-
Centric relation
- Optimum Condylar position
- MOST ANTERIOR AND SUPERIOR against the articular eminence of the glenoid fossa
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Glenoid fossa
the bone composing the glenoid fossa is thin in its most superior aspect and is not suited to be a stress-bearing area. However, the slope of the eminence in the anterior aspect of the fossa is composed of thick cortical bone that is capable of bearing stress.
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Articular disc:
- Biconcave
- devoid of nerves and blood vessels in the central area
- tough
- attached in the anterior aspect from the superior head of the lateral pterygoid muscle
- Attached to the condyle on its medial and lateral aspects and should be interposed between the condyle and articular eminence as function occurs
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Mandibular movement along Sagittal axis
When the mandible moves to one side, the condyle on the balancing side travels forward. As it does so, it encounters the eminentia of the glenoid fossa and moves downward simultaneously. non-working side mandible moves downward and forward.
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Centric interference
Premature contact caused by high cusps when returning to centric relation
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Working Interference
premature contact between posterior teeth on the working side, so heavy to disocclude anterior teeth.
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Nonworking Interference
- premature contact between posterior teeth on the nonworking side
- Particularly destructive
-
Protrusive Interference
premature contact between the mesial aspects of mandibular posterior teeth and the distal aspects of maxillary posterior teeth
-
Bilateral Occlusion
- have as many teeth touching as possible to stabilize the denture.
- causes wearing in excursive movements.
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Unilateral Occlusion
only one side touches in excursive movements
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Mutually Protected Occlusion
when one side contacts, the other side must disocclude.
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Nonadjustable articulator
- only a hinge opening
- The distance between teeth and horizontal axis is considerably shorter than it is in the skull, with a resultant loss of accuracy
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Semiadjustable articulator
- Close approximation of the anatomic distance, slight errors
- Intercondylar distances are not totally adjustable - some can choose small, medium, large
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Arcon Articulator
- Closer anatomy
- The condylar elements are placed on the lower member of the articulator
- The mechanical fossae are placed on the upper member of the articulator
- Non-arcon articulator has the reverse
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Materials Used to Restore the Tooth Structure
- 1. Cast Metal
- 2. Ceramic
- 3. Metal-Ceramic
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Crown Vs. Amalgam or Composite Restoration
- 1. Destruction of Tooth Structure
- i. If the amount of destruction suffered by the tooth leaves it weak, it is best to have it gain strength and protection through the crown
- 2. Esthetics
- i. If highly visible or the patient is highly critical
- 1) Metal-ceramic = for a single-unit anterior or posterior crown
- 2) All Ceramic = used on incisors or when the patient is ok with more frequent replacement.
- ii. Full veneers - ceramic or metal-ceramic should be used.
- 3. Adequate Plaque Control
- When inadequate, the use of crowns should be cautious
- 4. Financial Considerations - not as affordable as restoration
- 5. Retention
- i. FULL VENEER CROWNS = MOST RETENTIVE!!
- 1) less important for single tooth restorations than fixed partial denture retainers.
- a) Full Metal Crown > 7/8 Crown> 3/4 Crown> MOD Onlay (1/3 Retention of a Full Metal Crown)
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Intracoronal Restoration
most suitable when sufficient tooth structure exists to retain and protect a restoration under the stresses of mastication.
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Glass ionomer
- Leaches fluoride - further protecting the tooth, and can be used as a temporary treatment for mouths with rampant caries that needs to be controlled before restorative treatment.
- Highly retentive.
- Restore gingival abrasion or erosion and root caries.
- Tunnel Lesion Preps in Posterior Teeth proximal lesion
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Glass Ionomer - best indicated when:
- 1. the extension of the caries is low (limited)
- 2. the retention of amalgam or composite would not be likely
- 3. There is recurrent caries present
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Composite Resin
- Minor to moderate lesions in aesthetically critical area
- Mostly Used on Class III and V
- Can treat Class IV lesions, however, ultimately require a crown.
- Mixed results in posterior teeth, due to occlusal wear and the possible polymerization shrinkage if not applied in modest increments.
