The principal problem addressed by this paper is the treatment of the moderate/severe carious lesion. There are many treatment options available to dentists in order to restore these types of lesions including direct and indirect restorations. The purpose of this paper is to present evidence based data on the use of indirect esthetic restorations as a treatment option for the moderate/severe carious lesions. There are many indirect esthetic materials available, but particular attention will be paid to CEREC and Empress restorations.

Introduction

The moderate to severe lesion is defined as decay of tooth structure that extends at least 1/3 into dentin and may include more than one surface. The decay has spread laterally along the dentoenamel junction leading to an extensive lesion. Cavitation of the lesion is not a requirement, but it can be seen in more severe lesions. This type of lesion requires restoration in order to prevent bacterial attack on the pulp leading to pulpal inflammation and eventually necrosis.

Many options are available for restoring the moderate to severe dental lesion. Dentists can choose between direct and indirect restorations. Direct restoration options include the use of amalgam or composite. An indirect restoration is one that is fabricated outside of the mouth on a replica of the prepared tooth (Sturdevant, 1995). The indirect options include cast gold or porcelain crowns, inlays, and onlays. Due to the size of the moderate/severe lesion, direct restorations may compromise the strength of the remaining tooth thus decreasing long-term prognosis. Indirect restorations strengthen the remaining tooth and do not leave compromised cusps that could fracture in the future.

The indirect restoration options for the moderate/severe dental lesion include inlays, onlays, and crowns. Specifically, an inlay is an intracoronal restoration on an anterior or posterior tooth that does not include any cusp coverage (Phair, 2000). Inlays are preferred over onlays when high strength is required or when excellent control of contours and contacts is desired (Phair, 2000). An onlay is an intracoronal or extracoronal restoration on an anterior or posterior tooth that includes one or more cusps. Onlays are superior to inlays when there are weakened or fractured cusps (and buccal and lingual surfaces are intact), when alteration of occlusion is needed, and for non-vital teeth (Trushkowsky and Burgess, 2002). There are several advantages to using a partial coverage rather than a full coverage restoration. First, the margin may be finished directly on the tooth, resulting in improved marginal adaptation, margin visibility, and periodontal health. Second, the inlay/ onlay conserves tooth structure. Third, the partial restoration is easier to seat completely due to reduced hydraulic forces. Finally, an inlay/ onlay allows for easier pulp testing of the tooth compared to the full coverage crown (Phair, 2000). The contraindications for partial coverage would include an extremely weak remaining tooth structure, poor oral hygiene, non­adequate isolation, and teeth with short clinical crowns (Phair, 2000, Trushkowsky and Burgess, 2002). Full coverage veneers are needed in these situations.

When choosing a partial coverage indirect restoration, the choices include cast gold and porcelain. Gold has historically been considered the standard indirect material due to the excellent properties and its proven track record. Unfortunately, metal restorations are unable to fulfill the increasing demand of patients for restorations that are esthetically pleasing. All-ceramic restorations are an esthetic option to cast gold. Even when metal is used only as an underlying material, there is no possibility to obtain the same esthetic result as with porcelain (Christensen, 1999). Without a metal substructure, light is transmitted more easily through the crown, which improves the color and translucency of the porcelain (Douglas, 1999). In addition, metal has the potential for toxicity and allergic reactions, which is not a possibility with porcelain. Some patients suffer from "metalphobia," which is the fear possessed by many patients that metal causes imagined or real systemic problems. Since this phobia is increasing in prevalence, it is wise for the practitioner to become competent in the use of porcelain restorations (Christensen, 1999).

All-ceramic restorations have been utilized for over sixty years. Historically, porcelain restorations were considered to have excellent esthetics, biocompatibility, and compressive strength but lacked tensile strength resulting in fractures under shear forces. This limits the use of feldspathic porcelain to areas of low forces such as anterior teeth. Recently, new techniques and technology have lead to the development of stronger ceramics that are suitable for anterior and posterior teeth. However, porcelain restorations cannot always be used. Porcelain may be contraindicated in patients with para-functional habits such as bruxing and clenching, or those with economic concerns (Trushkowsky and Burgess, 2002).

