Evaluation of Partial Cut-out of Sacroiliac Screws From the Sacral Ala Slope via Pelvic Inlet and Outlet View.

Abstract

An anatomic and radiographic study of placement of sacroiliac screws. The aim of this study was to quantitatively assess the risk of partial cut-out of sacroiliac screws from the sacral ala slope via inlet and outlet view. The partial cut-out of sacroiliac screws from the superior surface of sacral ala can jeopardize the L5 nerve root, which is difficult to identify on the pelvic inlet and outlet views. Computed tomography images of 60 patients without pelvic ring deformity or injury were used to measure the width (on inlet view) and height (on outlet view) of the sacral ala. The angle of the sacral ala slope was measured on lateral view. According to the measured parameters, the theoretical safe trajectories of screw placement were calculated using inverse trigonometric functions. Under fluoroscopic guidance, a sacroiliac screw was placed close to the midline on both inlet and outlet views, including posterosuperior, posteroinferior, anterosuperior, and anteroinferior regions to the midline. The incidence of screw partial cut-out from the superior surface of sacral ala was identified. The measured widths and heights of the sacral alas were 28.1 ± 2.8 and 29.8 ± 3.1 mm, respectively. The average angle between the superior aspect of the S1 vertebral body and the superior aspect of the sacral ala was 37.2 ± 2.5 degrees. The rate of partial cut-out of the screws from the superior surface of sacral ala slope was 12.5% (5/40) in posterosuperior, 0% (0/40) in posteroinferior, 70% (28/40) in anterosuperior, and 20% (8/40) in anteroinferior. To avoid the risk of partial cut-out from sacroiliac screw placement, more precise description should be added to the conventional description: the sacroiliac screws should be placed at the inferior half portion on outlet view and at the posterior half portion on inlet view. 4.