Dental Ceramics Page 4

This is the fourth 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. 

Glass Ceramics

People who have read page 3 of this series know that older feldspathic dental porcelains started out as a form of domestic porcelain in which a refractory ceramic structure supported a vitrified feldspathic glass.  Later,  due to esthetic considerations, the refractory structure was removed producing a more esthetic, but weaker glass structure.  Finally, in the 1960's, the clinical failures experienced with porcelain jacket crowns drove the technology toward replacing the missing refractory structure by adding up to 50% by volume of fine aluminum oxide crystals to the glass recipe before fusing.  This produced the aluminous glass core (see the diagram on the previous page). 

Glass ceramics also contain a substantial refractory crystalline core.  However, they are not like aluminous glass since they start out as a pure glass in which finely dispersed crystalline structures are stimulated to "grow" within the solidified glass matrix by a process of controlled devitrification.  Devitrification means the formation of crystals on or within an amorphous glass, generally due to a prolonged cooling cycle.  The presence of native crystalline inclusions strengthens the glass and makes it more flexible, reducing the presence and severity of micro cracks and acting as crack stoppers,

There are four main advantage to these "home grown" crystals.

The size and distribution of the crystalline substructure within the glass can be precisely controlled, making it possible to fabricate cores and veneers out of the same glass.  The cores are strong and sometimes opaque while the veneer is translucent and esthetic, yet they are made out of the same glass ceramic.  This insures the best integration of the two components with the best combination of strength and esthetics for the finished crown. 

These native crystals are much more compatible with the existing glass chemistry and their complete integration with the glass gel allows for much better translucency than the older aluminous core crowns. In some cases, it is possible to create a porcelain that is strong enough to act as a core, while at the same time remaining esthetic enough to require no additional esthetic veneer.

The crystals formed within the glass lend the finished body various characteristics such as greater thermal expansion and elasticity which can be precisely controlled to suit the purpose of the specific porcelain.

Glass ceramics  are still a form of glass, and thus they can be etched.  This means that they can be bonded directly to tooth structure which improves the strength of the restoration tremendously.

The process of forming crystalline structures in (or on) a glass body is called devitrification.  In general, devitrification within a glass body creates opacity which makes the glass unusable for esthetic purposes unless a veneer of non crystalline feldspathic porcelain is fused on visible surfaces, and indeed, most glass ceramics are cloudy or opaque.  On the other hand, this is less of a problem with some of the glass ceramics, and these can be used without the addition of a feldspathic veneer.  The reasons for this will become apparent later.  The heat treatment that encourages the growth of these native crystals throughout the glass is called ceramming and it is a two step process.

Ceramming

Ceramming is a controlled crystallization (devitrification) of the glass that results in the formation of tiny crystals that are evenly distributed throughout the body of the glass structure.  The size of the crystals, as well at the number and rate of growth is determined by the time and temperature of the ceramming heat treatment

There are two parts to the ceramming process; crystal nucleation and crystal growth.  Each phase  happens because the glass body is held at a specific temperature for a specific length of time. 

Crystal nucleation

Crystals have a tendency to develop in a mixture of glass when it is held at a specific temperature, called the crystal nucleation temperature.  This means that when held at the crystal nucleation temperature, multiple seed crystals begin to grow throughout the glass body.  The longer the glass is held at this temperature, the more seed crystals will form.  Ideally, a glass ceramic will be strongest when there is a very large number of small crystals distributed evenly throughout its mass.  Once a seed crystal forms, it will also begin growing larger at this temperature, but quite slowly.  If the temperature of the glass body is held at the crystal  nucleation temperature for a very long time, a very large number of crystals of widely varying size will form.  The earliest to seed will be the largest while the crystals that have recently just begun to grow will be the smallest.

