Cons indirect materials Flashcards
what material categories are there for indirect restorations?
metals, ceramics, metal-ceramics, resin composites
what things affect the selection of metal alloys?
cost
castability and handling
physical properties
resin bonding
corrosion and tarnish
biocompatibility
what is a constituent of many precious alloys?
palladium
when does palladium have a strong whitening effect?
most will appear silver unless gold content is >40% and palladium is <6%
which alloys are more prone to corrosion?
those of low noble metal content
(noble metals are metals that resist chemical action and do not corrode and are not readily attacked by acids)
what are the noble metals? (relevant to dent)
silver gold platinum palladium
can crowns cause lichenoid reactions?
cheap alloys may but currently unknown effects
amalgams commonly cause lichen planus (and lichen planus near crowns may be due to an amalgam core)
describe nickel-chromium alloys - strength with luting agents? stiffness? sensitivity?
- good bond strength can be achieved between certain Ni-Cr alloys and resin luting agents
- they are stiffer than most noble alloys and thus can be used in slightly thinner sections
- avoid inhalation of Ni-Cr casting fumes
- avoid if sensitive to nickel
why might beryllium be used in Ni-Cr alloys?
improve physical properties of the alloy eg hardener and structure refiner
what are Ni-Cr actually used for?
they are base metal alloys used to make indirect restorations
when is rigidity in thin section needed?
preventing flexion of long span bridges
describe titanium
biocompatible
requires high temp for casting
what material properties must the alloy for cast post and cores need and why?
high modulus of elasticity
high yield strength
to avoid the post bending or breaking
why should you avoid cheap alloys for cast post and cores?
they may corrode or cast poorly
what does proper heat treatment of gold alloys for posts and cores ensure?
it will ensure a stiff post which is less likely to bend
what are dental ceramics are what is their structure?
also termed porcelains
composite structure consisting of crystalline phases or phases within a glassy matrix
consist of oxides, largely metals and silicsa, essentially inert materals
what can ceramics (although inert) be attacked by?
APF gel
(acidulated phosphate fluoride) gel
what is the most common material used for PJCs?
aluminous porcelain
aluminous porcelain is highly aesthetic. should we use it to restore teeth?
prone to fracture so not reliable for restoring posterior teeth
how are porcelains strengthened these days?
rely on having a crystalline phase dispersed within a glassy matrix
what porcelain materials are suitable for posterior crowns?
in-ceram and procera allceram
what is the most common technique of making PJCs and veneers?
sintering
how can ceramics be classified?
based on fusion temperature and mode of manufacture
what should you consider when choosing a ceramic (7)?
strength of the ceramic
tooth reduction
marginal fit
aesthetics
abrasion of opposing tooth
supported by studies?
cost
what is an important consideration for strength of ceramics?
resin cements - esp with veneers
what can affect the effectiveness of resin bonding?
a poor bond will not enhance strength of the restoration
some ceramic cores are smooth internally and resistant to etching which prevents effective resin bonding unless specific silanisation techniques are used (eg In-Ceram)
what is a typical PJC reduction?
1mm shoulder and 1.5mm incisal reduction for anterior teeth
2mm occlusal reduction for posterior teeth
destructive!!
what does loss of cement lute under crowns have indications for?
secondary caries and periodontal disease
what affects aesthetics of crowns?
glazing (adjustments and wear might ruin this)
translucency
colour
which metal-ceramic allots have advantages when rigidity is needed?
high palladium low gold alloys have significant advantages over high gold alloys where rigidity is needed
what can stress concentrations within PJCs lead to and what can stop this?
stress concentrations within PJCs often lead to cracks propagating outwards from the sit surface of the restoration
a tough metal coping bonded to the ceramic will help stop cracks developing in this way
what is used to promote adhesion of alloy to ceramic?
oxides of gallium, indium and tin
what might too thick an oxide layer result in?
too thick an oxide layer can result in ceramic debonding or discolouration
what is a coping?
a thin metal, resin or ceramic cap covering a prepared tooth
pros and cons of cast copings
good aesthetics hard to achieve
good method of strengthening porcelain
can create a metal occlusal surface (unlike foil copings)
are foil copings strong enough for posterior restorations?
no
are feldspathic porcelains strong or weak?
feldspathic porcelains are weak in tension and strong in compression
rely on bonding to metal and coping design to disspitate tensile stresses
composites vs porcelains for crowns
composites induce less wear against opposing teeth than porcelains
research still unclear
what pre-operative considerations must be made before making a crown?
- expectations of the pt, eg aesthetics, tooth retention,
- can pt tolerate the restoration eg can pt lie flat for long, degree of mouth opening, maintenance of restorations (from neuromuscular or skeletal disease)
- operator skill
- justification for the damage to the tooth
- can risk of disease/long term damage be minimised?
- is plaque control good enough?
- has risk of future caries been addressed? (ESP ROOT CARIES)
- has risk of damage from occlusion been minimised
- are the foundations biologically and mechanically sound? periodontal tissues, endodontic state?
- is tooth strong enough to receive a crown?
- will preparation be retentive enough?
- is there enough space for the restoration?
which teeth are most likely to suffer from restorations?
posterior teeth - furcations and multiple roots difficulty to clean, inaccesible locations
how do we combat posterior teeth being an issue?
shortened dental arch - give priority to anterior and premolar teeth (meets aesthetic and mechanical function)
what factors determine success of a crown to do with the oral environment?
plaque control, caries risk, occlusion
what can marginal deficiencies lead to?
rapid caries progression and potential tooth loss
esp in patients with inadequate plaque control
what is caries risk related to?
plaque and diet (and the presence of an RPD for root caries)
what is the solution for managing root caries?
biological not mechanical eg
hygeine and diet
how do you eliminate occlusion problems from crowns?
know where the contacts are on the tooth you are treating. especially those involved in guiding the jaw movement in lateral and protrusive excursions.
