| Part one Digital anatomical measurements of the acetabular posterior wallObjectiveThe anatomy of acetabular posterior wall was measured digitally to guide the screw and plate shape design for anatomic combined posterior wall plate and provide reference for screw placement in the posterior wall and column.MethodsThe pelvic CT data of 100 healthy adults in our hospital were collected,50 males and 50 females.The scan data was imported into Mimics 20.0 software in Dicom format to reconstruct the 3D model of the pelvis.The posterior wall of the acetabulum was divided into the high posterior superior wall area involving the acetabular roof and the low posterior acetabular wall area not involving the acetabular roof.Sections M,D,E and N were obtained by cutting the posterior upper wall of the acetabulum;sections F,G,H,I and J were cut from the posterior wall of the lower acetabulum.The following indicators were measured on the section:(1)screw entry angle;(2)screw entry length;(3)thickness of posterior wall;(4)arc length and radian of posterior wall;(5)posterior column wall width Y,acetabular width X and percentage of acetabular fossa width(X/Y×100%).Results(1)In the posterior superior acetabulum area,at points M1,D1,E1,and N1 5 mm away from the acetabular rim,the maximum safe screw entry angles for males were 79.26±4.53°,70.95±3.61°,58.34±3.49°,and 53.04±4.83°,respectively;women were 83.46±4.03°,69.95±3.62°,60.48±4.18° and 56.38±4.64°.At points M1,D1,E1,and N1,the screw lengths of males fluctuated at(40.65±2.74～46.56±2.42)mm,and the thicknesses fluctuated at(4.63±0.78～8.26±1.93)mm;females fluctuated at(36.18±2.41～41.18±2.22)mm,(4.63±1.00～8.69±1.31)mm,respectively.(2)In the posterior wall area of the low acetabulum,the maximum safe screw entry angle fluctuated between(49.41±3.78～76.02±2.31)° in men and(53.82±5.12～76.10±2.81)° in women at the entry point 5 mm away from the acetabular rim;men and women fluctuate at(59.66±3,95～82.15±3.36)° and(62.81 ±3.23～82.15±3.14)°at 10mm,respectively;(77.35±3.12～95.68±3.20)°,(79.07±3.67～96.22±2.22)° at 20mm,respectively;(88.12±4.25～109.10±3.40)° and(92.25 ±3.95～109.30±2.94)° at 30mm.In section F,the screw lengths at each entry point of males F1 to F4 fluctuated at(40.94±3.74～62.22±8.61)mm,and the lengths of females fluctuated at(35.88±3.76～54.40±8.14)mm;in section G,males fluctuated at(36.99±2.98～43.41±8.54)mm,women fluctuate at(32.31±3.14～36.04±8.74)mm;in section H,men fluctuate at(33.93±4.08～36.52±2.80)mm,women fluctuate at(27.44±4.64～32.01±2.46)mm;in section I,men fluctuate at(33.37±2.98～36.04±2.21)mm,women fluctuate at(28.57±3.93～32.17±2.63)mm;in section J,men fluctuate at(39.17±4.08～49.32±5.52)mm,women fluctuate at(36.33±4.06～45.07±6.59)mm.In sections F,G,H and I,the thickness of the posterior wall of the acetabulum fluctuated within(4.32±0.70～6.04±1.01)mm for men and(4.27±0.91～5.99±1.16)mm at the insertion point 5 mm from the acetabular rim;men and women fluctuate at(5.54±1.01～8.16±1.10)mm and(6.44±1.17～8.18±1.39)mm at 10mm,respectively;(16.01±1.50～21.90±2.26)mm and(16.96±1.78～20.03±1.86)mm at 20mm,respectively;(25.57±4.77～51.05±6.64)mm and(15.71 ±2.95～30.92±6.10)mm at 30mm.The width of the acetabulum on section J is smaller,and the thickness of the posterior wall measured at each screw entry point is quite different from that of other sections.(3)In the male,the arc length and radian of posterior-superior wall of were(27.38±2.19)mm and(0.84±0.11)rad,respectively.The posterior middle wall was(24.87±1.31)mm and(0.71±0.12)rad,respectively.The radian of the posterior-inferior wall was(0.64±0.11)rad.In the female,The arc length and radian of posterior-superior wall were(24.64±3.26)mm and(0.79±0.09)rad,respectively.The posterior middle wall was(21.93±1.51)mm and(0.65±0.08)rad,respectively.The radian of the posterior-inferior wall was(0.69±0.08)rad.The arc length and radian of male were significantly different from that of female(P<0.05).(4)The percentages of acetabular fossa width from slices F to J were:62.31%,73.98%,72.68%,63.04%and 42.71%,respectively;women were 61.42%,72.18%,71.24%,64.18%,45.27%,respectively.ConclusionsIn the posterior superior acetabulum area,at points M1,D1,E1,and N1 5 mm away from the acetabular rim,the maximum safe screw entry angles for men and women should not exceed 80°,70°,60°,and 50°,respectively;the screw length should not exceed 40 mm for men,35 mm for women,the length of the vertical posterior wall penetration for both men and women should not exceed 4.5 mm.In the posterior wall area of the low acetabulum,in sections F to J,with the increase of the distance from the point to the outer edge of the acetabulum,the maximum safe screw insertion angle gradually increased.For males and females,the