| Objective:1.To establish the “point cloud” database of the surface of acetabular posterior column and wall in male and female anddevelop a new anatomical locking plate for fixation of different types of acetabular posterior column and wall fracture.2.To construct three internal fixation models for fixation of acetabular posterior column and wall fracture,including new plate model,double plate model and single plate + spring plate model.Then to compare the fixation stability of three models by using finite element method.Methods:1.Establishment the “point cloud” database of the surface of acetabular posterior column and wall200 cases of CT data including 100 males with the average age of 48.5 and 100 females with the average age of 50.8 were collected.Suffering from pelvic acetabular fracture or bone tumor were excluded from this study.The three-dimensional models of200 pelvises were reconstructed by Mimics 17.0 software.The arc lengths from the highest point A of acetabular rim to the lowest point B of acetabular rim were measured in male and female.The fiftieth pelvises of male and female,respectively,were defined as the standard pelvisesaccording to the ascending order of AB values.The point cloud files of both standard pelvises were exported as the “point cloud” database of acetabular posterior column and wall.2.Design of the new plate of acetabular posterior column and wall The “point cloud” database was imported into Geomagic Studio 13 software,then the triangular mesh of acetabular posterior column and wall was reconstructed with the procession of optimize and smooth.The reconstructed models were saved as STL format into Imageware 13.0 software for the design of the undersurface of new plate.There were two types of new plate,namely high-position type and nonhigh-position type according to the types of acetabular posterior column and wall fracture.The high-position type platewas applied for fixation of the posterosuperior part of acetabular posterior wall fracture,which was consisted of three parts:(1)The posterosuperior part of the acetabulum:the platewas placed in the weight-bearing area ofacetabulum.(2)The acetabular posterior part: the plate covered most of the area of acetabular posterior column and wall.(3)The ischial part: the plateextendedfrom the border between the lower margin of acetabular posterior walland body of ischium to the posterior surface of body of ischium.The nonhigh-position type plate wasapplied for fixation of the posterior part of acetabular posterior wall fracture,which wasalsoconsisted of three parts.The ilium part of the platestarted theproximalilium of acetabular posterior wall,whichwas placed in the lateral of the superior gluteal vascular nerve tracts.The acetabular posterior part and ischial part were same as the high-position type plate.The two undersurfaces of the plate weredrawn on the corresponding position of acetabular posterior column and wall in Imageware 13.0 software and adjusted the control points to make them fit the bone surface of acetabular posterior column and wall.Then the undersurfaces of the plate were saved as IGS format into UG 9.0 software with the procession of thickness,rounded edge and rounded corner.The screw holes were added in three parts of the plate.(1)The posterosuperior part ofhigh-position type plate and ilium part ofnonhigh-positon type plate were added the common elliptical screw holes.(2)The lateral 1/3 area of acetabular posterior part of the plate was added the round locking holes.The directions of round locking screw holes were avoided the hip joint and the extension lines of each screw hole's direction didn't intersect.The inner 2/3 area was added the common elliptical screw holes.(3)The ischial part of the plate was added the common elliptical screw holes.All the directions of common elliptical screw holes were perpendicular to the plate.3.The finite element analysis of the plate The standard pelvic models were imported into Geomagic Studio 13 software to construct the NURBS closed surface models of pelvic,which were saved as IGS format.Then the models were input into Solid Works 2016 software to create high-position and nonhigh-position acetabular posterior column and wall fractures respectively.The new plate,the double plate and the single plate + spring plate were assembled on the appropriate position of the fracture models with the use of SolidWorks 2016 software.These assembled models were input into Hypermesh 12.0 software to generate mesh and saved as Inp format.Then these models were input into Abaqus 6.14 software and the material of pelvis,plates and screws were assigned respectively.The screw and bone were set as a binding connection.The static analyses of the models were as following:(1)Vertical direction: the bilateral acetabular fossae were confined and the activities of the six degrees of freedom were limited.500 N vertical load was applied on the surface of S1 vertebrae in the six models.(2)Anterio-posterior direction: the posterior sacrum was confined and the activities of the six degrees of freedom were limited,then 500 N anterio-posterior load was applied on the anterior surface of pubic symphysis in the six models.(3)Lateral direction: the right acetabular fossa was confined and the activities of the six degrees of freedom were limited.500 N horizontal load was applied on the surface of the left iliac wing in the six models.The von Mises stress and displacement distribution in the six models were analyzed under different loading conditions.Results:1.The average value of AB were(82.00±5.62)mm in male and(73.77±5.07)mm in female,there was significant difference between male and female(P<0.05).The values of AB of fiftieth pelvis in male and female,respectively,were 82.06 mm and 73.05 mm according to the ascending order of AB values.The standard pelvises were obtained and its point cloud files were exported as the “point cloud” database of acetabular posterior column and wall.2.The “point cloud” database was imported into Geomagic Studio 13 software,then the triangular mesh of acetabular posterior column and wall was reconstructed.The reconstructed models were saved as STL format into Imageware 13.0 software for the design of the undersurface of new plate.The undersurfaces of the plate weredrawn on the corresponding position of acetabular posterior column and wall in Imageware 13.0 software and adjusted the control points to make it fit the surface of acetabular posterior column and wall.Then the undersurfaces of the plate were saved as IGS format into UG 9.0 software with the procession of 2.5mm thickness,0.3mm rounded edge and 0.2mm rounded corner.The high-position type plate was consisted of three parts:(1)The posterosuperior part of the acetabulum: the plate was placed in the weight-bearing area of acetabulum with the width of 1.0cm and the length of 2.0cm.The front border of the plate was 2.0cm away from anterior inferior iliac spine and the lateral border of the plate was 0.6cm away from acetabular rim.This part of the plate was added 2-3 common elliptical screw holes,which long diameter was 6mm and short diameter was 4mm.