INTRODUCTION
Taggart
permanently changed the practice of restorative dentistry by introducing his
technique for cast gold dental restorations, such that, today, cast dental
restorations and appliances have become basic treatment modalities in
dentistry.
The
Class II onlay involves the
proximal surfaces of a posterior tooth, and caps all of the cusps.
The procedure requires two
appointments: the first for preparing the tooth and making an impression, and
the second for delivering the restoration to the patient. The fabrication
process is referred to as an indirect
procedure because the casting is made on a replica of the prepared tooth
in a dental laboratory.
INDICATIONS:
1.
In extensive proximal
surface caries in posterior teeth involving buccal and lingual line angles.
2.
In patients with good oral
hygiene and low caries index.
3.
Post endodontic restoration
are preferably restored by onlay to strengthen the remaining tooth structure
and to distribute occlusal forces.
4.
In teeth with extensive
restoration, sometimes fracture line is present in enamel and dentin.
Inlay/onlay can brace the tooth and prevent fragmentation of the tooth.
5.
To maintain and restore
proper interproximal contact and contour and for occlusal plane correction.
6. When
there are other teeth present already restored with cast metal restorations.
7.
In posterior teeth with
heavy occlusal forces and attrition.
CONTRAINDICATIONS:
1. Facial
and lingual (especially lingual) smooth-surface caries is indicative of a high
caries activity that should be brought under control before expensive cast
metal restorations are used.
2. With
younger patients amalgam or composite is usually the restorative material of
choice for Class I and Class II restorations unless the tooth is severely
broken down or endodontically treated.
3. The
dentist must consider the esthetic impact (display of metal) of the cast metal
restoration. This factor often limits the use of cast metal restorations to
tooth surfaces that are not visible at a conversational distance.
4. Because
of the success of both amalgam and composite, few cast metal inlays are done in
small Class I and II restorations.
ADVANTAGES:
1. The
inherent strength of dental casting alloys allows them to restore large
damaged or missing areas and be used in ways that protect the tooth from future
fracture injury.
2. As
previously mentioned, high gold dental casting alloys are quite unreactive in
the oral environment.
3. Castings
are able to withstand occlusal loads with minimal changes. This is especially
important in large restorations that restore a large percentage of occlusal
contacts.
4. Through
the use of the indirect technique, the dentist has great control over contours
and contacts.
5. Casting
techniques and materials are capable of reproducing precise form and minute
detail.
6. Cast
restorations can be finished, polished or glazed out-side the oral cavity.
DISADVANTAGES:
1. The
cast inlay requires at least two appointments and much more time than a direct
restoration, such as amalgam or composite.
2. Patients
must have temporary restorations between the preparation and delivery appointments.
3. In
some instances, cost to the patient becomes a major consideration in the decision to restore teeth with cast
metal restorations.
4. Every
step of the indirect procedure requires diligence and attention to detail.
Errors at any part of the long, multi step process tends to be compounded,
resulting in a less-than-optimal restoration.
5. Small
inlays may produce a wedging effect on facial and/or lingual tooth structure,
and thereby increase the potential for splitting the tooth.
6. Leakage
around and under a cast restoration is the most complicated and has the highest
dimension among all restorative materials. This leakage will be more pronounced
gingivally than at other parts of the restoration.
BASIC CONCEPTS OF DESIGN FOR CAST
RESTORATIONS:
There are 3 principles of
tooth preparation for cast restorations:
·
Preparation path
·
Apico-occlusal taper of the
preparation.
·
Circumferential tie
Preparation
path
The preparation
will have a single insertion path, opposite to the direction of the occlusal
loading. All reductions in tooth structure, whether intra- or extra-coronal,
should be oriented towards one path, the path of withdrawal and insertion of
the future wax pattern and restoration. This path is usually parallel to the
long axis of the tooth crown.
Apico-occlusal taper of a preparation
For maximum
retention in a cast restoration, opposing walls and opposing axial surfaces of
a tooth preparation should be perfectly parallel to each other. Since exact
parallelism can create technical problems in processing and in getting final
materials into and out of the preparation, a slight divergence of opposing
walls intracoronally are essential to facilitate cast fabrication with minimum
errors.
Circumferential Tie:
Special
attention should be paid to the marginal peripheries of the preparation, and
every effort should be made to design and prepare these marginal peripheries to create the most
favorable relationship with the restoring casting and luting cement & tooth.
