USE OF CERAMIC MATERIALS IN THE RESTORATION OF POSTERIOR TEETH P NEWSOME1
T
he demand for highly aesthetic dental restorations continues to grow, and as a result, fewer and fewer patients will now accept the inherent aesthetic limitations and potential toxicity of metal-based restorations. These restorations, such as amalgams and porcelain-fused-to-metal (PFM) crowns (shown in Figure 1) are rapidly falling out of favour as a result of their obvious aesthetic limitations, destructive tooth preparations, inability to bond to tooth structure and potential toxicity.
Figure 1: A porcelain-fusedto-metal restoration
Ceramic systems have grown in complexity over the years and now offer an extremely wide variety of options and choices. This is good in one sense as virtually every clinical scenario is catered for, but less good in another, in that it can be somewhat daunting for the practitioner who has to select a suitable ceramic, prepare the tooth correctly, direct the laboratory and finally choose an appropriate luting cement. Long gone are the days when PFMs (with their alltoo-familiar cervical shadowing, or metal
shine-through, at the cervical margins and their general lack of translucency) cemented with zinc phosphate were the only ‘aesthetic’ option.
Understanding ceramics
1
Philip Newsome
Hon Clinical Associate Professor, Faculty of Dentistry, University of Hong Kong
42
It is helpful to think about any particular dental ceramic as a composite material sitting at some point on a spectrum comprising, at one end, an unfilled predominantly glassy matrix and, at the other, a virtually wholly crystalline structure with little or no matrix at all. In between the two there exists a range of ceramics with varying amounts of particles within this glassy matrix. Conceptually, this is very similar to the
more familiar resin-based composite restorative materials, but with a glass matrix as opposed to resin.
Predominantly glassy ceramics At one end of this spectrum are largely vitreous, or glass, ceramics comprising irregularly-spaced three-dimensional networks of atoms displaying a characteristically amorphous structure. Dental ceramics in this category come from a group of mined minerals called feldspar and are based on silica (silicon oxide) and alumina (aluminium oxide), and hence belong to a family called aluminosilicate glasses. Such feldspathic ceramics exhibit low opacity, high translucency and low heat conductivity, and are hard, chemically inert and biocompatible. While this high degree of translucency means that they are unsuitable for masking dark teeth, their main drawbacks are undoubtedly their poor mechanical properties (flexural strength of 56MPa) and the high degree of shrinkage that occurs during firing. The result of all this is that they are nowadays never used as the sole porcelain component of the restoration, but rather as a veneer over a much stronger underlying core material.
Particle-filled glasses As one moves further along the continuum, filler particles are added in increasing amounts to the base glass matrix to improve mechanical properties and to control optical effects such as opalescence, colour and opacity. The introduction of the particles into the glass matrix can be achieved in a number of different ways; the process used will affect the properties of the material and so influence material choice. a) Leucite-reinforced glass ceramics The first leucite-reinforced system, IPS Empress, was developed at the
P R I M A R Y D E N TA L J O U R N A L
University of Zurich in the early 1980s and brought to market as IPS Empress 1 (subsequently renamed Empress Esthetic in 2004) by Ivoclar Vivadent in 1990.
Figure 2: This scanning electron microscope image of Empress ceramic illustrates the leucite crystals embedded in a glassy matrix. Image courtesy of Ivoclar Vivadent
These are particle-filled glasses, the particles being crystals of leucite (see Figure 2). The introduction of these crystals creates a castable glass ceramic using the familiar lost-wax process. The shape of the restoration is carved in wax, the pattern invested and heated in a furnace. A ceramic ingot is then heated to 1,200°C and forced under pressure into the mould. Using this technique it is possible to create either a coping, onto which a more translucent enamel veneering porcelain is pressed, or a monolithic restoration (ie. made from one ingot), which can then be characterised by painting high-chroma pigments onto its surface, followed by glazing to seal the surface.
