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Achieving an Ideal Restorative Material
There are various indications for using lithium disilicate in combination smile design cases.
By Thomas E. Dudney, DMD
In choosing a restorative material, dentists are often faced with many factors to consider, including the esthetic desires of the patient, functional requirements of the material, tooth color, core or abutment being restored, condition of the tooth, whether the restoration is anterior or posterior, and whether the dentist prefers cementing or bonding the definitive restoration.1 According to Spear, a philosophy shared by the author, “all teeth should be restored with the most conservative restoration that satisfies the patient’s esthetic and functional requirements.”1
For instance, a porcelain veneer may be the most conservative restoration, and may be functionally strong enough when bonded to enamel, but it may not satisfy the patient’s esthetic demands if it must mask a dark tooth. A veneer may be the most conservative restoration to satisfy the patient’s esthetic goals when tooth color is acceptable. However, if the patient is a bruxer, a veneer may not be functionally strong enough. Conversely, an all-ceramic crown might satisfy the esthetic and functional requirements when tooth color is acceptable, but would not be as conservative as a veneer.
In addition to enabling the most conservative treatment approach possible, esthetics and strength are among the most desirable characteristics for a restorative material. This is especially true when restoring anterior teeth. However, an ideal restorative material possesses other characteristics, and dentists have long sought one material that incorporates them all.2,3
To present esthetically, an ideal restorative material would mimic the qualities of natural teeth, such as translucency, opalescence, and fluorescence.4,5 The material also would be strong enough to withstand occlusal forces, demonstrating high flexural strength, high fracture toughness, high compressive strength, and high tensile strength. It also would be biocompatible, color stable, and gentle to opposing dentition but not easily abraded. The material should also demonstrate low solubility in the presence of oral fluids, be relatively easy to fabricate, adapt well to margins with different margin designs, be easy to polish in the mouth, and produce predictable results in a variety of applications. Unfortunately, there is no one material available today that provides the very best of all these characteristics. However, in this author’s opinion, lithium disilicate comes closer than any other material presently available to achieving the ideal.
Lithium disilicate is a ceramic material that contains approximately 70%, by volume, needle-like crystals in a glassy matrix.5,6 The controlled size, shape, and density of this unique crystalline structure results in a ceramic that demonstrates greater strength and durability, but a relatively low refractive index that can produce outstanding optical properties, such as translucency, with optimal esthetics.3,5,6
Lithium-disilicate ceramics are presently available in two different processing techniques: pressable and milled CAD/CAM. Pressable lithium-disilicate restorations (IPS e.max® Press, Ivoclar Vivadent, www.ivoclarvivadent.us) are fabricated with a lost-wax hot-press technique, while the milled CAD/CAM lithium-disilicate restorations (IPS e.max® CAD, Ivoclar Vivadent) are fabricated using either laboratory or chairside CAD/CAM technology.6,7 Although both products are composed of lithium disilicate, the flexural strength of the pressable material (400 MPa) is slightly higher than the milled CAD/CAM material (360 MPa), due to the size and length of the lithium-disilicate crystals (Figure 1 and Figure 2). All other material properties are similar, however, and the ability to fabricate restorations with two different techniques is advantageous. Furthermore, because of the material’s greater strength and the presence of a glassy matrix, dentists can either conventionally cement or adhesively bond lithium-disilicate restorations.6,8,9
Lithium-disilicate materials also exhibit higher edge strength, meaning they can be finished thinner without chipping. Additionally, the low viscosity of the heated ingots and greater dimensional stability of the materials allow restorations to be pressed as thin as 0.3 mm. This makes lithium disilicate an excellent choice for no-preparation and minimal-preparation veneers.3,10,11
The pressable and CAD/CAM forms of lithium disilicate (IPS e.max Press and IPS e.max CAD) are available in a variety of translucencies. These include a high translucency (HT) ingot ideal for anterior esthetic cases, as well as ingots with more opacity, which are ideal for cases involving dark teeth and core build-ups needing to be masked. The esthetic nature of these materials, especially for anterior smile design cases, can be further enhanced with either a cut-back and layer technique or a stain-and-glaze technique.5,7
In addition to veneers and thin veneers (press only), lithium disilicate can be used effectively to fabricate anterior three-unit bridgework (press only) up to the second premolar,8,12 as well as anterior and posterior crowns, inlays/onlays, telescopic crowns, and implant restorations. For posterior crowns, lithium disilicate does not require a layering ceramic when pressed or milled to full contour, exhibiting a monolithic strength of 360 MPa to 400 MPa.
