1. Oral Environment and Patient Considerations
Chapter 2

2. Oral Environment and Patient Considerations
To understand why we manipulate materials the way we do, we first must understand the challenges of the oral environment.
Materials to be used must be biocompatible, durable, nonreactive in acid or alkaline conditions, compatible with other materials, and esthetically acceptable.
Limitations regarding what can and cannot be used safely will vary from patient to patient.
Materials must be compatible in an environment of moisture and differing stresses, temperatures, and acid levels.

3. Biocompatibility Materials must be biocompatible.
This means that they must NOT adversely affect living tissue.
Adverse responses may include postoperative sensitivity, toxicity, and hypersensitivity.
Short-term and long-term functionality and biocompatibility must be considered when new dental materials are developed.
Adverse responses may be caused by the material itself or by the breakdown of its components in the oral environment

4. Biomechanics
Biomechanics applies the principles of engineering and biological systems to the design of new dental materials.
Application of a material will depend on the properties of the material and how they relate to the needs of the patient, along with the oral environment.
Excessive wear of a material may be due to variations in forces applied by stronger to weaker antagonists.

5. Force
When materials are subjected to the forces of mastication, they must withstand varying degrees of force and load.
Normal biting force varies among individuals and from one area of the mouth to another.
Biting force is largely a measurement of the strength of the muscles of mastication during the normal chewing of foods.
Normal masticatory force on the occlusal surfaces of molar teeth averages 90 to 200 pounds per square inch (psi) and can increase by as much as 28,000 pounds psi on a cusp tip while decreasing in the incisor area.

6. Types of Force Three basic types of force have been identified:
Compressive force: the force applied to compress an object. Posterior teeth are able to deal with this type of force because of their broad occlusal surfaces and multiple roots.
Tensile force: the force applied in the opposite direction as an object is pulled or stretched.
Shearing force: the force applied when two surfaces slide against one another. The function of incisors is an ideal example of the cutting action associated with shear.

7. 3 Basic Force Types

8. Stress and Strain
When force is exerted on a tooth, the tooth or material creates resistance to counteract the force.
The internal force that resists the applied force is called stress.
If the stress within an object cannot resist the force, distortion or deformation occurs.
The distortion or deformation produced by stress is referred to as strain.
Stress is the amount of force exerted from within an object, and strain is the amount of change that the force has produced

9. Stress and Strain (cont’d)
When we chew during mastication, many types of stress are involved and form complex stress combinations.
One such combination, tension and compression, is known as flexural stress, which is seen as bending.
An example of flexural stress in restorative dentistry involves dental bridges, when tension is placed on the occlusal surface, bending the bridge toward the tissue. The tissue side of the bridge then stretches upward in response.

10. Stress and Strain (cont’d)
Dental materials are structured for multiple purposes, but not every material is suited to all types of stress.
If force is exerted over a large area, tooth structures can more adequately handle the stress. When the force is exerted over a small area, the increase in pressure may result in a fracture
During mastication, stresses occur repetitively over time; this may cause failure of a material.
Microscopic flaws occur until the material fails, resulting in fracture. This is known as fatigue failure.

11. Moisture and Acid Levels
The oral cavity is continually in contact with moisture that varies from acid to alkaline.
The type of moisture in our mouths depends on what we eat or drink, what medications we take, and what quantity of acid-producing bacteria is present.
The normal pH of saliva is 6.2 to 7.0; this is fairly neutral but fluctuates during the day.

12. Effects of Moisture
During placement or over time, most materials react adversely to moisture.
The breakdown of most dental materials is brought about by moisture, acid, and stress.
Materials that are needed to last a long time must withstand this hostile environment.

13. Solubility A soluble material dissolves in fluid.
Desirable materials should have a low solubility.
Gold and porcelain are retained in the oral environment for many years because they are insoluble.
Materials that are tooth-colored are more soluble; therefore they do not last as long.

14. Water Sorption
Some materials have the undesirable characteristic of water sorption.
These materials may stain or may enlarge slightly because they absorb moisture.
Some materials stain as the result of water sorption caused by repeated exposure to coffee, tea, cola, and foods that contain dyes.

15. Metals
Metals, with the exception of noble metals such as gold, suffer the effects of moisture and acidity.
Many metals corrode because of their continued contact with an acidic environment.
Others discolor as the result of oxidation of metal surfaces. This is known as tarnish.
Dental amalgam is highly susceptible to both corrosion and tarnish.

16. Galvanism
An environment that contains moisture, acidity, and dissimilar metals makes possible the generation of electrical current. This is known as galvanism.
The current may result in stimulation of the pulp, called galvanic shock.
It also may create a metallic taste in the mouth.
Insulation of the restoration may help to decrease the effects of galvanic shock.

