The Evolution and Revolution in Prosthodontics Continues
John A. Sorensen, DMD, PhD, FACP
The rate of innovations in prosthodontics has been picking up considerable momentum and products have emerged, demonstrating superb strength and high esthetics. Trends indicate a preference for metal-free restorations, as the offerings of zirconia products have become more nuanced. Also, innovative scanning systems provide dental teams a greater flexibility and enhanced productivity.
The integration of materials and digital technologies has facilitated enormous advancements in fixed and removable prosthodontics in the last 4 years. This synergistic combination has helped the specialty evolve to provide both the clinician and dental laboratory technician with enhanced prostheses possessing an increased clinical reliability while enabling fabrication with increased efficiency, reduced fabrication times, and better economy. This is all while providing greater precision and a superior restoration for the patient.
At no other time in dentistry has there been such a rapid rate of development in prosthodontic materials, digital technology, software, and the integration of systems. The products that have benefited the most from the convergence of these elements are single-tooth implant surgical/prosthodontics and posterior conventional crowns. Dentistry has witnessed an ease of fabrication of posterior restorations using high-strength ceramics that are highly esthetic, possess a monolithic structure, and have excellent occlusal anatomy.
Zirconia and Lithium Disilicate
New trends have begun to emerge. In November 2010, Glidewell Laboratories1 reported that the fabrications of BruxZirTM (Glidewell Laboratories, https://glidewelldental.com) were surpassing porcelain-fused-to-metal restorations as the most-common material fabricated in the company’s laboratory. Then, Lab Management Today2 published a 2014 survey of dental laboratories, which revealed more metal-free restorations were fabricated than metal-based restorations.
The unprecedented rise in popularity of the full-contour zirconia ceramic crown has been indisputable. Perhaps demonstrating the extent to which clinicians have embraced monolithic zirconia crowns is the commercial success of the BruxZir crown with production of more than 100,000 units per month for at least the last 3 years. BruxZir crowns are even now made for CEREC® (Dentsply Sirona, www.dentsplysirona.com) and inLab® (Dentsply Sirona). A number of other dental laboratories have considerable sales numbers for their versions of monolithic zirconia ceramic restorations. Dentists prefer these types of monolithic restorations because they are virtually indestructible, are reasonably esthetic, do not chip, and are gentle on opposing dentition.3,4
To overcome the inherent opacity of high-strength zirconia ceramics, manufacturers have directed their research efforts toward developing high-translucency zirconia for anterior restorations (cubeX2 cubic zirconia, DAL DT Technologies, https://daltechsystems.ca; Katana™ Ultra Translucent Multi-Layered, Kuraray Noritake Dental, www.kuraraynoritake.com; Prettau® Anterior®, Zirkonzahn, www.zirkonzahn.com; Zenostar MT, Wieland Dental, www.wieland-dental.de). Their work has concentrated on increasing the cubic zirconia phase, which has led to a higher translucency. An increase in the Yttrium concentration is then required to help stabilize the metastable zirconia ceramic. The tradeoff for greater translucency conferred by increased cubic zirconia is reduced strength. However, at 550 MPa to 700 MPa, the product has ample strength for anterior or posterior single crowns. Using a different zirconia formulation, Sagemax® Bioceramics (www.sagemax-dental.com) has produced the NexxZr® Plus zirconia with greater translucency but a strength of 1000 MPa.
Another innovation making zirconia more esthetic is the presintering staining/coloring/ characterization of full-contour zirconia prostheses in the green state. This provides a predictable, simple, and effective natural shading that routinely produces highly esthetic results. A significant advantage of the presintering approach is that the coloring and effects are distributed throughout the body of the ceramic and are not just a surface phenomenon. The Colour Liquid Prettau® Anterior Aquarell (Zirkonzahn) is a coloring system facilitating rapid characterization of the monolithic crowns or fixed dental prostheses. LavaTM Plus High Translucency Zirconia Dyeing Liquid (3M ESPE, www.3mespe.com), NexxZr Color (Sagemax), and Zenostar Color Zr (Wieland) are also effective coloring systems. Other manufacturers have developed multilayered-gradient shaded milling disks (Katana™ Zirconia [ML], Kuraray Noritake Dental).
With a clinically validated track record of more than a decade of widespread use, the lithium disilicate ceramic (e.max®, Ivoclar Vivadent) has been extraordinarily successful. This ubiquitous system is the gold standard for the creation of tooth structure-preserving, highly esthetic, adhesive ceramic restorations. Clinicians trust the system for applications in the prosthodontic spectrum from minimally invasive veneers, to partial coverage posterior restorations, to full molar crowns. Throughout the years, the lithium disilicate system has evolved from a predominately lost-wax pressed format to being fabricated with CAD/CAM applications.
Sirona just introduced a rapid-sintering induction oven that purportedly transforms the milling of zirconia crowns into a chairside procedure. Sirona says the CEREC SpeedFire furnace can sinter a single zirconia crown in just 15 minutes, and with its milling system, the entire crown can be completed in well under an hour.
Recent research tested a novel zirconia surface-modification technology called femtosecond laser that can systematically create 50-μm holes on the bonding surface of zirconia to create mechanical retention for adhesive cements. The results demonstrated a twofold increase in shear bond strength to MDP-containing cements compared to airborne particle abrasion alone.5,6
Scanning Systems
Since the introduction of intraoral scanning (IOS) systems and other digital prosthodontics systems at the 2013 International Dental Show in Cologne, Germany, dentistry has seen an unprecedented rate of development of IOS, design software, and milling systems. In just the last 2 years, great improvement has occurred in the integration of software and digital devices so that workflow progresses seamlessly throughout the steps of acquisition, ie, CAD and CAM.
