Dental Materials at a Glance

J. Anthony von Fraunhofer

• Publisher: Wiley-Blackwell
• Year: 2013
• Print ISBN: 9781118459966
• eBook ISBN: 9781118646649
• Edition: 2
• Topic: Medicine
• Subtopic: Dentistry

Dental Materials at a Glance, 2 nd edition, is the latest title in the highly popular At a Glance series, providing a concise and accessible introduction and revision aid. Following the familiar, easy-to-use at a Glance format, each topic is presented as a double-page spread with key facts accompanied by clear diagrams encapsulating essential information.

Systematically organized and succinctly delivered, Dental Materials at a Glance covers:

• Each major class of dental material and biomaterial
• Basic chemical and physical properties
• Clinical handling and application
• Complications and adverse effects of materials

Dental Materials at a Glance is the ideal companion for all students of dentistry, residents, and junior clinicians. In addition, the text will provide valuable insight for general dental practitioners wanting to update their materials knowledge and be of immediate application for dental hygienists, dental nurses, dental assistants, and technicians.

Part I

Fundamentals

1

Properties of materials—tensile properties

Figure 1.1 Applied forces and specimen deformations.

Figure 1.2 Load versus stress for feet.

Figure 1.3 The stress–strain curve of a nonferrous metal.

Figure 1.4 Stress–strain curves for brittle, elastic, and ductile materials.

Figure 1.5 Elastic and plastic regions of a stress–strain curve.

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Box 1.1 Desirable properties of dental materials

Biocompatibility

Absence of toxicity

Aesthetic appearance

Strength and durability

Low solubility

Ease of manipulation

Long shelf life

Simple laboratory processing

Long working time

Rapid/snap set

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Table 1.1 Typical mechanical properties of dental biomaterials

Dental biomaterials are used in laboratory procedures and for the restoration and replacement of teeth and bone. Material selection must consider function, properties, and associated risks, and all dental biomaterials must satisfy certain criteria (Box 1.1).

Mechanical properties are important since teeth and restorations must resist biting and chewing (masticatory) forces. Typical material properties are given in Table 1.1.

Biting forces vary with patient age and dentition, decreasing for restored teeth and when a bridge, removable partial denture (RPD), or complete denture is present. Effects vary with the type of applied force and its magnitude. Types of applied force, and the resulting deformations, are shown in Figure 1.1.

1 Stress: σ, force per unit cross-sectional area

• Stress, the applied force and the area over which it operates, determines the effect of the applied load. For example, a chewing force of 72 kg (10 N) spread over a quadrant 4 cm² in area exerts a stress of 18 kg/cm² (1.76 MPa). However, the same force on a restoration high spot or a 1-mm² hard food fragment produces a stress of 7200 kg/cm² (706 MPa), a 400-fold increase in loading. This stress effect is one reason that occlusal balancing is essential in restorative dentistry. A more graphic example of the difference between applied force and stress is shown in Figure 1.2. This example also clearly indicates why it is more painful when a woman wearing high heels steps on you than when a man does!

2 Strength: The stress that causes failure

3 Ultimate strength: The maximum stress sustained before failure

4 Proportional limit: The maximum stress that the material can sustain without deviation from linear stress–strain proportionality

5 Elastic limit: Maximum stress that can be applied without permanent deformation

6 Yield strength: σ_Y, stress at which there is a specified deviation from stress-to-strain proportionality, usually 0.1%, 0.2%, or 0.5% of the permanent strain

7 Strain: ε, ratio of deformation to original length, ΔL/L; measures deformation at failure

8 Ductility: Percentage elongation, i.e. ΔL/L × 100%

• Ductile materials exhibit greater percentage elongation than brittle materials and can withstand greater deformation before fracture.

9 Burnishing index: Ability of a material to be worked in the mouth or burnished, expressed as the ratio of % elongation to yield strength

10 Poisson's ratio: ν, ratio of lateral to axial strain under tensile loading; denotes reduction in cross-section during elongation

• Brittle materials have low ν values, i.e. little change in cross-section with elongation, whereas ductile materials show greater reduction in cross-section, known as specimen necking.

11 Elastic modulus: E, ratio of stress to strain, also known as modulus of elasticity or Young's modulus; denotes material stiffness and is determined as the slope of the elastic (linear) portion of the stress–strain curve

12 Stress–strain curves: Generated by applying a progressively increasing tensile force while measuring applied stress and material strain until fracture occurs

The shape of the stress–strain curve indicates the properties of the material (Figure 1.3 and Figure 1.4):

• Nonferrous metals (e.g., gold and copper) show a continuous curve to failure whereas ferrous materials exhibit a “kink” in the curve, known as the yield point.
• The intersection of a line parallel to the abscissa (strain) axis from the failure point to the ordinate (stress) axis is specimen strength whereas the vertical line from the failure point to the strain axis is the ductility.
• High-strength, brittle materials show steep stress–strain curves with little strain at failure, e.g. ceramics.
• Strong ductile materials, e.g. metals, show moderate slopes in the stress–strain curve but good extension until failure.
• Soft ductile materials, e.g. elastomers, show long, shallow linear stress–strain behavior followed by a sharp rise in the curve when, with increasing applied force, the elastomer no longer extends linearly (or elastically) and failure occurs.

13 Resilience: Resistance to permanent deformation (i.e., energy required for deformation to the proportional limit); given by the area under the elastic portion of the stress–strain curve (Figure 1.5)

14 Toughness: Resistance to fracture (i.e., energy required to cause fracture); given by the total area (i.e., both the elastic and plastic regions) under the stress–strain curve (Figure 1.5)

15 Hardness: Resistance to penetration; a measure of scratch resistance

• Hardness is measured by several techniques, including the Barcol, Bierbaum, Brinell, Knoop, Rockwell, Shore, and Vickers tests.

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Table of contents

• Cover
• Title page
• Copyright page
• Dedication
• Preface
• Part I: Fundamentals
• Part II: Laboratory materials
• Part III: Dental biomaterials
• Glossary
• Index