Preparations for severely

 O N E O F T H E C R I T E R I A F O R T H E U S E O F A C A S T M E T A L ,
M E T A L - C E R A M I C , O R A L L - C E R A M I C R E S T O R A T I O N I S A T O O T H
T H A T H A S B E E N D A M A G E D T O T H E E X T E N T T H A T I T M U S T B E
R E I N F O R C E D A N D P R O T E C T E D .
 I T S H O U L D N O T B E S U R P R I S I N G T H A T U N M O D I F I E D C L A S S I C
P R E P A R A T I O N D E S I G N S A R E I N F R E Q U E N T L Y U S E D F O R T H I S
P U R P O S E .
 T H E T Y P E S O F D A M A G E T H A T M A Y B E E N C O U N T E R E D I N
D E B I L I T A T E D T E E T H I N C L U D E L O S S O F C R O W N , P U L P A L
I N V O L V E M E N T , L O S S O F A T T A C H M E N T , A N D L O S S O F R O O T ( S )
 M O S T I N D I V I D U A L T E E T H R E Q U I R I N G C E M E N T E D
R E S T O R A T I O N S , A S W E L L A S M A N Y F I X E D P A R T I A L D E N T U R E
A B U T M E N T S , H A V E B E E N D A M A G E D E N O U G H T O R E Q U I R E
M O D I F I C A T I O N O F A C L A S S I C P R E P A R A T I O N D E S I G N
Preparations for Severely
Debilitated Teeth

Tooth conditions and interventions

Principle of Substitution
Two rules should be observed to avoid excessive tooth
destruction while creating retention in an already weakened
tooth:
 The central “core” (the pulp and the 1.0-mm-thick surrounding layer of
dentin) must not be invaded in vital teeth. No retentive features should
extend farther into the tooth than 1.5 mm at the cervical line or from the
central fossa.
 If caries removal results in a deeper cavity, any part lying within the vital
core should be filled with glass-ionomer cement.
 Any preparation feature added for mechanical retention is kept
peripheral to the vital core.
 No wall of dentin should be reduced to a thickness less than its height for
the sake of retention. This may preclude the use of a full veneer crown,
or, if one must be used, it might first require the placement of a core or
foundation restoration.

 Box forms
 Small to moderate interproximal caries lesions or
prior restorations can be incorporated into a
preparation as a box form.

 Opposing upright surfaces of tooth structure adjacent to a damaged area
can be used to create a box form if at least half the circumference (180
degrees) remains in the area outside the lingual walls of the boxes
 If significantly less than 180 degrees of the tooth’s circumference
 remains between two boxes, the lingual cusp is susceptible to fracture
during
 function, upon removal of the provisional restoration, or at try-in of the
 permanent restoration.

Grooves
 Grooves placed in vertical walls of bulk tooth
structure must be well formed, at least 1.0 mm wide
and deep, and as long as possible to improve
retention and resistance.
 Multiple grooves are as effective as box forms in
providing resistance, and they can be placed in axial
walls without excessive destruction of tooth
structure.
 However, too many grooves in a crown preparation
can adversely affect the seating of a full veneer crown

Pins
 Pins effectively increase retention9,10 by generating
additional length internally and apically rather than
externally.
 They do not require vertical, supragingival tooth structure
for their placement, and they can be used where there is
insufficient axial wall length.
 They can extend apically beyond the gingival attachment
without harming it.

 Pins are commonly used in two ways:
(1) Pinholes parallel the path of insertion of the preparation, receiving
pins that are an integral part of the cast restoration
(2) nonparallel pins are placed in the tooth to retain an amalgam or
composite resin core in which a classic preparation for a cast restoration
can be formed

 Four guidelines should be followed in drilling
pinholes:
1. They should be placed in sound dentin.
2. Enamel should not be undermined.
3. Perforation into the periodontal membrane
should be avoided.
4. The pulp should not be encroached upon.

