PAPER 26: IS THERE A BIOMECHANICAL DIFFERENCE BETWEEN COMMERCIALLY AVAILABLE HYBRID EXTERNAL FIXATORS?

Abstract

Purpose: The objectives of this project were to compare four commercially available hybrid external fixator systems under multiple loading conditions, and to compare each system to an idealized «rigid» frame to distinguish between motion allowed by the pins and wires and motion allowed by frame deformation.

Methods: A mechanical testing investigation was performed using fifteen fiberglass composite tibias (Pacific Research Laboratories, Vashon Island, WA, USA) with a 2 cm proximal metaphyseal gap osteotomy (simulating OTA classification 41-A3.3) to test four hybrid systems (Ace-Fischer, DePuyACE, Warsaw, IN; Hoffmann II, Stryker Howmedica Osteonics, Rutherford, NJ; Synthes Hybrid, Synthes USA, Paoli, PA; EBI DynaFix®, EBI, Parsippany, NJ) and a custom built «rigid» frame. Setting: A biomechanics laboratory using a servohydraulic load frame (MTS Bionix 858, Minneapolis, MN). Measurements were made of the motion produced when loads were applied to the proximal tibia through a custom load plate.

Results: The only significant difference between commercial systems was in axial loading where the Ace-Fischer allowed less motion than the other frames. In all cases the «rigid» reference frame allowed significantly less motion than the commercial systems. Approximately 50% of the motion comes from frame deformation and 50% from deformation of the pins and wires.

Conclusions/Significance: There are few differences between commercially available hybrid external fixation systems for treating unstable proximal tibia fractures. Approximately half of the motion at the fracture site is due to bending deformation in asymmetric frame designs, which leads to unwanted angular and shear displacements at the fracture site.