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Design of Anatomic Locking Plate for Anterior Surface of Posterior Pelvic Ring and Finite Element Analysis

Author: ZhouChengPeng
Tutor: GaoShiChang
School: Chongqing Medical University
Course: Surgery
Keywords: Sacroiliac joint Anatomic lockingplate Reconstruction plate Finite element analysis
CLC: R318.08
Type: Master's thesis
Year: 2013
Downloads: 17
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Abstract


Objective:1.Application of computer-aided design(CAD) technology to create a“point cloud” database of the anterior surface of posterior pelvic ring.2.Design a new anatomical locking plate at the base of “point cloud”database,and research the position and direction of the screw.Completedhole design at the new anatomical locking plate.3.Application of computer-aided engineering (CAE) technology toestablish plate fixation of anterior wall of pelvic ring finite element modeland finite element analysis, comparison the new anatomical lock platesand reconstruction plates fixed stability of the posterior pelvic ring.Methods:1.Establishment “point cloud” database of the front surface ofthe posterior pelvic ring250cases of thin slice CT data of pelviswas collected and imported into Mimics10.01software for three-dimensional reconstruction, to measure the length from pointM,which is the vertex of sacroiliac joint, along the sacroiliac joint line tothe point N,which is at the true edge of pelvis. MN value was divided intothree groups from small to large by percentile, each group’s MN medianas the measure of small, medium and large basis of the posterior pelvicring.The small, medium and large pelvic triangulation reconstruction datawere imported into Imageware13.2software to create three-dimensionalcoordinate system and standard plane.1/20*MN value as a cuttingspacing, the front wall of the pelvis ring was plane cutting along X、Y-axisdirection, the cutting plane and the front wall of pelvic ring intersects inlines,the points of the intersection of the line were as the measuringpoints.Based on the coordinate position of the measuring points and thecutting spacing, we know that the measuring point (X, Y) values.Themeasurement distance of the points to the standard plane, we can obtainthe Z value of the measuring points.Through the above measurement, themeasuring point (X, Y, Z) value can be obtained. The measuring point’sthree-dimensional coordinate values are stored as TXT format and were asthe front wall of pelvic ring’s "point cloud" database.2.Design of the new plate of anterior surface of posterior pelvicringThe "point cloud" database was imported into Imageware13.2software, triangulation reconstruction of the front wall of the posterior pelvic ring. Design cutting points A, B, C, and D in pelvic reconstructionspecimen, Point A,2.5cm away from the sacroiliac joint line,was theupper inside point of the plate. A point was50mm along the X-axis to theouter edge of the plate.D point was the upper outside point of the plate.ADline upwardly inclined with the X-axis and tilting30°.Point B,1.5cmaway from the sacroiliac joint line,was the lower inside point of the plate.Point C was the lower outside point of the plate.BC line downwardlyinclined with the X-axis and tilting15°.Application Imageware13.2software to virtual out the bottom of the plate, and requirements inaccordance with the above cut-off point to cut the plate. We use0.1mm asminimum adjustment unit to adjust the adjustment point in order to fit theplate with sacroiliac joint’s anatomical features. ApplicationSolidworks2010software to pull out the plate’s thickness to4mm, and0.5mm rounded edge of the plate,5mm rounded corner of the plate.Theplate was imported into Mimics10.01software,and was placed in middleand upper of the front surface of the posterior pelvis ring, and researchscrew’s placement position and safe angle.Recorded screw’s two endpoints (X, Y, Z) values, minus the values to obtain screw’s vector(X1,Y1, Z1) values.Take human sagittal plane as reference plane,where α=arctan (X1/Z1),as screw’s intraversion and extraversion angle;Takehuman coronal plane as the reference plane,where β=arctan (Y1/Z1),asscrew’s forward and backward angle.The plate was imported into UG NX8.0software, with reference to the remote radial locking