Table of Contents
This is the fifth in a series of pages on dental ceramics. The material presented on each page is designed to stand alone, but a real understanding of this material relies on knowledge presented on the pages that precede it. Terms on this page that will be unfamiliar to the casual reader have been defined there. This series represents a mini course in ceramics for the beginner, and persons seriously interested in gaining a basic working knowledge of dental ceramics are advised to take the time to start at the beginning.
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 first all-porcelain core was fabricated in 1965. 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. 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 for molar crowns or to support multi unit bridges.
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, RGMI 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.
Aluminous cores are made by adding alumina to the glass system before the frit-sintering stage. In other words, the frit is manufactured with the aluminum oxide already present before it is applied by the technician. 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 in a hot kiln PRIOR to the infusion of the glass between the scintered particles. 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.
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 alumina 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 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.
(My thanks to H & O dental lab in Manchester NH for allowing me to tour their plant and obtain these images.) What you see below is a “green” ingot of zirconia which is placed in a cad-cam milling machine to be ground down into whatever shape is programmed.
Lava (3M ESPE), Cercon(Dentsply), Procera Forte (Noble Biocare) and Everest (Kavo) are all made from blocks of partially pre-sintered Yttrium stabilized zirconium dioxide. They can be used to fabricate an incredibly hard ceramic core (over1200MPa). This type of core is eventually overlain with specially formulated felspathic porcelain to fabricate an extremely esthetic restoration. This core material is strong enough to use as a framework for multi unit porcelain bridges. It also has a melting temperature of about 2700°C, so it is never used as a completely fused ceramic. A more complete sintering of the milled core is done at 1500°C for approximately 11 hours. The images presented here demonstrate the fabrication of a Lava zirconia coping.
Owing to the extreme hardness of the fully sintered ceramic, the dental lab works with a highly compressed, partially sintered “green” block of ceramic (image to the right). This is milled using the same method as the Procera All-Ceram cores described above except that the cores are milled substantially oversized. The reason for this is that the greenware will shrink to exactly the correct size necessary to fit the original die during its final sintering firing .
The green ceramic is delivered to the lab in a partially sintered “green” plug, mounted in a jig especially designed to fit the milling machine. The jig is fabricated from the same zirconium oxide as the plug itself. The two barcodes on the jig identify the patient, as well as the lot number of the green zirconium plug. This information tells the milling computer the expected shrinkage of that particular lot in order to calculate the exact size to which the final product must be milled. This is necessary because each lot will have slightly different shrinkage ratios when fired to its final sintered state. The image below shows what a green core looks like when the milling machine is finished with it.
In order to mill the shape needed, the trimmed die is placed in a special “camera” enclosure which consists of a non-contact optical scanning system. This uses a white light triangulation system to create an exact digital replica of the entire surface of the die.
The net result of this exercise is the creation of a cyber-copy of the surface topology of the die. Once digitized, the image can be rotated and manipulated in any number of ways. The red line on the first image below shows where the computer “thinks” the margin of the crown is (computers are dumb!). The thin white line is drawn by the technician to correct the computer’s misconception. The image on the bottom right below shows the prep from the top with the margin correctly positioned.
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 is 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.
After the zirconia core is fully fabricated, a feldspathic porcelain veneer is applied completing the esthetic restoration. The major disadvantage of Zirconia/porcelain restorations is the vast difference between the flexural strengths of the two materials. The zirconia core has a strength of 1000 MPa while the Veneer has a flexural strength of 80-110 MPA. The failure of restorations of this type revolves around the weakness of the porcelain veneer and the low bond strength between the core and the veneer.
The Success of fused zirconium oxide (zirconia) as an exceptionally strong esthetic core used underneath an applied porcelain veneer has lead to the development of all-zirconia appliances without the applied porcelain veneer. The advantage that these appliances have over those with applied porcelain is their tremendous toughness and strength. The vast majority of failures in crowns and bridges involves fracture of the porcelain, either from chipping through the porcelain itself, or in a failure at the porcelain/core interface. All-zirconia restorations bypass these failures entirely. They are amazingly strong, and much less tooth reduction is necessary than for a porcelain fused to gold or porcelain fused to zirconium appliance. They require as little as a half millimeter of clearance as opposed to 2 mm of clearance for a standard PFM or PFZ.
The flexural strength of the typical porcelain veneer used over a metal or zirconia core is in the vicinity of 100 MPa. The strongest cerammed dental porcelain (IPS e.max®) contains lithium disilicate crystals and has a flex strength of 400 MPa. On the other hand, the flexural strength of the typical all-zirconia appliance is approximately 1000-1200 MPa.
For a rather impressive YouTube demonstration showing the strength of an all-zirconia crown, click on the image below:
The esthetics of all-ceramic restorations is a bit more complex to explain. While these appliances can be fabricated using the same shade specifications provided by any major porcelain manufacturer, (and the shade will be accurate), the finished product will lack normal optical characteristics because zirconia is opaque. This problem can be mitigated to a certain extent since the zirconia may be characterized with stains during the manufacturing process, but the ability to match the adjacent teeth is dependent on the wavelength and intensity of the ambient light, which changes whenever the patient moves to different lighting conditions. In low light conditions, they may match adjacent teeth quite well, but when a bright light falls upon them, teeth with these restorations tend to look bright! In my own practice, I have found these appliances to be more acceptable in posterior positions, or when the patient is willing to accept the esthetic liability.