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Modern Indirect Restorations: The Right Material for Every Situation
Russell A. Giordano II, DMD, CAGS, DMSc
Dentists now have a wide offering of materials for indirect restorations, as a result of the growth in digital dentistry. Computer-aided design and computer-aided manufacturing (CAD/CAM) have also fostered a conversion from the use of porcelain-fused-to-metal restorations to all-ceramic restorations and factory-made composite resins. Today the myriad materials used for indirect restorations can be categorized according to composition, microstructure, translucency, mechanical properties, full-contour or substructure/frameworks, and chairside or laboratory fabricated.
In general, chairside single-visit materials are full-contour restorations fabricated from fully dense materials that require minimal post-machining processing. Laboratory-fabricated restorations may be machined or pressed, with many block materials having a corresponding pressable equivalent. The services of dental laboratories are needed to fabricate restorations for machining of large frameworks or full-contour materials that require extended processing such as most zirconia materials.
A clinician typically chooses a product that combines esthetics with functional requirements while keeping in mind any time constraints of the patient. Some materials function extremely well in low-stress areas, such as centrals and laterals, but may fail in high-stress posterior regions.
One of the most important restrictions in the posterior region is the amount of occlusal reduction that can be obtained. Many clinicians may find that they underestimate just how much room for restorative materials they actually have. The use of digital technology can aid the dentist in real time to make sure that the needed clearance is reached. However, dentists encounter many occasions in which the planned material cannot meet the actual minimum reduction possible. If the minimum thickness cannot be met, the load-bearing capacity will not be sufficient to withstand the forces in the mouth and risk for an unexpected early failure becomes high.1,2
Right Material for the Right Situation
Because numerous options for indirect materials are available, a material can be found for every clinical situation.
For machined restorations, porcelain blocks (eg, IPS Empress CAD, Ivoclar Vivadent, ivoclarvivadent.com; Vitabloc Mark II, Vita Zahnfabrik, vita-zahnfabrik.com) with corresponding pressable materials (eg, Empress, Ivoclar Vivadent; Vita PM9, Vita North America, vitanorthamerica.com) are available. These block materials possess high translucency and similar mechanical properties. In comparison to pressed restorations, block materials tend to have minimal porosity and better longevity. If the clinician can achieve the manufacturers’ recommended 2-mm occlusal minimum thickness, then success rates for bonding to the tooth are equivalent to dental zirconia materials. Most of these studies were performed using Vita Mark II and have posterior success rates of 95% to 98% up to 7 years.3
Another broad category with corresponding pressable materials is glass-ceramics, which start as glass. The application of a second heat cycle changes the ionic state, causing crystallization in the glass. This heat cycle is critical to producing the correct amount and size of crystals as well as the proper shade. Dental professionals should closely follow the manufacturers’ recommended cycles and make sure furnaces are properly calibrated. If crystallization is not performed correctly, then weak, soluble glass and improper shade can be produced. The most widely known glass-ceramic is the IPS e.max® CAD block (Ivoclar Vivadent, ivoclarvivadent.com) with the corresponding pressable e.max press (Ivoclar Vivadent). However, a number of other glass-ceramic blocks such as Celtra Duo (Dentsply Sirona, dentsply.com), Vita Suprinity® (Vita Zahnfabrik), and Obsidian® (Glidewell Laboratories, glidewelldental.com) are available. Celtra Duo block also has a corresponding pressable. However, pressables tend to have porosity due to the pressing method. Blocks are fabricated under strict conditions at the factory and tend to have minimal defects. Due to the relatively high crystal content of up to 70%, these materials tend to have lower translucency than porcelain. The improved mechanical properties and fracture resistance allow for the use of these materials at about 1.5-mm occlusal reduction. Although manufacturers state that these materials may be “cemented” with glass monomer, it is my opinion, based on clinical studies and laboratory research, that maximum resistance to failure is achieved by bonding with composite resin cement.