Figures 3a-c: A monolithic pressed leucite-reinforced glass ceramic restoration of a hypoplastic premolar
The introduction of leucite crystals in conjunction with the injection-moulding process reduces the risk of micro-crack formation and improves flexural strength (ranging from 95-180MPa, as compared with the 56MPa of feldspathic porcelain). The result is superb aesthetics, excellent marginal integrity and biocompatibility combined with ease of fabrication and an etchable fitting surface allowing bonding using silane/adhesive resin. As far as posterior teeth are concerned, indications are limited to inlays/onlays. Figures 3a-c show a case of a monolithic pressed Empress being used to restore a hypoplastic premolar. The restoration is retained achieved solely as a result of the etched surface of the ceramic which results in micromechanical retention of the resin luting agent. Even in these cases, great care must be exercised in case selection and treatment planning, especially in terms of margin placement (ideally within enamel) and occlusal loading (ideally minimal).
In all other respects, these restorations are essentially the same as their laboratory-made counterparts in terms of mechanical properties, clinical indications, preparation requirements and luting protocols. Subsequent developments of the CAD theme include:
Leucite ceramic CAD/CAM systems The discussion so far has focused on restorations made using traditional impressions sent to a laboratory and
Polychromatic blocks featuring a smooth transition between dentine and incisal areas, thus creating natural-looking results. This is aided by a combination
V O L 3 N O 2 M AY 2 0 1 4
Figure 3a
Figure 3b
fabricated by skilled technicians. However, leucite ceramics are now available in CAD/CAM form, thus allowing aesthetic restorations to be milled chairside (using technology such as CEREC) and placed immediately. This approach has the convenience of ‘one-stop’ restorations, albeit with the need for the prescribing dentist to carry out chairside polishing, staining and glazing.
Figure 3c
of a high level of chroma in the cervical area and incisal translucency. These effects obviate the need for further characterisation in many cases. Variable translucency blocks, which take the ability to customise chairsidemade restorations even further by offering high translucency and lower brightness values, and vice versa. The latter are especially useful in those cases where there is a likelihood of a dark underlying preparation ‘greying’ the final restoration. b) Lithium dislicate ceramics In 1989, a glass ceramic (IPS Empress 2, subsequently renamed e.max) containing 70% crystalline lithium disilicate was developed, with the specific goal of improving upon the mechanical properties of leucite ceramics without compromising aesthetics. As a result, flexural strength rose to 360-400 MPa.
43
USE OF CERAMIC MATERIALS IN THE RESTORATION OF POSTERIOR TEETH
The method of fabrication was essentially the same (ie. pressed ceramic) and again, restorations could be either monolithic or bi-layer, layered with a veneering ceramic specially designed for the lithium disilicate. The improved physical properties saw these ceramics being recommended for premolar crowns and short-span premolar bridges in those cases with sufficient space present to house a 4 × 4mm connector. The high crystal density also permits greater masking of discoloured teeth than do the more translucent leucite ceramics. Indeed, more opaque ingots have been specifically developed to cope in such clinical situations.
Figure 4: The densely-packed polycrystalline nature of zirconium ceramic can be clearly seen in this image
Lithium disilicate CAD/CAM systems As with leucite ceramics, lithium disilicate systems are available in CAD form, allowing practitioners to create restorations at the chairside. Again, the result is comparable with laboratorymade restorations in terms of mechanical properties, while final aesthetics largely depend on the dentist applying stain as and when necessary prior to cementation. It is worth noting that, for ease of milling, these CAD blocks are manufactured and supplied in their weaker lithium metasilicate state, therefore taking on a blue appearance. The milled blue blocks are then fully crystallised in a furnace, which transforms them to their full-strength lithium disilicate state. Wear characteristics of particle-filled glass restorations As far as abrasiveness is concerned, low-fusing ceramic has been shown to cause less wear of opposing teeth than conventional porcelain.1 It is also well documented that rough, abraded porcelain is extremely damaging to opposing unrestored teeth. One study in 2006 compared various ceramic materials with gold, and while gold, unsurprisingly, proved to be the least abrasive, polished low-fusing porcelains also resulted in minimal toothwear.2
44
This suggests that should intra-oral adjustment be required, thorough polishing will lead to an acceptably smooth surface.
