Principles of tooth preparation

PRINCIPLES OF
TOOTH PREPARATION
By Dr.Sayli Patil
2nd year PG
Dept. Of Prosthodontics

Contents
 Introduction
 Defination
 Principles of Tooth Preparation
 Biologic considerations
 Conservation of Tooth structure
 Mechanical Considerations
 Esthetic considerations
 Planning and Evaluating Tooth Preparations
 Recent advances
 Conclusion
 References

Introduction
 Once enamel or dentin is lost as a result of caries,
trauma, or wear, restorative materials must be used to
reestablish form and function.
 Teeth require preparation to receive restorations, and
these preparations must be based on fundamental
principles from which basic criteria can be developed
to help predict the success of prosthodontic treatment.
Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics
3rd ed St. Louis: Elsevier. 2001:108-35.

Defination
 Tooth Preparation:
the process of removal of diseased and/or
healthy enamel and dentin and cementum to shape a
tooth to receive a restoration. GPT 9.
 According to Tylman:
as the mechanical treatment of dental disease
or injury to hard tissue that restores a tooth to the
original form
Glossary of Prosthodontic Terms, Ninth Edition, GPT‐9. The Academy of
Prosthodontics Foundation. J. Prosthet. Dent.. 2017;117(5S):e1-05.

 Principles of tooth preparation may be divided
into:
Biologic considerations, which affect the
health of the oral tissues
Mechanical considerations, which affect
the
integrity and durability of the restoration
Esthetic considerations, which affect the
appearance of the patient
Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed
prosthodontics 3rd ed St. Louis: Elsevier. 2001:108-35.

Optimum restoration should satisfy biologic, mechanical, and esthetic
requirements

Design and Preparation of Tooth are
governed by 5 principles:
• Preservation of Tooth structure
• Retention and Resistance form
• Structural durability of the Restoration
• Marginal Integrity
• Preservation of the Periodontium
Shillingburg HT, Sather DA, Wilson EL, Cain JR, Mitchell DL, Blanco LJ, Kessler JC.
Fundamentals of fixed prosthodontics. Quintessence Pub Co (4th edn), Chicago. 2013.

BIOLOGIC CONSIDERATIONS
 The adjacent teeth, soft tissues, and the pulp of the tooth
being prepared are easily damaged in tooth
preparation.
 If poor preparation leads to inadequate marginal fit or
deficient crown contour, plaque control around fixed
restorations will become more difficult.
 This will impede the long-term maintenance of dental
health

PREVENTION OF DAMAGE DURING
TOOTH PREPARATION:
Adjacent Teeth:
 A metal matrix band around the adjacent tooth for
protection may be helpful; however, the thin band can
still be perforated and the underlying enamel
damaged.
 The preferred method is to use the proximal enamel of
the tooth being prepared for protection of the adjacent
structures.
 Teeth are 1.5 to 2 mm wider at the contact area than
at the cementoenamel junction (CEJ), and a thin,
tapered diamond can be passed through the
interproximal contact area to leave a slight lip or fin of
enamel without causing excessive tooth reduction or
undesirable angulation of the rotary instrument

Soft Tissues :
 Damage to the soft tissues of the tongue and cheeks can
be prevented by careful retraction with an aspirator tip, a
mouth mirror or a flanged saliva ejector.
 Great care is needed to protect the tongue when the
lingual surfaces of mandibular molars are being
prepared.

Pulp :
 Extreme temperatures, chemical irritation, or microorganisms can
cause an irreversible pulpitis, particularly when they occur on
freshly sectioned dentinal tubules.
 Pulp size can be evaluated on a radiograph, and it decreases with
age.
 Up to about age 50, it decreases more so occlusocervically than
faciolingually.

Causes of Injury
Temperature :
 The spray must be accurately directed at the area of
contact between tooth and rotary instrument.
 It also washes away debris, which is important because
rotary instrument clogging reduces cutting efficiency.
 Irrigation also prevents desiccation of the dentin.
 To prevent heat buildup,retention features should be
prepared at low rotational speed or with a high-speed
handpiece with a feather-light touch
Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics 3rd ed St. Louis:
Elsevier. 2001:108-35.

Chemical Action
 The chemical action of certain dental materials
(bases, restorative resins, solvents, and luting
agents) can cause pulpal damage, particularly
when applied to freshly cut dentin.
 Dentin bonding agents form an effective
barrier, but their effect on the retention of
cemented restorations is controversial.

Bacterial Action
 Pulpal damage under restorations has been
attributed to bacteria that either were left behind or
gained access to the dentin because of
microleakage.
 Because vital dentin seems to resist infection,the
routine use of antimicrobial agents may not be
advantageous.
 Of importance is that all carious dentin must be
removed before placement of a restoration that will
serve as a foundation for a fixed prosthesis.

