2. The Morphology of metal ceramic single crowns
& fixed partial dentures should closely simulate
atural 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
– Shoulder-bevel
• Margin design
4. Shoulder With A Beveled Finish Line
• Some people think it makes for a better fitting crown
• It is certainly acceptable to use such a finish line but it
does not make the crown fit better and it does not
work with a porcelain margin
5. Shoulder Finish Lines
• Shoulder finish lines provide greater cervical space for an
esthetic thickness of porcelain without overcontouring the
crown and can be used with porcelain margins
Shoulder with sharp
axiogingival line angle
Shoulder with rounded
axiogingival line angle
Chamfer
6. Why Use A
Shoulder Finish Line?
• Need for optimal porcelain thickness near
the margin for esthetics and particularly
when optimal reduction is not possible
10. Why Use A Shoulder With
A Rounded Axiogingival Line Angle?
• Easier to prepare
than
a sharp angle
• Required for any
crown
that will be milled
11. Why Use A Shoulder With
A Rounded Axiogingival Line Angle?
• Easier to prepare than sharp angle
• Required for any crown that will be milled
• Produces less tooth weakening than sharp
axiogingival line angles for small teeth or RCT,
teeth that will be subjected to greater force since
they are FPD abutments, and teeth with
substantial secondary dentin where there is very
little pulp space remaining (older patients)
12.
13.
14.
15. Key Tooth Preparation
Form Decisions
• Finish line form (3 types have
been used: chamfer; shoulder;
and shoulder-bevel)
• Margin design
18. Margin Design
• Metal collar – ease of fabrication
• Metal thinned to a fine line that is barely visible –
improved esthetics compared to metal collar and
greater strength than a ceramic margins
19.
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
21.
22.
23.
24. Why Use A Shoulder
Finish Line And Ceramic Margin?
• Enhanced cervical esthetics
• When there is gingival inflammation due
to overcontoured crowns and you want
to make slightly undercontoured crowns
to improve the gingival response
38. Axial Reduction
• The axial surfaces need to be
sufficiently reduced to provide
an esthetic thickness of
porcelain on all visible surfaces
• The lingual surface and some
proximal surfaces may not
require an ideal thickness of
porcelain because they are not
visible
39. It is common to remove
crowns and find inadequate
finish line depth
40.
41. The proximal surfaces of anterior teeth need
adequate reduction to allow reproduction of normal
translucency in the ceramic crowns
43. Facial Surface Reduction
• Using a coarse grit diamond instrument, prepare facial depth
grooves that follow the incisocervical 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.
44. • When teeth are prepared with
finish lines that actually
approach or reach 1.5 mm, the
teeth are usually structurally
compromised
46. • The combined thickness of enamel
and dentin on young teeth does
not support facial reductions that
approach 1.5 millimeters without
encroaching on the pulp
47. Facial Tooth Thickness (mm)
(pulp to external surface)
• 10-19 1.8 2.0-2.1 2.7
• 20-40 1.9-2.4 2.1-2.3 2.9-3.1
• 40-60 2.0-2.8 2.1-2.3 2.8-2.9
Oshshi, 1968, from Rosenstiel,
Contemporary Fixed Prosthodontics
Age Central Lateral Canine
48. • Available tooth structure
thickness at the cervical line
is limited and particularly on
the proximal surfaces
53. 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
54. MC Facial Shoulder Depths
• Incisors – 0.8 mm mean depth
• Canines – 0.9 mm mean depth
Poon, Quintessence Int 2001;32:603-610
55. MC Finish Line Depths
• 0.86 mm mean facial shoulder
• 0.7 mm mean chamfer depths on
mesial, distal, and lingual surfaces
• 0.45 mm mean facial shoulder on
mandibular incisors
Al-Omari, 2004
56. FL Dimension Before & After Preparation:
1 mm Coronal to Finish Line
• 250 teeth from 42 patients
• Maxillary anterior – 0.85 mm mean
• Mandibular anterior – 0.71 mm
Ehrlich, J Prosthet Dent 1981;46:153-156
57. Reduction depths in
excess of 1.0 mm can be
only achieved on mature
teeth where considerable
secondary dentin formation
has occurred and the pulp
dimensions have been
reduced
58. • 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
59. • 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 or exceeds that dimension on
mature teeth
63. • Following the depth of the grooves, uniformly reduce the facial
surface.
• Also, uniformly reduce both proximal surfaces while
establishing a finish line of the desired depth.
• A total occlusal convergence angle of 10 – 20 degrees is
established between the mesial and distal surfaces.
65. • When restoring the cervical aspect of the lingual surface with
metal, reduce the lingual surface (cervical to the cingulum)
and form a 0.3 – 0.5 mm deep chamfer finish line
Lingual reduction cervical
to the cingulum
66. A chamfer finish line was used on the lingual
surface where a metal margin was used
67.
68.
69. Aligning Tapered Round
End Diamond Instrument
With Facial Surface So
Lingual Can Be Reduced
A Lingual Chamfer
Finish Line Is Formed
That Is 0.3 – 0.5 mm
deep
70.
71. • When restoring the cervical aspect of the lingual surface
with porcelain, reduce the lingual surface (cervical to the
cingulum) and form a 0.5-0.8 mm deep shoulder finish line
with a rounded axiogingival line angle
72. • The total occlusal convergence between the facial
and lingual surfaces and also between the mesial
and distal surfaces should be between 10 and 20°.
73. 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.
74. The mesial surface has
been modified to include
a proximal groove that
will increase resistance
form
75. • 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
78. • Depths grooves, 2.0 mm deep, are prepared into the incisal
aspect of anterior teeth and they are then used as guides to
uniformly reduce the incisal edge, following its mesiodistal form
INCISAL REDUCTION
83. • The lingual surface of anterior teeth
should be reduced at least 1.0 mm for
occlusal clearance with the opposing
teeth when the lingual surface will be
veneered with porcelain.
LINGUAL REDUCTION
FOR OCCLUSAL CLEARANCE
85. • When the lingual surface of anterior teeth will be
restored with metal, reductions can be less than
1.0 mm but should still be at least 0.5 mm
LINGUAL REDUCTION
FOR OCCLUSAL CLEARANCE
88. • Line angles should be 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
• With CAD/CAM milled crowns, the milling tools cannot
get into sharp acute angles where surfaces meet
ROUNDING LINE ANGLES
90. 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 0.5 – 0.8 mm for porcelain
• Uniform axial reduction that produces 10 to 20
degrees of total occlusal convergence
• Incisal reduction of 2.0 mm for anterior teeth
• 1.0 mm of occlusal clearance for anterior teeth and
2.0 mm for posterior teeth when those surfaces will be
veneered with porcelain. Less reduction can be
adequate when using metal to restore occlusal
contact
• Rounded line angles
91. 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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