- Should be restricted to small occlusal and mesio-occlusal restroations on first premolars at most.
- The shrinkage and leakage can sometimes be combated by performing a composite resin inlay.
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Simple Amalgam
- without pins or other means of auxiliary retention
- mild to moderate cavitation in aesthetically non-critical areas
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Metal Inlay
- Mild to moderate lesion
- Low aesthetic requirements
- Prep should be narrow and is usually made of gold.
- Premolars should have one intact marginal ridge to reduce coronal fracture.
- Usually in an MOD configuration
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Ceramic Inlay
- Same idea, but the esthetic requirements are high.
- Because etching must occur with this type of restoration, there is evidence that the structural integrity of the tooth cusps may be stabilized by this bonding.
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MOD Onlay
- Moderately large lesions on premolars and molars with intact facial and lingual surfaces.
- Unlike an inlay, this will accommodate a wide isthmus and up to one missing cusp.
- MOD inlays CANNOT involve cusps.
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If a cast metal restoration is needed on a premolar with both marginal ridges compromised, it should include occlusal coverage to protect the remaining tooth structure. This restoration also can be considered an extracoronal restoration because of the occlusal coverage that overlays and protects the tooth cusps.
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Ceramic onlays without generous occlusal thicknesses are EXTREMELY susceptible to fracture.
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Complex Amalgam
- Moderate to severe lesion - less than half of the coronal structure remains.
- Can be employed as a final restoration when a crown is contraindicated because of limited finances or poor oral hygiene.
- Provides needed strength to the tooth.
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Extracoronal Restorations
- Crowns
- when not enough tooth structure is left or if you need to add tooth structure to improve esthetics or occlusion
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Partial Veneer Crown
- A crown that leaves one or more axial surfaces unveneered
- Can be used to restore a tooth with one or more intact axial surfaces with half
- or more of the coronal tooth structure remaining
- Must be a minimally extended prep and will meet moderate esthetic demands in the maxillary arch.
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Full Metal Crown
- multiple defective axial surfaces - provides max retention
- NO ESTHETIC EXPECTATIONS!! - Usually limited to second molars and mandibular first molars.
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Metal-Ceramic Crown
- multiple defective axial surfaces - provides max retention
- works well cosmetically.
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Ceramic Crown
- Combines full coverage and max esthetics
- NOT RESISTANT TO FRACTURE
-
Ceramic Veneer
- ALL ceramic and metal-ceramic crowns require the removal of LARGE quantities of tooth structure
- Veneers can produce a very cosmetic result on anterior teeth with a minimal tooth loss
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The design of a preparation are governed by
- 1. Preservation of tooth structure
- 2. Retention and resistance
- 3. Structural durability
- 4. Marginal integrity
- 5. Preservation of the periodontium
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Preservation of tooth structure
- Must preserve remaining tooth structure
- Intact surfaces of tooth structure that can be maintained while producing a strong, retentive restoration should be saved if patient acceptance and retention requirements will permit it.
- The rationale for the removal of 1 to 1.5mm of occlusal tooth structure when preparing for an MOD onlay.
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Retention and resistance
- The underlying tooth structure must be in a taper
- Opposing external walls must converge or opposing internal surfaces of tooth structure must diverge occlusally (3 degrees bur -> 6 degrees taper)
- The more parallel the opposing walls, the greater the retention. HOWEVER, 0 degree is impossible to create without generating undercuts.
- On anterior teeth you can have up to a 10 degree taper without loosing retention and a 22 degree on posterior - due to the mechanical forces of cement holding it to the tooth; the larger the surface area, the greater the hold.
- Limit displacement from torquing or twisting forces in a horizontal plane by generating 90 degree angles
- Length of a Preparation and its indication on Retention and Resistance
- - Longer preparations, more surface area, more retentive; the shorter the wall, the more important the inclination is.
- - The walls of shorter prep should have minimal taper, which still may not be enough
- - possible to restore a tooth with short walls if the tooth has a small diameter. 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 - short-walled prep on a large tooth can be improved by placing grooves in the axial walls, which reduces the rotational radius, and that portion of the walls of the grooves near the occlusal surface of the preparation will interfere with displacement
- Structural Durability - bulk of material adequate to withstand the forces of occlusion.
