4.3. Practical works (tasks) that are executed at the lesson:
By means of diagnostic models, thematic patients demonstratively examined students:
1. indication and contra-indication to making of the plastic crown.
2. sequence and maintenance of the clinical stages of making of the plastic crown.
3. sequence and maintenance of the laboratory stages of making of the plastic crown.
Content of the topic:
Technology of plastic crown.
Provisional restorations should protect the teeth (dentin and pulpal tissues) against invasive microorganisms, saliva, and food, which may penetrate the dentinal tubules as well as prevent thermal conduction for comfort. Proper marginal adaptation will also protect the finish line of the tooth preparation. Adequate maxillary and mandibular occlusal relationships as well as interproximal contacts will prevent the drifting and extrusion of teeth that can affect the fit of the final restoration. Maintaining these relationships between the teeth promote gingival health by preventing food impaction into the soft tissues. Proper contouring of provisional restorations can assist in the maintenance of periodontal health by allowing access to the soft tissues during patients’ oral hygiene procedures.
To identify an optimum treatment outcome before the completion of a definitive restoration, diagnostic changes in tooth morphology and occlusal contours that were established during a diagnostic wax up can be evaluated for function, phonetics, and esthetics. Provisional restorations can also be used for extended treatment intervals by providing long-term tooth protection and stabilization during adjunctive periodontal and endodontic treatment procedures.
The fabrication of provisional restorations requires an array of clinical activities, material selection, and techniques to fabricate acceptable interim restorations. Ideally, they should be similar to definitive restorations by providing pleasing esthetics to enhance the self-image of the patient and help guarantee cosmetic acceptability of the definitive restoration for the patient and the dentist. The provisional restoration must also provide adequate support and protection for the prepared tooth (teeth) while maintaining periodontal health. It should maintain or restore function for two weeks to six months.
Provisional restorations can be classified by whether they are prefabricated or custom made. Prefabricated forms include stock aluminum cylinders ("tin cans"), anatomic metal crown forms, clear celluloid shells, and tooth-colored polycarbonate crown forms. They can be used only for single-tooth restorations. Custom crowns and fixed partial dentures can be fabricated of several different kinds of resins by a variety of methods, direct or indirect.
Provisional restorations also can be classified by the method used for adapting the restoration to the teeth: the direct technique is done on the actual prepared teeth in the mouth, and the indirect technique is accomplished outside of the mouth on a cast made of quick-set plaster. The direct technique is inviting to novices, because it eliminates the alginate impression and the plaster cast.
However, the direct reline is very technique-sensitive. In today's computer terminology, it is decidedly "user unfriendly." If the direct technique has any place in restorative dentistry, it is in the hands of experienced operators using a resin other than poly(methyl methacrylate).
The indirect technique is preferred over the direct technique for its accuracy. To avoid locking into undercuts, a directly fabricated resin provisional restoration must be removed from the tooth before it has completely polymerized. Since poly(methyl methacrylate) shrinks approximately 8% when it polymerizes, polymerization outside the mouth without a supporting form results in distortion and a less than optimal fit. A study of the marginal adaptation of provisional restorations showed that the marginal fit of poly(methyl methacrylate) provisional restorations could be improved nearly 70% by fabricating them indirectly.
The fit of provisional restorations made from almost all resins can be improved by using the indirect technique. For some materials, the improvement in fit obtained by using the indirect technique is as much or more than the improvement seen with poly(methyl methacrylate). Monday and Blais found better margins on poly(vinylethyl methacrylate) crowns made indirectly than those made either directly or by relining.
The indirect technique also is preferred for the protection that it provides the pulp, particularly if poly(methyl methacrylate) is used. The placement of polymerizing poly(methyl methacrylate) on freshly cut dentin could lead to thermal irritation from the exothermic reaction or chemical irritation from the free monomer. It has been reported that this produces an acute pulpal inflammation, as evidenced by an accumulation of neutrophilic leukocytes in the pulp horns. This is another irritant added to a tooth that in most cases has already been subjected to caries, previous restorations, and high-speed cutting in the preparation of the tooth. It is an additional insult that should be avoided whenever possible. A further advantage of the indirect technique is that much of the work can be delegated to auxiliary personnel.
Resins for Provisional Restorations
There are several types of resins that can be used for making custom provisional restorations. Poly(methyl methacrylate) has been in use the longest. Poly(ethyl methacrylate), poly(vinylethyl methacrylate), bis-acryl composite resin, and visible light-cured (VLC) urethane dimethacrylate have come into common usage in recent years. Epimine resin, which for a decade also was used for this purpose, is no longer available. Domstic industry produces heat-cured plastics for fixed restorations – “Sinma-M” and “Sinma-74”.
