Evaluation of test protocol variables for dental implant fatigue research

Objective: This work begins to explore certain ISO-14801 fatigue protocol variables to rationalize implant testing procedures and lay groundwork for later validation of in vitro methods as reproducing clinically-relevant failure mechanisms. Methods: 24 Straumann implants (4.1mm x 12mm, 5.5mm solid abutments) were tested per ISO-14801 up to 5 million cycles at either 2Hz or 30Hz in room air or normal saline at 37°C (n = 6 per group) (30Hz versus 15Hz chosen to accelerate frequency as a stressor).  Fracture-surfaces were examined by SEM.  Two methods were developed to estimate fatigue crack growth rates (implant displacement and fracture surface measures). Results: Failures were found to be bi-modally distributed, either < 350,000 cycles or > 1.5 million cycles at both 2Hz and 30Hz.  Implant failure involved three distinct stages: (1) crack pop-in, (2) fatigue-crack growth and (3) ductile failure; with actual fatigue crack growth involving only 1,100–4,200 cycles.  Failure was more likely at 2Hz than 30Hz (c², p < 0.05) but testing in air and normal saline were equivalent in terms of likelihood of fracture versus runout (c², p > 0.6).  Fatigue crack growth rates increase with distance from the origin similarly at 2Hz and 30Hz, but may be slower in saline at 2Hz due to damage accumulation involving grain boundaries. Conclusion: Implant failure involved “classic” fatigue mechanisms.  Failure was more likely at 2Hz than 30Hz for reasons remaining to be elucidated.  Compared to total cycles-to-failure, greater than 97% of the lifetime of the implant was used to create an initial crack.  Saline may enable chemically-assisted crack growth involving grain boundaries during the stage of fatigue crack growth, but did not influence failure likelihood.  Bi-modal distributions may indicate distinct mechanisms for early versus late failure.  Supported by the ITI Foundation for the Promotion of Oral Implantology, Switzerland.