Principles of Tooth
Preparation
Rola Shadid, BDS, MSc, Associate Fellow AAID
Tooth structure is conserved by
using the following guidelines
1.
2.
3.
4.
5.
Use of partial coverage rather than complete
coverage restoration
Preparation of teeth with minimum practical
convergence angle (taper) between axial walls
Preparation of the occlusal surface so that
reduction follows the anatomic planes to give
uniform thickness in the restoration
Preparation of the axial wall so tooth structure is
removed evenly if necessary teeth should be
orthodontically positioned
selection of conservative margin compatible with
other principles of tooth preparation
Preservation of tooth structure
 Histologic studies showed that
specimen with dentin thickness greater
than 2 mm demonstrated very little or
no pulp response , while when cutting
was done within 1.5mm of the pulp,
23%of teeth developed an abscess
Reduction of axial walls with maximal thickness of
remaining dentin surrounded the pulp
Preparation of tooth with minimal practical
convergence angle (Taper) between axial walls
Preparation of occlusal surface following the
anatomical contour to give uniform reduction
Selection of marginal geometry which is conservative
and compatible with other principles
Avoidance of unnecessary apical extension of the
preparation
Reduce
Retention
Reduce
Resistance
Over
reduction
Increase
Hypersensitivity
Pulp
inflammation
and necrosis
Tooth fracture
Also you should be careful about:
1- Adjacent tooth
- by using matrix band of the adjacent tooth
- cutting in the enamel of prepared tooth with
fine tapered stone
2- Soft tissues
- by using mirror or the flange of saliva ejector
Retention &
Resistance
RETENTION FORM
 The feature of a tooth preparation
that resists dislodgement of a crown
in a vertical direction or along the
path of placement.
RESISTANCE FORM
 The features of a tooth preparation that
enhance the stability of restoration and
resist dislodgement along an axis other
than the path of placement
 It prevents dislodgement of a restoration
by forces directed in an apical, oblique or
horizontal direction.
Retention and resistance
1.
2.
3.
4.
Total occlusal convergence
Length of preparation
Diameter
Ratio of occlusocervical to
faciolingual dimension
5. Circumfencial morphology
6. Auxiliary retentive features
7. Cements
Total occlusal convergence
Factors that may create greater
total occlusal convergence
1. Posterior teeth are prepared with greater
total occlusal convergence than anterior
teeth
2. Mandibular teeth were prepared with
greater convergence than maxillary teeth
3. Faciolingual surfaces had greater
convergence than mesiodistal surfaces.
4. Abutments of fixed partial denture usually
prepared with greater total occlusal
convergence
5. Monocular vision created greater total
occlusal convergence than binocular vision
Parallel walls are impossible to
create in the mouth without
producing preparation undercuts.
The axial walls of the preparation
must taper slightly to permit the
restoration to seat.
 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.
 Ideal convergence angle of 4-10° is
seldom achieved
 Mack estimates that a minimum taper
of 12 degrees is necessary Just to
insure the absence of undercuts.
 A taper or total
convergence of 16
degrees has been
proposed as being
achievable clinically
while still affording
adequate retention. It
can be as low as 10
degrees on
preparations anterior
teeth and as high as 22
degrees on molars
 Resistance is more sensitive than retention
to changes in convergence angle
 As a rule we will go for the minimal
tapering without having undercuts in the
preparation
SURFACE AREA VS. RETENTION
Provided the restoration has limited path of
withdrawal, the greater the surface area of
a preparation, the greater is its retention.
Length….. Crowns with long axial walls
are more retentive…
Molar crowns are more retentive than
premolar crowns of same taper
The longer the occlusogingival height, the more the
surface area.
The more diameter of prepared
teeth, the more the surface area.
Length of preparation vs.
resistance
 Because the axial wall occlusal to the finish
line interferes with displacement, the length
and inclination of that wall become factors
in resistance to tipping forces.
 The shorter the wall, the more important its
inclination
 The walls of shorter preparations should
have as little taper a possible to increase the
resistance.
Width of prep. vs resistance
 A narrow tooth can have greater
resistance to tipping.
 The preparation on the smaller tooth will
have a short rotational radius for the arc
of displacement, and the incisal portion of
the axial wall will resist displacement.
 The longer rotational radius on the larger
preparation allows for a more gradual arc of
displacement, and the axial wall does not
resist removal
 Parker et al found that approximately
95% of anterior preparations analyzed
had resistance form, while only 46% of
those on molars did.