Polycrystalline ceramics At the other end of the spectrum to the predominantly glassy ceramics are the polycrystalline ceramics, which contain densely packed atoms with little or no vitreous glassy ‘matrix’ phase (Figure 4). This arrangement results in restorations that are more difficult to drive a crack through, in comparison to the less dense and irregular linkages found in glass structures. As a consequence, polycrystalline ceramics are generally much tougher and stronger than glassy ceramics. They are, however, much more opaque and more difficult to process into complex shapes than glass ceramics. It was only with the introduction of computer-aided manufacturing that well-fitting prostheses made from polycrystalline ceramics became possible. There are two basic types of polycrystalline dental ceramics currently available, namely, aluminium oxide and zirconium oxide. a) Aluminium oxide With aluminium oxide content as high as 99%, these materials offer extremely high flexural strength – between 400-700 MPa. These systems provide a solution when restorations need to exhibit high strength and aesthetic concerns are not paramount. The manufacture of these restorations uses a scanning process to machine the densely sintered ceramics and demands very careful tooth preparation if errors in scanning are to be avoided. b) Zirconium oxide Zirconium oxide-based restorations feature a wholly crystalline structure with no glassy matrix at all between the crystals. This makes for an extremely strong and tough structure (flexural strength of 1200MPa), but one that is extremely opaque with a high optical value. A
P R I M A R Y D E N TA L J O U R N A L
further concern has been the possibility of abrasion of opposing tooth surfaces.
properties of monolithic zirconium crowns and how they might best be used clinically.
As a result of these concerns, when zirconium was first introduced as a restorative material, it was used as coping material upon which a more aesthetic, but weaker, veneering ceramic (such as pressable lithium disilicate ceramic) could be added. Such restorations were well received, but problems with fracture of the overlying ceramic led to the development of monolithic zirconium restorations. The initial indications for zirconium-based restorations (which, by virtue of their physical properties and the technical complexity of production and handling, are only available through a dental laboratory) were primarily in posterior teeth as individual crown copings and bridge frameworks (see Figure 5). These included very long-span bridges (up to 12 units), for which, in previous years, only all-metal or PFM bridges were suitable.
Wear characteristics of MZ restorations The prospect of excessive antagonist toothwear has probably been the main factor deterring dentists from adopting MZ crowns. Recent research, however, suggests that this might not be cause for such concern. For example, a study comparing the in vitro wear of enamel antagonists produced by monolithic alloy, veneered zirconia and monolithic zirconium finished in four different ways (glazed using a glaze ceramic, glazed using a glaze spray, manually polished and mechanically polished) found that polished MZ was responsible for lower wear rate on enamel antagonists as well as within the material itself.4 Another in vitro study examined the wear characteristics of different dental ceramics after grinding and polishing treatments.5 Once again, the results confounded the conventional wisdom that zirconia led to opposing toothwear, concluding that zirconia ceramics yielded superior wear behaviour and lower antagonistic wear compared to porcelains. Perhaps unsurprisingly, in all cases higher ceramic and antagonistic wear was shown after grinding treatments.
Monolithic zirconium restorations The biggest drawback of any restoration combining a high-strength coping coupled with an aesthetic veneering ceramic is the possibility of the latter fracturing or delaminating, even in the absence of any contributory factors such as adverse occlusal loading or inadequate material thickness. A recent study found that fractures of layered ceramic crowns comprised the most common complication prompting replacement of ceramic crowns.3 Such failures are distressing for the patient and also for the clinician, who is faced with having either to repair the fractured area or to remove and remake the crown. Crown removal has proved to be particularly problematic in the case of zirconium-based crowns as they are usually placed with a highly retentive adhesive resin cement, and zirconium is extremely difficult to section intra-orally. The advent, however, of monolithic zirconium (MZ) prostheses presents the possibility of restoring posterior teeth with extremely durable aesthetic restorations. There follows a discussion of the various
V O L 3 N O 2 M AY 2 0 1 4
Figure 5
The message is clear: MZ restorations are unlikely to lead to excessive wear of opposing teeth provided that they are finished correctly. Rough surfaces should be avoided, and as with all ceramic materials (and indeed all restorative materials), injudicious grinding of restorative surfaces either prior to fitting or once cemented should be avoided at all costs.
