Dental ceramics: What's new?

K Supreetha S Naik; GS Tarun; BS Keshava Prasad; Karanam Apoorva Prakash; NV Sheela; Sindhu Haldal

doi:10.4103/ijmo.ijmo_16_21

REVIEW ARTICLE

Year : 2021 | Volume : 6 | Issue : 2 | Page : 46-48

Dental ceramics: What's new?

K Supreetha S Naik¹, GS Tarun², BS Keshava Prasad³, Karanam Apoorva Prakash¹, NV Sheela², Sindhu Haldal²
¹ Lecturer, Department of Conservative Dentistry and Endodontics, D A Pandu Memorial R V Dental College, Bengaluru, Karnataka, India
² Reader, Department of Conservative Dentistry and Endodontics, D A Pandu Memorial R V Dental College, Bengaluru, Karnataka, India
³ Professor and Head, Department of Conservative Dentistry and Endodontics, D A Pandu Memorial R V Dental College, Bengaluru, Karnataka, India

Date of Submission	21-Sep-2021
Date of Acceptance	26-Oct-2021
Date of Web Publication	24-Dec-2021

Correspondence Address:
Dr. K Supreetha S Naik
BDS, MSc (Dental Materials), Lecturer, Department of Conservative Dentistry and Endodontics, D A Pandu Memorial R V Dental College, Bangalore, Karnataka
India

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/ijmo.ijmo_16_21

Abstract

Dental restorations associated with the development of novel microstructures for ceramic materials have caused an important change in the clinical workflow for dentists and technicians, as well as in the treatment options offered to patients. New microstructures have also been developed by the industry to offer ceramic and composite materials with optimized properties, i.e. good mechanical properties, appropriate wear behavior, and acceptable esthetic characteristics. The objective of this literature review is to discuss the main advantages and disadvantages of the new ceramic systems and processing methods.

Keywords: Ceramics, composite resins, computer-aided design, dental materials, dental porcelain

How to cite this article:
Naik K S, Tarun G S, Keshava Prasad B S, Prakash KA, Sheela N V, Haldal S. Dental ceramics: What's new?. Int J Med Oral Res 2021;6:46-8

How to cite this URL:
Naik K S, Tarun G S, Keshava Prasad B S, Prakash KA, Sheela N V, Haldal S. Dental ceramics: What's new?. Int J Med Oral Res [serial online] 2021 [cited 2022 Jan 19];6:46-8. Available from: http://www.ijmorweb.com/text.asp?2021/6/2/46/333675

Introduction

Dental ceramics are materials that are part of systems designed with the purpose of producing dental prostheses that in turn are used to replace missing or damaged dental structures. The literature on this topic defines ceramics as inorganic, nonmetallic materials made by human by the heating of raw minerals at high temperatures.^{[1],[2],[3],[4],[5]} Ceramics and glasses are brittle, which means that they display a high compressive strength but low tensile strength and may be fractured under very low strain (0.1%, 0.2%). As restorative materials, dental ceramics have disadvantages mostly due to their inability to withstand functional forces that are present in the oral cavity. Hence, initially, they found limited application in the premolar and molar areas, although further development in these materials has enabled their use as a posterior long-span fixed partial prosthetic restorations and structures over dental implants.^{[6],[7],[8],[9],[10]} All dental ceramics display low fracture toughness when compared with other dental materials, such as metals.

Classification

Ceramics can be classified by their microstructure (i.e., amount and type of crystalline phase and glass composition). They can also be classified by the processing technique (power-liquid, pressed, or machined).^{[1],[2],[3],[4],[5],[11],[12],[13],[14],[15]}

Microstructural Classification

At the microstructural level, we can define ceramics by the nature of their composition of glass-to-crystalline ratio. There can be infinite variability of the microstructures of materials, but they can be broken down into four basic compositional categories, with a few subgroups:

Composition category 1 – glass-based systems (mainly silica)
Composition category 2 – glass-based systems (mainly silica) with fillers, usually crystalline (typically leucite or, more recently, lithium disilicate)
Composition category 3 – crystalline-based systems with glass fillers (mainly alumina) and
Composition category 4 – polycrystalline solids (alumina and zirconia).

Glass-Based Systems

Glass-based systems are made from materials that contain mainly silicon dioxide (also known as silica or quartz), which contains various amounts of alumina.

Aluminosilicates found in nature, which contain various amounts of potassium and sodium, are known as feldspars. Feldspars are modified in various ways to create the glass used in dentistry. Synthetic forms of aluminosilicate glasses are also manufactured for dental ceramics.

Composition category 2 – Glass-based systems with fillers

This category of materials has a very large range of glass–crystalline ratios and crystal types, so much so that this category can be subdivided into three groups. The glass composition is basically the same as the pure glass category.

The difference is that varying amounts of different types of crystals have either been added or grown in the glassy matrix. The primary crystal types today are leucite, lithium disilicate, or fluoroapatite.