- teeth involved in guidance
- is the tooth a deflective contact or interference to guidance
what things can result in gingival problems to do with crowns?
poor crown margins
inccorect emergency profiles
where should crown margins be placed?
SUPRAGINGIVALLY
what should you consider when placing a subgingival margin?
placed within the limits of the sulcus and the biological width is not encroached on
what is the biological width?
2.04mm of supracrestal connective tissue attachment and junctional epithelium
what happens if a restoration encroaches upon the biological width?
an inflammatory response occurs resulting in:
- attachment loss
- apical migration
- pocket formation
apart from periodontal issues, what other issues can placement of a subgingival restoration cause?
issues with accurate impression taking - causing poor marginal fit
what can poorly contoured temporaries cause?
problems with impressions because of poor gingival condition
what factors affect strength of a tooth for provision of a crown?
- endodontics weakens a tooth
- extent of existing restoration
what factors affect retention of preparation?
adequate crown height
if there is not enough space for a crown in OVD what can you do?
reduce height of opposing tooth (not recommended but maybe in toothwear cases)
make a local temporary increase in vertical dimension to promote axial orthodontic tooth movements (eg Dahl appliance or occlusal composite buildup) - can take several months
what should you do before placing your definitive crown when getting it back from the lab?
check it on the die
check it in pt mouth without cement
what common lab errors can occur affecting marginal fit of the crown?
tight proximal contacts
casting blebs on fit surface
overextended crown margins
underextended crown margins
damaged dies
no die spacer
what are the 3 types of hard cement?
conventional, resin, hybrid of the 2
examples of conventional cements and what do they rely on?
zinc phosphate
zinc polycarboxylate
glass ionomer
rely on acid-base reaction resulting in the formation of an insoluble salt (cement) and water
what do resin based cements set by?
polymerisation
what mechanisms are there by which cements secure restoration to prepared tooth?
non-adhesive luting
micro-mechanical bonding
molecular adhesion
what mechanisms correlate to what cements?
non adhesive luting and micromechanical retention are main actions for conventional cements
molecular adhesion for resin and hybrid cements
what are the advantages of zinc phosphate cement?
long track record
good compressive strength
good film thickness
reasonable working time
resistant to water dissolution
(initially acidic) but no adverse effect on pulp
disadvantages of zinc phosphate cement
low tensile strength
no chemical bonding
not resistant to acid dissolution
when would you recommend a zinc phosphate cement?
good for conventional crowns and posts with retentive preparations
action of zinc phosphate cement
micromechanical interlocking between surface irregularities of the crown and tooth
are ZOE cements hard or soft?
soft - not for definitive cementation of restorations
what are soft cements used for?
provisional cementation of definitive restorations when a trial assessment period is needed
eg if occlusion or aesthetics are being majorly altered
advantages of polycarboxylate cements
reasonable track record
good compressive strength
adequate working time
bonds to enamel and dentine
adequate resistance to water dissolution (not as good as ZP)
no adverse pulp effects, less acidic than ZP on mixing
disadvantages of polycarboxylate cements
low tensile strength
can deform under loading
can be difficult to obtain low film thickness
not resistant to acid dissolution
recommendations for polycarboxylate cements
traditionally for vital or sensitive teeth
occasionally useful to retain an unretentive provisional crown
action of zinc polycarboxylate cements
luting cement but can also bond to tooth tissue (and stainless steel!)
advantages of glass ionomer cements
same as polycarboxylate cement but similar acidity to zinc phosphate on mixing
good compressive strength (higher than ZP?)
fluoride release
disadvantages of glass ionomer cements
sensitive to early moisture contamination
low tensile strength (higher than ZP)
not resistant to acid dissolution
has been accused of causing post-op sensitivity (but no worse than ZP)
recommendations for GIC
conventional crowns where pt has had previously high caries rate
alternative default cement to zinc phosphate
action of GIC
forms considerable bond to tooth tissue
advantages of RMGI and compomers
good compressive and tensile strengths
reasonable working time
rsistant to water dissolution
fluoride release
disadvantages of RMGI and compomers
short track record
may expand and crack overlying porcelain because of water absorption
recommendations for RMGI and compomers
worth trying for metal or metal-ceramic crowns esp where preparation retention is borderline
unclear which RMGI cements can be used safely with ceramic crowns
what are the advantages of resin cements?
good compressive and tensile strengths
high tensile strength (in comparison to conventional)
resistant to water dissolution
relatively resistant to acid dissolution
can enhance strength of ceramic restoration if bond obtained
disadvantages of resin cements
film thickness varies substantially between materials
excess material extruded at margin may be difficult to remove especially proximally
marginal leakage due to setting shrinkage?
recommendations for resin cements
porcelain veneers, ceramic onlays, resin-bonded ceramic crowns
must be used with effective dentine bonding agent
good for when rententive preps are not possible
what is richwil crown remover?
material not unlike a sticky sweet
how can you remove definitive crowns that have been temporarily bonded on?
richwil crown remover
finger pressure
matrix band
impact mallet
what does die spacing do?
achieves space for the cement lute
technique for cementing a crown
clean prep amd crown with water spray
airdry but do not dessicate prep
mix cement according to manufacturer instruction
coat fit surface with cement - do not overfill
only apply cement to the preparation if cementing a post
seat quickly with firm pressure until all excess cement pressed from margins
when should you remove the cement
conventional - excess cement left until after cement sets
resin-based - removal before setting
when should adequate moisture control be maintained?
until cement has set to prevent moisture contamination of the unset material at the crown margin
how can you prevent interproximal cement excess?
floss down and pass through interdental space
