The Oral Cavity
Structure of Teeth
Potential Issues and Treatments Associated With Teeth
Categories of Dental Materials
Challenges of Dental Materials in the Oral Cavity
The Future Need for Dental Biomaterials
Organization of the Book
Auxiliary dental material Substance used in the construction of a dental prosthesis that does not become a part of the prosthesis.
Preventive dental material Cement, coating, or restorative material that either seals pits and fissures or releases a therapeutic agent, such as fluoride and/or mineralizing ions, to prevent or arrest the demineralization of the tooth structure.
Restorative dental material Metallic, ceramic, metal-ceramic, or resin-based substance used to replace, repair, or rebuild teeth and/or enhance esthetics. A direct restorative material is placed in the tooth preparation and is transformed to be a restoration. An indirect restorative material is fabricated extraorally to produce prostheses.
Temporary restorative material Cement or resin-based composite used for a period ranging from a few days to several months to temporarily restore or replace missing teeth or tooth structure until a definitive, lasting prosthesis or restoration can be placed.
Dentists and engineers have similar long-range objectives in their professions—that is, to design, construct, apply, and evaluate devices or structures of materials that can be subjected to a wide range of environmental conditions. They must have a thorough knowledge of the properties and behavioral characteristics of the materials they intend to use. However, dentists need to make proper diagnoses, prevent dental caries, and surgically treat the affected tooth structures. Subsequently, they must select a material, or materials, for either direct or indirect applications aiming to restore the patient’s intraoral functions. The science of dental materials covers a broad range of terminology, composition, microstructure, and properties used to describe or predict the performance of materials developed for dental applications. Previous courses in mathematics, chemistry, and physics should have prepared you to read this book and understand the terms and principles involved in describing the behavior of these materials as they are used clinically and in the testing laboratories of academia, governmental facilities, and industry.
Properties of materials can be categorized into chemical and physical properties. Chemical properties are generally composed of the composition and behavior of materials in a chemical environment, regardless of any interaction with other external influences. These properties will be presented in chapters where specific materials are discussed. Physical properties ( Chapter 3, Introduction ) are measurable variables that describe how an object looks, feels, or acts when the object is probed by external agents, such as heat, light, moisture, or force. Mechanical properties are an aspect of physical properties, primarily related to the behavior of materials in response to externally applied forces or pressures ( Chapter 4, What Are Mechanical Properties? ). In a clinical environment, the behavior of dental materials may depend on several variables simultaneously, but our ability to differentiate primary from secondary factors or properties will allow us to easily understand or predict a material’s performance. Furthermore, this potential to predict clinical performance will allow us to analyze the causes of structural degradation and failure of these materials when they no longer serve their intended functions in the oral cavity.
In this chapter, we will describe the function of the oral cavity, the structure of the tooth, potential issues involving teeth that require intervention, categories of materials by application, challenges to these materials in restoring the function of the teeth, safety issues of dental materials, the future need for dental biomaterials, and the organization of the book.
The Oral Cavity
As an anatomical space and part of the head and neck, the oral cavity consists of the lips, cheeks, minor salivary glands, gingiva, tongue, hard palate, and teeth. As part of human evolution, the oral cavity developed to allow humans to ingest food, chew, swallow, breathe, and speak.
In addition, the oral cavity is a food processor for the body. The presence and colonization of bacteria, along with distinct teeth anatomy, saliva, and chewing (motion), begin the breakdown of food and initiate the digestive process. Therefore humans are able to obtain the nutrients necessary for survival. Furthermore, both biting and chewing require upper and lower jaws, muscles, and teeth working in unity to achieve this goal. A distinctive component required in this interaction ( Figure 1-1, A ) to cut and grind food is force. You will find throughout this book that the term force is often used, so a proper presentation of the term is warranted.
How is force generated, and how do we measure the quantity of force?
What Is Force?
In classical physics, force is defined as the interaction between two objects during the action of push or pull. When opposing teeth come into contact during occlusion, an interaction occurs, and forces act upon the teeth in contact. Thus one can say that force exists as the result of an interaction, or when two objects contact one another. According to Newton’s third law of motion, for every action, there is an equal and opposite reaction, or in other words, for every interaction, there is a pair of forces going in opposite directions coming from both objects ( Figure 1-1, B ). When this interaction ceases, or when teeth do not occlude, the two objects no longer experience force. However, not all interaction requires physical contact. For example, objects fall to the ground because the earth’s gravity constantly pulls objects toward the earth.
Force is a vector that has direction and magnitude. The quantity of force is measured using a unit known as Newton, abbreviated with an “N.” One Newton is the amount of force required to give a 1-kg mass an acceleration of 1 m/s 2 , which means that 1 N = 1 kg · m/s 2 . Acceleration makes a stationary object come into movement. We know that the Earth’s gravitational acceleration is about 9.8 m/s 2 . Therefore gravity exerts a force of 9.8 N on an object with a mass of 1 kg. If this object is resting on a tabletop, a force of 9.8 N will be exerted on the area of contact. Furthermore, if we assume this area of contact is 100 mm 2 (= 1 × 10 –4 m 2 ), and we divide the applied force by the area, we obtain a value of 0.098 N/mm 2 (= 9.8 × 10 4 N/m 2 ) for the force, known as pressure, on the surface. The International System (SI) unit of pressure is N/m 2 , which is also called Pascal (Pa). The force applied by the weight of the object is distributed throughout the supporting substrate as internal stress and can cause a strain, or deformation of the substrate. Stress is calculated by dividing the force by the cross-sectional area of the substrate and has the same SI unit as pressure. The concept of stress and strain will be discussed in Chapter 4, Stress and Strain .
