Comparative Finite Element Analysis of Fixation Techniques for APC II Open-Book Injuries of the Pelvis

Open-book fractures are defined as the separation of the pubic symphysis or fractures of the rami and disruption of the anterior sacroiliac, sacrotuberous, and sacrospinal ligaments. They can be stabilized by fixation of the anterior arch. However, indications and advantages of additional placement of iliosacral screws remain unknown. A CT-based model of the healthy pelvis was created and ligaments were modeled as tension springs. Range of motion of the sacroiliac joint and the pubic symphysis, and bone and implant stresses were compared for the physiological model, anterior symphyseal plating alone, and additional posterior fixation using two iliosacral screws. The range of motion of the sacroiliac joint was reduced for anterior symphyseal plating alone and further decrease was noted with additional posterior fixation. Von Mises stresses acting on the symphyseal plate were 819.7 MPa for anterior fixation only and 711.56 MPa for additional posterior fixation equivalent with a safety factor of 1.1 and 1.26, respectively. Implant stresses were highest parasymphyseal. While bone stresses exhibited a more homogeneous distribution in the model of the healthy pelvis and the model with anterior and posterior fixation, pure symphyseal plating resulted in bending at the pelvic rami. The analysis does not indicate the superiority of either anterior plating alone or additional posterior fixation. In both cases, the physiological range of motion of the sacroiliac joint is permanently limited, which should be taken into account with regard to implant removal or more flexible techniques for stabilization of the sacroiliac joint.