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 metallic pressed crown.

2. sequence and maintenance of the clinical stages of making of the metallic pressed crown.

3. sequence and maintenance of the laboratory stages of making of the metallic pressed crown.

Content of the topic:

TYPES OF CROWNS

Full coverage

• Full veneer crowns (usually made of gold for posterior teeth)

• Porcelain jacket crowns (anterior teeth)

• Metal ceramic crowns.

Post crowns

• Cast gold core

• Prefabricated core.

Partial coverage

• Three-quarter crowns and reverse three-quarter crowns.

ASSESSMENT OF TEETH FOR CROWNS

Case selection is important. In order to plan treatment appro­priately, when considering crowns, assess: • tooth vitality • periodontal support and gingival condition • oral hygiene • caries control • occlusion • radiographic appearance • aesthetics (including patient's expectations) • adjacent teeth.

In some cases study casts, clinical photographs and a diagnos­tic wax-up of anticipated appearance may be useful.

CLINICAL STAGES IN MAKING CROWNS

1. PREPARATION

Crown preparation involves removal of enough tooth substance allowing sufficient thickness of material (from which the crown is to be made) to provide strength and aesthetics. Preparation must not damage the pulp. Preparation must provide sufficient retention for the crown. This can be achieved by taper of 5-20° (especially in cervical third of preparation), and inclusion of retention grooves or slots is useful in teeth of reduced occlusogingival height. Prepara­tion should involve minimal gingival trauma. Preparation should have smooth curves, not right angles or sharp edges. Finishing lines depend on the material from which the crown is to be made. Options for finishing lines:

Buttjoint e.g. porcelain jacket crown.

Chamfer e.g. palatal margin metal ceramic crown.

Taper e.g. full veneer gold crown.

Preparation is usually achieved by a selection of high-speed diamond burs.

2. TEMPORISATION

Prepared teeth require temporisation for aesthetics, pulpal protection and prevention of overeruption or drifting of opposing or neighbouring teeth.

Types of temporary crowns

Anterior teeth: • polycarbonate preformed crowns • polyethyl-methacrylate crowns fabricated using an initial irreversible hydrocolloid impression.

Posterior teeth: • stainless steel • polycarbonate or polyethyl-methacrylate.

Usually temporary crowns are cemented with temporary cement. Occasionally a more permanent luting cement may be used when the temporary crowns are to be worn for a prolonged period or preparations are of reduced occlusogingival height. Heat-cured acrylic temporary crowns may be used if temporisa­tion is for a prolonged period.

O3. IMPRESSIONS

An accurate impression of the preparation is essential if the crown is to fit. Materials used in crown impressions include polyvinylsiloxane, polyether, polysulphide (usually with an individual tray) and reversible hydrocolloid (needs special apparatus). To ensure an accurate impression: Follow the manufacturer's instructions for the particular material selected.

Obtain gingival retraction where a preparation is subgingival via use of appropriate thickness of retraction cord or occasionally using electrosurgery.

Examine the set impression critically, paying particular attention to air blows, voids, tears and shiny surfaces (lack of flow of impression), and if necessary repeat.

In difficult cases use of impression copings or proceeding to a trial of a casting may ensure a satisfactory end result.

An impression of the opposing arch in irreversible hydrocolloid is required.

Jaw registration is essential and is usually achieved by using wax, reinforced wax or silicone rubber.

4. PRESCRIPTION TO TECHNICIANS

The dentist should communicate information about crown shape, shade,irregularities and design (e.g. type of margin, type of material, rest seats/undercuts/guide planes) clearly to the technician.

5. CEMENTING A CROWN

On receipt of a crown from the laboratory check that: • the cast has been trimmed correctly; compare impression margin and cast margin • the neighbouring teeth on the cast have not been abraded • the crown fits the cast • the correct design features are present • the occlusion is correct • the shade looks broadly correct.

The temporary crown should be removed from the mouth and any adherent temporary cement removed (often this requires local anaesthetic).

The permanent crown is tried in. The following should be carefully checked:

Marginal fit

Contact point with neighbouring teeth This should be such that interdental cleaning is possible.

Gingival emergence angle

Occlusion In all mandibular movements.

When the dentist is satisfied, the patient should be shown the crown aesthetics and modifications made if required. When both dentist and patient are satisfied, the preparation is degreased (with alcohol), dried and the crown cemented with a permanent luting cement. If there is any doubt, it is prudent to use a temporary luting cement and review the situation. Excess cement must be removed from around the crown margin. The patient should be given oral hygiene instruction regarding the crown.

COMMON FAULTS WITH CROWNS

Despite careful attention to detail the following faults with crowns occur commonly:

Overhanging margin • Usually due to poor impression or poor technical work. • Can in some cases be corrected by trimming with a bur, but often requires a remake. • If uncorrected leads to plaque accumulation, gingival inflammation or recurrent caries.

