The Balance Between Function and Esthetics

In the dental office, dental hygienists are the patient’s advocate and information source for the often complex world of modern dentistry. Because they are consulted about all dental procedures, not only the ones they perform, dental hygienists need a broad body of knowledge that includes all facets of clinical dentistry. Functional tooth restoration is an important part of dental practice and significant scientific and clinical advances have been made in restorative materials over the past 2 decades.

As esthetic dentistry has evolved, the unnatural appearance of certain functional restorations has become socially unacceptable. Previously, functional restorations were the aim of restorative dentistry and metal showing in a patient’s mouth garnered few complaints. Recent advances in the esthetics, strength, and durability of restorative materials are encouraging dental practitioners to consider esthetic and less aggressive intracoronal restorations as an alternative to full coverage crowns to restore teeth with large caries or undermined tooth structure. Esthetic, tooth-colored restorative materials are now the first choice for intracoronal restorations and for replacement of failing metallic restorations, replacing traditional composite materials such as amalgam and cast gold.

Amalgam restorations are being replaced because of alleged adverse health effects and inferior esthetic appearance.¹ High noble metal cast gold restorations are still considered the gold standard of restorative materials for posterior teeth. They are biocompatible with excellent functional strength and they allow for conservative tooth preparation. However, cast gold is used less frequently for intracoronal restorations since the esthetic revolution of restorative dentistry began in the early 1970s.³ Gold-based alloys (a mixture of two or more metals) are more expensive than palladium-based alloys, but they have established their clinical integrity and are widely accepted in practice today.²

Inlays and onlays are intracoronal restorations placed to restore a tooth with a carious lesion or small fracture of the occlusal tooth structure. Inlays restore the occlusal surface without extending to the cusp tip of posterior teeth (Figures 1 and 2). Onlays are more extensive restorations that cover one or more cusps to restore tooth surfaces and establish occlusion (Figures 3 and 4). Upon replacement of a failing inlay or onlay or to improve esthetics, full coverage restorations, such as porcelain-fused-to-metal (PFM) crowns are used.⁴ This involves replacement of the failing inlay or onlay with composite core followed by extensive tooth structure removal to prepare the tooth for the PFM crown placement.

Although, this is a viable approach and has an increased esthetic appeal, the procedure involves extensive removal of tooth structure5 when compared to placement of full metal crowns or intracoronal restorations. Therefore, to retain viable tooth structure, conservative options for esthetic restoration should be considered.

CAD/CAM TECHNOLOGY

Computer-aided design/computer-aided manufacturing (CAD/CAM) technology is an important development that improves the design and manufacture of high-accuracy intracoronal restorations, which provide a conservative alternative approach to full coverage restorations. More tooth structure can be preserved with these restorations while the appearance remains esthetic. Dentists are able to achieve good clinical results and long-term success after short theoretical and practical training with a CAD/CAM system.⁶

CAD/CAM systems involve digitalization of geometry into data that are processed with a software system. Software processes the data and translates the information into a model to be fabricated. The product can also be manipulated by changing width, contact points, etc, by the dental professional. The addition of intraoral digital impression capability can capture highly detailed images using a powerful blue light emitting diode, replacing the tray and putty method. This rapid, easy imaging technique results in detailed CAD/CAM precision, clinical reliability, and efficiency.⁷

CAD/CAM technology can be used to fabricate restorations during one appointment. When the model is complete a milling machine carves the actual restoration. This eliminates the wait time encountered when a restoration has to be sent to a laboratory for fabrication. However, the restorations can also be fabricated in a laboratory. The dentist just sends the impression to the laboratory, where the scanning and fabrication of the restoration takes place.

RESIN-BASED COMPOSITES

Resin-based composites have been used as esthetic dental fillings for many years. Composites are a mixture of glass filler particles and polymerizable (either self- or light-activated) resin. This polymerization results in the formation of a solid material that is similar in color and translucency to the natural tooth. Composite restorations can be modified by adding tint and opaquers, which help match the color of a patient’s natural tooth. Clinical studies have compared the overall clinical success of composites against amalgam. According to one study, both composite and amalgam offer similar survival rates.^8,9 A study by Mair reported similar success and failure rates of both composite and amalgam but found an increased rate of mechanical failure in amalgam restorations and an increased rate of biological failure in composite restorations.⁹

The principal reason that amalgam and composite restorations need to be replaced is secondary caries. Material failures (marginal degradation, discoloration, bulk fracture, and loss of anatomic form) cause more composite restorations to fail than amalgam restorations.¹⁰ Clinical success of composite restorations depends on patient selection, tooth surface selection, size of the defect to be restored, and how well the guidelines

INDIRECT RESIN-BASED COMPOSITES

Indirect resin composites are fabricated in the laboratory. They minimize the shrinkage that occurs during the curing process as a result of the bulk of the restoration being cured outside the tooth. Indirect composites can improve the durability of the restoration by providing excellent marginal adaptation and function through restoration of the physiological interproximal contact. For posterior restorations, fiber-reinforced composite resin systems offer improved flexure strength compared to nonreinforced indirect composite resin.

