Design Of Two Types New Locking Plate For Fracture Of Acetabular Anterior Column And Finite Element Analysis

Objective:1.To create a "point cloud" database of the pelvic by application of reverse engineering technology and design two types new anatomical locking plate by using computer-aided design on the base of "point cloud" database.2.To make finite element models of two types plate fixing the acetabular transverse fracture and normal pelvic by application of computer-aided engineering technology, and to compare the fixation stability by analysis of strain and displacement of the two models.Methods:1. To create the "point cloud" database of the pelvic The CT scan data of 166 cases(90 males and 76 females) of adults pelvis were obtained, who underwent continuous slice pelvic 64-slice computed tomography scan, and imported into Mimics 14.11 software for three-dimensional reconstruction. Then the arc length of the true pelvic edge was measured by segmentation in male and female, respectively, the arc length was defined as AB, and calculated the average of AB value in males and females, respectively. The average of AB was viewed as the average size of acetabular anterior column. We found the pelvic who’s AB value is equivalent the average of AB value, its three-dimensional reconstruction data were exported by "Point Cloud" approach and were saved as TXT format2. To design the two types new plate of acetabular anterior column The "point cloud" database was imported into Imageware 13.2 software and reconstructed. Type one:the true pelvic edge plate, it’s from symphyses to the point where 5mm away from sacroiliac joint. The inner edge of the plate is along the true pelvic edge, and the outer edge is 10mm away from the inner edge. Then 0.5mm as adjustment unit was used to make it fit the plate with the anatomical features of surface of the true pelvic edge, the plate’s thickness was stretched to 2mm by applying UG NX 8.0 software, and 0.3mm rounded edge and 2mm rounded corner of the plate were carried out. One screw hole that the diameter is 4mm was designed on the plate every 12mm space, the number of the holes are 10 and 11 in male and female, respectively. For the direction of holes of the true pelvic edge plate:the direction of the first and the second hole parallel to sagittal plane, the angle between screws and coronal plane were on the base of the results of virtual screws. The direction of the third screw was on the base of the results of virtual screw. The direction of the last two screws parallel to coronal plane, the angle between the two screws and sagittal plane were on the base of the results of virtual screws. Direction of others perpendicular to the plate. The plates were placed on the acetabular anterior column, then the safe angle of screws were studied according to the above rules. The two end points(X1、Y1、Z1)、(X2、Y2、Z2) values of screws were recorded, and the differences of the values were the screw’s vector (X、Y、Z) values. The angles of external inclination are arctan(X/Z) on sagittal plane, and the angles of forward inclination are arctan(Y/Z) on coronal plane. Type two:the lliopubic edge plate, which is from symphyses to iliac crest. The distance from outer edge of the plate to the anterior superior iliac spine is 30mmm, and to the apex of the anterior inferior iliac spine is 10mm. The width of the plate is 10mm, the number of holes are 12 both in male and in female. The directions of holes on the lliopubic edge plate were followed as:the directions of the first three screws were similar to the directions of the first three screws of the true pelvic edge plate.The external inclination of the ninth, tenth and eleventh screws are 30°and 40°and 50°on sagittal plane, respectively, and the angle of forward of the three screws are according to the results of virtual screws. Others screws perpendicular to the plate. Design of others are similar to the true pelvic edge plate.3. Finite element analysis To obtain the smoother surface of the pelvic by reducing the triangular surface, filtering sharp-shaped triangle surfaces and surface smoothing. The model of three-dimensional reconstruction of the pelvis was imported into Geomagic Studio 12.0 software, then mesh grid of the pelvis was obtained by the model processed, which was saved into IGS format. The geometry model of two new type anatomic plate and the acetabular transverse fracture were imported into Solidwork 2012 software, and two different acetabular anterior column internal fixation models for acetabular transverse fracture were assembled. At last, two internal fixation models were imported into Hypermeshl2.0 software, and the pelvis fixed model’s meshing and normal pelvic model’s meshing were completed and saved into Inp format. The three mesh model were imported into Abaqus6.11 software, and the pelvis, plate and screw material property were assigned, the screw and the