CSAR Part1 Flashcards
Reasons for a patient needing a crown
-protect tooth with significant tooth tissue loss
-protection from flexion (compression forces instead)
-protect against future fracture
-materials are stronger
-Hard to recreate anatomy and occlusion with filling
-distribute the force
-loss of marginal ridges (these prevent flexion)
- heavily cavitated
-cracks into dentine
-after RCT
What are the risks and disadvantages of providing a crown
-Failure due to poor OH, ongoing caries and perio, poor fit at margins
-15% of crowns will lead to loss of pulp vitality due to thermal stimuli (handpiece), dentine being connected to pulp, contaminants, direct exposure, lots of tooth loss)
-Needs many appointments
- possible sensititvity after the prep
-Potential future costs, so Need to look after it
Pros and cons of an adhesive crown over a mechanical bonded
-Pros: more conservative prep -less pericervical tooth loss, don’t need ferrule
-needs periphery of enamel as more predictable bonding than dentine, maintains its bond over time, and doesn’t flex, resins cements are more aesthetic
-Cons: Adhesive resin cement needs isolation to be kept dry so more technique sensitive, cannot be used sub gingival
-resin is more expensive and difficult to clean up but stronger and good aesthetics
What are the criteria for a mechanically prepped tooth
To allow resistance and retention:
-minimal taper - < 20 degrees (for POI, retention, no undercut)
- need enough height (>3-4mm)
-ferrule to resist displacement (>2mm)
(potentially grooves and boxes)
-peri-cervical dentine
-good margin for space for crown, good seal, aids lab, respects the periodontium
-good occlusal reduction to protect tooth and allow enough room for material
Additional ways to improve retention of a crown
-grooves/ boxes
-resin cements
-surgical crown lengthening to create more ferrule for crown to engage and improve aesthetics (but margin narrower further up tooth)
Pros and cons of conventional cements (for mechanically retentive crowns)
-Using conventional cement (GIC, zinc phosphate) which is easier to see and remove excess as opaque, doesn’t need moisture control or enamel, low technique sensitivity, can use subgingival, cheaper, long lasting
-but weaker, less aesthetic as more opaque
Pros of metal crown over ceramic
-less destructive preparation as metal doesn’t need to be as thick as cermaic
-easier to adjust
- kinder to opposing teeth
-more durable
-less brittle, stronger in thin sections
-gives a more reliable marginal seal (high gold alloys can be burnished)
-good for patients not bothered about aesthetics and posterior teeth
What material options are there for indirect restorations
-Metal (precious [gold] or non-precious)
-porcelain fused to metal
-ceramic: Zirconia (non-etchable) and Emax (etchable, a glass)
-composite
Explain the following terms: 1) extra-coronal, 2) intracoronal, 3) indirect and 4) direct restorations
1) replacing cusp with indirect restoration
2) Restoration within the crown. form of tooth is fairly in contact with cusps remaining
3) eg. Crown: Preparing the tooth, taking a record of the tooth, restoration made outside mouth, then cemented into place
4) Cutting a cavity then directly replacing inside the mouth. Material is soft then becomes hard so can get in under cuts unlike in indirect restorations
The classifications of indirect restorations by coverage
-Full coverage: crown
-Partial coverage (3/4 crown): only (partial or full occlusal coverage)
-Intra-coronal: Inlay. No cusp coverage. Rarely done as direct restoration just as good
What cements are used in mechanical and adhesive crowns
-Mechanical: conventional cements (zinc phosphate, GIC, RMGIC) - passive bonding
-Adhesive: resin cement (stronger, active bonding)
What are the properties you would want a crown cement to have
-ideal viscosity, ST, WT,
-hydrophilic, not technique sensitive, low cost, opacity (easier to differentiate between tooth if cutting crown off), aesthetics,
-good compressive and tensile strength, durability,
-biocompatible, insulator
How properties of zinc phosphate and resin cements differ. Which lasts longer
1.Zinc phosphate: long lasting, much cheaper, looks like chalk so easy to distinguish with tooth when removing but less aesthetic. Mix on glass slab until cream consistency [zinc oxide + phosphoric acid], chewing gum consistency for temporary crowns, cool the glass slab before mixing to increase ST
-a conventional cement for mechanically bonded crowns- passive bonding
