April 1, 2014
by David Hornbrook, DDS, FAACD, FACE
Over the past 20 years, the trend in dentistry has been in search of metal-free alternatives to replace many of the traditional restorative materials that have been used for decades in both direct and indirect applications. This trend has been the focus of manufacturers, researchers, and clinicians for many reasons. Obviously the initial primary goal was to improve aesthetic value of the restorations we place. Elimination of metal will eliminate the compromising effect of employing a dark, opaque substructure that does not exist in natural dentition. As materials evolved, the aesthetic value was achieved, but even more importantly was all the other major benefits that metal-free restorations overcame that the traditional metal and metal-supported options did not offer including conservation of tooth structure, wear compatibility, strength and durability, and “bondability.”
Conservation of tooth structure should be of vital importance to all clinicians when restoring the dentition. The introduction of new materials has provided options that did not exist previously. This importance is accentuated by the patient’s desire to have minimally invasive procedures and maintaining as much natural, healthy tooth structure as possible. Numerous factors contribute to a long term, successful restoration and are interdependent. For instance, although the longevity of a restorative material depends upon its mechanical properties to that of natural dentition,1,2 the permanence and durability of the seal between the restoration and natural tooth structure, success of marginal integrity,3 and mechanical strength from adhesive effectiveness will consequently determine predictability, reliability, and durability of the restoration.1
Material strength and bonding/cementation alternatives required for the selected restorative treatment are predicated upon functional stresses such as wear and fracture patterns and the strength of masticatory forces endured.4,5 The location of the restoration in the mouth also determines strength and durability requirements, in addition to aesthetic considerations such as color, shape, translucency, and polishability, especially when restoring anterior teeth.1 The occasional need to combine restorative materials to satisfy treatment expectations further compounds the problem. To simplify material selection for the dentist and provide patients with long-lasting and aesthetic restorations, dental manufacturers continue to develop enhanced restorative materials that resolve the everyday challenges faced in both the dental practice and the ceramic studio and are suitable for a variety of indications. The ideal restorative material capable of solving the everyday challenges faced by today’s dentists should possess physical, material, and optical characteristics similar to natural teeth, demonstrating the ability to sustain equivalent masticatory forces and wear resistance.1
A question that is frequently asked by most clinicians is whether there is an ideal material for all applications. Certainly the answer to this question would be what is required of the restoration. Often times, the primary requirement is aesthetics and thus many materials traditionally used are eliminated. In determining the ideal material, there are several characteristics that must be considered. These include aesthetics, strength and durability, bondability, wear compatibility, marginal integrity, conservation of tooth structure or amount of tooth structure that must be removed to satisfy material usage, and ability to use different cementation options.
A material that has received much praise and clinical success since its introduction almost 10 years ago is a unique lithium disilicate material (IPS e.Max, Ivoclar-Vivadent, Amherst, NY) that can be used in a monolithic form to optimize strength or combined with an overlying flourapatite glass-ceramic to optimize aesthetics and mimic natural tooth structure6 (Figs. 1-7).
Lithium disilicate is a metal-free restorative material constructed by melting lithium dioxide, phosphor oxide, potassium oxide, and alumina and quartz powders into a glassy matrix composed of 70 percent “needle-like” crystals. This high crystal density results in outstanding mechanical properties and flexural strength values from 360 to 400 MPa. Flexural strength is defined as a material’s ability to resist deformation under load. The flexural strength represents the highest amount of stress within the crystalline material at its moment of rupture. This fracture resistance is almost four times that seen with traditional feldspathic ceramics that have been used as overlying material for metal copings in porcelain-fused-to-metal crowns.6,8-11
Another important factor in choosing ceramics, especially when wrapping over the incisal edge in the anterior or when utilizing the ceramic as a guide plane for laterotrusive movements, is the wear compatibility of the ceramic. Many traditional ceramic materials have shown to significantly wear of opposing enamel.12 The trend has been to develop ceramics that are not wear abrasive, yet still have excellent wear resistance. Ceramics with finer grained leucite particles and smoother polished surfaces, including lithium disilicate, have shown to have wear compatibility very similar to that of enamel against enamel.13
One of the many advantages of the lithium disilicate material is its versatility. With its strength and bondability to resin cement, this includes inlays/onlays where otherwise cuspal coverage or full crowns would be indicated (Figs. 8-11). Many studies have shown that intra-coronal bonded restorations can have “tooth-strengthening” properties that are superior to non-adhesively bonded restorations and can actually render the restored tooth less fracture resistant than a non-restored tooth.14,15
FIGURE 10. Large amalgam restorations removed from maxillary premolars. Lingual cusps were intact, as well as mesial contact of second premolar and mesial and distal contacts of first premolar. Treat planned was bonded all-ceramic onlays to replace defective restorations and restore aesthetics and function.