Crystal growth

In order to better control the esthetics of the finished product, the ideal glass ceramic will have crystals of a small, relatively uniform size.  Any form of devitrification in a glass structure will produce one degree or another of opacity.  Large crystals are more prone to making the glass opaque, while small crystals evenly scattered throughout the structure have less of an impact on the optical qualities of the finished product.  Thus it is of benefit to hold the temperature at the point of maximum seeding for a finite length of time in order to allow numerous tiny seed crystals to nucleate, and then to stop the nucleation process and encourage the ones that have already formed to grow to suitable size. 

Luckily for the dental ceramicists of the world, the ideal temperature range for crystal seeding in glass bodies is different than the ideal temperature range for the crystal growth, so it is possible to control both phases separately and precisely.  It's all in the thermal cycle.  The temperature of the melt is brought to the nucleation temperature and held there for long enough to allow large numbers of seed crystals to form.  Then the temperature is raised to the temperature where nucleation halts and growth of the existing crystals is accelerated.  It is held there for the time required to grow the crystals to the ideal size, and then the temperature is lowered fairly rapidly to the annealing temperature (the temperature where the glass is nearly hard, but the molecules are still mobile enough to move about in the matrix relieving stresses) and finally harden into the finished product.

opacity

Tiny crystals scattered throughout a glass body produce cloudiness and opacity in the same way that tiny water droplets suspended in the air produce fog.  This happen for two reasons:

If the particles are at least the size of the wavelength of the ambient light, the light waves will bounce off them and scatter, producing cloudiness, or if there are enough of them, total opacity.  On the other hand, if the particles are smaller than the wavelength of the ambient light, the light waves tend to bend around them, making them invisible, and the glass remains transparent.  Thus the opacity of the glass ceramics can be adjusted by controlling the size and concentration of the crystals, and this can be done precisely by controlling the firing cycle. 

If the index of refraction of both the crystals and the glass medium are the same, then light will not reflect off the crystals, and the glass remains transparent no matter how dense the concentration of crystals, or how large they grow.  Diffusion of light can take place only if the refractive indexes of the crystals and the glass are substantially different.  The greater the difference, the greater the reflection and refraction, and the more opaque the glass will appear.

Cerammed crystalline inclusions in glass ceramics

When feldspar is subjected to the process of ceramming , it undergoes incongruent melting to form crystals in a liquid glass.  Incongruent melting is the process by which one material melts to form a liquid plus a different crystalline phase.   This form of devitrification results in crystals that depend on the exact formulation of the feldspar and the exact ceramming temperatures and times. Different feldspathic formulations and different firing schedules will yield different cerammed crystals.  They all serve as crack stoppers, like the refractory structures that are part of an ordinary feldspathic porcelain, but each one delivers its own specific benefits.

Leucite (KAlSi2O6)

The original feldspathic glass used in the production of porcelain fused to metal restorations contained additional leucite crystals as crack stoppers.  In this case, the leucite was added because of its optical properties, and because its thermal expansion properties made it possible to more easily fit the veneer to the metal substructure.  These leucite containing porcelains had about the same flexural strength as the original feldspathic porcelains, about 70 MPa.  The crystals in cerammed leucite, however can be controlled more precisely, and their size and density within the glass matrix produces a much more translucent glass at a flexural strength of about 125-180 MPa.

Lucite was the first, and is still probably the most popular of the crystalline inclusions that form in cerammed feldspathic glass.  It forms when nearly any feldspathic glass is heated and held at temperatures between 1150°C and 1530°C.  (Note that the chemical formula for leucite contains the same elements as are found in ordinary feldspars.)   One of the main functions of these crystalline inclusions within the porcelain body is to act as crack stoppers, however the usefulness of leucite crystals goes beyond that.  It is a potassium-aluminum-silicate  mineral with a large coefficient of thermal expansion when compared with  non-cerammed feldspar glasses.  This property makes it especially useful because it becomes possible to adjust the thermal expansion of the glass body to suit the specific structures over which it is to be applied.  It is especially useful when formulating veneers that will be placed over a metal substructure as in porcelain fused to metal crowns and bridges.  In a porcelain jacket crown, the porcelain can be matched to the thermal expansion properties of the tooth structure over which it will be placed.

