Reflections on Modern Dental Ceramics

Reflections on Modern Dental Ceramics

Kenneth A. Malament DDS, MScD      

INTRODUCTION
An important clinical objective is dentistry that is both aesthetic and functional for the patient. The knowledge within dental technology, dental science, and the dental practice has dramatically expanded, leading to better quality, a higher level of artistry, and more standards-based clinical applications. Ceramics are the most consistently predictable aesthetic dental material, and dentists can now offer more treatment options for patients’ complex problems. Metal-ceramics continue to be the “state of the art” and profoundly affect complex prosthodontic care where splinted restorations are required. However, single phase or monolithic all-ceramic materials have become increasingly more popular and are far less likely to chip, as may all bi-layered ceramics. Monolithic all-ceramic dental materials already dominate the market and, with future development, even greater long-term success can be expected.

All-ceramic materials have been developed and improved to optimize strength, aesthetics, and fit of the restorations. Few researchers have studied their long-term use or performance factors, without modeling the data, until recently. Previous bi-layered all-ceramic crowns (eg, Procera [Nobel Biocare], In-Ceram [VITA North America], etc) for posterior teeth have reached their full potential. Despite substantial improvements in material strength and toughness, these restorations are still susceptible to failure because of breakage and chipping at relatively high rates. All-ceramics (such as lithium disilicate and zirconia), when used as monolithic (not layered) materials, are likely to change dentistry and the expectation for long-term survival. Zirconia monolithic and lightly bi-layered all-ceramic restorations continue to have significant clinical success. Research, with only a few years of observation, has proved the efficacy of the ever-changing zirconia restorations.

Clinicians face decisions daily about which restorative material will best meet the aesthetic and mechanical strength needs for a given patient. For each specific situation, benefit and risk should be evaluated in light of the following: the patient’s expectations and desires; therapeutic goals, based on the standards in prosthodontics; possible alternative treatments; anatomical limitations and prosthodontic limitations; and predictability. The standards in prosthodontic education can offer only a generalized opinion on a ceramic material choice because few research papers have described the survival of these different materials over time with the confounding variables that might affect survival.

Two classes of all-ceramic materials currently dominate the dental market, lithium disilicate (a polycrystalline glassy ceramic) and zirconia (a metal oxide ceramic, also known as zirconium dioxide). Unfortunately, there is a significant amount of advertising intended to influence the choice dentists make in treating their patients that is often not science-based nor based upon long-term clinical experience. Thus, the purpose of this discussion is to present an update, including concerns, related to these popular dental ceramic materials.

LITHIUM DISILICATE RESTORATIONS
IPS e.max (Ivoclar Vivadent), a lithium disilicate ceramic, was developed in part by Prof. Wolfram Holland at Ivoclar Vivadent. After the development of clinical applications, the material was released to the dental community about 12 years ago. This ceramic material is well researched, and many authors have described its physical properties. The effect that flaws in IPS e.max lithium disilicate or luting agent spaces have on fracture potential and tensile strength have been tested; as well as the effects of physiologic aging in a water environment, abrasiveness, wear, and surface roughness. Though it has been recognized that standard physical tests for new restorative materials may not provide the necessary information to establish success in long-term use, the author has made some profound clinical observations. One can easily see in Figures 1 and 2 that lithium disilicate (IPS e.max) is the most successful material (other than gold) in the author’s practice. It has met, or exceeded, almost all of the clinical requirements considered “ideal” for a dental ceramic used in clinical practice.

1. IPS e.max is a monolithic ingot manufactured to a very high standard in glass ceramics. The physical strength properties and ability to resist fracture have been well researched and above one can observe the clinical data compiled (by the author) for more than 10 years (as previously seen in Figures 1 and 2).
2. There are many different colored and variously saturated e.max ingots produced; there is even a multicolored ingot to be used either when pressing or with CAD/CAM technology. Research has proven that e.max has close to ideal color and light properties, considering the level of light absorption and reflection as compared to natural teeth. To allow for greater translucency and natural effects, its preferred veneering material is a fluorapatite ceramic creating stratification (Figure 3). Research has proved that thinly veneered e. max crowns have similar physical properties to monolithic restorations. During the 10-plus years of my observation, minimal chipping has been observed and there is no degradation of the physical properties with time. The crown color is stable because all the major color is built into the crystal structure.
3. Laboratory technology can create occlusion and full anatomy with either the lost-wax pressing technique or with CAD/CAM technology. Lithium disilicate wears almost the same as human enamel and, after occlusal adjustment or functional wear, this ceramic is minimally abrasive and can be easily polished.
4. In the author’s opinion, lithium disilicate is the most versatile all-ceramic material to be used in most areas of restorative dentistry. Minimally invasive (conservative) tooth preparation and aesthetic restorations are a currently a high consideration (Figure 4). Because lithium disilicate is a glassy polycrystalline ceramic and can be acid-etched and treated with silane (or a universal primer), it is an ideal material to use for complete or partial coverage restorations (Figure 5). These restorations can be luted using either an adhesive resin cement or a resin modified glass ionomer (RMGI) cement (as long as there is adequate resistance and retention form in the preparation). The survival of the ceramic especially in molar areas is significant, as can be seen in Figure 6.

