well-healed, a conventional RPD is constructed. In-mediate partial dentures are usually fabricated completely of a resin base and denture teeth.

OVERDENTURES

These prostheses include complete, partial, and immediate overdentures. A complete overdenture replaces the entire dentition and is constructed for insertion over one or more remaining prepared teeth, roots, or dental implants. A partial overdenture replaces the partially missing dentition and is constructed for insertion over one or more remaining prepared teeth or roots. If the overdenture is constructed for insertion immediately following the surgical removal of natural teeth, leaving no strategic teeth to support the denture, the prosthesis is considered an immediate prosthesis.

PROSTHODONTIC MATERIALS

Many dental materials are unique to prosthodontic procedures. The improper use of any of these materials could cause a delay in the treatment and an inconvenience to the patient. You should be familiar with the use, handling, reaction time, and storing procedures for these materials. This knowledge is necessary for your successful performance as a prosthodontic assistant.

DENTAL ALLOYS

Although you do not make dental prostheses as a basic dental assistant, you must know enough about the materials used in their construction to document properly the treatment patients receive. When a patient’s prosthesis is given to a dental lab for repair or change, they need to know its history to do the work properly, or a tragic result may follow. You should document al I laboratory requests and patient dental records with information, such as alloy type used, solder type, and tooth shade if applicable.

Dental alloys can be classified as precious, semiprecious, and nonprecious. For the purpose of training and clarification, we will classify them as noble metal or base metal alloys.

Noble Metal Alloys

Noble metals resist oxidation and corrosion. The four noble metals used primarily in dentistry are silver, platinum, palladium, and gold. Gold is very useful for dental put-poses. Although too soft for use alone, it can be combined with other metals in varying proportions to produce alloys of almost any desired properties. Other noble metals are used in most dental labs to fabricate crowns and FPDs because of the high cost of gold.

Base Metal Alloys

Since base metal alloys do not contain noble metals, they are much stiffer and harder. Thus, they are useful for constructing RPDs and certain types of FPDs.

IMPRESSION MATERIALS

Many types of impression materials are used in the dental clinic. However, no one material fulfills all requirements for making a perfect negative reproduction of the oral structures. The dentist will determine which material will best meet the requirements for each case. The two commonly used impression materials are alginate hydrocolloids and synthetic rubbers.

Alginate Hydrocolloids

Hydrocolloids that change state because of thermal changes are known as reversible hydrocolloids because the process can be changed back and forth by altering the temperature. Those that are altered through a chemical change are known as irreversible hydrocolloids. Once the chemical change has taken place, it cannot be reversed or turned back to the previous state.

Irreversible hydrocolloids, more commonly known as alginates, were developed from seaweed during World War II. Alginate impression material has largely replaced the reversible type for impressions. The advantages of alginate material are that it is easy to prepare and handle, it does not require excess equipment and advanced preparation, it is comfortable for the patient, and it is inexpensive. Alginate is used in making preliminary impressions for all study casts and most final impressions for RPD working casts.

According to the American Dental Association (ADA) specifications, alginate materials are divided into two types based on gelling time:

Type I - Fast set material, must gel in 1 to 2 minutes.

Type II - Regular set material, must gel in 2 to 4.5 minutes after the beginning of the mix.