angle of screw insertion at a distance of 5 mm from the acetabular edge should be less than 50°,less than 60° at 10mm,less than 80° at 20mm,and less than 90° at 30mm.In male sections F to J,the screw lengths of each section should be less than 40mm,35mm,30mm,30mm,and 40mm;for women,the lengths should be less than 35mm,30mm,25mm,25mm,and 35mm,respectively.In sections F,G,H and I,the thickness of the posterior wall gradually increased as the entry point moved away from the acetabular edge.For males and females,the length of the vertical posterior wall 5 mm from the acetabular edge should be less than 4 mm,and the length of the 10 mm from the acetabular edge should be less than 5 mm,6 mm,and less than 16 mm at 20 mm,and less than 25 mm,15 mm at 30 mm,respectively.The posterior acetabular wall margin is not regularly round,and the radian of the superior,middle and inferior parts of the posterior wall are different,and there are significant differences between male and female.The percentage of acetabular fossa width was the largest in the middle section G and section H of the posterior column of the acetabulum,and gradually decreased toward the distal and proximal ends.In sections G and H,the percentage of acetabular fossa width was 70%～75%,in sections F and I 60%～65%,and in section J 40%～45%.Part two Finite element analysis of anatomical combined plates for posterior superior wall fractureObjectiveThrough the finite element analysis and research,the mechanical stability of anatomical combined plates for fixating posterior superior wall fracture was discussed.MethodsCT images of the pelvis of a healthy adult male volunteer was selected.Using Mimics 20.0 software,ANSYS 12.0-ICEM software,Hypermesh 14.0 software and Abaqus 6.14 software to establish a three-dimensional finite element model of the pelvis and verify its effectiveness.The model of posterior superior acetabular wall fracture was established,and four groups of internal fixation models were established:TS group(two screws),TSRP group(two screws and reconstruction plate),MPRP group(mini-plate and reconstruction plate)and APSCP group(anatomical posteriorsuperior combined plates).Simulating the pelvic standing position,the overall stiffness,fracture line displacement and displacement difference,stress distribution in cortical bone of the fracture fragment and iliac bone were compared between the four internal fixation models and the fracture model(FM group)and the normal pelvis model(APM group),and the stress distribution of the four internal fixators was analysed.ResultsThe overall stiffness of each model in descending order is:NPM group>APSCP group>MPRP group>TSRP group>TS group>FM group.The APSCP group and NPM group had the highest coincidence degree in the displacement and displacement difference of each node,followed by TSRP group and MPRP group,and the deviation between TS group,FM group and NPM group was larger.The mean stress value of each node in the stress path above the cortical bone of the fracture was:NPM group>TSRP group>APSCP group>MPRP group>TS group>FM group.In the normal model,the maximum stress on the ilium was mainly concentrated at the posterior margin of the acetabulum and the greater sciatic notch.When a fracture occurs,the maximum stress occurs at the posterior fracture line.After internal fixation,the stress at the posterior margin of the acetabulum increases,while the stress at the posterior fracture line decreases.The maximum stress in TS group was in contact with cortical bone,the middle part of the screw is stressed at the fracture line of cancellous bone.In the TSRP group,the stress of the reconstruction plate located above the fracture is greater,the stress distribution of the two screws outside the plate was more balanced than that of the TS group.The stress distribution of the reconstruction plate in the MPRP group and APSCP group was more balanced than that in the TSRP group.The mini-plate in the MPRP group had a larger stress across the fracture line,while the anatomical mini-plate in the APSCP group had a balanced stress distribution.ConclusionsTwo-screw internal fixation alone lacks a reliable fixation effect in the treatment of posterior superior posterior wall fractures of the acetabulum.Two screws and reconstruction plate,miniplate and reconstruction plate,and anatomical combined plates all have good biomechanical stability,and under the fixation of the anatomical combined plates,the mechanical transmission characteristics of the fracture site are most in line with the physiological state,and it is safe and reliable to fix the posteriorsuperior posterior wall fractures involving the acetabular roof. |
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