Conservation of Tooth
Structure
Tooth structure is conserved through adherence to the
following guidelines:
 Use of partial-coverage
rather than complete
Coverage restorations.
 Preparation of teeth with
the minimum practical
convergence angle
(taper) between axial
walls

 Preparation of teeth
with the minimum
practical convergence
angle (taper) between
axial walls.
 Preparation of the
occlusal surface so that
reduction follows the
anatomic planes and
produces uniform
thickness in the
restoration

 Preparation of the
occlusal surface so that
reduction follows the
anatomic planes and
produces uniform
thickness in the
restoration.
A shoulder margin (2) is indicated when
esthetic restorations are planned, but it
is much less conservative than a chamfe
margin (1).
An anatomically prepared occlusal surface
results in adequate clearance without
excessive tooth reduction. A flat occlusal
preparation will result in either insufficient
clearance (1) or an excessive amount of
reduction (2).
Selection of a margin
geometry that is conservative
and yet compatible with the
other principles of tooth
preparation

 Avoidance of
unnecessary apical
extension of the
preparation which would
result in loss of additional
tooth structure
Apical extension of the preparation can
necessitate additional tooth reduction
because coronal diameter becomes
B, Preparations for periodontally
involvedteeth may necessitate
considerable reduction if the margins are
to be placed subgingivally for esthetic
reasons.
C, Supragingival margins are preferred
where applicable

MECHANICAL
CONSIDERATIONS
 Mechanical considerations can be divided into three
categories:
• Providing retention form
• Providing resistance form
• Preventing deformation of the restoration

Retention & Resistance form
 the feature of a tooth preparation that resists
dislodgment of a crown in a vertical direction or
along the path of placement- GPT 9
 Certain forces (e.g., when the jaws are moved apart
after biting on very sticky food) act on a cemented
restoration in the same direction as the path of
placement.
 The quality of a preparation that prevents the
restoration from becoming dislodged by such forces
parallel to the path of placement is known as its
retention form.
Shillingburg HT, Sather DA, Wilson EL, Cain JR, Mitchell DL, Blanco LJ,
Kessler JC. Fundamentals of fixed prosthodontics. Quintessence Pub Co (4th

 Resistance form: the features of a tooth preparation
that enhance the stability of a restoration and resists
dislodgment along an axis other than the path of
placement - Gpt 9
 Resistance prevents dislodgment of the restoration
by forces directed in an apical or oblique direction
and prevents any movement of the restoration under
occlusal forces - Shillingburg

 There are 4 factors which are in operators
control during tooth preparation which
influence retention.
a) Degree of Taper
b) Total Surface area of cement film
c) Area of cement under shear
d) Roughness of the tooth surface.

 The ability of a cement bond to withstand force depends
largely on the direction of he force in relation o the
cemented surfaces.
 From this we would expect that more parallel the
opposing walls of a preparation the greater will be the
retention.
 Jorgensen, found that retention decreases as taper
increases.
Extracoronal restoration (a) uses opposing
external surfaces for retention (b).
Intracoronal restoration (a) uses
opposing internal surfaces for retention

Taper
 the axial walls of the
preparation must taper slightly
to permit the restoration to
seat;
 ie, in the occlusal direction, two
opposing external walls must
gradually converge or two
opposing internal surfaces of
tooth structure must diverge.
 A tapered diamond or bur will
impart an inclination of 2 to 3
degrees to any surface it cuts if
the shank of the instrument is
held parallel to the intended
path of insertion of the
preparation.
 Two opposing surfaces, each
with a 3-degree inclination,
would give the preparation a 6-
 External (top row) and internal
(bottom row) opposing
surfaces

 By calculating the average
height-to-base ratio of
preparations of incisors,
canines, premolars, and
molars, minimally acceptable
guidelines for preparation
tapers by tooth group were
determined.
 The values expressed as total
occlusal convergence were
found to be 58 for incisors, 66
for canines, 20 for premolars,
and 16 for molars.
 These values are based on
resistance form
recommended for minimally
acceptable average tapers
 Resistance form is easily
obtained on anterior teeth,
although it is more difficult to
obtain with posterior teeth
because they are shorter and
wider, making the height-to-
base ratio less.
 Shillingburg recommended
valuefor average ideal tapers
(total convergence angle)
increase from anterior (10) to
posterior (19 to 22)
Parker MH. Resistance form in tooth preparation. Dental
Clinics. 2004 Apr 1;48(2):387-96.

