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 external 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 type II configurations were modeled for finite element analysis. Each model was evaluated with a standard stainless-steel and three different diameters of acrylic connecting bar. Displacements and stresses were calculated for the loading modes of axial compression, medio-lateral bending, cranio-caudal bending, and torsion. The location of the pin experiencing maximum stress was 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 equivalent stiffness and maximum pin stresses to those provided by the standard stainless-steel connecting bars (3.2- and 4.8-mm diameter, respectively). The largest diameter acrylic column tested (31.75-mm) produced lower stiffness and 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 column 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 required. CLINICAL RELEVANCE: The sizes of acrylic connecting bars for use in small and medium external fixator systems have been determined. Large systems should incorporate the standard stainless-steel connecting bar.