preparations for severely
Post on 13-Apr-2017
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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 TA L , M E TA 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 AT I O N I S A T O O T H T H AT 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 AT 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 AT U N M O D I F I E D C L A S S I C P R E PA R AT I O N D E S I G N S A R E I N F R E Q U E N T LY 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 AT M AY B E E N C O U N T E R E D I N D E B I L I TAT E D T E E T H I N C LU D E L O S S O F C R O W N, P U L PA L I N V O LV E M E N T, L O S S O F AT TA 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 UA 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 AT I O N S, A S W E L L A S M A N Y F I X E D PA R T I A L D E N T U R E A B U T M E N T S, H AV 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 AT I O N O F A C L A S S I C P R E PA R AT 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 ofdentin) must not be invaded in vital teeth. No retentive features shouldextend farther into the tooth than 1.5 mm at the cervical line or from thecentral 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 keptperipheral to the vital core.
No wall of dentin should be reduced to a thickness less than its height forthe 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 orfoundation restoration.
Box formsSmall 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.
GroovesGrooves 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 DamagedTeeth
Regaining interproximal spacein 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 crown3) 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
Technique1. 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 used9 .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 bandComposite core by using a clear crown (light-activated resin), or polycarbonate crown12. 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
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