 Resistance to displacement for a shortwalled preparation on a large tooth can be
improved by placing grooves in the axial
walls.
 In effect, this reduces the rotational radius,
and that portion of the walls of the grooves
near the occlusal surface of the preparation
will interfere with displacement
Ratio of occlusocervical to
faciolingual dimension
 The longer the faciolingual dimension of
prepared molars compared with other
teeth and shorter occlusogingival
dimension compared with anterior teeth
produce a lower ratio and lower resistance
to dislodgment of molar crown
 Weed and Baez recommended that the
occlusocervical / faciolingual ratio should
be 0.4 mm or higher for all teeth
STRESS CONCENTRATION
 If line angle between axial and occlusal
surface is sharp, it leads to concentration
of stresses around that junction
 Induced stresses exceeds the strength of
the cement
 Leads to cohesive failure of cement
TYPE OF RESTORATION VS RETENTION
 Full veneer crown has excellent retention
when compared to partial veneer crown
because reducing the path of insertion to
a narrow range.
TYPE OF RESTORATION VS RESISTANCE
 Partial coverage restoration may have
less resistance than a complete crown
because it has no buccal resistance area
Surface Roughness
 Adhesion of dental cements depends
primarily on projections of the cement
into microscopic irregularities.
 Jorgensen found retention of castings
cemented with ZnPO4 cement on test dies
with a 10° taper to be twice as great on
preparations with 40µm scratches than
10µm.
 Retention increases when restoration is
roughened or grooved
Materials being cemented vs
retention
 Retention is affected by both the casting
alloy and the core material.
 More reactive the alloy is more adhesion.
 Base metal alloys are better retained
than less reactive high gold content
metals.
 Type of luting agent: Studies show that
adhesive resin cements are more
retentive than conventional ZnPO4 and
GIC cements
PHYSICAL PROPERTIES OF LUTING
AGENT VS RESISTANCE
 Resistance to deformation is affected by
physical properties of the luting agent,
such as compressive strength and
modulus of elasticity
 Adhesive resin >GIC > ZnPO4>
Polycarboxylate > ZOE
Auxiliary retentive features
 Internal features such as the groove, the box
form, and the pin hole
 Secondary retentive features does not
significantly affect the retention because
the surface area is not increased
significantly.
 But where these features limits the path
of withdrawal, retention is increased
 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).
 Kent et al found that grooves and boxes
had less convergence than the
convergence of the axial wall to enhance
resistance
Circumferential morphology
 Pyramidal tooth preparations
provided increased resistance
because they had corners
when compared with conical
preparation. So it is
important to preserve
facioproximal and
linguoproximal corners of
tooth preparation
 Prepared mandibular molars
are rectangular while
maxillary molars are
rhomboidal and premolars and
anterior teeth possess an oval
form
Path of Insertion
 It is an imaginary line along which the
restoration will be placed onto or
removed from the preparation.
 It is of special importance when
preparing teeth to be fixed partial
denture abutments, since the paths of all
the abutment preparations must parallel
each other.
Path of insertion
Parallel to the long axis of the
tooth
Parallel to the incisal 2/3 of the
tooth
The path of insertion must be considered in
two dimensions:
 Faciolingually: the faciolingual
orientation of the path can affect the
esthetics of metal-ceramic or partial
veneer crowns.
 Mesiodistally
Structural durability
 A restoration must have sufficient
strength to prevent permanent
deformation during function
 “The ability of a restoration to withstand
destruction due to external forces “
Structural durability
 Adequate tooth reduction- occlusal
reduction - functional cusp bevel - axial
reduction
 Alloy selection
 Metal-ceramic framework design
 Margin design
Occlusal reduction
 Zurcherman has shown that the placement of
inclined planes on occlusal surfaces of a crown
preparation rather than flat surface increases
resistance form
Functional cusp bevel
 This includes wide bevel
on the lingual surface
inclines of maxillary
lingual cusps and buccal
inclines of mandibular
buccal cusps, this will
provide space for
adequate bulk of metal in
an area of heavy occlusal
contact
Axial reduction
Margin Placement
Biologic width
It is the dimension of space that the healthy
gingival tissues occupy above the alveolar
bone. It refers to the combined connective
tissue-epithelial attachment from the crest
of the alveolar bone to the base of the
sulcus (2mm;connective tissue-1.07mm and
epithelium-0.97mm).
