Fixed Prosthodontics Exam 2
What is the indication for Full Metal Crown? (FMC)
- First and second molar - Heavy grinder and bruxer patients - History of breaking PFM or ACC - Patient request
Bite registration steps
-A bite registration is made intraorally with a fast setting PVS material. The registration material is accurate enough to capture all the surface detail. -An impression is made of the teeth and poured into gypsum. Since there is some dimensional change with the impression material and the stone, the casts are not a perfect replica of the teeth. -The registration material will not fully seat on the casts because the material is more accurate than the cast in the embrasure areas. -The registration material is trimmed. -Be sure to trim away any areas that have "tooth to tooth" contacts. The registration material will prevent the two casts from coming into contact. -Once trimmed, the registration will seat fully on the cusp tips.
Wax up
-A full contour wax up is performed if fabricating a full metal crown. A cut back of the wax is performed from the full contour wax up when fabricating a metal coping for a PFM crown. -A crown preparation with the proper taper will allow for the wax pattern to be taken off the die without distorting or breaking.
Properly Trimmed die
-A properly trimmed die will mimic the root form of the prepared tooth. This facilitates the wax up of the crown by allowing the waxing instruments access to the margin. The margin is marked with a red pencil to clearly define the junction of the crown and the tooth. -The die is marked with a red pencil so that the margin is clearly defined during the wax up phase.
Needed to mount a maxillary cast
-Facebow, (mandibular= MI record) -
How to adjust crown with TT after recieved from lab
-First adjust inter-proximal contacts to get seated first -To see if crown all the way seated take BWX, then occlusion -check articulating paper- check that its like adjacent teeth -then check with lateral contacts, see the lines, first with red (tap tap), then use black (tap tap and use lateral), adjust lines, dont want to see the lines
Why do we get bubbles?
-Improper mixing, look at times and how long for vacuum The guesswork of repeatedly adding water and powder to achieve the proper consistency must be avoided, because this yields a lower strength and it may cause distortion, one of the main causes of inaccuracy in the use of gypsum products. The powder is incorporated during approximately 15 sec of mixing with a hand spatula, followed by 20 to 30 sec of mechanical mixing under vacuum. In this way, a properly mixed stone results in a solid cast. The strength and hardness obtained in such mechanical vacuum mixing usually exceed that obtained by 1 min of hand mixing.
Regisil vs. take 1 advanced
-Regisil has a longer working time (1m 10s), but is not as rigid. Do not confuse this material with the medium body PVS as it is also purple in color. This bite registration material can be used to make the template for provisionals instead of medium body PVS, since it sets faster. Flimsier and more rubbery- can be distorted. -> used more for face bow recored -Take 1, sets up extremely firm, but has a short working time. This registration material is to be reserved for simple straightforward cases such as single unit crowns because of its quick setting time. Bite records for cross arch situations are to be taken with Regisil. -> used more for bite registration
What to do if patient wants all porcelain but is a bruxer?
-do the porcelain, but tell the patient to wear a night gaurd -use same material on opposing arches, porcelain can wear metal faster bc metal is softer
High strength low expansion dental stone
-mostly used for dies because it has low expansion and high strength
DISSOLUTION-PRECIPITATION THEORY:
1. When the hemihydrate is mixed with water, a suspension is formed that is fluid and workable. 2. The hemihydrate dissolves until it forms a saturated solution. 3. This saturated hemihydrate solution, supersaturated in dihydrate, precipitates out dihydrate. 4. As the dihydrate precipitates, the solution is no longer saturated with the hemihydrate, so it continues to dissolve. Dissolution of the hemihydrate and precipitation of the dihydrate proceeds as either new crystals form or further growth occurs on the crystals already present. The reaction is continuous and continues until no further dihydrate precipitates out of solution.
A "rock" or tipping will exists between two casts when
A "rock" or tipping will exists between two casts when not enough teeth are present to "tripodize" them. In order to stablize the two casts together a "record base" needs to be fabricated on the master casts to provide a platform for a bite registration to be made intraorally.
