A dental cast is any dental prosthetic or device formed in a mold or used as a mold.
Purpose
Dental casts copy a patient's teeth and mouth structures for diagnostic purposes and are used as models for further casting of dental prosthetics such as bridges, crowns, implants, dentures, and partial dentures. Casts may also be used as education tools for dentists as they explain characteristics of a patient's bite or particular dental needs. Diagnostic casts can also show how a restoration will look when the work is complete.
Description
Dentists are challenged to create solutions for problems that patients present from chipped or missing teeth, a collapsed bite, or teeth that are irregular or misshapen. A clear diagnosis of the condition often is made through the use of plaster-like dental casts of the patient's teeth and gums. These diagnostic casts allow the dentist to examine how the teeth fit together and what may be hampering the patient from chewing or speaking well. Often, the dentist will recommend replacement prosthetic devices for missing teeth. In the case of severely crooked teeth, the dentist may urge the patient to have the teeth realigned through braces or other orthodontic appliances.
From these initial casts, dental prosthetics are made that will fit into a patient's jaw structure and resemble the other teeth there. A variety of prosthetic devices can be cast from these initial casts: individual teeth for implants, crowns, bridges, dentures, and partial dentures.
The dentist first makes an impression of the patient's teeth and gums. A variety of impression materials are available, some are firm when used (waxes, plasters, puttys, zinc oxide pastes) and some are more pliable (alginates and elastic silicones and polyvinyls). The elastic mediums are more stable and can stand longer before the casts are poured. Some impressions materials require that the cast be poured immediately or within a few hours. Some can wait several days or up to two weeks.
The impression produces a negative replica of the patient's teeth, which can then be cast in a variety of materials. Usually, plaster is used because it is inexpensive and sets up quickly. Other materials used include dental stone or special die stone. Both are very strong, but die stone is more abrasion resistant.
From these casts, the dental technician can create wax diagnostic models of the proposed restoration. These wax models are finely-detailed sculptures of the mouth, showing how the restoration will look. From these, the prosthetics are cast.
Operation
The dental assistant prepares the impression medium and fills an impression tray that fits over the patient's mouth. The patient's teeth are dried either with gauze or a chemical product before the impression tray is fitted over the patient's teeth. The tray is held in place by the dental assistant for a brief time. The tray is removed in one motion, not rocked, in order to have a clean impression.
The impression is rinsed and disinfected and left to set. The dental assistant pours in the casting material. Sometimes this is only a few minutes after the impression is made; sometimes it can be several hours or days later.
Plaster-type casts are usually made first. These are used in diagnosis, patient education, and the creation of dental restorations.
The dental technician makes a single wax model of the prosthetic device to fit the patient's mouth, using the plaster cast as a guide. An investment mold is poured around the wax model and fired. The wax melts and drains out of the investment mold, leaving behind a durable mold into which the technician can pour a casting material. Restorative casting materials can be metals (gold, silver, amalgam), resins, or ceramics. These are poured into the investment mold to craft individual teeth, crowns, and other dental prosthetics. The mold is broken or opened, depending on whether the mold is reusable. This technique is called lost wax casting and has been used for centuries to cast bronze sculptures or craft fine jewelry.
Maintenance
Alginate impressions are affected by water and can shrink or swell. This material often comes in powder form and should not be inhaled.
Casts made directly from impressions from plaster can be damaged through rough handling. Dental stone and dental die stone are more durable.
Dental prosthetics made from molds are long lasting and only require regular dental maintenance like tooth brushing and flossing. Dentures and partial dentures need to be rinsed and soaked to remove stains and odor. The dentist and dental assistant should inspect dental prosthetics often for any damage or wear, especially around the wires of partial dentures.
It should be noted that the impressions used to make plaster-like casts of the patient's mouth should be disinfected and dried before the casts are made. This protects dental personnel from any infectious agent attached to the impression when the cast was made and could be transferred to the cast.
Health care team roles
Dentists, dental assistants, and dental technicians work as a team to create and maintain dental casts. Dentists diagnose a particular problem in a patient's mouth and present solutions, depending on the specific nature of the case and the patient's comfort and budget. Sometimes, the exact solution is not fully determined until impressions of the patient's mouth are made and the dentist consults with the dental technician. Impressions are crucial to the problem-solving that the dentist and dental technician do. The dental assistant makes an impression of the patient's teeth and mouth structures and pours a hardened cast. The dentist examines this cast and sends it to the dental technician with recommendations for the creation of dental restorations. Dental technicians make wax diagnostic models from the mouth cast for various restorations (crowns, bridges, partial dentures, implants). These models are used by technicians to create metal frameworks and tooth structures in investment molds.
Training
Dental assistants and hygienists usually require some specialized training. Dental hygienists are often licensed. Dental technicians are trained at dental laboratories, though some dental schools offer coursework in dental restorations.
KEY TERMS
Bite—How the upper teeth and lower teeth fit together so that a person can chew and speak.
Bridge—A device that has at least one prosthetic tooth and two crowns. It is used to replace a missing tooth or teeth. The bridge is held in place when the adjacent crowns on the bridge are cemented over the two teeth on each side of the space left by the missing tooth or teeth.
Crown—A dental prosthetic that copies an existing tooth's shape (or ideal shape) and covers the tooth. It is cemented into place and feels like and is used as a normal tooth.
Denture—A dental prosthetic device consisting of a full set of teeth to fill the upper or lower jaw, or both. Also called false teeth.
Implant—A prosthetic tooth anchored permanently into the jaw bone by a post. It has the same strength and appearance as natural teeth.
Impression—An exact copy of the teeth and mouth using materials that will set sufficiently so that a more durable cast of the mouth can be made from plaster, dental stone, or other casting materials.
Investment mold—A plaster-like substance that is created around a wax model of an object that is to be cast in metal.
Lost wax casting—A process of casting metal that involves making a mold around a wax object that has been shaped exactly in the likeness of the intended finished object. The mold is then fired and the wax melts. The resultant mold is durable and capable of receiving molten metals which will be cast in the mold in the shape of the desired object.
Mold—A form or physical outline of an object used to hold a pliable material in order to copy the same shape or design.
Orthodontic appliances—Devices that help straighten the teeth; e.g. invisible braces, retainers, etc.
Partial dentures—A dental prosthetic of two or more teeth used to replace missing teeth.
Prosthetic device—A re-creation of a tooth or series of teeth to replace or improve the structure or appearance of a tooth or teeth.
Restoration—Any prosthetic device or process used to replace or improve the structure or appearance of a tooth or teeth.
ORGANIZATIONS
American Dental Association. 211 East Chicago Ave., Chicago, IL 60611. (800) 947-4746. (312)440-2500. <http://www.ada.org>.
OTHER
Christensen, Gorden J. Efficient Dental Assisting for Fixed Prosthodontics. [video] Provo, UT: Practical Clinical Courses, 1990.
"The Dental Impression." Quality Dentistry. April 2001. <http://www.qualitydentistry.com>.