Evaluation of the biological width by
radiographs, probing, and sounding of
bone
Location of restorative margins
 Supragingival
 Equigingival
 Subgingival
Indications for subgingival margins
Where to place subgingival
margin ?
 Wearhang reported that a
conventional tooth brush could
remove plaque only to a point
0.5mm subgingivally, so it is
important to ensure that
restorations are placed no deeper
than 0.5mm into sulcus when this is
possible
Where to place subgingival
margin ?
 It is generally taught that the gingival
margin should never enter the sulcus by
more than half the depth of the sulcus. In
the average case this equals a depth of 1.0
mm or less, with less being favored
Where to place subgingival
margin ?
 No margin should be placed nearer than 0.5
to 1.0 mm from the attachment area in
healthy sulcus
 Some authors suggested that restorative
margins should end 3-4 mm coronal to the
alveolar crest, they assumed the biologic
width is 2mm and additional 1mm will keep
the margin 1mm above the coronal extent
of junctional epithelium
 Gingival attachment is more reliable
reference point than marginal gingiva
Technique
 Packing a small length of retraction cord in
the area of the sulcus to receive the
subgingival margin works very well.
 Make a mental note of the relative
relationship of the free gingival margin and
roughed-in finish line prior to packing the
cord. Even small cords often achieve
remarkable gingival retraction. You may
not need to place the margin as far apically
as the level of the cord.
Technique
 The lingual and approximately half of the
interproximal margins should remain
supragingival (if possible).
 The labial and labial one-half of the
interproximal margins are brought down to or
near the level of the retraction cord.
 Be specifically aware of keeping the
interproximal margin subgingival until it will no
longer be visible when viewed at an angle. This is
a frequent area of margin visibility, especially for
porcelain veneers.
o D.A. Orkin et al and J.Valderhang et al
found out that there was significant
difference in bleeding between the
subgingival crowns and contralateral
teeth without crowns but the difference
was negligible with supragingival crowns
 Hatchy J. Sttler concluded that in the
presence of subgingival restoration, the
degree of gingival inflammation is
significantly greater in association with
narrow (less than 2mm ) zones of
keratinized gingiva than with those
greater than 2mm
The extent of disease depends
on:
 contour of restoration,
 relative position of cervical
margin,
 precision of fit,
 restorative material used
Marginal contours : Lang et al reported that
overhanging margin not only accumulate more
plaque but the plaque undergoes a change in
composition to that usually associated with
destructive periodontitis
Surface roughness :Donnan and Prince
evaluated the plaque accumulation in metal
ceramic restoration and discovered the
order of decreasing plaque accumulation:
Aluminum oxide (greatest), opaque porcelain,
polished metal, glazed porcelain (least)
In term of smoothness (generally):
glazed porcelain > gold > polished amalgam
>composite
Marginal fit : the severity of periodontal
disease was elevated with greater subgingival
marginal discrepancy
Surgical crown lengthening of crown
is necessary if restorative margin
would terminate at or below alveolar
crest
If restorative margin would terminate at
or below alveolar crest surgical lengthening
of crown is necessary , the procedure
involves reflection of full thickness flap to
expose alveolar crest around the teeth in
question, the distance from the bone crest
to margin of tooth preparation can be
measured with periodontal probe, the
distance should be 3-4 mm if the bony
crest is not reduced. At least 6-8 weeks
must elapse to allow for proper healing.
MARGINAL GEOMETRY OR
FINISH LINE
CONFIGURATION
The margin design depends on:
 type of restoration ,
 tooth morphology,
 position and alignment of the teeth
in the arch,
 esthetics
Guidelines for margin design
 Ease of preparation without overextension or
unsupported enamel
 Ease of identification in the impression and on the die
 A distinct boundary to which wax pattern can be
finished
 Conservation of tooth structure
 Sufficient bulk of material for esthetic and strength of
the restoration.
 The most important consideration in selecting a
cervical margin design is its ability to consistently and
predictably provide excellent marginal integrity.
Featheredge
Chisel
Chamfer Bevel
Shoulder
Sloped
Shoulder
Beveled
Shoulder
The shoulderless (featheredge)
margin
 Conservation of the tooth structure
 Permits an acute margin of the metal
 Insufficient removal of tooth structure at
cervical area. (Results in overcontouring )
 Impossible to identify the margin of prep.
