Properties of elastomers for small-joint replacements

Abstract

Silicones are used to manufacture finger and wrist joints. However these joints have fractured prematurely in vivo. There is a lack of literature on the mechanical properties of silicones. The aim of this thesis was to investigate the viscoelastic and related properties of elastomers such as silicones and polyurethanes (suggested as a possible substitute for silicones in the implants) and to relate the properties to how an implant may perform in vivo. The viscoelastic properties of medical-grade silicones and Elast-Eon$^{TM}$3 were found to depend on frequency in compression. Above a certain frequency, the silicones appeared to undergo a transition from the rubbery to the glassy state. There is a danger that this could lead to the creation of fracture surfaces. The viscoelastic properties of the silicones were not significantly affected by the temperature; pre-treatment of specimens had no appreciable effect on the results. When the viscoelastic properties were measured in tension, there was a significant difference between the properties measured in tension and compression. Accelerated aging significantly increased the moduli of Elast-Eon$^{TM}$3, which is of some concern. The cross-link densities of the silicones were measured using a solvent swelling technique and the Flory-Rehner equation. The results showed that this method is useful as an approximate model