All of the indirect esthetic materials discussed in this paper are prepared using the same principals. Preparations of porcelain inlays and onlays are similar to cast gold preparations but have several unique features. For both inlays and onlays, the occlusal depth should be 1.5 to 2.0 mm. For onlays, the functional cusps should be reduced 1.5-2.0 mm and the non-functional cusps should be reduced 1.5 mm. A functional cusp shoulder 1 mm in width should also be utilized (Phair, 2000). This amount of reduction allows for sufficient bulk of ceramic to avoid fracture and allow for a minimum of 1.0 mm thickness of translucent porcelain needed for proper color matching (Douglas, 1999). Second, all external and internal line angles should be rounded to avoid stress concentration reducing the chance for fracture (Sturdevant, 1995). Also, a 6-8 degree taper on proximal and occlusal walls is needed to allow for draw of the restoration (Excel Restoration Guide).

Cementation of porcelain inlays or onlays differs from that used for gold restorations. All-ceramic restorations should be cemented with resin composites. Glass ionomer cements can create stresses in the porcelain so their use should be avoided (Giordano, 2000). In order to bond the ceramic to the tooth with composite, both the preparation and the cavity side of the restoration need to be etched with hydrofluoric acid. The restoration is then treated with silanating agent to enhance bonding to the composite cement. The restoration and the preparation are both coated with a bonding agent, then dual-cure composite cement, and the restoration is inserted onto the preparation. The restoration needs to be light cured from the occlusal, facial, and lingual directions for a minimum exposure of 40-60 seconds in each direction (Sturdevant, 1995). Concerns regarding inadequate resin polymerization under the porcelain are unfounded if proper curing is utilized.

CEREC System

The CEREC system utilizes computer­aided design/computer-aided manufacture (CAD/CAM). Fabrication of high quality esthetic ceramic restorations at the dentist's chairside is the result. The development of the CEREC method began at the University of Zurich in 1980 and the first patient was treated with the CEREC at the University of Zurich in 1985. Over a decade of clinical research and documentation is available to support the clinical use of this technology.

In 1986 Siemens/Sirona acquired the licensing to market the CEREC equipment for further development. Siemens/Sirona has produced three versions of CEREC. CEREC 1, the first CEREC machine, was introduced in 1987. This version was only capable of fabricating all-ceramic inlays. The technology was very advanced for the time but had limited functionality preventing average dentists from adopting the technology. The next generation CEREC 2 was introduced in 1994. CEREC 2 was mainly intended for inlays, with the ability to create some onlays and anterior single tooth veneers. Unlike the CEREC 1, this version took advantage of advances in computing allowing CEREC 2 to be software driven and utilize a faster processor, a higher-resolution optical camera and a diamond-milling cylinder. Most importantly, software can be upgraded. In 1997, the company produced a software upgrade allowing dentists the ability to create crowns. Finally, in 2000 Sirona introduced the CEREC 3, more compact and user-friendly than its predecessors. The CEREC 3 consists of two components. The first component is the imaging unit where the optical impression is taken. The second component is a milling machine where the actual milling takes place. Individual components allow flexibility. For instance, the system's milling unit can be stationed outside of the operatory and connected via cable or radio signal. Because the software is Windows based, the user interaction is more or less common knowledge for anybody who operates a computer. In addition, Sirona continues to produce software upgrades that allow the system to be more functional and increase the lifespan of the system. To date, more than 2,000 dentists in the United States are providing CEREC restorations and this number continues to increase (Hehn 2002).

When utilizing the CEREC system, the three P's, preparation, powder, and picture, must be considered in order to produce acceptable restorations. The first P is preparation. The prep must have proper reduction and taper and be smooth and rounded. The second P refers to application of a contrast powder. First, a bonding medium is applied to the affected dentition, and a highly reflective titanium dioxide contrast powder is applied intra-orally to fully coat the preparations. Lastly, a quality picture must be taken. The infrared camera records the preparation contours and creates a 3D color rendering of the prepared tooth. The CEREC system allows the dentist to manipulate the designs to their satisfaction. The computer calculates the size of the blank necessary but it is up to the dentist to select the appropriate tooth shade. Place the proper size and shade porcelain blank into the milling machine and in approximately 12-15 minutes the patient has a very esthetic, functional restoration. More specific or localized color can be added by either fusing on a surface glaze or by applying composite shade pastes to the fitting surface as part of the cement layer.

There is a wealth of clinical data available to support the use of CEREC restorations as an alternative to cast gold restorations. Many factors must be considered when evaluating the two materials. First, the main advantage of all-ceramic restorations over gold is esthetics. Second, gold requires a material specific preparation whereas the CEREC restoration is more lesion-specific allowing conservation of tooth structure. The bond between the ceramic and the tooth has been demonstrated to be both reliable and durable which allows tooth substance that might not be viable when left as part of a conventional cavity preparation to be retained. Furthermore, large volume restorations are possible without resorting to full coronal coverage.

