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Influence of loading frequency on implant fatigue
M. KARL, University Clinic Erlangen, Germany, and J.R. KELLY, University of Connecticut, Farmington, USA | 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.
| Seq #122 - Implant Prosthodontics 3:30 PM-4:45 PM, Thursday, July 3, 2008 Metro Toronto Convention Centre Exhibit Hall D-E |
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