[Measurement of the maximum corridor parameters of infra acetabular screw and evaluation of the feasibility of screw insertion by digital analysis].

OBJECTIVE

To measure the maximum corridor parameters of the infra acetabular screw and evaluate the feasibility of screw insertion through digital analysis of the acetabular structure.

METHODS

The pelvic CT data of 100 patients who received plain pelvic CT scan from April 2013 to June 2015 were retrospectively analyzed. There were 50 males, aged 20 to 84 years, with an average age of (48.42±17.48) years, and 50 females, aged 18 to 87 years, with an average age of (55.02±19.54) years. Patients with acetabular fractures, hip dysplasia, and metal implants in the acetabulum were excluded. Import CT data into Mimics software in DICOM format to generate a three-dimensional model, and find the axialprojection of the infra-acetabular corridor in the middle of the pubis ramus in the inlet view. A virtual screw was 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 5 mm was defined as a cutoff for placing a 3.5 mm screw, and calculate the screw insertion rate.

RESULTS

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.

CONCLUSION

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.