TTU and extra Flashcards
(31 cards)
criteria for a crown impression
margin detectable and even
no air blows
no creases or folds
no sharp lines or pointed angles
retentive
occludes well
prep not undercut
opposing impression taken
steps in crown production
-take impression: of prepped tooth and occluding arch. Record an occlusal registration if occlusion not obvious
-working model made of hard die stone produced in lab. Mounted on articulator. Model sectioned
-occlusal contacts checked
-die spacer applied, and isolit to prevent sticking
-waxing: build up the wax to include cusp cones, contact points, marginal judges, triangular ridges, contact points
-wax pattern is sprued and invested
-metal crown adjusted to fit to the model, checking and restoring contacts
-contour and polish so it is shiny
how to build up the wax systematically when making the wax crown
-dip into yellow wax, 1mm short of gingival margin
-blue wax to create the margin
-red to create the medial and distal contact points
-yellow to create the cusp cones, ensuring the functional cusp occlude with opposing tooth
-blue to create marginal ridges which should be same height as adjacent teeth
-red for internal and external ridges
-secondary anatomy in green
-then fill in, and smoothen
material options for crowns and the methods used
gold - casting - lost wax technique
cobalt chromium tungsten - milling, using computer technology to scan the wax prep
Lithium disilicate ceramic (Emax) - pressing
Porcelain fused to metal
pros and cons of gold crown
Strong, can be used in thin sections so more minimal prep so low biological cost, biocompatible, lasts very long, No adverse wear of opposing dentition, BUT expensive, aesthetic issues.
-the preferred material
pros and cons of cobalt chromium tungsten crown
Strong, can be used in thin sections so more minimal prep so low biological cost, biocompatible, No adverse wear of opposing dentition, low cost - Milling machines cut and shape blocks of Cobalt Chromium Tungsten alloy from the information sent via computer on the digital file.
BUT aesthetic issues
pros and cons of ceramic crowns
Aesthetic, low cost,
BUT heavier tooth preparation required as ceramic can be brittle in sections which are too thin. So increased thickness means more tooth tissue removal. Damaging to opposing dentition
Pros and cons of porcelain metal crown
Combines strength and aesthetics if planned correctly, low cost, conserves tooth tissue in areas where there is metal, occasionally unsightly metal shine through at margin
What is an onlay and inlay
Onlay - A restoration designed to cover and protect the entire occlusal and cusp areas, so it spreads the load so less likely to fracture
Inlays just go into the hole, not on the occlusal surface
Criteria for a crown
-margins - no areas for micro leakage, not overhanging
-occlusion - no premature contacts to prevent opposing tooth fractures, and no low occlusion to prevent over eruption
-proximal contacts - required to prevent tooth drift and food trapping
-finish - smooth, no sharp edges
-aesthetics
why a post is needed. what will help prognosis of the post and core
When endodontic treatment is needed to restore a non vital tooth and there is inadequate tooth structure to support a crown
There are two posts required for an indirect post and core, identify which is needed for the impression and which is needed for the post fabrication and why
Smooth – impression (captured in model) smooth so can come out
Serrated – post fabrication (retention and additional surface area for cementation)
why is a die spacer applied to the cast before making the crown. how thick it should be
this should extend to 1mm short of the preparation margins and should be no thicker than 25µm. This spacer will ensure that the crown does not fit absolutely perfectly against the tooth, allowing space for the cement with which the crown will be secured.
why model hardener is applied to rest of the teeth of the cast
applied to the margin and the occlusal surface of the rest of the teeth on both jaws - this gives a harder surface and prevents damage to the casts during crown fabrication
Explain sprueing, investing and casting for gold crown production in the lost wax technique
-sprueing = preformed plastic sprue attached to thickest part of pattern at 45 degree angle to allow smooth flow into mould
-investing= pattern and sprue attached to sprue former and casting ring placed around. pattern sprayed with whetting agent to reduce surface tension and allow the pattern to be fully whetted by the investment. wax pattern coated in gypsum to create a mould
-casting = sprue former removed. 700 degree heat to burn away the sprue and wax. the crown is then cast with gold alloy
-devesting = investment scraped away
-crown trimmed, polished, adjusted
why there is a reservoir in the sprue
it is a slightly thicker part to the sprue, which reduces porosity
-when the metal cools after casting it contracts slightly, doing so from the thinnest parts first, and then from the thicker parts, because we have sprued from the thickest part of the pattern that will be the last part to cool. If we did not have a reservoir then we may see porosity in the sprued area of the crown.
what crown material is made by milling technique. Explain this technique
cobalt chromium tungsten, zirconia, non-precioous alloys
-(maybe Emax?)
special wax is used which is scanned on a digital scanner
very cost effective and requires minimal finishing in the lab
explain the production of e-max crown
After burning the wax pattern out in the furnace, a ceramic ingot (lithium disilicate) is placed at the entry channel for the sprue. placed in a special press furnace where the porcelain is heated and it is slowly pressed into the investment mould, filling the void left by the lost wax.
What Emax is made of. and its used in dentistry
-lithium disilicate glass ceramic
Veneers
Occlusal veneers
Inlays
Onlays
Anterior crowns
Posterior crowns
¾ Crowns
3 unit Anterior Bridge
3 unit Bridge up to the second premolar as the distal abutment.
how to take a post impression for an indirect post
- Prepare post channel.
- Shape coronal tooth tissue for crown
- Insert smooth, plastic impression post & record impression
- Make temporary post crown with smooth metal temporary crown post
- Send impression to lab
- Cast the impression
- Remove the smooth impression post from the model
- Insert a size-matched serrated post in the model & wax-up the core
- Remove the pattern from the model & cast it in hard gold or non-precious metal.
- Return the casting to the clinic.
- Fit and cement the post.
- After cementation the crown can then be fabricated in the normal manner.
how open apex teeth differ to normal teeth and how the approach to filling them differs
-can’t find WL, sealer more likely to ooze out end, root not as long, wider canals, root not as long, large pulp, narrower walls
-chemicaly remove pulp using micro brush or tepees as don’t want to remove dentine than necessary.
-find WL using x-ray. Or microscope and paper point until feel resistance and measure this length. MTA cement plug at 3-4mm apex
What sealer is used for RCT. is it hydrophobic or hydrophilic
AH plus
Hydrophobic
What are MTA and biodentine. How long it takes to set
calcium silicate cements
set well in water
take 2 days to set
what speed does gooseneck bur go
clockwise >1000rpm
