VITA Machinable Ceramics

VITABLOCS® MARK II: Background
Vitablocs Mark II blocks were specifically developed for the CEREC CAD/CAM system. Several conditions must be met for any successful CAD/CAM material. In general these include esthetics, biocompatibility and strength. However for a CAD/CAM restoration, the material must be resistant to machining damage, easily polished, and wear kind. Vitablocs Mark II was and still is a revolutionary material. It’s unique combination of feldspar materials and fine microstructure make it resistant to chipping, easily polished, and wear kind. Traditional veneering porcelains had coarse microstructures which make them difficult to polish, susceptible to machining damage and often cause extreme wear of natural tooth enamel. Vitablocs Mark II has over 20 years of clinical success in more than 14 million restorations.

VITABLOCS® MARK II: Properties
Mark II ceramic blocks are fabricated using fine grain powders which produces a nearly pore-free ceramic with fine crystals, Figure 1. This results in improved polishability, decreased enamel wear, and increased strength. The strength of this material is approximately 130 MPa when polished, but may be about 160 MPa or higher when glazed, which is about twice as strong as conventional feldspathic porcelains and somewhat higher than many pressable materials.^1,2 The ceramic can easily be acid etched and cemented with composite resin. The material possess excellent esthetic qualities and can be characterized using Vita external stains and VITAVM®9 porcelain. The fine particle size microstructure helps resist machining damage, improve mechanical properties, decrease polishing time and improve wear kindness of the finished restoration. In fact, Vita has been the leader in redesigning their porcelains to create “wear kind” fine particle size veneering materials.

Figure 1a - Vitablocs Mark II Block

Figure 1b - Pressed Ceramic

Figure 1b - Conventional Ceramic

One concern about using block materials is the fact that they are monochromatic. There are a variety of block shades available to match the patient’s natural dentition and these materials also exhibit a “chameleon’ affect such that they tend to blend in with the surrounding tooth structure. In order to improve upon the esthetic match, especially for anterior crowns, a multi-layer block from Vita was introduced. Vita TriLuxe® contains a graded variation in color saturation. The middle layer (body) represents a regular intensity; the top layer (enamel) offers a lower, less intense chroma with increased translucency and the lower layer (neck) offers high color saturation and a lower amount of translucency. Recently, Vita has refined the color gradation to provide a smoother transition between layers as well as increased number of layers with a block called Triluxe forte. This makes it possible to copy the optical characteristics of a natural tooth, including translucency and color intensity, which may enhance the integration of the restoration into the remaining natural dentition.

Wear of enamel has long been a concern when using ceramics. The surface finish and microstructure of materials greatly influence enamel wear. If the surface is polished or glazed and if the microstructure is fine grained, then “polishing” of enamel as opposed to wear will occur. A number of studies show that enamel wear of Vitablocs Mark II is essentially equivalent to that of enamel against enamel if the surface is polished or glazed.^{3 - 6} These materials were tested against natural human enamel in standard abrasion system where the volume loss of and the materials was recorded, Figure 2. A “wear ratio” is obtained which normalizes the data relative to enamel versus enamel in order to account for natural variations in tooth structure. The closer the value of the test materials is to 1, the more the material behaves like natural tooth structure with respect to enamel abrasion. In Figure 2, Mark II is close to 1; while MZ100 is higher indicating material loss but still good wear kindness. As part of this study, surface roughness was also measured before and after wear testing. Surface roughness did not significantly change for the Mark II, Figure 3. Rougher surfaces may be more likely to accumulate plaque and lead to periodontal disease or recurrent decay.

Figure 2 - Wear Testing

Figure 3 - Surface Roughness

Reliability and Clinical Success
The fact that these blocks are factory made under the same conditions for each block produces a more homogeneous, more dense, and reliable ceramic than what can often be produced conventionally by hand in a dental laboratory. A study of Mark II compared to conventional porcelains demonstrated this improved reliability.⁷