Methods to analyze resistance
form
 Lewis and Owen showed that for preparations
with straight walls, the dividing point between the
resistive and nonresistive sections of a
preparation wall is the point of intersection with
the perpendicular line from the center of rotation
on the opposing margin.
 Using an ingenious application of geometry,
Zuckerman used a boundary circle but centered
his circle at the center of the base of the
preparation with a radius one half of the base
radius used by Weed.

 Three techniques for evaluating
the resistance form of side AC.
The Lewis method of evaluating
resistance form of side AC is
applied by extending a
perpendicular line to side AC
from the center of rotation E at
the opposite margin.
 The intersection of the Lewis line
and side AC is point B, so all
points occlusal to B (section BC)
are resistive and all points
gingival to B (section AB) are not
resistive. The same results are
obtained with the Zuckerman
circle, where all points outside of
the circle (section BC) are
resistive and all points inside
(section AB) are nonresistive.
Parker MH. Resistance form in tooth preparation. Dental Clinics. 2004
Apr 1;48(2):387-96.

The most retentive preparation should be
one with parallel walls. However, parallel
walls are impossible to create in the mouth
without producing preparation undercuts
Preparation walls are tapered to allow their
visualization, prevent undercuts, compensate
for inaccuracies in the fabrication process, and
permit more nearly complete seating of
restorations during cementation.
A tapered diamond or bur will impart an inclination
of 2 to 3 degrees to any surface it cuts if the
shank of the instrument is held parallel to the
intended path of insertion of the preparation.
Two opposing surfaces, each with a 3-degree
inclination, would give the preparation a 6-degree
taper.

 Ward was one of the first to recommend tapering as
such, prescribing 5% to 20% per inch (3 to 12 degrees,
respectively).
 Jorgensen and Kaufman et al have demonstrated
experimentally that retention decreases as taper is
increased.
Recommendations for optimum axial wall taper of tooth
preparations for cast restorations have ranged from 3 to 5
degrees,6 to 6 degrees, 7 to 10 to 14 degrees.
To minimize stress in the cement interface between the preparation
and restoration, a taper of 2.5 to 6.5 degrees has been suggested
as optimum.
There is only a slight increase in stress as taper is increased from
0 to 15 degrees9; however, at 20 degrees, stress concentration
was found to increase sharply.

 Consciously
attempting to
create a taper can
easily result in an
overtapered and
nonretentive
preparation.
 It can be as low
as 10 degrees on
preparations on
anterior teeth and
as high as 22
degrees on
molars.

Freedom of displacement
 Retention is improved
by geometrically limiting
the numbers of paths
along which a
restoration can be
removed from the tooth
preparation.
 Maximum retention is
achieved when there is
only one path.
 A full veneer
preparation with long,
parallel axial walls and
grooves would produce
such retention
By limiting the paths of
withdrawal, retention is
improved. (b) A preparation
with unlimited freedom of
displacement is much less
retentive

(a) The walls of a groove that
meet the axial wall at an
oblique angle do not
provide the necessary
resistance.
(b) The walls of a groove must
be perpendicular to
rotating forces to resist
displacement.
(a) The facial and lingual walls of a
box will not resist rotational
displacement if they form oblique
angles with the pulpal wall.
(b) They must meet the pulpal wall at
angles near 90 degrees

Length
 Occlusogingival length
is an important factor in
both retention and
resistance.
The preparation with longer walls (a)
interferes with the tipping
displacement of the restoration better
than the short preparation (b).
preparation on a tooth with a smaller
diameter (a) resists pivoting
movements better than a preparation of
equal length on a tooth of larger
The shorter the wall, the more
important its inclination.
The walls of shorter
preparations should have as
little taper as possible to
increase the resistance.
However, even this will not help
if the walls are too short.

Substitution of internal
features
 Kent et al reported a marked
difference between the degree of
taper of full crown preparations (18.4
to 22.2 degrees) and that of boxes
and grooves in the axial surfaces of
those preparations (7.3 degrees).
 The taper of these internal features
is nearly the same as the taper of
the instruments used to cut them (4
to 6 degrees).
 These features offer an excellent
means of enhancing the overall
retention and resistance of an
otherwise overinclined axial wall.
Internal preparation features

Path of insertion
 The path of insertion is an imaginary line along which the
restoration will be placed onto or removed from the
preparation.
 The correct technique must be used to survey a
preparation visually because this is the primary means of
ensuring that the preparation is neither undercut nor
overtapered.
If the center of the occlusal surface of a preparation is viewed with one
eye from a distance of approximately 30 cm (12 inches), it is possible to
sight down the axial walls of a preparation with a minimum taper.
However, it is also possible to sight down the axial walls of a
preparation with a reverse (ie, undercut) taper of 8 degrees when both
eyes are open.
Therefore, it is important that preparations be viewed with one eye
closed

