2. The Morphology Of Posterior Metal
Ceramic Single Crowns & Fixed Partial
Dentures Should Closely Simulate
Natural Tooth Morphology
This is only possible with adequate tooth preparation
3. Key Tooth Preparation
Form Decisions
• Finish line form (3 types have been
used: chamfer; shoulder; and
shoulder-bevel)
• Margin design
• Total occlusal convergence
• Amounts and form of reduction
• Need for auxiliary resistance form
features
4. Metal Ceramic Crown
Finish Lines• CHAMFER
• Shoulder
• Shoulder-
bevel
It has a continuous cervical slope that
produces a decrease in reduction
depth as the finish line is approached.
It is formed
by using a
tapered
round-end
diamond
instrument
and using ½
of its tip
diameter
5. Chamfer Finish Lines
• Easy to form
• Conserves cervical tooth
structure
• Depth decreases as finish line is
approached and therefore
cervical color is not as good
• Well suited for crowns where a
cervical collar of metal will be
present
6. Metal Ceramic Crowns
• Chamfer
• SHOULDER –
Round axiogingival
line angle
Some people call this
type of finish line a
heavy chamfer.
Flat area
lateral to
rounded
line angle
7. Shoulder with rounded axiogingival
line angle was used because it is
relatively easy to form and provides
adequate space for an esthetic
thickness of cervical porcelain.
8. Metal Ceramic Crowns
• Chamfer
• SHOULDER –
Round Axiogingival Line Angle
Sharp Axiogingival Line Angle
Used where optimal
cervical finish line depth
is not possible to provide
a little more space for
porcelain color
Used on discolored teeth where porcelain margins are needed for esthetics but
the porcelain needs optimal thickness to mask the discoloration.
10. Metal Ceramic Crowns
• Chamfer
• Shoulder
• SHOULDER-BEVEL
A CONCEPT HAS EXISTED FOR OVER 75
YEARS THAT BEVELED FINISH LINES MAKE
CROWNS FIT BETTER
11. Early Results of Shoulder-
Bevel Crown Fit
• Metal distortion from porcelain fusion was
significantly less with a shoulder-bevel (6 µm) and
shoulder (10 µm) than chamfer (47 µm) and chamfer-
bevel (29 µm)
Shillingburg, 1973
• Metal distortion from porcelain fusion was
significantly less with a shoulder-bevel (13 – 16 µm)
and shoulder (15 – 19 µm) than a chamfer finish line
(21 – 35 µm)
Faucher, 1980
12. Subsequent Results Of
Shoulder- Bevel Crown Fit
• No significant difference in marginal
distortion from porcelain fusion (shoulder,
shoulder-bevel, chamfer, chamfer-bevel)
Hamaguchi, 1982
• Finish line form did not affect the fit of
metal ceramic crowns as a result of
porcelain fusion
Richter-Snapp, 1988
13. Subsequent Results Of Marginal Fit
• With uncemented crowns, there were
no significant differences between the
axial and marginal fit of crowns with
shoulder-bevel (32 µm), shoulder (22
µm), and chamfer (26 µm) finish lines
Syu, 1993
14. Marginal Fit after Cementation
• No significant differences in marginal fit
between metal shoulder-bevel, metal
shoulder, and porcelain shoulders either
before or after cementation.
Belser, 1985
• Finish line form does not significantly
affect the fit of cemented crowns.
Byrne, 1992
15. Metal Ceramic Finish Line
Selection Is Not Based On
Crown Fit. It Is Based On:
• Personal preference
• Formation ease: (chamfer is easiest;
shoulders are harder)
• Cervical crown esthetics: (shoulder
provides greater porcelain thickness)
• Margin design (metal or porcelain):
porcelain margins require a shoulder
16. Key Tooth Preparation
Form Decisions
• Finish line form
• Margin design
• Total occlusal convergence
• Amounts and form of reduction
• Need for auxiliary resistance
form features
17. Margin Design
• Metal collar – ease of fabrication for
non-visible or non-esthetic situations
18. Margin Design
• Metal collar – ease of fabrication for non-esthetic situations
• Metal thinned to a fine line that is barely visible – improved
esthetics compared to metal collar but more cervical opacity
than porcelain margin. Opacity has to be masked by surface
coloring.