With the increased patient awareness of conservative or minimally-prepared veneers, there is an increased indication for materials that will provide high aesthetic value coupled with marginal integrity. The e.Max lithium disilicate, using a wax and pressed technique, can easily be fabricated to a thinness of 0.2 mm. The high flexural strength of this material and the ability to achieve very accurate margination even with thin application makes it ideal for minimally-invasion dentistry16 (Figs. 12-16). This ability to fabricate these restorations accurately in very thin restorations makes it an ideal material to replace the compromises with marginal integrity and margination accuracy often seen with the materials marketed for the “prepless” or “minimally-prepped” technique. The author has placed hundreds of these very thin veneers, even with no chamfer margin placement and the fit and marginal contour would rival any “prepped” veneer preparation.
The high flexural strength also permits fabrication of three-unit bridges replacing premolars forward. Research has shown the lithium disilicate material is clinically successful when replacing a pontic of 11.0 mm or less in the anterior and 9.0 mm or less in the posterior. This makes it ideal for three-unit bridges replacing a single missing anterior incisor or a premolar17 (Figs. 17-21).
FGURE 17. 3 unit lithium disilicate bridge and single crown.
Since the lithium disilicate can be bonded into place and fabricated in very thin layers, the necessary reduction of existing tooth structure is significantly reduced. With a porcelain-fused-to-metal restoration, typically a ceramist requires 1.5 mm to 2.0 mm of thickness to obtain aesthetic results because of the thickness of the metal and the opaquer required to hide the metal. Since the lithium disilicate is translucent, the thickness of the facial/buccal and lingual/palatal dimensions can be fabricated in thicknesses less than 1.0 mm. This makes it ideal as an “enamel-replacement” only restorative and significantly reduces the need for aggressive preparation of tooth structure, yet still delivers excellent results (Figs. 22-24). Not only is it indicated for traditional bridge applications, it is an excellent alternative to metal when placing a resin-bonded bridge, both in the anterior and posterior (Figs. 25-33).
FIGURE 22. Patient presented with amelogenesis imperfecta. Treatment plan was full mouth rehabilitation to replace defective enamel using all-ceramic restorations.
With the increasing number of anterior implants placed, there is an increased need for a predictable “tooth-colored” abutment that a very translucent and natural appearing overlying crown can be placed. With metal abutments, the compromise has always been hiding or diluting the influence of the underlying “grey” or “gold” abutment. Many clinicians advocate the use of zirconium oxide abutments to overcome the compromises of the metal, yet the adhesion to the zirconium and also the potential fracture of the zirconium at the implant-abutment has always been a concern. Since the lithium disilicate abutment is waxed and then pressed or milled from a solid block of lithium disilicate, with the accuracy and control that dentistry has seen with cast gold, it is an ideal material to be used as the suprastructure for a titanium-based “UCLA-Style” abutment. The lithium disilicate suprastructure is then adhesively bonded to the titanium base, which yields the benefits of the accuracy and strength of a machined titanium base of the abutment against the titanium implant and a “tooth-colored,” very bondable, abutment for the overlying crown. With very thin gingival biotypes, and bone-level implants, the lithium disilicate can be shaded “pink” or “gingival-shaded” where the abutment remains below the gingival margin. This reduces any graying or shine-through of the abutment through the gingival tissue (Figs. 34-41).
FIGURE 34. Patient presented with missing right lateral incisor due to extraction after endodontic failure. Treatment plan was placement of a dental implant followed by a custom abutment and all-ceramic crown.
The examples of the cases presented in this article exhibit the versatility of lithium disilicate in diverse applications. The innovations of modern dental materials, especially with metal-free restorations, have helped clinicians resolve and overcome daily challenges of trying to meet the aesthetic and functional demands of their patients, while being as conservative as possible. Combining strength, marginal integrity, and high aesthetic value, the evolution of dental restoratives will help provide long lasting results.OH
Dr. David Hornbrook graduated from UCLA School of Dentistry and practices advanced restorative dentistry in San Diego, California. He has lectured internationally on all facets of aesthetic and restorative care, and has authored numerous articles in most of the leading dental journals. He was the original co-founder of the Las Vegas Institute and then founded and directed the PAC~live live-patient program. He is an adjunct faculty member at several dental schools, and is the director of the educational entity, the Hornbrook Group. He can be reached at www.davidhronbrook.com.
The Author would like to acknowledge creamists Hakjoo Savercool, Rob Maatta, Matt Roberts, Marv Staggs, and Jeff Austin for their ceramic artistry in the manuscript. Oral Health welcomes this original article.
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