A number of polymer-based materials have been developed for use as permanent machined restorations. These include traditional composite resin such as LavaTM Ultimate (3M ESPE, 3mespe.com) and GC CerasmartTM (GC America, gcamerica.com). Both materials have a silica glass (Cerasmart) and/or zirconia silicate glass filler (Lava Ultimate). Lava Ultimate has about 65% volume filler and Cerasmart has approximately 55%. These materials have good translucency and excellent fracture resistance. However, they do have much higher flexibility than other block materials and still may exhibit wear and discoloration over time. Usage indications for Lava Ultimate include inlays and onlays; however, the crown indication has been removed due, in part, to issues related to debonding. Cerasmart still retains all its indications. The issue of bonding is critical, and again careful attention must be paid to the manufacturer’s recommendations for restoration and tooth preparation when bonding these restorations. The high degree of cure of the polymer resin and the individual filler particles make it more difficult to achieve a good bond. Minimum occlusal thickness is also recommended as 1.5 mm. A number of dental practices have eliminated the use of direct-fill composite resins and switched to machined composite resins. The relatively fast machining, high density (bubbles in hand layered), and full control of the contours generally provide superior restorations and may take less time than hand layering composite.
A completely different polymer type is called polyaryletherketones (PAEKs), which are thermoplastic materials with different properties relating to their exact chemical composition. Three basic types are becoming increasingly used for frameworks on which composite resins or ceramics may be bonded. These include poly-ether-ether-ketone (PEEK) (PEEK-Optima, Invibio Biomaterial Solutions, invibio.com) and poly-ether-ketone-ketone (PEKK) (Cendres+Métaux, cmsa.ch). PEEK is an amorphous material that may be compounded with fibers to create a fiber-reinforced polymer, and PEKK is a crystalline material with higher mechanical properties. An increasingly popular use of these materials is for implant-supported frameworks. Although PAEKs may have high strength values, they are highly flexible similar to indirect composite resins and have much higher flexibility as compared to the composite resin blocks.4 Only fiber-reinforced materials have flexibility similar to composite resins.
Interpenetrating Phase Ceramics
A unique category of materials, interpenetrating phase ceramics are essentially comprised of two completely interconnected networks—a ceramic and polymer. One might think of a ceramic sponge being filled with a polymer such that the ceramic and polymers are completely connected to themselves and to each other. The bulk of the material is the ceramic network, 75% by volume. Because the ceramic is dominant, issues such as color stability are eliminated, as color is dependent on the ceramic and not the polymer. The flexibility issues seen with conventional composites are also nonexistent due to the interconnected ceramic “backbone.” Interpenetrating phase ceramics such as Vita Enamic® (Vita Zahnfabrik) may represent a new area of development for advancing machined materials that are resistant to damage and easy machinability. It is a first-generation material with a unique microstructure, easy to machine (4 minutes for a crown), is bur kind, and requires polishing only. Enamic tends to be more resistant to chipping than feldspathic material or glass-ceramics. Due to the unique structure, load-bearing capacity is higher and requires only 1.0 mm of occlusal thickness. The US Food and Drug Administration has recently approved Enamic blocks for implant abutments or one-piece implant/abutment crowns. Enamic should be acid etched and bonded just as conventional feldspathic and glass-ceramic blocks.
Polycrystalline materials include alumina and zirconia. Alumina was first fabricated for all-ceramic restorations by Nobel Biocare (nobelbiocare.com) and marketed as NobelProcera. Since then, zirconia has become the most dominant machinable material, technically called yttria-partially stabilized zirconia (Y-TZP). In the past year, some important developments have occurred in the material type and processing of the zirconia family. Zirconia (ZrO2) is the oxidized form of zirconium (Zr) just as alumina (Al2O3) is an oxide of aluminum (Al).
Zirconia exists in three major phases: monoclinic, tetragonal, and cubic. Monoclinic is the largest form, tetragonal is the intermediate, and cubic is the smallest. Biomedical and structural/functional applications of zirconia typically do not use pure zirconia. The addition of other ceramic components may stabilize the monoclinic phase at room temperature. If the right amount of component is added, then a fully stabilized material can be created. The addition of smaller amounts (5 percentage by weight [wt%]) produces a partially stabilized zirconia. Although stabilized at room temperature, the tetragonal phase may change under stress to monoclinic with a subsequent 3% volumetric increase. This property is called transformation toughening.
