Deflection Fatigue of Ti-6Al-7Nb, Co-Cr, and Gold Alloy Cast Clasps

Objective: This study evaluated deflection fatigue of cast clasps made from some partial denture alloys. Methods: Pre-formed clasp patterns were cast in one of three alloys including Ti-6Al-7Nb (T-alloy Tough, GC), Co-Cr (Biosil-L, Degussa) and type IV gold alloy (Degulor M, Degussa). Each of thirty three Ti-6Al-7Nb, thirty two Co-Cr, and fifteen gold alloy clasps was fixed to a micro material testing machine and subjected to sinusoidal cyclic deflection in the radial direction in a frequency of 5 Hertz. The deflection of each clasp was chosen from one of the four pre-determined values: 0.25 mm, 0.375 mm. 0.50 mm, and 0.75 mm. The test was continued until the applied force was reduced to 15 % of the initial force, or until one million cycles were completed. Finite element clasp models were also created to calculate the principle stress within the clasps. Results: Permanent deformation was significantly greater in Co-Cr clasps than those seen in the other clasps, indicating less adaptation of the Co-Cr clasps to the tooth surface. Average permanent deformations for 0.50 mm preset deflection were 27 µm for Ti-6Al-7Nb, 111 µm for Co-Cr, and 40 µm for the gold alloy clasps. Ti-6Al-7Nb clasps showed significantly less permanent deformation. When under relatively large deflections such as 0.50 mm and 0.75 mm, the gold alloy clasps showed significantly longer fatigue life while Ti-6Al-7Nb clasps showed less. Conclusion: Ti-6Al-7Nb clasps showed superior elastic recovery, while the gold alloy demonstrated longer fatigue life.