Dentists and allied dental professionals often seek CE courses from ADA CERP recognized providers to fulfill their CE requirements for re-licensure.  Most state and provincial licensing boards will accept CE credits issued by ADA CERP recognized providers. In the spring of 2003, the FDI World Dental Federation became the first internationally based CE provider to be granted ADA CERP recognition. Please contact your state board directly for their specific rules and regulations. Most states approve supervised self-study courses that are ADA CERP accredited. Those interested in receiving 3 continuing education credits for this course may take the 20 question test at a cost of $39 and receive their certificate immediately by clicking here.

 

Ceramics 1 The basics from pottery to porcelain

Ceramics 2 Glass

Ceramics 3 Dental porcelain 1--traditional porcelain to feldspathic glass

Ceramics 4 Dental Porcelain 2--Glass ceramics

Ceramics 5 Dental porcelain 3--Advanced ceramic cores (this page)

Table of contents (page 5)

• What is a core?

• Aluminous cores

• Ceramic cores

  • No Glass? NO bond!

  • Glass infused core systems

  • Pure alumina cores

  • Zirconium Dioxide (Zirconia) cores

What is a core?

In dental ceramics, a core can be loosely defined as a rigid and durable structure designed to closely fit one or more abutment teeth and used as a framework to support a tooth colored, translucent esthetic veneer.  The majority of core materials are opaque and therefore cannot be used without an esthetic veneer.  This is not true in every case as a few core materials are esthetic enough to stand alone as tooth colored restorations without the addition of a feldspathic veneer.

The first esthetic frameworks were gold crowns and bridges with mechanical retention used to retain an acrylic veneer.  These were invented in the 1940's.  The first true porcelain-bearing core structures were made of cast gold alloys that displayed metallic oxides on their surfaces.  The metal oxides on the surface of the framework made it possible to chemically bond a porcelain veneer directly to the metal.  Metallic frameworks gave support to the otherwise weak porcelain making possible the production of both crowns and multi unit bridges.  The porcelain fused to metal technique was invented in the late 1950's and patented in 1962.  PFM restorations are discussed in detail on my Dental Alloys page. 

The aluminous core

The first all-porcelain core was fabricated in1965.  This was the aluminous core, and it remained the workhorse of all-porcelain anterior crowns from that time until the early 1990's when the first glass ceramic restorations entered the market.  Aluminous cores are fabricated on a refractory die by a dental laboratory technician using the powder condensation technique technique.  The first layers of the frit are compacted tightly against the die since the die tends to draw the water out of the slurry.  The porcelain remains on the die through all subsequent firings and additions of frit.  After the the main body of the crown has been fully fired, the technician switches to feldspathic frits to complete the buccal esthetic veneer.  While the flexural strength of "plain" feldspathic porcelain is around 50-60 MPa (Mega Pascals), that of an aluminous core is  between 120-130MPa.  Even with the increase in strength, aluminous cores still are not strong enough to support multi unit bridges. 

Ceramic cores

No glass?...No bond!!

Porcelain Jacket crowns, Glass ceramic crowns and veneers, and aluminous core crowns are all variations of feldspathic porcelains.  As such, they are made of glass infused with various inclusions and thus may be etched and bonded to enamel and dentin in the same way that composite  restorations are bonded.  However, the restorations in the following sections are built on ceramic cores that do not contain glass.  Therefore, etching these restorations would accomplish nothing.  Resin bonding is not necessary when cementing these restorations to the teeth.  They are cemented using zinc phosphate or glass ionomer cements, the same as the dentist uses for PFM crowns and bridges, although many dentists still etch and bond the tooth structure under the core. 

Glass infused ceramic core systems

Aluminous cores are made by adding alumina to the glass system before the frit-sintering stage.  This method of manufacture limits the addition of alumina to no more than 40-50% by volume.  On the other hand, glass infused ceramic cores are built using pure alumina, spinel or zirconia which is sintered PRIOR to the introduction of the glass.   Thus these cores achieve a much higher proportion of refractory crystalline filler than is possible with traditional aluminous core techniques. 

