External fixation of the pelvic ring - An experimental study on the role of pin diameter, pin position, and parasymphyseal fixator pins

Background The mechanical properties of current external fixator systems for unstable (type C) pelvic ring fractures are inferior to internal fixation, and are not optimal for definitive treatment. We explored methods to increase stability of external fixator constructs. Methods An experimental model was used for load tests. The same pelvic fixator was used while different pin diameters, pin positions, and modes of pubic symphysis fixation were tested. Results Changing of the pin diameter of the unthreaded part from 6 to 8 mm resulted in an increase in stiffness of 20%. An increase in stiffness by a factor of 1.9 was found by placing a pin on the iliac crest and one supra-acetabular. An additional increase by a factor of 3.6 was obtained by adding pubic symphysis plate fixation. Parasymphyseal pin fixation instead reduced stiffness, but not so much as when parasymphyseal pins were connected to the external fixator of the pelvic ring. The final configuration was at least 6 times stiffer than the initial configuration. Interpretation The new concept of parasymphyseal pin fixation connected to an external fixator of the pelvic ring produces a considerable increase in stability for the treatment of type C pelvic ring injuries, as does an increase in pin diameter and alternative pin positioning.