This peripheral marginal anatomy of the preparation is called "circumferential
tie",
If the margin ends on
enamel, enamel walls should fulfill all requirements advocated by Noy for an
ideal cavity wall, namely:
o
Enamel
must be supported by sound dentin;
o
Enamel
rods forming the cavosurface margin should be continuous with sound dentin;
o
Enamel
rods forming the cavosurface margin should be covered with the restorative
material and
o
Angular
cavosurface angles should be trimmed.
Cast restorations are the
only restorations that can fulfill these requirements, for the simple reason
that they are stronger than
tooth structure.
Types and Design Features of Occlusal and
Gingival Bevels
According to their shapes and types of
tissue involvement there are six types of bevels:
Partial bevel:
This involves part of the enamel wall, not exceeding two-thirds of its
dimension. This is usually not used in cast restorations, except to trim weak enamel
rods from margin peripheries.
Short bevel
This includes the entire enamel
wall, but not dentin. This bevel is used mostly with Class I alloys specially
for type 1 and 2.
Long bevel
This includes
all of the enamel wall and up to one-half of the dentinal wall. This is the
most frequently used bevel for the first three classes of cast materials. Its
major advantage is that it preserves the internal "boxed-up"
resistance and retention features of the preparation.
Full bevel
This includes
all of the dentinal and enamel walls of the cavity wall or floor. Although it
is well reproduced by all four classes of cast alloys,
Counterbevel
When capping
cusps to protect and support them, this type of bevel is used, opposite to an
axial cavity wall, on the facial or lingual surface of the tooth, and it will
have a gingival inclination facially or lingually.
Hollow ground (concave)
bevel:
All of the aforementioned types of bevels are
in the form of a flat plane, but any of them, especially the last three, can be
prepared in a concave form. This allows more space for cast material bulk, a
design feature needed in special preparations to improve material's castability
retention and better resistance to stresses.
For the facial and lingual
proximal walls in intracoronal cavity preparations for castings, flares are
used, which are the flat or concave peripheral portions of the facial and
lingual walls.
Types and Design Features of Facial and
Lingual Flares
There are two types of
flares.
a. The primary flare is the conventional and basic part of
the circumferential tie facially and lingually for an intracoronal preparation.
It is very similar to a long bevel formed of enamel and part of the dentin on
the facial or lingual wall. Primary flares always have a specific angulation,
i.e., 45° to the inner dentinal wall proper. They may be hollow ground if they
are part of the circumferential tie and the preparation is for a non-noble
alloy or cast ceramics.
Functions
and indications for primary flares
These design features
perform the same functions as bevels. In addition, they can bring the facial
and lingual margins of the cavity preparation to cleansable-finishable areas.
They are indicated for any facial or lingual proximal wall of ah intracoronal
cavity preparation. If they fulfill the objectives of a preparation
circumferential tie they will be the most peripheral part of the proximal
preparation; if not, a secondary flare must be placed peripheral to them.
b. The secondary flare is almost always a flat plane
superimposed peripherally to a primary flare. Sometimes it is prepared in a
hollow ground form to accommodate materials with low castability. Usually it is
prepared solely in enamel, but sometimes it may contain some dentin in all or parts of its
surfaces. Unlike primary flares, secondary flares may have different
angulations, involvement and extent, de-pending on their function.
Functions
and indications of secondary flares
In addition to performing
the functions mentioned for bevels, secondary flares have other specific
indications. In very widely extended lesions bucco-lingually, the buccal and lingual
tooth structure will be badly thinned; the primary flare will end with
acute-angled marginal tooth structure, occasionally with un-supported enamel. A
secondary superimposed flare at the correct angulation can create the needed
obtuse angulation of the marginal tooth structure. This is done without any
sacrifice in the preparation resistance and retention, because the wall proper
and primary flares are maintained at their proper locations and angulations.
In very broad
contact areas or malposed contact areas, the primary flare will not bring the
facial and/or lingual margins to finishable-cleansable areas. However, a
secondary flare will accomplish this without changing the fixed 45° angulation
of the primary flare necessary for resistance and retention.
