Morphological Study Of The Infra-acetabular Screw Corridor Based On A Three-dimensional Model

Objective: To study the morphology of infra-acetabular screw corridor through the application of Mimics software to construct a three-dimensional model of the pelvis.Methods: 1.The pelvic CT data of 100 patients who received plain pelvic CT scan in our hospital from April 2013 to June 2015 are retrospectively analyzed.There are 50 males,aged 20-84 years,and 50 females,aged 18-87 years.Patients with acetabular fractures,hip dysplasia,and metal implants in the acetabulum are excluded.Import CT data into Mimics software in DICOM format to generate a three-dimensional model,and find the axial projection of the infra-acetabular corridor in the middle of the pubis ramus in the inlet view.A virtual screw is placed in the infra-acetabular space and measure the parameters including the diameter and the length of the maximum corridor,the distance from the insertion point to the pubic symphysis,to the anterosuperior iliac spine and to the medial edge of the pelvis.Then import the pelvic model into 3-matic software,establish the pelvic model anterior pelvic plane and median sagittal plane,and measure the angle between the screw axis and the two planes.A minimum corridor diameter of at least 5mm is defined as a cutoff for placing a 3.5mm screw,and calculate the screw insertion rate.2.The pelvis computed tomography data of 187 patients who underwent pelvic CT plain scan during May 2018 to September 2020 at our hospital are imported into Mimics software in DICOM format to generate a 3D model.The anterior pelvis plane is used as the reference plane to measure the diameter of the optimum IAC when the pelvis model is tilted forward by 5°,15°,25°,35°and 45°.The diameter of at least 3.5mm is defined as the cutoff for placing a 3.5 mm screw,the rate of infra-acetabular screw insertion is calculated,and the mean length of the IAC and the mean tilt of the corridor axis in relation to the sagittal midline plane are measured.Results: 1.In 100 cases,49% of patients had a infra-acetabular corridor with a diameter≥5 mm,and the rate of screw placement in men was significantly higher than that in women.The average diameter of the maximum corridor of infra-acetabular screw was(4.86 ± 1.72)mm,the average length was(94.04 ± 8.29)mm,the average distance from the insertion point to the pubic symphysis was(60.92 ± 4.84)mm,to the anterosuperior iliac spine was(85.15 ± 6.85)mm,and to the medial edge of the pelvis was(6.12 ±3.32)mm.The mean angle between the axis of the screw and the median sagittal plane was(-1.38 ± 4.74)°,and the mean angle between the axis of the screw and the anterior pelvic plane was(56.77 ± 7.93)°.There are significant differences between male and female measured parameters,except for the angle between the screw axis and the anterior pelvic plane.There was no statistically significant difference in the maximum corridor parameters of infra-acetabular screw on both sides of the pelvis.2.The largest mean diameters of the IAC can be found under fluoroscopy at 5°–35°.The largest mean diameter of the IAC is 4.08 ± 1.84 mm and the highest screw insertion rate is 60.42% at15° and 25°,whereas the diameter and insertion rate are lowest at 45°.The corridor length increases with increasing fluoroscopic angle,and the angle of the corridor axis to the SMP decreases gradually.Conclusion: 1.This study shows that the insertion rate of infra-acetabular screws is low in local patients,and the feasibility of screw insertion should be fully evaluated before surgery.2.The traditional 45° pelvic inlet radiograph is not suitable as the fluoroscopic angle for IAS insertion.The parameters of the IAC vary under different fluoroscopic angles,so a surgeon can select the appropriate fluoroscopic angle in accordance with the type of fracture and the fracture line angle.