Symphyseal internal rod fixation versus standard plate fixation for open book pelvic ring injuries: a biomechanical study

Purpose This study investigates the biomechanical stability of a novel technique for symphyseal internal rod fixation (SYMFIX) using a multiaxial spinal screw-rod implant that allows for direct reduction and can be performed percutaneously and compares it to standard internal plate fixation of the symphysis. Methods Standard plate fixation (PLATE, n = 6) and the SYMFIX ( n = 6) were tested on pelvic composite models with a simulated open book injury using a universal testing machine. On a previously described testing setup, 500 consecutive cyclic loadings were applied with sinusoidal resulting forces of 200 N. Displacement under loading was measured using an optoelectronic camera system and construct rigidity was calculated as a function of load and displacement. Results The rigidity of the PLATE construct was 122.8 N/mm (95 % CI: 110.7–134.8), rigidity of the SYMFIX construct 119.3 N/mm (95 % CI: 105.8–132.7). Displacement in the symphyseal area was mean 0.007 mm (95 % CI: 0.003–0.012) in the PLATE group and 0.021 mm (95 % CI: 0.011–0.031) in the SYMFIX group. Displacement in the sacroiliac joint area was mean 0.156 mm (95 % CI: 0.051–0.261) in the PLATE group and 0.120 mm (95 % CI: 0.039–0.201) in the SYMFIX group. Conclusions In comparison to standard internal plate fixation for the stabilization of open book pelvic ring injuries, symphyseal internal rod fixation using a multiaxial spinal screw-rod implant in vitro shows a similar rigidity and comparable low degrees of displacement.