In-Ceram by Vita was the first high strength alumina core system, achieving approximately 85%  by volume of sintered alumina in its core.  These are fabricated using a slip casting process.  A slip is simply a clay mixed with enough water to make it a creamy texture.  In-Ceram uses a slip made of water mixed with a suspension of finely ground alumina particles.  The slip is used to coat a porous die in the shape of the final coping.  In slip casting, the die is designed to absorb the water in the slip.  This causes the suspended ceramic particles to condense tightly against the die.  The "green" ceramic body is fired on the die at 1120°C for 10 hours.  This temperature is too low to completely fuse the silica, but it produces a sintered framework with a fairly dense structure and little or no shrinkage.  The sintered body by itself is not especially strong, but it has a porous texture and when infused with a low viscosity glass, it creates a thin coping with great strength.  This coping is then overlain with feldspathic dental ceramic to fill out the form of the tooth.  This creates a somewhat opaque restoration that can be used on molars.  The strength of this core material is not quite sufficient to be used as a framework for posterior bridges.  This type of core is known as a glass infused ceramic core.  An In-Ceram all-alumina core's flexural strength is about 500 MPa. 

It should be emphasized that the listed flexural strengths for this, and the other cores shown on this page are for the cores themselves.  The flexural strength of the finished appliance, including both the core and the applied veneer is considerably less due to the inherent low strength of the veneered porcelain.

Vita has created other glass infused core systems replacing the sintered alumina with other oxides and oxide mixtures.  In-Ceram-Spinel (ICS) uses spinel (MgAl2O4) in sintered form to produce a more translucent and esthetic version of its original In-Ceram at the cost of slightly reduced flexural strength (~350MPa).  ICS is indicated for anterior crowns.  In-Ceram--Zirconia (ICZ) uses a mixture of aumina and zirconium oxide crystals to produce a glass infused ceramic that is even stronger than the original In-Ceram (~700MPa).  ICZ is used for posterior crowns and bridges, but not indicated for anterior restorations due to its opacity.

Pure alumina cores

Pure alumina will fuse at between 1600°C to 1700°C, but it will sinter at a much lower temperature.  Procera (Noble Biocare) AllCeram cores were the first CAD-CAM (Computer Assisted Design-Computer Assisted Manufactured) dental substructures made.  A standard die made from an impression taken by a dentist is digitized using a specially designed mechanical scanning device (pictured at left) and a computer that turns the shape of the die into digitized data.  The data is then used to fabricate an oversized die to which 99.9% pure alumina is dry pressed.  The pressed, oversize green body is then removed from the die and sintered, thus shrinking it to the correct size and creating a hard core to which a feldspathic porcelain veneer can be applied. Cores like this are about as strong as In-Ceram-Zirconia (~700MPa), but the coping is said to be more translucent and to give better esthetics.  While they can be used for posterior crowns, posterior bridges are not advised.  Procera Forte is a newer product in which the same mechanical technique is used to scan the the model for fabrication of a milled, sintered zirconia product which is sufficiently strong to be used as a framework for posterior bridges.   Click on the image to see more on the Procera scanner,the computerized images it creates and some of the ceramic substructures fabricated from the process.

 

The technician then proceeds to block out all the undercuts, round any sharp line angles and make any other corrections that will be necessary in order for the finished coping to fully seat on the prep and not place too much pressure on any one area.  This is all done with digital "wax" on the computer monitor.

Once the milling machine has done its work, another technician cuts off the sprues still connecting the finished green coping in the jig.  He then proceeds to thin out the marginal edges using specially designed rubber wheels and a handpiece.

The image below shows seven crown copings and two three unit bridge frameworks, all in a green state.  The milling machine automatically mills the walls of the copings to exactly the correct thickness. This if fairly thin, because of the inherent strength of the zirconia material.  The thinness of the copings leaves more room for the esthetic veneer that will overlay it, making for a more esthetic result.

The substructures you see in the image above are in a "baking" tray waiting to be dipped into the coloring baths you see in the image below.  The coloring of the frameworks takes place before the sintering process according to the prescribed shade.  There are seven possible shades, each corresponding to the approximate color of the normal dentin of a tooth that would be the same shade of the one prescribed by the dentist.