 For a preparation to be
surveyed in the mouth,
where direct vision is
rarely
 possible, a mouth mirror
is used. It is held at an
angle
 approximately ½ inch
above the preparation,
and the image is viewed
with one eye.
 The path of insertion must
be considered in two

The path of insertion of a
preparation must parallel the
adjacent
proximal contacts (a) or it will
be prevented from seating (b).
(a) The path of insertion of a
preparation for a metalceramic
crown should parallel the long axis
of the tooth. (b) If the path is
directed facially, the prominent
facioincisal angle may create
esthetic problems of
overcontouring or opaque show-
through. (c) However, if the path is
directed lingually, the facial surface
will intersect the lingual surface,
creating a shorter preparation. It
also may encroach on the pulp.

The correction of inadequate retention and resistance form
should always be attempted first through the application of
compensatory measures (principles) to the primary factor
(axial walls) and then, if necessary, through the addition of
secondary factors .
Gilboe DB, Teteruck WR. Fundamentals of extracoronal tooth preparation. Part
I. Retention and resistance form. Journal of Prosthetic Dentistry. 1974 Dec

Structural Durability
 A restoration must contain a bulk of material that is
adequate to withstand the forces of occlusion. This bulk
must be confined to the space created by the tooth
preparation.
Inadequate occlusal reduction does not
provide the needed space for
a cast restoration of adequate thickness
Occlusal reduction should reproduce basic
inclined planes rather than being cut as
one flat plane

Occlusal reduction:
 One of the most important features for
providing adequate bulk of metal and strength
to the restoration is occlusal clearance.
 For gold alloys, there should be 1.5 mm of
clearance.
 Metal-ceramic crowns will require 2.0 mm of
clearance to receive ceramic coverage. There
should be 2.0 mm of clearance on
preparations for all ceramic crowns.

Functional cusp bevel:
 A wide bevel on the palatal inclines of the maxillary
palatal cusps and the facial inclines of mandibular facial
cusps provides space for an adequate bulk of metal in
an area of heavy occlusal contact.
Lack of a functional cusp bevel
can cause a thin area or
perforation in the casting.
Lack of a functional cusp bevel
may result in overcontouring
and poor occlusion.
Overinclination of the facial
surface will destroy excessive
tooth structure and lessen
retention

Axial reduction:
 If restorations are made with normal contours over
preparations with inadequate axial reduction, they
will have thin walls that will be subject to distortion.
Laboratory technicians often
attempt to compensate for this by
overcontouring the axial surfaces.
While this intended solution to the
problem strengthens the
restoration, it can have a
disastrous effect on the
periodontium.
Inadequate axial reduction can
cause thin walls and a weak
restoration (a) or a bulbous,
overcontoured restoration (b).

There are other features that
serve to provide space for
metal that will
improve the rigidity and
durability of the restoration: the
offset, the occlusal shoulder,
the isthmus, the proximal
groove, and the box.
The isthmus connects the
boxes, and the offset ties the
grooves together to enhance
the reinforcing “truss effect.”
(a) The three-quarter crown is
reinforced by the bulk of gold that fills
the offset and grooves. (b) The occlusal
shoulder strengthens the lingual
margin, and the isthmus and boxes
reinforce the main body of an MOD
onlay

Marginal Integrity
 The configuration of the preparation finish line
dictates the shape and bulk of restorative
material in the margin of the restoration. It also
can affect both marginal adaptation and the
degree of seating of the restoration.
 Bevels have been advocated as a means of
diminishing marginal discrepancy

 The following criteria for margin selection seem
reasonable:
 1. The selected margin must provide a
predictable level of marginal integrity.
 2. To minimize plaque accumulation, the
selected margin must present smooth
materials to the gingival sulcus.
 3. In some situations, the margin also must
provide acceptable esthetics.
Donovan TE, Chee WW. Cervical margin design with contemporary esthetic
restorations. Dental Clinics of North America. 2004 Apr;48(2):vi-417.