 Pinholes should be placed vertically in shoulders or ledges halfway between the
outer surface of the tooth and the pulp, surrounded by at least 0.5 mm of
dentin
 The safest locations for pinholes are the line angles or corners of the teeth
 The least desirable area for placing pinholes is midway between the corners,14
especially in regions overlying the furcations
 If bleeding occurs during drilling of a pinhole, it should be determined whether
the misdirected drill has gone into the pulp or the periodontal membrane. If it
is in the pulp, endodontic therapy is performed before the procedure continues

 Although retention increases as the number, depth, and
diameter of pins increases,
 a point of diminishing returns occurs after four or five
pins are placed. This confirms the clinical
recommendations that one pin should be used for each
missing cusp, line angle or axial wall (a maximum total of
four in any case).
 Self-threading pins are nearly five times more retentive
than cemented pins and need to be placed to a depth of
only 2.0 mm.
 However, cemented pins that are an integral part of the
restoration need to extend 4.0 mm into the tooth.

Bases and Cores
Bases
 Cement bases are used only to protect the pulp and to eliminate
undercuts in defects in tooth structure produced by the removal of
caries or old restorations.
 They are used if there is adequate bulk of tooth structure to resist
occlusal forces and enough axial wall surface to provide retention for
the definitive restoration.
 Glass-ionomer and polycarboxylate cements are excellent materials for
this purpose. They are nonirritating to the pulp and have some adhesive
properties that make them less likely to become dislodged during
subsequent preparation of the tooth.
 Deep areas of the preparation near the pulp may be covered with
calcium hydroxide
 Cement bases do not have sufficient strength to effectively replace
weakened dentinal walls, unless there are two walls of tooth structure
remaining, Amalgam or composite resin should be used for that
purpose

Cores
 If one-half or more of the clinical crown has been destroyed, an
amalgam or composite resin core should be placed in the tooth
 If less than half of a clinical crown has been destroyed, a
preparation design that will employ auxiliary features for added
retention in the area of missing cusps can be used
 Pin-retained cores have been used to retain cast restorations on
severely damaged teeth for nearly 50 years.
 Both amalgam and composite resin have been used for this purpose.
 Composite resins are favored by some because they are easily
molded into large cavities and they polymerize quickly, allowing the
crown preparation to be done at the same appointment.
 However, composite resin cores exhibit greater microleakage than
doamalgam cores,and they are not as dimensionally stable as
amalgam

 Dentin chambers, or “pot holes,” 2 to 3 mm deep can
be placed with a no.1156, 1157, or 1158 bur. When
amalgam is condensed into these holes, they become
“amalgapins.
 A properly contoured amalgam core can serve as a
provisional restoration for several weeks, giving the
tissue an opportunity to recover while more urgent
treatment is being performed.

Modifications for Damaged Vital Teeth
In the preparation of a damaged tooth, an orderly sequence should
be followed to take full advantage of the remaining tooth structure
and attain the most retentive preparation possible:
1) The first step is to evaluate pulpal health. If it is questionable, or if
there is an exposure, however small, endodontic therapy should
be done before placing a cast restoration
2) The second step is to assess the periodontal condition.
Periodontal tissues are examined for deep subgingival extensions
of caries, fractures, or previous restorations. Finish line
extensions that violate the biologic width of 2.0 mm of tissue
attachment may require periodontal surgery before a restoration
is made.
3) Next, a preliminary preparation design is made. A general concept
can be formulated in advance, but the specific features to be used
and their location cannot be ascertained until the initial phases of
the preparation have been completed.

Orthodontic Adjuncts to Restoring Damaged
Teeth
 Regaining interproximal space
in case of teeth getting close due to interproximal
caries, we need to change their position in to the basic
form which can be achieved by applying elastic
separator and then copper wire that screwing once a
week .

