TRY-IN PROCEDURE FOR CROWNS

Lect. 6. Crown and Bridge Dr. Ameer Hamdi AL-Ameedee
TRY-IN PROCEDURE FOR CROWNS:
A- Checking the crown on the die:
We must check the fit of the restoration on the cast model before trying it in the patient mouth:
1- Marginal fit.
2- Esthetics and articulation can be anticipated prior to try in.
3- Fitness of the crown surface for defects and the die for damage, preferably with a good light and under magnification.
4- Detecting these problems before try-in will allow you either to address the problem yourself or to negotiate with your laboratory from a position of strength. It is surprising how often clear ledges or deficiencies can be detected at this stage.
5- Subsequent stages then rely on the crown being checked in the mouth and then often back on the die again when there is a problem with the fit.

B- Seating the crown:
Having checked the crown on the patient mouth:
1- Any temporary restoration is removed and the preparation is carefully cleaned of all residues of temporary cement, especially in retention grooves.
2- The try-in procedure can normally be accomplished without the need for local anesthetic.
3- The crown should be tried in without forcing it onto its preparation.

The frailer to seat because of reasons localize problems:
a- First, ensure there is no retained temporary cement or trapped gingival tissue.
b- Then check and adjust tight proximal contacts as these often prevent seating.
c- Check the original cast for damage to the stone in these contact areas.
d- Then re-check the crown for the most obvious laboratory errors, including casting blebs, damaged or chipped dies or grossly overextended margins.
e- Over-extended margins should be adjusted from the axial surface, not from underneath by using bite-wing radiograph.
f- Where the crown still does not seat burnish marks on the internal walls of a sandblasted metal crown may indicate where it is binding.

Error Cause Treatment
Tight proximal contacts Imprecise die location or abrasion of the adjacent stone contact points Check for displacement of the dies contacts the adjacent stone contact points when the crown is seated on the working cast. Identify tight contact by interposing articulating paper, grind and polish
Casting blebs on fit surface Air bubbles trapped during investment Identify under magnification and fit surface remove with small round bur
Over-extended
crown margins Poor impression, poor die trimming surplus untrimmed wax or porcelain Trim from axial surface and polish— consider returning crown to lab
Under-extended
crown margins Poor impression, poor die trimming, difficulty identifying finish line If under-extension obvious and impression satisfactory have crown remade. Alternatively retake impression
Damaged dies Finish line chipped because of careless handling or abraded when casting reseated with blebs or overextended margins Always try and determine why the die is damaged. If the crown does not fit after adjusting blebs or over-extended margins return it to laboratory
No die spacer (Space needed to accommodate cement lute) Technician not aware of technique or forgot to apply Lack of spacer results in a tightly fitting crown which may not seat during try in and may ‘lift’ further after cementation

Assessment of the seated crown:
1. Proximal contacts: The tightness of proximal contacts can be tested with dental floss and should offer some resistance but not make its passage too difficult. If these are too tight they can be ground a little at a time and polished.
2. Assessment of marginal fit: A combination of clinical experience and empirical data suggest a marginal opening of 100 ?m2 is at the borderline of acceptability, ill-fitting margins will render the tooth more susceptible to cement dissolution, plaque retention and recurrent caries. Defective sub-gingival margins compromise gingival health by an alteration in local bacteria.
3. Assessment of esthetics: The minimal adjustment to crown may effected on shape and shade, so that the crown can be glazed when you are patient agree with the final appearance.
4. Assessment of the occlusion: The occlusion is the last assessment to make, but there is no point thinking about making any adjustment to the occlusal surface until the crown is finally seated. Posterior teeth, both restoration and adjacent teeth should hold shim stock firmly in the intercuspal position (ICP).
5. Finishing and polishing: The final stage prior to cementation is polishing. A rough surface, especially in porcelain, will rapidly wear the opposing tooth6 and so it is very important to use a sequence of abrasives designed for the material in question to achieve a smooth surface. Alternatively, a metal ceramic crown can be reglazed.
CEMENTS:
When the fit of the crown is considered satisfactory and all adjustments have been made, the crown can be cemented. There are essentially three types of hard cement: conventional, resin or a hybrid of the two. Conventional cements (eg zinc phosphate, zinc polycarboxylate and glass ionomer) rely on an acid-base reaction resulting in the formation
of an insoluble salt (the cement) and water. Resin cements set by polymerisation. The mechanisms by which cements secure restoration to prepared tooth include non-adhesive luting, micro-mechanical bonding and molecular adhesion.

Resin cements:
Resin cements are composites composed of a resin matrix, eg bis-GMA or urethane dimethacrylate, and a filler of fine inorganic particles. They have been available as direct filling materials since the early 1950s26 but it was not until the early 1970s that a composite resin was introduced for crown and bridge cementation.

Advantages:
• Good compressive and tensile strengths.
• High tensile strength (relative to conventional cements).
• Resistant to water dissolution.
• Relatively resistant to acid dissolution.
• Can enhance strength of ceramic restoration if bond obtained.
Disadvantages:
• Film thickness varies substantially between materials.
• Excess material extruded at margin may be difficult to remove especially proximally.

Recommendations:
• Must be used with or incorporate an effective dentine bonding agent.
• Material of choice for porcelain veneers, ceramic onlays and resin bonded ceramic crowns
• May be used to improve retention where preparation geometry sub-optimal, but clinical studies needed to determine long-term success.