Biomechanical stability evaluation of isolated straddle fracture using finite element analysis

Straddle fractures, bilateral fractures of the superior and inferior pubic rami, are considered mechanically stable when the posterior pelvic ring remains intact. However, progressive displacement or delayed union, even in isolated straddle fractures, raise concerns about their true biomechanical stability. This study aimed to quantitatively evaluate the biomechanical stability of isolated straddle fractures with an intact posterior pelvic ring using finite element analysis based on computed tomography-derived three-dimensional pelvic models. Two models were developed: model A, an isolated straddle fracture model with an intact posterior pelvic ring; model B, a posterior fracture model with sacroiliac joint fracture. Under a 600 N axial load, model A exhibited a maximum displacement of 2.03 mm within the physiological safety limit and a maximum von-Mises stress of 100.6 MPa, lower than the ultimate tensile strength (UTS) of the cortical bone (110 MPa). Conversely, model B exhibited severe instability, with a displacement > 50 mm and a stress surpassing the UTS. Model A maintained biomechanical stability under a 600 N bending load and 10 Nm torsional moment, with displacements < 2 mm and stresses remaining below the UTS. These findings indicate that an intact posterior pelvic ring provides sufficient biomechanical stability for isolated straddle fractures, supporting conservative management when posterior ring integrity is maintained. In contrast, injury to the posterior pelvic ring results in severe mechanical instability, emphasizing its crucial role in pelvic load transfer. Collectively, this quantitative biomechanical evidence refines traditional treatment principles for isolated straddle fractures and supports clinical decision-making in pelvic ring injuries.