Full metal & all ceramic restorations Flashcards
• The restoration can survive in the biologic environment of
the oral cavity only if
the margins are closely adapted to the
Cavo surface finish line of the preparation.
The configuration of the preparation finish line dictates the
shape and bulk of restorative material in the margin of the
restoration.
Marginal Integrity
• It also can affect both
marginal adaptation and the degree of
seating of the restoration
CERVICAL FINISH LINES
Marginal Geometry – Margin Types
(5)
•Knife Edge
•Chamfer
•Shoulder
• Radial (Modified) Shoulder
•Bevel (Beveled Shoulder)
Knife Edge Finish Line (Metal)
(5)
• Permits very acute margin of
metal
• Most conservative preparation type
• Flame or needle diamond
• Generally contraindicated
• Difficult to see; difficult to cast
Knife Edge Finish Line (Metal)
Other Disadvantage’s:
(2)
ØThe axial reduction may fade out
instead of terminating in a
definite finish line.
ØSusceptible to distortion in the
mouth when the casting is
subjected to occlusal forces.
Knife Edge Margin
• Difficult to wax / cast →
(2)
• Produces thin margin susceptible to
distortion (B)
• OR Leads to over-contoured surface (C)
• (Poor emergence profile)
Knife Edge Finish Line (Metal)
• Indications:
very convex axial
surfaces; lingual surface of
mandibular posteriors; tipped teeth
where the axial surface is at an
angle >15o
• It will produce a distinct finish
line
• Less destruction of tooth
structure
Chamfer Finish Line: Metal
• Margin Geometry – Narrow Chamfer
(3)
• (0.3mm) – 0.5mm axial depth
• Produces 30o – 45o acute metal margin / rounded internal angle
• Margin combines an acute edge (adaptability / seating) with a
nearby bulk of metal (strength).
Chamfer Finish Line: Metal
Round-end or torpedo diamond as axial wall is being produced
(1/3 – 1/2 diameter of rotary instrument).
• Axial preparation deeper than ½ diameter
results in
an acute, unsupported “lip” of tooth
structure at the margin.
Chamfer Finish Line: Ceramics
• Marginal Geometry
• Heavy Chamfer Metal-ceramic / Ceramic Crowns
- Adequate thickness for ceramic support
- Internal rounded angle ↓ stresses
- 1 – 1.2 mm axial depth
• Large round-end or chamfer diamond
Shoulder Finish Line
(2)
• 90o Cavo surface angle
• Flat-end diamond
Shoulder Finish Line
• Indications:
(2)
• Metal Ceramic Margins
• All-Ceramic Crowns
Modified (Radial) Shoulder
• Rounded internal axio-gingival
line angle
Shoulder Finish Line
• Cavosurface angle of 90o
(2)
• Supports adequate thickness of ceramic
• Rounded axiogingival line angle – decreased stresses
• Finish line most used for ceramic margins
Shoulder Finish Line
Shoulder Finish Line
Generally not used for cast metal
• WHY NOT?
Shoulder with Bevel
• Added to a shoulder, a bevel creates an acute edge of metal
(30o – 45o) at a margin.
- Inlay / Onlay
- Partial Veneer Crown
- Metal-Ceramic Crown (metal collar)
Chamfer is well suited for a complete cast crown:
(3)
• Ease of placement
• Ease of detection on impression; waxing, casting and adaptation to the tooth
• Acute in cross section while providing adequate bulk for strength
Marginal Integrity Problems:
Roughness
(2)
• Roughness of the tooth surface finish line can prevent
close adaptation of the margin.
• Rough margin → plaque retention
• Preparation:
high speed handpiece / air-water
spray with diamond burs
• Finishing:
Slow speed w/ diamond or carbide
burs; hand instruments
• Caries removal:
slow speed with a #4-6 round
carbide bur; spoon excavators
• Raising pulp temperature 6oC ↑ pulp death by —%.
15
• Increases of 10oC → –% incidence of pulp death
95
• Thermal Responses:
(3)
• Tooth preparation (↑5o-20oC)
• Light curing units (↑5o-20oC)
• Heat of polymerization: resins, provisional materials (↑5o-45oC)
• Thermal Responses:
(3)
• Tooth preparation (↑5o-20oC)
• Light curing units (↑5o-20oC)
• Heat of polymerization: resins, provisional materials (↑5o-45oC)
Effect of Temperature Increase on the Pulp
how to avoid (4)
§Use copious water spray to cool dentin / pulp
§Use sharp burs / diamonds / high speed, light pressure
§Do NOT marginate crowns “dry”
§Prepare teeth as quickly as feasible / avoid desiccation
Instrumentation: Diamond Stones
(4)
• Remove enamel and dentin by grinding
• Most efficient (2-3X faster than carbide burs)
• Basic shapes: round end, flat end, (taper or cylinder);
flame / needle, wheel, torpedo, football / egg
• Diamond cleaning stone (wet)
Tungsten carbide burs
(2)
• milling cutters that shear tooth structure from
surface
• cut precise preparation features and smooth
surfaces in enamel and dentin
Twist drills
(2)
• cutting edges on the tip and fluted sides
• bore small-diameter holes in dentin for pins
Preparation finish line smoothness is important to
marginal fit and longevity.
