Dental pt4 Flashcards
polymer
chain molecule many units
olymer
short polymer 2,3,4 mers units
mer
simplest repeating chem units -> make up polymer
monomer
molecule forms polymer
Polymerization
mono forms poly thru primary covalent bond
Copoly
2/more diff mono joined
types: random, block
Molecular weight poly
sum MW mers made up
M= m+m+m
Degree polymerization
total mers/ poly
Spatial structure poly
linear
branched
cross linked
Effect heating on linear, branched molcule
weak, physical bonds
heating -> bonds break -> slide -> softened material -aka thermoplastic
Thermoplastic - heat -> softened ; cooling - hardened
Ex thermoplastic
polystyrene
polyvinyl acrylics
polymethyl methacrylate PMMA
Ex cross linked polymer
cis polyisoprene
cis bisphenol
cis silicones
cross linked PMMA
Charac cross linked polymer
strong covalent bond -> strong bonded unit
Effect heating on spatial molecule
NO chain sliding -> NO softening aka thermosets
Prop poly for strength, hardness, resistance
longer chain
higher molecular weight
many cross linked
-> poly increase strength, hardness, resistance creep along increase brittleness
Prop poly affected by
degree poly
no. cross linked chain
chem comp
amount crystallinity
plasticizer: add to stiff uncross -> reduce rigidity (small molecule surrounds big -> move easily) aka decrease Tg
Prop elastomeric impression
individual coiled chains
few cross links
Prop dental polymers
crystalline/ amorphous
Crystalline poly
atoms w/ reg arrangements
Charac crystalline
stronger
absorb < water
Charac amorphous poly
irreg arrangements
Glass transition temp def
temp range poly rigid glassy -> soft
Glass transition temp measured in terms of
stiffness
modulus
Temp < Tg
brittle
Temp > Tg
rubber
room temp»_space;> Tg
elastomer
(memorize as in room temp aka room temp high - high Tg - elastomer)
room temp «< Tg
rigid
(memorize as in room temp - room temp low aka low Tg -> rigid)
Effect plasticizer
added to stiff uncross linked polymer -> reduce rigidity aka decrease Tg
How plasticizer effects
small molecules surrounds large -> large move more easily -> decrease Tg
Prop after poly
poly shrinkage
water absorption
warpage: change shape when heated
(unmono poly)
what cause poly shrinkage
mono in poly arranged at covalent bond distance aka < og distance unpoly mono
effect water absorption
expansion
HEMA hydrophil mono > tendency cause water absorption
effect unpoly mono
toxicity
Types poly
add. poly - free radical add. poly
condensation
Charac, ex, use add. poly
NO by product formed
ex:
PMMA - use: denture
bis GMA - use: matrix resin of composite
Charac condensation
form low MW by product (H2O/ OH)
ex:
polysufide
silicon rubber impression material
Free radical add. poly accelerated by
heat
light
small amount peroxides
Steps free radical add. poly
initiation:
produce free radical
forms activated mono
propagation:
radicals (activated mono) attacks = bond -> add. mono to free radical -> chain grows
termination:
inhibitors (hydroquinone) -> increase storage life -> reduce initiation -> forms branches, cross links
Initiaion step in free radial add. poly
produce free radical
initiator:
1. Thermal initiator
have 1 weak bond -> breaks at high rate - moderate temp -> free radicals (peroxides O-O / azo N=N)
2. Photo dissociation
UV light -> breaks bond -> radical
3. Redox rxn
transfer e- -> radical
Poly in dentistry
denture base poly
Def denture base
material which teeth of denture are set -> rests on supporting tissue when denture in mouth
Component denture base poly
simple stiff base w/ teeth
stiff base + resilent liner
ad of resilent liner + stiff base
greater retention, comfort
When to use viscoelastic gel aka tissue conditioner
tissue under loose denture traumatized b/c constant motion hard plastic over mucosa
Classification denture, soft liners, tissue conditioners
- Denture based material
- heat cured (conventional, high impact)
- autopoly PMMA
- injection molded - Soft liners
- acrylic
- silicon (room temp vulcanizing RTV, heat cured - Tissue conditioners
- plasticized acrylics