Excalibur
A Type IV high strength-low expansion die stone, Excalibur, is used for crown and bridge cases at ASDOH.
A record base
A record base can be made out of pink TRIAD (UDMA). A wax rim is added to the base to allow for quick adjustment of the height during the record appointment. 2-3 mm of space is created between the rim and teeth. V-notches are placed in the wax, so that the registration can be placed back on in the exact orientation every time.
■ Phosphate:
Above 650 C ● Increased in metal ceramic prostheses necessitates use of higher melting gold alloys that do not cast well into gypsum ● These are more difficult to remove but provide satisfactory results
After firing in the laboratory -phases
After firing in the laboratory, dental ceramics consist of about 15-25% volume tetragonal leucite crystals dispersed in a glassy matrix ○ Glassy amorphous phase ■ Has properties of typical glass ■ Low toughness and strength but high translucency ● Similar to enamel ■ The structure of the glassy matrix is a random Si-O network ● Silicon atom combines with four oxygen in a tetrahedral configuration ○ Crystalline phase ■ Crystalline leucite ■ Makes up 15-25% volume ■ Provides strength to ceramic ■ Highly ordered
An investment material is primarily composed of
An investment material is primarily composed of a binder and the refractory material. The binder holds the ingredients together and provide rigidity to the material. The three types of binders are gypsum, phosphate, and ethyl silicate. The refractory material is composed of silica (quartz and cristobalite), and its function is to regulate thermal expansion of the investment material.
An investment must fulfill three important requirements:
An investment must fulfill three important requirements: 1. It must reproduce precisely the detailed form of the wax pattern. 2. It must provide sufficient strength to withstand the heat of burnout and the actual casting of the molten metal. 3. It must expand sufficiently to compensate for the solidification shrinkage of the alloy. Binder - provides rigidity -> Gypsum, phosphate or ethyl silicate Refractory - regulates thermal expansion -> Silica
PFM noble vs. base metal
Because of lower cost, base metal alloys are used for metal-ceramic restorations ○ These alloy systems undergo continuous oxide formation ○ Because extent of oxide formation cannot be easily controlled, there is a potential for failure through thick oxide layers ■ Too thin: not thick enough to create the bond ■ Too thick: weak bond created
Gypsum:
Below 650 C ● Represent the type used for casting of gold inlays, onlays, crowns, and FPD ● Limiting to high gold containing alloys with melting ranges below 1000 C ● Rapid growth in ceramic prostheses has resulting in decrease in gypsum investment material
Body vs incisal Porcelain
Body Porcelain -Is fired onto the opaque layer -It provides some translucency and contains metallic oxides that aid in the shade matching. Incisal Porcelain -Is usually translucent -The final shade and color is influenced by the color of the underlying body porcelain -> if it is wrong color, send back to lab, dont add external staining, remove part of crown, re-do it and add correct stain to the porcelain -take picture of tooth adjacent with the shade tab and email to lab
Die spacer
Die spacer is added onto the die to create space for cement upon delivery of the crown. The recommended thickness for die spacer is 20 to 40 µm. The taper and type of cement will influence the ideal amount of die space.
