Evaluation of test protocol variables for dental implant fatigue research

This work begins to explore the influence of cycling rate and environment on fatigue testing of dental implants according to the ISO protocol 14801.

Twenty-four Straumann implants (4.1 mm x 12 mm) were tested up to five million cycles per ISO 14801: loaded at either 2 or 30 Hz in room air at 25 degrees C or normal saline at 37 degrees C (n=6 per group). Implant displacements/cycle were captured during all testing. Fracture-surface features were examined using scanning electron microscopy (n=12). Two complimentary methods were developed to estimate fatigue crack growth rates.

Failures (bulk fracture) were found to be bi-modally distributed, either <350,000 cycles or >1.5 million cycles at both cycling rates. Following initial crack formation, fatigue crack growth required merely 1100-4200 cycles to failure. Initial crack pop-in was statistically more likely under 2 Hz than 30 Hz (chi(2), p<0.05) but testing in air and normal saline were equivalent in terms of likelihood of fracture versus runout (chi(2), p>0.6). On a microscopic level, fatigue crack growth rates appears to be similar at 2 and 30 Hz, but may be slower in the presence of saline versus dry at 2 Hz.

Implant failure under fatigue conditions involved "classic" damage mechanisms. Failure appears more likely at 2 Hz than 30 Hz for reasons that remain to be elucidated. Saline may enable chemically assisted crack growth involving grain boundaries during the stage of fatigue crack growth, but did not influence likelihood of failure.