Deflection fatigue of Ti-6Al-7Nb, Co-Cr, and gold alloy cast clasps

STATEMENT OF PROBLEM: There is little information about the deflection fatigue of clasps in relation to stress distribution. PURPOSE: The aim of this study was to investigate the fatigue resistance and permanent deformation of cast clasps made of titanium and other dental alloys and to relate the fatigue resistance with the calculated stress values. MATERIAL AND METHODS: Twenty-five Ti-6Al-7Nb, 25 Co-Cr, and 15 Type IV gold alloy clasps were subjected to cyclic deflection of preset values of 0.25 mm, 0.50 mm, or 0.75 mm, for 10 6 cycles (n = 14). Finite element models were created to calculate principal stresses within the specimens. Fatigue life, retentive force, and permanent deformation were recorded, and the fracture locations were determined microscopically. The results were characterized in relation to the stress within the clasps. One-way analysis of variance and Tamhane's post-hoc tests were used to compare the results of the 9 material-deflection groups (alpha=.05). RESULTS: Ti-6Al-7Nb clasps exhibited significantly less permanent deformation than the other clasps under relatively greater deflections, indicating better adaptation to the tooth surface. However, the fatigue life of the Ti-6Al-7Nb clasps under 0.75-mm deflection, with the stress above the alloy's 0.2% yield strength, was significantly shorter than those under smaller deflections. The gold clasps showed significantly longer fatigue life than the other clasps under the 0.50-mm deflection. High-stress areas within the fatigue clasp specimens coincided with the fracture locations. The probabilities of fatigue fracture and permanent deformation were closely related to the material strengths and the preset deflections. CONCLUSION: To minimize fatigue failures, the cast clasp should be designed with consideration of the stresses distributions within the clasps.