2.Resin cement: more expensive, etch and bond needed, dual cure (light and chemical), slower setting time. Once cemented, remove excess then cure for few seconds then remove more. Cure for 1 min at each angle
-for adhesive crowns - active bonding
Does ceramic have an elastic modulus similar to enamel or dentine
-enamel
-similar stiffness and rigidity
-enamel is 80, Emax is 95
-dentine is around 15 as it is less stiff for shock absorbance
Questions to ask yourself when considering a tooth for a crown
1)Patient factors- money, time, importance to patient
2) Is it restorable
3) Indirect or direct
4) Mechanical or adhesive (can it be isolated, ferrule, height, taper
5) What material
Risk assessment to assess restorability of a tooth for a crown
-perio status. No PPDs >3mm
-sufficient structure (per-cervical and height) for ferrule
-endo status -no PAP, no exposed root filling
Explain to a patient the option of a crown, why it is needed, advantages, disadvantages, the process
-You have a broken down weak tooth that is at risk of fracturing further if a normal filling was done
- we want to stop it fracturing and keep it in mouth for as long as possible by protecting it with a crown which is cap
-No guarantee how long it will last, but on average it is 5-8 years
-We will prepare tooth, take impression, put a temporary crown on while it is made in lab, then will cement a definitive crown on. Likely to take 2-3 appointments
-Risks: may experience some sensitivity after which should resolve after a couple days. Risks that the nerve will die (15% chance) which could cause pain and swelling which would mean RCT to save the tooth or would need to take the tooth out. This depends on how compromised the tooth was initially, how well you look after it, reducing sugar in diet. If it does fail then it can become costly in the future for RCT extraction.
-Patient’s choice, I can only guide you. But I believe the patient would benefit from this treatment
-Summarise, check understanding, document
Importance of appropriate tooth preparation (issues that arise due to over and under prep)
-Over prep= weakens tooth, compromises pulp, less resistance form
-Under-prep= thin weak crown or bulky crown (occlusal problems), poor aesthetics as shine through, perio problems as difficult to clean due to ledge/ overhangs
-knowing the dimensions required for different materials, putty matrix and depth cuts help
Explain the types of margin design: 1) chamfer, 2) shoulder, 3) Knife-edge
1) rounded
2)flat ended bur to create sharp edge (this can crack)
3) Vertical prep. No definitive edge for lab
How much occlusal and margin reduction is required for a metal crown
-Occlusal= 1.0-1.5mm
-0.2-0.5mm light chamfer
How much occlusal and margin reduction is required for an Emax crown
-1.5-2.0mm occlusal
-0.7-1.2mm champher
How much incisal and margin reduction is required for a Emax veneer.
1.5mm incisal
0.3mm light chamfer at gingival margin, 0.5-0.7mm facial reduction
Reduction required for a PFM crown
-More conservative on lingual and palatal, more reduction on buccal for more aesthetic to accommodate more room for porcelain
-occlusal buccal= 2.0mm
-occlusal lingual= 1.0-1.5mm
-buccal margin= 0.7-1.2mm deep chamfer
-lingual margin= 0.2-0.5 light chamfer
Pros and cons of a sub gingival or supra gingival margin. Reasons for a sub
-Ideally keep it supragingival as easier to prep, take impression and cement, but sometimes hard to avoid. Pulpal considerations (deeper puts pulp more at risk) Easier to clean
Sub factors:
-Quality and location of existing restorations
-Remaining tooth structure - margins on sound tooth structure
-Preparation height (optimising ferrule effect >4mm)
-Aesthetic requirements (subgingival more aesthetic)
-Periodontal health
*Want margin in sulcus but not impeding on epithelial attachment (biological width) as this causes inflammation and recession
What material is Emax. Is it etchable. Properties
-a lithium disilicate ceramic - glass
-etchable
-no porosity, strong
-translucent so very aesthetic (but conventional cement would shine though)
-monolithic (made from solid block)
What are the properties of zirconia
-very strong
-very white/opaque (poor aesthetics, so layering needed)
-cannot be etched
-difficult to bond to teeth and difficult to remove from teeth
-delamination is a problem
-polycrystaline
Why is zirconia so strong
If a crack starts forming, Transformation from tetragonal structure to monolithic blunts the crack and so you get higher mechanical properties.