Dentistry typically has used Y-TZP with about 5 wt% yttria. Another key component is a small amount of alumina to help prevent uncontrolled transformation that would result in cracking and failure. The “standard” zirconia has about 0.25 wt%. Yttria is responsible for the zirconia’s ability to resist damage and stop cracks, while alumina prevents wholesale transformation leading to failure of the material under aging. Due to transformation toughening, this type of zirconia has excellent fracture resistance. This property, in part, allows clinicians to use this material at only an occlusal thickness of about 0.8 mm. However, it remains a traditionally brittle ceramic. Therefore, extreme caution should be used when going below the 1.0-mm thickness.
“Ultra/Super/Mega” Translucent Zirconia
With only about 0.05 wt% alumina and 9.0 wt% yttrium, “Ultra/Super/Mega” translucent zirconia has arrived in the marketplace in the past year. Research conducted at Boston University has revealed that the crack-stopping/damage-resistant property in a standard zirconia is not present in the high translucent material.5 In fact, typical procedures that may be performed in the laboratory or by the dentist when adjusting this material can reduce the strength from an untouched value of about 700 MPa to 400 MPa after using a 125-micron wheel and only 300 MPa once sandblasted with 50-micron alumina.5 Thus, caution must be used when considering this material for various clinical applications and particularly if adjustments need to be made chairside. Usage may be best for low-stress areas such as centrals and laterals. Studies into the fatigue resistance are needed to fully determine proper clinical use.
Single-Unit Standard Zirconia
Advances in production of single-unit standard zirconia have taken two approaches. Both might allow for single-visit chairside fabrication of zirconia restorations. One is by Glidewell Dental with the release of BruxZir® Now, which is a traditional Y-TZP zirconia that is already fully dense as opposed to the porous blocks that require sintering. The block and bur are supplied together for single use. Machining time is approximately 45 minutes, and the standard 6-hour sintering cycle is eliminated. Only polishing or polishing and glazing is required before placement of the restoration.
Dentsply Sirona has taken a different approach for single-visit zirconia crowns. A porous zirconia block is machined to produce a crown. The crown is then placed in a special furnace, CEREC SpeedFire (Dentsply Sirona), which allows for sintering and glazing of the crown in approximately 15 minutes. Findings from a recently completed, yet-unpublished, double-blind study performed at Boston University revealed the strength of the speed-fired zirconia and conventionally fired zirconia (6-hour cycle) was statistically the same. In other research using fast firing with a different furnace and different zirconia, the zirconia was found to be significantly weaker.6
Finishing Machined Restorations
Finishing procedures for machined restorations should include a polishing step before glazing. The surface finish on machined restorations tends to be rough; even “wear kind” block materials can abrade the opposing dentition if the surface is not smooth. Although glaze may fill in this rough surface after initial application, glaze tends to wear off within 3 years, leaving a rough and damaged surface.7-10 Polishing zirconia is particularly important. In general, the use of polishing wheels with embedded diamonds followed by the use of diamond paste produces a smooth surface just as good as a glaze.
Proper luting of any of these materials is important to the long-term success of the restoration. It is my opinion that materials with a glassy matrix should be bonded with composite resin cement. These include feldspathic materials, glass-ceramics, interpenetrating phase ceramics, and composite resins. Properly preparing the restorations by using acid etching and silane treatment is extremely important. Different materials require different etching times. Also, some etchants have concentrations higher than those listed in the directions; typically etching times are based on 5% hydrofluoric acid. However, 9.6% etchants are also popular. Over-etching and under-etching can both lead to debonding from the restoration. If bonding cements are not used, then a traditional retention form must be employed. Zirconia restorations are typically cemented with glass ionomers and often debond because of lack of proper retention form. One possible solution is the use of cements with monophosphate agents that bond to zirconia, alumina, and metals.
The large variety of materials for indirect restorations allows for selection of the most appropriate material to suit the patient’s needs. Clinicians should keep in mind the minimum thickness required as well as proper luting techniques for each material. If these requirements are followed, then the use of any of these materials can lead to a successful outcome for the patient.
Dr. Giordano is a paid consultant for Vita and Dentsply Sirona. He has received honoraria from Dentsply Sirona and Vita, and grant/research support from Dentsply Sirona, Vita, Ivoclar Vivadent, and 3M ESPE.
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