A new silicone elastomer provides maximum resistance to flexural fatigue, crack growth, and the high durability required in an elastomer used as a material of construction for the flexible-hinge finger and toe joint implants designed by A. B. Swanson, M. D. The high-performance characteristics of this new elastomer are derived from its excellent physical properties, particularly its exceptionally high tear propagation strength (ASTM Test for Tear Resistance—Rubber Property (D 624-73), Die B) and high resistance to crack propagation (ASTM Test for Rubber Deterioration—Crack Growth (D 813-59)). In vitro testing indicated its use in finger joint implants would provide excellent flexural durability. Biocompatibility evaluations to qualify this new material for clinical use included chronic biodurability studies. Test specimens were implanted subcutaneously in dogs and retrieved at various time intervals for periods up to two years. Physical properties of test and control specimens were compared. The properties measured included tensile strength, tensile stress, ultimate elongation, tear strength (Die B), and flaw propagation characteristics. All testing was done by ASTM standard methods except for flaw propagation measurements where implantability limitations in specimen size mandated use of a smaller specimen than required in standard test methods. Thus, a flaw propagation test method was specifically designed for this experiment.

The findings indicated that this new high-performance silicone elastomer had excellent biodurability. The performance characteristics evaluated in test and control specimens were stable over the two-year implant time. Lipid absorption stabilized at approximately 1.5%, a level reached relatively early after implantation and remaining constant thereafter, apparently representing the lipid saturation level when this elastomer is implanted subcutaneously in the dog.