19. In some locations, thinning the metal collar to a fine line
can work very well because there is no esthetic deficit.
20. Margin Design
• Metal collar – ease of fabrication for non-
esthetic situations.
• Metal thinned to a fine line that is barely
visible – improved esthetics compared to
metal collar but more opacity than porcelain
margin.
• Porcelain (collarless metal ceramic crown) –
most esthetic result but requires very skillful
technician.
25. Key Tooth Preparation
Form Decisions
• Finish line form
• Margin design
• Total occlusal convergence
• Amounts and form of reduction
• Need for auxiliary resistance
form features
26. Some Textbooks & Articles
Recommend 3-5°
• This recommendation is not realistic because it is
not an achievable number.
• Only occasionally is this minimal angle achieved.
27. What TOC Angles Are Typically
Achieved By Students?
• 13.5° on dentoform teeth in laboratory
• 25° during 1st clinical examination
• 22° during clinical examination at the
end of the senior year
Weed, 1984
28. Additional Clinical Data
Regarding Student TOC
• Ohm, 1978 190 dies 19-27°
• Mack, 1980 132 dies 17°
• Weed, 1984 73 dies 22-25°
• Noonan, 1991 909 dies 16-19°
• Annerstedt, 1996 127 dies 19°
• Sato, 1998 63 dies 19°
• Poon, 2001 151 dies 11-26°
• Al-Omari, 2004 157 dies 16-37°
29. • Eames, 1978 n/a 20°
• Leempoel, 1987 132 dies 16-30°
• Nordlander, 1988 175 dies 20°
• Kent, 1988 418 dies 14°
• Annerstedt, 1996 351 dies 22°
What TOC Angles Are Clinically
Achievable By Practitioners?
32. Key Tooth Preparation
Form Decisions
• Finish line form
• Margin design
• Total occlusal convergence
• Amounts and form of reduction
• Need for auxiliary resistance form
features
33. Finish Line Reduction Depth
• There have been recommendations in
textbooks and articles that finish lines should
be 1.2 to 1.5 mm deep.
• However, available tooth structure thickness
on most teeth in young to middle age patients
does not permit these reduction depths,
particularly when you consider TOC.
35. • When a 1.2 mm deep finish line is
combined with total occlusal
convergence angles of 15 to 20
degrees or more, the amount of
remaining tooth structure is minimal
on young teeth
36. Maxillary Premolar
Dentin Thickness & TOC
(20 adolescent first premolars)
• 1.2 mm deep finish line (F,L,M,D)
• 5, 10, 15, 20° TOC
• 1.2 mm + 20° TOC = 0.3 mm dentin
• 1.2 mm + 15° TOC = 0.5 mm dentin
• 1.2 mm + 10° TOC = 0.7 mm dentin
Doyle, Int J Prosthodont 1990;3:241-248
REDUCTIONS THAT MEET OR EXCEED 1 mm CANNOT BE PRODUCED ON
YOUNG TEETH WHERE THE PULPS ARE RELATIVELY LARGE
38. Mean Facial Shoulder Depth
• 24 extracted teeth prepared by 3 different
faculty members with at least 6 years of
experience.
• No shoulders were within the common.
recommendations of 1.2 – 1.5 mm depth
• Mean depth of 0.75 mm.
• Range from 0.47 to 1.1 mm.
Seymour, 1996
39. MC Facial Shoulder Depths
• Premolars – 0.7 mm mean depth
• Molars – 0.8 mm mean depth
Poon, Quintessence Int 2001;32:603-610
41. • Reduction depths in excess of 1.0 mm can
only be achieved on mature teeth where
considerable secondary dentin formation
has occurred and the pulp dimensions
have been reduced.