Marginal integrity is another important factor. The marginal gap of cast gold is considered to be less than CEREC restorations. A number of in vitro studies have evaluated the fit of CEREC restorations. Martin (2000) evaluated the fit of CEREC MOD inlays at 21 points, and reported that the average cavosurface marginal gap was 50 um +/- 15 um. Another group reported a mean marginal fit of 56 microns +- 27 microns for CEREC inlays (Mormann 1997). Gaining consistency of fit at these levels depends on the operator being competent with the technology. Cavity preparations for CEREC require smooth and even margins and roughly finished cavities may cause a lesser quality of fit. Also, both the CEREC 3D camera and the milling unit must be properly and accurately calibrated. Although CEREC restorations have slightly larger marginal gaps, they are bonded to the tooth unlike gold thus effectively sealing the gap. The difference in the size of the marginal gaps is therefore clinically insignificant.

Questions relative to clinical longevity, post-operative sensitivity and restoration fracture are best answered by long-term clinical studies. A systematic review of 29 clinical studies of CEREC ceramic inlays between the years 1986 and 1997 examined a total of 2862 restorations. The review reported a survival rate of 97.4% over a period of 4.2 years (Martin 1999). The most common reason for failure of the restoration was fracture of the ceramic. Heymann and co-workers (1996) reported on 50 CEREC inlays over 4 years of clinical service. They reported no significant changes in the inlays during the four years. There were no restoration fractures, no post-operative sensitivity, and no failures reported. Reiss (2001) followed 1,010 CEREC placed between 1987-1990 and followed those restorations over a ten-year period and found the success rate to be 90%. Impressive results even considering these are first generation CEREC restorations. Four other significant findings should also be discussed. First, the number of surfaces restored was significant. Smaller restorations (1-surface) had an 80% success rate compared to multiple surfaces which all exceeded 90%. Second, premolar restorations had a success rate of over 95% compared to molar restorations which fell to 85% over 10 years. Third, the use of dentin adhesives is a factor in longevity. The use of a dentin adhesive decreased fractures and hypersensitivity leading to success rates of 94%. Restorations without dentin adhesives had success rates that dropped to below 85%. Finally, this study provides evidence that CEREC restorations should be avoided on non-vital teeth. Success rates dropped to 65% for non-vital teeth contrasted with vital teeth with success over 92%. These studies collectively show that the longevity of CEREC restorations is similar to that of cast gold when using a dentin adhesive and avoiding non-vital teeth and areas of very high stresses.

Apart from restoration failure, the fatigue processes that occur in the remaining tooth substance are becoming increasingly recognized. Machined restorations are dimensionally stable from the time of construction and can be cemented passively onto the tooth and via bonding to the tooth reinforce the strength of the remaining tooth structure. Gold restorations do not have this capability. In addition, ceramics have a coefficient of thermal expansion that is relatively well matched to that of tooth substance and therefore place little stress on the tooth during thermal changes. Machined ceramics reduce the total stresses on the tooth than other types of restorations (Martin 1996).

Another advantage of CAD/CAM technology is that very precise esthetic restorations can be placed in a single visit to the dentist. This is convenient for the patient and the dentist. The advantages are not just economic either. The clinical advantage to a single visit is reduced trauma to the pulp. The dentin exposed during tooth preparation procedures remains isolated under the rubber dam for the whole session and is then sealed adhesively before the patient leaves. Exposed dentin need never come into contact with oral fluids and is not repeatedly traumatized as temporary restorations are placed, removed, and then replaced with the definitive restoration. Economically, CEREC is a sound choice because lab costs are avoided and multiple appointments are not needed. The infection control turnaround of the treatment room for a second visit is avoided, which is a significant cost savings (Martin and Jedynakiewicz, 2001). Of course, CEREC technology has a large upfront cost, approximately $90,000. Most dentists choose to lease the CEREC for approximately $2000 a month. Supplies per unit milled are approximately $25 for burs, materials and ceramic blocks which equals the cost of impression materials used with traditional methods. Current calculations put the break-even point for CEREC versus outside lab fees of somewhere between 8 and 20 units per month dependent on current lab expense (Blair 2002). For example, a dentist works 17 days out of the month, has a $2000 lease payment, and substitutes 3 restorations with a $135 lab fee with 3 CEREC restorations. The extra revenue generated is ($135 x 3 = 405 x 17 = $6,885 - $2000 = $4,885). This extra $4,885 turns into about $60,000 per year and $300,000 over five years (Morin 2001). A great return on the initial investment.

