Orthodontic Applications of Biomaterials

A Clinical Guide

Theodore Eliades,William A. Brantley

• Publisher: Woodhead Publishing
• Year: 2016
• ISBN:
  9780081003992
• Topic: Medicina
• Subtopic: Tecnologia e forniture mediche

Orthodontic Applications of Biomaterials: A Clinical Guide reviews the applications of biomaterials and their effects on enamel preparation, bonding, bracket and archwire ligation, mechanotherapy, debonding, and long-term enamel structural, color, and surface effects.

The book provides a step-by-step analysis of the phenomena occurring, their clinical importance, and their underlying cause without the use of complex mathematical or physical-chemical analyses, with the goal of providing 'digestible' evidence for the clinician.

• Serves as a reference source of the spectrum of biomaterials used in orthodontics
• Presents the most current evidence of state-of-the-art methods of materials research
• Provides substantiation for the effects occurring during the materials' uses

Part One

Structure and properties of orthodontic biomaterials

Outline

1. Structure/property relationships in orthodontic alloys

2. Structure/property relationships in orthodontic polymers

3. Structure/property relationships in orthodontic ceramics

1

Structure/property relationships in orthodontic alloys

W. Brantley, D. Berzins, M. Iijima, E. Tufekçi, and Z. Cai

Abstract

Metallic materials occupy a central place among orthodontic materials, being used for archwires, brackets, and a wide variety of appliances. The orthodontist needs to understand how the basic principles of metallic bonding lead to the general properties of metals and how the manner in which orthodontic alloys are fabricated and manipulated, along with their compositions, lead to their physical, mechanical, and biological properties, which determine clinical performance. This chapter will first review the nature of metallic bonding and how this mode of interatomic bonding leads to the general properties of metals. Then the commonly used fabrication methods for alloys employed in orthodontics are described, along with how these methods affect their properties. The compositions and structures of the four major orthodontic alloy types are then presented, along with their uses. The chapter closes with a summary of the important methods employed to obtain information about the structures and properties of orthodontic alloys, with comparisons of clinically relevant properties of these alloys.

Keywords

Beta-titanium; Copper Ni-Ti; Electrochemical corrosion; Elgiloy; Metal injection molding; Micro X-ray diffraction; Orthodontic alloys; Scanning electron microscope; Temperature-modulated differential scanning calorimetry; Three-point bending; Transmission electron microscopy; Vickers hardness test

1.1. Introduction

Metallic materials occupy a central place among orthodontic materials, being used for archwires, brackets, and a wide variety of appliances. The orthodontist needs to understand how the basic principles of metallic bonding lead to the general properties of metals and how the manner in which orthodontic alloys are fabricated and manipulated, along with their compositions, lead to their physical, mechanical, and biological properties, which determine clinical performance.

This chapter will first review the nature of metallic bonding and how this mode of interatomic bonding leads to the general properties of metals. Then the commonly used fabrication methods for alloys employed in orthodontics are described, along with how these methods affect their properties. The compositions and structures of the four major orthodontic alloy types are then presented, along with their uses. The chapter closes with a summary of the important methods employed to obtain information about the structures and properties of orthodontic alloys, with comparisons of clinically relevant properties of these alloys.

Much of the information in Sections 1.2 and 1.3 that follows should be familiar to residents in orthodontics, who generally have a predoctoral course in dental biomaterials. Textbooks^1,2 should be consulted as needed for greater detail and illustrative figures about the background information for metals presented in these two sections. Additional background is available in a previous textbook³ on orthodontic materials.

1.2. Metallic bonding and general properties of metals

Materials with metallic bonding have valence electrons which are loosely bound to atoms, in contrast to the ionic and covalent modes of interatomic bonding which involve localized valence electron transfer or sharing between adjacent atoms, respectively. Because the valence electrons in metals move with facility under the influence of electrical potential differences or in the presence of elevated temperatures, metals are characterized by high values of electrical and thermal conductivities. Metals are also crystalline materials, having long-range three-dimensional repeating patterns of the ionic cores (atoms without their valence electrons) or crystal structures, in contrast to dental ceramics and dental polymers, which are generally noncrystalline with only short-range order in their atomic or molecular arrangements. Consequently, positively charged cations form in solution when a metal is attacked by an acid. The opaque character of metals also arises from the metallic bonding in a complex manner that is beyond the scope of this book.

The nonlocalized nature of metallic bonding enables metals to possess generally the property of ductility, that is, the capability of undergoing permanent deformation when the applied force or stress is sufficiently high. The principal mechanism for permanent deformation of metals is the movement of dislocations, line defects in the atomic arrangement that move on certain interatomic planes, termed slip planes because movement of the dislocation causes an offset (slip) with respect to adjacent planes. Dislocations can only occur in crystalline materials, which accounts for the brittle character of the noncrystalline dental ceramics and dental polymers. Vast numbers of dislocations are generated and move in the atomic structure during the macroscopic permanent deformation of metals; the movement of a single dislocation across a slip plane will generate an offset between adjacent planes of only one interatomic distance. The ease of dislocation movement is dependent on the metal crystal structure, which determines the number of slip systems (combinations of a slip plane and the slip direction from the dislocation movement). The face-centered cubic structure has the largest number of slip systems, followed by the body-centered cubic and hexagonal close-packed crystal structures. The facility of dislocation movement is much less for metals having other crystal structures.

Twinning is another mode of permanent deformation in metals and occurs at suf...