crowns

Question 1

indications

Answer 1

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

Question 2

contraindications

Answer 2

active caries and PDD
more conservative options available
unable to provide post and core
lack of tooth tissue for prep
unfavourable occlusion

Question 3

principles of crown prep

Answer 3

preservation of tooth structure
retention and resistance
structural durability
marginal integrity
preservation of the periodontium
aesthetic considerations

Question 4

preservation of tooth structure

Answer 4

avoid unnecessary weakening and damage to pulp

balance against criteria for retention and resistance and structural durabillity

Question 5

under prep

Answer 5

poor aesthetics

over-built crown - periodontal and occlusal consequences

Question 6

over prep

Answer 6

compromised pulp and tooth strength

Question 7

retention

Answer 7

prevents removal of restoration along the PofI or the long axis of the tooth prep

Question 8

resistance

Answer 8

prevents dislodgement of the Rx by forces directed in an apical or oblique direction and prevents any movement of the Rx under occ forces

Question 9

features aiding retention and resistance

Answer 9

taper
length of walls
grooves/slots for extra retention
PofI

Question 10

taper

Answer 10

ideal inclination of opposing walls 6 degrees

Question 11

length of walls

Answer 11

longer walls interfere with tipping displacement

Question 12

design of grooves/slots

Answer 12

should be parallel to LA of prep

Question 13

PofI

Answer 13

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

Question 14

structural integrity

Answer 14

Rx must contain a bulk of material that is adequate to withstand occlusal forces

Question 15

how is structural integrity achieved?

Answer 15

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

Question 16

marginal integrity - options

Answer 16

knife edge
bevel
bevelled shoulder
chamfer
shoulder

Question 17

chamfer

Answer 17

veneers
palatal of MCCs
all metal

Question 18

shoulder

Answer 18

MCC - buccal aspect

Question 19

preservation of the periodontium

Answer 19

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

Question 20

aesthetic considerations

Answer 20

smile line

aesthetics or fct? - material

Question 21

biological width

Answer 21

top of alveolar bone to bottom of sulcus

don’t enroach on this as causes recession over time

Question 22

metal crowns - occlusal reduction

Answer 22

NFC - 0.5-1mm

FC - 1.5mm

Question 23

metal crowns - axial reduction

Answer 23

0.5-1mm

Question 24

metal crowns - finish

Answer 24

chamfer 0.5mm

Question 25

traditional porcelain crowns - occ reduction

Answer 25

NFC 1-1.5mm

FC 1.5-2mm

Question 26

traditional porcelain crowns - axial reduction

Answer 26

1-1.5mm

Question 27

traditional porcelain crowns - finish

Answer 27

shoulder to support ceramic

Question 28

MCCs - anteriors reduction

Answer 28

metal - chamfer 0.5mm
incisal reduction 1.5-2mm
MC - shoulder 1.3mm (1.5mm reduction)

Question 29

MCCs - posterior reduction

Answer 29

occlusal
 - NFC 1.3mm
 - FC 1.8mm
MC - shoulder 1.3mm (0.4mm metal) 1.5mm reduction
M - chamfer 0.5mm

Question 30

MCCs reduction requirements

Answer 30

rounded internal line angles

occlusal contacts 1.5mm or more away from porcelain metal jct

Question 31

what do you need in order for porcelain to bond to alloy?

Answer 31

mononuclear oxidative layer

Question 32

all ceramic crowns - bonded porcelain to Al/Z framework

Answer 32

axial reduction 1.5mm
occlusal reduction
 - FC 2mm
 - NFC 1.5mm
chamfer 1-1.5mm

Question 33

porcelain layers

Answer 33

opaque - masks dark oxide layer and provides PM bond
body/dentine - contains most of shade
incisal/enamel - most translucent

Question 34

stages in crown prep

Answer 34

occlusal reduction
separation
buccal reduction
palatal/lingual reduction
shoulder/chamfer finish
check occlusal surface and clearance
 - in ICP and in excursive movements

Question 35

occlusal reduction

Answer 35

reduce cusps and MRs
fissure bur diamond tapered
check cusp alignment and occlusal clearance

Question 36

separation

Answer 36

long diamond tapered bur

hold at LA

Question 37

buccal reduction

Answer 37

2 planes to reflect bulbosity of crown whilst still providing reduction
avoid buccal pulp horn
IP finishing line should follow gingival contour

Question 38

palatal/lingual reduction

Answer 38

premolars and molars - 1 plane
incisors and canines - follow contour
diamond chamfer

Question 39

crown failure

Answer 39

crown fracture
untreatable caries
root fracture at post level not attributable to trauma (stress relief)
traumatic fracture
furcation perf due to D pins