Biomechanical properties of different external fixator frame configurations.

Purpose: External fixators are easy to apply and maximize soft tissue preservation. However, frames need providing an adequate stiffness in order to avoid excessive interfragmentary movement during the healing period. We characterized the stiffness of four different configurations of the newly developed Hoffmann 3 external fixation system.
Methods: A synthetic fracture gap model was stabilized using four different frame configurations: a double-∅ 11 mm rod configuration (group DR), a hybrid double-∅ 8 mm rod configuration (group H), a single ∅ 11 mm rod direct link configuration (group DL) and a single ∅ 11 mm rod side arm configuration (group SA). The stiffness of each configuration was measured under anterior-posterior bending, medial-lateral bending and axial torsion loading directions and the results statistically compared.
Results: The basic frame construct (group DR) showed the highest bending and torsional stiffness properties while the single rod side arm configuration (group SA) the lowest.
Conclusions: The diameter and the amount of used connecting rods as well as the adequate placement of these rods towards the main loading directions determine the construct stiffness. These results could help the surgeons estimating how different frames can potentially affect the interfragmentary motion. This information might help in choosing specific configuration when treating different fracture types on given patients.