plate (U.S.Smith&Nephew Inc. company) locking screw holes、the ordinary nailholes characteristics and the screw’s simulation results, we cut out thescrew holes.Where an the sacral side,two locking holes A11, A12were atthe first row; one lock hole A21was at the second row; one locking holeA31at the third row.Where at the iliac side, two locking holes B11, B13and an ordinary hole B12were at the first row; two locking holes B21,B22were at the second row; two locking holes B31, B33and an ordinaryhole B32were at the third row.At sacral side, two Kirschner holes C1, C2were added;at the iliac side,four Kirschner holes D1, D2, D3, D4wereadded(Figure20).3.One case of normal adult thin data of pelvis was imported intoMimics10.01software,image deleted the left side of sacroiliac joints andpubic symphysis to separation5mm and three-dimensional reconstructionof the pelvis.Adopt Mimics software’s own Magics9.9software to rebuildthe specimens through reducing the triangular surface, filteringsharp-shaped triangle surfaces, and surface smoothing. Thethree-dimensional reconstruction of the pelvis model was imported intoGeomagic Studio9.0software. After points phase,wrap phase,polygonphase and shape phase processing,we obtain mesh grid of the pelvis, savedinto IGS format.Referring to the Zimmer reconstruction plate, weapplicated Solidworks2010software and UG NX8.0software to design virtual reconstruction plate and pre-bending.Pelvis’s geometricmodel,reconstruction plate’s geometry model, the new anatomic lockingplate’s geometry model were imported into Autodesk InventorProfessional2012software, we assembled two different methods ofinternal fixation.Two internal fixation models were imported intoHypermesh10.0software, we checked the model mesh surface’s holes andpatched.We used Hypermesh software’s Tetra Mesh function to completethe the pelvis fixed model’s meshing, and saved into Inp format file.4.Both fixed mesh model were imported into Abaqus6.11software,given the pelvis, plate and screw material property,and set the screw andthe bone as a binding connection.Vertical direction static analysis of thehuman pelvis: the acetabular fossa was fixed and limited six degrees offreedom of activities.500N axial compressive load was applied on the topsurface of sacral one vertebrae in the two types of plate fixed pelvicmodel,respectively, and load uniformly distributed over the top surface ofthe sacral vertebrae. Front and back direction static analysis of the humanpelvis: the back of the sacrum was fixed and limited six degrees offreedom of activities.500N axial compressive load was applied on thefront surface of pubic united in the two types of plate fixed pelvicmodel,respectively, and load uniformly distributed over the frontsurface.of the pubic united.Side direction static analysis of the human pelvis: the outermost edge of the surface of the right iliac was fixed andlimited six degrees of freedom of activities.500N axial compressive loadwas applied on the the outermost edge of the surface of the left iliac in thetwo types of plate fixed pelvic model,respectively, and load uniformlydistributed over the outermost edge of the surface of the left iliac.Analysisvon Mises stress and strain distribution of the two internal fixation modelin the case of three loading situation.Results:1.By measuring the250cases of normal human pelvis specimens,MN’s maximum value is71.27mm, minimum value is36.54mm. Themedian value of MN of big、middle、small group were:54.13mm,48.7mm,43.11mm. Application imageware13.2Softwares, measuring the verticaldistance from the measuring point to the standard plane, and earn the Zvalues of the measuring point. Combining the values of the measuringpoint (X, Y), we obtain the three-dimensional coordinates (X, Y, Z) valuesof the measuring point, the datas were stored in the text file,which is the"point cloud" database of front wall of posterior pelvic ring.2.The "point cloud" database was imported into Imageware13.2software, triangulation reconstruction of the front wall of the posteriorpelvic ring. We design the bottom of plate at the basis of reconstructionspecimen of anterior wall of posterior pelvic ring.We cut out the shape of the plate in accordance with the cutting points, and adjust the adjustmentpoint in order to fit the plate with sacroiliac joint’s