Influence of loading frequency on implant fatigue

Purpose: ISO 14801:2003 allows loading frequencies ranging from 2Hz to 15Hz for fatigue testing of implants.  Preliminary studies suggested that fractures were more likely to occur at 2Hz. This investigation further explores a possible frequency effect and underlying damage mechanisms.  Methods: 36 implants (Straumann solid screw) were loaded up to 5x10⁶ cycles (20N – 420N) per ISO 14801 at frequencies of 2Hz and 30Hz (chosen to accelerate frequency as the stressor).  Absolute strain magnitudes and strain rates were measured using one strain-gauged implant (polished collar) loaded cyclically (120N, 220N, 320N, 420N) at frequencies from 2Hz to 30Hz.  Cycles/time and strain-rate/time were analyzed by both Weibull and life data analysis (RealiaSoft Corp.).  Measured strain was used to validate an FEA model for calculation of strains at failure origins (root of first prominent thread).  Fracture surfaces were examined by SEM.  Results:  19/36 implants failed (2Hz 13/18; 30Hz 6/18).  Number-per-group and failure-rates-per-time differed significantly (p < 0.05) but Weibull moduli did not.  Strain was independent of frequency at all loads.  Strain rates were highly dependent on frequency (linear r² > 0.99) and differed significantly under failure conditions (420 N): 2Hz = 8,292 µstrain/sec; 30Hz = 92,956 µstrain/sec.  Measured and FEA-calculated strains were similar.  Fracture surfaces were indistinguishable (2Hz versus 30Hz).  Conclusion:  Fatigue failure is more likely at 2 Hz than 30 Hz.  Since absolute strain (and therefore stress) is identical at these frequencies our results suggest a strain-rate sensitivity for this titanium implant.  Both the Weibull and SEM analyses support an identical failure mechanism with damage accumulation more severe at lower frequencies; an interpretation consistent with strain-rate sensitivity.  Validation of the FEA model supports further investigation of conditions at failure origins. This project was supported by a grant from the ITI Foundation for the Promotion of Oral Implantology, Switzerland.