In ovoid teeth peripheral
marginal undercuts are especially apt to be present occlusoapically on the
facial and/or lingual peripheries of the cavity preparation. A cast restoration fabricated in an
indirect technique for this preparation will definitely end with marginal
failure (misfit). Elimination of these undercuts via wall proper or primary
flare extension will unnecessarily involve and weaken tooth structure. However,
a secondary flare superimposed on a primary flare in the correct angulation and
extent can eliminate these undercuts with minimal sacrifice of tooth structure,
Surface defects or decalcifications, etc., facial or lingual to the primary
flare's facial or lingual margin respectively, can be involved in the
preparation with a secondary flare without the need to extend or angulate the
primary flare more than indicated.
AUXILLARY
MEANS OF RETENTION
In addition to the
principal retention forms previously described (parallelism, dovetails,
surface area frictional retention, circumferential tie, and masticatory loads
directed to seat the restoration), there are numerous auxiliary means of
retention for cast restorations:
a. Luting cements
Their action is primarily
mechanical, locking the cast to tooth structure by filling the space between
them, wetting the details of both the casting and tooth preparation and filling
in these vacancies or irregularities. Cements remain in one solid mass,
immobilizing the cast to the tooth. Some cements can create a physico-chemical
bonding with parts of the tooth substance. Others can do the same thing with
some tooth components and a treated tooth surface of the casting. Some can etch
enamel to allow the enamel surface to be wetted with the cement. These ways can
work together to retain the casting in or on the tooth. However, all of them
are still considered auxiliary means since none of them can substitute for
frictional retention. All cements are susceptible to dissolution by oral
fluids. In addition, they are far weaker than the casting or the tooth
structure.
b. Grooves
All types of grooves should
be located completely in dentin. “They can be internal in an intracoronal
preparation cut at the expense of the dentinal portion of the facial or lingual
walls or the gingival floors proximally.
Generally,
internal grooves are indicated when the dimensions of the walls are fairly
limited and a locking mechanism for the restoration is needed proximally
(facially and lingually) in addition to an occlusal dovetail. Internal grooves
are contraindicated if there is any danger of impinging on the pulp chamber or
root canal system, undermining or involving an axial angle of the tooth or
undermining adjacent enamel.
c. Reverse
bevel
This is placed at the
expense of the gingival floor, creating an internal dentinal plane inclining gingivally-axially,
locking the restoration and preventing proximal displacement. It always has a
flat dentinal transition between it and the proximal gingival bevel. It can only be used when the gingival floor
has sufficient dimension to accommodate
it without decreasing the resistance form of the restoration (flat portion of
gingival floor).
Ideally, reverse
bevels are prepared using gingival margin trimmers.
TOOTH
PREPARATION FOR FULL CAST METAL ONLAYS:
The cast metal onlay
restoration spans the gap between the inlay, which is primarily an intracoronal
restoration, and the full crown, which is a totally extracoronal restoration.
The full onlay by definition caps all of the cusps of a posterior tooth and can
be designed to help strengthen a tooth that has been weakened by caries or
previous restorative experiences. It can be designed to distribute occlusal
loads over the tooth in a manner that greatly decreases the chance of future
fracture.4,6 It is more conservative of
the tooth structure than the full crown preparation, and its supragingival
margins, when possible, are less irritating to the gingiva. Usually, an onlay
diagnosis is made pre-operatively because of the tooth's status. Sometimes, the
diagnosis is deferred until the extension of the occlusal step of an inlay
preparation facially and lingually to the limits of the caries lesion shows
that cusp reduction is mandatory. The mandibular first molar is used to
illustrate one mesio-occluso-distal preparation for a full cast metal onlay.
Initial Preparation
Occlusal
Reduction
As soon as the decision is
made to restore the tooth with a full cast metal onlay, the cusps should be
reduced because this improves the access and the visibility for subsequent
steps in tooth preparation. With the cusps reduced, the efficiency of the
cutting instrument and the air-water cooling spray is improved. Also, when the
cusps are reduced, it is easier to assess the height of the remaining clinical
crown of the tooth, which determines the degree of occlusal divergence
necessary for adequate retention form. Using the No. 271 carbide bur held
parallel to the long axis of the tooth crown, a 2-mm deep pulpal floor is prepared
along the central groove. To verify the pre-operative diagnosis for cusp
reduction, this occlusal preparation is extended facially and lingually just
beyond the caries to sound tooth structure. The groove should not be extended
farther, however, than two thirds the distance from the central groove to the
cusp tips because the need for cusp reduction is verified at this point. With
the side of the No. 271 carbide bur, uniform 1.5-mm deep depth cuts are
prepared on the remaining occlusal surface. Depth cuts usually are placed on
the crest of the triangular ridges and in the facial and lingual groove
regions. These depth cuts help prevent thin spots in the final restoration. If
a cusp is in infraocclusion of the desired occlusal plane before reduction, the
amount of cusp reduction is less and needs only that which provides the
required clearance with the desired occlusal plane. Caries and old restorative
material that is deeper in the tooth than the desired clearance are not removed
at this step in preparation.