Figure 1. Author’s gross all-ceramic data. The physical strength properties and ability to resist fracture of lithium disilicate (IPS e.max [Ivoclar Vivadent]) have been well researched and above one can observe the clinical data compiled (by the author) for more than 10 years.	Figure 2. Author’s IPS e.max full-coverage restoration data (during 10-plus years).

Figure 3. Patient presented with decayed and failing ceramic veneers (left). IPS e.max with fluorapatite veneering ceramic layered crowns (right) were luted with Variolink 2 (Ivoclar Vivadent) cement.

Figures 5a and 5b. Patient presented with decay and a failing silver amalgam restoration. The tooth was prepared as an inlay where the inner angles of the cusps were hollow ground to allow more space to bond to enamel. An IPS e.max monolithic inlay was luted with Variolink 2 cement.

Figure 6. Author’s IPS e.max partial coverage restoration data.

  
If, after endodontic therapy, if there is adequate resistance and retention due to significant axial wall height, an IPS e.max core can be created to restore lost tooth structure. This ceramic core can be etched, silanated, and cemented into an acid-etched and dentin-bonded tooth (Figure 7). In this scenario there is no need to enter the often delicate roots in an attempt to gain retention. This can be accomplished as an endo inlay on any tooth that has had endodontic therapy, creating an excellent seal for the endodontically treated canals. The author’s IPS e.max endodontic restoration data can be seen in Figure 8.

IPS e.max can be pressed to a prefabricated gold post (Figure 9) by extending the wax pattern and pressed ceramic surface over and on to dentin wall structure; in this way, one can significantly increase ferrule for retention or even the need to block out discolored dentin.

To replace one tooth, a one-piece crown and cantilever IPS e.max restoration can be used whether for space reasons or even as a more aesthetic provisional restoration. (The author’s IPS e.max crown-cantilever restoration data can be seen in Figure 10.) There are reports that a “Maryland” type bridge can be successfully used, but one should be cautious in placing prosthesis of this type, as there have been reported connector failures.

IPS e.max can be luted to a stock titanium implant abutment (Figure 11) to provide an excellent and aesthetic custom implant abutment (Figure 12).

Figure 7. Endodontic therapy was completed on a molar after tooth preparation for a complete coverage crown. There was adequate axial wall height. An IPS e.max core was created and cemented with a universal resin cement (Multilink [Ivoclar Vivadent]) after etching the dentin.

Figure 8. Author’s IPS e.max endodontic restoration data.

Figure 9. Lithium disilicate (e.max) pressed to gold post and core.

5. Bacterial pellicle and bacterial colonies do not form an aggressive connection on lithium disilicate. This would be the same for any dense, nonporous, and smooth/polished ceramic.

6. One issue to consider is that because lithium disilicate is a very stiff and tough material, post-cementation restorations are very difficult to remove; these restorations are not easily retrievable. (Note: There have been articles in Dentistry Today by Dr. Glenn van As [September 2012] and most recently by Dr. Jeff Cranska [June 2013 and March 2015] reporting the successful removal of all ceramic restorations using an erbium YAG laser.)

Figure 10. Author’s IPS e.max complete coverage crown and cantilever restoration data.

Figure 11. IPS e.max lithium disilicate cemented to an implant abutment with a universal adhesive resin cement (Multilink).

ZIRCONIA RESTORATIONS
Zirconia restorations continue to be successful (from the standpoint of fit, retention, wear, and fracture resistance), and yet controversial in some clinical leader and academic circles, for use in simple single unit and more complex restorations.

