Comparison of the mechanical performance of three types of external fixators: Linear, circular and hybrid

Different configurations of the Monticelli-Spinelli and llizarov external fixation systems were tested to define their mechanical properties. In five configurations the external fixator consisted of rings with tensioned wires (circular), while in one configuration two pairs of the tensioned wires and their correspondent ring were replaced by threaded pins (hybrid). Testing was performed in axial compression, bending and torsion. The results were compared to the characteristics of a selected linear fixator group. Both the circular and the hybrid configurations were non-linear in compression. In bending, circular fixators had a similar pattern in both anteroposterior and oblique loading directions. The bending load-displacement pattern for the hybrid fixators was similar to the linear fixators, higher stiffness in the plane of the pins. Torsion was linear for both circular and hybrid fixators, as for the linear fixators. By combination of wires and pins (hybrid configuration), the mechanical behaviour had characteristics from both linear and circular fixators. it is concluded that the three studied groups own different mechanical performance and can be considered as different types of fixators. While it has been demonstrated that osteogenesis can be achieved independently of the mechanical behaviour of the fixator, this study supports the suggestion that some complications can be related to the mechanical behaviour of the fixator. Relevance-This study provides some guidelines for selecting the appropriated external fixator for clinical use depending upon the stiffness requirements of a specific clinical case. Furthermore, the information on the mechanical characteristics of the different systems of external fixation permits some conclusions on the possible failures of the fixators during their clinical application. However, the mechanical differences observed cannot be utilized to support differences on the biology of the fracture or osteotomy foci where these types of fixators are used.