Inlays and Onlays
Definitive proof of success is clinical trial data. A paper by Martin and Jedynakiewicz summarizing 29 trials over 1- 10 years (mean 4.2 years) and nearly 3,000 restorations shows a success rate of 97.4% for inlays.⁸ The material used in the clinical studies was predominately Mark II. In a study by Mörmann examining only Mark II inlays, success rate after 6 years was about 99% ^9,10 In a study by Posselt, 2328 ceramic inlays were placed in 794 patients and the survival rate was 95.5% after 9 years.¹¹ We can compare these clinical results to conventional castable glass ceramics as reported by Sharer, a 5% failure rate of Empress I inlays after 5 years and for Empress I crowns an 11.6% failure rate after 6 years with a predicted rate of 14.5% after 7 years - most failures were from the canine posteriorly.^12,13 In a review article by Fasbinder, clinical success of several trials involving predominately Mark II inlays and onlays was presented. Overall success rates were about 95 – 97% after 5 years and 90 – 95% after 10 years.¹⁴

Crowns
With respect to crowns two recent studies demonstrate the effectiveness of Mark II restorations. In a study by CRA, results of a seven year ongoing clinical trial revealed a 94% success rate for Vitabloc Mark II, a 71% success rate for Procad and only a 50% success rate for Paradigm MZ100.¹⁵ The differences in porcelain materials may be related to microstructure and response to machining damage; studies have shown less chipping occurs for the Vitabloc Mark II.^16,17 MZ100 is a composite resin and issues of lower modulus/stiffness and more wear of the material may have contributed to the lower success rates. In a separate study by Bindl and Mörmann, Vitabloc Mark II molar crowns had a 97% success over the trial period of 3.5 – 6 years.¹⁸ The data clearly indicates that Vitablocs Mark II milled restorations are clinically reliable. One of the reasons for the good clinical success may be in the ability to restore the mechanical properties of the tooth using these bonded milled materials. The stiffness of a tooth may be restored to nearly its original level (96%) by using bonded ceramics.^19,20 The properties of the ceramic closely match that of enamel and the bonded enamel-ceramic-dentin complex may mimic that of natural tooth structure. In a recent study examining load to failure of restored teeth with Mark II and a conventional feldspathic porcelain; the teeth restored with the milled crowns failed at load equivalent to that of unrestored natural teeth which was also significantly higher than the teeth restored with the conventional porcelain.²¹ It is important to note that bonding is required to achieve this reinforcement. Although lab studies may show that glass matrix high strength crowns do not need to be bonded, clinically cementing crowns using a conventional glass ionomer cement may not provide the same type of reinforcement due to a much lower bond to both the tooth and restoration. It is important to keep this in mind when using any type of glass matrix ceramic.

LABORATORY–BASED FRAMEWORK MATERIALS
There are several materials available for frameworks using the inLab system. These include the Vita In-Ceram family of materials, Vita Alumina cubes, as well as yttria partially stabilized zirconia materials, so called “pure” zirconia. This partially stabilized zirconia material is the VITA InVizion ® system (In-Ceram YZ zirconia and VITAVM ®9 wear-kind veneering porcelain).

In-Ceram belongs to a class of materials known as interpenetrating phase composites.²²They consist of at least two phases which are intertwined and extend continuously throughout the material. These materials possess improved mechanical and physical properties relative to the individual components due to the geometrical and physical constraints that are placed on the path that a crack must follow to cause fracture. Porous blocks of In-Ceram materials are milled to produce a framework. The blocks are then infused with a glass in different shades to produce a 100% dense material which is then veneered with porcelain. In-Ceram is available in three types, designed for specific regions of the mouth. In-Ceram Spinell is the most translucent with moderately high strength (350 MPa) for anterior crowns, followed by In-Ceram Alumina (450 – 600 MPa) high strength and moderate translucency for anterior bridges, anterior/ posterior crowns, and In-Ceram Zirconia with very high strength (650-700 MPa) and lower translucency for three unit anterior/posterior bridges and anterior/ posterior crowns. One advantage of In-Ceram frameworks is that they may be repaired by the laboratory using the conventional In-Ceram slip material or a wax based margin correction material. Pure zirconia block frameworks cannot be repaired.

