Biomechanical analysis of anterior ring fixation of the ramus in type C pelvis fractures

Abstract

This biomechanical study compared the stability of four different ramus fracture fixation methods for Type C pelvic ring injuries in the absence of posterior fixation. A 5-mm vertical osteotomy of the mid-superior and inferior pubic ramus was created in 12 synthetic pelvic models. Four surgical constructs were compared: (1) two-pin AIIS external fixation, (2) 3.5-mm reconstruction plating, (3) bicortical, fully threaded 3.5-mm, and (4) 6.5-mm pubic ramus screws. Specimens were tested in a simulated one-legged stance on a hemiarthroplasty implant in three stages: (1) no applied load, (2) application of the loading fixture preload to the sacrum (6N), and (3) following six cycles of a 250N load. Stability was assessed based on resultant displacement of the fracture sites at the superior ramus and the anterior sacroiliac joint. The bicortical, fully threaded 6.5-mm pubic ramus screw provided the most stable ramus fracture fixation (0.5 ± 0.4mm) displacement under load and was the only construct to finish testing without gross posterior pelvic disruption. Plate constructs finished the final loading stage with only a small increase (3.1 ± 2.3mm) in ramus fracture gap size, but had significant displacement at the SI joint (>20mm). 3.5-mm screw constructs had 1.6 ± 0.7mm of ramus displacement in the preload stage, but had complete posterior pelvic disruption (>20mm) that prevented further testing. External fixation was unstable at the ramus and sacroiliac sites in the initial setup. The bicortical, fully threaded 6.5-mm pubic ramus screw was the only anterior fixation construct tested that controlled motion at both the anterior and posterior pelvic rings in the absence of posterior fixation.