Heat cured denture base material component powder, liquid: initiator, chem accelerator, filler, plasticizer, opacifier, inhibitor
(starting -> faster -> more -> flexibility -> opaque, stoping)
powder:
-poly beads PMMA polymethyl metacrylate (poly)
-benzoyl peroxide (initiator)
-inorg glass beads/ beads coupling agent Triydroxy -Silane
-dibutyl pthalate (plasticizer)
-Zn
-Mercuric Cadmium Sulfate (pigments) - colored nylon fibers -> stimulate blood vessels
(powder in heat -> NO run -> NO accelerator -> No one stop it -> NO inhibitor)
(heat -> started by using benzoyl peroxide -> see Lane? -> Silane -> Zack makes it opaque (Zn - opacifier) -
liquid:
-mono beads MMA methyl metacrylate
-dibutyl pthalate (plasticizer)
-3rd amine/ sulfinic acid
-hydroquinone
-cross linking agent w/ reactive CR=CR group opposite ends molecules (reduce denture base solubility to org solvents ; reduce craze)
(plasticizer + cross linking agent and things powder dont have -> plasticizer + chem accelerator + inhibitor)
Powder liquid rxn poly
powder liquid -> dough -> MMA (mono) into PMMA (poly) -> swell -> entanglements btw poly -> bead-monomer mixture cohesive gel -> benzol peroxide diffuse out bead -> interstitial space -> benzoyl peroxide initiates dough curing
Cond poly to convert to solid
min length 5000 beads
MW 150k
Effect of improper hardening heat cured
weakens material
distortion -> trauma to soft tissue
swelling
poor adhesion denture
discoloration
Autopoly/ pour type PMMA denture based materials charac
similar to heat cured denture
but have reducing agent (3rd aromatic amine)
(auto turn on gas but need agent -> auto same as heat but need RD agent)
Effect reducing agent on autopoly
mono + reducing agent (3rd aromatic amine) + benzoyl peroxide at room temp -> peroxy free radicals -> initiate free radical poly
Cond of autopoly
size, MW, plasticizer content balanced -> high penetration mono into bead w/o too early increase viscosity -> allow pouring acrylic into mold, good wetting plastic teeth
Ad, disad Injection denture based materials
Ad:
constant MW
replacement metal denture for patient sensitized methacrylate/ Ni/ Co (ỊNection -> meth -> methacrylate ; in -> ni -> Ni)
high flexibility
Disad:
high cost
low craze (heat) resistance - b/c NO crosslinks
low teeth adhesion
(injection -> expensive -> no need heat -> low craxe resistance -> if injection too much -> no teeth aka low teeth adhesion)
Types injection denture based material
low MW linear PMMA
Polycarbonate
Nylon (Sunflex) and Polyamid
Light activated material components
UDMA urethane dimethtacrylate
Silica filler
(U LIGHT up the world SILLY-> light - UDMA - sillica)
Steps light activated material components
form base plates -> light chamber 400-500nm
Ad, disad light activated material components
Ad: quick
Disad: expensive
def, func soft liners
resilient (elasticity) poly
replace fitting surface hard plastic denture
improve retention denture
Types soft liners
permanent soft liners (acrylic, silicon)
temporary soft liners/ tissue conditioners
Permanent soft liners components
acrylic
silicone
(acrylic nails, silicon boobs are permanent)
Charac acrylic in permanent soft liners
highly plasticized aka Tg < oral temp
-> soft liner elastic in mouth
high dibutyl pthalate -> high HEMA made -> H2O absorbs -> swelling -> deform
(acrylic is plastic
-> high plasticizer -> high dibutyl pthalate -> high HEMA -> swelling -> deform
-> highly plasticized aka soft liner on mouth oral temp)
Types silicon soft liner
room temp vulcanizing RTV
heat cured
Disad silicon soft liner
poor tear resistance
NO adhesion acrylic denture base
high effect osmotic pressure
(silicon boobs -> NO tear -> NO adhesion acrylic nails -> flying planes -> osmotic pressure effect)
Temporary soft liners ex
PEMA - primarily acrylic gel
PMMA - polyethylmetaacrylate
Basic info alloy
elements alloyed tgt -> change prop -> single melting temp change to range temp - eq btw solid crystal nucleated in liquid metal
Liquidus temp
upper temp for liquid solid alloy range
Liquidous temp def
