Technology of Fixed Prosthodontics (2) - DEP3202 - Spring 2023, Slides of Biomaterials

Download Technology of Fixed Prosthodontics (2) - DEP3202 - Spring 2023 and more Slides Biomaterials in PDF only on Docsity! Ss KANTARA SINAL UNIVERSITY Dental Alloys in Fixed Prosthodontics (1) Dr Ahmed Elbieh BDS, M.Sc., PhD SU SINAI UNIVERSITY * Alloys are used for fixed prostheses rather than pure metals because pure metals do not have the appropriate physical properties to function in these types of restorations. * Thus, the use of alloys provides physical, mechanical and biologic properties that are required for successful, long-term fixed prostheses. SU SINAI UNIVERSITY Requirements of dental casting alloys: 9. Biocompatibility: Alloys should not be harmful, toxic, allergic, or carcinogenic. 10. Phase structure: Single-phase alloys are generally easier to manipulate (i.e., easier to cast) and have lower corrosion rates than multiple phase alloys; however, multiple-phase alloys may be etched for bonding and may be significantly stronger than single-phase alloys. 11. Grain size: For gold-based alloys, a small (30 1) grain size has been shown to improve tensile strength and elongation. For base-metal alloys, the grains are large and may approach 1 mm in diameter. 12. Alloy solidus and fit: The higher the solidus temperature, the more shrinkage occurs resulting in ue crowns. The shrinkage must be compensated by expansion of the die, application of die spacers, and the use o special expanding investment mechanisms. 13. Soldering: Gold-based alloysare most easily soldered compared with palladium-, nickel-, silver-, or nickel- based alloys. Furthermore, the heating that occurs during soldering is more likely to cause distortion of the casting. SU SINAI UNIVERSITY Classification of dental casting alloys: 1. According to use (gold alloys): . Type I: Simple inlays (restorations subjected to low stresses, class Ill, V). Maximum yield strength: 140Mpa; Minimum % of elongation: 18%. . Type Il: Complex inlays (restorations subjected to moderate stresses, class |, Il, IV). Maximum yield strength: 140-200Mpa; Minimum% of elongation: 18%. . Type Ill: Crowns and FPDs and short span bridges (restorations subjected to high stresses). Maximum yield strength: 200-340Mpa; Minimum % of elongation: 12%, e Type IV: RPDs and pinledges and long span bridges (restorations subjected to very high stresses). Maximum yield strength: >340Mpa; Minimum% of elongation: 10%. SINAL UNIVERSITY Ss KANTARA 1. According to physical properties (specifically their hardness) (1965): Type I: Soft. Type I: Medium. Type If: Hard. Type IV: Extra hard. SINAI UNIVERSITY Available alloy systems for metal-cersHe restorations: 1. High noble metal alloys 2. Noble Metal Alloys 3. Predominantly base-metal alloys High noble metal alloys: SUAS The high-noble metal content alloys contain a minimum of 60% by weight of noble elements; at least 40% is gold. There are three systems in this class: gold-platinum-palladium(Au-Pt- Pd), gold- palladium-silver(Au-Pd-Ag), and gold-palladium(Au-Pd). These alloys are single-phase with a low modulus of elasticity. The corrosion of these alloys is generally low but may be higher if multiple phases are present. They may be used for full cast or metal-ceramic applications. Au-Pt-Pd (Au 88%) These were the first casting alloys formulated to bond with dental porcelain. The addition of platinum & palladium to gold increase the melting range & decrease the coefficient of * thermal expansion of the alloy to a value closer than that of porcelain. Advantages: » Excellent castability. » Excellent porcelain bonding. » Easy to adjust and finish. » Burnishable. » Excellent corrosion resistance and biocompatibility. » Yellow color which imparts a natural looking appearance of the final restoration. SU SINAI UNIVERSITY Disadvantages: Poor sag resistance (unsuited for long span FPDs). + Low hardness. + Expensive. High density. The alloy requires a minimum coping thickness of 0.5mm due to low modulus of elasticity which may result in over contoured restoration resulting in gingival irritation.