Traditionally, posterior crowns have been fabricated with a metal or high-strength oxide ceramic core (eg, alumina or zirconia) that must be layered or veneered with a more esthetic glass ceramic. Depending on the material used, layering ceramics possess a flexural strength in the range of 90 MPa to 160 MPa, which makes them less likely to resist chipping and fracturing during function than a crown pressed or milled to full contour with lithium disilicate.6,13
Ideally, in all smile design cases, the treatment plan would call for minimal-preparation or no-preparation veneers on teeth with acceptable color, using a translucent powder-liquid porcelain or pressed glass-ceramic restorative material to achieve the desired esthetic result. Unfortunately, clinical situations often involve worn, eroded, broken down, missing, or dark teeth and/or core build-ups, as well as old or unesthetic restorations. These issues necessitate a treatment plan that incorporates a combination of different types of restorations and materials within the same arch.
When faced with challenging cases such as these, the dentist and ceramist may benefit from using lithium disilicate because of its high strength, esthetic qualities, physical properties, ease of fabrication, versatility, and predictable results.6 The following case illustrates the use of lithium disilicate (IPS e.max Press) as the restorative material chosen to satisfy the esthetic and functional goals of treatment as conservatively as possible.
A 52-year-old man who was unhappy with the appearance of his teeth and concerned about gingival recession and wear presented for a consultation to discuss treatment options (Figure 3 and Figure 4). He stated that, although he realized the worn teeth did not show when he smiled, he was worried about their worsening condition (Figure 5). Additionally, the patient was interested in treatment options that would not only address his restorative needs but also provide a more attractive smile and appealing color.
An examination was performed, which included a medical history, periodontal charting, radiographs, joint and muscle evaluations, and digital photographs. The patient’s medical history was non-contributory and periodontal health was acceptable, with pocket depths within normal limits; the patient reported no soreness or tenderness in the muscles or in the joints when loaded. In addition to the areas of recession, abfractions, and incisal wear already noted, the clinical examination and radiographs revealed several old restorations, some with recurrent decay and some teeth with interproximal caries.
The patient also was congenitally missing both maxillary lateral incisors, and the mesial repositioning of the canines resulted in spaces where the canines would have normally been. These spaces had been restored with porcelain-fused-to-metal (PFM) bridges, including a two-unit cantilever bridge on teeth Nos. 5 and 6, using the first premolar as the abutment, and a three-unit bridge on teeth Nos. 10 through 12 using the canine in the lateral position and the first premolar as abutments ( Figure 6 , Figure 7 , Figure 8 and Figure 9). Both the unrestored canine (tooth No. 6) and the restored canine (tooth No. 11) had been shaped to simulate lateral incisors.
At the first appointment, polyether impressions (Impregum Soft™, 3M ESPE, www.3mespe.com), a facebow transfer (Artex®, Jensen Industries, www.jensendental.com), and a centric-relation bite registration were taken and forwarded to the laboratory. Digital pictures were also sent to the laboratory, along with instructions to fabricate a diagnostic wax-up, a putty matrix for provisionals, and preparation guides. When the patient returned for the second appointment, all of the old restorations were removed, as well as any recurrent and interproximal decay, and the preparations were refined (Figure 10 , Figure 11 and Figure 12). Provisional restorations were fabricated using the putty matrix and a temporary material in shade B-1 (Luxatemp®, DMG America, www.dmg-america.com) (Figure 13).
The provisionals were instrumental to the success of the case, as they provided invaluable information for laboratory communication while allowing the patient to “preview” a simulation of the final result before completion of the case. They also made it possible to evaluate esthetics, function, and phonetics. Therefore, it is always advisable to have patients return for a postoperative appointment during which specifics such as length, shape, color, speech, and the patient’s bite can be checked and adjustments made, where necessary. Equally important, this information can be communicated to the laboratory via photographs, models, and written instructions, as was done in this case after minor adjustments and patient acceptance.