Ceramic selection It should be clear by now that there is no single ceramic that is suitable for every clinical situation. In order to make a rational decision one needs to ask the following questions: Does the restoration need to mask tooth discolouration? If so, how severe is this discolouration? Many ceramic
45
USE OF CERAMIC MATERIALS IN THE RESTORATION OF POSTERIOR TEETH
Does the patient exhibit signs of toothwear or parafunctional habits? Such habits can be particularly damaging for ceramic restorations, leading to localised fracture, or worse, complete fracture of the restoration. If the decision is made to proceed with ceramic restorations under such circumstances then all steps (such as pre-treatment counselling, night guards, etc.) should be taken to minimise the destructive effects of the patient’s habits.
restorations are used specifically to hide the effects of discolouration caused by such conditions as loss of vitality, tetracycline staining and general ageing. Generally speaking, in the first instance it is always preferable to attempt to counter such intrinsic discolouration by conservative treatment means such as tooth whitening or microabrasion before considering more invasive procedures. Often by doing so the patient is happy not to proceed any further but should more destructive techniques be found necessary, then the dentist’s task is always made easier if the degree of discolouration is lessened in advance.
How will the restoration be retained? Will there be an element of conventional retention (as in a full-coverage crown), or will the restoration be retained more by the adhesion of the resin cement than by conventional retentive features (as in, for example, a ceramic onlay)? As will be described below, some ceramics can be retained solely by means of micromechanical retention while in others (eg. zirconium), factors such as restoration height/depth and degree of taper are critical determinants of success.
What is the functional loading on the new restoration? Great care should always be taken to assess the likely damage to any proposed ceramic restorations by occlusal forces. Tell-tale signs such as wear facets, crack lines and chipped and fractured teeth should always alert the practitioner to the likelihood of similar damage being inflicted on any planned restorations. It is always a wise precaution to plan for the worst rather than hope for the best, and it is always good practice to hold a detailed conversation with the patient about the likely failure rates of different restorative materials under conditions of occlusal loading. It may well be that after such a discussion, the patient decides to eschew the most aesthetic option for a more durable material such as gold.
REFERENCES 1 2
3
46
Christenson GJ. Treating bruxism and clenching. JADA 2000;131:233-5. Elmaria A, Goldstein G, Vijayaraghavan T, Legeros RZ, Hittelman EL. An evaluation of wear when enamel is opposed by various ceramic materials and gold. J Pros Dent 2006;96:345-53. Dhima M et al. Practice based clinical evaluation of ceramic single crowns after at least five years. J Prosthet Dent 2014;111(2):124-30.
By balancing the requirements for aesthetics, strength, retention and avoidance of further toothwear, one can start to choose a porcelain that is most suitable for any given clinical scenario. A further consideration is the abrasiveness of the particular ceramic against natural tooth structure. Ideally, this should match that of natural tooth enamel. It is this interplay between light transmission, strength and clinical requirements which must be considered when choosing the type of porcelain to be used.6
4
5
6
Stawarczyk B et al. Two-body wear of monolithic, veneered and glazed zirconia and their corresponding enamel antagonists. Acta Odont Scan 2013;71:102-112. Preis V et al. Wear performance of dental ceramics after grinding and polishing treatments. J Mech Behav Biomed Mater 2012;10:13-22. Fons-Font A et all. Choice of ceramic for use in treatments with porcelain laminate veneers. Med Oral Patol Oral Cir Buccal 2006;11:297-302.
For example, in certain clinical situations, there is a need to mask deep discolouration, and a highly opaque porcelain is therefore desirable. So for example, if one is planning to restore a heavily discoloured premolar tooth in a bruxing patient by means of a full coverage crown, then a monolithic zirconium is likely to combine the required opacity with the strength required to resist the heavy occlusal loading. If however, an onlay is being considered, then an aluminium oxide ceramic might be more appropriate, as the densely packed crystalline nature of zirconium does not permit selective etching of the fitting surface, which is required to achieve the micro-mechanical retention found in particle-filled ceramics. In other clinical situations, where there is less need for such a high degree of opacity, and where occlusal forces are not so damaging, then a lithium disilicate material would be suitable. If the restoration under consideration is a fixed bridge, then the choice lies between a pressed particle-filled ceramic or zirconium, depending on the length of span, and the aesthetic and functional requirements. When the span is short and the bridge is located in the premolar region, consider using a lithium disilicatereinforced material. However, when the bridge is located in the molar region or is greater than three units, then zirconium would likely be the material of choice.
Conclusion The modern practitioner is extremely fortunate to have such a wide array of excellent ceramic materials at their fingertips. While these materials undoubtedly allow the creation of extremely lifelike restorations, meticulous case selection, treatment planning and material selection is vital to avoid failure and disappointment.