Facebow vs. triple tray
Facebow= more accurate hinge axis Triple tray only has 1 side-> limits ability to know whats happening when we move into lateral excursion ○ Therefore, a disadvantage of a triple tray is that there is a different arc of movement and therefore it is not accurately depicted, increasing adjustments and chair time ○ Since there is not contralateral canine, the disclusion pattern of the left laterotrusive movement is guided by the second molar ■ In the patient's mouth, the left canine would have separated the posterior teeth on the right side ■ Movement on the triple tray does not replicate what the patient's mouth does -You get lingual with facebow, -Lose right lateral excursion with tripletray -more interferences when you use triple tray-> may cause PDL problems later on, you would see interferences with FB and you could correct that in the lab -dont use triple tray for anteriors
PREPARATION ideal reductons
Grind tooth circumferentially and on top ○ Functional cusp: 1.5 mm ■ Porcelain coverage ○ Central groove: 1.5 mm ■ Porcelain coverage ○ Non-functional cusp: 1.5 mm ■ Porcelain coverage ○ Facial axial reduction: 1.5 mm ○ Lingual axial reduction: 1.0 mm ○ Path of withdrawal: parallel to long axis ○ Total occlusal convergence: 10 degrees ○ Chamfer width: 0.5 mm ○ Shoulder width: 1.2 mm ○ chamfer/ shoulder placement: 0.5 mm from GM ○ Interproximal clearance: 0.5 mm from adjacent tooth ○ Margin uniformity: no irregularities ○ Rounding of line angles: no sharp angles ○ Overall smoothness: no deep cuts or scratches
If the bite registration is not trimmed,
If the bite registration is not trimmed, it will not seat fully on the stone cast. Remember, the bite registration is made on the teeth in the mouth, and then seated onto the casts. The dimensional changes in the material (ex: pvs, alginate and stone) is enough to cause a discrepancy.
Where should the margin end?
In the simulation clinic, the margin should be kept 0.5 mm above the free gingival margin. This is to protect the gingival margin from being nicked by the bur. In the clinic, the margin placement will vary based on the clinical situation.
Hot spot
Left: A hot spot may retain a localized pool of molten metal after other areas of the casting have solidified. This in turn creates a shrinkage void, or suck-back porosity. Right: A complete casting of a restoration.
Metals are classified by their
Metals are classified by their noble metal content. For the purposes of dental classification of noble metals, Silver (Ag) is not considered a noble metal.
Most porcelains have coefficents of thermal expression between
Most porcelains have coefficents of thermal expression between 13 and 14 x 10^-6 ■ The difference of 0.5 x 10^-6 in thermal contraction between metal and porcelain causes the metal to contract slightly more than the ceramic during cooling ● This condition puts porcelain under slight residual compression, which makes it less sensitive to tensile stresses induced by mechanical loading ■ In order to keep porcelain under compression, you want CTE to be slightly less than metal ■ CTE= how much it expands when heated
Three mesh pattern castings represent
Three mesh pattern castings represent the range of various alloys in a castability study. The higher noble content alloys result in crowns that have better adaptation to the die/preparation. Better adaptation to the preparation is advantageous because there is less reliance on the cement to hold the crown onto the tooth.
Repeatable positions
ONLY CR, not protrusive record
what are the 3 reference points
Orients in three dimensions: ■ Ears ■ Teeth ■ Nasion
PURPOSE of the facebow
PURPOSE of the facebow is to orient the maxillary cast relative to the hinge axis, need for both mandibular and maxillary prep ->facebow shows relationship of maxilla to TMJ -use MI/CR record with bite registration to mount Mandibular -Use when patient has group function or multiple units -do not use when patient has good occlusion, single unit with stable teeth on either side, and canine guidance
Feldspar use
Pigments, opacifiers, and glasses are added to control the fusion temperature, sintering temperature, thermal contraction coefficient, and solubility ○ Feldspars used for dental porcelains are relatively pure and colorless ○ Pigments must be added to produce hues of natural teeth or the colors of tooth-colored restorative materials in adjacent teeth
Plaster Type i/II vs Stone type III/IV
PlasterI/II: less strong, more space, less organized. -β-hemihydrate -Large particles -Irregularly shaped Type I and II stones are produced with larger and irregularly shaped ß-hemihydrate. Since there is more space between the particles a larger water to powder ratiois required when mixing. On the other hand, Type III to V have smaller and regularly shaped particles allow for crystals to be tightly packed. This increases thestrength of the stone and does not require as much water when mixing. Stone III/IV: more organized, stronger used more for fabricating prosthetics, stone has more surface area -α-hemihydrate -Small particles -Prism/rod shaped
PFM Secondary plane of reduction
Secondary plane of reduction ○ Required at the occlusal third of the lingual axial wall in order to maintain uniform reduction ○ Use contours of the adjacent teeth as a guide to orient the angulation of the bur ■ Much shallower than functional cusp bevel
Chamfer
Since the lingual aspect of a crown is not in the esthetic zone, it is acceptable to have a metal margin in that area. The reduction for a metal margin is more conservative (0.5 mm) than a procelain margin (1.2 mm) because metal has superior strength. A porcelain margin requires greater material thickness to prevent it from fracturing. In addition, the marginal seal of a casted metal junction is superior to that of a porcelain margin.