Leucite reinforced ceramic glasses produce cores with about the same flexural strength as the older aluminous porcelain cores (125-180 MPa).  Since both types of cores can be etched and bonded directly to tooth structure, the major advantage of the glass ceramic core is the translucency that these cores display when compared with that of their aluminous competitors.   The internal surface micro crack problem still remains, but is greatly diminished due to the increased elasticity lent by the leucite crystals.   The techniques that allow for strong bonding of the internal surface of the PJC with the surface of the prepared tooth compensate for any remaining weaknesses due to internal micro cracking.   Optec HSP, and Fortress are two examples of leucite reinforced glass ceramics that are condensed and sintered over a refractory die like traditional feldspathic porcelain crowns. These restorations can be used on anterior teeth, but are still too weak to fabricate bridges, or crowns for posterior teeth.

Empress (Ivoclar-Vivadent) and Optec OPS

These are brand names. They are manufactures using a lost wax technique to press glass ceramic crowns rather than the powder condensation technique used by dental lab technicians .  The glass ceramic is supplied in ingots in which the leucite particles (about 35% by volume) have been previously formed in a ceramming process done by the manufacturer.   A wax pattern is made in the form of a crown and invested in a refractory die material.  The wax is burnt out to create the space to be filled by the leucite reinforced glass ceramic.  A specially designed pressing furnace is then used to melt the glass ingot and infuse the mould with the glass ceramic melt.

Lumineers®

Lumineers® are a veneering system made by DenMat, which can be fabricated so thin that tooth reduction is not usually necessary. These veneers are built to be 0.3 mm thick and are  returned to the dental office fully etched and ready for bonding to uncut, etched tooth structure.  If these veneers were made of ordinary feldspathic porcelain, or even most other cerammed porcelains, they would be too thin to handle without breaking prior to being bonded to the tooth.  Lumineers® are, however, surprisingly strong, and this is due to the internal structure of the porcelain used to fabricate them.  There are several varieties of lumineers.   One type is made from  cerammed lithium disilicate which is very strong, and somewhat opaque.  Lumineers® are made from pressed feldspathic porcelain reinforced with leucite nano-crystals and are much more translucent than the lithium disilicate variety. DenMat calls this type of porcelain Next-Generation Cerinate.  Both types are pressed in a way similar to the method used for pressing Empress crowns, and can be made to be as thin as 0.3 mm.   The leucite reinforced feldspathic glass also is used in the form of a frit for use with the powder-condensation method to make "stackable" veneers.  Stacking glass powders makes it possible to characterize the veneers more accurately.

In-Office CAD/CAM ceramics

CEREC 1, the first  CAD/CAM system for in-office milling of porcelain crowns was released in 1985 by Sirona Dental systems.  The earliest porcelain blocks used in the milling process were Viatabocks Mark I, replaced in 1987 by Vitablocks Mark II.  Vitablocks Mark II are still in use, and are made of a fine-grained, high glass content feldspar.  ProCAD (made by Ivoclar Viadent), was an early competitor.  ProCAD was improved to make it stronger and sold as IPS Empress CAD.  This porcelain is composed of 40 percent leucite embedded in feldspathic glass.  In 2007, Sirona Dental Systems introduced CEREC blocks, which are similar to Vitablocks Mark II, but with different shading nomenclature.  All of these blocks are manufactured using a ceramming process. 

In general, the less glass a ceramic material contains, the stronger it is flexurally.  On the other hand, the less glass a ceramic material contains, the less translucent it will be. In 2006, Ivoclar Viadent introduced a lithium disilicate ceramic called IPS e.max which has the least glass of all the CAD/CAM "glass ceramics".   IPS e.max blocks are sold in a pre crystallized state.  They are cerammed using a kiln after milling.  While IPS e.max contains the most crystalline material, and is therefore the strongest of the ceramics in this category, it is also somewhat less esthetic than the others due to its increased opacity. Crowns and veneers made from any of these systems are bonded onto the underlying tooth structure after the internal surfaces are sand blasted, etched, and silane is applied.  IPS e.max may alternately be cemented to the preparation using standard luting cements.

