Non-Metallic Biomaterials for Tooth Repair and Replacement

P Vallittu

• Publisher: Woodhead Publishing
• Year: 2012
• Print ISBN:
  9780857092441
• eBook ISBN:
  9780857096432
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

As the demand for healthy, attractive teeth increases, the methods and materials employed in restorative dentistry have become progressively more advanced. Non-metallic biomaterials for tooth repair and replacement focuses on the use of biomaterials for a range of applications in tooth repair and, in particular, dental restoration.Part one reviews the structure, modification and repair of dental tissues. The properties of enamel and dentin and their role in adhesive dental restoration are discussed, along with biomineralization and biomimicry of tooth enamel, and enamel matrix proteins (EMPs) for periodontal regeneration. Part two goes on to discuss the processing, bonding and wear properties of dental ceramics, glasses and sol-gel derived bioactive glass ceramics for tooth repair and replacement. Dental composites for tooth repair and replacement are then the focus of part three, including composite adhesive and antibacterial restorative materials for dental applications. The effects of particulate filler systems on the properties and performance of dental polymer composites are considered, along with composite based oral implants, fibre reinforced composites (FRCs) as dental materials and luting cements for dental applications.With its distinguished editor and international team of expert contributors, Non-metallic biomaterials for tooth repair and replacement provides a clear overview for all those involved in the development and application of these materials, including academic researchers, materials scientists and dental clinicians. - Discusses the properties of enamel and dentin and their role in adhesive dental restoration - Chapters also examine the wear properties of dental ceramics, glasses and bioactive glass ceramics for tooth repair and replacement - Dental composites and antibacterial restorative mateirals are also considered

Part I

Structure, modification and repair of dental tissues

1

Structure and properties of enamel and dentin

V.P. Thompson, NYU College of Dentistry, USA

N.R.F.A. Silva, Federal University of Minas Gerais, Brazil

Abstract:

This chapter addresses the mineralized tissues of teeth – enamel and dentin – and how they develop into structural components with unique physical properties. Tooth structure includes an epithelium-derived outer shell of enamel that is highly mineralized, hard, stiff and wear resistant. This is supported both mechanically and biochemically by a mesenchyme-derived dentin, which is vital, less mineralized, softer and more compliant. The dentin is maintained by the dental pulp, which is cellular and innervated, and has a vascular plexus.

Key words

dentin

enamel

mechanical properties of tooth structure

mineralized tissues

1.1 Introduction

Much is known about teeth and their structure. Teeth have long been studied by paleontologists, since they degrade much more slowly than bone; in fact, they are the source of our primary knowledge of many ancient species. Nonetheless our understanding of their intriguing structure is still incomplete. Human teeth are generally representative, with an epithelium-derived outer shell of enamel that is highly mineralized, hard, stiff and wear resistant. The enamel is supported both mechanically and biochemically by a mesenchyme-derived dentin, which is vital, less mineralized, softer and more compliant. Dentin is maintained by the dental pulp, which is cellular and innervated, and has a vascular plexus. In this chapter we give details of each of the mineralized tissues and how they develop into structural components with unique physical properties.

1.2 Enamel

1.2.1 Development

Tooth enamel is the hardest tissue in the body, with a hardness comparable to that of window glass, and is highly fatigue-and wear-resistant. Human enamel is laid down by cells in a programmed temporal and spatial sequence to provide the overall shape of the tooth. The cells that make enamel develop from the invagination of epithelial tissue during fetal development. In what is known, because of its shape, as the ‘bell stage’ of tooth development (ca. 14th week of intrauterine life), the epithelial cells on the inside of the bell align with a concentration of mesenchyme cells in what appears to be a one-to-one relationship. More accurately the latter are ‘ectomesenchyme’ cells, as the first branchial arch, whose ectodermal cells migrates into the mesenchyme in the area of the developing jaws (Nanci, 2008). During this alignment an extracellular collagen network is created that extends from the epithelial cells to the mesenchyme cells. The epithelial cells begin to elongate and transform into ameloblasts, and the mesenchyme cells transform into odontoblasts (Nanci, 2008). The elongation of the ameloblasts when compared with the odontoblasts leads to pulling on the collagen network formed between the two, creating a local puckering of this structure that will become the dentin–enamel junction (DEJ). Seen in cross-section the DEJ appears as scalloped, but viewed in three dimensions (3-D), when the enamel has been dissolved, the circular ridges and pits of the DEJ structure become apparent. The gene expression controlling this process is not fully understood, but a large number of genes involved in tooth development have been identified (Nieminen, 2007).