SPRUE:
Spruing involves the flow of molten metal straight from the casting crucible to the pattern area in the ring. The sprue must be attached to the thickest portion of the wax pattern. It is essential to take advantage of centrifugal and gravitational forces by positioning the wax patterns so the alloy is cast toward thinner sections, such as the margins. Therefore, position the waxed restorations so the margins of the patterns face the trailing edge in the casting ring.
Studies show that alloys will higher noble metal content
Studies show that alloys will higher noble metal content fair better in castability compared to base metal alloys. The advantage of base metal alloys is their low cost and rigidity in long span bridges/frameworks.
METAL:
The amount of noble metal content affects the chemical and mechanical properties of the alloy. The higher the noble content will result in the following properties when compared to base metal alloys: Melting temperature - decrease Shrinkage on cooling - decrease Castability - better Hardness - decreased Corrosion - decrease (more resistant to corrosion)
WATER POWDER RATIO:
The amounts of water and hemihydrate should be gauged accurately by weight. The ratio of the water to the hemihydrate powder is usually expressed as the W/P ratio, or the quotient obtained when the weight (or volume) of the water is divided by the weight of the powder. The W/P ratio is an important factor in determining the physical and chemical properties of the final gypsum product. For example, as the W/P ratio increases, the setting time increases, the strength of the gypsum product decreases, and the setting expansion decreases.
Can't we just set an average value for the degrees of condylar inclination?
The condylar inclination can be set to a shallow angle in order to minimize possibility of lateral interference ■ By setting the angle shallower than the average (45) angle measured clinically, the space between the teeth on the articulator will be larger than the space between the teeth in the mouth ■ A shallower angle would make the crown shorter, help avoid interferences -set in between F and B
The investment material used at ASDOH
The investment material used at ASDOH is a phosphate bonded investment material.
DIE TRIM:
The pindex system prepares the master cast for crown fabrication. The advantage of the pindex system is that the die can be removed individually for ease of waxing. The interproximal regions and margins can be easily accessed and waxed with the die removed from the base. The die and wax up can then be placed back onto the base to wax up the crown to the proper interproximal and occlusal contacts.
PFM Refine
The preparation is refined with the fine grit diamond burs. The red stripped 8847KR-018 and 8856-016 are used to blend and smooth line angles, axial walls, and margins. Sharp points are to be avoided to make the subsequent crown fabrication steps easier. Sharp areas are commonly found at the marginal ridges and functional cusp bevel. Lower the speed of the handpiece to increase tactile control and to avoid over reducing during the refining step.
BITE REGISTRATION:
The purpose of the bite registration is to assist in mounting the maxillary and mandibular casts together. This is also known as a "jaw relation record" because it helps to relate the jaws to each other accurately. In situations where MIP is stable, the most accurate way to mount is to hand articulate the casts together. The bite registration helps to verify an accurate mounting, but the case can still able to be mounted without the bite registration. However when MIP is not achievable (ex: flat worn teeth, unable to tripodize due to missing teeth), a bite registration is needed to mount the cast accurately.
CASTABILITY:
To achieve accurate details in a cast framework or prosthesis, the molten metal must be able to wet the investment mold material very well (demonstrated by a sufficiently low contact angle) and flow into the most intricate regions of the mold without any appreciable interaction with the investment and without forming porosity within the surface or subsurface regions. The castability of some base metals is extremely challenging in this regard, because these alloys tend to readily form oxides or interact chemically with the mold wall during the casting process. In addition, these cast alloys tend to be more difficult to separate from the casting investment after cooling to room temperature.