Laboratory scanners such as the D2000 lab scanner (3Shape, www.3shape.com) boast improved accuracy, a significant increase in scanning speed, and an ability to scan two casts simultaneously. Additional innovations include calibrated transfer plates enabling capture of the occlusion and model position in the articulator, all-in-one scanning of bite-impression tray, and improved scanning of impressions.
Studies have established the high accuracy of intraoral scanners for single-tooth preparation scanning7,8 and now full-arch scanning.9 Improved technology and software facilitate extremely rapid intraoral scanning with new systems being more intuitive and easier to stay on track.
Milling
Many improvements have occurred with milling machines such as high-capacity systems holding eight milling disks (Wieland Zenotec® select hybrid, Ivoclar Vivadent) and high-capacity wet-milling machines for lithium disilicate restoration fabrication (IPS e.matrix™, Wieland, Ivoclar Vivadent). Another high-capacity system is the M4 Milling Unit (Zirkonzahn, www.zirkonzahn.com), which has an extra-large milling area (15 inches by 7 inches) suitable for production of up to 20 full arches at once. The tool-changer function holds up to 32 milling tools, and a large variety of materials can be processed in one milling process.
In addition to the developments for large milling centers, even more exciting was the introduction at the 2015 International Dental Show of 4-axis wet-milling machine systems in the mid $20,000 range (Biodenta DS 1300, Biodenta, www.biodenta.net). Also, a similarly affordable system was introduced at the recent Chicago Lab Management Today meeting (DWX-4W, Roland DGA, www.rolanddga.com). This affordability greatly expands the use of CAD/CAM systems to smaller dental laboratories and to dental practices for chairside applications to mill ceramics, PMMA, composite, and titanium.
A master’s degree thesis project measured the accuracy of chairside milling of custom titanium abutments to within 25 μm to 30 μm for precision.10 With the economical wet-milling machines, IOS, and available software, the concept of a desktop dental lab was introduced in December 2015 in which a clinic could use software to integrate the cone-beam computed tomography and IOS scans, treatment plan implants, mill out a surgical guide, design and mill a provisional crown, and design and mill an abutment and crown from a variety of materials.11
Digital Workflows
Digital technologies have also touched removable prosthodontics. To varying degrees, the Avadent (www.avadent.com), DENTCA (www.dentca.com), and Digital Denture (Ivoclar Vivadent) systems have incorporated digital workflows. The Ivoclar Vivadent system is cleverly designed in that the dentist can work at a traditional level of analog border-molded impressions and, through the process, merely make a few numeric settings that allow the dental laboratory to work in a nearly completely digital workflow. With all these systems, the denture base is milled from a factory-fabricated pink PMMA disk that is dense and free of voids and flaws. The fit of the intaglio surface of the milled base is perfect, unlike traditional heat-processed PMMA in which the 8% to 9% polymerization shrinkage pulls away from the palatal aspect creating poor adaptation. DENTCA introduced a 3D-printable denture base system.
Other exciting developments in removable prosthodontics include a system in which the doctor could perform an intraoral scan of the prepared teeth and soft tissues, and then send STL files to the laboratory. The laboratory would then 3D print the resin model, use software to design the RPD framework, and then print the CrCo metal framework. Finally, the powder alloy is laser sintered to produce a precise-fitting metal framework (3DRPD USA, www.3drpd.com).
Conclusion
Recent innovations in modern materials, digital technology, software, and integration of systems make the future appear bright for prosthodontics. With the control, reduced capital investment, added capabilities, increased flexibility, and the precision available, never has there been a more exciting time in fixed and removable prosthodontics.
References
1. DiTolla M. Glidewell Laboratories website. The BruxZir Phenomenon-A Clinician's Perspective. https://glidewelldental.com/wp-content/uploads/2016/02/bruxzir-solid-zirconia-science-guidea.pdf. Accessed May 9, 2016.
2. LMT Research Department. The Answers Issue. Lab Management Today. March 2016.
3. Sorensen JA. Three-body wear of enamel against full crown ceramics. J Dent Res. 2011;90: Spec Issue abstract 1652.
4. Stawarczyk B, Özcan M, Schmutz F, et al. Two-body wear of monolithic veneered and glazed zirconia and their corresponding enamel antagonists. Acta Odontol Scand. 2013;71(1):102-112
5. Yavuz T, Aslan MA, Akpinar Y, et al. Evaluation of femtosecond laser treatment on zirconia-resin cement bonding. J Dent Res. 2015; Spec Issue abstract 174.
6. Yavuz T, Özyilmaz, Dilber E, et al. Effect of different surface treatments on porcelain-resin bond strength. J Prosthodont. 2015 [published ahead of print].
7. Hack GD, Patzelt SBM. Evaluation of the accuracy of 6 intraoral scanning devices: an in vitro investigation. ADA Professional Product Review. A Publication of the Council on Scientific Affairs. September 25, 2015.
8. Mehl A, Ender A, Mörmann W, Attin T. Accuracy testing of a new intraoral 3D camera. Int J Comput Dent. 2009;12(1):121-128.
9. Sorensen JA. Accuracy of full-arch scanning with intra-oral scanners. J Dent Res. 2014;93: Spec Issue Abstract 52.
10. An H. Accuracy of milled custom abutments fabricated from a semi-prefabricated abutment blank. MSD Thesis, Graduate Prosthodontics, University of Washington. 2015.
11. Sorensen JA. The desktop dental lab. Inside Dentistry. 2015;11:54-61.
About the Author
John A. Sorensen, DMD, PhD, FACP
Professor, Department of Restorative Dentistry
Associate Dean for Clinics
Director
Biomimetics Biomaterials Biophotonics Biomechanics & Technology Laboratory
School of Dentistry
University of Washington
