In vitro Assessment of Clasps of Cobalt-Chromium and Nickel-titanium Alloys in Removable Prosthesis

It has been since a long time that the use of base-metal alloys has significantly increased in the area of prosthetic science. One of the factors contributing for the increasing use of base-metal alloys is the increasing and high cost factor of noble metals. Although numerous materials have been tried since the recent past, Cobalt-Chromium (CoCr) alloys still form the most commonly used ones because of the numerous advantages they offer. Titanium alloys are also significantly being used in this field because of their high resistance strength and high corrosion resistance. Therefore, we aimed to evaluate the flexure strength and fatigue resistance of clasps made up of CoCr alloys and Nickel-titanium (NiTi) alloys.

We evaluated the retentive forces of CoCr and NiTi clasps two different sizes engaging retentive undercuts of different depths (0.25 and 0.50 mm). Mandibluar second premolar was prepared to frame the prosthesis for missing mandibular first and second molar. A total of 50 clasps were included in the study out of which 30 were NiTi clasps and 30 CoCr clasps. We evaluated the loading of the force at the rate of half Newton(N)/second at the central part of the specimen until fracturing of the specimen occurred and finally measurement of the modulus of elasticity was also done. Statistical analysis was carried out; unpaired "t" test was used for evaluating the level of significance.

The mean load needed for CoCr clasps was 0.7450 and for NiTi clasp was 0.6140 Kgf for producing a deflection of 0.25 mm. As far as flexibility is concerned, more value was seen in NiTi group than CoCr group. For deflecting the clasp up to 0.50 mm, the mean load needed for CoCr clasps was 1.4102 and for NiTi clasp was 0.8260 Kgf. The results were statistically significant. While measuring the flexibility, more value (p < 0.05) was seen in NiTi group than in CoCr group. NiTi alloy clasps had mean flexural strength of 1640 MPa and modulus of elasticity of 32 GPa.

For producing deflection of 0.25 and 0.50 mm, the mean loading force was higher for NiTi alloys. Although being increasingly used these days, NiTi alloys have limited use as compared with CoCr alloys due to high cost of titanium and certain other limitations.