Mount
Turn the impression into stone and put it on an instrument that simulates the movements of the mouth -Mount w facebow (instrument to take relationship bw maxillary arch and TMJ)
Type V gypsum is used
Type V gypsum is used in conjunction with base metals in order to offset for the higher shrinkage found in base metals.
When casting multiple restorations,
When casting multiple restorations, a runner bar is added to act as a reservoir for molten metal. The reservoir should be placed in the center of the ring, since that is where the heat center is located. All castings contain porosity in the area that solidifies last. Therefore, correct placement of the wax pattern will ensure that the porosity will exist in the reservoir and not on the crown.
When fractures DO occur due to unsupported porcelain, they are typically found at the
When fractures DO occur due to unsupported porcelain, they are typically found at the marginal ridges for posterior teeth and at the corners of the incisal edge for anterior teeth ■ Occurs when cutback was not done, leading to uneven thickness
Facebow
a caliper-like instrument used to record the spatial relationship of the maxillary arch to some anatomic reference point or points and then transfer this relationship to an articulator; it orients the dental cast in the same relationship to the opening axis of the articulator.
What type of stone is the cast made of?
`Gypsum: more commonly known as "dental stone" ○ Classified class I to class V based on strength ○ Expands on setting ○ Type 1: 4.0 MPa (mounting plaster) ○ Type 2: 9.0 MPa (model plaster), rarely used ○ Type 3: 20.0 MPa- fabricating diagnostic and complete denture cast ■ Used to fabricate dentures ○ Type 4: 35.0 MPa ○ Type 5: 35.0 MPa ■ Type 4 and 5 further classified by Setting Expansion Range (%) ● Type 4 is low expansion, noble and high noble metal and PFM ● Type 5 is high expansion, use when metal shrinks a lot , base metal alloy or chrome novote alloy ○ Both used to fabricate crowns
Bite registration
put on prep with out the provisional on the tooth -Very accurate
why use regiseal
setting of regiseal is longer working time than take 1
Right lateral excursion
shows difference in path that jaw will move with TT vs. FB for group function -In a right laterotrusive movement, the ML cusp of the maxillary molar will take a different path.
Why do we want a Chamfer for metal and SHOULDER for porcelain margins?
• Metal can be used in non-esthetic areas • Chamfer is more conservative • Metal is stronger than porcelain • Marginal seal is better with metal • Porcelain is strongest under compression • Porcelain needs more reduction to hide the metal
Failures ○ Metal ceramic bond fails in one of three locations
■ Highest bond strength leads to failure within the porcelain (c) ● Observed with some alloys that were properly prepared with excellent wetting- a COHESIVE failure ■ Within the oxide layer (B) ● COHESIVE failure ■ Failures occurring at the interface between metal and oxide layer are called ADHESIVE failures (A) ● Metal alloys that are resistant to forming surface oxides ● Base metal alloys commonly exhibit failures within the oxide layer if an excessively thick oxide layer is present -oxide layer too thick=base metal alloy, high noble and noble if leave coping too long in furnace
How to Make a Crown ● Preparation Questions: How much do we need to reduce the tooth? Where should the junction end? How parallel should the walls be? What shape should the junction of the tooth and crown look like?
■ How much do we need to reduce the tooth? -Depends on impression material ■ What shape should the junction of tooth and crown look like? -Flat and rounded margin ■ Where should the junction end? -At, above, below gumline? Depends ■ How parallel should the walls be? -Divergent?
ASDOH steps
■ Select and evaluate closed-mouth tray- patient should be able to close to MIP without interference ■ Apply tray adhesive to tray walls, load both sides ■ Concurrently, remove cord and apply impression materials onto critical areas ■ Place loaded tray into position and have patient close properly- check contralateral side to verify MIP ■ Remove polymerized impression, helping patient open the mouth by applying pressure to the set material or tray border on the non- prepared side
Porcelain technique
■ Technique always produces uneven porcelain thickness with increased potential for material fracture as a result of porcelain's lack of proper support ■ If porcelain thickness is not well controlled, appearance suffers as well because the shade of the final crown depends on porcelain thickness ■ Framework must be carefully designed and shaped ● A cut back from full contour wax up yields most predictable result -need proper thickness (0.3) of metal
Why do we see a difference?
