Digital Anatomy Of Modified Sacroiliac Screw Technique And Its Finite Element Analysis And Biomechanical Check

Part 1 Three-dimensional digital anatomical measurement of modified sacroiliac screwsObjective To rebuild a model of the pelvis and effectively simulate the trajectory of modified sacroiliac screws,we measured the parameters of each screw and screw channel and assessed the safety and feasibility of the parameters in adults,to provide a new method of screw placement for sacroiliac complex injuries.Methods One hundred(50 males and 50 females)normal adult pelvic CT(computed tomography)images were randomly selected and imported into Mimics 20.0 software to rebuild the three-dimensional pelvis model.In these models,each ideal channel of modified screws was simulated,and then we obtained the precise parameters of screws and channels using Mimics 20.0 and Three-matic 13.0 software,SPSS(Statistical Product and Service Solutions)25.0 was applied for statistical analysis of various parameters.Results(1)The results of the comparison(right vs.left)showed that there were no significant differences(P>0.05)in any of the angles,radius or M1SI parameters(the first modified sacroiliac).However,one parameter(BS)of M2SI(the second modified sacroiliac),two parameters(AP and BS)of M3 SI(the third modified sacroiliac),and three parameters(AP、BS、L)of M4SI(the fourth modified sacroiliac)were statistically significant(P<0.05).(2)The result of comparison(between genders)showed that there were no significant differences in M1SI and M2SI;in contrast,the radius,length and the α angle of M3 SI and M4SI were significantly different between genders(P<0.05),and the radius of M4SI required special attention.If the radius of the limiting screw channel was>3.50 mm,52 cases(52%,24 males and 28 females)could not complete the M4SI screw placement among 100 samples.If the radius of the limiting screw channel was>3.0 mm,a total of 10 cases(10%,2 males and 8 females)could not complete the M4SI screw placement.Conclusion(1)Through the measurement of 100 healthy adult real threedimensional pelvic models,we obtained the parameters of each modified sacroiliac screw and measured the three angles of each screw based on international coordinates for the first time,which can instruct clinical application.(2)There are M1SI-M4SI screw placement channels in the adult pelvis,and only one ideal screw with the maximum radius can be inserted.(3)There is a radius requirement for M4SI implantation,which should be measured in advance before surgery.Part 2 Establishment and verification of three-dimensional finite element model of pelvisObjective Pelvic CT data were collected from a healthy adult volunteer,threedimensional finite element pelvic model with high simulation of pelvic bone,ligament,sacroiliac joint cartilage and pubic symphysis was established by reverse engineering software,after verification,it provides a reliable three-dimensional digital finite element model for the biomechanical study of pelvic internal fixation and pelvic fracture.Methods Selected a healthy adult volunteer,Imaging examination ruled out bone tumor,tuberculosis,traumatic fracture,congenital pelvic structural abnormalities and other conditions,pelvic data obtained after spiral CT scan(SAVED in Dicom format),This data was imported into Mimics software to reconstruct a three-dimensional digitized model of the pelvis,saved in "Stl" format and imported into Geomagic Studio reverse engineering software,after various commands and operations,obtained the"NUBS" surface of two hips and one sacrum,it was saved in "Step or Stp" format and imported into Solidworks software for materialization of pelvic bones,the construction of "pubic symphysis and sacroiliac articular cartilage" was completed and assembled in this software,Saved the file in "X-T" format and imported it to "Ansys Workbench",ligaments were created by "Spring" according to the starting and ending points of the pelvic ligament and meshed simultaneously,material values were assigned to each different part of the pelvis and boundary conditions were constrained,applied a certain load,set the analysis step,and viewed the analysis results of strain,stress,and displacement distribution of the pelvis after solving.Results(1)By using reverse engineering software,we established a threedimensional finite element model of pelvis with high simulation including bone,ligament and articular cartilage,including 1269162 nodes and 881652 units;(2)There was no statistical difference in the distance between the original pelvic model and the