P R I M A R Y D E N TA L J O U R N A L
T
he demand for highly aesthetic dental restorations continues to grow, and as a result, fewer and fewer patients will now accept the inherent aesthetic limitations and potential toxicity of metal-based restorations. These restorations, such as amalgams and porcelain-fused-to-metal (PFM) crowns (shown in Figure 1) are rapidly falling out of favour as a result of their obvious aesthetic limitations, destructive tooth preparations, inability to bond to tooth structure and potential toxicity.
Figure 1: A porcelain-fusedto-metal restoration
Ceramic systems have grown in complexity over the years and now offer an extremely wide variety of options and choices. This is good in one sense as virtually every clinical scenario is catered for, but less good in another, in that it can be somewhat daunting for the practitioner who has to select a suitable ceramic, prepare the tooth correctly, direct the laboratory and finally choose an appropriate luting cement. Long gone are the days when PFMs (with their alltoo-familiar cervical shadowing, or metal
shine-through, at the cervical margins and their general lack of translucency) cemented with zinc phosphate were the only ‘aesthetic’ option.
Understanding ceramics
1
Philip Newsome
Hon Clinical Associate Professor, Faculty of Dentistry, University of Hong Kong
42
It is helpful to think about any particular dental ceramic as a composite material sitting at some point on a spectrum comprising, at one end, an unfilled predominantly glassy matrix and, at the other, a virtually wholly crystalline structure with little or no matrix at all. In between the two there exists a range of ceramics with varying amounts of particles within this glassy matrix. Conceptually, this is very similar to the
more familiar resin-based composite restorative materials, but with a glass matrix as opposed to resin.
Predominantly glassy ceramics At one end of this spectrum are largely vitreous, or glass, ceramics comprising irregularly-spaced three-dimensional networks of atoms displaying a characteristically amorphous structure. Dental ceramics in this category come from a group of mined minerals called feldspar and are based on silica (silicon oxide) and alumina (aluminium oxide), and hence belong to a family called aluminosilicate glasses. Such feldspathic ceramics exhibit low opacity, high translucency and low heat conductivity, and are hard, chemically inert and biocompatible. While this high degree of translucency means that they are unsuitable for masking dark teeth, their main drawbacks are undoubtedly their poor mechanical properties (flexural strength of 56MPa) and the high degree of shrinkage that occurs during firing. The result of all this is that they are nowadays never used as the sole porcelain component of the restoration, but rather as a veneer over a much stronger underlying core material.
Particle-filled glasses As one moves further along the continuum, filler particles are added in increasing amounts to the base glass matrix to improve mechanical properties and to control optical effects such as opalescence, colour and opacity. The introduction of the particles into the glass matrix can be achieved in a number of different ways; the process used will affect the properties of the material and so influence material choice. a) Leucite-reinforced glass ceramics The first leucite-reinforced system, IPS Empress, was developed at the
P R I M A R Y D E N TA L J O U R N A L
University of Zurich in the early 1980s and brought to market as IPS Empress 1 (subsequently renamed Empress Esthetic in 2004) by Ivoclar Vivadent in 1990.
Figure 2: This scanning electron microscope image of Empress ceramic illustrates the leucite crystals embedded in a glassy matrix. Image courtesy of Ivoclar Vivadent
These are particle-filled glasses, the particles being crystals of leucite (see Figure 2). The introduction of these crystals creates a castable glass ceramic using the familiar lost-wax process. The shape of the restoration is carved in wax, the pattern invested and heated in a furnace. A ceramic ingot is then heated to 1,200°C and forced under pressure into the mould. Using this technique it is possible to create either a coping, onto which a more translucent enamel veneering porcelain is pressed, or a monolithic restoration (ie. made from one ingot), which can then be characterised by painting high-chroma pigments onto its surface, followed by glazing to seal the surface.
