Anterior supra-acetabular external fixation for tile C1 pelvic fractures: a digital anatomical study and a finite element analysis.

Abstract

Investigating the anatomical characteristics of supra-acetabular screw corridor as well as comparing the biomechanical stability between semi- and full-length screw external fixations for Tile C1 pelvic fractures. 50 male and 50 female uninjured pelvic CT data were converted into three-dimensional models and the supra-acetabular corridors were reconstructed by the Mimics software. The horizontal slice was redefined passing through the bilateral anterior inferior iliac spine and bilateral posterior superior iliac spine at the same time. On every horizontal slices, a 5mm diameter virtual screw was placed along with the unilateral anterior narrowing to posterior narrowing of the supra-acetabular corridor, and the relevant parameters of the semi- and full-length screw fixations were measured from inferior to superior at an interval height of 2.5mm until the width of the corridor was less than 5mm. Thus, the insertion position was located, as well as the inclination angle ranges were measured. The same procedure was done on a bilateral supra-acetabular corridor for each patient. Last, a Tile C1 pelvic finite element model was simulated and fixed with either semi- or a full-length anterior supra-acetabular external fixator. A 500N loading was tested on the model with three directions (cranial-caudal, anterior-posterior and lateral-medial) and the stiffness was determined by the max displacements. The insertion points of supra-acetabular screw for 66% (66/100) of men and 78% (78/100) of women were located at the outer lower part of the anterior inferior iliac spine (AIIS) (p < 0.001). The medial inclination angles of the screw were 29.50 ± 4.05° for men and 28.00 ± 3.78° for women (p = 0.007). The cranial inclination angles of the screw were 29.40 ± 5.38° for men and 28.57 ± 6.01° for women (p = 0.306). The safe inclination angle ranges of the semi-length screw were 3°-6° bigger than the full-length screw. For semi-length corridor, the widths were 14.07 ± 2.39mm for men and 11.29 ± 1.81mm for women (p < 0.001) and the depths were 71.77 ± 5.96mm for men and 69.26 ± 5.21mm for women (p < 0.001). For full-length corridor, the widths were 10.61 ± 2.10mm for men and 8.44 ± 2.03mm for women (p < 0.001) and the depths were 133.37 ± 7.60mm for men and 129.01 ± 8.12mm for women (p < 0.001). In the finite element analysis, the max displacements of the pelvis under three loading modes (cranial-caudal, anterior-posterior and lateral-medial) for semi-length fixation were 0.772mm, 0.409mm and 0.331mm and for full-length fixation were 0.727mm, 0.385mm and 0.262mm. The outer lower part of AIIS is recommended as an insertion point of the supra-acetabular screw with about 30° medial and cranial inclination angles, and the semi-length screw is safer for placing and has satisfactory biomechanical stability by compared with a full-length screw.