optimized three-dimensional finite element pelvic model in the landmark structure,that is,the real shape and size of the pelvis did not change significantly;(3)By applying load on the sacrum,we obtained the displacement and torsion angle of the sacroiliac joint,which are basically consistent with the results of cadaveric mechanical experiments reported in the relevant literature;(4)The stress conduction direction of the established three-dimensional finite element pelvic model was from the sacral wing through the bilateral sacroiliac joints along the iliac arcuate line to the bilateral acetabulum(standing position)or the ischial tuberosity(sitting position),which was consistent with the mechanical conduction direction reported in the literature;(5)The strain of the pelvic model began to change around the sacrum and gradually weakened to both sides,and the iliac bone strain on both sides was basically symmetric.The results obtained were not much different from previous studies on the pelvic finite element model,and the established model was effective.Conclusion The three-dimensional finite element model of pelvis established by reverse engineering software objectively and truly reflects the morphology and biomechanical data of cadaver biomechanical pelvis.The established model is real and effective,and can be used for subsequent application and research.Part 3 Finite element analysis of modified sacroiliac screw and common sacroiliac screwObjective A finite element pelvic model was established to simulate the right sacroiliac joint injury.Modified sacroiliac screws and common sacroiliac screws were inserted:(1)The strain of two kinds of pelvic models after screw fixation was compared and analyzed by finite element model of pelvis;(2)The stress and distribution of pelvis and screws after two kinds of screw fixation were compared and analyzed by finite element model of pelvis;(3)According to the experimental data of finite element model,the fixing efficiency and fatigue resistance of the two kinds of screws were analyzed and summarized.Methods Adult male volunteers were recruited and DICOM data were obtained by CT scanning of the pelvis.The data were imported into Mimics software to establish the pelvic bone mask and re-establish the three-dimensional pelvic model,which was saved in "STL" format and imported into Geomagic studio software at the same time.After the commands of "relaxation,fast smoothing","Delete the nail","redraw the grid","detect the contour","construct the surface piece","construct the grid",and so on,the surface was finally fitted to form the NURBS surface,In this software,the "offset"function was used to generate "cancellous bone",and the two hip bones and sacrum were saved in the format of "Wrp"or "Stp/Step",and then imported into Solidworks software one by one for diagnosis.The real "cortical bone" was generated by"combination" function,and the sketch was created.Generated "sacroiliac articular cartilage,pubic symphysis,"etc,created screws in the sketch,according to the screw entry point,all the pelvic entities and screws were assembled,and the fixed working conditions of eight screws were divided and checked without "interference".After saving in "X-T" format,the screws were imported into Ansys 19.0 software to define material properties,assign material values,optimize mesh,set boundary conditions,and constrain bilateral ischial tubercles.Load 500N vertical upper sacral endplate load.After all the Settings are completed,the Job is submitted and solved by JOB,and the analysis results are completed.Results After finite element solution with Ansys 19.0 software,we obtained the following results for eight working conditions,The global displacement of the pelvis;Pelvic maximum stress value and stress distribution;Maximum stress value of screw;The maximum displacement,average displacement and their difference values of the right sacroiliac joint surface on the hip side and the sacral side;The displacement values of the sacrum in the direction of vertical load were compared according to the three conditions of single screw,two screws and mixed.Except for M3 SI,the values of single or two modified sacroiliac screws were better than those of ordinary sacroiliac screws in the above results.Conclusion(1)In the finite element model,the fixation efficiency of single modified sacroiliac screw(except M3 SI)and two modified sacroiliac screws was better than that of common sacroiliac screws with the same number of screws;(2)Compared with the common sacroiliac screws,the modified sacroiliac screws have more