Margin Placement: Whenever possible, the
margin of the preparation should be supragingival.
 Advantages of supragingival
margins:
1. Can be easily finished
without associated soft
tissue trauma.
2. More easily kept plaque
free.
3. Impressions are more easily
made, with less potential for
soft tissue damage.
4. Restorations can be easily
evaluated at the time of
placement and at recall
appointments.
 Subgingival margins
indications:
1. Dental caries, cervical erosion,
or restorations extend
subgingivally, and a crown-
lengthening procedure is
contraindicated.
2. Proximal contact area extends
apically to the level of the
gingival crest.
3. The margin of an esthetic
restoration is to be hidden
behind the labiogingival crest.
4. Axial contour modification is
indicated
prosthodontics 5th ed St. Louis: Elsevier. 2015 Jul 28

Margin Adaptation
more precisely the restoration is adapted
to the tooth, the lower is the risk for
recurrent caries or periodontal disease
castings that fit to within 10 μm34 and
porcelain margins that fit to within 50
μm,35 provided that the tooth was
properly prepared.
Rough, irregular, or “stepped” junctions
between tooth and restoration greatly
increase overall margin length
Poor preparation design, leading to
increased margin length. C, A rough
irregular margin makes the
fabrication
of an accurately fitted restoration
almost impossible.

Margin Geometry : Guidelines for margin
design should be considered:
Ease of preparation without overextension or
unsupported enamel at the cavosurface line angle
Ease of identification in the impression or optical
scan and on the (virtual) die
A distinct boundary to which the wax pattern can
be finished or the design terminated
Sufficient bulk of material
Conservation of tooth structure
Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics 5th ed St.
Louis: Elsevier. 2015 Jul 28

Finish line configurations
 The preferred gingival finish line
for veneer metal restorations is
the chamfer,it has been shown
experimentally to exhibit the
least stress, so the cement
underlying it will have less
likelihood of failure.
 Deep chamfer (heavy chamfer)
is used to provide a 90-degree
cavosurface angle with a large-
radius rounded internal angle.
 It provides better support for a
ceramic crown than does a
conventional chamfer, but it is
not as good as a shoulder.
Chamfer finish line demonstrated on
a full veneer crown preparation
(a) Deep chamfer on a preparation for
an all-ceramic crown. (b)90 degree
finish line formation.

Lip formation
an undesirable fragile lip of
enamel at the cavosurface.This
friable, unsupported enamel is
very easily fractured during or
after cementation of the
restoration.
Classic shoulder on a preparation
for an all-ceramic crown
wide ledge provides resistance
to occlusal forces and minimizes
stresses that might lead to
fracture of the porcelain
a) A radial shoulder on an allceramic
crown preparation combines maximum
support of the ceramic with a stress
reducing, rounded gingivoaxial angle.
(b) Minimal radius of curvature reduces
stress

It can be used in those
situations where a shoulder is
already present either because
of
destruction by caries or the
presence of previous
restorations. for preparations
with extremely short walls
finish lines that permit an acute
margin of metal is the knife
Edge. It may have to be used
on the lingual surface
of mandibular posterior teeth,
on teeth with very convex axial
surfaces, and
on the surface toward which a
tooth may have tilted
Shoulder with a bevel on the occlusal
shoulder of an MOD onlay
Knife edge on the lingual of a mandibular
three-quarter crown

Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics 5th ed St. Louis:
Elsevier. 2015 Jul 28

Becker CM, Kaldahl WB. Current theories of crown contour, margin placement,
and pontic design. The Journal of prosthetic dentistry. 1981 Mar 1;45(3):268-77.
Crown contours which promote favorable tissue response
follow these guidelines:
(1) buccal and lingual contours are flat
(2) embrasure spaces should be open
(3) contacts should be high (incisal one third) and buccal to
the central fossa (except between first and second
molars)
(4) furcations should be ‘‘fluted’’ or ‘‘barreled out.’’ Margins
should be supragingival where possible.
(5) The pontic design of choice is the modified ridge lap for
posterior spaces and the ridge-lap facing for anterior
spaces.

Preservation of the
Periodontium
Whenever possible, the
finish line should be
placed in an area where
the margins of the
restoration can be
finished by the dentist
and kept clean by the
patient. The deeper the
restoration margin
resides in the gingival
sulcus, the greater the
inflammatory response
Caution should be
exercised if conditions
require that the finish
line be placed any
closer to the alveolar
crest than 2.0 mm,
which is the combined
dimension of the
epithelial and
connective tissue
attachments
Placement of a
restoration margin in
this area probably will
result in gingival
inflammation,loss of
alveolar crest height,
and formation of a
periodontal pocket

Instrumentation
The excavation of caries should be
accomplished with sharp spoon
excavators and round burs (no. 4 or
no. 6).Small diamond points, used
with an air-water spray in a high-
speed handpiece, will remove
precisely amounts of tooth structure
Cross-cut or dentate burs are
employed for removal of old
restorations, but the horizontal
ridges they leave on tooth structure
make them unacceptable for planing
tooth surfaces.
Acceptable finish lines on vertical
flares can be obtained through the
use of abrasive paper disks.
Nondentate tapered burs (169L,
170L, and 171L) are used for
grooves, boxes, isthmuses, and
offsets where they are needed.
Gross reduction is most efficiently
accomplished with coarse
diamonds.
Using fine diamonds of the same
size and shape, it is possible to
maintain the finish line configuration
developed by the coarse diamond
instrument.