With the depth cuts serving as guides
for the amount of reduction, the cusp reduction is completed with the side of
the No. 271 bur. When completed, this reduction should reflect the general
topography of the original occlusal surface. The operator should not attempt to
reduce the mesial and distal marginal ridges completely at this time to avoid
hitting an adjacent tooth. The remainder of the ridges are reduced in a later
step when the proximal boxes are prepared.
Throughout the next steps
in the initial preparation, the cutting instruments used to develop the
vertical walls are oriented continually to a single draw path, usually the long
axis of the tooth crown, so that the completed preparation has draft (i.e., no
undercuts). For mandibular molars and second premolars whose crowns tilt
slightly lingually, the bur should be tilted slightly (5–10 degrees) lingually
to help preserve the strength of the lingual cusps. The gingival-to-occlusal
divergence of these preparation walls may range from 2 to 5 degrees from the
line of draw, depending on their heights. If the vertical walls are unusually
short, a minimum of 2 degrees occlusal divergence is desirable for retentive
purposes. Cusp reduction appreciably decreases the retention form because it
decreases the height of the vertical walls, so this minimal amount of
divergence is often indicated in the preparation of a tooth for a cast metal
onlay. As the gingivo-occlusal height of the vertical walls increases, the
occlusal divergence should increase, allowing 5 degrees in the preparation of
the greatest gingivo-occlusal length. The latter preparations present difficulties
during pattern withdrawal, trial seating and withdrawal of the casting, and
cementing, unless this maximal divergence is provided.
Occlusal
Step
After cusp reduction, a 0.5-mm deep
occlusal step should be present in the central groove region between the
reduced cuspal inclines and the pulpal floor. Maintaining the pulpal depth
(0.5 mm) of the step, it is extended facially and lingually just beyond
any carious areas, to sound tooth structure (or to sound base or restorative
material if certain conditions, discussed subsequently, have been met). Next,
the operator extends the step mesially and distally far enough to expose the
proximal DEJ. The step is extended along any remaining facial (and lingual)
occlusal fissures as far as they are faulty (fissured). The facial and lingual
walls of the occlusal step should go around the cusps in graceful curves, and
the isthmus should be only as wide as necessary to be in sound tooth structure
or sound base or restorative material. Old restorative material or caries that
is deeper pulpally than this 0.5-mm step is not removed at this stage of tooth
preparation.
As the occlusal step
approaches the mesial and distal surfaces, it should widen faciolingually in
anticipation of the proximal box extensions. This 0.5-mm occlusal step
contributes to the retention of the restoration and provides the wax pattern
and cast metal onlay with additional bulk for rigidity.
Proximal
Box
Continuing with the No. 271
carbide bur held parallel to the long axis of the tooth crown, the proximal
boxes are prepared as described in the inlay section.
Final
Preparation
Removal
of Infected Carious Dentin and Defective Restorative Materials and Pulp
Protection
If the occlusal step and
the proximal boxes have been extended properly, any caries or previous
restorative materials remaining on the pulpal and axial walls should be
visible. They should be removed as described previously.
Preparation
of Bevels and Flares
After the cement base (when
indicated) is completed, the slender, flame-shaped, fine-grit diamond
instrument is used to place counterbevels on the reduced cusps, to apply the
gingival bevels, and to create secondary flares on the facial and lingual walls
of the proximal boxes. First, a gingival retraction cord is inserted, as
described in the previous inlay section. During the few minutes required for
the cord's effect on the gingival tissues, the diamond instrument is used to
prepare the counter bevels on the facial and lingual margins of the reduced
cusps. The bevel should be of generous width and should result in 30-degree
marginal metal. The best way to judge this is to always maintain a 30-degree
angle between the side of the instrument and the external enamel surface beyond
the counterbevel. The counterbevel usually should be wide enough so that the
cavosurface margin is beyond (gingival to) any contact with the opposing
dentition. If a facial (lingual) surface fissure extends slightly beyond the
normal position of the counterbevel, it may be included (removed) by deepening
the counterbevel in the region of the fissure. If the fissure extends
gingivally more than 0.5 mm,
A counterbevel is not
placed on the facial cusps of maxillary premolars and first molars where
esthetic considerations may dictate using a stubbed margin by blunting and
smoothing the enamel margin by the light application of a fine-grit sandpaper
disk or the fine-grit diamond instrument (flame-shaped) held at a right angle
to the facial surface. The surface created by this blunting should be
approximately 0.5 mm in width. For beveling the gingival margins and
flaring (secondary) the proximal enamel walls.
