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Abstract
Metal plates are commonly used in the operative treatment of bone fractures. Rigid metal plates stabilize the fracture site, maintain good contact between bone fragments and allow early weight bearing and patient mobility. However, treatment with rigid metal plates can cause localized bone atrophy due to stress-shielding, and the weakened bone can refracture after plate removal. A hybrid bone plate system that combines the torsional and bending rigidity of a metal plate with the axial compliance of a polymer insert has been designed. A three-dimensional, quarter-symmetric finite element model was generated for a canine femur diaphysis (shaft of thigh bone) plated with this metal/polymer hybrid design. A model with a standard metal fixation plate was also generated for comparison purposes. The stress state in the underlying bone was examined for several loading conditions taken from published in vivo studies. The finite element model was used to study the performance of biodegradable polymer inserts in the plate system. An in vitro pilot study of the polymer's degradation behaviour was completed to determine appropriate material properties for the biodegradable inserts. The polymer inserts had a small but significant effect on the stress state under the plate when subjected to an axial load. The bending strength of the plate was not compromised by the addition of the polymer inserts. Biodegradable inserts further enhanced the performance of the new plate design, transferring less of the axial load to the plate as the inserts broke down.