Rotary instruments for tooth preparations
Shape S i l houette Use
Round-end tapered
diamond
Depth orientation grooves ,Occlusal
reduction,Functional cusp bevel
Round-end tapered
finishing diamond
Occlusal finishing, Functional
cusp bevel finishing
Flat-end tapered
diamond
Axial reduction(MCR,all ceramic
crown) Shoulder(MCR,all ceramic
crown)
Flat-end tapered
finishing diamond
Axial wall finishing (MCR, allceramic
crown)Radial shoulder(MCR,
allceramic crown
Tapered torpedo
diamond
Axial reduction Chamfer finish line
Tapered torpedo
finishing diamond
Axial wall Finishing Chamfer finishing
Short needle
diamond
Initial proximal reduction (posterior
teeth)

Shape S i l houette Use
Long needle
diamond
Initial proximal reduction (anterior
teeth)
Small football shaped
diamond
Lingual Reduction (anterior teeth)
Small footballshaped
Finishing diamond
Lingual reduction finishing (anterior
teeth)
Flame diamond Proximal Flare Gingival bevel
Flame finishing
diamond
Flare and bevel finishing
Tapered fissure
bur
Seating groove ,Proximal groove
(posterior teeth)Offset,Occlusal
shoulder,Isthmus,Proximal box,
Smoothing and finishing,Occlusal
and
Incisal bevels
Tapered groove
alignment fissure burs
Initial groove alignment Angles of
Proximal boxes Smoothing and

ESTHETIC CONSIDERATIONS
 Options for esthetic restorations include partial veneer
crowns, which maintain an intact labial or buccal surface
in original tooth structure;
 metal-ceramic restorations, which consist of a metal cast
substructure that in visible areas has an esthetic
porcelain veneer; and all-ceramic restorations

All-Ceramic Restorations
 Not all ceramic crown preparations are
conservative of tooth structure, in as much as a
wide 90-degree heavy chamfer margin must be
prepared around the entire tooth to ensure
increased material thickness and material
strength.
 For the same reason, additional reduction on the
lingual surface is needed for these restorations
 A minimal material thickness of approximately 1 to
1.2 mm is necessary to ensure optimal esthetics.
 This limits the use of these restorations on
faciolingually thin teeth and on teeth with large
pulps, as in young individuals.

Metal-Ceramic Restorations
 Facial Tooth Reduction:
 A minimum reduction of 1.5 mm is typically required for
optimal appearance.
 Adequate thickness of porcelain is needed to create a
sense of color depth and translucency.
 The labial surfaces of anterior teeth should be prepared
for metal-ceramic restorations in two distinct planes .
 If they are prepared in a single plane, the reduction in
either the cervical or the incisal area of the preparation
is insufficient
Rosenstiel SF, Land MF, Fujimoto J. Contemporary
fixed prosthodontics 5th ed St. Louis: Elsevier. 2015

Recommended tooth preparation
for maxillary
(A) and mandibular (B) metal-
ceramic restoration. In each
case,
the facial reduction has two distinct
planes.
Adequate porcelain thickness is
essential for preventing direct light
reflection from the highly pigmented
opaque porcelain.
The most critical areas are the
gingival and incisal thirds; in practice,
opaque modifying stains are often
used in these areas.

Incisal Reduction: An incisal reduction of 2 mm is
recommended for good esthetics.
Excessive incisal reduction must be avoided because it
reduces the resistance and retention form of the
preparation.
Proximal Reduction: The proximal surfaces of anterior
teeth look most natural if they are restored at the incisal
edges, without metal backing
Optimal esthetics require proximal
light transmission through the esthetic
veneer. Occluding lingual surfaces
are
in metal, which extends into the
proximal aspect.

 Labial Margin Placement: subgingival margins may
be indicated for esthetic reasons, particularly when the
patient has a high lip line and when the use of a metal
collar labial margin is contemplated.
 If the patient has a low lip line, a metal supragingival
collar may be placed because the metal is not seen
during normal function.
 The periodontium must be healthy before the tooth is
prepared. If periodontal surgery is needed, the sulcular
space should not be eliminated completely; rather, a
postsurgical depth of about 2 mm should be the
objective.
 Margins should not be placed so far apically that they
encroach on the attachment; extension to within 1.5
mm of the alveolar crest leads to bone resorption

Partial-Coverage
Restorations
 Whenever possible, an esthetically acceptable result
without the use of complete crowns is preferred because
tooth structure is conserved and because no restorative
material can approach the appearance of intact tooth
enamel.
 Esthetic partial-coverage restorations depend on
accurate placement of the potentially visible facial and
proximal margins.