Negative margin • Usually due to poor finishing line delin­eation, over-trimming of die or over-vigorous polishing of crown margins. • Often patient feels sensitivity. • Risks recurrent caries or poor aesthetics.

Poor gingival emergence angle • Usually due to lack of communication between dentist and technician. • Overbulking of material at the gingival margin leads to plaque accumulation.

Poor contact point • Usually due to under-preparation of mesial and distal walls or overbulking of interdental area by technician. • Hinders interdental cleaning.

Poor aesthetics • Can be due to incorrect shade, shape or under-preparation leading to insufficient space for material. • Occasionally patients have unrealistic expectations.

Persistent debonding • Often due to inadequate retention on preparation • May be due to occlusal interference. • In post crowns may be due to poor post design or longitudinal root fracture.

ANTERIOR CROWNS

Indications

• Protection of heavily restored teeth

• Aesthetics

• Bridge retainer

• Tooth wear.

TYPES OF ANTERIOR CROWNS

Metal-ceramic crown • Used when limited occlusal space and high functional loads. • Relies on ability of porcelain to bond to metal oxide. • Modern metal-ceramic crowns have excellent aesthetics. • Can have metal (when very limited occlusal space) or porcelain palatal surface. • Often have butt joint labially (1.5 mm shoulder to allow adequate metal and porcelain for aesthetics) and chamfer margin palatally.

Porcelain jacket crown • Used when aesthetics of prime concern.* Problem in high-load situation as porcelain in thin section and liable to fracture. Not usually suitable for posterior teeth.* Usually butt joint around whole preparation (minimum 1 mm shoulder to allow adequate porcelain for aesthetics).* Need 1.5 mm thickness of porcelain incisally.

Other anterior crowns Porcelain crowns with superior aesthetics and with higher tensile strength than conventional porcelain jacket crowns are finding increasing use - employ sintered alumina cores or injection moulding of ceramic. Require even reduction; preparation similar to a conventional aluminous

Note: rounded preparation, i.e. no sharp angles

POSTERIOR CROWNS

Indications

• Aesthetics (some posterior teeth only)

• Bridge retainer

• Tooth wear

• Protection of heavily restored teeth

• Partial denture abutments.

TYPES OF POSTERIOR CROWNS

Metal-ceramic crown Used when insufficient occlusal space, high functional loads, or aesthetics important. Metal (when limited occlusal space) or porcelain occlusal surface. Junction of metal and porcelain should not be in area of high occlusal stress. Can have metal or porcelain (superior aesthetics) labial margin. Often have butt joint labially (1.5 mm shoulder to allow adequate metal and porcelain for aesthetics) and chamfer margin palatally or lingually. Functional cusps (in Class I occlusion upper palatal cusps and lower buccal cusps) need further tooth reduction by means of a functional cusp bevel.

Full-veneer crown Used when aesthetics of minimal concern (usually second or third molars). Usually made of cast gold. Tooth preparation should be as conservative as possible with the following features: • buccolingually and approximally, a 5° taper is ideal; chamfer margin removing all undercut areas. • Should finish supragingivally - not always possible as preparation should extend more gingivally than existing restorations so that prepara­tion finishes on sound dentine. • Require minimum of 1 mm reduction occlusally to allow for gold to cover preparation. • Functional cusp bevel is needed to allow more occlusal clearance (1.5 mm) over functional cusps. •

Other posterior crowns Cast gold partial-veneer crowns such as three-quarter crowns are occasionally useful to preserve a single intact cusp (usually mesiobuccal cusp of upper first molar). Porcelain crowns using sintered alumina cores or injection moulding of ceramic are finding increasing use in posterior crown situations.

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. The least grind hard fabrics from the transferred constructions of artificial crowns requires:

A. Plastic crown.

B. Metallic pressed crown.

С. Cast crown.

D. A crown by Belkin.

Е. Ceramic crown.

2. During preparation of solution for an imprint at making of the metallic pressed crown it follows a gypsum to use:

А. 1% solution of salt.

B. 2% solution of salt.

С. 3% solution of salt.

D. 5% solution of salt.

Е. 7% solution of salt.

3. For making of metallic stamps it is needed to use:

А. Stainless steel.

В. КХС.

С. Fusible alloy.

D. Silver alloy.

Е. Solder.

4. Which surface of the tooth is prepared first:

А. Vestibular .

B. Approximal.

С. Palatal .

D. Cutting edge.

Е. Occlusal.

5. A thickness of the metallic pressed crown must be:

А. a 0,28 mm

In. 0,33 mm

С. 0,34 mm

D. 0,4 mm

Е. 0,44 mm

6. For polishing of the metallic pressed crown it is necessary to use:

А. Paste "Crocus".

In. Pumice.

С. Solution of chalk.

D. Paste of ДОІ.

Е. Diamond.

7. Removing slander from the surface of metallic shells is possible a way:

А.Boiling in water.

B. Boiling is in salt to acid.

С. Heating.

D. Treatment a monomer.

Е. Treatment an insulant.