 No control over reduction of cervical tooth
structure
 No control in placem. of subgingival margin
 Poor resistance to marginal distortion during firing
of porcelain to the gold alloy.
 difficult to accurately wax and cast
 more susceptible to distortion in the mouth when
the casting is subjected to occlusal forces
Indications for featheredge
 Not recommended
 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.
 Could be Used for the
full metal crown prep
The shoulder (Butt joint) margin
 Adequate removal
of tooth structure
at cervical area.
 It is possible to
identify the
margin of prep.
 Good control over
reduction of
cervical tooth
structure
The shoulder (Butt joint) margin
 Control in placem.of
subgingival margin
 Adequate resistance
to marginal distortion
during firing of
porcelain to the
metal alloy.
 It does require the
destruction of more
tooth structure than
any other finish line.
The shoulder (Butt joint)
margin
 Finish line of choice for the all-ceramic
crown and porcelain labial margin
 The wide ledge provides resistance to
occlusal forces and minimizes stresses that
might lead to fracture of the porcelain. It
produces the space for healthy restoration
contours and maximum esthetics.
The shoulder (Butt joint)
margin
 The sharp, 90-degree
internal line angle
associated with the
classic variety of this
finish line
concentrates stress in
the tooth and is
conducive to coronal
fracture.
The radial shoulder
 A modified form of shoulder finish line
 A small-radius rounded internal angle is
instrumented by an end-cutting parallel
sided carbide finishing bur, and finishing is
completed with a specially modified binangle chisel.
 The cavosurface angle is 90 degrees, and
shoulder width is only lessened by the
rounded internal angle.
135 sloping shoulder
Very useful preparation
to use where the labial
shoulder for MC crowns
is extending well into the
root face, e.g. canines
with marked gingival
recession. The finishing
line can be placed subgingivally and the axiogingival floor line angle
left at a higher level.
135 sloping shoulder
 provides for more conservative
preparation especially in root
surface
 it requires metal collar
 porcelain butt margin is not
recommended with the sloping
shoulder
The shoulder-bevel
 It is obtained by preparing
shoulder at the height of healthy
gingival margin, then adding 0.51.25 mm bevel.
 It has the same advantages of
straight shoulder with optimum
opportunity of marginal fit
provided by the bevel
 The beveling removes
unsupported enamel, may allow
some finishing of the metal, &
minimize margin discrepency….
 Used for ceramo-metal, full metal
 This design can also be used for the
facial finish line of metal-ceramic
restorations where gingival esthetics
are not critical. 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.
 It is also a good finish line for
preparations with extremely short
walls, since it facilitates axial walls
that are nearly parallel.
A shoulder or sloped shoulder is
preferred to shoulder with bevel for
ceramometal restorations due to
biological & esthetic considerations
(the metal margin can be thinned to a
knife edge & hidden in the sulcus
without the need for positioning the
margin closer to the epithelial
attachment)
Chamfer
 Conservative type
when compared with
shoulder finish line.
 control over
reduction of cervical
tooth structure,
 control in placem.of
subgingival margin
Chamfer
 Margin of prep.is distinct
 Degree of marginal
distortion during firing of
porcelain directly related
to the thickness of metal
at the margin
Chamfer
 The preferred gingival finish
line for full veneer metal
restoration and the metal
only portion of MC crown
Chamfer

Porcelain margin is not recommended
because it will lack mechanical
resistance and depth of translucency
√
X
Heavy chamafer
 A heavy chamfer is used to provide a 90-degree
cavosurface angle with a large-radius rounded
internal angle
 It is created with a round-end tapered
diamond.
 In the hands of an unskilled operator, this
instrument can create an undesirable fragile
"lip" of enamel at the cavosurface.
 The heavy chamfer provides better support for
a ceramic crown than does a conventional
chamfer, but it is not as good as a shoulder.
Margin forms for MC
 Metal collar
 Metal feather –edge
(Disappearing margin)
 Porcelain margin
Margin forms for MC
Metal collar
Advantages :
1. good marginal seal
2. preservation of periodontal health
3. rigidity during cementation
4. wide facial metal collar (.8mm) gives
sufficient rigidity against distortion
caused by porcelain shrinkage in
comparison of the feather edge collar
5. can be used with any of the finish
lines described previously
Disadvantages :
1. difficult to conceal in a shallow
crevice or with a thin translucent
gingival margin
2. they display in case of gingival
recession
3. display of metal becomes very clear
with high lip line
Metal feather–edge (Disappearing
margin)
 Reduction of the labial metal collar has
been described as: feather – edge ,
triangular formation , hairline collar .
