Methods: From a total of 40 edentulous participants (mean age 64 yrs; range 55-76yrs) with existing mandibular two-implant overdentures, 20 were randomly allocated to having three maxillary implants splinted with bar units to support opposing maxillary overdentures. The prosthodontic maintenance was documented related to complications with the splinted bar units where matrix maintenance was required or where bar fractures occurred. Bar units were examined under the SEM to identify potential sites and aetiology of existing or possible long-term failure. Potential sites of failure initiation were identified.
Results: Prosthodontic maintenance requirements throughout the two years of the bar units revealed only one bar fracture in year 1 and two bar fractures in year 2. A total of three fractured bars as well as two additional intact bars revealed signs of stress corrosion. In the fractured bars, there was evidence of corrosion in the grain boundaries of the solder joining the gold bar to the gold cylinders which undermined the solder joint leading to crack initiation and growth of the crack resulting in failure of the bars. In the two additional intact bars, evidence of corrosion in an advanced stage was observed with the first signs of cracking observed in one of the bars.
Conclusion: Bar-attachment and bar-joint substructures with removable implant overdentures are subject to torquing mechanical forces in the solder joints that may cause stress corrosion and breakage. There is a need to establish the appropriate safe design and fatigue resistance; physical and metallurgical properties of bar-joint and bar-attachment solder joints for implant overdentures in terms of various soldering materials and methods.
Supported by NobelBiocare, Australia, and Southern Implants, South Africa