Denis pattern–dependent biomechanical behavior of posterior trans-iliac plate fixation compared with bilateral triangular osteosynthesis in unilateral sacral fractures: a finite element study

Posterior trans-iliac plate osteosynthesis (TPO) is commonly used for the stabilization of unilateral vertically unstable sacral fractures. However, its biomechanical performance across different Denis fracture patterns remains unclear. This study aimed to investigate the fracture pattern–dependent biomechanical behavior of posterior trans-iliac plate osteosynthesis (TPO) in unilateral vertically unstable sacral fractures, using bilateral triangular osteosynthesis (BTO) as a high-stability reference construct.

A validated three-dimensional finite element model of the lumbopelvic complex was developed to simulate unilateral vertically unstable sacral fractures corresponding to Denis type I, II, and III patterns. Two posterior fixation strategies, TPO and BTO, were constructed for each fracture type. Seven physiological loading conditions were applied, including standing, flexion, extension, axial rotation, and lateral bending. Von Mises stress distribution and fracture displacement were evaluated. Fracture micromotion was quantified by calculating relative displacement changes between paired points along the fracture gap.

When interpreted relative to the high-stability BTO reference construct, TPO showed fracture pattern–dependent variation in fracture displacement. The difference was most pronounced in Denis type I fractures and progressively decreased in Denis type II and III patterns.

The biomechanical performance of posterior trans-iliac plate fixation is influenced by the Denis fracture pattern. Fracture micromotion associated with the non-reinforced TPO construct evaluated in this study decreased as the fracture line approached the sacral midline, indicating improved mechanical compatibility in more medially located fracture patterns. These findings provide biomechanical reference data for understanding fracture pattern–dependent fixation compatibility of TPO in unilateral sacral fractures. However, it remains unclear how much fixation strength is actually required to achieve sufficient construct stability for fracture healing and how postoperative management strategies, such as weight-bearing status, may influence this requirement.