This document provides guidelines for tooth preparation for anterior metal ceramic crowns. It discusses the key decisions of finish line form and margin design. It recommends a shoulder finish line with a rounded axiogingival line angle and describes facial, incisal, proximal, and lingual reduction steps. Facial reduction of 0.8-1mm is recommended when possible, with incisal reduction of 2mm and lingual reduction of 1mm for porcelain surfaces. Uniform axial reduction with 10-20 degrees of convergence is advised. Rounded line angles are also recommended. The goal is to provide adequate space for an esthetic thickness of porcelain while preserving tooth structure.

1. Tooth preparation guidelines for
anterior metal ceramic crowns
Charles J. Goodacre, DDS, MSD
Professor of Restorative Dentistry
Loma Linda University School of Dentistry
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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

9. Finish Line Form Can Affect The
Potential For Gingival Trauma

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)

15. Key Tooth Preparation
Form Decisions
• Finish line form (3 types have
been used: chamfer; shoulder;
and shoulder-bevel)
• Margin design

16. Margin Design
• Metal collar – ease of fabrication
for non-esthetic situations

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

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

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

28. At Cementation
4 Weeks Postcementation

29. 2013 – 4 year PO
2009 - Pretreatment

35. Pretreatment Cementation - December 2011
2 month PO February 2012
9 month PO August 2012 17 month PO May 2013

36. TOOTH PREPARATION
STEPS FOR ANTERIOR
METAL CERAMIC
CROWNS & FIXED
PARTIAL DENTURES

37. AXIAL REDUCTION
Facial
Proximal
Lingual

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

41. The proximal surfaces of anterior teeth need
adequate reduction to allow reproduction of normal
translucency in the ceramic crowns

42. Facial Surface Reduction

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

45. 0.8 mm 1.0 mm

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

49. Maxillary Tooth Thickness
(At the cervical line) (Patient age range: 28-37)
• Central Incisor 2.3 2.2
• Lateral Incisor 2.1 1.8
• Canine 2.8 2.1
Stambaugh, J Prosthet Dent 1977;37:537-546
Tooth Facial Mesial

50. REDUCTIONS THAT MEET OR EXCEED 1
mm CANNOT BE PRODUCED ON YOUNG
TEETH WHERE THE PULPS ARE
RELATIVELY LARGE

51. Gingival recession and the position of abutments relative to
each other can limit the depth and location of facial finish lines

52. Data Regarding Finish Line
Depth

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

60. 1 mm diameter
instrument

61. The reduced
facial surface
should follow the
curvature of the
facial surface on
adjacent
unprepared teeth

62. AXIAL REDUCTION
Proximal Surfaces

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.

64. AXIAL REDUCTION
Lingual Surface

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

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

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

76. Proximal
transition from
shoulder to
chamfer

77. INCISAL REDUCTION

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

79. INCISAL REDUCTION

80. • The available incisal thickness
of enamel and dentin safely
supports reductions of 2.0 mm,
even on young teeth
INCISAL REDUCTION

81. Incisal Tooth Thickness
(pulp to incisal edge)
• 10-19 4.7-4.8 3.9-4.3 4.4
• 20-40 4.8-5.5 4.8-5.2 4.6-4.8
• 40-60 6.2-6.3 4.8-6.0 4.8-5.4
Oshshi, 1969, from Rosenstiel,
Contemporary Fixed Prosthodontics
Age CI LI Ca

82. LINGUAL REDUCTION FOR
OCCLUSAL CLEARANCE

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

84. 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

86. LINGUAL REDUCTION
FOR OCCLUSAL CLEARANCE

87. ROUNDING LINE ANGLES

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

89. 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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