Mica (Fluoromica glass ceramics)

Mica is a naturally occurring mineral with numerous compositional formulas.  The crystals form in very thin, flat sheets, and tend to be optically clear, but in natural formations, the mineral looks like a glassy, silvery rock with a stepped flat surface. The crystals are stacked on top of each other like pages in a book, and it is easy to pry paper thin sheets of mica off the top using the point of a pin.  These thin sheets are composed of even thinner sheets which could be separated from each other if one has a fine enough pointed instrument to do it with.  In the detail on the right above, the flat plane structure of mica crystals is visible from the top. 

Though no longer sold, the first commercially available castable dental ceramic was Dicor.  It was developed by Corning glass works and marketed by Dentsply.  The mica crystals formed in Dicor are based on the composition SiO2 · K2O · MgO · Al2O3 · ZrO2. and fluorides are added to the mixture to help produce a degree of fluorescence in the finished prosthesis.  For this reason, this formulation is called a fluoromica glass ceramic.  A wax pattern is fabricated by a lab technician, just as it would be done for a gold casting.  An ingot of the castable ceramic is placed in a special crucible, melted, and centrifugally cast at a temperature of 1380°C.  Ceramming is done at this stage, and results in the nucleation and growth of needle-like crystals which form at random angles rather than the plate-like crystals of naturally occurring mica.  This has the advantage of forming an interlocking matrix which gives added flexural strength to the ceramic body. 

One of the peculiarities of this glass ceramic is that crystal growth can be controlled so that the crystals that form can be smaller than the wavelength of visible light.  This property, combined with the fact that the refractive index of the tiny mica particles is close to that of the surrounding glass means that the ceramic body can be nearly transparent.  For practical reasons, the mica crystals are allowed to grow to a larger size in order to produce a translucency close to that of enamel.  Proper shading and characterization is produced by sintering a layer of self glazing shading porcelain over the surface of the finished glass ceramic body. 

Fluoromica glass ceramic restorations have a flexural strength in the region of 120-150 MPa, slightly more than that of the leucite containing glass ceramics.  Their strength makes them adequate for fabricating bonded crowns for premolars.

Lithium Disilicate (Li2Si2O5 and Li3PO4) and Apatite Glass Ceramics

Empress II (today called Eris) and Optec OPS 3G are not so much glass ceramics, but glass ceramic systems.  Two unique glass ceramics had to be developed in order to build complete prostheses using Lithium disilicate core systems.

The lithium disilicate core is somewhat opaque, but is the strongest of all the glass ceramics (~350-450MPa).  This makes the core strong enough to fabricate crowns for molars, and adequate for the fabrication of anterior three unit bridges. 

Like Empress IPS, this glass ceramic is pre-cerammed by the manufacturer and supplied in ingots for pressing in a pressing furnace.  The crystalline phase that forms during the ceramming of this glass is lithium disilicate (LiS2O5), which makes up about 70% of the volume of this glass ceramic.  This microstructure is unusual because it consists of many small interlocking plate-like crystals that are randomly oriented.  The interlocking nature of the crystals, as well as their high density gives this glass ceramic very high flexural strength.  The ceramic body is said to be highly translucent because the high optical compatibility between the crystals and the glassy matrix minimizes internal scattering of light. 

Apatite Glass Ceramic

The lithium disilicate ceramics are different from other glass ceramics in that it has an unusually high coefficient of thermal expansion, and ordinary feldspathic glasses cannot be sintered over the lithium disilicate substructure.  Therefore, a new esthetic glass ceramic with a higher thermal expansion had to be invented to overlay the thick framework.  This new layering ceramic is an apatite glass ceramic. The crystals formed on ceramming have the composition Ca10(PO4)6 · 2OH.  This is the same basic constituent in natural tooth enamel. 

