Does spinopelvic alignment affect fixation stability in pelvic ring fractures?: A finite element study.

Background: The pelvic ring plays a pivotal role in maintaining biomechanical stability during upright posture. While lumbopelvic fixation is effective for stabilizing unstable pelvic fractures, the influence of spinopelvic alignment, particularly sacral slope on fixation mechanics and stress on adjacent joints, has not been adequately investigated.

Methods: A finite element model of the lumbar spine, pelvis, and femur was used to simulate a pelvic ring fracture. Three spinopelvic configurations were analyzed (sacral slope = 20°, 26°, 32°), each stabilized with lumbopelvic fixation, with or without a cross connector. Biomechanical parameters including fracture displacement, lumbar intersegmental mobility, intervertebral disc stress, sacroiliac joint motion, and implant stress were evaluated under physiological loading.

Findings: A steeper sacral slope increased mobility at the L5 to S1 segment, nucleus pulposus stress, and stress at the sacroiliac joint, suggesting potential for degeneration and mechanical overload. Rod stress was also the highest in models with a steeper sacral slope. Models with a shallower sacral slope showed reduced rod and disc stress but greater pelvic tilt. Cross connectors reduced motion and stress in all configurations, especially under steep sacral slope conditions. Displacement at the sacral and pubic fractures was also greater with steep sacral slope. Fixation stability was optimal in the model with normal alignment and compromised in the steeper alignment.

Interpretation: To our knowledge, this is the first biomechanical study investigating how sacral slope influences the mechanical behavior of lumbopelvic fixation constructs in the setting of unstable pelvic ring fractures using finite element methods. Understanding the biomechanical effects of variations in spinopelvic parameters can aid in decision making for implant selection strategy as well as preoperative planning in high-risk patient populations. The finite element simulations suggest that higher sacral slopes may biomechanically reduce fixation stability, highlighting the potential importance of spinopelvic assessment in preoperative planning. Cross-connectors demonstrated mechanical benefits in this FE model and may be considered in high SS cases, pending further clinical validation.