After beveling and flaring,
any sharp junctions between the counterbevels and the secondary flares are
rounded slightly. The fine-grit diamond instrument also is used to bevel the
axiopulpal line angles lightly. Such a bevel produces a stronger wax pattern at
this critical region by increasing its thickness. Any sharp projecting corners
in the preparation are rounded slightly because these projections are difficult
to reproduce without voids when developing the working cast and often cause
difficulties when seating the casting. The desirable metal angle at the margins
of onlays is 40 degrees except at the gingivally directed margins, where the
metal angle should be 30 degrees.
When deemed necessary,
shallow (0.3 mm deep) retention grooves may be cut in the facioaxial and
the linguoaxial line angles with the No. 169L carbide bur. These grooves are
especially important for retention when the prepared tooth is short, which is
often the case after reducing all the cusps. When properly positioned, the
grooves are entirely in dentin near the DEJ and do not undermine enamel. The
direction of cutting (translation of the bur) is parallel to the DEJ. The long
axis of the No. 169L bur must be held parallel to the line of draw, and the tip
of the bur must be positioned in the gingival box internal point angles. If the
axial walls are deeper than ideal, however, the correct reference for placing
retention grooves is just inside the DEJ to minimize pulpal impacts but avoids
undermining enamel.
Modifications
in Full Onlay Tooth Preparations
Skirt
Preparation
Skirts are thin extensions
of the facial or lingual proximal margins of the cast metal onlay that extend
from the primary flare to a termination just past the transitional line angle
of the tooth. A skirt extension is a conservative method of improving the
retention and resistance forms of the preparation. It is relatively atraumatic
to the tooth because it involves removing very little (if any) dentin. Usually,
the skirt extensions are prepared entirely in enamel.
When the proximal portion
of a Class II preparation for an onlay is being prepared and the lingual wall
is partially or totally missing, the retention form normally provided by this
wall can be developed with a skirt extension of the facial margin. Similarly,
if the facial wall is not retentive, a skirt extension of the lingual margin
supplies the desired retention form. When the lingual and facial walls of a
proximal box are inadequate, skirt extensions on the respective lingual and
facial margins can satisfy the retention and resistance form requirements. The
addition of properly prepared skirts to three of four line angles of the tooth
virtually eliminates the chance of post-restorative fracture of the tooth
because the skirting onlay is primarily an extracoronal restoration that
encompasses and braces the tooth against forces that might otherwise split the
tooth. The skirting onlay is often used successfully for many teeth that
exhibit split-tooth syndrome.
The addition of skirt
extensions also is recommended when the proximal surface contour and contact
are to be extended more than the normal dimension to develop a proximal
contact. Extending these proximal margins onto the respective facial and
lingual surfaces aids in recontouring the proximal surface to this increased
dimension. Also, when improving the occlusal plane of a mesially tilted molar
by a cusp-capping onlay, reshaping the mesial surface to a satisfactory contour
and contact is facilitated when the mesiofacial and mesiolingual margins are
extended generously.
Skirting
also is recommended when splinting posterior teeth together with onlays. The
added retention and resistance forms are desirable because of the increased
stress on each unit. Because the facial and lingual proximal margins are
extended generously, the ease of soldering the connector and finishing of the
proximal margins is increased.
A disadvantage of skirting
is that it increases the display of metal on the facial and lingual surfaces of
the tooth. For this reason, skirts are not placed on the mesiofacial margin of
maxillary premolars and first molars. Skirting the remaining three line angles
of the tooth provides ample retention and resistance forms.
