Rigidity of unilateral external fixators - A biomechanical study

Introduction: External fixation is the primary choice of temporary fracture stabilisation for specific polytrauma patients. Adequate initial fracture healing requires sufficient stability at the fracture site. The purpose of this study was to compare the rigidity of the Dynafix DFS®Standard Fixator (4 joints) with the Orthofix ProCallus Fixator®(2 joints), which differ in possibilities for adapting the configuration for clinical needs. Materials and methods: Both devices were tested 10 times in a standardised model. In steps of 10 N, loading was increased to a maximum of 160 N in parallel, transversal and axial direction (distraction and compression). Translation resultant and rotation resultant were calculated. Results: With a force of 100 N in parallel direction the mean translation resultant (Trmean) of the Dynafix DFS®Standard Fixator (6.65 ± 1.43 mm) was significantly higher than the ProCallus Fixator®(3.29 ± 0.83 mm, p < 0.001; Student's t-test). With a maximum load of 60 N in transverse direction the Trmeanof the Dynafix DFS®Standard Fixator was significantly lower (8.14 ± 1.20 mm versus 9.83 ± 0.63 mm, p < 0.005). Translation was significantly higher with the Dynafix DFS®Standard Fixator, for both distraction (2.13 ± 0.32 mm versus 1.69 ± 0.44 mm, p < 0.05) and compression (1.55 ± 1.08 mm versus 0.15 ± 0.33 mm, p < 0.005). The mean rotation resultant (Rrmean) at 160 N distraction was lower for the Dynafix DFS®Standard Fixator (0.70 ± 0.17° versus 0.97 ± 0.21°, p < 0.005). Conclusions: Both fixators were most sensitive to transverse forces. The Dynafix DFS®Standard Fixator was less rigid with parallel and axial forces, whereas transverse forces and rotation at distraction forces favoured the Dynafix DFS®Standard Fixator. Repeated heavy loading did not influence the rigidity of both devices