FABRICATION OF THE 3-UNIT METAL-CERMAIC FIXED PARTIAL DENTURE

Andrea D. Jackson, DDS, MS, FACPHoward University College of Dentistry

GOAL:Expose the student to the laboratory fabrication of a posterior three-unit

metal – ceramic fixed partial denture. The student will also be exposed to the

soldering procedure for gold and ceramo - metal alloys and the uses of solder.

OBJECTIVES:The student upon completion of the instruction session and suggested

reading assignments should be able to:

A. Understand the requirements for the wax pattern of a three-unit metal – ceramic fixed partial

denture.

B. Understand the concept of cut-back for porcelain application in the wax pattern

C. Understand the various pontic designs.

D. Understand the indirect spruing procedure and the reason for it.

E. Understand and select the proper investment for metal – ceramic alloys and

the investing procedures

F. Understand the burnout, casting, and divesting procedure and how it differs from gold alloys.

H. Understand the finishing procedure and the preparation of the surface for porcelain application.

I. Understand the reasons for uses of solder

J. Understand the process involved in soldering

K. Understand the difference between pre and post solder

L. Understand the classification and requirements of a solder

Clinical Considerations

Clinical execution of tooth preparation

Finish-line design

Proper axial reduction

Proper occlusal/proximal reduction

Reduction appropriate for selected restorative materials

Clinical procedures necessary for proper laboratory fabrication

Impression making

Placement of retraction cord

Capturing details of preparation

Inter-occlusal registration

Articulation of cast

TERMS/DEFINITIONS

Framework – Fixed partial denture or metal occlusal single unit

Coping – single tooth thimble

Sequence of Procedures and Clinical Considerations:

Tooth Preparation/Design

Combination

Shoulder

Chamfer

Location

Proper Taper

Adequate tooth reduction

parallelism

Metal-ceramic restoration design

Metal-Ceramic Restoration

Ceramic layer

Metal understructure

Framework

Copings

Esthetics

LABORATORY PROCEDURES

Waxing

Wax add-on technique

Full-cast crown

PFM crown

Pontic

Full anatomic wax-up

Specifications for FPD

Tooth #19 – full cast metal retainer

Tooth #20 – PFM pontic

Tooth #21 - PFM

LABORATORY PROCEDURES

Wax cutback of pontic and retainer unit

Well-rounded

No undercuts

No concavities

No sharp corners

No angles

Finished Wax Patterns

No inner surface defects

Smooth, polished outer surfaces

COPING/FRAMEWORK DESIGN

There are four features of importance to be considered when designing the metal coping for a metal-ceramic restoration:

1.Thickness of metal underlying and adjoining the

porcelain

2.Placement of occlusal and proximal contacts

3.Extensions of the area to be veneered for porcelain

4.Design of the facial margin

LABORATORY PROCEDURES

Coping design

Thin porcelain of uniform thickness

Rigid metal

Convex surfaces

Noble vs. base metals

Dental porcelains (which are, chemically

speaking, glasses) resist compressive

loading but tend to succumb to tensile

stress.

Therefore, the metal substructure must

be designed so that any tensile stresses

in the porcelain are minimized.

Ideally, an esthetic restoration should wear at approximately the same

rate as the enamel it replaces (about 10 µm per year5.) In addition, the

restoration should not increase the wear rate of an opposing enamel

surface.

Dental porcelain is more abrasive of enamel than of other restorative

materials (e.g., gold or amalgam, and has been implicated in severe

occlusal wear, particularly when the porcelain is not glazed or highly

polished.

This factor should be considered whenever a metal-ceramic restoration

is being designed.

Restorations with porcelain occlusal surfaces must be planned carefully. Although they are esthetically very acceptable, these restorations have disadvantages, especially wear of the

opposing enamel.

Akram Abdulla

Pencil

Types of Facial Margins

1. Metal margins

1. – unesthetic

2. Porcelain margins on metal collar

1. – over contoured

3. All porcelain margins

1. – most esthetic

To avoid fracture, the thickness of a ceramic veneer must not exceed 2

mm; however, a To avoid fracture, the thickness of a ceramic veneer

must not exceed 2 mm; however, a minimum thickness of 1 mm is

needed for an esthetically pleasing restoration.