What are the 2 essential requirements for doing an adhesive restoration
-Enough enamel (Bonding to enamel is more predictable than to dentine as dentine can be sclerotic or carious and it has fluid linked to the pulp) Maintains its bond over time, and doesn’t flex
-Good isolation as needs kept dry (resin cement is technique sensitive)
Properties of dental porcelain. What is its structure
-inorganic. Ionic bonds. Ceramic is crystalline, glass is amorphous
-Really good, long lasting aesthetics (resistant to stains)
-Good strength in compression, but weak in tension (as porous)
-high hardness
-Brittle material (so need to be thick)
-Low toughness - notch sensitive
-pigmented
-erosion resistant
Difference between brittle and ductile
-ductile materials: can permanently deform without fracture (eg. metals and alloys)
-brittle: fracture when deformed by 0.1% strain. Cannot plastically deform (eg. ceramics and glass)
What are the 2 ingredients in dental porcelain. How is it made
-Contains silica and feldspar
-ingredients are fired, quenched in cold water, ground to a powder, pigments added. Water added to form paste, layers built-up. Blotting to reduce shrinkage. Then device is sintered (fired)
Why an alloy sub-layer is used with porcelain (PFM)
-High Stiffness: Reduce tensile stresses forming in ceramic
-High Strength
Using both materials to get the best of both worlds. Strength from the metal, aesthetics from the porcelain
Cons for a PFM
-Weak point at material interface
-Risk of chipping/fracture
-metal collar at cervical margin- unaesthetic
-destructive to tooth tissue to accommodate material thickness (Porcelain needs to be thick enough so not brittle and so it blocks out the metal shining through on buccal side)
-Lots of work/time for technician = increased cost
-porcelain causes damage to opposing teeth
Considerations when choosing the alloy and porcelain for PFM crown
-When firing porcelain on top of alloy coping, need to be careful of melting and recrystalisation temperature of alloy. Alloy needs to have a melting temperature of > 100 degrees than the firing temperature of the ceramics. If too close, can get recrystalisation of alloy (mechanical properties drop) or get deformation. So careful to choose alloy wisely.
-Porcelain needs to be opaque enough to block the appearance of the metal from showing through
-thermal expansions need to be well matched to prevent cracking
What are the 3 bonding mechanisms between metal and porcelain for PFM crown
1) Thermal contraction
2) Mechanical interlocking: Alloy crown roughened. Ceramic flows into undercuts during firing
3) Chemical bonding: Ceramic reacts with oxide layer on alloy
Material options for the alloy coping for PFM crown. Pros and cons for each
-Gold alloy- better if low gold content for better stiffness. can deform during firing
-Silver palladium alloy: stiffer than gold alloys, no sag, cheaper. Good bonding. But porosity can occur
-Ni/Cr alloy: very high MP so not affecting my porcelain firing, high stiffness. But weak bonding as thick oxide layer, and casting shrinkage
-cobalt chromium tungsten
3 methods of crown production
- Casting (investing, lost wax technique, cast)
- CAD cam (milling)
- Pressing (lost-wax hot-pressing)
Explain the lab stages for making a PFM crown with CAD CAM technique
-Coping made via CAD CAM: cast is scanned, coping is designed, milled/cut from cobalt chromium tungsten
-Sandblasted and steam cleaned
-Porcelains are layered: opaque layer to mask dark alloy, dentine for bulk of crown [pink powder], enamel for translucency [light blue powder]
-Fired. Trimmed and shaped
-Glazed and stains added
-Fired
Why alloy cannot have copper in PFM crown
Needs to be copper free as green/blue oxides discolour the ceramic
What is a ceramic core. What is added and what properties does it give it
-Cracks tend to be on interior fitting surface and especially on palatal aspect, so ceramic-core is needed (inner layer) to provide strength and toughness
[-then a layering a pretty feldspathic porcelain over the top which is weaker]
-alumina is added to improve strength. It increases resistance to crack propagation, improves flexural strength and increases elastic modulus (harder to deform/break)
-However alumina is opaque so poorer aesthetics so another ceramic layer needed over the top
Which is etchable: Emax or zirconia. Why
-Emax is etchable so micro-mechanical bonding
-Emax is a glass. Glass ceramics shouldn’t be sandblasted (roughens surface) as it can initiate cracks and weaken the material.
