3.7 MCRs, PFM and Cantilever bridges

what are examples of metal ceramic restorations (MCR)?

• crowns

• inlays

• veneers

what does MCRs consist of generally?

• metal casting or coping (acts as base for ceramic layering) over tooth preparation

• Ceramic fused to coping

What are the 3 layers of porcelain on metal copings of MCRs?

1. Opaque porcelain - conceals the metal underneath, initiates development of the shade, plays important role in development of bond between ceramic and metal

2. Dentin (body) porcelain - makes up bulk of restoration, providing most of the colour or shade

3. Enamel (incisal) porcelain - imparts translucency to the restoration

other than the main types of porcelains which other porcelains are used within the 3 basic layers of metal copings for special effect and characterisation?

• opaque or dentine modifiers

• porcelain stains

• porcelains of varying translucencies

what are the types of alloys used for fabricating MCRs?

• high noble

• noble

• predominately base

what are the high noble alloys used in fabricating MCRs?

• Gold-platinum-palladium

• Gold-palladium-silver

• Gold-palladium

what caution should be taken when using Gold-palladium-silver when making MCRs?

• if silver in dental alloy is more than 5%, technician must finish the restoration only with special porcelain masses otherwise there will be oxides on surface of framework, which will be blue/green

• So must be cleaned very carefully and only this special porcelain mass can be used

what are characteristics of high noble alloys?

• most expensive alloy

• warm colour

• natural look

• good aesthetic (especially in front teeth)

Disadvantages of high noble alloys?

• cost

what are the noble alloys that are used to make MCRs?

• Palladium-Silver

• High palladium

what considerations should be taken, when handling palladium-silver

• if silver in dental alloy is more than 5%, technician must finish the restoration only with special porcelain masses otherwise there will be oxides on surface of framework, which will be blue/green

• So must be cleaned very carefully and only this special porcelain mass can be used

characteristics of noble alloys?

• moderate cost

• adequate aesthetic

disadvantages of noble alloy usage in fabrication of MCRs?

• cost

• aesthetic

what are the predominately base alloys used to make MCRs?

• Nickel-chromium

• Nickel-chromium-beryllium

• Cobalt-chromium

what considerations should be taken when handling Nickel-chromium-beryllium?

• Beryllium is dangerous to the technician, during cleaning and trimming the beryllium can go inside the body of technician

• Berylliosis disease, granuloma in lungs, very dangerous

what are characteristics of predominately base alloys usage in making MCRs?

• Most popular base alloys

• Need to check patient for nickel allergy

disadvantages of predominately base alloys?

• patient safety

• technician safety

• material hardness

what are the 6 features of importance to be considered when designing the metal coping for metal-ceramic restorations?

1. thickness of porcelain veneers

2. Support of porcelain veneer

3. Thickness of metal underlying and adjoining the porcelain

4. Placement of the occlusal and proximal contacts

5. Extent of the area to be veneered for porcelain

6. Design of the facial margin

what considerations should be taken on thickness of porcelain?

• Porcelain at minimum thickness that still maintains good aesthetic

• thin porcelain, uniform thickness, supported by rigid metal is the strongest

• absolute minimum - 0.7mm

• Desirable thickness is 1-1.5mm

• Porcelain extension above 2mm is prone to fracture, even if not in are of force concentration

• Important to check coping thickness using Iwanson thickness gauge

What are the 4 theories made to explain process of porcelain-to-metal bonding?

1. mechanical retention

2. compressive bonding

3. Van der waals forces

4. Direct chemical bonding

What does a PFM (porcelain fused to metal ) involve?

• metal framework covered with opaque ceramic that is then fired

• Dental technician creates a wax up over this structure and fixes the sprue formers.

• After investing and wax elimination, additional ceramics are pressed over the metal and opaque ceramic framework.

what is key difference between PFM and MCRs?

• the additional layer of ceramics in PFM

what is an important consideration to be taken when it comes to contact between tooth and metal ceramic crown in PFMs?

• important for the contact to be on the metal or ceramic part, not on the border

• contact on border can increase risk of fracture

what does PFM consist of?

• metal inner structure and a porcelain cover

what is the inner structure of PFMs usually made of?

• Cr

• Co alloy

• Cr-Ni alloy

• Titanium

advantages of PFMs(porcelain fused to metal)

• affordable option

disadvantages of PFMs

• after a few years of using, the crowns will lose their original brightness. metal may oxidize.

• The extent and speed of oxidation depend on the type of metal and other factors. This causes the roots to have a metallic shine and the gumlines to look bruised, affecting the overall appearance.

• As a result, porcelain-fused-to-metal crowns are not commonly indicated for the restoration of front teeth.

PFMs are not indicated commonly for use on?

restoration of front teeth

advantages of precious metal crowns or golden crowns

• most durable type of crowns

• last from several dozens of years to a lifetime.

• Don't change color at the base so don't affect the natural pinkish color of the gums.

• Have sturdy linings that don't crack or break even if they're extremely thin.

• highly accurate, thus, prevent cavities, chipped teeth, stuck food particles, bacterial buildups, etc.

• Have a naturally beautiful color thanks to the inner golden shine (compared to the normal dull shine of metals).

• high biocompatibility, are comfortable for your natural teeth and gums

• non-toxic

• don't oxidize

• don't have a taste or odor

• Minimize the loss of tooth tissue, resulting in stronger teeth

• anti-inflammatory properties as gold is naturally antibacterial.

disadvantages of Precious metal crowns and golden crowns

• don't have the disadvantage of the aforementioned porcelain-fused-to- metal crowns.

• As they are made of precious materials and require more advanced technique, they are more expensive.

what do Precious metal crowns and golden crowns consist of?

have the inner structure made of precious metals such as gold or platinum

advantages of Zirconia crowns

• pleasant appearance but also stability, durability, and accuracy

• most beautiful and natural-looking crowns

• Don't oxidize - no discoloration at the roots after years of use.

• High biocompatibility, thus are comfortable with real teeth and gums.

• don't have an odour and don't cause allergy.

• Zirconia frame is sturdy and durable. It can withstand large forces and is hard to break

• CAD/CAM technique offers the most accurate crowns, effectively preventing cavities and loose crowns.

what does a pure zirconia crown consist of?

has an inner structure made of Zirconia. They are finished with layers of porcelain

what is a cantilevar bridge?

type of bridge that has an abutment or abutments at one end only, while the other end of the pontic (false tooth) remains unattached.

In general, a cantilever bridge is designed to replace only one tooth and typically has two abutments, although there can be exceptions to this rule.

what is the problem that can occur when using cantilever bridge design?

• this design can potentially be destructive due to the lever arm created by the pontic.

• The lever arm can put excessive stress on the supporting tooth, which may lead to complications or damage over time

where is pontic positioned in cantilever bridge?

pontic, or false tooth, is usually positioned on the mesial side, but there can be variations depending on the specific case.

how is stress minimised in cantilever bridges?

important to ensure that the length of the lever (the unsupported portion of the bridge) is shorter than the support (the abutment tooth or teeth) to minimize stress and potential complications.

what is a issue (not lever arm) faced when using cantilever bridge design?

Complications of cantilever bridge include peridontal abscess.