 Proximal Margin
Correct placement of the mesial margin of a partial-coverage
restoration is essential for good esthetics. To allow proper
access for finishing, the restoration must extend just beyond the
contact area, but the metal must remain hidden from the
casual observer. B, The tooth should be prepared in its long
axis; otherwise, metal is displayed.

Facial Margin
The facial margin of a partial
cast crown should
be shaped so that light is not
reflected directly to the observer.
B, A three-unit fixed dental
prosthesis.
The mesial abutment is
a canine, shaped to look like a
lateral incisor. The distal
abutment is a partial cast crown,
which proved to be esthetically
acceptable because the metal
had been correctly contoured.

PLANNING AND EVALUATING
TOOTH PREPARATIONS
 Diagnostic Tooth Preparations:
 Diagnostic tooth preparations are performed on
articulated casts before the actual clinical
preparation
 Selecting the appropriate path of placement for a
fixed dental prosthesis, particularly for abutment
teeth that are tilted, are rotated, or have an
atypical coronal contour
 Deciding on the amount of tooth reduction
necessary to accomplish a planned change in the
occlusion.
 Determining the best location for the facial and
proximal margins of a partial-coverage restoration
so that the metal is not visible.

Goodacre CJ, Campagni WV, Aquilino SA. Tooth preparations for complete crowns: an art
form based on scientific principles. The Journal of prosthetic dentistry. 2001 Apr 1;85(4):363-
Teeth should be prepared so that they exhibit the
following characteristics:
 10 to 20 degrees of total
occlusal convergence, a
minimal occlusocervical
dimension of 4 mm for molars
and 3 mm for other teeth, and
an occlusocervical-to-
faciolingual dimension ratio of
0.4 or greater.
 Facioproximal and
linguoproximal line angles
should be preserved whenever
possible.
 When the above features are
missing, the teeth should be
modified with auxiliary
resistance features such as
axial grooves or boxes,
preferably on proximal
surfaces.
 Finish line selection should be
based on the type of
crown/retainer, esthetic
requirements, ease of
formation, and personal
experience
 Expectations of enhanced
marginal fit with certain finish
lines could not be validated by
recent research.
 Esthetic requirements and
tooth conditions determine
finish line locations relative to
the gingiva, with a
supragingival location being
more acceptable.
 Line angles should be
rounded, and a reasonable
degree of surface smoothness
is desired.

Recent advances
 the study evaluated the operator preferences and tooth preparation
performance of electric handpieces compared with those of
traditional air-turbine handpieces.
 Conclusion:
1. Despite heavier weight, poorer grip feel, and less flexibility, the
electric handpiece emitted lower noise, had better feel of the tooth
preparation in general, and was preferred for its greater
smoothness in the finishing step of tooth preparation than the air-
turbine handpiece.
2. The surface roughness of the prepared teeth and the crown fit
between the teeth and ceramic crown were not affected by the
choice of air-turbine or electric handpiece
Pei DD, Meng YC, Fayed AS, You YF, Wu ZX, Lu Y. Comparison of crown fit and
operator preferences between tooth preparation with electric and air-turbine
handpieces. The Journal of Prosthetic Dentistry. 2020 Feb 7.

 Green et al 2018,The purpose of this study was to measure
the influence of tooth substrate type (all enamel, all-dentin, or
half-dentin-half-enamel) and veneer thickness on the loads
needed to cause initial and catastrophic porcelain veneer
failure.
 Conclusion:
 1. Porcelain veneers bonded to enamel were substantially
stronger and more damage-tolerant than those bonded to
dentin or mixed substrates.
 2. Veneers on half-enamel-half-dentin substrates were no
more resistant to catastrophic failure than those placed
completely on dentin.
 3. Increased porcelain thickness substantially raised the loads
to catastrophic failure on all-enamel substrates.
 4. Increased porcelain thickness only moderately raised the
loads to catastrophic failure on all-dentin or half-enamel-half-
dentin substrates.
Ge C, Green CC, Sederstrom DA, McLaren EA, Chalfant JA, White SN. Effect of tooth
substrate and porcelain thickness on porcelain veneer failure loads in vitro. The Journal of
prosthetic dentistry. 2018 Jul 1;120(1):85-91.

