Monolithic Vs. Layered Restorations: Considerations for Achieving the Optimum Result
Q: What are your parameters when considering placing monolithic versus layered restorations?
The primary goal of either monolithic or layered restorations is to reintegrate form, function, and esthetics with minimal damage and maximum longevity to the remaining natural dentition. Today’s state-of-the-art technology available in both realms is capable of yielding from above average to excellent esthetic results. The clinical choice between one or the other can depend on several factors that include strength and esthetics and whether restoring the anterior or posterior segments. The layering porcelain that is stacked over the core of all restorations is the weakest link that gives “under shear” or flexural loads between 90 and 140 MPa. Because of high flexural strength (380-1,000 MPa), monolithic restorations are ideally indicated for stress-bearing areas and can be used as a single bulk material without the need for the weaker outer layer of stacked porcelain, especially in the posterior zone or in the form of short-span anterior or posterior bridges.
Most powder/liquid porcelains will present color and optical properties that most closely match that of natural dentin and enamel, which is an advantage over monolithic ceramics. The greatest challenge with monolithic restorations has been optimizing esthetic outcomes. Newer blocks and ingots that present improved color and optical properties minimize the use of surface stains. CAD/CAM technology now allows the milling of blocks that have a dentin-like color bulk (eg, CEREC® Block PC), with more pronounced hue and chroma, topped with an enamel-like, more translucent layer. The milling can be adjusted to achieve the desired color result while final staining is still an option for further customization. This option is convenient in the posterior segment where esthetic challenges are not as great. Other CAM/CAM systems (eg, Lava™ DVS) allow characterization to be applied internally, rendering the restoration more polychromatic and natural looking. Other monolithic systems now present a high-translucency coping material that precludes the use of a veneering layer due to improved optical characteristics (eg, e.max® HT). This restorative option is particularly important in cases of anterior veneers on bruxers or where occlusion poses a challenge.
There is a wide range of all-ceramic choices. The outer esthetic layer can be accomplished with conventional powder and liquid porcelain or pressed over the ceramic coping. The latter seems to be gaining greater acceptance due to the ease of fabrication and precision of the marginal fit. Layered all-ceramic restorations comprise veneers, inlays/onlays/overlays, full crowns, and bridges. The principal difference in layered all-ceramic restorations lies in the ceramic used for the coping, which include zirconia, alumina, and lithium disilicate.
The good news for clinicians is that more products are entering the marketplace. While monolithic lithium disilicate (e.max®) and full-contour zirconia (BruxZir™) have been the dominant materials and have performed excellently, manufacturers are increasingly developing competitive materials, which is paving the way for more innovation. In the zirconia area, translucency is being improved. Zirconia is a wonderful material to work with, however translucency can be an issue. My recommendation today would be to use full-contour zirconia mostly on the posterior teeth. In clinical situations where cement is needed, conventional cements can be used such as phosphate or glass ionomer. This is because the strength of the restoration is not enhanced by the bonding procedure.
One of the premier benefits of lithium disilicate is that it is easy to bond. It is an etchable ceramic. I differentiate those two when I have a partial preparation or a conservative preparation when doing essentially non-retentive preparations. This is clearly an indication for e.max or other similar products. For a more “normal” crown situation, involving a retentive-type preparation or subgingival margins that can’t be bonded, or if not in the anterior, full-contour zirconia would be recommended—ie, one of the more translucent versions and one on which some surface color could be added. In the anterior, for monolithic restorations a high-translucency material is needed for esthetics purposes, which would be lithium disilicate or e.max. A new version coming onto the market from VITA, which is already on the market in Europe, is called Suprinity®, a zirconium-reinforced glass ceramic. Also, DENTSPLY has introduced new Celtra™ and a machinable version, Celtra™ Duo. Like e.max, Suprinity and Celtra require machine crystallization. The Duo version does not, however it offers about half the strength, making it well suited in situations where time is an issue and excessive strength is not needed. In the author’s experience testing these newer materials, translucency is not an issue. They can be made opaque or translucent based on the version of the material—therein lies the advance.
Relative to anterior teeth and monolithic, my preference continues to be to layer, whether using e.max, veneer, or zirconia coping. If there is space to layer and if strength is not an issue, 3-dimensional color can be created. Monolithic systems can provide good esthetics if the correct translucency is chosen and surface color is applied effectively.
Monolithic restorations are being promoted by dental laboratories heavily for their strength and reasonable cost, with the most popular being lithium disilicate and zirconia. They are both good restorations, but they should be used in different areas of the mouth for maximum strength and esthetics. In my practice, I use full-zirconia restorations sparingly in the posterior region of the mouth for full-coverage molar crowns, mostly when the patient does not want a gold restoration and there is not enough room for a porcelain-fused-to-metal crown. The esthetics can range from bad to good depending on the laboratory that fabricates the restorations. These restorations can be conventionally cemented and are very cost effective. However, they are surface stained, so if the occlusion needs adjustment, the restorations may need to be reglazed. The other drawback is if the crown needs to be removed or endodontics performed, it is very hard to drill through. As manufacturers continue to create more translucent zirconia, technicians will be able to create anterior restorations that are monolithic with minimal to no addition of layered porcelain.
Lithium-disilicate monolithic restorations are more esthetic, but the material is not as strong. They have about 400 MPa of strength and are used in the molar region and some second bicuspids. The material is more translucent than zirconium, but the final esthetics still depends on the laboratory that fabricates them. They also are cost effective because they are easily fabricated, and are popular with many laboratories. For maximum esthetics the lithium-disilicate material should be cut back and a layer of porcelain fired over the core. This would be necessary in the anterior region.
ABOUT THE AUTHORS
Newton Fahl, Jr., DDS, MS
Private Practice, Curitiba, Brazil; Director, Fahl Art & Science in Aesthetic Dentistry Institute in Brazil; Founding Member and Past-President of the Brazilian Society of Aesthetic Dentistry
Edward A. McLaren, DDS
Professor, Founder, and Director, UCLA Post Graduate Esthetics; Director, UCLA Center for Esthetic Dentistry; Founder and Director, UCLA Master Dental Ceramist Program, UCLA School of Dentistry, Los Angeles, California; Private Practice, prosthodontics and esthetic dentistry, Los Angeles, California
Robert Margeas, DDS
Adjunct Professor, Department of Operative Dentistry, University of Iowa College of Dentistry, Iowa City, Iowa; Private Practice, Des Moines, Iowa