Extrusion of teeth
 The options When all tooth structure has been lost to the level of the
alveolar crest or beyond because of either fracture or caries:
1) Designing post and core which will encounter u with challenges as:
 Brittle post due to lack of ferrule effect
 Finish line violates the biologic width resulting in low marginal
adaptation

2) Crown lengthening which makes unstable and ugly crown
3) Need to extruding tooth and also gingival surgery for leveling the
gingiva with extruded tooth

Important to Remember:
The distance that the tooth is to be extruded is
calculated by adding
the distance from the most apical point of fracture or
caries to the alveolar crest (if the damage extends
subcrestally), 2.0 mm for the biologic width, at least
1.0 mm to prevent placement of the crown margin
too far subgingivally.
If the damage is flush with the alveolar crest, a
minimum of 3.0 mm of extrusion is required

Restoration of Endodontically Treated Teeth
 The restoration to be used on an endodontically treated tooth is dictated by the
extent of coronal destruction and by the type of tooth
Rationale
 If a moderate-sized anterior tooth is intact except for the endodontic access
and one or two small proximal lesions, composite resin restorations will
suffice. Placement of a dowel in such a tooth is more likely to weaken it
than to strengthen it.

 The use of a dowel requires that the canal be obturated with
gutta-percha. It is difficult to ream out a canal filled with a silver
point or other hard material.
 If a dowel is used, its extension into the root must at least equal
the length of the crown for optimum stress distribution and
maximum retention, or the dowel should be two-thirds the
length of the root, whichever is greater .
 A minimum length of 4.0 mm of gutta-percha, and more if
possible, should remain at the apex to prevent dislodgment and
subsequent leakage.
 If it is not possible to meet these criteria, the prognosis for the
restoration will be compromised.
 The minimum treatment indicated for an endodontically treated
molar or premolar is the placement of a cast restoration with
occlusal coverage, such as an MOD onlay.

 Maxillary premolars often have drastically tapering roots, thin root walls, and
proximal root concavities or invaginations, all of which are predisposing factors
to perforation or fracture.
 Care must be exercised in the selection of restorations for teeth that have no
remaining coronal tooth structure. The encirclement of 1.0 to 2.0 mm of
vertical axial tooth structure within the walls of a crown creates a ferrule effect
around the tooth to protect it from fracture.
 If a minimum of 1.0 mm of vertical axial wall cannot be covered by a crown on
a premolar that is to serve as an abutment, the tooth should be extracted.
 Endodontically treated teeth should not be used as abutments for distal
extension removable partial dentures because They are more than four times as
likely to fail compared to pulpless teeth not serving as abutments
 Pulpless fixed partial denture abutment teeth fail nearly twice as often as single
teeth

 A pulpless molar with a moderately damaged clinical crown can be
built up with an amalgam or composite resin core prior to
placement of an artificial crown
 If there is one sound cusp, the core may be retained by gross
extension of the amalgam into the pulpal chamber alone,or in
conjunction with pins, peripheral slots, or dentinal wells
 Advantages of dowel-core as a two-unit system:
 The restoration can be replaced at some future time if necessary,
without disturbing the dowel core.
 If a dowel is necessary, the choice is not limited to a custom cast
device.
 If the endodontically treated tooth must serve as a fixed partial
denture abutment, it is not necessary to make the root canal
preparation parallel with the path of insertion of other preparations.

Prefabricated dowel with amalgam or
resin core
 Prefabricated dowels with amalgam or composite resin
cores are the most commonly used dowel cores today,
and there is a wide variety of dowel systems available.
 Although Amalgam provides greater strength but
Composite resin remains popular because it is easily
placed, polymerizing in minutes and allowing work on
the core preparation to progress almost immediately.
Resin requires less bulk of material than does amalgam,
which makes it useful on small teeth
 A dowel increases resistance to lateral forces applied to
the crown from 15% to 48%.

 The preferred materials in light of current knowledge of galvanism
and corrosion are titanium, high platinum, and cobalt-chromium
molybdenum alloys.90 The least desirable are brass and chromium-
nickel steel
Technique
1. 1) The preparation for a dowel core is begun by preparing the
coronal tooth structure for the crown that will be the definitive
restoration for the tooth
2. 2) A Peeso reamer is measured against a radiograph of the tooth
being restored to determine the length to which the instrument
(and later, the dowel) will be inserted into the canal
3. 3) A silicone rubber endodontic stop is slid onto the shank of the
reamer, aligning it with a landmark such as the incisal edge of the
adjacent tooth to ensure insertion of the instrument to the proper
depth in the tooth.