temp solid crystal dissolve during heating
temp solid crystal nucleate during cooling
Solidus temp def
temp point last liquid solidifies on cooling
last solid melts on heating
Formation alloy
cooling - liquid metal nucleate -> atoms joining crystal from packing arrangement in space characteristic of metal/ alloy at eq
unit cell
smallest division crystalline metal
repeated unit cell -> crystalline structure of crystalline solid
atom at each corner shared among adj 8 unit cells
Formation nucleation, polycrystalline grain structure
cooled melted metal -> cluster atoms forms solid crystal nuclei -> stable -> grow into crytsallites/ grains if E favorable
aka
if increase interfacial SA -> E lost by bonding > gain
Process nucleation
Homogeneous nucleation
Heterogeneous nucleation
Homogeneous nucleation
rapid cooling nuclei
-> > E loss when liquids bond solid
-> > nuclei formed/ unit volume -> form < size polycrystalline grains -> irregular polycrystaline grains
relationship nuclei, grain size
more nuclei formed - smaller grain size
Heterogeneous nucleation
add foreign solid particle (fine high melting metal/ oxide powder) in melted metal -> atoms attracted -> decrease grain size
Ad decrease grain size
increase yield stress
increase ductility
increase strength
Alloy def
2 metals form solution in liquid state -> atoms mix randomly
Combining metal for alloys cond
same atomic lattice type
similar atomic radii
same valence number
form bond to other atoms with strength similar to those form among themselves
Bravais lattice
14 diff grid structure for symmetry
Types structure
Face centered cubic FCC
Body centered cubic
Hexagon closed packes
Face centered cubic FCC example
Au
Al
Cu
Ni
(Face -> Apply All Completely Neck -> Au, Al, Cu, Ni)
Body centered cubic BCC
Fe
Cr
Mo
Ta
W
(body -> Fe Cr Mo Ta W)
Hexagonal closed packed
Co
Ti
Mg
Cd
Zn
(Hex -> sounds bad -> mad -> Co Ti Mg Cd Zn)
Methods ID crystalline structure
X ray diffraction
Electron diffraction
Neutron diffraction
Single crystal
periodic arrangement atoms in total volume matter
made up from 1 single crystal
Ex single crystal
quartz
Polycrystal
same structure - diff orientation small single crystal
Ex polycrystal
alpha phase titanium
Crystal defects types
vacancy - misses lattice
interstitial - particle btw rows lattice
substitutional - substitution (ex: K+ for Na+ in KCl)
dislocation - NOT big enough for vacancy
Effect crystal defects
determine:
electricity
strength
malleable
ductility
Steel formation, effect
C in Fe
stronger
more brittle
Movement atoms in crystal defects
Vacancy: diffusion (atom move -> create new vacancy)
Interstitial:
1. fast diffusion small size atoms (ex: C,N,B)
2. solid dissolution small size atoms (ex: C in Fe - steel)
Substitution: solid dissolution
(ex: Bronze - Sn solid dissolution in Cu)(bronZE -> use Sn)
(ex: Brass - Zn solid dissolution in Cu)(braSS -> use Zn)
Types of linear defects, dislocations
edge
screw
Edge
extra half plane thru solid structure
Screw
1 side crystal displaces -> distortion
(screw up -> 1 side displaces)
Effect of dislocate planes
weakens metal
reversible called hardening
Effect of temp during melting
temp constant
Effect when freezing/ solidification
high E liquid -> low E solid + heat
What happens in super cooling phase
crystallization on pure metal
When does crystallization complete
crystals form -> release latent heat fusion -> temp rise to freezing point -> constant -> complete
Formation crystallization metals
nuclei -> crystallizes -> pure metal
pure metal dendrites shape (branch, elongated crystal)
Types of homogeneous
pure metal
metal compound
solid solution (substitutional, interstitial)
Types of heterogeneous
eutectic
eutectoid
(HEterogeneous -> eu -> eutectic, eutectoid)
Effect pure metal
miscible in liquid
(mix)
Solid solution (Formation alloy)
cooling mixed alloy -> atoms randomly distributed on unit cell lattice sites in each crystal grain
Charac phase diagram
NO calc eq composition, structure depends on temp
MUST ID experimental
Func phase diagram
structure alloy