- - Occlusal Reduction, aka Occlusal Clearance, provides this bulk.
- -- Gold alloys: 1.5mm clearance functional cusps, 1.0mm nonfunctional cusps.
- -- Metal-ceramic: 1.5-2.0mm functional cusps; 1.0-1.5mm nonfunctional ones.
- -- all-ceramic crowns: 2.0mm
- - OCCLUSAL REDUCTION SHOULD REPRODUCE BASIC INCLINED PLANES RATHER THAN ONE FLAT PLANE.
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Functional Cusp Bevel
- a wide bevel on the lingual inclines of the maxillary lingual cusps and the buccal inclines of the mandibular bucal cusps - space for adequate bulk of metal in an area of heavy occlusal contact.
- Otherwise, casting may be extremely thin between the occlusal and axial reduction, or remove too much tooth structure.
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Bevels have been advocated as a means of diminishing marginal discrepancy. However, wide, shallow bevels nearly parallel with the outer surface of the tooth should be avoided - overcontouring, or thin, unsupported wax breaks or distorts
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Heavy chamfer
- Provides a 90degree cavosurface angle with a large-radius rounded internal angle
- A lip can be generated with this technique
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Shoulder
- has long been the finish line of choice for the all-ceramic crown
- The wide ledge provides resistance to occlusal forces and minimizes stresses that
- might lead to fracture of the porcelain
- Produces the space for healthy restoration contours and maximum esthetics. Destructs more tooth structure than others
- The sharp, 90degree internal line angle concentrates stress, conducive to coronal fracture
- Generally not used as a finish line for cast metal restorations
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Radial shoulder
- A modified form of shoulder finish line
- The cavosurface angle is 90 degrees, and shoulder width is only slightly lessened by the rounded internal angle
- Stress concentration is less and support for ceramic restoration walls is good. Destruction of tooth structure not significantly less than classic shoulder, however.
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Shoulder with a bevel
- Used in a variety of situations
- - gingival finish line on the proximal box of inlays and onlays
- - occlusal shoulder of onlays and mandibular three-quarter crowns
- - facial finish line of metal-ceramic restorations w/ noncritical gingival esthetics
- - shoulder present w/ destruction by caries or previous restorations
- - extremely short walls by facilitating nearly parallel axial walls
- Possible to create an acute edge of metal at the margin
- Not be used routinely for full veneer restorations because the unnecessary axial reduction
- May afford some resistance against distortion during porcelain firing.
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Knife edge
- Mostly for metal
- Axial reduction may fade out
- Margin of the restoration may be difficult to accurately wax and cast
- More susceptible to distortion under occlusal forces
- Used for lingual surfaces of the mandibular posterior teeth.
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Full Metal Crown
- Used when all axial surfaces of a posterior tooth - decalcified, carious, or previously restored
- Strengthen and support the tooth by tying together the remaining tooth structure
- In order to prep, necessary to remove a significant amount of tooth structure, but not as much as for a full ceramic or a metal-ceramic crown.
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FULL VENEER CROWNS SHOULD NOT BE USED IN THE MOUTHS OF THOSE WITH UNCONTROLLED DENTAL CARIES.
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Metal-Ceramic Crown Preparation
- aka: porcelain-fused-to-metal restoration
- Allows desired esthetics while be stronger and more resistant than pure porcelain.
- Anterior Metal-Ceramic Crowns
- - uniform reduction of approximately 1.2mm over facial surface
- - Incisal should be 2.0 mm deep towards the gingiva.
- Posterior Metal-Ceramic Crowns
- - Only considered for posterior teeth in the "appearance zone": Maxillary premolars and first molars, Mandibular first premolars and sometimes the second
- - Occlusal Reduction = 1.5 to 2.0 mm
- - 1.0 mm shoulder
- - 1.2 for a base metal with a cermic overlay
- - 1.4 for a nobel metal with a ceramic overlay.