1.2.2 Enamel prisms

The ameloblasts are arranged in a close, overlapping array. Each cell has a tail that extends between its neighbors (see Fig. 1.1), so that if observed from above the DEJ, they interdigitate.

1.1 Ameloblasts arranged next to one another (upper right). Each cell has a head (dotted black oval) and a tail (dotted black box) that extends between its neighbors. Observe the discontinuity of the enamel crystallites. Asterisk shows secondary territories. Each arrow in the upper right denotes a sectioning plane through the enamel. Each arrow points to the diagram depicting the microscopic view of that sectioning plane in the enamel. Image modified from Boyde (1989).

Once aligned with their neighbors, the ameloblasts begin to mature and to lay down the enamel structure. The maturation of ameloblasts starts from what will become the cusp tip or the incisal edge of the tooth (but at this stage is the inner top of the bell) and proceeds apically. The last enamel to begin formation will be that closest to the cement–enamel junction (CEJ). The ameloblast at its terminal end (nearest to the DEJ) takes on a ‘brush border’ appearance and begins to excrete proteins, in particular amelogenins; these are the template molecules for the nucleation of calcium phosphate to form, with maturation, ribbons of dense hydroxyapatite (HA). In this process each ameloblast will create one enamel prism of approximately 5 μm in diameter, which is also referred to as an ‘enamel rod’ (Fig. 1.2). Individual prisms are currently thought to extend from the DEJ to the enamel surface through various paths and not to change diameter.

1.2 Scanning electron micrograph of enamel rods: alignment of enamel prisms observed when the enamel surface is etched by acid.

Prisms are joined to their neighbors by a thin organic layer referred to as a ‘prism sheath’. When loaded to the point of cracking, the resultant cracks preferentially propagate through the protein sheath, going around and along the prism (Fig. 1.3).

1.3 Cracks (crenellations) propagating through the protein sheath going around and along the prisms following Vickers indentation.

The tensile strength of enamel is lower when loaded perpendicular to the prism direction (11.4 ± 6.3 MPa) than when it is when loaded parallel (24.7 ± 9.6 MPa) (Carvalho et al., 2000). When acid etched, the shear bond of adhesive applied end-on to the prism direction (enamel surface) is approximately 40% higher than when the adhesive is applied parallel to the enamel prism direction (Ikeda et al., 2002). However, self-etch adhesives, which do not employ a separate etching step, do not result in a significant difference in bond strength relative to enamel prism orientation (Shimada and Tagami, 2003).

The laying down of enamel by the ameloblasts proceeds at a rate of about 4 μm per day (Dean, 1998). If an ameloblast were to migrate directly to the enamel surface, the fastest it could reach the outer dimension of a 1.2-mm-thick enamel cusp would be (1200/4 =) 300 days, but we note that ameloblasts do not proceed directly radially from the DEJ to the surface (as discussed below), so much more time is necessary to develop enamel for permanent teeth. Molar enamel thickness varies by cusp from 1.2–1.7 mm (Mahoney, 2008), increases from the first molar to the third (Grine, 2005) and is generally slightly thicker for females (Smith et al., 2006). The enamel thickness on the facial or incisal of a central incisor is approximately 1.3 mm (Shillingburg Jr. and Grace, 1973). Once ameloblasts reach the outer extent of the enamel they transform to a more cuboidal shape and die. What signaling controls this process is not known. The calcification of the developing enamel prism occurs gradually and continues for a some time even after the tooth erupts into the mouth. This makes newly erupted teeth sensitive to decalcification and caries for more than a year.

Enamel growth periods are seen via structural features in the enamel. Ameloblasts mature in layers or fronts from the cusp toward the DEJ, resulting in layers called ‘striae of Retzius’. These appear at the external surface of the tooth as ‘perikymata’, more pronounced layers in the ce...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Woodhead Publishing Series in Biomaterials
• Foreword
• Part I: Structure, modification and repair of dental tissues
• Part II: Dental ceramics and glasses for tooth repair and replacement
• Part III: Dental composites for tooth repair and replacement
• Index