Clinical trials of VITA In-Ceram Alumina restorations have demonstrated a high success rate; 95 - 98% success rate after 7 – 10 years in clinical trials including anterior and posterior crowns and three unit anterior bridges.^{23 -26} Clinical data on In-Ceram Zirconia is limited to that reported by the inventor, M. Sadoun who has found a 98% success rate in three unit posterior bridges over 7 years.²⁷ Indeed, there is overwhelming clinical evidence to demonstrate that the In-Ceram materials perform at least as well as porcelain fused to metal restorations. In studies of PFM bridges, success rates were approximately 95% – 98% at 5 years and dropped to about 84% - 87% at 10 years.^{28 – 30}

Pure Zirconia and Pure Alumina
Partially stabilized zirconia is one of the materials which allows production of reliable multi-unit all–ceramic restorations for high stress areas, such as the posterior region of the mouth. Due to its high strength and toughness, zirconia may be a “universal” ceramic restorative material, able to be used anywhere in the mouth. There is some confusion as to whose zirconia does what and why zirconia might be advantageous. Zirconia (ZrO₂) may exist in several crystal types (tetragonal, monoclinic and cubic phases) depending upon the addition of minor components such as Calcia(CaO), Magnesia (MgO), Yttria (Y₂O₃), or Ceria(CeO₂). Specific phases are said to be stabilized at room temperature by the minor components. If the right amount of component is added, then one can produce a fully stabilized cubic phase – cubic zirconia of mega carat “faux” diamond fame. If smaller amounts are added, 3 – 5 weight %, then a partially stabilized zirconia is produced. The tetragonal zirconia phase is stabilized at room temperature, but under stress the phase may change to monoclinic with a subsequent 3% volumetric size increase. This dimensional change takes energy away from the crack as well and can stop it in its tracks. This is called “transformation toughening”, Figure 4. Also, the volume change creates compressive stress around the particle which provides a further inhibition to crack growth. Transformation toughening helps give the zirconia its excellent mechanical properties; high flexural strength, about 1.0 GPa and toughness, 7 – 8 MPam^-0.5. Other beneficial properties include good biocompatibilty. The mechanical properties may allow for decreased coping thickness and connector sizes. Failure in all ceramic bridges often occurs at the connectors, so connector sizes must be larger, particularly for lower strength frameworks. However, both connector size and coping thickness of zirconia frameworks approaches that of conventional metal restorations. Also, longer span bridge frameworks may be made – 4 or even more units. The framework is milled oversized and then fired at about 1500°C to fully densify the zirconia. Each block has a barcode which tells the computer the density in order to properly mill the framework oversized. YZ frameworks may be intrinsically shaded using colorant solutions which can help eliminate the need for any opaque or liner and enhance the esthetic result. There is no effect of colorant on the strength of the zirconia.

The Vita alumina cubes are in chalky form similar to the YZ zirconia; they are milled oversized and sintered to full density. The alumina has a fine particle size of about 1 micron and a strength of about 600 MPa and is designed for anterior, posterior single units as well as anterior three unit bridges. The alumina is similar to Procera All-Ceram but may be produced in a variety of shades using Vita’s liquid colorants specifically designed for the alumina cubes.

Figure 4 - Transformation Toughening Mechanism

Zirconia: Is it All the Same?
A question which is often asked about zirconia is: Is it all the same? Initially, many believed that it probably was very close. An analysis of two leading brand name zirconia materials (Vita In-Ceram YZ and 3M Espe Lava) showed no significant difference in chemical composition, Figure 5. However, studies over the past year have demonstrated a significant difference in the strength and homogeneity of various generic zirconia blocks produced for use by the inLab system. In order for the frameworks to fit accurately, the manufacturer must carefully measure the density of the blocks and ensure that the blocks are homogeneous. In careful testing of blocks, it was discovered that areas of blocks may not be homogenous. Bars sectioned from certain areas of the block, usually the outer surfaces, produced warping. This may be due to differences in density in the block which arise during the pressing procedure. Some blocks had warping confined to the outer few mm, many blocks had warping which extended several mm into the block and a few blocks produced warped bars throughout. This can cause frameworks not to fit and may lead to open margins or a framework that rocks. In particular, bridges are most affected since these frameworks often span the entire length of a block. Overall, generic blocks will not mill uniformly and consistently. Also, strength values for several blocks were significantly lower (as low as 600 MPa) than the 900 – 1000 MPa expected for a “pure” zirconia material, Figure 6a and 6b. Finally, a comparison of strengths of various CAD/CAM restorative materials is presented in Figure 7. Our research laboratory continues to study zirconium oxide blocks in order to help the dental community provide esthetic, high quality, and reliable restorations for patients.

Figure 5 - Chemical Composition Energy Dispersion Analysis (EDS)

Figure 6a - Zirconia Materials: Flexural Strength (MPa)

Figure 6b - Zirconia Materials: Strength

Figure 7 - Mean Flexural Strengths of CAD/CAM Materials

References

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