Dental Ceramics
Ceramics are compounds of one or more metals with a nonmetallic element. This includes lithium disilicate, zirconium oxide and aluminum oxide. They are rigid, brittle, and inert nonconductors of thermal and electrical energy. The laboratory uses ceramics to create tooth coloured restorations.
This chapter aims to cover the modern applications of dental ceramics. Their manufacturing methods will be covered as well as their advantages, disadvantages, indications and contraindications in a clinical setting.
The first all-ceramic dental restoration was the ‘porcelain jacket crown’ patented 1889 by Land. The crown was feldspathic porcelain and cemented with silicophosphate cement. It was prone to cracking and needed to have a thick cross section to be durable. From 1950 the main function of dental ceramics was to cover ceramo-metal
restorations. This helped improve the mechanical properties of restorations and decreased ceramic failures. In 1965 new technologies emerged leading to the resurgence of all ceramic restorations. Today ceramic technology allows fabrication of small inlays and veneers to full-arch implant prostheses.
How Are They Made?
Ceramics are traditionally made with powder and liquid on a platinum foil or refractory die which is then sintered in a porcelain furnace and finally glazed. Modern methods of manufacturing ceramic include hot press injection moulding. This means an ingot is melted and pressed into an investment. This investment contains the space of an eliminated wax or acrylic burnout pattern.
Another method is CAD CAM manufacturing. A ceramic disc or block is machined to a shape and size designed with computer software. The machined ceramics can be in a precursor state or ‘green’ state of partial crystallisation to reduce wear on the machines.
They then require further heat treatment to reach their crystallized toothcoloured state. The wax or acrylic patterns used for hot pressing can be made with CAD CAM. Those are machined with blocks or 3D printed then invested to be eliminated before hot-pressing.
In a ceramo-metal restoration, the alloy core enhances the strength of the overlying ceramic. All-ceramic restorations may combine a strong ceramic core with a veneer ceramic. The veneer ceramic is weaker but can add colour and translucency to improve esthetics. An anatomical monolithic structure, which is stronger and more crack resistant can also be made. Monolithic restorations can be made with multicoloured ingots or blocks/discs to add colour or are stained externally.
Classifications of Dental Ceramics
Dental ceramics can be classified in a number of different ways, including by their composition, processing method, fusing temperature, microstructure, translucency, fracture resistance, and abrasiveness.
A classification introduced in the International Journal of Prosthodontics in 2015 has simplified the categorization of dental ceramics without overlaps. It simply separates ceramics into:
- Glass matrix ceramics
- Polycrystalline ceramics.
Those are what are referred to as ‘porcelains’. They have a structure consisting of a crystalline phase suspended in a glassy phase. Their main constituent is feldspathic porcelain, which is very translucent but has very poor mechanical properties.
Feldspathic porcelain restorations are usually limited to anterior teeth. It is manufactured by condensing powder and liquid onto a platinum foil or refractory die. Blocks of fine-grained feldspathic ceramic can be machined to be monolithic restorations or further veneered by powder and liquid. Notable brands of machinable feldspathic porcelain are Vita Mark I and Vita Mark II by VITA Zahnfabrik.
Disadvantages:
- Poor mechanical properties due to low flexure strength and high brittleness. Leucite is the main strengthening component of feldspathic porcelain. It is formed when the porcelain powder is sintered in a porcelain furnace. It consists of 5% of the porcelain mass. This low leucite concentration is the primary reason for its poor mechanical properties.
Indications:
- Anterior crowns and veneers. Specific indication is when a single restoration is to be placed beside very translucent and characterized natural teeth.
Contraindications:
- Posterior restorations. Not an absolute contraindication if occlusal factors are ideal but other restorative materials have a higher success rate.
- Highly destructed teeth. Feldspathic porcelain restorations derive their strength from bonding and predictable bonding requires copious amounts of healthy enamel
- Fixed Prostheses.
For porcelain to attach to metal predictably there must be a uniform difference in coefficient of thermal expansion (CTE). This lets the porcelain shrink wrap on the metal while cooling after sintering. In addition, oxides in the ceramic will bond to oxides in the metal. The metal is also surface treated to create mechanical interlocks for the ceramic. The ceramic is applied by powder and liquid sintering or hot pressing molten ceramic onto the metal framework (press to metal technique).
Advantages
- High strength of metal and esthetics of ceramics
- With good clinical and laboratory technique, can be easily characterized to match natural teeth
- Complete coverage restorations will have high retention in preparations with core buildups and can be easily cemented with conventional cements.