microstructural predictions when alloy need heat treatment
Ex eutectid alloy
Ag-Cu
(tectid -> Ah Cu -> Ag Cu)
Charac eutectic system
binary alloy system
NO complete solubility in solid, liquid states
Effect:
-2 phases nucleate separate grains
-physically distinct
-diff mech prop
(eutectid - eu - 2 ->
binary alloy system
NO complete solubility -> 2 phases separate grain -> diff phys prop - diff mech prop)
Ex dental eutectic system
porcelain - multiphase alloy: diff composition -> weaker
Casting steps alloy
- prepare wax pattern
- embedding wax pattern (wax pattern in investment)
- prep mold (investment around wax pattern)
- burn wax pattern
- casting
- cooling, remove investment, sand blasting
- gross trimming
- electrogalvanic bath (remove sharp edges)
Def lost wax casting process
mold burns at high temp -> reveal wax pattern
Types alloy
non nobel
nobel
Non nobel alloy
cobalt chromium
nickel chromium
Ti based alloy
(non nobel -> kids -> TiNiCo)
Nobel alloy
gold containing alloy
palladium containing alloy
(nobel -> fancy -> gold, palladium)
Classification nobel alloy gold content alloy
High gold content alloy
1. soft
2. medium
3. hard
4. extra hard
Medium gold content alloy
Low gold content alloy
(decrease -> increase -> high med low (soft med hard extra hard)
Charac type 1 gold alloy
aka inlay gold
burnishable (burn - burnishable)
Use type 1 gold alloy
small inlays - NO great stress
class3,5 restorations
easily
Charac type 2 gold alloy
aka inlay gold
Use type 2 gold alloy
inlay - moderate stress
onlay abutments (core of bridge)
full crowns
pontics (bridge connections)
Use Type 3 gold alloy
inlay - high stress
fixed bridgework
denture bases
short span fixed partial denture
cast backing abutments(“bridge with metal root”)
pontics
full crown
Charac type 3 gold alloy
age hardened
Use Type 4 gold alloy
thin crowns - high stress
long span fixed partial denture
bars, clasps
fixed bridgework
Charac type 4 gold alloy
age hardened by heat
Use of medium, low gold content alloy
rarely for inlay
for full cast crowns
(med, low gold content alloy - cheaper than high -> can do a lot of it -> full cast crowns ; high gold content -> expansive - little work only -> inlay)
Relationship decrease Au to hardness, proportional limit, strength, ductility, corrosion resistance
increase hardness
increase proportional limit
increase strength
decrease ductility
decrease corrosion resistance
(working-> low Au -> have to work -> higher hardness, strength, temper (prop limit) - no time for yoga -> decrease ductility, corrosion resistance (antioxidants)
Types Palldium nobel alloy
Au-Pd-Ag
Au-Pd
Pd-Ag
Pd-Cu
Pd-Co
(Au, Pd ->
foundation: Au-Pd-Ag
1st 2: Au-Pd
2nd: Pd-Ag
3rd,4th, use Pd only -> Pd-Cu, Pd-Co)
Charac, disad AuPdAg
corosion resistance
disad: color change to yellow
(yellow foundation-> corrosion resistance ; color change to yellow)
Charac, dis PdAg
corrosion resistance
good porcelain bond strength
disad: porcelain color change to green b/d Ag diffusion to porcelain; gold agents reduce
(take last 2 of AuPdAg -> foundation -> 1. corrosion resistance, 2. porcelain bonding ; color change to green)
Charac, dis AuPd
corrosion resistance
rigidity for partial dentures
good porcelain metal bonds
disad: high expansion porcelains -> thermal expansion incompatibility
(take 1st 2 of AuPdAg -> foundation -> 1. corrosion resistance, 2. porcelain metal bonds)
(1st 2 -> partially more responsibility -> rigid
-> partially -> partial dentures
-> deal with heat -> thermal expansion incompatibility w/ high expansion porcelains)
Charac, dis PdCu
Cu cause probs in porcelain bonding but NOT in high Pd
high melting point compare to PdAg
use: large span fixed partial denture
(Cu
-> large span partial denture
-> better than PdAg -> higher melting point)
Charac, dis PdCo
high coeff thermal expansion
limited use
disad: forms dark oxide
(Co -> handles lots heat -> high coeff thermal expansion)
(Co -> old -> forms dark oxide)
Classification porcelain fused metal alloy
- Base
a. NiCrBe
b. NiCr
c. CoCr
d. Ti, Ti alloys - Noble