(2)The acetabular posterior part: the lateral border of the plate was 0.6cm away from acetabular rim.The maximumwidths of the plate in male and female were 3.0cm and 2.5cm,respectively.The lateral 1/3 area of the plate was added 5-6 round locking holes,which diameter was 2.7mm and the inner 2/3 area of the plate was added 7-11 common elliptical screw holes as the previous parameters.(3)The ischial part: the length of the plate was approximately 2.0cmwith width of 1.0cm.It was added 2 common elliptical screw holesas the previous parameters.The nonhigh-position type plate was also consisted of three parts.The length ofilium part was approximately 2.0cm with width of 1.0cm.It was added 2-3 common elliptical screw holesas the previous parameters.The acetabular posterior part and ischial part were same as the high-position type plate.The directions of common elliptical screw holes were perpendicular to the plate.The directions of round locking screw holes were avoided the hip joint and the extension lines of each screw hole's direction didn't intersect.3.Six finite element analysis models for fixation of acetabular posterior column and wall fracture were successfully obtained.The fracture line of the high-positionmodel of acetabular fracture was close to the level of the greater sciatic notch with the width of 0.5mm.The fracture fragments were located the posterosuperior part of the acetabulum,including the proximal fragment with the maximumsuperior-inferior diameter of 6.5mm and the maximuminner-lateral diameter of14 mm and the distalfragment with the maximumsuperior-inferior diameter of 18 mm and the maximum inner-lateral diameter of 12 mm.The fracture line of the nonhigh-position model of acetabular fracture was close to the middle part of acetabular posterior column with the width of 0.5mm.The fracture fragments were located the posterior part of the acetabulum,including the proximal fragment with the maximumsuperior-inferior diameter of 6.4mm and the maximum inner-lateral diameter of 19 mm and the distalfragment with the maximumsuperior-inferior diameter of 19 mm and the maximuminner-lateral diameter of 12 mm.The fracture lines of the middle part of inferior ramus of pubis were made in the two fracture models with the width of 0.5mm.The von Mises stress and displacement distribution in the six models were analyzed under vertical static load.(1)Three fixation models for fixation of acetabularhigh-positionfracture: the maximum displacements of new plate and double plate and single plate + spring plate were 0.16 mm,0.12 mm,0.17 mm,respectively.The positions of the maximum displacement were the left iliac crest.The maximum stresses of the pelvis were 24 MPa,15MPa,31 MPa and the maximum stresses of the plate were 48 MPa,127MPa,38 MPa.(2)Three fixation models for fixation of acetabularnonhigh-positionfracture:the maximum displacements of new plate and double plate and single plate + spring plate were 0.17 mm,0.15 mm,0.18 mm,respectively.The positions of the maximum displacement were the left iliac crest.The maximum stresses of the pelvis were 26 MPa,34MPa,39 MPa and the maximum stresses of the plate were 37 MPa,213MPa,113 MPa.The von Mises stress and displacement distribution in the six models were analyzed under anterio-posterior static load.(1)Three fixation models for fixation of acetabularhigh-positionfracture: the maximum displacements of new plate and double plate and single plate + spring plate were 0.19 mm,0.21 mm,0.19 mm,respectively.The positions of the maximum displacement were the fracture position of the left inferior ramus of pubis.The maximum stresses of the pelvis were 45 MPa,72MPa,23 MPa and the maximum stresses of the plate were 5.9MPa,9MPa,9.8MPa.(2)Three fixation models for fixation of acetabularnonhigh-positionfracture:the maximum displacements of new plate and double plate and single plate + spring plate were 0.19 mm,0.22 mm,0.24 mm,respectively.The positions of the maximum displacement werethe fracture position of the left inferior ramus of pubis.The maximum stresses of the pelvis were 46 MPa,72MPa,79 MPa and the maximum stresses of the plate were 8.9MPa,18 MPa,9.5MPa.The von Mises stress and displacement distribution in the six models were analyzed under lateral static load.(1)Three fixation models for fixation of acetabularhigh-positionfracture: the maximum displacements of new plate and double plate and single plate + spring plate were 2.30 mm,4.59 mm,4.85 mm,respectively.The positions of the maximum displacementwere the right iliac crest.The maximum stresses of the pelvis were 168 MPa,330MPa,272 MPa and the maximum stresses of the plate were 268 MPa,683MPa,782 MPa.(2)Three fixation models for fixation of acetabularnonhigh-positionfracture:the maximum displacements of new plate and double plate and single plate + spring plate were 2.04 mm,2.90 mm,5.60 mm,respectively.The positions of the maximum displacement were the right iliac crest.The maximum stresses of the pelvis were 108 MPa,196MPa,269 MPa and the maximum stresses of the plate were 334 MPa,213MPa,113 MPa.Conclusion:1.Acording to the anatomicalcharacteristics of surface of acetabular posterior column and wall,two types of new anatomical locking plate fitted the surface of acetabular posterior column and wall with different gender were designed successfully.2.The new anatomical locking plate was composed of one body and two wings.Thescrew holes of the plate were consisted of common elliptical screw holesand round locking screw holes.The common elliptical screw holes can adjust the screw's direction according to the position of the fracture line,which were added in the two wings of the plate and the inner 2/3 part of the body of the plate.Theround locking screw holes can be used for fixation of the small fracture fragments ofacetabular posterior wall,which were added in the lateral 1/3 of the body of the plate.Compared with double plate and single plate + spring plate,the new plate had advantages of wider coverage and more uniform stress distribution. |
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