KATANA Zirconia UTML (Photograph: Kuraray Noritake)
Cementation of zirconia
Ten-methacryloyloxydecyl dihydrogen phosphate is a bit of a tongue-twister for anyone not a chemist by profession, so in everyday communication, this adhesive monomer is referred to by its three initial letters, MDP. The monomer was developed by Kuraray Noritake Dental in 1981 to improve the adhesive strength to hydroxyapatite and has proved its strength ever since. It is indispensable in dentistry. For example, reliable adhesive cementation of zirconia restorations would not be possible without MDP.
Requirements for indirect restorations
Indirect restorations in modern dentistry must fulfil at least three requirements. First of all, they must preserve tissue. This implies that a full crown is not the first choice, because approximately 70 per cent of the tooth tissue has to be sacrificed for such a restoration. Nevertheless, full crowns are often still indicated by virtue of their mechanical retention. However, given that the retention that can be achieved by adhesive cementation is now sufficiently reliable, a less invasive restoration than a crown should be chosen more often. And this choice may very well be for a zirconia restoration. Combined with the preliminary sandblasting of such an adhesive restoration, this choice is now an appropriate one, owing to MDP.
Durability is the second requirement for an indirect restoration. This property of a restoration is largely associated with the flexural strength of the restorative material. While it has become clear that zirconia achieves the best durability scores, it should be pointed out that the cementation method also contributes significantly to the durability of a facing, inlay, onlay, adhesive bridge, etc., which can nowadays all be realised in zirconia.
Aesthetic acceptability is the third requirement for a modern indirect restoration. This means that porcelain baked on metal has become a thing of the past; all-ceramic materials are now the standard. Zirconia still has a relatively poor reputation in terms of aesthetics owing to the extreme whiteness of the earliest examples from the turn of the century. Zirconia is now available in varying translucencies, and there are even multilayer varieties (KATANA Zirconia ML, STML and UTML; all Kuraray Noritake), and these new zirconia materials no longer have to be porcelain-baked. Obviously, baking is still possible, and partial baking is a choice that is frequently made. One of the results of a multilayer build-up is that the transparency is higher incisally than cervically, as it is in natural dentition; the light falls through the incisal margin, but is blocked at the cingulum of the restoration. With a modern zirconia material such as KATANA Zirconia ML, this variable transparency goes hand in hand with a natural colour gradient from cervical to incisal. In a given colour, A1 for example, the cingulum has the corresponding dentine shade body and it passes to incisal in the appropriate enamel colour via two transition shades.
The new zirconia materials are changing the way dental technicians operate, as illustrated by the experience of Daniele Rondoni, a prominent dental technician from Savona in Italy, who has specialised in the use of multilayering technology for ceramic materials. According to his philosophy, the choice of restorative materials should be sufficiently wide to realise customised solutions. Among other things, he believes that there is still room for baked porcelain on a core of lithium disilicate or zirconia. Using baked porcelain, the dental technician can modify the surface texture of an aesthetic restoration to lend a certain age to the restoration.
As for surface structure, that the material allows for the smooth polishing of the occlusal plane is crucial to counteract abrasion by the antagonist and to maintain the occlusal balance. In this respect, the hardness of the material selected is not the main factor; the smoothness and resistance of the surface are far more important.
When selecting material for restorations, the dental technician has the option to choose KATANA Zirconia Ultra Translucent Multi-Layered (UTML) for veneers or anterior crowns, a zirconia with a translucence comparable to that of glass. This translucence is especially important for an anterior restoration that is to be fitted between flawless natural teeth, a situation that often occurs after anterior trauma. Such a restoration effected using KATANA Zirconia UTML harmonises with the neighbouring natural teeth, not least because this type of zirconia does not cause the white appearance common in anterior crowns.
The second-generation aesthetic zirconia materials are sintered at a temperature of 1,550 °C. This temperature is maintained for 2 hours. The dental technician needs to be aware that this temperature differs from the sintering temperature for KATANA Zirconia High Translucent Multi-Layered (1,500 °C).
Wide-span bridges can be realised with the latter product, whereas the size of bridges made from KATANA Zirconia Super Translucent Multi-Layer (STML) remains limited to a maximum of four dental elements. KATANA Zirconia UTML can be used for small anterior bridges, but is more suitable for anterior crowns and veneers. The reason for this is that the flexural strength of these highly aesthetic zirconia materials is lower than that of the standard zirconia, of which the flexural strength—1,125 MPa—is sufficient for the production of durable wide-span bridges. The flexural strength of the highly aesthetic zirconia varieties (approximately 750 MPa [STML] and 550 MPa [UTML]) is quite sufficient to ensure the durability of single aesthetic restorations and limited-span bridges.
Flexural strength is not the only decisive factor for durability; the method of preparation is also crucial to properties of this material. Chamfer preparation is the required form of preparation, with no knife-edge outline, no deep shoulder and, obviously, no undercuts. Since the restorations are fixed adhesively, parallel walls or grooves in the preparation are undesirable, and sharp edges and transitions must be rounded off. If a preparation for a full crown has nevertheless been made, a substantial height difference between the vestibular and palatal/lingual outline is contra-indicated.