 The metal and opaque layer porcelain meet
simultaneously on the external edge of the
tooth preparation
 Shoulder preparation is needed for this
design to provide rigidity of metal in the
cervical area
Disadvantages :
1. the design is technique sensitive and
difficult to achieve without overcontouring
the cervical aspect or exposing opaque
layer
2. difficult finishing and polishing ,
microscopically the surface remains rough
3. marginal adaptation after porcelain firing is
subject to some distortion ,
Porcelain margin
Advantages:
1. esthetic improvement because of :
a. facial metal collar elimination
b. depth in cervical translucency
c. possibility of light transmission
through the root area
2. less plaque accumulation than metal
because of low adhesive forces
between plaque and ceramics
 porcelain margin is used with shoulder finish line
(1.2mm wide ) internally rounded at a 90-100 angle
to root surface with regular and smooth outline
 chamfer and sloping shoulder finish lines are
contraindicated with porcelain margin because :
a. porcelain margin would be very thin and prone
to chipping
b. difficult to achieve satisfactory marginal
adaptation because porcelain shrinkage occurs
toward the greatest bulk during firing
Porcelain construction :
 various techniques of porcelain margin
construction have been described using :
platinum matrix , refractory dies , separating
varnish , wax, or resin binders
 with conventional porcelain margin materials
rounded edges with rough and
heterogeneous surfaces were more likely to
occur using direct lift off technique than
platinum matrix substrates
For all metal restorations, chamfer finish line
are frequently used the advantages of
chamfer finish line
1. They produce the less stress and marginal
opening of metal
2. They are easy to form with a tapered round
end diamond instrument
3. They posses adequate bulk for restoration
4. They are visible on prepared teeth
5. Their depth is sufficient to permit the
concept of normal axial contour .
Standard metal-ceramic crown involving
only the anatomical crown and where
aesthetics is of primary importance
Labial – Flat shoulder (90°)
Lingual – Deep chamfer with metal collar
ANTERIOR METAL-CERAMIC
CROWNS
A UNIFORM
REDUCTION OF
APPROXIMATELY
1.2 MM IS NEEDED
OVER THE ENTIRE
FACIAL SURFACE.
ANTERIOR METAL-CERAMIC
CROWNS
TO ACHIEVE ADEQUATE
REDUCTION WITHOUT
ENCROACHING UPON THE
PULP – FACIAL SURFACE
PREPARED IN TWO
PLANES THAT
CORRESPOND ROUGHLY
TO THE TWO GEOMETRIC
PLANES PRESENT ON THE
FACIALSURFACE OF AN
UNCUT TOOTH
ANTERIOR METAL-CERAMIC
CROWNS
FACIAL SURFACE
PREPARED IN ONE
PLANE THAT HAS
ADEQUATE FACIAL
REDUCTION IN THE
GINGIVAL ASPECT:
Inadequate space for a
sufficient thickness of
ceramic material- Poorly
contoured restoration affecting
both esthetic & health of the
surrounding gingiva.
ANTERIOR METAL-CERAMIC
CROWNS
FACIAL SURFACE
PREPARED IN ONE
PLANE THAT HAS
ADEQUATE FACIAL
REDUCTION IN THE
INCISAL ASPECT- FACIAL
SURFACE OVERTAPERED
AND TOO CLOSE TO THE
PULP.
ARMAMENTARIUM
ANTERIOR METAL-CERAMIC
CROWNS
SILICONE INDEX
MADE BEFORE
TOOTH
PREPARATION
TOOTH BADLY
BROKEN DOWN,
INDEX MADE ON WAXED
UP DIAGNOSTIC CAST.
ANTERIOR METAL-CERAMIC
CROWN PREPARATION
PLACEMENT OF DEPTH
ORIENTATION
GROOVES - ( 1.2MM )
THE LABIAL GROOVES
CUT IN TWO SETS
1. ONE SET PARALLEL
WITH THE GINGIVAL
HALF OF LABIAL
SURFACE
2. ONE SET PARALLEL
WITH THE INCISAL
HALF OF LABIAL
SURFACE
ANTERIOR METAL-CERAMIC
CROWN PREPARATION
ANTERIOR METAL-CERAMIC
CROWN PREPARATION
ANTERIOR METAL-CERAMIC
CROWN PREPARATION
INCISAL
REDUCTION(2MM)
ROUND –END
TAPERED DAIMOND.