Mastication and Clenching Forces
The range of biting forces varies markedly from one area of the mouth to another and from one individual to another. The most quoted maximum bite forces range from 400 to 890 N for molar teeth, 222 to 445 N for premolars, 133 to 334 N for canines, and 89 to 111 N for incisors. Although there is considerable overlap, biting force generally is greater for males compared with females and for young adults compared with children.
A 2002 study reported a mean clenching force of 462 N, with a range of 98 to 1031 N for individuals between the ages of 28 and 76 (mean age = 46) who had lost their posterior teeth. In comparison, subjects with a complete dentition exerted a mean clenching force of 720 N with a range of 244 to 1243 N. If a force of 756 N is applied to a cusp tip over an area equivalent to 3.9 mm 2 , the compressive stress would be 193 MPa (1 MPa = 1 × 10 6 Pa). If the area is smaller, then the stress within the cusp would be proportionately greater. The Guinness Book of Records (1994) lists the highest human bite force as 4337 N sustained for 2 s. The average maximum sustainable bite force is approximately 756 N.
Taking this information into consideration, one may ask what attributes or characteristics allow the tooth to sustain such force. Let us pause and familiarize ourselves with the structures of the tooth.
How does the structure of the tooth enable resistance to fracture from occlusal loading?
Structure of Teeth
In the oral cavity, teeth are firmly joined to the upper and lower jaws by tooth-supporting connective tissues (cementum, periodontal ligament). This assembly of tissues ensures enough flexibility to withstand the forces of mastication and act as thermal and chemical insulators. Teeth perform important functions in the oral cavity. The front teeth can grab and cut food to a size that is suitable for the mouth (bite-size). They also have a role in speech and contribute to facial aesthetics. The posterior teeth’s morphology is designed to grind the bite-size food into smaller sizes, which facilitates food passage from the throat to the stomach.
At the teeth’s full formation and physiological capacity, they will have the following structures: enamel, dentin, enamel-dentin junction, pulp, and cementum ( Figure 1-2 ).
Dental enamel is a biologically ceramic composite made of precisely arranged 20-nm-diameter fibrous apatite crystals (92 to 94 vol%). The meaning of composite will be discussed in Chapter 2, Composite Materials . The remaining nonmineral content (2 to 4 wt%) is represented by water, lipids, and several peptides. This small amount of nonmineral components, along with its hierarchical anisotropic structure, regulates the mechanical properties of enamel to respond to the tooth’s functional needs, such as strength and resistance to wear upon loading. On a micrometer scale, enamel contains rod and interrod structures ( Figure 1-3 ), beginning at the enamel-dentin junction and extending to the tooth surface. The ability of the rigid rod structure, along with the organic interrod component, to weaken the stress concentration at the crack tip, if present, improves the resistance of the enamel to fracture from the stress generated at the surface contact.
Dentin is a composite mineralized tissue that contains less mineral than enamel and is composed of nanocrystalline carbonated hydroxyapatite ( Figure 1-4 ). The organic content (30 vol%) is almost exclusively type I collagen fibrils plus noncollagen proteins, such as proteoglycans. In addition, this tissue has a peculiar morphological feature: the presence of tubules extending from the enamel-dentin junction to the pulp. These tubules have diameters varying from 2.5 μm near the pulp tissue to 0.8 μm at the enamel-dentin junction. They also run transverse to the root or in an S-shape in the crown. This anisotropic, hierarchically oriented, and less mineralized microstructure serves as a foundation, shapes the roots, and protects the pulp. When loaded intraorally and force is applied parallel to the tubules, dentin responds better mechanically than when loads are applied perpendicularly.
Between the harder, brittle enamel and the softer, durable (tough) dentin, a functionally graded junction, the dentin-enamel junction (DEJ), is present, allowing a smooth transition of loads from the enamel to the dentin ( Figure 1-5 ). This interface inhibits the propagation of cracks from the enamel to the dentin, thus supporting the tooth’s integrity during masticatory actions. Even though fracture lines between enamel and dentin appear as a result of continuous masticatory action and/or occasional impact loading, enamel infrequently debonds from dentin, making the DEJ highly resistant to damage.
Cementum is a mineralized tissue covering the entire root surface of the tooth. Cementum is composed of water, organic matrix, and mineral. About 50% of the dry mass is inorganic and consists of hydroxyapatite crystals. The remaining organic matrix is largely made up of collagen and, to a lesser degree, glycoproteins and proteoglycans. The main function of cementum is to support or anchor the tooth, together with the principal periodontal fibers and alveolar bone.
