Alloys in prosthetic dentistry

Why alloys instead of pure metals?

In dental practice, exclusively metal alloys are used to create structural elements. The use of metals in their pure form is ruled out for two main reasons: they have too low strength to withstand masticatory pressure and quickly undergo corrosion in the aggressive environment of the oral cavity.

Mixing several elements makes it possible to obtain a material with predetermined parameters, ideal for a specific part of a dental prosthesis. The properties of such structural materials are usually divided into three large groups:

• Physical and mechanical.
• Chemical.
• Technological.

Key physical and mechanical characteristics

For a prosthesis to last a long time, without breaking or deforming, the alloy must possess a complex of physical and mechanical properties:

• Strength — a characteristic that allows a material to withstand external loads and not fail when attempts are made to deform it.
• Elasticity — the ability of a material to completely restore its original shape after the action of external forces has ceased.
• Plasticity — the property opposite to elasticity. It is the ability to change configuration under pressure without the formation of cracks and to retain the acquired outlines after the load is removed.
• Hardness — resistance to plastic deformation in situations where another, harder metal attempts to penetrate the structure.
• Fluidity (castability) — a critically important parameter for casting, reflecting the ability of the molten mass to fill the mold.

Special attention should be paid to deformation — a change in the size and shape of a body under external pressure. It comes in two types:

1. Elastic deformation — disappears without a trace after the load is removed, without causing changes in the volume, structure, or properties of the alloy.
2. Plastic (residual) deformation — is not eliminated after pressure is removed. Its consequences are very serious: it causes a change in structure and volume, decreases the material's density, increases its electrical resistance, and alters magnetic properties.

Classification and basic compositions

According to their composition, all applied materials are divided into two large groups. The first is metal alloys, which consist entirely of metals or contain a small amount of non-metals. The second group is non-metallic, which includes glass, porcelain, and glass-ceramics (sitalls).

Speaking of metal bases, three main subgroups are most in demand in prosthodontics:

• Based on noble elements: gold, silver, and palladium.
• Based on iron, chromium, cobalt, and nickel (base alloys for strong frameworks).
• Based on copper, nickel, titanium, aluminum, niobium, and tantalum.

Requirements for dental alloys

Any material entering a patient's oral cavity undergoes rigorous selection. The requirements are divided into two vectors.

General medical properties guarantee safety: the material must be biocompatible, exert no toxic effect on tissues, and not provoke allergic reactions.

Technological properties ensure the dental technician's ease of work and the longevity of the structure. These include:

• High corrosion resistance.
• Optimal combination of hardness and strength.
• Minimal shrinkage during casting and good fluidity.
• Low melting point.
• Ability to undergo forging, soldering, and welding.
• Excellent electrolytic and mechanical indicators, which are necessary for high-quality processing and final polishing of the finished product.

The role of adhesive systems in fixation

For a reliable connection of orthopedic structures with dental tissues, adhesive systems are used, having progressed from generation I to VII. Early generations (I and II) consisted of several components, had weak adhesion (up to 15 MPa), and required additional retaining devices. Starting from generation IV, the concept of total etch and hybridization was introduced, and the bond strength to dentinum (dentin) increased to 17–25 MPa.

Modern representatives (generations V–VII, for example, I Bond or Single Bond) have significantly simplified the clinician's work. Generation VII systems combine etching, desensitization, application of primer and adhesive into a single step. They require no mixing, almost completely eliminate hypersensitivity, and provide an excellent bond with metal.

Main rule of selection: it is necessary to use all components of the system exclusively from a single manufacturer. Combining materials from different brands does not guarantee strong adhesion.

Class and exam questions

What else to study on this topic

• Detailed analysis of the characteristics of adhesive systems from generation I to VII.
• Mechanisms of hybridization and wet bonding in modern adhesive protocols.
• The influence of plastic deformation on the magnetic and electrical properties of metals.
• Features of non-metallic alloys (glass-ceramics, porcelain, glass) in prosthetics.
• Specifics of using noble metal-based alloys (gold, silver, palladium).

Other topics in "Chapter 8. Dental Materials"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.