■ They follow a different arch of movement, therefore teeth move across each other in a different pattern ■ From this, we can see that a crown fabricated on a triple tray may lead to an interference when seating the crown in the patient's mouth ● Longer appointment due to adjustments needed chairside ■ In a right laterotrusive movement, the ML cusp of the maxillary molar will take a different path ■ Arc of movement is much larger and at a different angle when a facebow is used
PFM Lingual margin
○ 0.5 mm chamfer ○ Use 6856-016 ○ Bur is 1.0mm at the tip, sink to half depth ○ Avoid J margin
PFM Buccal margin
○ 1.2 mm wide and kept 0.5 mm above free gingival margin ○ Use 6847-018 (green) ○ As long as bur is parallel to the long axis of the tooth, natural taper of bur will prevent any undercuts ○ Tip of bur is used as a measuring tool
PFM Central groove reduction
○ 330 D or carbide ○ Ensures proper reduction ○ Can be extended into marginal ridge ■ Interproximal contact broken with thin tapered bur
PFM Functional cusp bevel
○ 45 degree angle ○ Match with cusp angle of the opposing tooth ○ Line formed from functional cusp bevel and buccal reduction should match with line of functional cusps of adjacent teeth ○ Completed so that the functional cusp has the same angle as the non-reduced teeth
What is Frankfort Horizontal Plane?
○ A plane established by the lowest point on the orbitale and the highest point on the margin of the external auditory meatus ○ The average distance from the nasion to the Frankfort Horizontal Plane is 23 mm ■ The 3rd point of reference allows us to triangulate the measurements so that the maxillary cast can be mounted accurately in all three dimensions ■ This allows us to set the occlusal plane of the maxillary cast relative to the horizon
WHAT IS A TRIPLE TRAY IMPRESSION?
○ A triple tray, or dual arch impression, captures teeth on the prepared arch, opposing arch, and jaw relation record ○ A triple tray impression should have a "show through" in all areas that have tooth to tooth contact ■ If there is not, it is an indication that the patient did not bite down properly and the jaw relation is not correct ■ There should be NO show through over a prepared tooth -if see light going through prep area -> shows under reduced or there was jaw movement ● If there is, indicates under reduced preparation -use for final impression (instead of alginate) for single tooth, and is bounded by natural teeth adjacent if there is a class I relationship and good canine guidance, if not this condition cannot use
How do we find the patient's condylar inclination?
○ By make a protrusive record-> This "protrusive record" can be transferred onto the stone cast to set the condylar inclination. The angle in which the teeth are fully seated on the bite registration is the correct condylar incliniation. ○ Instruct patient to protrude mandible to edge to edge ■ Will cause separation in posterior region where you can place bite reg material ○ Transferring to cast: ■ Unlock hinge and slide cast to edge to edge ■ Put bite reg between the teeth and alter angle so that bite reg fully sits
In what scenarios can we use a triple tray?
○ Class I occlusion, tooth bound, single unit cases that have canine guidance will have the least potential for chairside adjustments when triple tray is used ■ Canine guidance allows for separation of posterior tooth in order to eliminate posterior interferences on the working side ■ Still missing contralateral canine, so there may be some interference on non-working side ○ Key questions: ■ Does the time/ cost savings of triple tray outweigh time/ cost saving of delivery? ■ Will chairside adjustment of the crown be so significant that it will make it unacceptable to esthetics and function?
PFM Buccal cusp reduction
○ Completed by connecting the depth grooves ○ Bring bur up towards to cusp tip in an upward direction to maintain uniform reduction ○ Mimic anatomical form ○ Carry to marginal ridge, short of breaking contact ○ Use 6847KR-018 (green) which is 1.2 mm at the tip and 1.8 mm at the shank ○ Place grooves at developmental grooves ■ Bur should be buried into the central groove until the the tip reaches the depth established by central groove reduction ■ Ensure angle of the bur matches the cusp angle of original anatomy ■ Use adjacent teeth/ cusp as visual guide to ensure proper angulation is achieved
● What are the advantages of triple tray?