At the insertion appointment, the provisional restorations were carefully removed, beginning with the maxillary arch, and the teeth were cleaned with hydrogen peroxide and a disinfectant rinse (Consepsis®, Ultradent Products, Inc., www.ultradent.com). The definitive restorations were tried individually and then together to evaluate fit, interproximal contacts, and color. Try-in pastes were not used in this case but are often valuable at this stage when assessing color and selecting a cement shade. As previously stated, a distinct advantage of lithium disilicate is that it can be either adhesively bonded or conventionally cemented.
The material chosen for the restorations in this case (IPS e.max Press) is composed of lithium-disilicate crystals embedded in a glassy matrix, and, therefore can be etched with hydrofluoric acid like other glass-ceramics (ie, feldspathic and pressable) and adhesively bonded to enamel and dentin. Adhesive bonding with a total-etch system and resin cement offers such advantages as higher bond strengths, better wear resistance, and lower solubility. It is, however, more technique sensitive, and strict adherence to proper protocols is essential to ensure success and to prevent postoperative sensitivity and/or debonding and microleakage. A concerted effort must also be made through isolation and tissue management to prevent the prepared teeth from being contaminated by blood, saliva, or crevicular fluids. Therefore, the proper steps, including manufacturer’s instructions, must be carefully followed.
In this case, the resistance and retention form was adequate for conventional cementation of the two three-unit bridges and the single crown. However, it was necessary to adhesively bond the veneers and onlay veneers. Because adherence to protocol for bonding to all teeth was possible, the decision was made to do so. A silane coupler was applied after cleaning and etching the intaglio surface of the definitive restorations with hydrofluoric-acid gel. The silane coupler was allowed to sit for 1 minute to dry before the internal aspect was painted with a thin coat of bonding liquid.
While the restorations were being prepared for bonding, the teeth were isolated with a rubber dam and the preparations were cleaned with the disinfectant (Consepsis, Ultradent Products, Inc), rinsed, and lightly dried. A phosphoric-acid gel was then applied for 15 seconds—with care taken to avoid any contact with the gingival tissues—and rinsed off, removing excess moisture but not over-drying the prepared teeth. A desensitizing agent (Gluma®, Heraeus Kulzer, www.heraeus-dental-us.com) was applied, followed by multiple coats of a fifth-generation prime and bond liquid (ExciTE®, Ivoclar Vivadent), which was lightly air-dried to evaporate the solvent and then light-cured.
At this stage, the operatory light was turned off, and the veneers were seated with a light-cure resin cement, shade white (Variolink® II, Ivoclar Vivadent), while the onlay veneers, bridges, and crown were placed with a dual-cure resin cement (Variolink® II base and catalyst, Ivoclar Vivadent), shades white and transparent. After careful cleaning of the excess cement with cotton rolls and brushes, the restorations were tacked in place, flossed interproximally, and light-cured from the facial and lingual aspects for 1 minute.
Next, the margins and interproximal contacts were polished, the rubber dam was removed, and the same steps were repeated for the mandibular arch. After completion of the cementation procedure, the occlusion was checked, adjustments were made where necessary, and the definitive restorations were polished. The patient returned to the office 1 week later for a postoperative evaluation, at which time minor adjustments were made. The patient was very pleased with the results ( Figure 14 , Figure 15 , Figure 16 , Figure 17 , Figure 18 , Figure 19 and Figure 20).
In recent years, patients have begun to place greater importance on the appearance of their teeth and the value of an attractive smile. Furthermore, they have higher expectations for what dentistry can provide, which is likely a result of an increased awareness obtained from various media sources, the marketing efforts of dentists, and information available on the internet. These expectations also exist for clinical situations that have traditionally been more difficult to treat such as combination cases, dark teeth, missing teeth, etc. Fortunately, manufacturers have continued to strive to develop new materials to meet these challenges, and dentists, ceramists, and technicians have continued to improve their techniques and abilities through education and sharing of knowledge.
These efforts have resulted in improved restorative materials and techniques, as well as a better understanding of their use. Lithium disilicate is a material that not only allows dentists to be as conservative as possible while still satisfying esthetic and functional goals, but it also possesses outstanding physical properties, is easy to fabricate, delivers predictable results, and is versatile in application. These characteristics make it an ideal choice for a variety of indications, from a single crown to more complex restorative cases like the one presented herein.
The author would like to thank Frontier Dental Laboratory for fabricating the restorations in this case.
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