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All Ceramic

Dental Technology

1


Introduction

2

Over the past 40 years metal-ceramic restorations

have become the standard in fixed prosthetics.

Consistent material improvements have resulted in

optimized aesthetics, especially in the cervical

region. PFM crowns are regarded as safe and

clinically proven. They are the standard for

evaluating any innovative restoration methods in

dental prosthetics.


Introduction

3

However, the PFM is limited by its opacity on

aesthetic success. By comparison the all-ceramic

restoration can offer a higher aesthetic potential,

through improved translucency and transparency.


Introduction

4

Ceramic also has a better biocompatibility, less

plaque attaches to ceramic. These advantages have

led to constant research over the past 30 years to

develop a ceramic system which is easy to

manufacture while maintaining optimum strength,

fracture resistance and aesthetics.


Introduction

5

Some of the previous all-ceramic systems designed

were the injectable ceramics or glass of the late 70’s

and 80’s called Dicor. These failed due to their

transparent ‘glass-like’ appearance and poor

fracture resistance. There were the aluminous

crowns conventionally built-up on a platinum matrix,

followed by the infiltrated Alumina ceramics of the

early 80’s called Inceram. These aluminous cores

were made in a special high temperature sintering

furnace of that time.


Introduction

6

In all-ceramic technology today we need to look

at the developments in different types of allceramic

materials and also the techniques

developed for all-ceramic fabrication.


Introduction

7

The 3 main materials over the past 10 to 15 years

proving to be successful are:

1. Pressable ceramics in ingot form

2. Spinell

3. Alumina sintered blocks with

Alumina infiltration

4. Zirconia

5. Veneering porcelains for Alumina and

Zirconia


Introduction

8

The three main fabrication techniques for

all-ceramic restorations are:

1. Press Technology or pressed ceramics

2. CAD/CAM technology or milling

3. Infiltration Process


Materials

9

1. Pressable Ceramics

These are in ingot form and are based on silicates.

Eg; Lithium disilicate glass-ceramic, nano leucite

ceramic or Flouro apatite ceramic. They have

sufficient strength for single units and some ingots

have the strength for 3 unit anterior bridges. Their

translucency is regarded as the best. This material

is stronger than the veneering porcelains (PFMs).


Materials

10

1. Pressable Ceramics (cont’d)

The improved homogeneity and density of the

crystals support natural light scattering through

the material, providing a balanced chameleon

effect. However their main disadvantage has

been the strength and fracture resistence. This

has been overcome to some degree with the

adhesive luting cementation process necessary

for these restorations but they are still not as

strong as Alumina and Zirconia.


Materials

11

2. Spinell

This material is not strong as

Alumina Oxide, and Zirconium

Oxide but is still an alternative

to press ceramics. It comes in

sintered block form and milled

using CAD/CAM technology.

These blocks are used for

anterior crowns only.


3. Alumina

Materials

12

Alumina Oxide(Al 2 O 3 ), is the middle range

material with excellent translucency and stronger

than Spinell and the press ceramics, but not as

strong as Zirconia and has less fracture

resistance. Crown cores and veneers can be

milled from sintered blocks or presintered blocks

where after milling infiltration of alumina can take

place in a special sintering furnace and process.

This is what happens with Procera alumina

cores.


Materials

13

4. Zirconia

Zirconium Oxide (ZrO2) or Yttrium stabilised –

zirconia are the strongest all-ceramic materials in

use. This material is part of the latest technology

with successful clinical trials. It has been used for

medical prosthesis such as hip joints for decades

and is well proven to be biocompatible and strong.

However, it was always a problem how to

maunufacture this hard material, in order to make

crown and bridgework.


Materials

14

4. Zirconia (cont’d)

A way was devised to mill the material in its ‘green

state’ or presintered state where its strength is not

complete thus it is easier to mill. Once the core or

bridge shape has been milled the final high

strength is now able to be acheived. The milled

core is placed in a sintering heat furnace rising to

a temperature of 1350°C for 6 hours. The zirconia

sinters, forming the final 100% density of

zirconium oxide as it goes through a controlled

shrinkage process of 25-30%.


Materials

15

5. Veneering Porcelains for Pressed Ceramic,

Alumina and Zirconia

The milled cores and frameworks plus the pressed

ceramic ingots, Alumina a Zirconia all have

differing coefficients of thermal expansion.

Consequently, like the metal-ceramic crown, the

veneering porcelains have to be developed with

matching coefficients to the appropriate

framework.


Materials

16

5. Veneering Porcelains for Pressed Ceramic,

Alumina and Zirconia (cont’d)

As a result there are many veneering or layering

porcelains to accomodate this new technology.

There are also porcelains designed to be layered

onto titanium frameworks as well. This is why as a

dental ceramist you must be aware of matching the

right porcelains to your associated frameworks.


Fabrication Techniques

17

Press Technology or Pressure-Casting

One of the inherent problems with the conventional

sintering process of building up a metal-ceramic

restoration is the porosity which develops, and the

shrinkage which accompanies firing. This means that

the accuracy of a restoration is a matter of

anticipating the shrinkage. If it is possible to mould

or machine a ceramic to shape, this shrinkage will be

eliminated or reduced. Casting or pressing the

ceramic into a mould is one way to do this.