4. The dowel space preparation is begun by first removing gutta-percha in
the canal with a hot endodontic condenser. Enlargement of the canal begins
with the largest Peeso reamer or Gates Glidden drill that will fit into the
canal
5. In the area of greatest bulk between the canal and the periphery of the
tooth, one or two 0.6-mm pinholes are drilled to a depth of 2.0 mm.
6. A thin mix of cement is made, and the dowel is coated with it. Cement is
introduced into the dowel space with a plastic instrument.
7. A Lentulo spiral is used to ensure that the walls of the canal are
completely coated with cement.
8. Retention can be increased by as much as 90% if a Lentulo spiral is used
9. The dowel is pushed slowly into place, allowing the excess cement to
escape.
10. The dowel is held in place with finger pressure until initial set occurs.
Then excess cement is removed from around the dowel head and pins.

11. Core making:
Amalgam core by using copper band
Composite core by using a clear crown (light-activated
resin), or polycarbonate crown
12. The gingival finish line must be on tooth structure.

Custom cast dowel cores
 The direct method for fabrication of a dowel core is accomplished in three
steps:
1. Canal preparation
 All caries, bases, and previous restorations are removed, and the remaining
tooth structure is evaluated to determine which areas are sound enough to be
incorporated into the definitive preparation.
 Thin walls of unsupported tooth structure should be removed at this time
 The instruments of choice for removing the gutta-percha and enlarging the
canal are Peeso reamers.
 The size of reamer used will depend on the diameter of the tooth. As a general
rule, it will be no greater than one-third the diameter of the root at the
cementoenamel junction, and there should be a minimum thickness of 1.0 mm
of tooth structure around the dowel at mid root and beyond
 The keyway should be cut to the depth of the diameter of the bur
(approximately 0.6 mm) and up the canal to the length of the cutting blades of
the bur (approximately 4 mm).

 If there is supragingival tooth structure, a flame diamond is used to place a
contrabevel around the external periphery of the preparation. This feature provides
a metal collar around the occlusal circumference of the preparation to aid in bracing
the tooth against fracture of the remaining tooth structure.
2. Resin pattern fabrication
 A Duralay plastic pin (Reliance) is trimmed so that it will slide easily into the canal
to the apical end of the dowel preparation.
 The canal is lubricated with petrolatum on a small piece of cotton on a Peeso
reamer. The orifice of the canal is filled as full as possible with acrylic resin
 When the acrylic resin has become tough and doughy, the pattern is pumped in and
out to ensure that it will not lock into any undercuts in the canal
 The dowel core pattern is wiped with an alcohol sponge to remove an residual
lubricant that could displace investment or promote bubble formation.
3. Finishing and cementation of the dowel core

Finishing and cementation of the dowel core
 The dowel core pattern is sprued on the incisal or occlusal end
 Extra water in the amount of 1.0 to 2.0 mL is added to 50 g of investment,
and a liner is not used in the ring. These measures will result in a slightly
smaller dowel core that should have less tendency to bind in the canal.
 The invested pattern should remain in the burnout oven for 30 minutes
longer to ensure complete elimination of the resin.
 Any shiny spots are relieved.
 A groove is cut on the side of the dowel from apical end to
 contrabevel to provide an escape vent for cement.
 On maxillary premolars with two canals, one canal is employed for the
dowel preparation, and a stabilizing keyway is placed in the other

 Cast dowel cores are very rarely done on molars because
they have divergent canals that require elaborate,
interlocking multipiece castings.
 If endodontic therapy must be done on a tooth after it
has received a crown, the access opening will diminish
crown retention by approximately 61%.
 If a tooth preparation fractures under a crown, a retrofit
dowel core can be fabricated under the dislodged
crown.The crown is cleaned out, lubricated, and used as
a matrix for forming the core portion after the dowel
segment of the pattern has been completed in the usual
manner.

Thanks for your
attention
By:Ensi Kolyaei

Preparations for severely

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