- Zirconia can be sandblasted (cement can mechanically interlock) to it doesn’t need etching
Advantage over CAD CAM technology over casting. Which materials can be milled with this technique
-Eliminated porosity (no layering required, no casting)
-Milling limited by bur size and is expensive
-not all materials can be cast/pressed
-doesn’t require high technician skill
-High strength polycrystalline ceramics (zirconia) can be used, as well as cobalt chromium tungsten. And EMAX I think
Which materials can be casted, milled (CAD CAM), and pressed
-cast-gold alloy, cobalt chromium tungsten, nickel chrome
-milled (CAD CAM)- cobalt chromium tungsten, zirconia, Emax
-Pressed - Emax lithium disilicate
Why precious alloys are cast, but not non-precious alloys
Precious:
-very accurate for gold alloys and margins can be burnished, easier to mask with feldspathic/layered porcelains as the metal is slightly ‘warmer’. But expensive
Non-precious: – better resin bond strength and cheaper. But more Shrinkage, less accurate, masking of metal tricky. So CAD CAM used
How does flexural strengths and aesthetics of metal, zirconia and Emax compare
-highest strength= metal, then zirconia then Emax
-reverse order for aesthetics
(Broadly, as flexural strength increases, aesthetic potential decreases)
Difference between layered and monolithic ceramic crowns
-Monolithic= made from single block of material, milled from block, not layered. No porosity, stronger, cheaper, less likely to chip. Less tooth prep but poorer aesthetics. Can cause wear to opposing teeth so needs polished
-Layered means you can have a strong core (but not as aesthetic) with a pretty porcelain layered over the top
Pros and cons of indirect composite restorations, compared to ceramics
-Cheaper and fast to mill (no firing required) -Can mill in thinner section than ceramic without fracture
-Easy to adjust/repair, not damaging to opposing teeth
-Good when supported by enamel (wear cases)
-Less biological cost
-More Flexible than ceramics so puts a greater strain on bond potentially making failure more likely
If into dentine requires same prep as ceramic (which is much stronger)
-more maintenance, more wear over time, loses its shine (loses its filler)
Which crown materials need to be mechanically bonding and which need adhesive bonding
-Strong materials (metal, PFM, zirconia) can be used with conventional cement- don’t need to be bonded to underlying tooth to gain strength, BUT need a retentive prep
-Emax is less strong. Work well with conventional cements (min thickness 1.2mm axial walls) but adhesive resin cements advised
Which has better bond strength: precious or non-precious metals
-Non-precious metal bond strengths are higher than precious metal
Crown material of choice for: 1) low aesthetic concern 2) mechanically bonded anterior tooth 3) resin bonded anterior tooth 4) all 4 incisors
1) metal (either resin or conventional bonded depending on prep)
2) PFM or zirconia
3) Emax
4) monolithic restorations would be a reasonable choice, as they would all look the same- they don’t have to be matched to a natural tooth. [less prep needed but more opaque] - zirconia
The steps for a crown
- Check perio, endo, restorability of tooth
- study models if needed
- record shade at start (including stump)
- LA
- Record putty matrix of tooth (for provisional). And record impression of opposing dentition
- Prep the tooth
- Gingival retraction if <1mm supra gingival or sub.
- Silicone impression of tooth and arch- check accuracy
- Record jaw reg if needed (if block needed then done in other appt) and confirm which opposing pairs of teeth have shim stock contacts
- Facebow if needed
- Use matrix to form a provisional
- Cement provisional
- Remove provisional and cement with excavator and USS
- Check occlusion that it is the same as before
- Check fit of crown on cast and on tooth - ensure correct height, occlusion, contacts, margins, alignment, feel for patient.