a. Pd - PdAg ; PdCu ; PdCo
b. Au - AuPtPd ; AuPd ; AuPdAg
(base -> aka simple aka non nobel -> TiNiCo -> Ti: TiTi alloy ; Ni: NiCrBe, NiCr ; Co: CoCr)
Ad porcelain fused metal alloy PFM
physical prop
chem prop
biocompatibility
workability
porc compatibility
Types PFM
noble
precious
semiprecious
nonprecious
Noble charac
resistance OXdation
NOT altered by acids
(noble -> rich -> access to antioxidant -> resistance OX, NOT altered by acid)
Precious metal charac
expensive
Precious alloy often used in dent
gold based alloy
silver palladium PdAg
Non precious alloy often used in dent
Ti based
NiCr
CoCr
(non precious - non nobel - TiNiCo)
Charac semiprecious
alloy contain silver, precious metal
Charac non precious
non precious ingredient except for common inclusion 1-3% Be
Physical prop choosing alloy contains
noble (corrosion resistance, inert)
hardness (occlusal wear resistance)
strength (load bearing)
elongation (imp in partial veneers crowns, abutment)
fusion temp
melting range
sag resistance (amount distortion at high temp)
thermal expansion (O residual stress)
bond strength
composition alloy (porc discoloration)
(budget -> comp alloy ; noble -> phys prop: hardness, strength -> bond strength -> elongation -> chem prop: fusion temp, melting range, thermal expansion, sag resistnce)
Alloys for removable denture
non nobel
Ti
NiCr
CoCr
CoCrNi
(removable -> unimportant -> non nobel -> have CoCrNi)
extra composition NiCr
Ni
Cr
Al
Be
Charac Ni
cause allergy
Charac NiCr
Ni3Al: intermetallic compound
increase strength
Charac Cr alloy
Cl attacks -> NO use for cleaning appliances made from Cr alloy
brittle
ZrO2 thin layer -> decrease Cr release level
(Cr -> Cl attacks -> ZrO2)
Charac Ni in CoCrNi
increase elongation
(Ni -> mimi -> fat, allergic to her-> elongation , allergy)
Charac Ti
diff manipulation
high casting temp
rapid OXdation
melting done best in Ar atmosphere
(Ti -> diff manipulation -> hot tempered aka high casting temp -> wrinkles -> rapid OXdation -> cooling down aka melting best in Ar atmosphere)
Alloys for fixed partial denture
non nobel
(normal TiNiCo)
Titanium
NiCr
CoCr
diff btw composition NiCr removable, fixed denture
removable: have Al, Be
fixed: Be
Charac NiCr fixed denture
etched electrochemically
better porc bonding than Be free NiCr
Charac Ti fixed denture
alternative to gold for inlay, onlay
non nobel, not precious
high biocompatibility
low cost
low density
corrosion resistance
room temp lattice: hexagon closed packs (alpha)
high temp lattice: body centered cubic (beta) - stronger, < ductile (high temp aka hot aka hot body -> body-beta -> stronger, less ductile)
(Ti -> non nobel -> low cost -> low density -> alt gold inlay onlay -> high biocompatibility -> high corrosion resistance)
Charac NiCr, CoCr fixed denture
sensitive to routine lab
cast NOT accurate comparing type3,4 gold, low gold alloys
Why non nobel for fixed dentur
high strength
high hardness
high stiffness
low cost
Effect elements released in mouth during corrosion
toxicity
allergy
carcinogenesis
Potent allergens
Ni
Co
Carcinogen element
Be
(be -> little -> carcinogen -> Be carcinogen)
Surgical alloy
Cr
a. CoCr
a. NiCoCr
Classification physical prop
- Mech prop
a. Strength (tensile, compressive, shear)
b. Hardness (Birnell, Vickers, Knoop, Rockwell, Shore A)
c. Elastic (elastic modulus, resilience)
d. Plasticity (ductility, percent elongation, yield strength) - Electrical, electrochem prop
a. Electrod potential
b. Electrical resistivity - Thermal prop
a. Thermal expansion
b. Heat flow (Thermal conductivity, diffusivity)
Vicks harness
diamond pyramid
square indent
are square indent depends on force, resistance material, angle
Func Vicks hardness
resistance of material
Shore hardness
A: truncated cone shape
B: cone shape
Func shore hardness
ID hardness rubber, polymer
Birnell
stainless steel ball
Knoop
diamond pyramid
1 diagonal longer
(K -> 1 long line, 1 short -> pyramid w/ diagonal longer)
formula compressive stress
stress= F/A
3 point tensile strength/ Flexural strength - measure compression force