Fabrication Techniques

18

Press Technology (cont’d)

A wax pattern is prepared and

invested in a phosphate investment

as though metal was going to be cast

into it. Instead, a glass is cast, or

pressure cast into the mould at

about 940°C. The ceramic material is

not as fluid as molten metal, it

becomes like chewing gum. Pressure

is needed to fill the mould with the

heated ceramic ingot.


Fabrication Techniques

19

Press Technology (cont’d)

The resulting ceramic casting will be

an accurate fit once cooled to room

temperature since its coefficient of

expansion is similar to dental alloys.

A more accurate size is achieved than

by the conventional sintering process.

Because the ceramic has been cast

from a temperature where it has

considerable fluidity, it has no particle

boundaries, and little or no porosity.


Fabrication Techniques

20

Press Technology (cont’d)

In the past the ingots were nearly transparent. To

obtain the controlled translucence of a natural

tooth, the old ‘castable’ or press ceramics were

subsequently heat treated to produce a precipitate

of fine crystalline particles. This ceramming

process was carried out at about 1100°C. The

crown produced was a translucent white and shades

were added using surface coats of coloured ceramic

which fired at a lower temperature.


Fabrication Techniques

21

Press Technology (cont’d)

A recent development has resulted

in the ingots already having the

precipitate of fine particles. There are now many

varied colour choices and translucency levels of

ingot to press. Once pressed no other process needs

to be carried out except to trim the restoration,

stain or layer the veneer porcelain over the core and

then glaze.


Fabrication Techniques

22

Press Technology (cont’d)

The strength of this material is greater than the old

aluminous cores as the ceramic is a finer

microstructure and homogeneously sintered in ingot

form. However they are not as strong as the current

processing to produce Alumina oxide cores (which

are milled and specially infiltrated and sintered by

the manufacturers).


Fabrication Techniques

23

Press Technology (cont’d)

Press technology is widely used for veneers and

single anterior restorations. Now special ingots are

designed to press ceramic over zirconia frameworks.

These ceramic ingots have a CTE compatible with

zirconia. Also some ingots have the strength to

produce 3 unit bridges. Furnaces are now designed

to be press furnaces and (veneer porcelain) firing

furnaces.


Fabrication Techniques

24

Press Technology (cont’d)

Press technology is now widely accepted and used

for anterior work. Some doubts in the past have

been raised about the resistance to water

penetration of ceramics which contain the amount of

network modifiers needed to increase the fluidity of

the material for pressing, but these concerns have

been overcome. Also, now with the press

temperatures being much lower, the material has a

greater opalescence and translucency providing

excellent aesthetics.


Fabrication Techniques

25

CAD/CAM Technology or Milling

The problem in CAD/CAM technology has been to

make the manufacturing processes amenable to

normal laboratory processing and to tailor the

expansion coefficient of the Zirconia and Alumina

to be workable.


Fabrication Techniques

26

CAD/CAM Technology or Milling (cont’d)

The basic process is as follows:

The stone model of the patient’s preparation is

scanned and reduced to a series of coordinates.

Upon the profile this generates, the shape of a full

ceramic restoration is designed. The scanned

coordinates, which define the shape of the

restoration, are sent to a laboratory which has

invested the large amount of money in the computer

based machinery.


Fabrication Techniques

27

CAD/CAM Technology or Milling (cont’d)

Manufacture the Scanning model of from the a model Designing high-quality, the framework Milling in The the milled

dimensionally stable and scannable in plaster the CAD unit CAM unit framework


Fabrication Techniques

28

CAD/CAM Technology or Milling (cont’d)

The scanned shape is used to drive milling drill

pieces or laser cutting machinery, which will carve

any shape from a crown to a full denture out of a

solid block of ceramic. This is then returned to the

technician, who applies matching shades and

completes the final fit.


Fabrication Techniques

29

CAD/CAM Technology or Milling (cont’d)

This technique has sufficiently reduced in price

allowing much stronger factory made ceramics to

be used as a core for full ceramic techniques.

There are no problems with accuracy and none

with sintering porosity. The question now is how

many units of restoration the strength of the

ceramic will allow to be made successfully. At

present large span bridges are being made even

up 14 units but the success rates are still being

monitored.


Fabrication Techniques

30

Infiltrated Ceramics

The normal aluminous core ceramic is made from a

sintered glass with a higher softening temperature,

and reinforced with crystalline particles of alumina

(Al 2 O 3 ). Alumina is known to be stronger than silica,

but the firing temperature for sintering alumina

particles is high.


Fabrication Techniques

31

Infiltrated Ceramics (cont’d)

How can more alumina be incorporated

One answer is to obtain a general shaped sintered

alumina core and infiltrate it. The core contains an

amount of porosity, but still has more alumina

than would be the case with a normal sintered

aluminous porcelain. The core is then coated with

a silica ceramic with a lower softening

temperature.


Fabrication Techniques

32

Infiltrated Ceramics (cont’d)

When fired under vacuum, this silica infiltrates the

pores of the alumina. The resulting core has more

alumina and less porosity than a normal aluminous

porcelain core, and is claimed to have flexural test

results twice as strong.

Shade layers are added to the outside of the

infiltration layer.


Fabrication Techniques

33

Infiltrated Ceramics (cont’d)

The purpose of having stronger porcelains is to

allow the full ceramic technique to make

multiple unit restorations successfully. Higher

strength and fracture resistance of the new

ceramics makes all-ceramic restorations a more

viable option for the future.

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