New Kids on the Block by Dr. Jeffrey Horowitz

A look at how hybrid milling blocks can be an efficient alternative to lithium disilicate and zirconia

by Jeffrey W. Horowitz, DMD, FAGD

With CAD/CAM technology becoming commonplace in general dentistry, practitioners are faced with indirect material options that may or may not be suitable for a given clinical application. In this article, the differences in block materials will be discussed with a focus on the newest generation of hybrid (nanoceramic and resin) materials.

Milled zirconia, both in-office and laboratory fabricated, has certainly gained popularity as the costs associated with these restorations have dropped over the years. The primary indication for zirconia is a full-coverage crown with the ability to add a veneering ceramic in an aesthetic zone. The advantages of this material are its overall strength and that when used as a monolithic restorative, there is no ceramic interface to serve as a point of failure. Because the bond strengths to these restorations are limited by many factors, they are not indicated where there is not adequate cohesive retention.^1,2 Careful attention must be given to the polish because these restorations can also be quite abrasive to the opposing dentition.

There can be no discussion on indirect restoratives without lithium disilicate (E.max) entering the conversation. This material now has a proven track record for beauty, strength and reliable bond strengths.³ It must be noted that when milled, lithium disilicate loses some of its reported strength when not bonded adhesively.⁴ Even with this material’s pressed form, the profession has seen a higher fracture rate in first and second molar applications. ⁴

One drawback to milling both lithium disilicate and zirconia restorations in the office is that additional time is required under high heat to fully crystalize these materials. This requires extra equipment as well as extra time with a patient occupying a dental chair. Another common issue with milling ceramic is that thin margins may chip, and the materials can be hard on the milling burs.⁴

In a perfect world, a milled restorative would not need to go in a crystallization or sintering oven and would have excellent strength, favorable bonding characteristics, resilience and, of course, be reasonably aesthetic. One such product that follows these guidelines is Voco’s Grandio Blocs (for in-office milling, discs for laboratories). The blocks are made of a fully cured resin matrix that is highly filled (86?percent) with nanoceramic technology. The product, with high flexural and compressive strength, can be used for inlays, onlays, full crowns, implant crowns and bite rehabilitation cases.

Case Report
A 65-year-old male presented with an existing amalgam in a first molar (#3). The tooth exhibited a fracture on the mesial marginal ridge as well as visible recurrent decay (Fig. 1). The tooth was prepared with cuspal coverage to reduce flexure⁵ while leaving a 1mm ring of enamel to enhance the adhesive protocol (Fig. 2).⁶ The quadrant was then scanned with a Cerec Omnicam (Dentsply Sirona). Grandio Blocs was selected as the restorative material and subsequently milled with the Cerec MCXL (Dentsply Sirona) milling unit (Figs. 3–5). After rough finishing with OS1 F and EF burs (Microcopy), the restoration was polished with ASAP polishers (Clinician’s Choice) at 15,000rpm, leaving a highly polished surface (Figs. 6–8).

The intaglio surface of the restoration as well as the preparation were then micro-etched using a Prep Start (Zest-Danville) with 27-micron aluminum oxide powder.⁷ After this step, the etched surface of the block was prepared for bonding with the application of Ceramic Bond (Voco), allowing adhesion to the ceramic filler throughout the resin matrix (Fig. 9).

The tooth was prepared for bonding with a Wedge-Guard (Triodent/Ultradent) placed on the proximal surfaces. After this, a total etch technique was performed, etching the enamel ring for 15 seconds, followed by a 10-second dentin/enamel etch (Fig. 10). After a thorough rinsing protocol, a single layer of Futurabond?U (Voco) primer/adhesive (Fig. 11, p. 80) was brushed onto both the dentin and enamel for 20 seconds. The material was thinned by a gentle stream of warm air produced by a warm-air dryer (A-dec). The adhesive was then light-cured for 20 seconds.

Bifix QM (Voco) resin cement (Fig.?12, p. 80) was then placed in the onlay and the restoration was placed with firm pressure until a tack-cure could be applied. After thorough curing and a five-minute period to allow for full polymerization, the restoration was finished as any other resin restoration. The final restoration blended beautifully with the surrounding tooth structure and the patient was quite satisfied with the end result (Figs. 13 and 14). Grandio Blocs, with its compressive and three-point flexural strength, allow me to create good-looking restorations in less than one hour of chair time.¹⁰

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Figure 13

Figure 14

References
1. Tzanakakis, Emmanouil-George C. et al. Is there a potential for durable adhesion to zirconia restorations? A systematic review Journal of Prosthetic Dentistry, Volume 115, Issue 1, 9 – 19
2. Skienhe H, Habchi R, Ounsi H, Ferrari M, Salameh Z. Evaluation of the Effect of Different Types of Abrasive Surface Treatment before and after Zirconia Sintering on Its Structural Composition and Bond Strength with Resin Cement. Biomed Res Int. 2018; 2018:1803425. Published 2018 May 27. doi:10.1155/2018/1803425
3. A. Rauch, S. Reich, O. Schierz Chair-side generated posterior monolithic lithium disilicate crowns: clinical survival after 6 years Clin Oral Invest, 2 (2017), pp. 2083-2089
4. Willard A, Chu T-M. The science and application of IPS e.Max dental ceramic The Kaohsiung Journal of Medical Sciences Volume 34, Issue 4, April 2018, Pages 238-242
5. Larson TD, Douglas WH, Geistfeld RE. Effect of prepared cavities on the strength of teeth. Oper Dent. 1981;6(1):2-5.
6. Gamborgi GP, Loguercio A, Reis A. Influence of enamel border and regional variability on durability of resin–dentin bonds. J Dent. 2007;35(5):371-376.
7. Güngör MB, Nemli SK, Bal BT, Ünver S, Dogan A. Effect of surface treatments on shear bond strength of resin composite bonded to CAD/CAM resin-ceramic hybrid materials. J Adv Prosthodont. 2016;8(4):259-66.
8. Misevska C, Grozdanov A. Contemporary Dental Ceramic Materials, A Review: Chemical Composition, Physical and Mechanical Properties, Indications for Use. Open Access Maced J Med Sci. 2018;6(9):1742-1755. Published 2018 Sep 24. doi:10.3889/oamjms.2018.378
9. Silva LH, Lima E, Miranda RBP, Favero SS, Lohbauer U, Cesar PF. Dental ceramics:a review of new materials and processing methods. Braz Oral Res. 2017;31(supple 58):133–46.
10. Gerstofer JG et al; University of Tubingen. Report to Voco. 2016