• When teeth are prepared with finish lines
that actually approach or reach 1.5 mm,
the teeth appear to have been
compromised.
43. Key Tooth Preparation
Form Decisions
• Finish line form
• Margin design
• Total occlusal convergence
• Amounts and form of reduction
• Need for auxiliary resistance form features
when TOC is > 25 degrees and / or prepared
tooth is short (↓ OC – less than 4 mm on
molars).
44.
45. • When there is a need for even greater
auxiliary resistance form, grooves can
be added to the tooth preparation
46.
47. Steps in the preparation of
posterior teeth for metal-ceramic
crowns:
Step 1: AXIAL REDUCTION
Facial, Proximal, & Lingual Surfaces
48. Facial Surface Reduction
• Using a coarse grit diamond instrument, prepare facial
depth grooves that follow the occlusoocervical curvature
of the facial surface.
• The depth of the grooves varies with patient age. It is
recommended that the grooves be 0.8 – 1.0 mm deep in
young patients and 1.0 – 1.2 mm deep in older patients.
Depths of 0.8 – 1.0 may not be possible on small teeth,
teeth with large pulps, and teeth where the finish line will
be located apical to the cervical line. Finish line depths of
1.2 – 1.5 mm produce excessive reduction of tooth
structure on most teeth.
49. • The use of depth grooves assures uniform
reduction that enhances the esthetic result by
providing uniform space for the ceramic
material that overlays the metal casting.
50. • Select a diamond instrument with a tip diameter
of about 0.8 to 1.0 mm, depending on tooth and
pulp size. Prepare facial grooves until their
depth matches the instrument diameter.
1 MILLIMETER
TIP DIAMETER
51. The depth groove form should follow the
occlusocervical curvature of the facial surface.
52. • Following the depth of the grooves, the facial
surface is reduced while establishing the
desired finish line.
• The proximal surfaces are also reduced.
• A total occlusal convergence angle of 10 – 20
degrees is established between the mesial and
distal surfaces.
53. • When restoring the lingual and proximal surfaces
with metal, reduce the lingual surface (cervical to
the cingulum) while forming a 0.3 – 0.5 mm deep
chamfer finish line.
54. • When restoring the lingual surface with porcelain,
reduce the lingual surface to a depth of 0.5 - 0.8 mm
• This reduced lingual surface should produce a total
occlusal convergence angle of 10 to 20 degrees
between the facial and lingual surfaces
TOC = 10-20°
55. Greater faciolingual convergence is sometimes needed so crowns & FPDs
can have compatible morphology with adjacent unprepared teeth that have
considerable occlusal convergence to their facial & lingual surfaces.
In these situations, the loss of
resistance form should be
compensated for by increasing
the occlusocervical dimension,
adding proximal grooves/boxes
or both.
56. • Creating a somewhat abrupt transition between the
deeper facial reduction and the proximal surfaces
enhances resistance form by functioning like one-half
of a groove when greater resistance is needed.
72. • Line angles should be rounded or beveled 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
• With CAD/CAM milled crowns, the milling tools
cannot get into sharp acute angles where
surfaces meet
73. Tooth Preparation Review
• Facial reduction of 0.8 – 1.0 mm whenever possible. Depths
may have to be less than 1.0 mm in younger patients. Depths
of 1.2 – 1.5 mm are not usually possible cervically
• A proximal and lingual chamfer of 0.3 – 0.5 mm for metal and
0.8 mm for porcelain
• Uniform axial reduction that produces 10 to 20 degrees of
total occlusal convergence
• At least 2.0 mm of occlusal reduction is required for posterior
teeth when those surfaces will be veneered with porcelain.
Less reduction (1.0 -1.5 mm) can be adequate when using
metal to restore occlusal contact
• Line angles should be rounded
74. Thank You For Your Kind Attention
Charles J. Goodacre, DDS, MSD
Professor of Restorative Dentistry
Loma Linda University School of Dentistry
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