Figures 3a-c: A monolithic pressed leucite-reinforced glass ceramic restoration of a hypoplastic premolar
The introduction of leucite crystals in conjunction with the injection-moulding process reduces the risk of micro-crack formation and improves flexural strength (ranging from 95-180MPa, as compared with the 56MPa of feldspathic porcelain). The result is superb aesthetics, excellent marginal integrity and biocompatibility combined with ease of fabrication and an etchable fitting surface allowing bonding using silane/adhesive resin. As far as posterior teeth are concerned, indications are limited to inlays/onlays. Figures 3a-c show a case of a monolithic pressed Empress being used to restore a hypoplastic premolar. The restoration is retained achieved solely as a result of the etched surface of the ceramic which results in micromechanical retention of the resin luting agent. Even in these cases, great care must be exercised in case selection and treatment planning, especially in terms of margin placement (ideally within enamel) and occlusal loading (ideally minimal).
In all other respects, these restorations are essentially the same as their laboratory-made counterparts in terms of mechanical properties, clinical indications, preparation requirements and luting protocols. Subsequent developments of the CAD theme include:
Leucite ceramic CAD/CAM systems The discussion so far has focused on restorations made using traditional impressions sent to a laboratory and
Polychromatic blocks featuring a smooth transition between dentine and incisal areas, thus creating natural-looking results. This is aided by a combination
V O L 3 N O 2 M AY 2 0 1 4
Figure 3a
Figure 3b
fabricated by skilled technicians. However, leucite ceramics are now available in CAD/CAM form, thus allowing aesthetic restorations to be milled chairside (using technology such as CEREC) and placed immediately. This approach has the convenience of ‘one-stop’ restorations, albeit with the need for the prescribing dentist to carry out chairside polishing, staining and glazing.
Figure 3c
of a high level of chroma in the cervical area and incisal translucency. These effects obviate the need for further characterisation in many cases. Variable translucency blocks, which take the ability to customise chairsidemade restorations even further by offering high translucency and lower brightness values, and vice versa. The latter are especially useful in those cases where there is a likelihood of a dark underlying preparation ‘greying’ the final restoration. b) Lithium dislicate ceramics In 1989, a glass ceramic (IPS Empress 2, subsequently renamed e.max) containing 70% crystalline lithium disilicate was developed, with the specific goal of improving upon the mechanical properties of leucite ceramics without compromising aesthetics. As a result, flexural strength rose to 360-400 MPa.
43
USE OF CERAMIC MATERIALS IN THE RESTORATION OF POSTERIOR TEETH
The method of fabrication was essentially the same (ie. pressed ceramic) and again, restorations could be either monolithic or bi-layer, layered with a veneering ceramic specially designed for the lithium disilicate. The improved physical properties saw these ceramics being recommended for premolar crowns and short-span premolar bridges in those cases with sufficient space present to house a 4 × 4mm connector. The high crystal density also permits greater masking of discoloured teeth than do the more translucent leucite ceramics. Indeed, more opaque ingots have been specifically developed to cope in such clinical situations.
Figure 4: The densely-packed polycrystalline nature of zirconium ceramic can be clearly seen in this image
Lithium disilicate CAD/CAM systems As with leucite ceramics, lithium disilicate systems are available in CAD form, allowing practitioners to create restorations at the chairside. Again, the result is comparable with laboratorymade restorations in terms of mechanical properties, while final aesthetics largely depend on the dentist applying stain as and when necessary prior to cementation. It is worth noting that, for ease of milling, these CAD blocks are manufactured and supplied in their weaker lithium metasilicate state, therefore taking on a blue appearance. The milled blue blocks are then fully crystallised in a furnace, which transforms them to their full-strength lithium disilicate state. Wear characteristics of particle-filled glass restorations As far as abrasiveness is concerned, low-fusing ceramic has been shown to cause less wear of opposing teeth than conventional porcelain.1 It is also well documented that rough, abraded porcelain is extremely damaging to opposing unrestored teeth. One study in 2006 compared various ceramic materials with gold, and while gold, unsurprisingly, proved to be the least abrasive, polished low-fusing porcelains also resulted in minimal toothwear.2
44
This suggests that should intra-oral adjustment be required, thorough polishing will lead to an acceptably smooth surface.
