posterior crowns Flashcards
What are the reasons to restore a compromised tooth?
- Restoring function (first) and aesthetics (second)
- Restoring structural integrity and resisting fracture
- Integrating with other prosthesis (incorporate the partial denture)
- Preserving remaining tooth tissues and increasing fracture resistance
- Improves patient confidence and psychology
What is special about endodontically treated teeth?
Weakened due to access cavity prep
Loss structural integrity associated with loss of roof of the pulp chamber
Loss of dentine elasticity (becomes brittle)
When is cuspal protection required in a posterior tooth?
Loss of marginal ridges
Loss of substantial tooth structure
Heavily restored tooth
How is cuspal protection achieved?
Adhesive restorations - can be direct or indirect (composite, ceramic or metal)
Cusp-coverage cast restorations - prepare the tooth as well as cuspal reduction
Full coverage restorations
What are the different design considerations for posterior crowns ?
Is the tooth in function? Appearance Adjacent teeth Periodontal tissues Pulp Retention of the crown to the tooth Materials
What needs to be considered with the periodontal tissues when designing a posterior crown?
Margins are plaque retentive - if have perio disease more prone to plaque retention
What are the different ways of gaining retention for the crown on posterior teeth
Dentine bonded is bonded to the tooth - requires less prep.
All ceramic: requires bigger prep - prep includes retention
What processes during the crown prep can lead to pulpal death?
Thermal damage
Local anaesthesia - adrenaline reduces blood supply to the tooth.
Desiccation - open tubules and keep drying the tooth by spraying air
Bacterial contamination
What is the probability of pulpal death following crown preps?
10-19%
if do a 1.2mm shoulder prep, what is the resulting dentine width?
0.7mm
What are the different threats when preparing the tooth?
Bacterial toxins Desiccation Thermal Chemical Osmotic
What are the different options of material that can be used for the posterior crown?
Metal - minimal tooth reduction, strong in thin section, can be adjusted intra orally
Metal-ceramic - metal core layered with porcelain
Ceramic - dentine bonded and high strength core - strong core layered with translucent porcelain
What is type I gold alloy?
Was hard enough to stand up to biting forces but soft enough to burnish against the margins of a cavity prep
Was used for one-surface inlays
What is type II gold alloy? (soft)
Was less burnishable but hard enough to stand up in small, multiple surface inlays that did not include buccal or lingual surfaces
What is type III gold alloy? (medium)
This one is most commonly used for all-metal crowns and bridges
What does a typical type III gold alloy include? (hard)
Gold 75%, silver 10%, copper 10%, palladium 3%, zinc 2%
What is type IV gold alloy? (extra hard)
Was used for partial denture frameworks but was not used for fixed prosthetic’s
How long do you expect a full gold crown to last?
more than 10 years long-term
What are the advantages of a full gold crown?
Minimal prep
High longevity
Adjust occlusion in mouth
nice marginal adaptation
What are the disadvantages of a metal-ceramic crown?
There’s extensive buccal tooth reduction.
Only the metal component can be adjusted intra-orally
What are the 3 PFM allow types?
High-nobel alloys
Nobel alloys
Base-metal alloys
With high-nobel being the more expensive
What are high-noble alloys?
have a minimum of 60% noble and a minimum of 40% by weight of gold
Also contains a small amount of tin, indium or iron which provides an oxide layer to provide chemical bond to porcelain
What are noble alloys? (gold, palladium or silver)
contains at least 25% by weight noble metal .They have high strength, durability, hardness and ductility
What are base metal alloys?
Contains less than 25% noble metal
Harder, stronger, and have twice the elasticity.
Castings can be made thinner and still remain the rigidity needed to support the porcelain.
Were heavily used because of their low cost and high strength characteristics
Nickel and beryllium are the 2 most commonly used constituents - can cause allergic reactions when in contact with gingiva
How long does PFM last?
5 year survival 93.3%
What are the positives and negatives of all ceramic posterior crowns?
High strength ceramic core Most aesthetic Low edge strength Requires extensive reduction Intra-oral adjustment not possible
How long does all ceramic last?
5 year survival = 90%
What are the shillinberg principles of tooth prep?
Preservation of tooth structure Retention - axial direction Resistance Structural durability marginal integrity
What does the retention form prevent?
Dislodgement of the crown in an axial direction
What does resistance form prevent?
dislodgement of the crown due to rotation from a lateral load
What is important when doing the tooth prep design?
A clear finish line at margins
Ceramic margins should be butt-joint rounded shoulder 90 degrees and follow the gingival margin
Metal margins = chamfer
All preparation line angles and point angles are best rounded
What is the tooth prep for all ceramic?
Minimum occlusal reduction = 1.5mm >2mm in areas of stress
Shoulder margin
5 degree taper
What is the tooth prep of PFM crown
Deep prep on the labial for metal and porcelain
Buccally lighter <1mm on the lingual side
functional cusp bevelled for more material thickness
Smooth shoulder with adequate depth
What is the full gold crown prep?
1mm chamfer, parallel, sightly tapered
What happens at phase 1 of the clinical stage?
Pre-op clinical radiographic assessment further investigation study models diagnostic wax up periapical radiograph treatment planning informed consent
What happens at phase 2 of the clinical stage?
Tooth build up and preparations impression shade selection occlusal record temporisation fabrication (lab)
What happens at phase 3 of the clincal stage?
Removal of temp crown try-in of definitive crown cementation of definitive crown occlusal check review