(Preparation surface roughness does not affect
retention.)
Instrumentation
• Hand instruments
smoothly finish
enamel finish lines
• Refine internal line angles
• Chisel
• Hoe
• Hatchet
• Margin trimmer
Advantages of Full Metal Crowns
(3)
1) Complete coverage affords greatest retention and
resistance form
2) Allows for the greatest change in esthetic tooth form
and occlusion
3) Makes it successful in a wide range of situations such as
tooth form or alignment is not ideal and therefore a less
than perfect tooth preparation will be a likely result
Disadvantages of Full Metal Crowns
(3)
1) Complete metal crowns are not esthetic to many people
and therefore they are often limited to non-visible areas
of the mouth
2) They require more extensive tooth preparation than
partial coverage crowns
3) If the finish line is subgingival around the entire tooth, it
is more likely there will be a less than optimal gingival
response
Chamfer finish lines are recommended for all – metal crowns because they are easy to form,
adequately distinct on the tooth and die, and
provides sufficient space that the metal is
adequately rigid. A depth of — is
recommended
0.3 – 0.5 mm
Chamfer finish lines exhibit a —.
They are formed by using a diamond
instrument with a rounded tip and a diameter of —
continuous cervical slope
0.6 – 0.8 mm
The tooth is reduced until — of the instrument
diameter is located within the tooth. The rotary
instrument is then moved around the tooth
perimeter while maintaining the desired depth
½
Axial Reduction
The axial surfaces (facial, lingual, mesial,
distal) are reduced using a
tapered round
– end diamond instrument so they
converge occlusally within the range of
10 to 20 degrees of total convergence
REDUCTION FOR OCCLUSAL CLEARANCE
• Occlusal depth grooves are prepared to a
uniform depth of — using a coarse
grit diamond. They should follow the pathways
and depths of the developmental grooves.
1.0 – 1.5 mm
REDUCTION FOR OCCLUSAL CLEARANCE
A depth groove prepared along the entire
— length of the central developmental
groove is very helpful in achieving adequate
reduction in an area that is commonly underreduced.
mesiodistal
Line angles are rounded to facilitate pouring impressions without
trapping
air bubbles and investing wax patterns without air inclusions.
Additionally, should nodules occur in the casting, they are easier to remove
when the
line angles have been rounded.
Rounding Lines Angles for Cast Metal Crowns*
(3)
• Facilitates pouring impressions without voids in
the dies
• Facilitates investing wax patterns without air
inclusions and resulting casting nodules
• When casting nodules are present, they are
easier to remove.
Final Finished Surface Of The
Preparation
• The prepared surfaces and finish line are
smoothed using a fine grit diamond that
is the same size as the coarse grit
diamond used for the initial reduction
• A coarse grit diamond that has been used
multiple times for initial tooth reduction
and sterilized multiple times can also be
used as it will no longer create the
roughness it once produced
Complete Metal Crown - Tooth
Preparation Review
•A chamfer finish line that is — mm in depth
•Axial reduction with — degrees of total
occlusal convergence
•Reduction for occlusal clearance of — mm
•Auxiliary axial resistance form features as needed
•Rounding of all line angles
0.3 – 0.5
10 to 20
1.0 – 1.5
Advantages of Glass-Ceramic Crowns
(3)
1) Esthetic potential / translucency
2) Good tissue response
3) Conservative tooth preparation (??)
i. Compared to MCC preparation (lingual / occlusal metal)
ii. Facial versus lingual
Disadvantages of Glass-Ceramic Crowns
(6)
1) Reduced strength
i. No metal substructure
ii. Brittle nature of material
2) Substantial tooth reduction required
3) Technique sensitive
i. Clinical tooth preparation
ii. Resin cementation
4) Opposing tooth wear
5) Greater fracture potential – posteriors ( Zirconia ?)
6) Fixed partial dental prostheses limitations (Zirconia ??)
Glass-Ceramic Crowns
Indications (4)
1) High esthetic requirement
2) Favorable distribution of
occlusal load
3) Optimal tooth preparation is
possible
i. Ceramic support
ii. Circumferential shoulder or
heavy chamfer
4) When a more conservative
restoration is not indicated
Glass-Ceramic Crowns
Contraindications (4)
1) Molar teeth (FC Zirconia)
i. Increased occlusal load
ii. Decreased esthetic demand
2) Unfavorable occlusion
i. Anterior: edge-edge or cervical 1/5
ii. Bruxism / clenching
3) Inadequate tooth length for
support
i. Ceramic thickness >2-3 mm
ii. Short clinical crowns
4) Inability to produce uniform
circumferential margin
Modified Shoulder or
Heavy Chamfer
(5)
marginal integrity
structural durability
periodontal preservation
resistance
esthetics
Axial Reduction
(4)
retention and
resistance
structural durability
esthetics
Vertical Lingual Wall
(1)
retention and resistance
Concave Cingulum Reduction
(1)
structural durability
Rounded Line Angles
(1)
structural durability
Design Specifications: All Ceramic Crown
• Incisal reduction
• Facial axial reduction
• Lingual axial reduction
• Marginal reduction
2.0 – 2.5 mm
1.2 – 1.7 mm
1.0 – 1.2 mm
1.0 – 1.2 mm
The Benefits of All-Ceramic Crowns
• Enhanced esthetics due to the absence of metal and improved light
transmission. Most beneficial for teeth with normally colored dentin
• With some dark teeth, can use — to mask dark dentin
zirconia core
Ceramic Susceptibility to Fracture
(2)
• Microcrack formation and propagation
- Stress concentrations
- Surface flaws
• No mechanism for yielding to stress w/o fracture (plastic deformation).