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Full Ceramic Crown Preparation
- Maximizes esthetics
- Prone to fracture.
- Incisal grooves = 2.0 mm
- Labial grooves = 1.2 to 1.4 mm.
- Shoulder should be at least 1.0 mm wide.
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Temps should:
- 1 Provide pulpal protection -> no conductance of temp extremes - Margins should be well adapted for prevention of leakage
- 2 Provide positional stability -> no extrusion or drifting
- 3 Provide good occlusal function -> patient comfort, stop tooth migration, and prevent joint or neuromuscular imbalance
- 4 Be easily cleaned -> to keep gingival tissues healthy, less problems with the final restoration
- 5 Have non-impinging margins -> impinging gingiva causes inflammation, proliferation, recession, hemorrhage during impression or cementation; damaging overhang from bad contouring or incorrect trimming; underextended margins.
- 6 Provide strength and retention -> A broken temp can accelerate tooth movement
- 7 Esthetics -> Particularly important on anterior teeth and premolars
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Prefabricated vs. Custom
Prefab can only be used for single-tooth restorations
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Direct vs. Indirect
- Direct: done on the actual prepared teeth in the mouth
- - No alginate or plaster cast
- - Saves time
- - Technique sensitive
- - Better for more experienced dentists and with the use of a resin
- - Remove before complete polymerization to avoid locking into undercuts - shrinks and distorts (8%).
- Indirect: done on a cast made of quick-set plaster
- - Better accuracy
- - Marginal fit can be improved by 70%
- - Good pulpal protection (better with polymethyl)
- - Can delegate work
- - The polymer can create heat, damage the freshly cut dentin, leukocytes aggregate, inflammation
- Better to do indirect and to use a polymethyl methacrylate
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Resins for Provisionals
- No one resin is superior in all respects
- Polymethyl methacrylate - Jet; good marginal fit & transverse strength, polishable, durable; high heat, low abrasion resistance, toxic free monomer, high shrinkage.
- Polyethyl methacrylate - Snap; polishable, low heat & shrinkage, stain resistant; low surface hardness & transverse strength, not durable
- Polyvinyl methacrylate - Trim; polishable, low heat, stain & abrasion resistant, flexible; low surface hardness & transverse strength, not esthetic
- Bis-acryl composite - Protemp II; good marginal fit, abrasion resistance, transverse strength, low heat & shrinkage; low surface hardness, stain resistance, limited shade selection and polishability, brittle
- VLC urethane dimethacrylate - Triad; High surface hardness, abrasion resistance, transverse strength, controllable working time, color stable; low marginal fit, stain resistance, limited shade selection, expensive, brittle
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pBest Provisional Restoration
a custom, indirect restoration.
-
Custom Provisional Restorations
- Custom Indirect restorations are best
- Elastomeric and alginate overimpressions are used to shape the temp
- - Made on the diagnostic cast using a vacuum forming machine or an impression tray with silicone putty
- - Overimpression = an impression made of the tooth before the tooth is prepared. Made of algenate
- - Elastometric = same as overimpression, except material is more expensive & stable
- Fill template with resin and apply to prep or plaster cast of the prep
- Can make thin shell crown of fixed partial denture
- Temps can be made prior to prep appt and relined after the prep is made
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Overimpression-Fabricated Provisional Crown
- Alginate overimpression - For tooth w/ obvious defects, alginate on diagnostic cast after built up with wax
- Alginate of the prepped tooth -> pour up in plaster
- Place resin in the overimpression and seat it on the plaster cast of the prep
- Use acrylic burs or coarse Moore discs to trim the excessive resin
- Cementation
- - Seat and check with articulating paper
- - Polish the restoration with pumice and then polishing compound - Less likely to discolor and easier to clean
- - To fit under an existing fixed partial denture -> undercontour the crown
- - Cement with a temporary cement of moderate strength (zinc-oxide eugenol) - Add petrolatum (5-10% of cement volume) to reduce the strength of the cement; skip petrolatum if prep is short or lacks retention.