bone were set as a binding connection. The following static analysis was completed:1. Vertical direction of the pelvis:the both sides acetabular fossa was fixed and 6 degrees of freedom were limited.400N axial static load was carried on the top surface of S1 vertebrae in the three models, respectively.2. Front and back direction of the pelvis:the back of the sacrum was fixed and six degrees of freedom were limited.400N axial load was carried on the midpoint of front surface of pubic symphysis in the three models, respectively.3. Side direction of the pelvis:the contralateral acetabular fossa was fixed and six degrees of freedom were limited.400N axial load was carried on the the edge of the surface of the left iliac wing in the three models, respectively. The von Mises strain and displacement distribution of the three models in the three loadings were analyzed.Results:1. The AB average values (length of true pelvic edge) are 128.14±7.31mm and 140.53±8.15mm in males and females, respectively, and the differences of the values of AB between the male and female had statistical significance(P<0.01). The "point cloud" databases of male and female pelvic were obtained by three-dimensional reconstruction data exported by approach of "point cloud".2. The pelvic’s "point cloud" database was imported into Imageware13.2, and the pelvic was triangulated reconstruction. The bottom of plate was designed and adjusted to make it fit with the front surface of acetabular anterior column. The bottoms of the plate was imported into UG NX 8.5 and were stretched to 2mm thickness, then the edges and corners of plate were rounded.The width of the two types of plates were 10mm. The true pelvic edge plate:the length of plate are 120m and 132mm in male and female, respectively, and the number of the holes are 10 and 11 in male and female, respectively. The angles of the first three screws are 0°,0°,20° on coronal plane, respectively, and the angle of forward inclination are 25°,30°,40° on sagittal plane, respectively. The direction of the last two holes of screws are parallel to coronal plane, and the angle of the external inclination are 25° and 20° on sagittal plane, respectively. The direction of others perpendicular to the plate. The lliopubic edge plate:The length of it is 144mmm, and the amounts of the hole on the plate are 12 both in male and female, and the direction of the first three screws are similar to the directions of the true pelvic edge plate. The external inclination of ninth, tenth and eleventh screws are 30°,40° and 50° on sagittal plane, respectively, and the angle of forward inclination are 15°,10° and 10° on coronal plane, respectively. The other screws perpendicular to plate.3. Finite element analysis model of two types anatomic locking plate fixed the transverse fracture of acetabulum and normal pelvic were reconstructed successfully.4. Vertical direction static analysis of the models of two fixation: the von Mises strains and displacement cloud images of two fixation models of acetabular transverse fracture were analyzed. The maximum displacement of the true pelvic edge plate model is 0.440 mm, the maximum stress of the pelvis is14.92MPa, the maximum stress of the true pelvic edge plate is 98.64MPa. The maximum displacement of the lliopubic edge plate model is 0.327mm, the maximum stress of the pelvis is 7.64MPa, and the maximum stress of the lliopubic edge plate is 86.71 MPa.Front and back direction static analysis of the models of two fixation:The maximum displacement of the true pelvic edge plate model is 0.587mm, the maximum stress of the pelvis is 19.38MPa, and the maximum stress of the true pelvic edge plate is 158.00MPa. Maximum displacement of the lliopubic edge plate model is 0.610mm, the maximum stress of the pelvis is 21.28MPa, and the maximum stress of the lliopubic edge plate is 167.50MPa.Side direction static analysis of the models of two fixation:The maximum displacement of the plate of edge of true pelvic model is 0.973mm, the maximum stress of the pelvis is 28.45MPa, and the maximum stress of the true pelvic edge plate is 34.35 MPa. Maximum displacement of the lliopubic edge plate model is 0.799mm, the maximum stress of the pelvis is 20.74MPa, and the maximum stress of the plate of edge of lliopubic is 43.28 MPa.Direction of the vertical and the front and back and the side static analysis of the model of normal pelvis:the maximum stress of it were 2.03 IMPa,10.16MPa,17.54MPa, respectively, and the maximum displacement were 0.019mm,0.214mm,0.730mm, respectively.Conclusion:1. It is practicable to design two type new anatomic locking plate of the anterior column of acetabular fracture by using reverse engineering technology and computer-aided design technology to get "point cloud" database of the pelvis.2. The lliopubic edge plate is more stable than the true pelvic edge plate for fixation of acetabular transverse fracture.