No one resin is superior in all respects, and the restorative dentist must assess the advantages and disadvantages of each in selecting which to use.
Clinical Phases
1st - Tooth preparation and impression obtaining.
Fig. Tooth after preparation. a -front view, b- side view
Accurate impressions of both dental arches are required. Flaws in the casts can lead to an inaccurate occlusal relationship. Irreversible hydrocolloid impressions are sufficiently accurate and offer adequate surface detail for casts. It is recommended the casts be mounted with the aid of an inter-occlusal record either on a non-adjustable or semi-adjustable articulator.
2nd- Fitting and fixing the crown.
Laboratory Phase
1. Models making, die construction.
2
.
Wax up.
Fig. Adjacent teeth to be restored can be waxed to ideal contours on the cast.
3. Flasking.
Plaster cast is trimmed (Fig 2) and is kept in cold water for five to eight minutes. Soft-mix plaster is poured onto the base of the flask (Fig 3) and the model embedded in it ( fig.4). After the plaster has set it is thoroughly smoothed out and kept in water for some time or covered with fat and the top part of the flask is then put on the base, loaded with soft-mix plaster, and closed with a lid.
Fig. 2 Trimmed plaster cast Fig. 3 Flask for fixed dentures
Fig 4 Types of cast placing in the flask
4. Wax wash
When the plaster has set the flask is kept in boiling water for five to ten minutes to melt the wax and then removed and opened carefully. The remains of wax are washed out with a jet of boiling water.
After the wax has been removed the flask is cooled.
7. Packing the resin
The plastic dough is prepared according to instructions supplied with the material, e.g. three parts by weight powder and one part by weight liquid (11.0 to 12.0 g powder and 4.0 to 4.5 g liquid) are combined in a cylindrical glass or porcelain vessel and stirred with a glass spade. The vessel is covered with a glass plate to saturate the powder with the monomer and prevent the latter from evaporating. When the powder swells and the mass acquires the consistency of soft dough and no longer sticks to the spatula and walls of the vessel the material is ready for moulding. It is removed from the vessel, put into the flask, and pressed into the sites which were freed of the wax. It is then covered with moistened cellophane and the flask is assembled, placed under the screw press and pressed gradually. After pressing the flask is opened, excess plastics removed or new portions added if needed, and the flask is again assembled and finally pressed.
8. Curing
After being kept under the press for three minutes the flask is transferred to a clamp and placed in a vessel filled with cold or warm water. The water is then slowly, over a period of 50 to 60 minutes, brought to boiling point and kept at that temperature for 60 minutes; heating is then stopped and the ring is left in the hot water for another 15 minutes. It is then cooled by air or in room-temperature water.
9. Deflasking
A crown removed from the plaster has extra plastic material and a rough surface and must be trimmed, given a finishing, and polished.
10. Polishing
Materials for self-control:
Materials for self-control:
1. Influence of preparing is on hard tissues of tooth and oral cavity.
2. Fusible alloys: composition, properties, applications.
3. Bleachers for stainless steel: composition, properties, applications.
4. Abrasive materials: composition, properties, applications.
5. Cements for fixing of the metallic pressed crown : properties, applications.
Test tasks:
1. Which plastic is used for plastic crown making:
A. Ftorax.
B. Protacril-м.
С. Bacril.
D. Aethacril.
Е. Sinma-м.
2. Taper for anterior teeth
A. Not more than 13-15 degrees.
B. Not more than 3-5 degrees.
С. Not more than 20-25 degrees.
D. There is not a value.
Е. Not more than 5-7 degrees.
3. In preparation of plastic dough it is necessary to follow rules:
A. To the monomer to add a polymer for strengthening.
B. To полімера to add a monomer.
С. To bring in simultaneously a monomer and polymer in crucible.
D. The methods of preparation do not influence on quality of plastic.
Е. To warm up during mixing.
4. Substituting of beeswax by a plastic at making of plastic crown is conducted on the next laboratory stage:
And. Third.
In. Second.
С. Fourth.
D. First.
Е. Fifth.
5. Specify a plastic which is used for making of two-color plastic crown :
A. Ftorax.
B. Protacril-м.
С. Bacril.
D. Aethacril.
Е. Sinma-м.
6. In what stage, plastic dough is used at making of plastic crown :
A. Doughy consistency.
B. Swelling.
С. Rubber consistencies.
D. Sandy consistency.
Е. All of the above.
7. Specify material of impression, which is used for making of plastic crown, :
A. Silicon.
B. Thermoplastic.
С. Compounds.
D. Hydrocolloid.
Е. All answers are correct.