specimen into machine 2 support points
measured during applied compression force vertically
meausure tensile strength
dog bone shape
measure tensil
Diametral tensile strength
cylindrical specimen
diametral position
cross sectional tensile force vertically to applied compression
ID poly shrinkage based on
Archimedes principle
volume changes btw poly, unpoly based on density determination
Fracture toughness Kic def
E to break surface intergrity
regarding fragile material (Ceramic)
Measure fractur toughness by
3 point flexural w/ rectangular specimen -> applied force -> cracking -> Kic calc -> ID applied fprce, para specimen, notch
Shear bond strength measure
sample attach to base surface
compression force vertical to adhesive layer surface
Use micro tensile bond strength
composite
dentin bod strength measurement
micro tensile bond strength
cavity prep -> filling material placed -> tooth filling sliced in microtome 2 directions -> test tensile
What does viscosity measures
resistance- gradual deform
by shear, tensile stress
Newton viscosity cond
parallel
steady flow
Newtonian fluid
shear stress proportional to shear rate
viscosity constant, independent shear rate
Pseudoplastic
flow index < unity
-> increase shear rate
-> non proportional increase shear stress
-> decrease viscosity (thining)
Dilatant
flow index > unity
-> increase shear rate
-> proportional increase shear stress
-> increase viscosity (thickening)
Thixotropy
time dependent shear thining (pseudoplasticity)
thick (viscous) gel, fluid -> shaken -> thin (less viscous) -> fixed time -> return viscous
Ex thixotropy
polyether impression material
Aim casting
produce metallic respiration
Cond to correspont investment material at application casting
investment material compensate thermal volume changes of wax pattern, used alloy
elasticity lowest - highest ceramic metal polymer
polymer -> metal -> ceramic
(elasticity PMC)
Length plastic deform lowest - highest ceramic metal polymer
ceramic - metal - polymer
Charac cross linked polymer
strong covalent bond -> strong bonded unit
Free radical add. poly accelerated by
heat
light
small amount peroxides
use of soft, med, hard, very hard Au content precious alloy
soft: 80-90% small inlay
med: 71-79% inlays
hard: 62-70% fixed prosthetics (if Cu min 11%)
very hard: 50-61% long span bridges, partial dentures (if Cu min 11%)
Ad precious alloy in dent
biocompatibility
good casting into molds
ductile (deform NO cracking)
easy polish
Formation/ Hardening process precious alloy
homogenization 700celcius
harden 400celcius 30min
cool down
Homogen
entire specimen in 1 phase
same physical, chem prop
Heterogen
2/more phase in specime
Why make alloy
better material prop
better financially
better processing in lab
Intermetallic compound
material all comp are metals
Charac intermetallic compound
brittle
break into pieces > alloy
Special types precious
-white gold
-very low gold content alloys divided into med, low
+med of very low gold content alloy: 40-60% gold - rest: metals (Ag, Pd)
+low of very low gold content alloy: 15-20%
Use, charac of med of very low gold content special precious alloy
subs hard, very hard
low ductility
use:
retainers (orthodontics)
partial denture
(almost last on list -> sometimes bad sometimes good
-> flexible -> ductility
-> at teenage age/ old -> need retainers/ partial denture -> use: retainers, partial denture)
Charac of low of very low gold content special precious alloy
worst mech prop
NO homogeneous
high melting point
subs hard
(last on the list -> worst b/c unlike others, hot tempered
-> worst mech prop
-> NO homo
-> high melting point)
White gold def, charac
Ag+Pd - NO gold
high strength
high melting point
expensive
use: inlays, crowns, bridges
More ex non precious
steel
brass
CoCr - 55-65%Co ; <30% Cr
Charac, use CoCr (same for NiCr)
easy to cast
high melting point
hard, strong
acid resistant
low density
use: bridges, crowns
(Co - not heavy, resistance to acid
-> low density, acid resistant
-> can carry over bridges -> become crown -> use: bridges, crowns)