reasonable overall stress distribution,smaller maximum stress value,stronger fatigue resistance and less breakage.Part 4 Biomechanical study of modified sacroiliac screw and common sacroiliac screwObjective The sacroiliac complex is the most important stability structure of the posterior pelvic ring,and whether fracture or dislocation is not given appropriate internal fixation,it will lead to serious sequelae,i.e.Among various internal fixation methods,percutaneous sacroiliac screw fixation is the most popular posterior ring fixation method at present.However,the placement of this screw is demanding,the learning curve is long,the screw tolerance rate is low,and injuries to important blood vessels and nerves in the pelvic cavity are likely to occur with low safety.In this experiment,modified sacroiliac screws and common percutaneous sacroiliac screws were tested for biomechanical stability on pelvic cadaveric specimens,and the biomechanical stability of the two screws was compared to provide a reference for clinical practice.Methods Six normal adult embalmed pelvic specimens(4 males and 2 females provided by Binzhou Medical College)were collected.The cadaver was transected at the level of the fourth lumbar vertebrae and the middle and upper 2/3 of both thighs,the lumbar vertebrae and soft tissues were removed,the hip joint was severed,and the muscles and fat attached to the pelvis,as well as the soft tissues of the pelvic cavity and pelvic floor viscera were carefully removed.The symphysis pubis,bilateral sacroiliac joints and their sacroiliac ligament tissues were kept intact,and the ischial tuberosity was fixed with dentate powder.The pelvis was placed on the Instron biomechanical experimental machine to simulate the normal sitting posture of human body.The displacement of the sacrum relative to the ilium was measured by applying 500N vertical pressure from the upper endplate of the sacrum under the following conditions:(1)Vertical displacement and Angle of the sacrum of the intact pelvis under 500N load;(2)The model of sacroiliac joint dislocation was established,and the vertical displacement and Angle of the sacrum were inserted after modified sacroiliac screws with different fixation methods and common sacroiliac screws with different fixation methods;(3)The original data were statistically analyzed by SPSS software to compare the statistical significance between different fixation methods of modified sacroiliac screws and ordinary sacroiliac screws.Results(1)The stiffness value of the pelvic model can be obtained under the vertical load of 500N:pelvic A=187.97N/mm;Pelvis B=124.69 N/mm;Pelvic C=148.85 N/mm;Pelvic D=170.65 N/mm;Pelvis E=161.29 N/mm;Pelvic F=200.16 N/mm;Rotation Angle of sacrum relative to ilium in sagittal plane:pelvis A=1.63°;Pelvic B=2.75°;Pelvic C=2.55°;Pelvic D=2.01°;Pelvic E=2.23°;Pelvic F=1.42°;(2)Displacement values under a vertical load of 500 N:M1SI=3.23±0.97 mm;M2SI=4.12±1.16 mm;M1SI+M2SI=2.97±0.65 mm;CISS1=3.51 ± 1.13 mm;CISS2=4.16± 0.91 mm;CISS1+CISS2=3.27± 0.43 mm;angle of sacral rotation relative to the ilium in the sagittal plane:M1SI=2.23°±0.34°;M2SI=2.31°±0.43°;M1SI+M2SI=1.39°±0.21°;CISS1=2.74°± 0.56°;CISS2=2.97°±0.16°;CISS1+CISS2=2.10°±0.47°;(3)Compared M1SI+M2SI with M2SI,t=2.82 p<0.01;compared M1SI+M2SI with CISS2,t=3.66 p<0.01;compared M1SI with CISS2,t=2.65 p=0.01;compared CISS1+CISS2 with CISS2,t=2.70 p=0.01.There was significant difference in screw fixation among the above four groups,but there was no significant difference between the other screws.Conclusion(1)The modified sacroiliac screw was successfully implanted on pelvic cadaveric specimens,and the biomechanical stability experiment was conducted to confirm the existence of the modified sacroiliac screw channel,providing a new method and idea for the implantation of sacroiliac screw;(2)Most of the experimental data of the two groups with different fixation methods were not statistically significant.However,in terms of compression displacement reflecting the stability of the screws,the compression displacement of the modified sacroiliac screw was smaller than that of the common sacroiliac screw,which proved that the modified sacroiliac screw was more stable than the common sacroiliac screw,and the modified sacroiliac screw was safer to be inserted.(3)The conclusions of this part of the study are consistent with the conclusions of the finite element analysis,namely,the modified sacroiliac screw is superior to the ordinary sacroiliac screw in terms of screw fixation efficiency,screw breakage rate and surgical risk.