Coping Design - Thickness of Metal Thin uniform thickness of porcelain supported by

rigid metal is strongest

Absolute minimum thickness of porcelain is .7mm

Desirable thickness for porcelain is 1mm

Minimum thickness with good esthetics

Always compensate with extra thickness of metal when the tooth preparation allows for more clearance

Maximum restoration strength and longevity is achieved by coping rigidity

Coping design continued

Surface should be convex

Occlusal contact must be 1mm from porcelain-metal junction

Minimum thickness is .3 -.5 mm for noble metals

Higher yield strength base metals copings can be as thin as .2mm

The ultimate goal of 1mm of uniform porcelain thickness will determine thickness of coping

Proximal contacts on anterior teeth should be in porcelain for esthetic reasons

Porcelain opposing natural teeth can cause wear of the opposing teeth-must make patient aware that a restoration may be needed in the future

Full contour wax- up and cut-back

The correct steps in the fabrication of a metal-

ceramic restoration: A, full-contour wax pattern;

B, coping wax pattern cut back; C, porcelain

addition to metal coping.

If the coping pattern (A) is the first step in

fabrication, the porcelain veneer on the final

restoration may have contours that are not

continuous with those of the unveneered coping

(B).

Full contour > > > Cut - Back

METAL – PORCELAIN DESIGN

CUT BACK >>>COPING DESIGN

Alloys

Classification for Dental Casting Alloys

Noble metal content: Au, Pt, Pd

(minimum percentage by weight)

High-noble metal

60% (>40% gold)

Noble metal

25% (no gold requirement)

Predominantly base metal

<25% (no gold requirement)

Au, gold; Pd, palladium; Pt, platinum.

Alloys Used for Fabricating Metal-Ceramic Restorations

High noble

Gold-platinum-palladium

Gold-palladium-silver

Gold-palladium

Noble

Palladium-silver

High palladium

Predominantly base

Nickel-chromium

Nickel-chromium-beryllium

Cobalt-chromium

Dental Casting Alloys

Classifications High-Noble metal content

Noble-noble metal content of 60% or greater. At least 40% of the alloy must be gold.

Noble metals must be at least 25% noble metal.

Predominately Base a noble metal content of less than 25%.

Cost Precious Semi-precious nonprecious

Titanium Alloys

Titanium based alloys have been studied since the late 1970’s as potential casting alloys.

Advantages of titanium:

Excellent bio-compatibility and corrosion resistance

Titanium oxide surface

Low density in comparison to gold or palladium resulting in lighter and possibly less expensive restorations

Titanium Disadvantages

The dental casting of titanium and titanium

alloys poses special problems because of the

high melting point of titanium (1668° C)

Its strong tendency to oxidize and react with

other materials.

Special casting machines needed that provide either a

vacuum environment or an argon atmosphere

must be used.

Dental Casting Alloys

Predominantly base alloys

Noble metal < 25%

Challenge to dentists and technicians

Physical properties

Handling characteristics

Fabrication techniques

Higher fusing elements

higher melting temperature 2300-2600 degrees Fahrenheit

Necessitates use of gas-oxygen torch and phosphate-bonded investment with high-heat burnout

Titanium disadvantages

Selecting a dental laboratory experienced in fabricating these castings is essential, and such dental laboratories are not common in the United States. Further research is needed to optimize the metallurgical structure and casting technology for titanium alloys,

Base Metal Alloys

The potential health problems associated with beryllium- and nickel-containing alloys have led to the development of another alternative base metal alloy system: cobalt-chromium.

The representative Co-Cr alloys have higher hardness than the Ni-Cr alloys suggests that finishing restorations made with the former alloys may be more difficult.

Clinical Case

LABORATORY INSTRUMENTS FOR FABRICATION OF CAST RESTORATIONS

CONSTRUCTION OF WORKING CASTS and die

CONSTRUCTION OF A WAX PATTERN

LABORATORY INSTRUMENTS FOR FABRICATION OF CAST RESTORATIONS

CONSTRUCTION OF WORKING CASTS

CONSTRUCTION OF A WAX PATTERN

LABORATORY PROCEDURES

Pontic Design Modified ridge lap pontic

Other pontic designs

Saddle

Hygienic

Ovate

PONTIC DESIGN

Size and shape of connectors

Posterior Anterior compared to posterior

METAL – CERAMIC RELATIONSHIP

The melting range of the alloy used in the coping must be 170 to

280°C (300 to 500°F) higher than the fusing temperature of the

porcelain applied to it.