Using the new zirconia materials means that a thickness of only 0.4–0.8 mm need be removed for a veneer in the incisal and cervical areas, and only 0.5 mm is required in the labial plane, which corresponds with the requirement to preserve tissue. For inlays too, only 1 mm is sufficient to achieve a durable result. If the inlay is extended to an onlay, 1 mm is also sufficient for the area where the cusps are capped. For a full crown in the lateral parts, a 1 mm space must be kept as a minimum and this thickness must also be maintained for the upright walls of the preparation.
Minimum wall thickness of KATANA
† Maintain 0.8 mm thickness of pressed ceramic in all areas. When trimming the zirconia framework, the framework should be at least 0.4 mm.
A wide range of possibilities has already been suggested for the durable fixation of zirconia-based restorations. All of these options have also been researched, but there is no point in conducting further research into the best cementation procedure, according to Prof. Matthias Kern from the University of Kiel in Germany. As a scientist and practitioner, Kern has been involved in the adhesive cementation of zirconia for nearly 20 years. Based on his wide experience, Kern is convinced that three requirements have to be met to achieve the reliable cementation of zirconia. First of all, a rubber dam must be applied for the procedure, which is obviously easier for partial restorations than for total restorations. It is not only from the perspective of tissue preservation that it is useful to keep the preparation limited for this reason. The second condition is that micromechanical adhesion needs to be achieved. For zirconia restorations, the necessary adhesion is obtained by sandblasting the surface. Obtaining chemical adhesion is the third requirement.
Based on extensive research, Kern is convinced that chemical adhesion can only be achieved by using MDP. His first publication on this subject dates back to 1998. It was the use of PANAVIA (Kuraray Noritake), which did indeed contain MDP, that made it possible to achieve durable synthetic resin bonding to zirconia after sandblasting.
Dentists and dental technicians are apparently somewhat averse to sandblasting,‡ as evidenced by the extensive research conducted in an attempt to find an alternative. No such alternative has yet been found. Efforts have been made to fuse a silica layer on to the zirconia to improve bonding, but, according to Kern, the results of this procedure—the Rocatec method, for example—have been disappointing. Nor is the silanisation of a zirconia restoration effective, because zirconia does not react to silane. In order to achieve durable cementation of zirconia restorations, dentists therefore have no other option than the purchase of a sandblasting device.
Sandblasting can be carried out in a small cabin to prevent the surrounding area of the practice from being affected. Soft air abrasion is carried out at 0.5 bar, while tight air abrasion is performed at 2.5 bar. The exact pressure is not all that crucial to the adhesion of the zirconia, provided that it is between 0.5 and 2.5 bar.§ Kern advises sandblasting at a pressure of 1 bar, so that the surface to be bonded becomes somewhat rougher without this being visible to the naked eye. Obviously, the part of the restoration that does not require bonding, such as the outside of a veneer or the dummy of an adhesive bridge, has to be protected from the effect of the abrasive particles. It is also advisable to apply a colourant (waterproof marker pen) to the area to be sandblasted prior to the procedure. The colour disappears during sandblasting, making it easy to check that the entire adhesive surface has actually been abraded.
The restoration surface can be cleaned using alcohol after the sandblasting process. This step is optional. If the alcohol becomes contaminated, for example, by saliva residues, the effect will be negated, because the sandblasted surface would be contaminated as well. The choice of the fixation procedure is relatively simple, provided MDP is used. MDP is not present in glass ionomer cements, which are also sometimes used for the cementation of zirconia restorations because of their ease of use. Kern advises against using these. It is clear from all of the studies that the composite cements containing MDP provide the most durable results. The oldest known cement in this category is PANAVIA EX, which was introduced in 1983. The optimised PANAVIA V5 was presented recently as the single one cement for all cementation indications guaranteed to work according to a predictable procedure. All of the cements and bonding materials produced by Kuraray Noritake contain MDP.
Possibly because Kern conducted his research in Maryland in the US for two years, he has recorded remarkable results with adhesively cemented Maryland bridges (adhesive bridges). It has also become apparent that, most of the time, an adhesive bridge functions best with only one wing. For example, if a one-wing zirconia adhesive bridge is cemented using a cement containing MDP, replacing a maxillary lateral incisor adhesive, such an adhesive bridge may remain in place for up to 20 years, to the satisfaction of both dentist and patient. This restoration, with its survival rate of 95.2 per cent after five years, therefore qualifies as a permanent restoration. And the same goes for an onlay bridge made from zirconia.
In summary, sandblasting and MDP are essential for the durable bonding of zirconia restorations. Furthermore, MDP is an extremely durable product.
‡ It had long been assumed that the tetragonal and/or cubic structure would relapse into a monoclinic state owing to sandblasting, as a result of which fracturing would occur because of the associated expansion. Sandblasting with aluminium oxide particles of 50 µ at a maximum and a maximum air pressure of 2.5 bar does not, however, cause any damage.
§ Air pressure is different from the manufacturer’s recommendation.