Disadvantages:
- Preparation to be veneered by ceramic is more invasive as the porcelain margin will need a shoulder or deep chamfer margin. Clinical experience suggests the veneering ceramic should be about 1–2 mm thick for adequate strength and aesthetics. This invites the controversy that an esthetic creamo-metal crown will need an extensive preparation, therefore it is invasive. Good clinical judgement is to minimally prepare non-esthetic zones of a tooth the be metal only and only keep areas of extensive preparation to ‘visible’ parts of the tooth. Areas which are in the esthetic zone but will not be visible on smiling can have metallic margins which are less invasive.
A ceramo-metal crown for a lower canine. Note the partial veneering as only visible areas were prepared with a deep chamfer for ceramic. Also note the cingulum rest prepared in the metal for a partial removable prosthesis
- More challenging esthetics compared to all ceramic crowns. There can be no translucency in the area occupied by the alloy core, so it cannot be placed beside translucent teeth. If the alloy core is base metal it needs an opaque layer, so the area to be veneered needs a more aggressive preparation. A precious metal alloy core such as gold palladium can mimic the the yellowness of dentin.
Ceramo-metal crowns on the left canine and lateral central incisor, Note the difference in translucency to the composite veneers in the rest of the anterior teeth
Indications:
- Highly destructed teeth. Where there are no large amounts of healthy enamel to allow bonding for all-ceramic restorations, a core build up and a ceramo-metal crown is more ideal as strength will be provided by the metal substructure.
- Subgingival margins. Bonding protocols require good isolation, so in the case of subgingival margins where this is not easily done, ceramo-metal restorations are indicated as they are routinely cemented conventionally with zinc phosphate or glass ionomer.
- Retainers for small to long span bridges. It is suitable for anterior and posterior bridges on natural teeth and implants.
A ceramo-metal bridge on implants 12-14 beside a single crown on 15. Due to limited inter-arch space the palatal and occlusal surfaces were not veneered
Stronger glass matrix ceramics were developed with increased concentrations (40–55% mass) of leucite. It can be traditionally sintered from powder and liquid, hotpressed or machined from blocks. A notable brand is Empress I (block form is Empress CAD) by Ivoclar Vivadent. Despite its unremarkable strength (160MPa flexure strength), it is popular for resin bonded veneers and crowns due to its excellent esthetics.
Advantages
- High translucency second only to feldspathic porcelain
- - Can be bonded
Disadvantages
- Unremarkable strength (160MPa flexure strength)
Indications
- Single anterior crowns and veneers. Very remarkable in shade, translucency and texture
- matching with neighboring natural teeth
Contraindications
- Posterior restorations. Not an absolute contra-indication and they were used for fabricating inlays and inlays but largely replaced by lithium disilicate.
- Fixed Prostheses
- Highly destructed teeth and subgingival margins. Strength is derived from bonding which will be compromised in such cases
Lithium disilicate was first introduced in the 1990’s as a strengthening agent for feldspathic porcelain. Lithium disilicate ceramics are manufactured by hot pressing or CAD CAM machining. To allow machining, blocks may come in a precursor state containing a lower concentration of crystals, requiring heat treatment after machining to increase the lithium disilicate content to 70%. In the case of e.max for example, the blocks may be blue then become tooth-coloured after the heat treatment.
Advantages
- Considered a ‘strong’ glass ceramic. At 70% concentration of lithium disilicate by mass, the result is more than three times higher flexural strength compared to high concentration leucite ceramics. This increases its success rate and indications
Disadvantages:
- Slightly different refractivity to enamel and dentin, therefore may represent a challenge in matching to natural teeth. As a general guideline, laboratory technicians prefer monolithic restorations with some external staining and micro layering when fabricating several adjacent restorations. However then matching a highly translucent and characterized natural tooth, a core ceramic of appropriate tranlsucency is fabricated first then layered with fluoroapatite ceramic. If the neighboring tooth is still more translucent or there is only space for a monolithic restoration then a switch to leucite or feldspathic porcelain may be the solution.
- Still requires bonding for durability.
Indications:
- Full Crowns and veneers. Monolithic Lithium disilicate crowns on the second molar have shown success.
- Posterior inlays and onlays. Lithium disilicate has been shown to be the optimum material for partial coverage posterior restorations due to combining strength, esthetics and the ability to bond to enamel
- Implant crowns. Can be used for single implant crowns or as single crowns on full arch frameworks made of polymer or metal.
A tooth wear case rehabilitated with lithium disilicate veneers and onlays
Contra-indications:
- Fixed dental prostheses. Not an absolute contraindication but it is best limited to anterior short span bridges with a monolithic structure if necessary.
- Highly destructed teeth with subgingival margins. Even though dentin bonded crowns have shown success, the poor bonding conditions may necessitate a choice of ceramo-metal or zirconia crowns.