 The purpose of this in vitro study was to compare the marginal
and internal fit accuracy of lithium disilicate glass-ceramic
inlays fabricated with conventional, milled, and 3-dimensional
(3D) printed wax patterns.
 Conclusion:
Lithium disilicate glass-ceramic inlays produced from digital
impressions and subtractive milling of wax patterns resulted in
better marginal and internal fit accuracy than either
conventional impression/fabrication or additive 3D
manufacturing. Three-dimensional printed wax patterns
yielded fit values similar to those of the conventionally waxed
inlays
Homsy FR, Özcan M, Khoury M, Majzoub ZA. Marginal and internal fit of pressed lithium
disilicate inlays fabricated with milling, 3D printing, and conventional technologies. The
Journal of prosthetic dentistry. 2018 May 1;119(5):783-90.

 The purpose of this clinical study was to examine the 10-year
survival of pressed lithium disilicate glass-ceramic restorations
and the relationship between clinical parameters on
outcomes.
 Conclusion: The probability of survival for lithium disilicate
monolithic complete-coverage restorations (n=1410) was
96.5% over 10.4 years and 100% over 7.9 years for the
bilayered group.
 Pressed lithium disilicate restorations in this study survived
successfully over the 10.4-year period studied with an
overallfailure rate below 0.2% per year and were primarily
confined to molar teeth.
Malament KA, Natto ZS, Thompson V, Rekow D, Eckert S, Weber HP. Ten-year survival of pressed,
acid-etched e. max lithium disilicate monolithic and bilayered complete-coverage restorations:
Performance and outcomes as a function of tooth position and age. The Journal of prosthetic
dentistry. 2019 May 1;121(5):782-90.

Conclusion
 The principles of tooth preparation can be categorized into
biologic,mechanical and esthetic considerations.
 Often those principles conflict, and the clinician must decide
how the restoration should be designed.
 Each tooth preparation must be measured by clearly defined
criteria, which can be used to identify and correct problems.
 It is important to understand the pertinent theories underlying
each step.
 Successful preparations can be obtained by systematically
following these steps.
 It is important to critically evaluate each step before
proceeding to the next step to ensure an optimal quality final
restoration, which will serve the patient for a longtime.

References
 Shillingburg HT, Sather DA, Wilson EL, Cain JR, Mitchell DL, Blanco LJ, Kessler JC.
Fundamentals of fixed prosthodontics. Quintessence Pub Co (4th edn), Chicago.
2013.
 Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics 5th ed St.
Louis: Elsevier. 2015 Jul 28.
 Goodacre CJ, Campagni WV, Aquilino SA. Tooth preparations for complete crowns:
an art form based on scientific principles. The Journal of prosthetic dentistry. 2001
Apr 1;85(4):363-76.
 Ge C, Green CC, Sederstrom DA, McLaren EA, Chalfant JA, White SN. Effect of
tooth substrate and porcelain thickness on porcelain veneer failure loads in vitro. The
Journal of prosthetic dentistry. 2018 Jul 1;120(1):85-91.
 Homsy FR, Özcan M, Khoury M, Majzoub ZA. Marginal and internal fit of pressed
lithium disilicate inlays fabricated with milling, 3D printing, and conventional
technologies. The Journal of prosthetic dentistry. 2018 May 1;119(5):783-90.
 Gilboe DB, Teteruck WR. Fundamentals of extracoronal tooth preparation. Part I.
Retention and resistance form. Journal of Prosthetic Dentistry. 1974 Dec
1;32(6):651-6.

 Pei DD, Meng YC, Fayed AS, You YF, Wu ZX, Lu Y. Comparison of crown fit and
operator preferences between tooth preparation with electric and air-turbine
handpieces. The Journal of Prosthetic Dentistry. 2020 Feb 7.
 Becker CM, Kaldahl WB. Current theories of crown contour, margin placement, and
pontic design. The Journal of prosthetic dentistry. 1981 Mar 1;45(3):268-77.
 Donovan TE, Chee WW. Cervical margin design with contemporary esthetic
restorations. Dental Clinics of North America. 2004 Apr;48(2):vi-417.
 Malament KA, Natto ZS, Thompson V, Rekow D, Eckert S, Weber HP. Ten-year
survival of pressed, acid-etched e. max lithium disilicate monolithic and bilayered
complete-coverage restorations: Performance and outcomes as a function of tooth
position and age. The Journal of prosthetic dentistry. 2019 May 1;121(5):782-90.
 Glossary of Prosthodontic Terms, Ninth Edition, GPT‐9. The Academy of
Prosthodontics Foundation. J. Prosthet. Dent.. 2017;117(5S):e1-05.
 Parker MH. Resistance form in tooth preparation. Dental Clinics. 2004 Apr
1;48(2):387-96.

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