Extra-articular screw placement in the true pelvis for fixing quadrilateral plate fractures remains challenging. We aimed to define the "safe zone" on the quadrilateral surface to facilitate safe plate-screw placement. Twenty cadaveric hemipelves were sectioned and assembled to define the projection of the acetabular boundary on the quadrilateral surface. Three lines (X, Y, and Z) were drawn tangent to the projection, with X parallel to the iliopectineal line, Y perpendicular to the iliopectineal line, and Z parallel to the posterior border of the ischial body. Then, the distances between X and the iliopectineal line (D1), Y and the sacroiliac joint (D2), and Z and the posterior border of the ischium (D3) could be used to determine a "safe zone" on the quadrilateral surface for screw insertion. We included 15 patients whose conditions satisfied the definition of a comminuted quadrilateral plate fracture and applied two-ended buttress plates for treatment in accordance with this "safe zone." The average D1 was 50.0mm, the average D2 was 30.6mm, and the average D3 was 12.4mm. For all 15 patients with comminuted quadrilateral fracture who were treated, no intraoperative or postoperative screw penetration of the acetabulum was identified, and no loss of reduction was observed during an average follow up of 17.7 months. The "safe zone" established in this study simplifies extraarticular screw placement for managing quadrilateral plate fractures in the true pelvis. As a result, two-ended buttress plate fixation in the true pelvis becomes safe, therefore, treatment with two-ended buttress plates may represent a viable alternative to single-ended elastic fixation in the management of comminuted quadrilateral fractures.