-
Template-Fabricated provisional Fixed Partial Denture
- Template-Fabricated VLC Provisional Restoration
- Shell-Fabricated Provisional Restoration
- Overimpression-Fabricated Bis-acryl Composition Crown
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Techniques for Prefabricated Provisional Restorations
- Anterior polycarbonate crown
- M For a single anterior tooth
- M Require extensive alteration
- M Must be carefully contoured to prevent horizontal overhangs
- M Must be relined with a resin
- 1 Take an alginate impression of the prepped tooth and pour up a plaster cast
- 2 Use the mold guide to determine the proper size of the crown form
- 3 Make a mark on the labial surface of the chosen mold guide crown form near the gingival portion - the mark shows the discrepancy in crown length that will be trimmed away with a bur
- 4 Trim crown form down to the correct length
- 5 Place mixed resin in the crown form and seat on the cast (that is coated in alcote) - Can be done on the prep in the mouth if the tooth is non-vital or not using polymethyl
- 6 Place cast in hot water to accelerate setting
- 7 Use a coarse garnet disc to trim the temp
- 8 Check occlusion and polish temp
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Provision Crown for an Endodontically Treated Tooth - Preformed Anatomic Metal Crown
- Emergency cases of fractured molars
- Zinc oxide and eugenol alone will not adhere to the tooth
- Prevents fractured tooth and prevents irritation of the tongue and mucosa
- Procedure consists of:
- - Minimal tooth prep
- - Measurement and selection of crown
- - Trimming and adaptation of gingival margin
- - Occlusal adjustment
- - Cementation
- 1 Prep tooth minimally to create space for the restoration - 1.0mm nonfunctional and 1.5mm functional occlusal reduction; Proximal reduction only enough to seat the crown
- 2 measure the mesiodistal width of the crown to select the proper crown form
- 3 Try crown form on the tooth and place crown form on the stretching block if the gingival margins are too tight
- 4 Remove amount at the gingival margin so that the marginal ridge height is equal to that of adjacent teeth
- 5 Check occlusion and correct proximal contacts by burnishing from inside of crown
- 6 Seat with zinc oxide-eugenol cement - Burnish margins of crown before the cement hardens
-
The impression materials most frequently used for cast restorations are elasticwhen removed from the mouth.
-
An impression for a cast restoration should meet the following requirements:
- 1. Exact duplication of the prepared tooth
- - including all of the preparation and enough uncut tooth surface beyond the preparation - location and configuration of the finish line.
- 2. Exact duplication of the Surrounding teeth
- 3. Free of bubbles - especially the finish line and occlusal surfaces
-
Impression Materials
reversible hydrocolloid, polysulfide, condensation silicone, polyvinyl siloxane, and polyether.
-
The choice is based on:
- 1. personal preference
- 2. ease of manipulation
- 3. economics
- Accuracy is not a consideration in the choice among these materials because there are no clinically significant differences.
-
Wettability
wettable by gypsum (hydrophilic) vs. resistant to wetting (hydrophobic)
-
Irreversible hydrocolloid (alginate), reversible hydrocolloid, and polyether are hydrophilic and the easiest to pour. Polysulfide, polyvinyl siloxanes, and condensation reaction silicones are the most hydrophobic, in ascending order, as indicated by their high contact angles. The more hydrophobic, the greater the contact angle, the greater the probability of air entrapment during pouring.
-
Shear Rate
the speed at which a liquid flows under external forces.
-
shear thinning
- Materials exhibit lower viscosity when the shear rate increases, when expressed through a syringe.
- This is why a single viscosity "monophasic" material can be placed in a tray, where "false body" (a higher apparent standing consistency) permits the material to stay in the tray without sagging or dripping, and yet the same material still can have sufficiently high fluidity (low viscosity) to be used in a syringe.
- A material with such property - thixotropic
- The force required of the dentist is dependent on both the material and the syringe used.
-
Reversible hydrocolloid (agar)
- Impt for gold restorations.
- Begins in a semi-solid gel
- Liquefied in a hydrocolloid conditioner in boiling water
- Cooled in two stages: storage and tempering. tempering also helps to increase the viscosity of the material in the tray so that it is more easily managed.