Empress, In-Ceram, Procera All-Ceram

Other indirect esthetic restorative options are available. Three popular all-ceramic systems include IPS Empress made by Ivoclar Vivadent, In-Ceram, and Procera AllCeram. These three systems represent the most popular categories of all ceramic crowns. The IPS Empress represents a popular choice in the pressed ceramic crown category. The In-Ceram system is an all-ceramic slip cast that is made by Vident. The Procera AllCeram exemplifies the last major category which employs Cad-Cam technology to fabricate a solid all-ceramic restoration from a single solid block (Christensen, 1997).

First the IPS Empress system will be described. To fully understand this system a review of the manufacture process will be imperative. IPS Empress is the product of Ivoclar Vivadent and was introduced to the United States in 1991. This system is a leucite reinforced castable ceramic that is designed primarily for single unit restorations including full coverage veneers, inlays, onlays, and some anterior three unit bridges (Neiva et al., 1998). An indirect procedure is done in the lab by waxing the restoration, investing, burning out and casting similar to cast gold restorations. Heat and pressure are applied in the furnace during casting, referred to as the "heat-press" technique. The hot pressing allows maturing of the crystals, which improves mechanical properties, shading and glazing (Lehner et al., 1998).

Preparation of Empress restorations is similar to the principals previously discussed for all-ceramics. A shade match should be taken before and after preparation to obtain the enamel and dentin shades. An impression is taken with an addition-cured silicone and temporized using a eugenol-free material (Krejci et al., 1992). The second appointment consists of cementing the Empress restoration with resin cement. The fitting surface of the crown is etched in the lab and during try in is protected by using Variolink try-in paste which is water-soluble. In clinic a rubber dam would be applied, the temporary removed and the prep cleaned with pumice and water. Try in the restoration with Variolink paste and if the restoration fits remove it and wash the excess paste off the fitting surface. Apply 37% phosphoric acid for 15 sec. to remove salivary contaminants, wash and then dry. Coat the fitting surface with Monobond S, a silane-bonding agent, for 60 sec. and allow to evaporate. Apply the unfilled resin, Heliobond, to the fitting surface of the crown. Acid etch the remaining tooth leaving the dentin moist. Apply Syntac Primer to the dentin, wait 15 sec. and then blow dry; apply Syntac Adhesive and dry thoroughly. Apply Heliobond and blow to a thin layer but do not polymerize. Prepare the Variolink luting cement by mixing the base and catalyst of the desired shade together. Apply the cement and seat the restoration. Polymerize the restoration starting occlusally then polymerize 40 sec. for each free surface restored. Finish with microfine diamonds, interproximal strips and then polish. (Krejci et al., 1992)

Clinical data to evaluate IPS Empress restorations has been collected. Strength of the restorative material is a very important issue and is determined by measuring flexural strength. Empress has been found to have a flexural strength value of 350 MPa (Giordano, 2000). Thus, Empress restorations are recommended for inlays, onlays, anterior crowns and anterior bridges. A subsequent concern with all-ceramic restoration system is marginal fit and adaptability. A study comparing the marginal integrity of IPS Empress, In-Ceram, and Procera Crowns showed the IPS Empress restoration to have the least marginal discrepancy. In-Ceram exhibited the greatest marginal discrepancy (161 um), followed by Procera (83 um), and IPS Empress (63um) (Sulaiman et al., 1997). These findings support manufacturer claims that the pressed ceramic method of fabrication decreases marginal discrepancy. Enamel wear is another concern with all-ceramics but Empress restorations have been found to have wear resistance comparable to that of enamel (Fradeani et al., 1997). Regarding longevity, Lehner et al. (1998) followed 155 Empress inlays and onlays for six years and found the survival rate to be 95%. Another study concludes that Empress restorations had a failure rate of 15% over seven years. However most of the failures occurred from the canine posterior. Anterior restorations had a success rate of 97% (Scharer et al 1998). These results are very favorable but long-term studies are still needed.