Pressed lithium disilicate crowns

Because lithium disilicate is a glass ceramic, it can be handled in the molten state much like high leucite glass ceramics such as Empress (see above).  It can be provided by the manufacturer in ingot form and pressed into the shape of a finished crown.  Crowns made of pressed lithium disilicate are not very translucent, but they are much stronger (350-450 MPa) than crowns made from other pressed leucite reinforced glass ceramics (125-180 MPa) and thus are suitable for posterior teeth.  The major brand of lithium disilicate crown at the time of writing is IPS-e.max®. 

Bonding

Both aluminous core and glass ceramic restorations remain, at base, fortified glass bodies.  This means that the internal surfaces of these vitrified cores can be acid etched using hydrofluoric acid.  Thus they can be luted directly to the teeth using standard bonding procedures.  The crystalline inclusions in these glasses act to reduce the tendency for micro cracks to form on the internal surfaces of these restorations, and the bonding technique turns the tooth structure itself into a sort of unbreakable core.  This further reduces the likelihood that any relatively minor cracks that may be present on the internal surface of the ceramic body will actually cause a catastrophic fracture. 

Bonding to glass is principally a mechanical process, but it may be assisted by chemical bonding if a silane agent is used.  The laboratory generally deals with the mechanical preparation of the prosthesis.

Mechanical bonding

The inside of the crown or bridge retainers are sandblasted  with 50 micron silica particles to roughen the porcelain and to increase the surface area for bonding.   This creates a series of microscopic hills and valleys over the internal surface.  Then a solution of 9.6% hydrofluoric acid gel is applied to the sandblasted area.  The hydrofluoric acid dissolves the surface of the glass in uneven patterns creating even tinier microscopic mountain ranges over the surface of the sandblasted hills and valleys.  This serves as mechanical retention. When bonding resin flows over the etched surface, it flows into all the tiny surface imperfections, and when the resin hardens, the imperfections, being at odd angles, act in concert as undercuts firmly bonding the resin in place.

Chemical bonding

Chemical adhesion of the resin to the etched porcelain is generally done by the dentist when inserting the restoration.  This is done by the application of silane to the prepared porcelain.

Silane coupling agents

Silane coupling agents are a class of silicon based molecules.  Boiled down to its essentials, a silane is a double ended molecule centered around a silicone atom.  One end of the molecule can bond to a glass substrate, while the other end can bond to the methyl methacrylate  in composite resin. 

Silicon is like carbon in a number of ways.  It can bond covalently with four oxygen atoms, and it can form long chains, very much like carbon.  This ability to form chains makes an entire chemistry based on silicon possible, so like carbon based products, silicon can be the basis for lubricants, rubber, adhesives, solvents, water proofing agents and a long list of other products, all of which have organic (carbon based) analogs. 

A manufacturer starts by bonding silicon atoms to methyl chloride in the following reaction:

Si + n(Ch3Cl) --> a mix of methyl chlorosilanes including:

(CH3)2SiCl2 and CH3SiCl3 and a number of other combinations

Note: The CH3 groups are called methyl groups. 

It is easy to substitute virtually any other organic molecule for the chlorine and methyl groups in the methyl chlorosilane molecule.  Since methyl groups (CH3) form the entire basis of organic chemistry, other organic molecules can also attach directly to them.  In the diagram below, one of the methyl groups in the original methyl chlorosilane molecule has been substituted with an ethyl group.  Acrylate groups are frequently used for this purpose instead.  Methylmethacrylate is the chemical unit of acrylic, and acrylic is the foundation of resin bonding. 

It is also easy to substitute oxygen atoms for one or more of the chlorine atoms on the same methyl chlorosilane molecule.  Those oxygen atoms can come from the silica molecules in the glass matrix, as is represented in the illustration above.  This provides the "hook" necessary for bonding the other side of the same silane molecule to the glass in the porcelain core.

Thus when the dentist applies a silane to the porcelain before bonding, he is applying a multi-sided molecule, one side of which bonds to the silica in the porcelain, the other side of which bonds to the acrylic bonding agents he uses to bond his restoration.  In combination with the mechanical bonding described above, this makes for a strong bond indeed.

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