Failure of Soldered Bar Attachments in Implant Overdentures

Objectives: To identify the physical and metallurgical properties of failed fatigue-cycled implant-overdenture bar attachment solder joints using nano-indentation and SEM/EDS.

Methods: Crack initiation and propagation behaviour of Type-IV gold “Dolder” cantilever bars (DB) soldered (using two heating methods – flame and furnace) to non-oxidising gold cylinders (NGC) with a gold solder (n=5), was investigated by examining the fatigue-cycled joints under SEM after every 1000 cycles until failure. Other NGCs were soldered to DBs using three melting-point gold-solders with a flame and furnace technique (n=1). The joint areas were sectioned, polished and nano-indented to assess the modulus and hardness of the parent alloys, solder and interface zones, then examined under SEM. Elemental compositions of the metallic phases were identified using EDS.

Results: SEM analysis showed the development of several distinct eutectic phases within the solders at the solder / NGC interface and a distinct diffusion zone of +-5µm depth within the NGC. Fatigue cracking initiated in the solder and quickly migrated to the NGC diffusion zone where instantaneous brittle failure occurred within the diffusion zone. EDS analysis of the phases showed migration of elements between the NGC and the solders. Specifically, the migration of zinc, copper and silver from the solders into the NGC to form a diffusion zone. There was no migration or phase changes at the DB / solder interface. Nano-indentation across the DB / solder / NGC joint areas showed higher values for E-modulus and hardness of the NGC diffusion zone.

Conclusion: Fatigue cracking starts in the gold solder and migrates quickly into a diffusion zone that resulted from the migration of zinc, copper and silver from the solders into the NGC. This diffusion zone shows an increase in brittleness and hardness, which leads to brittle failure of the joint.