In-ceram and Procera All-ceram are two newer all-ceramic materials. In-Ceram is based on an industrial process modified for dental uses. They contain a core of alumina, which can be considered a ceramic sponge, which is subsequently infused with molten glass. The result is a ceramic material which has improved physical and mechanical properties. Crack propagation is reduced because the crack must pass through alternating layers of the materials. In-Ceram is classified into three types, Spinell, Alumina, and Zirconia. All are fabricated the same way but contain different materials, translucencies, and strength. Alumina was the first type and provides decent esthetics and strength. Spinell is a material that has improved esthetics because it is very translucent allowing excellent color matching. It has the lowest strength and is only used in anterior areas. Zirconia is the newest type and is by far the strongest with flexural strength of 750 MPa. The esthetics are inferior to Spinell and Alumina but are acceptable. Due to its increased strength, Zirconia is the only all-ceramic material that can be used for posterior bridges. Studies indicate that the In-Ceram restorations display acceptable marginal integrity (80-150 um) and strength (Sulaiman et al. 1997). Three-year survival rates are 96% for all In-Ceram restorations placed (Mclaren et al., 2000). One study performed in private practice indicates that over six-years 546 In-Ceram restorations had a 99.1% success rate, 70% of which were posterior restorations (Segal 2001). Long-term data is still needed. Procera is a highly pressed alumina fired at extremely high temperatures resulting in a very strong ceramic which can be milled to create inlays, onlays, anterior crowns, and posterior crowns. Procera may be utilized in the future as a bridge material, but currently soldering crowns together is not possible (Giordano, 2000). Both In-Ceram and Procera have recently been introduced as blanks for use with CAD/CAM technology. The result of these stronger materials may be longer life spans and wider indications. Table 1 provides a summary of characteristics and uses.

Table 1 (Adapted from Giordano 2000)

Type	Translucency Flexure	Strength (MPa)	Inlays And Onlays	Ant. Crowns	Post. Crowns	Ant. Bridges	Post. Bridges
Empress	2	350	Yes	Yes	?	Yes	No
Procera	3	600	Yes	Yes	Yes	(Future)	(Future)
In-Ceram Spinell	1	350	Yes	Yes	No	No	No
In-Ceram Alumina	3	450	Yes	Yes	Yes	Yes	No
In-Ceram Zirconia	4	750	Yes	Yes	Yes	Yes	Yes

Conclusion

The goal of this paper was to introduce indirect esthetic options for treating the moderate/severe dental lesion. This paper has presented evidence-based data to support the use of all-ceramic restorations. Obviously, any new type of restoration must be shown to have superior or at the least equivalent properties to the current materials, thus the need to compare cast gold with all-ceramic restorations. The problem with gold is that the esthetic needs of the patient are not meet. More and more patients do not want any metal in their mouth. We as dentists must respect the patients' demands and adopt different treatment options, such as all-ceramic restorations. Besides esthetics, porcelain demonstrates many desirable properties such as biocompatibility, chemical resistance, and minimal plaque accumulation. The big problem with porcelain has been weak strength leading to fractures after short periods of time. Low fracture strength is no longer a problem. By utilizing new technologies and techniques, all-ceramic restorations have improved strength and clinical properties comparable to those of cast gold. For example, an acceptable marginal fit for gold is 50 um. New ceramics such as Empress are capable of achieving approximately 60 um margins. Even if ceramic materials produce larger marginal gaps, it is of no clinical significance. Why? Gold needs to have precise margins because there is no bonding to the tooth. A ceramic material which has a gap of 100 um is not a clinical problem because the space is sealed with a bonded resin. No leakage can occur with proper techniques. Bonding also allows conservation of tooth structure because there is no reason to remove weakened tooth structure. Therefore, a situation where full coverage is needed when using gold instead we can utilize partial coverage all-ceramic restorations. Of course, these new all-ceramic materials do not have the history of clinical success as gold restorations have. All that can be relied on at this time are short-term studies which all indicate with proper technique success rates comparable to gold can be achieved. Disregarding new ideas because there is long-term evidence lacking is not the proper approach. The literature shows that all-ceramic restorations are highly esthetic, clinically acceptable restorations for their indicated uses.

More and more dentists will be adopting these new technologies and techniques, such as CEREC, to better treat the needs of their patients. Traditional methods do not need to be abandoned but rather supplemented. Our duty as future dentists is to educate ourselves and provide the best possible care for our patients. Presently, we believe that there is no evidence to support neglecting the use of all-ceramic restorations.

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