17. Temperature
With few exceptions, all forms of matter expand and contract with temperature changes. These changes result in dimensional change within the restoration.
Ingestion of foods and beverages and smoking may alter the temperature of the oral cavity.
Dental materials should have expansion and contraction rates similar to those of teeth.

18. Temperature (cont’d)
Expansion and contraction are measured by using the coefficient of thermal expansion (CTE).
The CTE measures changes in volume or length related to changes in temperature.
Materials such as composites or amalgams should be similar to tooth structure so that marginal integrity can be maintained.

19. Temperature (cont’d)
If the CTE is significant, then repeated shrinkage and expansion of the material within a tooth will open gaps in the interface between the restoration and the tooth.
This action, known as percolation, will allow bacteria and oral fluids to breach the interface, causing recurrent caries, staining, and pulpal irritation.

20. Temperature (cont’d)
Another consideration associated with temperature is thermal conductivity.
This is the rate at which heat flows through a material.
Metals such as gold and amalgam are good conductors of temperature; nonmetals are poor conductors of temperature.

21. Other Temperature Considerations
Aside from the temperature changes associated with food and drink, one must consider chemical reactions within the material itself.
Some materials when mixed will produce heat.
This exothermic reaction must be minimized to protect tooth structures from excessive heat, which may cause sensitivity or pulpal damage.

22. Retention
One important factor related to the selection of materials is how they will be retained in the preparation or on the tooth surface.
Retention may be mechanical or chemical through adhesion, or it may reflect a combination of the two.
Mechanical retention places undercuts in the preparation.
Chemical retention works through adhesion or bonding.
Chemical adhesion offers several advantages in producing retention:
Requires less removal of tooth structure b/c no undercuts are necessary
Produces stronger retentive force between tooth and restoration
It can seal the margin of restoration to prevent seepage of bacteria and fluids through percolation

23. Retention (cont’d)
Most of today’s dental materials use a combination of mechanical and chemical retention.
This may be helped or hindered by the viscosity of the material.
Viscosity is a material’s ability to flow.
The thicker the material, the harder it is for the material to flow.
Film thickness, the minimum thickness attainable, is important when dental cements are used.

24. Wetting
Wetting is the degree to which a liquid adhesive is able to spread over the surface of the tooth and the restorative material.
The better the adhesive is able to coat the surface, the better the retention will be.

25. Surface Preparation
Another consideration that affects retention is the condition of the tooth or restoration surface.
Moisture contamination, surface texture, and energy will affect how well the two bond.
Even slight contamination of moisture or debris will compromise the integrity of the process.

26. Surface Energy
Surface irregularities may prevent complete wetting of the surface.
Microscopic irregularities trap air as the adhesive flows over them.
This diminishes the surface energy and the numbers of atoms attracted to the bonding surface.

27. Microleakage
The space between the walls of the preparation and the preparation is called the interface.
If the interface is not sealed, fluids and microorganisms can penetrate between the tooth surface and the restorative material.
This seepage of harmful materials, or microleakage, results in tooth sensitivity, recurrent decay, and marginal staining.

28. Esthetics Materials used in dentistry must be esthetically acceptable.
The human eye senses light through the cone cells in the retina in three different ranges of wavelength: red, green, and blue.
Mixture of the three is interpreted by the brain to determine the color we see.

29. Color The three components of color result in hue, chroma, and value.
Hue is the dominant color of the wavelength detected.
Teeth are seen predominantly in the yellow and brown ranges.
Chroma is the intensity or strength of a color.
Value describes how light or dark a color is.

30. Color (cont’d)
The color of teeth is also determined by the amount of light that passes through them.
If light passes easily through a tooth, it is said to be transparent.
If light is completely absorbed, then the tooth is opaque.
A combination of the two is typical; the term used is translucent.

31. Detection of Restorative Materials
It is important that oral health care professionals are able to identify restorative materials within the oral cavity.
Metal restorations are easier to identify than esthetic restorations.
Heavy pressure during scaling and polishing may damage the surface of some restorations (e.g., porcelain, composite); therefore it is vital that the restoration is identified before patient care is provided.

32. Detection of Restorative Materials (cont’d)
When tooth color is well matched, it can be difficult to identify restorations.
This may be accomplished by using location, tactile sensitivity, and radiographs, as well as air, magnification, and adequate illumination of the areas in question.

33. Summary
The oral environment is hostile, and the materials we use must be compatible, must last for the intended length of time, and must be of the highest esthetic and functional quality.
Materials are constantly changing, improving, and evolving.
Keeping abreast of these changes allows dental professionals to educate their patients and provide the highest quality of care.