The preparation
of a skirt is done entirely with the slender, flame-shaped, fine-grit diamond
instrument. Skirt preparations follow the completion of the proximal gingival
bevel and primary flares. Experienced operators often prepare the skirt
extensions at the same time that the gingival bevel is placed, however, working
from the lingual toward the facial, or vice versa. Maintaining the long axis of
the instrument parallel to the line of draw, the operator translates the
rotating instrument into the tooth to create a definite vertical margin, just
beyond the line angle of the tooth, providing at the same time a 140-degree
cavosurface enamel angle (40-degree metal angle). The occlusogingival length of
this entrance cut varies, depending on the length of the clinical crown and the
amount of extracoronal retention and resistance forms desired. Extending into
the gingival third of the anatomic crown is usually necessary for an effective resistance
form. In most instances, the gingival margin of the skirt extension is occlusal
to the position of the gingival bevel of the proximal box.
The operator
should use less than half the tip diameter of the flame-shaped diamond
instrument to avoid creating a ledge at the gingival margin of the skirt
extension. Using high speed and maintaining the long axis of the diamond
instrument parallel with the line of draw, the operator translates the
instrument from the entrance cut toward the proximal box to blend the skirt
into the primary flare and the proximal gingival margin. The operator must
ensure that the line angle of the tooth is not over-reduced when preparing
skirt extensions. If the line angle of the tooth is over-reduced, the bracing
effect of the skirt is diminished. Holding the diamond instrument at the same
angle that was used for preparing the counterbevel, the operator rounds the
junction between the skirt and the counterbevel. Any sharp angles that remain
after preparation of the skirt need to be rounded slightly because these angles
often lead to difficulties in the subsequent steps of the restoration.
Collar
Preparation
To increase the
retention and resistance forms when preparing a weakened tooth for a
mesio-occluso-distal onlay to cap all cusps, a facial or lingual “collar” or
both may be provided. To reduce the display of metal, however, the facial
surfaces of maxillary premolars and first molars usually are not prepared for a
collar. The operator uses a No. 271 carbide bur at high speed parallel to the
line of draw to prepare a 0.8 mm–deep shoulder (equivalent to the diameter
of the tip end of the bur) around the lingual (or facial) surface to provide
for a collar about 2 to 3 mm high occlusogingivally. To provide for a
uniform thickness of metal, the occlusal 1 mm of this reduction should be
prepared to follow the original contour of the tooth, and any undesirable sharp
line angle formed by the union of the prepared lingual and occlusal surfaces should
be rounded. This aspect of the preparation is completed by lightly beveling the
gingival margin of the shoulder with the flame-shaped, fine-grit diamond
instrument to achieve a 30-degree metal angle at the margin.
Slot
Preparation
Occasionally, the use of a
slot in dentin is helpful in creating the necessary retention form. An example
is the mandibular second molar that has no molar posterior to it and requires a
mesio-occlusal onlay restoration that caps all of the cusps. The distal,
facial, and lingual surfaces are free of caries or other injury, and these
surfaces also are judged not to be prone to caries. After cusp reduction, the
vertical walls of the occlusal step portion of the preparation have been
reduced so as to provide very little retention form. The necessary retention
can be achieved by cutting a distal slot. Such a slot is preferred over
preparing a box in the distal surface because (1) the former is more conserving
of the tooth structure and of the strength of the tooth crown, and (2) the
linear extent of marginal outline is less.
To form this slot, the
dentist uses a No. 169L carbide bur with its long axis parallel to the line of
draw (this must be reasonably close to a line parallel with the long axis of
the tooth). The slot is cut in dentin so that it would pass midway between the
pulp and the DEJ if it were to be extended gingivally. The position and
direction of the slot thus avert (1) the exposure of the pulp, (2) the removal
of the dentin supporting the distal enamel, and (3) the perforation of the
distal surface of the tooth at the gingival termination of the slot. The slot
should have the following approximate dimensions: (1) the width (diameter) of
the bur mesiodistally; (2) 2 mm faciolingually; and (3) a depth of
2 mm gingival of the normally positioned pulpal wall. To be effective, the
mesial wall of the slot must be in sound dentin; otherwise, the retention form
obtained is insufficient.
A comparable
situation occurs occasionally: The maxillary first premolar requires a
disto-occlusal onlay restoration to cap the cusps, and the mesial surface is
non-carious and deemed not prone to caries. To reduce the display of metal and
to conserve the tooth structure, a slot similar to that described in the
preceding paragraph (except that it is mesially positioned and 1.5 mm wide
faciolingually) may be used for the production of adequate retention. The
mesio-occlusal marginal outline in this preparation should be distal of the
height of the mesial marginal ridge.
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