The metal coping is an important part of the metal-ceramic

restoration, and one that unfortunately is often overlooked. Its

design can have an important effect on the success or failure of the

restoration.

LABORATORY PROCEDURES

Spruing Indirect (Runner Bar

Technique) Feeder sprues

Transverse bar

Manifold sprues

Direct Main sprues

6 mm from wax pattern to end of ring

6 mm from top of crucible former

Patterns for metal-ceramic fixed partial dentures must be sprued by an indirect method because the alloys used fuse and solidify at much higher temperatures.

Because the ambient air is much colder than the molten metal, the exposed button is likely to solidify while the metal at the center of the ring is still liquid.

This means that the button cannot serve as a reservoir to prevent shrink-spot porosity. Instead, a bulky horizontal runner bar is placed between crucible former and pattern.

runner bar – stabilizes pattern; serves as reservoir for solidification and contraction,

equalizes flow of metal through all parts of mold,

Pattern for a metal-ceramic fixed

partial denture is sprued indirectly.

The feeder sprues and the horizontal

runner are 8 gauge, and the

manifold sprues are 10 gauge.

Molten alloy swirls through the manifold system,

raising the temperature of the surrounding invest-

ment (shaded area).

LABORATORY PROCEDURES

Surrounding wax pattern with material that duplicates its shape and anatomic features

Requirements Reproduce precisely

Sufficient strength to withstand burn-out/casting

Expand to counter solidification shrinkage

Solidification shrinkage of metal: multi-metal shrinks as cooling occurs

Smaller casting as a result

Expansion of Mold (> original pattern) 4 mechanisms

1.Setting expansion (silica particles interfere with forming crystalline structure of gypsum-bonding)

2.Hygroscopic expansion (hydration process)

3.Wax pattern expansion

4.Thermal expansion of the investment

INVESTING

LABORATORY PROCEDURES

Investing De-bubblizer

Reducing surface tension

Liner Cellulose ***

Unrestricted thermal expansion

Ceramic

Mixture 60 grams powder : 9.5 cc liquid

Mix under vacuum (vac-u-spat)

Bench set for one hour after investing

Scrape top of casting ring after setting Allow gases to escape during

burn-out

It may be necessary to paint phosphate-bonded

investment into the wax pattern with a small brush.

LABORATORY PROCEDURES

Phosphate bonded investment

Higher fusing alloys

Stronger & withstands higher temperatures

Used for investing casting alloys with higher temperatures

Silver palladium

Gold platinum

Gypsum bonded investment

fuse <1975 degrees Fahrenheit

Gold alloys

Type I: small inlays

Type II: larger inlays and onlays

Type III: onlays, crowns, short-span FPD

Type IV: long-span FPD; thin veneer crown; RPD

LABORATORY INSTRUMENTS FOR FABRICATION OF CAST RESTORATIONS

CONSTRUCTION OF WORKING CASTS

CONSTRUCTION OF A WAX PATTERN

INVESTING AND CASTING

INVESTED WAX PATTERN

LABORATORY PROCEDURESBURNOUT

Removal of wax pattern to create mold into which molten alloy is placed

Thermal expansion of the mold Investment allowed to harden, Scrape top to allow gasses to escape Place in oven at 600 degrees F for 30 minutes Then to 1300 degrees Fahrenheit for1 hour If longer, investment may start to break down Investment and ring expand to compensate for shrinkage

LABORATORY PROCEDURES

Casting -Crucibles

-Gas-Oxygen systems

-Induction casting machine

-Electric melting machines

Gas-air torches for lower temperatures

As the alloy begins to solidify, the heat around the

manifold (dark shading) keeps it molten longer,

preventing porosity in the bridge.

)>

LABORATORY PROCEDURES

Divesting

Bench cool for 1 hour

Break investment with pointed instrument

Aluminum oxide abrasive to clean casting

Ultrasonic cleaning

Finishing Remove sprue and button

from casting

Use stones, sandpaper, and burs to finish

Sprue immediately adjacent to the

casting is removed with a separating

disc.

LABORATORY PROCEDURES

Polishing

Silicone wheels and cones

Abrasive paste or compound

Common shapes of abrasive stones are: cone (CN), flame (FL), cylinder (CY), barrel (BA), wheel (WH), inverted cone (IC), knife edge (KN), round (RD), round edge (RE).