ANTERIOR METAL-CERAMIC
CROWN PREPARATION
Inadequate incisal reduction
results in poor incisal
translucency
ANTERIOR METAL-CERAMIC
CROWN PREPARATION
LABIAL REDUCTION
(INCISAL HALF)
ROUND- END
TAPERED DAIMOND.
ANTERIOR METAL-CERAMIC
CROWN PREPARATION
LABIAL REDUCTION
(GINGIVAL HALF)
ROUND-END
TAPERED DAIMOND
ANTERIOR METAL-CERAMIC
CROWN PREPARATION
LINGUAL
REDUCTION
(0.7 -1MM )
SMALL WHEEL
DAIMOND.
ANTERIOR METAL-CERAMIC
CROWN PREPARATION
SMOOTHENING
THE SHARP ANGLES
ANTERIOR METAL-CERAMIC
CROWN PREPARATION
RADIAL SHOULDER
MODIFIED FORM OF
SHOULDER
SMALL RADIUS
INTERNAL ANGLE
WITH 90-DEGREE
CAVOSURFACE
ANTERIOR METAL-CERAMIC
CROWN PREPARATION
POSTERIOR METALCERAMIC CROWNS
POSTERIOR METALCERAMIC CROWNS
STEP NO : 1
OCCLUSAL REDUCTION
FOLLOWED BY
FUNCTIONAL CUSP
BEVEL
POSTERIOR METAL-CERAMIC
CROWNS
STEP NO : 2
DEPTH
ORIENTATION
GROOVES
POSTERIOR METAL-CERAMIC
CROWNS
STEP NO :3
FACIAL REDUCTIONOCCLUSAL HALF
POSTERIOR METAL-CERAMIC
CROWN
STEP NO :4
FACIAL REDUCTION
GINGIVAL HALF
POSTERIOR METAL-CERAMIC
CROWN
STEP NO: 5
PROXIMAL
AXIAL
REDUCTION
POSTERIOR METAL-CERAMIC
CROWN
STEP NO :6
LINGUAL AXIAL
REDUCTION
POSTERIOR METAL-CERAMIC
CROWN
STEP NO : 7
AXIAL FINISHING
POSTERIOR METALCERAMIC CROWN
POSTERIOR METALCERAMIC CROWN
POSTERIOR METALCERAMIC CROWN
The recommended dimensions
for metal ceramic crown
Metal ceramic crown prep.
Remove any unsupported enamel
Avoid traumatizing gingiva during
subgingival preparation
Common Faults in Preparation
 Insufficient removal of buccal or labial enamel
particularly at the labio-incisal one-third of the
preparation
 Insufficient removal of occlusal enamel in
posterior teeth particularly at the cusp tips
 Insufficient removal of lingual enamel which
may force the ceramist to widen his occlusal
table or reduce the gold coping thickness which
can increase the risk of metal deformation.
Common Faults in Preparation
 Failure to round off all internal line and point
angles, thereby creating stress concentration
areas which may cause “pop-off” of the
porcelain veneer.
 Flattening occlusal tables in the preparation
instead of following the line of the cusp angles.
 Inadequate removal of approximal enamel,
particularly on the front teeth, leaving
insufficient space for metal and porcelain at the
cervical third of the tooth.
Some guidelines for preparing teeth
for metal ceramic complete crown:
 The total occlusal convergence
should range between 10 and 20
degrees.
 The minimal occlusocervical
dimension of molars should be 4 mm
when prepared with 10 to 20
degrees total occlusal convergence.
 Many molars need auxiliary grooves
or boxes to enhance resistance
form.
 Axial grooves/boxes should be used
routinely with mandibular molars .
 When tooth conditions and esthetics
permit, finish lines should be
located supragingivally.
 Check the sub-gingival margins for
any deposits of calculus. These
must be removed prior to taking the
impression. In particular, calculus in
the approximal regions is much more
easily removed at the time of crown
preparation and often can be
present despite careful preoperative prophylaxis.
 Immediate dentine sealing of
prepared teeth with a dentine
bonding agent (DBA) removes the
smear layer, seals patent dentine
tubules, halts bacterial ingress,
reduces postoperative sensitivity
and results in superior bonding of
the definitive restoration when
using a resin-based cement.
References
 Principles of Tooth Preparations ;
Preparations for Full Coverage Crowns,
Fundamentals of Fixed prosthodontics, 4th
Ed. Shillingburg, Quintessence publishing.