- After the tray filled with tempered sol is placed in the mouth, cool tap water is circulated through the double-walled jacket of the tray to complete the gelation process.
- The material begins to gel near the cool tray first, spreading to the material adjacent to the oral tissues.
- Rapid cooling by excessively cold water can result in stress concentrations near the tray with possible distortion of the impression.
- The temperature should be 65°F to 70°F.
- When the material is completely gelled, the impression is removed from
- the mouth and is ready to be poured.
- Hydrocolloid is approximately 85% water, critical to the impression's accuracy.
- Syneresis = water seeping from the surface
- Imbibation= absorb water (if placed in contact with it).
- The agar in hydrocolloid is a polysaccharide (a sulfuric ester of a linear polymer of galactose), which is obtained from seaweed. Sodium tetraborate increases the strength of the gel and the viscosity of the sol.
-
Three Baths for an Ideal Pour-up:
- 1. Liquefying bath: 10 min at 212°F
- 2. Storage bath: 150°F for a minimum of 10 minutes
- 3. Tempering bath: Loaded impression trays are tempered in this bath at 110°F to 115°F for 5 to 10 minutes immediately before placing them in the mouth. Heavier-bodied materials require less time, and a lower temperature requires less time. The tray should remain in the tempering bath for no less than 3 minutes.
-
Custom resin trays
- Utilized in elastomeric impression techniques
- more accurate in uniform, thin layers of 2 to 3 mm
-
Polysulfide
- Rubber base
- Much greater dimensional stability than hydrocolloid.
- Contracts during curing
- Should be poured within approximately 1 hour of the impression to obtain the maximum accuracy
- Hydrophobic - no moisture on the preparation when the impression is made; especially important by the margin, where there is the potential for bleeding.
-
Condensation-reaction silicones
- Named for the nature of their polymerization reaction.
- The reactor, a viscous liquid, consists of a crosslinking agent, ethyl silicate, with an organo-tin activator, tin octoate. When the two are mixed, the materials are
- crosslinked by a reaction between terminal hydroxyl groups on the polymer and ethyl orthosilicate.
- The condensation reaction occurs by the elimination of ethyl or methyl alcohol.
- The evaporation of this alcohol is believed to be responsible for shrinkage of the material and results in poor dimensional stability.
-
Polyvinyl siloxane silicone
- Dimensional stability much better than condensation silicone.
- Least affected by pouring delays, or by second pours - used for separate die and cast technique; accurate, even 1 week after impression.
-
Polyether:
- Elastomeric impression material that has become popular
- Hydrophilic
- Accuracy is on par with, or superior to others - excellent dimensional stability for delayed pouring
-
Requirements for good casts:
- 1. Bubble free, especially along the finish lines of the prepared teeth.
- 2. Distortion free.
- 3. Trimmed to insure access for carving wax pattern margins.
-
Working Cast With a Separate Die
- The simplest means of fabricating a working cast and die - no procedures are required to create a die other than making a sectional cast and a full-arch cast.
- Keeps the relationship between abutments fixed and immovable - accurately orienting the preparation models to each other, an important step in minimizing casting adjustments; no improper seating
- Easier to obtain physiologically harmonious restoration contours when fabricating the wax pattern, due to the intact gingival tissue and other landmarks are intact, unlike the other dies that are ditched in order to allow for reconstruction.
- The wax pattern must be transferred from one to the other - Inexperienced technicians are prone to do this more often than necessary, and in the process they destroy some of the internal adaptation of the wax pattern.
- The working cast and the sectional cast for the die can be obtained from separate impressions or by pouring an elastomeric full-arch impression twice.
- - If a double pour is utilized, the first cast is used for fabrication of the die.
- - Can be used only with elastomeric impressions, since hydrocolloid is torn and distorted too much to be used for an accurate second pour.
- Must use high-strength type IV (class II, "Densite") or high-strength, high-expansion type V stones for fabricating the die.
- The wettability or pourability can be improved by using a surfactant
- The surface should be free of visible water, but it should still be shiny. If the surface of the hydrocolloid appears dull, it has been overdried and some distortion may occur.