CASTING DEFECTS

LABORATORY INSTRUMENTS FOR FABRICATION OF CAST RESTORATIONS

CONSTRUCTION OF WORKING CASTS

CONSTRUCTION OF A WAX PATTERN

INVESTING AND CASTING

PREPARATION FOR PORCELAIN APPLICATION – preparing the metal

The veneering area is

prepared with aluminum

oxide stones.

The coping thickness is checked

with an Iwanson thickness gauge.

Do not use any polishing compounds,

as they may contaminate the surface

of the metal to be veneered later.

PREPARING THE METAL

Must be cleaned and uncontaminated from oils from the skin or debris from finishing with steam

The final step in metal preparation is reduction

of the oxide layer on the part of the coping to be

veneered with porcelain by air abrading with

50 μm aluminum oxide.

.

Metal framework finishing

LABORATORY INSTRUMENTS FOR FABRICATION OF CAST RESTORATIONS

CONSTRUCTION OF WORKING CASTS

CONSTRUCTION OF A WAX PATTERN

INVESTING AND CASTING

PORCELAIN APPLICATION

Heat Treatment – Oxidation cycle

Coping is placed in furnace and the temperature raised from 300 – 400 oC

Directions are alloy specific

Removal of hydrogen gases that was incorporated in the casting process

Porcelain buildup - Opaque application

The veneering surface of the coping

is wetted with distilled water or special

liquid recommended by the manufacturer.

Two layers of opaque are baked

individually

Done to mask the metal, supply the base shade, and

creates the initial bond

Application of Dentin porcelain/ Body porcelain

Made with a brush

Tissue is used to absorb water

Over build contour to compensate for shrinkage

A sable brush is used for condensing and final shaping

Cut back and apply Incisal/ enamel porcelain Create translucency and esthetics

Done with a spatula on the inciisal edges and proximal contact areas cutback

Cut back and apply Incisal/ enamel porcelain

Incisal porcelain ready for

firing

Adding small amount to proximal

Contact before firing

Firing / Baking porcelain

Requires more than one bake to make adjustments in the contours

A clean green stone is used for contouring

Finishing Porcelain

Shade modification done prior to final finish if necessary

Auto glaze – glazes itself during firing process

Must avoid over firing

Applied glaze - a low fusing clear porcelain is applied and baked to the surface

Polishing – done on small areas that may have been adjusted in the mouth

May be less destructive to opposing tooth structure

*Finishing the metal is the same as any other metal

LABORATORY INSTRUMENTS FOR FABRICATION OF CAST RESTORATIONS

CONSTRUCTION OF WORKING CASTS

CONSTRUCTION OF A WAX PATTERN

INVESTING AND CASTING

PORCELAIN APPLICATION

Bisque bake try-in

Cementation of definitive restoration

JOINING METALS/ SOLDERING

The addition of filler

Uses: Soldering/joining units of

conventional gold alloy (FPD’s, splints)

Adding proximal contacts

Repairing casting voids

Soldering porcelain fused to metal alloys

Soldering

Classification

Fineness (gold content)

Characteristics

Corrosion resistant

Lower fusing

Non-pitting

Strength

Free-flowing

Welding (melting)

.

Pre and Post Soldering for metal ceramic restorations

Pre-Soldering Done prior to placing the

porcelain

Post – soldering Done after the porcelain is

placed

Wax patterns for metal-ceramic fixed partial dentures are

invested and cast as one unit whenever possible because of the

difficulty encountered in soldering the alloys used for this type of

restoration

Gold Solder Fineness

Refers to parts per thousand of the solder that is gold

Ex:

600 fine solder is 600 parts of gold per 1,000 or

60% gold. (minimum that should be used is 580 fine)

The higher the fineness, the higher the melting range and the more corrosion resistance

A casting alloy is designated by carat (refers to parts per 24

ex: 18K = 75% gold

A solder designated 18K means it should be used with 18K casting alloy

Soldering

Solder should possess a fusion temperature that is about 60 degrees C (100 – 150 F) below that of the metal being soldered

Four- and five-unit fixed partial dentures joined by soldering have better-fitting margins than do one-piece castings of the same length.

Any fixed partial denture larger than three units should still be cast in two pieces and soldered.

The SOLDERING procedureAn index is made in the mouth, the restoration is invested and soldered.

1. Applying anti-flux 2. Heating the area to be joined

3. Adding solder 4. Completed solder joint