| Objective:1.To explore the entry point and orientation of the supra-acetabular screw as well as compare the safe zone size of half-way and full-way fixation.2.To compare the stability of supra-acetabular screws fixation with other mini-invasive fixators in Tile C1 pelvic fractures by finite element analysis.Methods:1.An Imaging Anatomy Study of Safe Fixation Zone for Supra-acetabular Screw:100(male:50,female:50)CT data of pelvis were collected in the first affiliated hospital of Chongqing Medical University.The three-dimensional model of supra-acetabular(oriented from anterior inferior iliac spine(AIIS)to posterior superior iliac spine(PSIS))zone were reconstructed by Mimics software.The width of the corridor was measured from lower to higher and finally the location of the central pin was determined.Then the angles between the central pin and sagittal as well as transverse planes were measured and the entry point of the central pin was determined.At last,the safe ranges of half-way and full-way supra-acetabular pins were measured.2.The Stability Comparison of Supra-acetabular Screws Fixation of C1 Pelvis Fracture With Other Mini-invasive Fixator by Finite Element Analysis:The standard pelvic model was imported into Geomagic Studio 13 software to construct the NURBS closed surface model of pelvis,which were saved as IGS format.Then the model was put into SolidWorks 2016 software to create Tile C1-3a2 pelvis fracture which means the fractures of pubic symphysis and unilateral sacral foramen.The fixators were assembled on the appropriate position of the fracture models with the use of SolidWorks 2016 software.These assembled models were put into Hyper mesh 12.0 software to generate mesh and saved as Inp format.Then these models were put into Abaqus 6.14 software and the material of pelvis,pins and connectors were assigned respectively.The pins and the connectors as well as the 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.500N vertical load was applied on the surface of S1 vertebrae in the models.(2)anterior-posterior direction:the poster part of bilateral iliac crest were confined and the activities of the six degrees of freedom were limited,then 500N anterior-posterior load was applied on the anterior surface of pubic symphysis.(3)lateral direction:the bilateral acetabular fossae were confined and the activities of the six degrees of freedom were limited.500N horizontal load was applied on the surface of the left iliac tubercle.The von Mises stress and displacement distribution in the models were analyzed under different loading conditions.Results:1.An Imaging Anatomy Study of Safe Fixation Zone for Supra-acetabular Screw:There were two narrowing in the supra-acetabular corridor.From inferior to superior,the width of the anterior narrowing has the characteristic of narrow-wide-narrow and the posterior narrowing becomes wider and wider.The width of the safe corridor also has the characteristic of narrow-wide-narrow.The depth of the full-way was about 130mm,the width of the central corridor for 90%male and female were larger than 7mm and 6mm;The depth of the half-way was about 70mm,the width of the central corridor for 90%male and female were larger than 11mm and 9mm.The length from the entry point to anterior narrowing was 50mm and to posterior narrowing was 100mm.The angle between the central pin and sagittal plane or transverse plane was 30 degrees.The safe range for half-screw was bigger 3 to 6 degrees than full-screw.71%of the entry point located at the inferior-lateral of the center of the AIIS and about 90%located in the lateral quadrant.2.The Stability Comparison of Supra-acetabular Screws Fixation of C1 Pelvis Fracture With Other Mini-invasive Fixator by Finite Element Analysis:In all,we constructed thirteen fixators.The number one is S1 and S2 sacroiliac screws.The number two and three are the INFIX with the fixation depth of 5cm and 7cm,respectively.The number four is pelvic C clamp.The number five to eight are single-pin supra-acetabular external fixation with the fixation depth of 5cm,7cm,10cm and 13cm,respectively.The number nine to twelve are anterior pelvic external fixator with the supra-acetabular and iliac double-pin.The fixation depths of supra-acetabular pins are 5cm,7cm,10cm and 13cm,respectively,and the fixation depth of iliac pin is 5cm.The number thirteen is anterior pelvic external fixator with iliac double-pin and the fixation depth was 5cm.In the finite element analysis,when the vertical static load was loaded on the surface of S1 vertebral body,the maximum displacements of 13 models were 0.061mm,0.797mm,0.541mm,0.364mm,0.813mm,0.772mm,0.743mm,0.727mm,0.740mm 0.703mm,0.682mm,0.676mm and 0.808mm,respectively;when the anterior-posterior static load was loaded on the anterior surface of pubic symphysis,the maximum displacements of 13 models were 0.725mm,0.381mm,0.380mm,0.958mm,0.440mm,0.409mm,0.394mm,0.385mm,0.392mm,0.386mm,0.380mm,0.367mm and 0.418mm,respectively;when the lateral static load was loaded on the surface of the left iliac tubercle,the maximum displacements of 13 models were 0.161mm,0.311mm,0.260mm,0.227mm,0.335mm,0.331mm,0.296mm,0.262mm,0.261mm,0.260mm,0.254mm,0.243mm and 0.266mm,respectively.Conclusion:1.For the supra-acetabular screws fixation,the entry point is located at the lateral of the center of the AIIS and the inward and cranial inclination angles of the screw are both about 30 degrees.The half-way fixation is safer than full-way fixation.2.In emergency,the external fixation of supra-acetabular longer screws combined with anterior superior iliac spine screws can be used to stabilize the posterior ring in unstable pelvic fractures as a temporary device. |
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