Subcategory 2.1

Low-to-moderate leucite-containing feldspathic glass – these materials have been called “feldspathic porcelains” by default. Even though other categories have a feldspathic-like glass, this category is what most people mean when they say “feldspathic porcelain.”

Subcategory 2.2

High-leucite-containing (approximately 50%) glass – again, the glassy phase is based on an aluminosilicate glass. These materials have been developed in both powder/liquid, machinable, and pressable forms.

Subcategory 2.3

Lithium-disilicate glass ceramic is a new type of glass ceramic introduced by Ivoclar as IPS Empress^® II (now called IPS e.max^®), where the aluminosilicate glass has lithium oxide added.

Composition category 3 – Crystalline-based systems with glass fillers

Glass-infiltrated, partially sintered alumina was introduced in 1988 and marketed under the name In-Ceram. The system was developed as an alternative to conventional metal ceramics and has met with great clinical success.

Composition category 4 – Polycrystalline solids

Solid-sintered, monophase ceramics are materials that are formed by directly sintering crystals together without any intervening matrix to form a dense, air-free, glass-free, polycrystalline structure. There are several different processing techniques that allow the fabrication of either solid-sintered aluminous-oxide or zirconia-oxide frameworks.

Classification Based on Processing Technique

A more user-friendly and simplistic way to classify the ceramics used in dentistry is by how they are processed. It is important to note that all materials can be processed by varied techniques. However, in general, for dentistry, they can be classified as:

Powder/liquid, glass-based systems
Machinable or pressable blocks of glass-based systems
Computer-aided design/computer-aided manufacturing or slurry, die-processed, mostly crystalline (alumina or zirconia) systems.

Powder/liquid, with or without crystalline fillers

These are the porcelains that are made for veneering cores made from either metal, alumina, or zirconia but can be used for porcelain veneers on either a refractory die or platinum foil technique.

Manufactured blocks, with or without crystalline fillers

Vitabloc Mark II for the CEREC and pressable and machinable versions of IPS Empress are the primary materials available in this classification. These materials are ideally suited for inlay and onlay restorations, anterior crowns and veneers, and possibly bicuspid crowns. They have to be bonded and can be used full contour as there are polychromatic machinable versions.

Computer-aided design/computer-aided manufacturing or slurry/die-generated mostly or all-crystalline alumina- or zirconia-based systems

Alumina materials in this classification are Procera, which is solid-sintered alumina, and In-Ceram, which is glass infiltrated. These materials work well for cores for single crowns that are veneered with a powder/liquid glass-based material (porcelain).

Strength and Fracture Toughness

There are two interrelated properties that often are quoted regarding ceramics intended for structural purposes:^[8],[9],[10]

Strength
Fracture toughness.

Strength

Mechanical failure of ceramic materials is almost completely controlled by brittle fracture. Usually, this brittle behavior combined with surface flaws resulted in relatively low ceramic strengths. Increased crystalline-filler content within the glass matrix, with a more even distribution of particles and finer particle size, has yielded significant improvements in the flexural strength of ceramic materials. However, strength improvements are still limited by the inherent weakness of the glass matrix. All ceramics fail because of crack propagation at a critical strain of 0.1%. Applied stresses can cause a crack to grow throughout the matrix, causing the ultimate failure of that restoration.

Fracture toughness

A more important physical property is fracture toughness, which has been reported to be between 8 MPa m1/2 and 10 MPa m1/2 for zirconia. This is significantly higher than any previously reported ceramic and roughly twice the amount reported for the alumina materials. Fracture toughness is a measure of a material's ability to resist crack growth (i.e., a measure of the amount of energy necessary to cause crack growth). Clinically, restorations are not loaded to failure as is done in a flexural strength test; instead, millions of subcritical loads (chewing) are applied. Materials ultimately fail because of this cyclic fatigue by crack propagation. Thus, materials with higher fracture toughness are more ideal clinically as it takes more energy to cause crack growth. Other factors such as stress corrosion (chemically assisted crack growth) and residual flaws in the material greatly affect the final strength of finished material.

Mechanisms that can lead to toughened or strengthened ceramics can be categorized into the following three types

Crack tip interactions

These occur when obstacles in the microstructure act to impede the crack motion. These are generally second-phase particles and act to deflect the crack into a different plane so that it is no longer subject to the normal tensile stress that originally caused its propagation.

Crack tip shielding

These are a result of events that are triggered by high stresses in the crack tip region that acts to reduce these high stresses. Transformation toughening and microcrack toughening are two mechanisms that have been identified as leading to crack tip shielding.

Crack bridging

This occurs when the second-phase particles act as a ligament to make it more difficult for the cracks to open. Crack bridging is best understood for bonded fiber composites. This mechanism has been shown to be important in large-grain Al₂O₃ and possible whisker-reinforced ceramic materials.

Conclusion

The new generation of ceramic materials presents interesting options, both in terms of material selection and in terms of fabrication techniques. A closer understanding of the dynamics of the materials with respect to design of the restoration and the intended use is required to enable these restorations to perform productively.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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