- Make any adjustments
- Cement the crown and remove excess
- Check occlusion
What to do if patient experiencing pain with their temporary crown
-If lots of pain then delay putting permanent crown on
-If mild pain, this is probably due to dentine sensitivity or temporary crown leakage so fine to proceed
Indications for veneers. And contraindications
-TSL, peg shaped laterals, diastemas, discolouration (but first choice is bleaching, micro abrasion, resin infiltration, composite masking), fluorosis, hypoplasia, trauma discolouration due to necrosis (first choice is internal- external bleaching)
-contra- involved in heavy occlusion (bruxism), lots of restorations (crown may be better), inadequate tooth structure for bonding, poor OH
Using a bur that is too narrow can create a lip on the margin. What are the consequences of this
-stone cast will be weak so tendency to fracture. Lab will make crown to the fractured model so it will not re-seat. No defined margin for lab to follow.
-it can also fracture in mouth meaning the crown won’t fit
-use chisel or wider bur to remove lip
Consequences of having an over tapered prep. How to manage
-less mechanical retention so enamel, isolation and strong adhesive cement is essential
-if adhesive approach is not an option then lower the margin with an upright bur
-if already sub gingival, then create grooves for a definitive POI (but last resort as removing causes fracture)
Why moisture control is essential for taking crown prep impressions. What techniques are used to control moisture
-controlling saliva, blood, GCF because silicones are mildly hydrophobic
-ensure perio health is stable before hand to minimise bleeding (removing plaque retentive factors, OHI)
-haemostatic agents to vasoconstriction vessels
-LA with a vasoconstrictor
-ask patient to swallow prior to impression
-lightly dry teeth with gauze or 3 in 1
-saliva ejectors
-possibly use check retractors
Why a first and second cord is used for taking impression. What sizes are used
-Used for equi or sub gingival margins
-First cord is small diameter (00 or 000) soaked in astringent to go fully sub gingival for moisture control and stays in during impression
-Thicker cord (1,or 2) soaked in astringent is not fully sub gingival and expands the sulcus, deflecting the gingiva so silicone can capture >0.5mm beyond the margin. but does not stay in during impression taking as gingiva is viscosity-elastic (keep in for ~6 mins) If need to retake imp then must place the second cord again
What astringent is used with retraction cord. What to warn your patient about it
-15.5% ferric sulphate
tastes awful
-Precipitate out proteins when in contact with blood and these physically obstruct vessels. Proteins are black/blue which stains gingiva so aesthetic concern for anteriors. But takes 1-2 days to resolve
Alternative option to retraction cord for gingival deflection during impression taking
-Electrosurgery/rotary curettage, laser: Cuts the inner epithelium and creates that space in the sulcus
-BUT Causes post op pain, Unpredictable healing and possible recession , Can damage tissues around the site
-if put 2nd cord in and tissue start to enroach, then use larger cord or add another
Gold standard material for crown impression
Addition silicone (condensation has a biproduct so less dimensionlly stable)
-Light body wash over prep for accuracy, with heavy body around so doesn’t go down patient’s throat
-(putty is a cheaper alternative to the heavy body but tends to be too thick and displaces the light body and they aren’t very compatible)
What are the properties of addition silicone. Properties of light body compared to heavy
-dimensionally stable-doesn’t lose water (unlike condensation silicone) so can leave it up to a month
-elastic
-captures fine details
-tear resistant.
-BUT expensive, hydrophobic so problem with subgingival margins
-Light body: lower viscosity, flows better so picks up very fine details of the crown prep and occlusion of adjacent teeth. But messier so just used for area of impressions. Heavy body for rest of arch
What are the aims of taking a crown prep impression
- To capture excellent occlusal detail of the surrounding teeth and also of the opposing teeth
- To capture a very detailed recording of the prepared tooth, showing the margin clearly
-no air blows, material not pulling away from tray, correct seating of tray, correct tray size
How far below the margin must be captured during the impression and why is it important
-At least 0.5mm of unprepared axial surface beyond the margin
1. to allow the lab to determine where the margin ends
2. so the lab can gauge the emergence profile of the crown- Can visualise the crown shape as it moves from the tissues. Without this crowns often tend to have bulbous emergence profiles which are hard to clean and look bulky
What 3 techniques can be used for making provisional crowns. What materials are most common
1- freehand (composite)
2- shell technique (metal, poly carboxylate, PMMA)
3- matrix technique (bis-acryl most common)
Explain the matrix technique for provisional crowns
- Pre-op Matrix made by placing alginate or silicone putty over tooth
- Prep tooth
- Temporary bisacryl material loaded into matrix then placed over prepped tooth (remember no spacer unlike in lab so little room for cement so only put on walls)
- Trim excess material, flick off of the prep or matrix. Make adjustments using shofu
- Cement with temp bond zinc oxide (or may not need cement if still contracting around tooth and firm)
- POIG- slight sensitivity expected, some bleeding if subgingival, temporary may break
Advantages and disadvantages to using bis-acryl provisionals with matrix technique
Advantages:
-Easy to use (auto-mix)
-Quick to set
-Low exo-therm and low shrinkage
Disadvantages
-Hard to add to, although possible by roughening then using bond and composite
-Brittle – no flex (so don’t leave too long before removing from matrix)
-Not great for multiple unit work
Bis-acryl provisionals can be hard to bond to composite core. So what can be done to improve bonding
-Need to get rid of the oxygen inhibition layer (unreacted surface monomer) Use glycerine then cure it.