Polycrystalline ceramics At the other end of the spectrum to the predominantly glassy ceramics are the polycrystalline ceramics, which contain densely packed atoms with little or no vitreous glassy ‘matrix’ phase (Figure 4). This arrangement results in restorations that are more difficult to drive a crack through, in comparison to the less dense and irregular linkages found in glass structures. As a consequence, polycrystalline ceramics are generally much tougher and stronger than glassy ceramics. They are, however, much more opaque and more difficult to process into complex shapes than glass ceramics. It was only with the introduction of computer-aided manufacturing that well-fitting prostheses made from polycrystalline ceramics became possible. There are two basic types of polycrystalline dental ceramics currently available, namely, aluminium oxide and zirconium oxide. a) Aluminium oxide With aluminium oxide content as high as 99%, these materials offer extremely high flexural strength – between 400-700 MPa. These systems provide a solution when restorations need to exhibit high strength and aesthetic concerns are not paramount. The manufacture of these restorations uses a scanning process to machine the densely sintered ceramics and demands very careful tooth preparation if errors in scanning are to be avoided. b) Zirconium oxide Zirconium oxide-based restorations feature a wholly crystalline structure with no glassy matrix at all between the crystals. This makes for an extremely strong and tough structure (flexural strength of 1200MPa), but one that is extremely opaque with a high optical value. A
P R I M A R Y D E N TA L J O U R N A L
further concern has been the possibility of abrasion of opposing tooth surfaces.
properties of monolithic zirconium crowns and how they might best be used clinically.
As a result of these concerns, when zirconium was first introduced as a restorative material, it was used as coping material upon which a more aesthetic, but weaker, veneering ceramic (such as pressable lithium disilicate ceramic) could be added. Such restorations were well received, but problems with fracture of the overlying ceramic led to the development of monolithic zirconium restorations. The initial indications for zirconium-based restorations (which, by virtue of their physical properties and the technical complexity of production and handling, are only available through a dental laboratory) were primarily in posterior teeth as individual crown copings and bridge frameworks (see Figure 5). These included very long-span bridges (up to 12 units), for which, in previous years, only all-metal or PFM bridges were suitable.
Wear characteristics of MZ restorations The prospect of excessive antagonist toothwear has probably been the main factor deterring dentists from adopting MZ crowns. Recent research, however, suggests that this might not be cause for such concern. For example, a study comparing the in vitro wear of enamel antagonists produced by monolithic alloy, veneered zirconia and monolithic zirconium finished in four different ways (glazed using a glaze ceramic, glazed using a glaze spray, manually polished and mechanically polished) found that polished MZ was responsible for lower wear rate on enamel antagonists as well as within the material itself.4 Another in vitro study examined the wear characteristics of different dental ceramics after grinding and polishing treatments.5 Once again, the results confounded the conventional wisdom that zirconia led to opposing toothwear, concluding that zirconia ceramics yielded superior wear behaviour and lower antagonistic wear compared to porcelains. Perhaps unsurprisingly, in all cases higher ceramic and antagonistic wear was shown after grinding treatments.
Monolithic zirconium restorations The biggest drawback of any restoration combining a high-strength coping coupled with an aesthetic veneering ceramic is the possibility of the latter fracturing or delaminating, even in the absence of any contributory factors such as adverse occlusal loading or inadequate material thickness. A recent study found that fractures of layered ceramic crowns comprised the most common complication prompting replacement of ceramic crowns.3 Such failures are distressing for the patient and also for the clinician, who is faced with having either to repair the fractured area or to remove and remake the crown. Crown removal has proved to be particularly problematic in the case of zirconium-based crowns as they are usually placed with a highly retentive adhesive resin cement, and zirconium is extremely difficult to section intra-orally. The advent, however, of monolithic zirconium (MZ) prostheses presents the possibility of restoring posterior teeth with extremely durable aesthetic restorations. There follows a discussion of the various
V O L 3 N O 2 M AY 2 0 1 4
Figure 5
The message is clear: MZ restorations are unlikely to lead to excessive wear of opposing teeth provided that they are finished correctly. Rough surfaces should be avoided, and as with all ceramic materials (and indeed all restorative materials), injudicious grinding of restorative surfaces either prior to fitting or once cemented should be avoided at all costs.
