Methods: Failed bar-attachment systems from three maxillary overdentures in a randomised controlled clinical trial and two mandibular overdentures from routine treatment were examined under SEM to identify sites and aetiology failure. A further six non-failed maxillary bar-attachment systems were examined (five from the randomised controlled clinical trial at the two-year recall stage and one routine treatment at the five-year recall stage) under SEM to identify potential sites of failure initiation. The clinical failure process is compared with a simple flexure and fatigue laboratory study of similar cantilever soldered joints.
Results: All the failed bar-attachment systems, as well the intact bar-attachment systems, revealed signs of stress corrosion. In the failed bars, there was evidence of corrosion in the grain boundaries of the solder joining the gold bars to the gold cylinders which undermined the solder joint leading to crack initiation and growth resulting in classic fatigue behaviour and failure of the bars. In the intact bars, evidence of localised corrosion was observed in areas of high stress concentration with crack initiation observed in two of the bars.
Conclusion: Bar-attachment systems with removable implant overdentures are subject to torquing and flexing mechanical forces resulting in high stress concentration at the solder joints that appear to initiate stress corrosion cracking and breakage. There is a need to establish appropriate safe design and fatigue resistance criteria along with better physical and metallurgical properties of bar-attachment systems and soldering materials for implant overdentures as well as methods of manufacture.
Supported by: ITI Research Foundation for the Promotion of Implantology, Switzerland (Grant for Clinical Research RCL 252/2002), NobelBiocare, Australia, and Southern Implants, South Africa.