Such ceramics show higher flexural strength but higher brittleness. They can be manufactured
the same way as lithium disilicate and have the same clinical applications but are still under evaluation to determine if they are clinically better. Notable brands include Vita Suprinity by VITA ZAHNFABRIK and Celtra by Dentsply Sirona.
Polycrystalline ceramics have a uniform crystal lattice structure. Their main use is to
create the core or framework of a restoration to be layered by glass ceramic afterwards.
Recently polycrystalline ceramics have broken ground in the creation of
strong and aesthetic monolithic restorations. Another less common application of
polycrystalline ceramics is glass infiltration, namely the In-Ceram System.
Alumina
Such ceramics show higher flexural strength but higher brittleness. They can be manufactured
the same way as lithium disilicate and have the same clinical applications but are still under evaluation to determine if they are clinically better. Notable brands include Vita Suprinity by VITA ZAHNFABRIK and Celtra by Dentsply Sirona.
Zirconia
Zirconia (ZrO2) is an oxidized form of the zirconium metal. Zirconia restorations are manufactured by CAD CAM machining of discs or blocks. Because they are only partially fired, the blocks are weak but easy to mill. However, the milled framework must be fired for 6 to 8 hours to increase the density of the restoration. A large amount of shrinkage occurs, and this volume difference is calculated using the CAD-CAM software.
Advantages
- High strength. Zirconia has a flexural strength of up to 1400 MPa. Even high translucency zirconia which is weaker (600MPa)is considerably stronger than lithium disilicate ceramic. This gives it a wide range of indications and allows it to be cemented conventionally with zinc phosphate or glass ionomer.
- Crack resistant. Zirconia has the benefit of transformation toughening, in which the crystal structure can change to stop crack propagation. Translucent zirconia does not have that benefit.
Disadvantages:
- Poor translucency. The main disadvantage of tetragonal zirconia is the low translucency resulting in a plain white colour after sintering. There are techniques to colour the zirconia and it can be stained externally but the result will not be as aesthetic as a layered restoration. Monolithic zirconia restorations are best kept to the posterior segments of the mouth. In the aesthetic zone, tetragonal zirconia is the framework or core of the restorations and that is veneered by glass ceramic dentin and enamel layers. The glass ceramic is applied by brush condensation, hot pressing, or sintering a machined veneer on the core. Lithium disilicate can be pressed on zirconia to create very strong aesthetic restorations (press to zirconia technique). In thin sections some brands of high translucency zirconia are as translucent as lithium disilicate. This allows monolithic full coverage restorations in the anterior zone where multiple teeth are being restored to allow easy colour matching.
- Unreliable bonding. Zirconia products are resistant to acids and will resist acid etching. This makes adhesive surface treatment challenging so they are best applied in full coverage restorations cemented by glass ionomer.
Indications:
- Full crowns. Zirconia margins can be feather edge or 0.3mm thin, making them a conservative material for monolithic crowns. This has proven very useful in restoring lower anterior teeth. Veneered sections may need a thicker margin.
- Destructed teeth with sub gingival margins. Since zirconia does not need to be bonded to tooth structure for strength, the same protocols used for ceramometal crowns can be done for zirconia crowns. Zirconia restorations are more conservative than ceramometal as the zirconia core does not need masking.
A porcelain fused to zirconia crown used to restore the upper right lateral incisor. The
lack of enamel after caries excavation would have made bonding a glass ceramic
crown unreliable.
Contra-indications:
- Partial coverage restorations. Not an absolute contra-indication but the preparation will need to be mechanically retentive like metal onlays, which opposed the conservatism provided by adhesive restorations
- Veneers. Bonding zirconia restorations is still systematically unreliable and glass ceramics are better suited for adhesive properties and even more predictable esthetics
Future Developments
Those materials combine glass ceramic and resin components. They are also known as hybrid ceramics, reinforced composite or polymer-infused ceramic network according to the brand. They show lower fracture strength than glass ceramics but greater resilience, allowing them to resit cyclic loading. Another advantage is they do not need glazing, allowing for their occlusal and marginal finishing after cementation followed by polishing. This is useful in minimal prep veneer restorations
In the 1990s it was discovered that organophosphates have been able to prime sandblasted
zirconia, allowing it to bond to resin cement. Other methods include laser, selective
glass infiltration and silicated sandblasting. The main method is using an
MDP primer to sandblasted zirconia. Bonded zirconia is still in the evaluation period
to confirm quality comparable to etched and silanated glass ceramics but it can allow
applications
- Contemporary Fixed Prosthodontics by Rosenstiel
- High Strength Ceramics: an Inderdiciplinary Perspective by Ferencz
- Extracoronal Restorations by Wassel
