crowns Flashcards
indications
protect weakened tooth structure improve/restore aesthetics restoration of RCT tooth use as retainer for fixed bridgework restore tooth fct indicated by design of an RPD
contraindications
active caries and PDD more conservative options available unable to provide post and core lack of tooth tissue for prep unfavourable occlusion
principles of crown prep
preservation of tooth structure retention and resistance structural durability marginal integrity preservation of the periodontium aesthetic considerations
preservation of tooth structure
avoid unnecessary weakening and damage to pulp
balance against criteria for retention and resistance and structural durabillity
under prep
poor aesthetics
over-built crown - periodontal and occlusal consequences
over prep
compromised pulp and tooth strength
retention
prevents removal of restoration along the PofI or the long axis of the tooth prep
resistance
prevents dislodgement of the Rx by forces directed in an apical or oblique direction and prevents any movement of the Rx under occ forces
features aiding retention and resistance
taper
length of walls
grooves/slots for extra retention
PofI
taper
ideal inclination of opposing walls 6 degrees
length of walls
longer walls interfere with tipping displacement
design of grooves/slots
should be parallel to LA of prep
PofI
imaginary line along which the Rx will be placed onto or removed from the preparation
is set before prep is begun and all features of the prep must coincide with that line
improve retention by limiting the number of PofIs
no UCs when viewed from side
structural integrity
Rx must contain a bulk of material that is adequate to withstand occlusal forces
how is structural integrity achieved?
occlusal reduction - maintain basic morphology
fct cusp bevel - prevents thin areas at line angles where perfs can occur
axial reduction
- axial profile constantly changing - don’t do in straight line - pulp, unaesthetic, weak points, adequate thickness
marginal integrity - options
knife edge bevel bevelled shoulder chamfer shoulder
chamfer
veneers
palatal of MCCs
all metal
shoulder
MCC - buccal aspect
preservation of the periodontium
margins of Rx should be:
- smooth and fully exposed to cleansing action
- placed where dentist can finish them and pt can clean them
- placed at gingival margin wherever possible
- supragingival easier to clean and maintain but poorer aesthetics
don’t enroach on biological width
aesthetic considerations
smile line
aesthetics or fct? - material
biological width
top of alveolar bone to bottom of sulcus
don’t enroach on this as causes recession over time
metal crowns - occlusal reduction
NFC - 0.5-1mm
FC - 1.5mm
metal crowns - axial reduction
0.5-1mm
metal crowns - finish
chamfer 0.5mm
traditional porcelain crowns - occ reduction
NFC 1-1.5mm
FC 1.5-2mm
traditional porcelain crowns - axial reduction
1-1.5mm
traditional porcelain crowns - finish
shoulder to support ceramic
MCCs - anteriors reduction
metal - chamfer 0.5mm
incisal reduction 1.5-2mm
MC - shoulder 1.3mm (1.5mm reduction)
MCCs - posterior reduction
occlusal - NFC 1.3mm - FC 1.8mm MC - shoulder 1.3mm (0.4mm metal) 1.5mm reduction M - chamfer 0.5mm
MCCs reduction requirements
rounded internal line angles
occlusal contacts 1.5mm or more away from porcelain metal jct
what do you need in order for porcelain to bond to alloy?
mononuclear oxidative layer
all ceramic crowns - bonded porcelain to Al/Z framework
axial reduction 1.5mm occlusal reduction - FC 2mm - NFC 1.5mm chamfer 1-1.5mm
porcelain layers
opaque - masks dark oxide layer and provides PM bond
body/dentine - contains most of shade
incisal/enamel - most translucent
stages in crown prep
occlusal reduction separation buccal reduction palatal/lingual reduction shoulder/chamfer finish check occlusal surface and clearance - in ICP and in excursive movements
occlusal reduction
reduce cusps and MRs
fissure bur diamond tapered
check cusp alignment and occlusal clearance
separation
long diamond tapered bur
hold at LA
buccal reduction
2 planes to reflect bulbosity of crown whilst still providing reduction
avoid buccal pulp horn
IP finishing line should follow gingival contour
palatal/lingual reduction
premolars and molars - 1 plane
incisors and canines - follow contour
diamond chamfer
crown failure
crown fracture untreatable caries root fracture at post level not attributable to trauma (stress relief) traumatic fracture furcation perf due to D pins