-Glycerine facilitates removal but remember to wash off before cementing permanent crown
-use non-euganol cement
When would the shell technique be better over the matrix technique
- if the tooth is very broken down to begin with, or area that are under contoured, or in highly aesthetically demanding cases, or when 3+ teeth are involved
-preformed crown shell
Explain the Shell technique for provisionals. What cement is used
-Select metal or poly carboxylate crown that is roughly the right size and shape, then trim around margin of the crown and cut down until rough fit so good marginal adaptation
-Use polymethylmethacrylate temporary acrylic resin inside fitting surface of the shell
-used firm pressure to apply to tooth and remove excess spilled out.
-then remove. ^ I think you cement on the crown and leave it on
What cements are used for bonding Bis-acryl provisional crowns
- Temp-Bond -zinc-oxide
(Most common. Not adhesive)
[Need to use non-eugenol with composite cores as eugenol can denature composite resins] - Zinc Polycarboxylate -if >20 convergence (as needs to be adhesive)
- Precursor to GICs, lightly bonds to most things
Useful when lacking resistance form
But harder to remove provisionals and from the tooth so may need gentle ultrasonics/ sand blasting to remove before permanent cementation
Why are provisional crowns essential
-worn during the 2 weeks gap while lab makes permanent crown
-prevents tooth fracture
-protects exposed dentine, preventing sensitivity, pulpits, loss of vitality
-prevents tooth drift and overeruption
-aesthetics
-function
-help stabilise gingival inflammation and also develop gingiva contour (to improve aesthetics) before putting on a permanent
The use of provisional crowns in cracked tooth syndrome
-can place provisional crown after restoring and monitor symptoms. By providing tooth coverage it may resolve the problem or not. If deteriorates it is likely pulpitis so can treat accordingly.
-therefore it helps assess the tooth and make a pulpal diagnosis as symptoms can present very similarly
Why never clean non-precious metals with phosphoric acid
-decreased bond strength
3 contraindications for all ceramic crowns
-Insufficient occlusal clearance or sharp internal angles on the tooth preparation.
-Bruxism - brittle so likely to fracture
-Atypical occlusal loading
-opposing tooth at risk of wear from an abrasive ceramic guidance surface.
Brands of resin cements
Panavia
Multilink
Rely-X
If you need to retake impression, do you need to put the deflection cord back in
The gingival displacement created by removing deflection cord only lasts 20-40 seconds and will need repeating each time you want to attempt an impression.
The 4-5 minute wait is also required each time to produce the viscoelastic phenomenon that creates the displacement.
Negative outcomes if temporary crown comes off and not replaced for 2 weeks
pulpits, loss of vitality, gingiva overgrowth, unable to seat definitive crown
- will not cause fracture, occlusal disturbance, discolouration (as not in occlusion)
Is feldspathic porcelain or composite weaker
feldspathic porcelain (it is a layering pretty porcelain)
What substances are applied to the ceramic before cementing porcelain crown chair side
-Zirconia- sandblasted by lab. Primer with MDP
-Emax Apply hydrofluoric acid to the ceramic in lab. Apply primer with silane
-Clean ceramic with phosphoric acid
Monobond has MDP and silane
What is the active chemical in non precious metal alloy bonding
10 MDP
