An implantable prosthesis is provided having a radially-expandable tubular body and at least one skirt extending therefrom. The skirt terminates in a peripheral edge, wherein at least portions of the peripheral edge are free and displaceable to a greater diameter of the tubular body. Thus, with the implantable prosthesis being a stent-graft used to treat an aortic aneurysm (e.g., abdominal aortic aneurysm (“AAA”)), the skirt may be used to inhibit Type I endoleaks upon its selective displacement in response to irregular aortic shaping and/or aneurysm neck expansion. The skirt may actively inhibit Type I endoleaks by forming a physical barrier against flow between the tubular body and the aortic wall. In addition, the skirt may passively inhibit endoleak formation by sufficiently restricting blood flow to allow coagulation and clot formation, which would act as a barrier against endoleakage.