anatomical features.The bottom of the plate was imported into Solidworks2010software andwas pulled out a4mm thickness, and added the rounded edges androunded corner features. Small, medium, large plate having upside lengthof52mm,52mm,52mm,respectively; having sacral side length of28mm、31.5mm、33mm,respectively;having downside length of36.5mm、37mm、40mm,respectively;having ilium side length of60.5mm、65mm、65mm,respectively.The angle between the extension lines of top andbottom sides of small, medium, large plate are all45°.Plate wasimported into Mimics10.01software, and placed in middle and upper ofthe front surface of the posterior pelvis ring.Adjustment screw introversionand extroversion to avoid the sacroiliac joint and sacral foramina, andforward and backward to avoid the screws "fight". We got:Where at thesacral side,two locking screws A11, A12were at the first row,screw A11forward25°,introversion9°;screw A12forward30°,introversion20°;onelocking screw A21was at the second row,screw A21forward25°,introversion15°;one locking screw A31was at the third row,screw A31forward40°,introversion25°。Where at the iliac side, two locking holesB11, B13and an ordinary hole B12were at the first row,screw B11forward35°,introversion20°; screw B12perpendicular to the plane of theplate; screw B13forward40°,extroversion30°. Two locking holes B21, B22were at the second row,screw B21forward30°,introversion15°;screw B22forward35°,extroversion20°.Two locking holes B31, B33andan ordinary hole B32were at the first row,screw B31forward40°,introversion10°; screw B32perpendicular to the plane of the plate; screwB33forward30°,extroversion15°。At sacral side, two Kirschner holes C1,C2were added,C1, C2were all both forward30°,introversion20°;at theiliac side,four Kirschner holes D1, D2, D3, D4were added,D1, D2, D3,D4were all perpendicular to the plane of the plate.Reference to the lockscrew hole features of remote radial locking plate (U.S. Smith&NephewInc. company), the ordinary nail hole characteristic and design result ofthe screw direction,we used UG NX8.0software added screw holes to theplate.Locking hole was the taper hole, which had the large diameter for5.5mm and5°taper.Common hole was the simple hole, which had thediameter for6mm.The Kirschner hole was the simple hole,which had thediameter for1.6mm.3.We successfully constructed finite element analysis model ofanatomic locking plate fixed and reconstruction plate fixed of pelvic ring.4.Vertical direction static analysis of the human pelvis,we get vonMises stress, strain cloud images of the two internal fixation model(Figure7、8).Maximum displacement of anatomic locking platemodel:5.056×10-2mm,the maximum stress of the pelvis is:11.42MPa,the maximum stress of the Anatomic locking plate:41.58MPa.Maximumdisplacement of reconstruction plate model:1.205×10-1mm,the maximumstress of the pelvis is:43.43MPaMPa,the maximum stress of thereconstruction plate:196.4MPa.Front and back direction static analysis of the human pelvis,we getvon Mises stress, strain cloud images of the two internal fixation model(Figure9、10).Maximum displacement of anatomic locking platemodel:1.978mm,the maximum stress of the pelvis is:157.5MPa,themaximum stress of the Anatomic locking plate:300.4MPa.Maximumdisplacement of reconstruction plate model:24.93mm,the maximum stressof the pelvis is:450.3MPa,the maximum stress of the reconstruction plate:3282MPa.Side direction static analysis of the human pelvis,we get von Misesstress, strain cloud images of the two internal fixation model(Figure11、12).Maximum displacement of anatomic locking platemodel:3.377mm,the maximum stress of the pelvis is:309.6MPa,themaximum stress of the Anatomic locking plate:457MPa.Maximumdisplacement of reconstruction plate model:33.59mm,the maximum stressof the pelvis is:671.7MPa,the maximum stress of the reconstruction plate:3686MPa. Conclusion:1.It is practicable to use reverse engineeringtechnology and computer-aided design (CAD) technology to get "pointcloud" database the front surface of the posterior pelvis ring and designthe anatomic locking plate of the anterior wall of posterior ring ofpelvis.2.New anatomic locking plate is significantly stability thanreconstruction plate fixing pelvis,which is Tile C1type fracture.

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