Crowns Flashcards
What is a crown?
Extra-coronal restoration that is constructed in a lab and covers the entire coronal aspect of the tooth.
Why might you choose to provide a patient with a crown?
Tooth fracture/cuspal fracture
Structural weakness caused by caries or large restorations
Pre-existing failed restorations
Restoration of root treated teeth
High aesthetic demand
Onlay not possible- too much tooth tissue lost
What are the principles of crown prep?
Aesthetics
Structural durability
Resistance and retention
Maintain as much tooth tissue as possible
Marginal integrity
Preservation of the periodontium
Describe the stages of crown preparation.
Occlusal reduction
Separation
Buccal reduction
Palatal or lingual reduction
Shoulder and chamfer finish
Occlusion check/adjustment
Polishing the preparation
What materials might be used for a crown?
All metal
Porcelain fused to metal.
Lithium Disilicate (EMAX)
Zirconia
Describe the prep for a porcelain-fused to metal crown and an all metal crown.
Porcelain fused to metal
- Occlusal reduction first- 2.5mm
- Separation- 1-1.5mm
- Functional cusp reduction- 2mm
- Non-functional cusp reduction- 1.5-2mm
- 1.5mm buccal shoulder and 1mm palatal chamfer.
- 5-7 degree taper
- Bevel the functional cusp
Metal
- Occlusal reduction first- 1-1.5mm
- Separation- 1-1.5mm.
- Functional cusp reduction- 2mm
- Non-functional cusp reduction- 1-1.5mm.
- 0.5-1mm chamfer all the way round.
- 5-7 degree taper
- Bevel the functional cusp
No undercuts, rounded internal line angles.
Check the occlusion and polish.
Describe the prep required for an all ceramic crown?
Occlusal reduction- 2.5mm
Functional cusp reduction- 2mm
Non-functional cusp reduction- 1.5mm
Separation
1.5-2mm buccal shoulder circumferentially.
5-7 degree taper
Bevel the functional cusp
No undercuts
No sharp internal line line angles.
Check occlusion and polish.
What are the indications for a providing a crown for an anterior tooth?
Trauma
Root treated tooth
Not enough tooth tissue left for a direct restoration
Caries has significantly weakened the tooth
Other direct restorations have failed.
Tooth wear
Describe the process of taking definitive impressions for a crown.
Assess the gingival condition- if the gingivae look inflamed or traumatised from the prep- consider postponing the impression until a later appointment- give OHI advice.
Select your tray for impression.
Apply appropriate adhesive to the tray- leave for 10 minutes.
Apply retraction cord to the.
- can use one (size 0 or 1) or two retraction cord (size 00) depending on the gingival biotype.
- Pre-soak the retraction cord in haemostatic agent- Ferric sulphate.
- For porcelain use aluminium chloride to avoid staining.
Use angulated cord packer to walk the retraction cord around the gingivae into the crevice and leave for 2 minutes.
Remove the retraction cord and evaluate the gingivae and the crown prep.
Wash and dry the preparation0 ensure blood and haemostatic agent is removed.
Then use the one stage or two stage impression technique
- use other a light body silicone or medium body polyether.
Take opposing arch impression.
Take a bite registration.
What should you ask the lab for?
What tooth you want to restore
The restoration- i.e. crown
Material
Shade
Ask for casts to be poured and articulated according tot he bite reg.
If lithium disilicate- request hydrofluoric acid treatment
If Zirconia- surface must be sand blasted
Describe the process of crown cementation.
Check fit of the crown on the working model.
Apply LA and remove temp crown.
Re-check reference contact points using shim stock and articulating paper.
Try in crown without cement- check occlusion, lateral excursions and aesthetics.
Prepare the crown for cementation- ceramic should be sandblasted and treated with all in one etch and primer.
Lithium dislocate should be treated with 5% HF etch.
Prepare the tooth
- apply rubber dam, PTFE tape to protect adjacent teeth from bonding, pumice the tooth and etch/bond depending on the cement used- think only resin needs etched and bond?
Load the crown/onlay with cement- filling it only halfway.
- light cure according to manufacturers instructions if it is a resin cement.
Floss and remove excess cement.
Apply glycerine and cure fully.
Check occlusion and adjust/polish.
Describe the process of dentine sealing.
Dentine sealing is done after the cavity has been prepped and all caries removed.
Apply rubber dam (should already be in place)
Etch the surface with 37% orthophosphoric acid.
Thoroughly rinse with 3 in 1.
Dry the tooth.
Apply bond and light cure.
Block out undercuts with flowable composite.
Then composite core.
Then do your crown prep.
What are the advantages of immediate dentine sealing?
Less bacterial leakage/contamination of dentine during temporisation
Reduced post-operative sensitivity
Increased bond strength as bonding to fresh cut dentine
Better fitting restoration as uniform bonding surface.
