Relative stiffness and stress of type I and type II external fixators, acrylic versus stainless-steel connecting bars - A theoretical approach

Objective-To compare the stiffness and pin stresses of three sizes of exter nal fixator systems with stainless-steel and acrylic connecting bars. Study Design-Finite element analysis. Methods-Small, medium, and large external fixator systems of type I and typ e II configurations were modeled for finite element analysis. Each model wa s evaluated with a standard stainless-steel and three different diameters o f acrylic connecting bar. Displacements and stresses were calculated for th e loading modes of axial compression, medio-lateral bending, cranio-caudal bending, and torsion. The location of the pin experiencing maximum stress w as determined for all configurations and loading modes. Results-Acrylic column diameters of 9.53 mm for the small external fixator system and 15.9 mm for the medium external fixator system provide equivalen t stiffness and maximum pin stresses to those provided by the standard stai nless-steel connecting bars (3.2- and 4.8-mm diameter, respectively). The l argest diameter acrylic column tested (31.75-mm) produced lower stiffness a nd higher maximum pin stresses than the standard stainless-steel connecting bar (11.1-mm diameter). Conclusions-When applying a small or medium external fixator, an acrylic co lumn of 9.53-mm or 15.9-mm diameter, respectively, can be used. For a large external fixator system, an acrylic column of diameter >31.75 mm is requir ed. Clinical Relevance-The sizes of acrylic connecting bars for use in small an d medium external fixator systems have been determined. Large systems shoul d incorporate the standard stainless-steel connecting bar. (C)Copyright 200 0 by The American College of Veterinary Surgeons.