-
Die and Cast
- The handle of the die should be Parallel to the long axis of the tooth and octoganol shaped in cross section.
- Trimming should be done APICAL to the finish line.
- If the die is ditched in an undercut, it is very problematic and can produce a restoration that will damage the gingiva
- After the die has been trimmed, the finish line should be highlighted with a sharp Colorbrite red pencil
- Relief applied to the preparation area of the die to provide space for cement .
- - The thickness of the overall relief varies with:
- -- the number of coats applied
- -- the brand used
- -- the care with which it is applied - As a bottle of enamel or lacquer ages, the contents thickens due to evaporation.
- Full veneer crowns with grooves will seat more completely if a spacer is used, whether or not it is actually placed in the grooves.
- Thicker relief agents tend to pool in the ends and corners of grooves.
- Die hardening agent cyanoacrylate or acrylic resin lacquer) can be applied to the finish line area of a die to prevent abrasion by waxing instruments during the fabrication of the wax pattern. However, the coating material should be used with care. It must have a low viscosity, and it must be applied lightly.
-
Working with a cast with a Removable Die
- convenient to use because wax patterns or copings need not be removed from their respective dies when they are transferred to the working cast (less distortion)
- - particularly important when making ceramic restorations, as the unfired material is quite fragile.
- - Also, there can be discrepencies between the dies, and because of this, the separate dies can not fit the same.
- The principal disadvantage of a removable die system is not seating the die back properly.
- If a removable die system is used, it should satisfy these requirements:
- 1. The dies must return to their exact original positions.
- 2. The dies must remain stable, even when inverted.
- 3. The cast containing the dies must be easy to mount on an articulator.
-
Die systems:
- Single dowels are simple to use, but not as much antirotational resistance as double dowels.
- Two separate dowels also can be cemented into parallel pin holes drilled in the underside of a cast, using a special drill press
- 1. Straight dowel pin
- 2. Curved dowel pin
- 3. Pindex system
- 4. Di-Lok tray
-
Straight Dowel Pin
- Most of the dowel systems are modifications of it.
- - The brass dowel pin is one of the most accurate dowel types in terms of resisting horizontal displacement
- - the second lowest in vertical deviation of four types of removable dies.
- - A dowel pin is positioned over each prepared tooth in the impression.
- DOWEL PLACEMENT CAN BE A PROBLEM!! If the dowel pins are positioned inaccurately, they may:
- - impinge on the margins
- - weaken the die
- - prevent the die from being easily removed from the cast
- The pin should parallel the long axis of the preparation, and it must not touch the impression.
- Easier if the teeth DISTAL are removed.
-
Curved Dowel Pin
- Fixing the dowels to the impression before it is poured
- Or cementing the dowels into holes drilled in a previously poured cast.
- The tail of the dowel normally points facially. However, if a tooth is in linguoversion, reverse the direction of the dowel for easier removal.
- The dowel should not touch the impression, and its head should approximately parallel the long axis of the tooth.
- Repeat this procedure for all prepared teeth and any pontic areas. If the restoration is a fixed partial denture, a dowel must also be placed near the center of each segment of unprepared teeth. This will allow removal of these segments of
- the stone cast to provide better access to the gingival margins of the retainer wax patterns.
- To place the dowels after the cast has been made, pour the impression with die stone to form a horseshoeshaped working cast. Trim the bottom of the cast flat on a model trimmer to a level no more than 10 mm from the necks of the teeth. Thin casts require less sawing, and short dies are more stable than longer ones. Drill a 5.0mm deep hole in the bottom of the cast directly under the center of each prepared tooth, pontic area, and segment containing unprepared teeth. This can be done with the pindex system.
-
Di-Lok Tray
- A snap-apart plastic tray with internal orienting grooves and notches also can be used to reassemble the working cast and die.
- Have the least vertical error
- With a saw frame and a thin saw blade, cut between the prepared tooth and the adjacent tooth
- - cut start in the interdental papilla area and extend downward on a very slight taper.
- - The die will be slightly wider mesiodistally at its base than at the gingival finish line of the prepared tooth.
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