| Mechanical failures in removable partial denture (RPD) frameworks do not usually occur immediately, but after years of use. Some reports indicate that the components of the framework undergo fatigue, a mode of failure whereby a material or structure eventually fails after being subjected to repeated stresses. Studies have shown that normal mastication can produce thousands of stress cycles per day. Thus fatigue resistance is an important factor in the clinical durability of dental materials.Ti-6Al-7Nb alloy has been considered as an alternative to Co-Cr and other existing RPD alloys because it offers excellent biocompatibility, good resistance to abrasion, and other advantages for relatively low cost. The present study appraised the fatigue resistance of Ti-6Al-7Nb alloy under the process of heat-treatment and laser-welding, also simulated the variation of three types of alloy including Co-Cr, TA2, and Ti-6Al-7Nb alloy. This study will provide a solid basis for clinical application of Ti-6Al-7Nb alloy.Objective and methods:1. The influence of different heat-treatments on fatigue strength of Ti-6Al-7Nb alloy castings was investigated. The Ti-6Al-7Nb alloy was casted into a certain shape according to ASTME466-07 and were performed with different heat-treatments: annealed at 950℃for 1h then aged at 560℃for 2h as the solution annealing and age harden group (treatment A); annealed at 750℃for 1h as the recrystallization group (treatment B); aged at 250℃(treatment C1), 300℃(treatment C2), 350℃(treatment C3),and 400℃(treatment C4) for 10 min after treatment B as the isothermal annealing groups. And the treatment D as control groups. The castings were tested by Force-Controlled Axial Fatigue Testing procedure, the fatigue failure loading cycle numbers (FFLCN) of the castings were recorded, so were the tensile strength.2. This study evaluated the effect of different irradiation power on the fatigue resistance and tensile strength of laser-welded wrought and cast specimens fabricated from TA2 and Ti-6Al-7Nb alloy, which were cast into a certain shape according to ASTME466-07 and joined with laser welding in three kind of electrical current (125mA 150mA 175mA). The castings were tested by Force-Controlled Axial Fatigue Testing procedure, the FFLCN of the castings were recorded, the tensile strength as well.3. The objective of this study was to compare the retention force of 120# clasps made of Co-Cr, TA2 and Ti-6Al-7Nb alloy. The retention force and circulation was tested by an insertion/removal testing apparatus designed at the Fourth Military Medical University was used. The apparatus allowed the placement (insertion) of the framework to its predetermined terminal position and its subsequent removal from the matrix, thus simulating the placement and removal of an RPD 4. .Results:1. Tensile strength of treatment A is the highest (P<0.05); The FFLCN of treatment C2, C3, C4 are the highest. The FFLCN of treatment C1 is lager than treatment A, B and D(P<0.05). Treatment A and D are the lowest. Therefore, the isothermal annealing groups including group C2, C3, C4 improved fatigue resistance of Ti-6Al-7Nb alloy most significantly during this test.2. The 175mA bilaterally welded TA2 and Ti-6Al-7Nb specimens were nearly as strong as their corresponding control specimens.3. The data were subjected to analysis of correlation and the linear regression test. The Student t test was used to compare the same alloys with different undercuts. The correlation is significant (α=0.05), it shows an attenuation trend in circulation (P < 0.01), and it can be concluded that the differences between the slopes are extremely significant. The variations of the retention in the circulation are obvious from the different undercut depths.Conclusions:1. The isothermal annealing groups including group C2,C3,C4 improved fatigue strength of Ti-6Al-7Nb alloy most significantly during this test.2. When a large proportion of the cross-sectional area of the joint is laser welded, the fatigue resistance and tensile strength of the laser-welded portion of the cast Ti and Ti-6Al-7Nb may approach or equal that of the non-welded metal frameworks. 3. In the first 1.5 years of use, the retention force of Ti-6Al-7Nb alloy clasps are better than that of TA2, but less than that of Co-Cr alloy clasps. Since the loss of retention force of Co-Cr alloy clasps are much greater than that of Ti-6Al-7Nb alloy clasps, the retention force of Ti-6Al-7Nb alloy clasps became greater than that of Co-Cr alloy clasps after 1.5 years of use. The Ti-6Al-7Nb alloy clasps showed adequate fatigue resistance in these three kind of clasps, which next to that of Co-Cr alloy clasps and better than TA2.
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