Ceramic selection It should be clear by now that there is no single ceramic that is suitable for every clinical situation. In order to make a rational decision one needs to ask the following questions: Does the restoration need to mask tooth discolouration? If so, how severe is this discolouration? Many ceramic
45
USE OF CERAMIC MATERIALS IN THE RESTORATION OF POSTERIOR TEETH
Does the patient exhibit signs of toothwear or parafunctional habits? Such habits can be particularly damaging for ceramic restorations, leading to localised fracture, or worse, complete fracture of the restoration. If the decision is made to proceed with ceramic restorations under such circumstances then all steps (such as pre-treatment counselling, night guards, etc.) should be taken to minimise the destructive effects of the patient’s habits.
restorations are used specifically to hide the effects of discolouration caused by such conditions as loss of vitality, tetracycline staining and general ageing. Generally speaking, in the first instance it is always preferable to attempt to counter such intrinsic discolouration by conservative treatment means such as tooth whitening or microabrasion before considering more invasive procedures. Often by doing so the patient is happy not to proceed any further but should more destructive techniques be found necessary, then the dentist’s task is always made easier if the degree of discolouration is lessened in advance.
How will the restoration be retained? Will there be an element of conventional retention (as in a full-coverage crown), or will the restoration be retained more by the adhesion of the resin cement than by conventional retentive features (as in, for example, a ceramic onlay)? As will be described below, some ceramics can be retained solely by means of micromechanical retention while in others (eg. zirconium), factors such as restoration height/depth and degree of taper are critical determinants of success.
What is the functional loading on the new restoration? Great care should always be taken to assess the likely damage to any proposed ceramic restorations by occlusal forces. Tell-tale signs such as wear facets, crack lines and chipped and fractured teeth should always alert the practitioner to the likelihood of similar damage being inflicted on any planned restorations. It is always a wise precaution to plan for the worst rather than hope for the best, and it is always good practice to hold a detailed conversation with the patient about the likely failure rates of different restorative materials under conditions of occlusal loading. It may well be that after such a discussion, the patient decides to eschew the most aesthetic option for a more durable material such as gold.
REFERENCES 1 2
3
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Christenson GJ. Treating bruxism and clenching. JADA 2000;131:233-5. Elmaria A, Goldstein G, Vijayaraghavan T, Legeros RZ, Hittelman EL. An evaluation of wear when enamel is opposed by various ceramic materials and gold. J Pros Dent 2006;96:345-53. Dhima M et al. Practice based clinical evaluation of ceramic single crowns after at least five years. J Prosthet Dent 2014;111(2):124-30.
By balancing the requirements for aesthetics, strength, retention and avoidance of further toothwear, one can start to choose a porcelain that is most suitable for any given clinical scenario. A further consideration is the abrasiveness of the particular ceramic against natural tooth structure. Ideally, this should match that of natural tooth enamel. It is this interplay between light transmission, strength and clinical requirements which must be considered when choosing the type of porcelain to be used.6
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Stawarczyk B et al. Two-body wear of monolithic, veneered and glazed zirconia and their corresponding enamel antagonists. Acta Odont Scan 2013;71:102-112. Preis V et al. Wear performance of dental ceramics after grinding and polishing treatments. J Mech Behav Biomed Mater 2012;10:13-22. Fons-Font A et all. Choice of ceramic for use in treatments with porcelain laminate veneers. Med Oral Patol Oral Cir Buccal 2006;11:297-302.
For example, in certain clinical situations, there is a need to mask deep discolouration, and a highly opaque porcelain is therefore desirable. So for example, if one is planning to restore a heavily discoloured premolar tooth in a bruxing patient by means of a full coverage crown, then a monolithic zirconium is likely to combine the required opacity with the strength required to resist the heavy occlusal loading. If however, an onlay is being considered, then an aluminium oxide ceramic might be more appropriate, as the densely packed crystalline nature of zirconium does not permit selective etching of the fitting surface, which is required to achieve the micro-mechanical retention found in particle-filled ceramics. In other clinical situations, where there is less need for such a high degree of opacity, and where occlusal forces are not so damaging, then a lithium disilicate material would be suitable. If the restoration under consideration is a fixed bridge, then the choice lies between a pressed particle-filled ceramic or zirconium, depending on the length of span, and the aesthetic and functional requirements. When the span is short and the bridge is located in the premolar region, consider using a lithium disilicatereinforced material. However, when the bridge is located in the molar region or is greater than three units, then zirconium would likely be the material of choice.
Conclusion The modern practitioner is extremely fortunate to have such a wide array of excellent ceramic materials at their fingertips. While these materials undoubtedly allow the creation of extremely lifelike restorations, meticulous case selection, treatment planning and material selection is vital to avoid failure and disappointment.
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