○ Cost savings- use less PVS ○ Save time by not having to take opposing impression and bite reg
Does a facebow record affect occlusal contacts in MIP?
○ Distance between point of rotation and contact dictates jaw movement ○ Even though the arc of movement of the mandible is different between the two, the point at which the teeth come into contact is the same ○ The facebow record does NOT impact contact points in MIP ■ Arc of movement comes into play only when the mandible moves away from starting position (MIP) ○ Contact point in MIP is same regardless of whether a facebow is used
PORCELAIN BUTT MARGIN
○ Extension of porcelain onto a shoulder preparation ○ Often used on PFM to prevent visible metal margin ○ Maximizes aesthetics (only reason) ○ More work to seal nicely
For high noble and noble metal alloys, elements having a major role for porcelain adherence are For base metal alloys in which the principal metals are
○ For high noble and noble metal alloys, elements having a major role for porcelain adherence are ■ Iron ■ Tin ■ Indium ■ Gallium ○ For base metal alloys in which the principal metals are nickel and cobalt, ■ Chromium oxidation ■ Titanium oxidation for titanium casting alloys
Unsupported porcelain
○ High rigidity of structure is needed to prevent fracture of porcelain ○ Very little flexibility is sustained by porcelains because of high modulus of elasticity and low tensile strength ○ Only limited elastic deformation of the porcelain (less than .1%) can be tolerated before fracture occurs ○ Sufficient bulk of material is necessary to provide the proper rigidity ○ Minimal metal coping thickness necessary in the occlusal region is 0.3 mm ○ Some technicians attempt to fabricate framework by dipping die into molten wax, obtaining an uneven thickness
Coefficient of thermal expansion
○ High thermal residual stress between the metal and porcelain can lead to failure ○ If metal and ceramic have largely different thermal expansion coefficients, the two materials with contract at different rates during cooling and large thermal residual stresses will form along the metal-ceramic interface ■ If these forces are very high, (either compression or tension) the porcelain will crack or delaminate from the metal ■ Even if these stresses do not cause immediate failure, they can still weaken the bond and lead to delayed failure ■ To avoid these problems, porcelains and alloys are formulated to have adequately matched thermal expression coefficients
Mandibular movement
○ Mandibular movement occurs as a complex series of interrelated three-dimensional rotational and translational activities ○ It is determined by the combined and simultaneous activities of both TMJs ■ Although TMJs cannot function entirely independently of one another, the also rarely function with identical movements ○ Can move: ■ Rotation around horizontal axis ■ Rotation around vertical (frontal) axis ■ Translation
Ceramco firing chart
○ Paste Opaque: 975 ○ Margin Improved: 965 ○ Op/Dentin/Enamels: 960 ○ Natural Glaze: 945 ■ Decrease sintering temperature as you add more layers ■ If you were to increase it, would lose structure/ melt away
PFM Lingual cusp reduction
○ Similar as above ○ Use 6847KR-018 (green) which is 1.2 mm at the tip and 1.8 mm at the shank ○ Place grooves at developmental grooves ■ Bur should be buried into the central groove until the the tip reaches the depth established by central groove reduction ■ Ensure angle of the bur matches the cusp angle of original anatomy ■ Use adjacent teeth/ cusp as visual guide to ensure proper angulation is achieved
How does condylar inclination effect occlusion?
○ Steep condylar inclination leads to greater separation of the posterior teeth as the condyle travels down the articular eminence ○ As a result, the cusp angle can be made taller and steeper
What is condylar inclination?
