complications as adjacent segment diseases(ASD).For such a reason,The investigation for the variation of postoperative biomechanical environment caused by different grades of osteotomy in the treatment of different types of LDH(armpit,preraticular and shoulder types of LDH)could provide theoretical guidance for the selection of indication of novice surgeons in IES.Methods:Finite element analysis(FEA)models from L3 to S1 were constructed by standard curves.Bone structures include cortical,cancellous,and posterior structures reconstructed in our previous studies which were used as the templates for model construction.Different parts of the templates were layered,and the outline in each layer was traced by fitted curves.Nonbony components include intervertebral discs,facet joints,and ligaments.Intervertebral discs were also constructed by curves.Thickness of the cortical and the endplates was set at 0.8 mm,and the gap between the facets was set as 0.5 mm and ligamentum structures were constructed in the FEA preprocessing phase by tension-only cable elements.IES simulation were constructed on the right side of the L4-L5 segment and were divided into the armpit,periradicular,and shoulder types of LDH.4 mm incisions on the annulus were made in models with difffferent types of LDH,respectively;one in the medial border of the pedicle;one in the posterior midline and another one in the midline between the other two.To represent discectomy,one-third of the nucleus was deleted in corresponding regions.To simulate the osteotomy by novice and experienced PED surgeons,13.5 and 11.5 mm diameter circles around the central point of annulus incision were drawn,and osteotomies of the laminar and facet joints were outlined diameter of the endoscopic working tube in our department was 7.5 mm,3 and 2 mm enlargement was defined in all directions to simulate the needed operation spaces for novice and experienced surgeons.Facet cartilages and joint capsules within the range of osteotomy were excised together.In addition,one-third of the ligamentum flavum on the surgical side for armpit and periradicular types and quarter of which for shoulder-type LDH was removed.Models were constructed to be symmetric in the sagittal plane.Different sizes of tetrahedron elements were used in the mesh generation,and the mesh was refifined in thin structures and the structures with large deformation.Average mesh quality was greater than 0.75 in current models and greater than 0.8 in structures with mesh refifinement.Bounded contact type was used for all surfaces except for facet cartilages,in which contact was defifined as frictionless.All degrees of freedom were fifixed below S1,and stress was applied superior to L3.Then,to validate whether current FEA models are adequate representations of real situations in different body positions,the values for range of motion(ROM)in the surgical segment(L4-L5)were computed and compared with those from widely cited in vitro studies under same loading conditions.Biomechanical indicators used to evaluate the risk of ASD were computed under different directions of 10 N·m moments without vertical compression.The intact model was strictly symmetric and placed symmetrically along the sagittal plane,so that lateral bending and axial rotation were computed only in the rightward direction.However,PED models were not symmetric;therefore,they were computed in both leftward and rightward directions.Results:The computational result of model validation indicated that the current lumbo-sacral model could make a good represent for real biomechanical situations,(i.e.the difference between computational bioimechanical indicators with values from widely cited in-vitro studies was less than one standard deviation).Variation of biomechanical indiators indicated that a large ranges of osteotomy will increase the risk of biomechanical deterioration and resulting risk of ASD.In which the von-Mises stress of endplates increased slightly,and the dramatically increase of the annulus maximum shear stress,the intradiscal pressure(IDP)and ROM can be observed in the shoulder type LDH model after a larger grade of osteotomy in IES,and this trend was most signifificant in the shoulder-type LDH model.Conclusion:There was a credible relation between the "damage of bone structures"and "injury of tendons" in the development of LDD.Hence,in order to decrease the risk of biomechanical deterioration and resulting risk of ASD caused by the large grade of osteotomy after IES by novice surgeons,armpit and periradicular types of LDH can be seen as suitable indications for the training of novice PEID surgeons,and shoulder-type LDH should be excluded from such indications until novices can perform PEID within a relatively small range of osteotomy.