Biomechanical Factors: Overcoming
Deficiencies in Glass Ceramic Restorations
• Methods of strengthening brittle materials:
(2)
- Reinforced Glass Ceramics; Polycrystalline - Zirconia
- CAD-CAM
Biomechanical Factors: Overcoming
Deficiencies in Glass Ceramic Restorations
• Methods of design to optimize success:
(4)
- Management of occlusal forces
- Restoration design and material
- Cementation (resin bonding)
- Tooth preparation design
Particle-filled Glass-Ceramics
Lithium Disilicate (e.max)
Layered or Monolithic
• Indications:
(3)
• Anterior / Posterior crowns
• Anterior FDP
• Inlays / Onlays, Veneers
Particle-filled Glass-Ceramics
Lithium Disilicate (e.max)
Layered or Monolithic
• Strength:
360-500 Mpa
(Moderate)
Particle-filled Glass-Ceramics
Lithium Disilicate (e.max)
Layered or Monolithic
• — cementation
• Requires — occlusal thickness and — shoulder
Resin
1.5-2.0mm
1mm
Glass Ceramics and Opposing Occlusal Forces
Anterior teeth
(4)
• Lower occlusal forces / less strength requirement
• Higher esthetic requirements
• Unfavorable occlusion – edge-edge and cervical 1/5
• Must have adequate tooth support (long walls)
Zirco(YTZP): Core / Ceramic Veneer or Full
Contour Crown
Core thickness :
0.4mm minimum (layered with 1–2mm porcelain)
Full-contour zirconia with porcelain glaze (complete metal prep requirements)
Glass Ceramics and Opposing Occlusal Forces
Posterior teeth
(3)
• Higher occlusal forces → greater strength requirements
• Less esthetic requirement
• Increased failure on molars (zirconia or MCC or metal)
Glass Ceramics and Opposing Occlusal Forces
Bruxism / clenching
(2)
• Contraindication for glass-ceramic crowns / FDPs
• Suggest occlusal night guard with ACCs
50% increase in fracture strength ACC with adhesive
resin cement
Layers of Porcelain and High-Strength Core
Necessary reduction for thickness:
0.4mm zirconia core
+1.0 – 2.0 mm porcelain
≈1.2-1.4mm (axial) – 2.0-2.5mm (incisal) reduction
ACC Preparation Design Considerations
(6)
• Flat planes at right angles to forces
• Modified shoulder (or heavy chamfer) margin
• Avoid sharp line angles (no grooves)
• Avoid steep proximal angles in finish line
• Adequate length of preparation for support
• Uniform axial reduction and minimal taper
Sharp line angles promote fracture
(3)
• A sharp incisal edge may be reduced until a flat area is obtained.
• Incisal reduction should not exceed 3mm.
• Incisal edge ≥ 1mm wide (CAD-CAM)
Adequate Tooth Preparation Length for Support
Short preparations cause considerable stress and may lead to fracture.
• When a load is applied from a lingual direction, the facial shoulder is placed
under compression and only the incisal-lingual length of the preparation provides
significant resistance to this force
Effect of Taper on Strength
(2)
• Excessive porcelain bulk decreases strength.
• It is not the bulk of porcelain that gives strength to the crown, but the
resistance to flexure provided by support from the tooth preparation and
accuracy of fit.
Shoulder Provides Support at Margin
(3)
• Occlusal loading → tensile/shear stress → fracture
• 90o shoulder helps provide support for porcelain
- Heavy Chamfer adequate with resin bonded cementation
• Excessively sloped finish line or bevel creates thin unsupported margin of
porcelain – prone to fracture
All-Ceramic Margin Designs
(2)
• A rounded internal angle on the modified shoulder
decreases stress at the axiogingival line angle.
• A heavy chamfer is relatively simple to form.
- Supragingival (subgingival difficult to avoid “lip”)
- Enamel / resin bonding
Effect of Finish Line Form
(3)
• Avoid excessive cervical slope / bevel
• Avoid rapid occluso-cervical undulations
• Uniformity and smoothness improves marginal fit
TOOTH PREPARATIONS
CAD/CAM All-Ceramic Restorations
• Adequate occlusal reduction (2 mm) and lingual reduction (1 mm)
- Inciso-cervical height should not be greater than 10.5mm – length of milling
diamond)
• Well-defined, smooth finish lines
• Shoulder finish lines with rounded axio-gingival line angles
• Rounded line angles
- Incisal edge and internal angles ≥ 1mm wide