The dental pulp is centrally located in the pulp cavity and quite often resembles the external surface of the tooth. This specialized, loose, fibrous connective tissue is composed of collagen fibrils and organic ground substance composed of 75% water and 25% organic material. As an organ, the pulp fulfills key physiological functions, namely, formative or developmental (generates dentin), nutritive (supplies nutrients and moisture through the vascular system), protective (responds to injury and noxious stimuli), and sensory (nerve fiber network transmits afferent pain). Whenever possible, especially while using dental materials, preserving the health of such vital tissue is highly desired.
Potential Issues and Treatments Associated With Teeth
Taking into consideration the lifespan and lifestyle of humans, there are three categories of issues that may affect and change the normal appearance, structure, position, and function of teeth to undesirable outcomes that require intervention to restore their function. These categories are biological, genetic, and mechanical in nature.
The oral cavity is an open ecosystem with the most diverse microbial communities in the body and is constantly under threat. The oral cavity is also where digestion of food begins (solid and liquid) and the apparatus that humans use to communicate. In this environment, the inevitable interaction between bacteria, saliva, oral hygiene, and diet can lead to health or disease. If such interaction favors a balance among these factors, then oral health is promoted. On the other hand, if an imbalance takes place (i.e., poor oral hygiene and high-sugar diet), carious lesions will develop, leading to superficial (S), moderate (M), and deep (D) active lesions ( Figure 1-6 ).
Superficial active carious lesions are usually located at the enamel level only and appear chalky ( Figure 1-6, A ). If these lesions are located on smooth buccal or lingual surfaces, they are known as white spot lesions (WSLs). WSLs can be treated by application of either highly fluoridated materials or through the infiltration of polymerizable, unfilled, low-viscosity adhesive resins. If the superficial lesions are present in grooves, pit and fissure sealants are used. Clinically, placing sealants in noncarious grooves is recommended for pediatric and adolescent patients at high caries risk. All treatments discussed do not involve surgical intervention but utilize, in part, the porosity of tooth structure caused by the carious process for infiltration of fluoride ions or fluid polymerizable monomers for protection against further acid attack. Moderate to deep carious lesions indicate loss of tooth structures, quite often with exposed dentin substrate ( Figure 1-6, A & B ). Unfortunately, these clinical occurrences require the use of sharp burs and/or hand instruments to remove the affected tissues and reshape the remaining tooth structures to accommodate the restorative dental biomaterial selected. The materials can be composites, ceramics, or metals, as will be discussed later. A myriad of preparation designs (reshaping), such as cavity preparation, inlays, onlays, and full crowns, are available. In cases of extensive carious lesions that affect several teeth, partial or even full-mouth teeth extraction is the only solution. To recover the patient’s ability to chew and speak in the case of tooth loss from extraction or trauma, prosthetic appliances such as dental bridges, implants, and complete dentures are used.
Across the world, in all countries and cultures, observing people with misaligned or deficient bites when they smile or speak is not uncommon. Such phenomena, termed malocclusion, can be caused by extra teeth, lost teeth, impacted teeth, or abnormally shaped teeth. A small underdeveloped jaw, caused by a lack of masticatory stress during childhood, can cause tooth overcrowding ( Figure 1-7, A ). Crowding of the teeth can be treated with orthodontics, often through planned tooth extractions, clear aligners, and/or dental braces. Braces are sets of flexible metal wires and brackets made of metal or ceramic materials. Brackets are cemented to the teeth ( Figure 1-7, B ) , and wires are tightened and adjusted over time so that the elastic properties of the wire gradually apply enough force to move the teeth into the desired alignment. However, teeth naturally tend to drift out of place, even after treatment with braces. As a result, wearing a retainer made of wires and acrylic resin or wires bonded to the tooth ( Figure 1-7, C ) may be needed to keep newly aligned teeth from moving.
The interaction between opposing teeth that enables the size reduction of food also leads to the wearing down of teeth, which is known as attrition and is often associated with masticatory force and parafunctional activity. Attrition mostly causes wear of the incisal and occlusal surfaces of the teeth. Although a certain degree of attrition is normal, unnecessary loss of tooth structure at the cementum-enamel junction can also occur in the presence of unbalanced stresses, friction, biocorrosion (chemical, biochemical, and electrochemical degradation), or a combination of these. Studies have reported that the prevalence of attrition in the population can reach as high as 75%. Clinical examples are bruxism/clenching, caused by repetitive jaw-muscle parafunction activity leading to occlusal excessive wear; abfraction (static stress biocorrosion); and abrasion/biocorrosion, caused by the action of acidic food and/or beverages, as well as toothbrushes, on the surface of the teeth ( Figure 1-8 ). Treatment options that require the use of dental materials include fabrication of an occlusal guard to deprogram masticatory muscles and avoid grinding and clenching, restorations to prevent further wear and decrease sensitivity, and “full-mouth rehabilitation” for severe cases. In cases of tooth fracture, the extension and remaining amount of the tooth structures dictate the approach (i.e., restoration or extraction) and the selection of the restorative dental biomaterial.