Imaging study of unilateral/bilateral dual second sacral alar-iliac screws placement in adults

English Keywords:Sacral alar-iliac screw  Internal fixation  Pelvis  CT measurement

English Abstract:

  【Abstract】 Objectives: To simulate placing two ipsilateral S2AI screws in the pelvis of normal adults and measure the imaging parameters of the screw trajectories, and to investigate the feasibility and safety of such technique. Methods: The imaging data of patients who received plain CT scan and three-dimensional reconstruction of the pelvis in our hospital from January 2019 to July 2020 were selected, and 50 patients with no anatomical abnormalities in the pelvis were also selected. There were 25 males and 25 females, aged from 25 to 75 years old, with an average of 55.7±12.3 years old. The fixation trajectories of two S2AI screws on the same side was simulated on the Light Speed system: the intersection point of the vertical line 1mm at the outer edge of the 1st dorsal foramen and the horizontal line 1mm at the upper edge was selected as the anchoring point of the proximal S2AI screw, and the midpoint of the connection between the 1st dorsal foramen and the outer edge of the 2nd dorsal foramen 1mm was selected as the anchoring point of the distal S2AI screw. Then the three-dimensional pelvis images were rotated and cut with the two anchoring points as the center, so as to obtain the ideal trajectories. After the construction of the trajectories, the imaging parameters were measured, including sagittal angle(SA), transverse angle(TA), maximal length(ML), sacral length(SL), and skin distance(SD). Paired-samples′ t test was used to compare the parameters of S2AI screws in the same gender, and independent-samples′ t test was used to compare the parameters between different genders. Results: The trajectories of virtual bilateral dual S2AI screws could be constructed on every patients′ pelvic CT scan images. The imaging parameters of male′s proximal S2AI screw were SA: 42.31°±3.71°, TA: 37.31°±2.39°, ML: 105.51±5.92mm, SL: 29.32±3.87mm, SD: 26.52±3.31mm, and of the distal S2AI screw were: SA: 24.57°±2.17°, TA: 41.65°±2.39°, ML: 125.89±6.01mm, SL: 48.57±11.34mm, SD: 43.53±8.01mm. There were statistically significant differences between the SA, TA, ML, SL(P<0.05), but there was no statistical difference between the values of SD(P=0.078). For female, the imaging parameters of proximal S2AI screw were: SA: 43.05°±3.05°, TA: 36.53°±1.98°, ML: 102.38±7.44mm, SL: 31.31±4.11mm, SD: 28.01±2.91mm, and the imaging parameters of distal S2AI screw were: SA: 29.38°±2.67°, TA: 40.96°±2.58°, ML: 118.74±6.68mm, SL: 51.87±10.52mm, SD: 47.72±12.63mm. There were statistically significant differences between the SA, TA, ML, SL(P<0.05), but there was no statistical difference between the values of SD(P=0.223). The screw trajectories parameters of two S2AI screws from patients of different genders were compared and analyzed. Both proximal and distal screws showed that males had larger ML(P<0.05). At the same time, for distal S2AI screws, the difference in SA between men and women was statistically significant(P<0.001). Conclusions: Generally, the proximal S2AI screw is more tailward and adducted than the distal S2AI screw. And the females′ screw trajectories are more tailward and shorter than males′.