Oral Environment and Patient Considerations
KEY TERMS defined within chapter
Therapeutic Agents materials used to treat disease
Restorative Agents materials used to reconstruct tooth structure
Compressive Force force applied to compress an object
Flexural Stress bending caused by a combination of tension and compression
Solubility susceptible to being dissolved
Water Sorption the ability to absorb moisture
Coefficient of Thermal Expansion the measurement of change of volume or length in relationship to change in temperature
Percolation movement of fluid in the microscopic gap of the restoration margin as a result of differences in the expansion and contraction rates of the tooth and the restoration with temperature changes associated with ingestion of cold or hot fluids or foods
Thermal Conductivity the rate at which heat flows through a material
Insulators materials having low thermal conductivity
Adhesion the act of sticking two things together. In dentistry, it is used to describe the bonding or the cementation process. Chemical adhesion occurs when atoms or molecules of dissimilar substances bond together and differs from cohesion in which attraction among atoms and molecules of like (similar) materials holds them together
Viscosity the ability of a liquid material to flow
Surface Energy the electrical charge that attracts atoms to a surface
Transparent light passing directly through an object
Vitality a life-like quality
In the selection, manipulation, and handling of dental materials, it is important that the student have an appreciation for the complexity and challenges of the oral environment. Materials placed and used within the oral cavity must be biocompatible, durable, nonreactive in acid or alkaline conditions, compatible with other materials, and esthetically acceptable. All of these factors must be considered within a very unique environment. The oral environment produces many limitations—limitations on what can and cannot be used safely, limitations in the type and long-term clinical needs of the treatment, and limitations in the conditions of the oral cavity. These limitations may vary somewhat from patient to patient or in specific circumstances.
Materials must be compatible in an environment of moisture and differing stresses, temperatures, and acid levels. The degree of compatibility may depend on how and how long the materials are expected to be used. Therapeutic agents, those used to treat disease, are generally used for short periods, whereas restorative agents, those used to reconstruct tooth structure, are expected to remain in contact with tissues for indefinite lengths of time. Consider the following cases. If a therapeutic agent is being used to treat a specific condition, such as a denture sore, it would have to be biocompatible with the tissues but would not require an extreme amount of longevity. If the material were being placed as a permanent restoration, such as a gold crown, biocompatibility and longevity would be of great concern.
Patient concerns, questions, and demands must also play a part in the decision process. The patient should be brought into the decision process very early. Tooth-colored materials are frequently requested by the patient but may have limited use under certain circumstances. The patient needs to be educated on the limitations of his or her particular situation and the appropriate restorative choices. The allied oral health care practitioner is frequently involved in this education.
Materials must be biocompatible, that is, they must not impede or adversely affect living tissue. However, all materials contain potentially irritating ingredients. Responses may include postoperative sensitivity, toxicity, and hypersensitivity. A material may be acceptable for use or fabrication on hard tissues (tooth structure), while it may not be acceptable for use on soft tissues. Some materials may be therapeutic in small quantities or if in contact with tissues for short periods of time but may be irritating or toxic with longer or larger doses. Topical fluoride is highly beneficial when used according to the manufacturer’s directions, but it can be irritating to soft tissues and can even excessively etch enamel if used improperly. Dentistry is not alone in its concern for the development of biocompatible materials; practitioners of orthopedics must consider biocompatibility in the placement of joint prostheses and cardiology in the placement of catheters and prosthetic heart valves. All must consider the short-term and long-term functional and biocompatible responses of any material.
Postoperative sensitivity is often associated with operative procedures. This may be due to the toxicity of the restorative, preventive, or therapeutic material or to bacterial invasion into or near the pulpal tissues.
Adverse responses may be caused by the material itself or by the breakdown of its components in the oral environment. Frequently, materials are used in combination to produce the restoration, as when a porcelain-fused-to-metal crown is cemented with glass ionomer cement. The use of multiple materials makes adverse responses more difficult to evaluate. It has been reported that significant percentages of people have skin allergies or hypersensitivity to some metals, in particular nickel and acrylics, and should avoid these materials. A complete health history and questioning of the patient can help to identify those individuals. In general, materials intended for permanent replacement of tooth structures should exhibit no adverse biological responses.
In subsequent chapters, the limitations and precautions for the use of each material will be clearly outlined.
The success or failure of a restoration may be related to its performance in any given situation. The function of a material is dependent on the properties of that material and on what the material is expected to do. These design considerations define the biomechanics of the material, that is, the application of engineering principles to biological systems. Much as an engineer would do, a dentist must design a bridge while taking into consideration load, span, and supporting structures. Excessive wear of a material may be due to the forces of a stronger material against a weaker material, such as porcelain against enamel, and may be intensified with the addition of/>