1. There are perhaps 50 different zirconia ceramics on the market and, often, the laboratory technician or clinician have no idea what the manufacturing standards were for a given zirconia block or disk used to fabricate restorations coming from different sources. The physical strength and ability to resist fatigue fracture is dependent upon the manufacturing standard that is applied by a specific manufacturer.
2. Zirconia is susceptible to damage accumulation. The CAM (milling) process of fabrication in the powder state has been reported to place damage in the restoration that persists even after it has been heated to the crystallization temperature. If there is grinding of the restoration, after it has been manufactured in order to finish it, serious damage and even phase transfer to a weakened state has been described. There is research that has raised concerns of hydrothermal degradation in CAD/CAM zirconia that could prove over time to be a major problem, although with the short clinical observation there is little evidence that this has yet occurred.
3. Zirconia is inherently an opaque material and although there are now more translucent zirconia materials available, these latter materials have reduced strength and resistance to fracture compared to the more opaque zirconia materials. Zirconia can be used as a core to block out discolored dentin effectively but then a feldspathic veneering ceramic needs to be built up to create proper aesthetics. In the dental laboratory, serious chipping of the feldspathic porcelain has been reported and confirmed by clinical observation. However, it has been reported clinically that specific zirconia and feldspathic porcelain combinations demonstrate significantly less chipping. In addition, if colorants are infiltrated directly into the zirconia, minimal chipping has been reported.
4. There are few reports in the dental literature that represent survival with a minimum of 5 years observation. The information that exists today is certainly encouraging, but insufficient to prove efficacy during many years.
5. Laboratory technology can develop occlusion easily in its complete form with CAD/CAM technology. This minimizes the likelihood of chipping that may be induced by damage during and after adjustment. It has been demonstrated that zirconia, in its smooth fired state or properly polished, wears the opposing enamel much like gold and enamel against enamel. However, if adjusted and not properly polished by the clinician, it could be more abrasive to natural tooth structure than the in vitro studies that have involved using meticulously polished zirconia.
6. Although zirconia can be used in thinner dimensions in full-coverage crowns successfully, utilizing CAD/CAM to create partial-coverage restorations is very challenging. Because the zirconia internal surface is so smooth, there have been some reports that restorations are spontaneously (or otherwise) coming out (Figure 13). On the other hand, others are claiming that zirconia can be handled in ways that make the material as retentive as an acid-etched ceramic. In point of fact, any effort to created roughness either by improper/excessive sandblasting or grinding the internal surface of a restoration has been found to create damage that may ultimately lead to fracture and there is active research attempting to solve this dilemma. Instead of using sandblasting to clean a restoration after try-in, universal cleaners have been developed (such as Ivoclean [Ivoclar Vivadent]) that can be used to easily and quickly decontaminate the internal surfaces after restorations try-in and before cementation.
7. Zirconia can be luted to a stock titanium implant abutment to provide an excellent and aesthetic custom implant abutment. It can be used as a crown-cantilever restoration or as a large- or small-span fixed bridge. It can be utilized for full-arch implant-supported prostheses, but one should be aware that there are many different levels of sophistication and technical excellence that could potentially affect significant short- or long-term chipping/fracturing.
8. Zirconia has been proven to be bacterial plaque resistant.
9. Zirconia is a very stiff material and removal by preparation is difficult, but there continues to be improvements in bur technology by the manufacturers by making diamond burs that are more resistant to degradation (even if they do not cut significantly faster).

Figure 13. A zirconia full-coverage restoration dislodged when chewing candy.

Zirconia full-coverage restorations present a minimal risk when placed in incisor, premolar and molar areas. Since zirconia cannot be etched, partial-coverage restorations and cores do present a risk. The long-term success of zirconia, full-arch implant-supported reconstructions is dependent on the quality of the zirconia used as well as the level of sophistication and expertise of the laboratory team. The use of zirconia in full-arch implant-retained complex prosthodontics has an acceptable level of risk, but discussion with the patient should include potential remake fees in case of a possible future catastrophic failure.

CLOSING COMMENTS
What has been presented here documents the many points that clinicians and dental laboratory technicians must take into consideration to evaluate “best practices.” Every all-ceramic material discussed herein has a significant place in the current dental practice. Their aesthetic and functional advantages and risks have been described. Within the limitations of this discussion, and in the author’s opinion based upon much clinical experience with these materials, it is concluded that the lithium disilicate all-ceramic single-tooth restoration represents a significant advantage over zirconia and metal-based ceramic materials.

Acknowledgments
The author would like to thank Thomas Sing, MDT, for completed the laboratory technology; Brendon Cornell for the fabricated of the IPS e.max inlay; and Marc Nevins, DMD, MScD, for completing all periodontal therapy.

Dr. Malament received his DDS from New York University College of Dentistry and a specialty certificate and master’s degree from Boston University School of Graduate Dentistry. Dr. Malament has a full-time practice limited to prosthodontics in Boston that includes a dental laboratory with master dental technologists. A past president of the American Board of Prosthodontics, he is a clinical professor at Tufts University and a course director in the postgraduate department of prosthodontics. He is a Fellow of the American College of Prosthodontists, Academy of Prosthodontics, Greater New York Academy of Prosthodontics, and Northeastern Gnathological Society. He is an active member of many dental organizations including the International College of Prosthodontists, American Academy of Fixed Prosthodontics, American Academy of Esthetic Dentistry, Academy of Osseointegration, and Northeastern Prosthodontic Society. A past president of the Greater New York Academy of Prosthodontics, Northeastern Gnathological Society, and the Northeastern Prosthodontic Society, he has served as the secretary and director of the American College of Prosthodontists and secretary-treasurer of the International College of Prosthodontists. He presently serves on the Board of Directors of the Academy of Prosthodontics and the American Academy of Esthetic Dentistry and is the past president of the American Academy of Dental Science. Dr. Malament can be reached via email at ken.malament@verizon.net.