Types of dental casts
The impression obtained in the clinic is sent to the dental laboratory, where a cast is poured from it. The cast is an exact replica reproducing the shape of the future appliance or the patient's anatomical structures. Depending on the clinical stage and purpose, casts obtained from impressions are divided into three main groups:
- Diagnostic — used by the clinician to assess the initial clinical picture, make a diagnosis, and plan the design.
- Working (master) — used directly for the fabrication of the dental prosthesis on the working jaw.
- Auxiliary — these are casts of the antagonist jaw (opposite to the one being treated). They are necessary for the correct adjustment of dental occlusion.
General principles of prosthetic treatment
When planning treatment for patients with partial secondary edentulism (loss of some teeth), the clinician must choose the optimal design. In such cases, removable partial cast-framework dentures (clasp dentures) are recommended as the preferred choice, as their use significantly reduces the side effects typical of other types of removable appliances.
To systematize dental arch defects, Kennedy classification is traditionally used. However, it is important to understand that in real clinical practice, "pure" Kennedy classes are extremely rare. Much more frequently, a clinician has to deal with subclasses and complex combinations of defects of different classes in the same jaw.
Fixed prostheses: fixed partial dentures (bridges)
In addition to removable prosthetics (such as clasp dentures), fixed prosthetic appliances are actively used to restore dental arch defects. The most common option is a fixed partial denture (bridge). This is a fixed appliance supported by the natural teeth bounding the dental arch defect (usually, bilateral support is used).
Biomechanics of the bridge: Supported by the patient's remaining natural teeth, the prosthesis receives masticatory pressure and transfers it directly to the periodontium of the abutment teeth.
Structural components of a fixed partial denture:
- Abutment elements — parts of the prosthesis fixed to the teeth. These include: full metal crowns (stamped or cast), metal-ceramic and veneered crowns, half-crowns, post crowns, pin-retained crowns, and inlays/onlays.
- Pontic (intermediate part) — located between the abutment elements and represents a single block of artificial teeth replacing the missing ones.
Classification and materials of fixed partial dentures
Fixed partial dentures are classified according to several criteria. First of all, they are categorized by the manufacturing method:
- Soldered — parts of such a prosthesis (crowns and pontic) are manufactured separately and then joined together with a special solder. The main disadvantage of such designs lies in the composition of the solder: it contains metals (zinc, copper, bismuth, cadmium) that can cause allergic reactions and intolerance phenomena in some patients.
- All-cast — have a single, cast-in-one-piece framework. They are completely devoid of the disadvantages of soldered prostheses since solder is absent.
According to the material used, fixed partial dentures can be all-metal, acrylic, porcelain, and combination (e.g., metal-acrylic or metal-ceramic).
The following materials are used for their fabrication:
- Metal alloys: nickel-chromium, cobalt-chromium, silver-palladium.
- 900 fine gold.
- Various acrylic resins.
- Dental porcelain.
Briefly: answers to topical questions
What are the types of dental impressions?
Dental impressions are classified according to several main criteria. Based on the method of border molding, anatomical and functional impressions are distinguished. According to the number of teeth and the extent of the prosthetic bed tissues covered, full impressions taken from the entire dental arch and partial impressions are differentiated. Depending on the degree of pressure on the mucosa of the prosthetic bed during their taking, impressions are subdivided into compressive ones, which are further divided into voluntarily compressive and functionally compressive.
What are the classes of dental arch defects according to Kennedy's classification?
Kennedy's classification of dental arch defects is based on the localization of the defect and the position of the abutment teeth and includes four classes. Class I represents bilateral distal extension defects formed due to the loss of posterior teeth. Class II is characterized by a unilateral distal extension defect. Class III represents a bounded (intercalated) defect in the lateral segment of the dental arch on one side. Class IV includes defects where only anterior teeth crossing the midline are missing, and this class has no subclasses.
What groups of impression materials are used to obtain dental impressions?
To obtain dental impressions, a classification of materials according to their physical state after setting is used, dividing them into three main groups: rigid, elastic, and thermoplastic impression materials. Rigid materials include gypsum and zinc oxide-eugenol pastes. Elastic materials are represented by alginate and silicone impression compounds. Thermoplastic materials are subdivided into reversible and irreversible ones, which soften upon heating and harden upon cooling.
What quality requirements apply to the finished dental impression?
Strict requirements are imposed on the quality of the finished dental impression to ensure the accuracy of tissue reproduction. The impression must have a clear relief without pores, smudges, or tears in the material. After removal from the oral cavity, delamination of the wash material from the base impression is not allowed. To produce accurate models, maximum reproduction of the prosthetic bed relief and detailed transfer of the relationship between the tooth crown and surrounding tissues are also required.
Class and exam questions
What is the fundamental difference between an impression and a model?
An impression is a negative (reversed) reproduction of the jaw tissues, obtained using an impression tray. A model is a positive, accurate reproduction of those same tissues, poured into the impression.
What is the purpose of an auxiliary cast?
The auxiliary cast reproduces the antagonist jaw (opposite to the one being restored). It is necessary for the dental technician to correctly model the occlusion of the teeth of the future prosthesis.
Why are soldered fixed partial dentures abandoned in modern practice?
Soldered prostheses use a solder containing zinc, copper, bismuth, and cadmium. These metals frequently cause galvanism and individual intolerance in patients.
What else to study on this topic
- Kennedy's classification of dental arch defects: main classes and subclasses.
- Peculiarities of using removable partial cast-framework dentures in partial secondary edentulism.
- Indications for choosing various abutment elements (half-crowns, inlays, post crowns) for fixed partial dentures.
- Comparative characteristics of metal alloys (nickel-chromium, cobalt-chromium, silver-palladium) used in prosthetics.
Master the entire topic in the «Propaedeutics of Dentistry» course
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Other topics in the section "Chapter 10. Methods of restoring dental arch defects (orthopedic design)"
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