○ The angle at which the condyle moves away from a horizontal reference plane is called the "condylar guidance angle" ○ Generated by the condyle and articular disc traversing the contour of the glenoid fossa ○ Rate at which it moves inferiorly as the mandible is being protruded depends on the steepness of the articular eminence ■ If the surface is quite steep, the condyle will take a steep, vertically inclined path ■ If it is flatter, the condyle will take a path that is less vertically inclined ○ Angle: made between the Frankfort Horizontal (upper part of articulator) Plane and the slope of the articular eminence
Mutually protected occlusion
○ The masticatory system, a functional unit of the body, is primarily responsible for chewing, speaking, and swallowing ○ The system is made up of bones, joints, ligaments, teeth, and muscles ○ An intricate neurologic system controls, regulates, and coordinates all structural components
Hinge axis- how should maxillary cast be mounted?
○ The maxillary cast should be mounted in the same 3D orientation as the maxilla is related to the hinge axis ■ Hinge axis is the line that intersects between the two TMJs ● How do me mount the maxillary cast in the correct orientation? ○ With the use of a facebow! ○ A facebow is a caliper like device that facilitates the mounting of the maxillary cast onto the articulator in relation to the patient's hinge axis in all three dimensions ○ Distance from condyle to where the teeth are
Sintering
○ The process of compacting/ forming a solid mass of material by heat or pressure without melting to the point of liquefaction ○ As sintering of the particles begins, porcelain particles bond at their points of contact ○ As the temperature is raised, the sintered glass gradually flow to fill up the air spaces ○ Air becomes trapped in the form of voids because the fused mass is too viscous to allow all the air to escape ■ An aid in reduction of porosity in dental porcelain is vacuum firing
Opaque Layer
○ The purpose of the opaque layer is to block out the metal coping and is responsible for the metal ceramic bond ○ Darkness of the metal will affect the value (brightness) if not masked ○ The thickness of opaque layer should be minimized to 0.1 mm ■ Maximizes porcelain for "pretty layers"
ASDOH Lab Submission Protocol -indications -Contradictions
○ Triple tray impression technique (closed-mouth impression technique/ dual arch impression technique) when performed properly in situations in which they are indicated can produce excellent results ■ Technique sensitive procedure ○ Indications: ■ Angle Class I occlusion ■ Mutually protected occlusion ■ Single crown bound by in tact teeth ■ Teeth opposing prepared tooth should have intact occlusal surfaces ■ MI should be achievable without interference ○ Contraindications: ■ Multiple fixed units ■ FPD ■ Survey crowns ■ Implant supported fixed restorations ■ No cases with Class II or III occlusion
PFM Interproximal Reduction
○ Use 856-012 bur to break the contact in the buccal-lingual direction ○ Keep bur parallel to the long axis of the tooth
Does a facebow record affect the occlusion in lateral movements?
○ When using a facebow, there is sufficient clearance between posterior teeth when moving the mandible to the right side ■ In harmony with canine guidance ○ Using the same wax up and impression from the same typodont, there is an interference on the working side from the lingual view with the triple tray impression
Does a facebow record affect the occlusion in lateral movements?
○ Without the facebow, the movement derived from the mounting does not mimic the path that exists in the patient's mouth ○ As a result, a crown that appears to have no lateral interference when checked on the articulator may actually have interferences in the patient's mouth
Feldspar -chemical
● Feldspar is the main raw component of dental porcelains ● In it's mineral state, it is crystalline and opaque ● Chemically designated as: ○ Potassium aluminosilicate (Potash feldspar) ■ Most veneering ceramic are derived from Potash feldspar although some are based on soda or a combination ○ Sodium aluminosilicate (Soda feldspar)
Purpose of articulator -degrees?
● Helps to better replicate movements of the jaw ● Average of humans: 45 degrees
Ethyl Silicate: RPD
● Losing popularity because more complicated and time consuming procedures involved ● Used in construction of base metal partial denture alloys ● Silica gel that reverts to silica on heating
Porcelain Oxide layer
● To establish the chemical bond between metal and porcelain, a controlled oxide layer must be created on the metal surface ● The oxide layer is typically obtained by placing the substructure on a firing tray, and raising the temperature to a specified level that sufficiently exceeds the firing temperature of the porcelain ● Manufacturers incorporate in the casting alloy composition small amounts of certain metals that form OXIDES and contribute chemical bonding to the metal-chemic adherence