The limitations of resin composites are most evident when direct composite restorations are used in posterior dentition. Composite restorations can fail to withstand the stresses of posterior occlusion and masticatory forces. The use of composites to restore posterior teeth can result in restoration fracture, microleakage, surface discrepancy, secondary caries, symptoms of postinsertion sensitivity, occlusal and proximal surface wear due to masticatory forces, and difficulty in achieving and maintaining interproximal contact. Indirect laboratory-prepared composites avoid the large-volume polymerization shrinkage but the problem of shrinkage still exists during cementation of the restoration.

Future resin-based materials may be improved through the use of nanotechnology (the creation of functional systems smaller than 100 nanometers). Dental composites may reach a completely new level of success by incorporating new trends from materials science, such as introducing antimicrobial properties, stimuli responsive capabilities, the ability to promote tissue regeneration, and the possibility of self-repair.¹¹

PORCELAIN RESTORATIONS

Posterior porcelain restorations have become increasingly popular because of patient demand for improved esthetics. Conservative ceramic inlay preparations can be made with minimal tooth removal. Studies show that many all-ceramic restorations demonstrate acceptable longevity when compared with conventional restorations.¹² All-ceramic restorations can be developed with glass ceramic core, which is manufactured by heat pressing. Machinable feldspathic porcelain, which is composed of 63% silicon dioxide and 19% aluminum oxide, is used to fabricate onlays, three-quarter crowns, and veneers. Alumina-based ceramic restorations are prepared with a slip-casting technique. They have high strength and can be used in low stress-bearing anterior teeth. Zirconia-based ceramics and yttrium oxide partially-stabilized zirconia can be prepared with the help of CAD/CAM technology. Zirconia-based ceramics have high initial flexural strength and fracture toughness. These materials offer excellent mechanical properties and can be used for posterior teeth.

Survival rates for porcelain inlays and onlays range from 96% at 4.5 years to 91% at 7 years.13 Most failures are due to bulk fracture.¹³ The survival rates of porcelain crowns range from 92% to 99% at 3 years to 31?2 years.¹³ Crown failure is also mainly due to fracture.¹³

GLASS CERAMIC

Castable-glass ceramic provides excellent accuracy and detail. Glass ceramic is cast with the lost-wax technique (creating a wax replica of an item that is to be replicated). The casting process is similar to that used for cast gold restorations. This technique requires an additional appointment when the occlusal adjustments are made prior to sending the restoration to the laboratory for surface color stain application. To achieve excellent retention and to enhance the bonding strength of the resin cement to the tooth, the bonding surfaces of the restoration are etched with 10% ammonium bifluoride. The survival rates of acid-etched restorations luted to dentin preparations are significantly better than those of nonacid-etched restorations luted to dentin.¹⁴ Acid-etched restorations luted with resin composite have higher survival rates than those luted with glass ionomer and zinc phosphate.¹⁴

The primary advantage of castable glass ceramic is improved fit and decreased amount of resin bonding cement at the margin, which further decreases the potential of ditching. These restorations also have high flexural strength. Disadvantages of castable glass ceramic include the need for additional chairside time and the additional appointment to adjust occlusion prior to application of surface colorants. If any further occlusion adjustment is required on the restoration surface, an unesthetic opaque area results. The processing of cast-glass restorations is very technique-sensitive. In a study evaluating the failure rate of castable-glass ceramic inlays placed by senior dental students, the failure rate was as high as 25% after 4 years.¹⁵ Class I and class II glass ceramic inlay restorations had higher failure rates than amalgam restorations.¹⁵

LEUCITE-REINFORCED CASTABLE GLASS CERAMIC

Leucite-reinforced castable glass ceramic material contains feldspathic porcelain to which leucite crystals are added. These restorations are cast with the lost-wax technique. At a high temperature of 1200°C a presintered glass–ceramic ingot is placed in the furnace and pressed with an aluminum oxide plunger to achieve precise details. After the casting process, the restoration is finished in one of two ways depending on the application and esthetic demand of the restoration. To fabricate crowns, a layering technique is used where the dentinal portion of the restoration is made by a dentine shade ceramic ingot. Then multiple thin incremental enamel layers are added to achieve optimum esthetics. The shading technique can be used for fabrication of inlays and onlays, where the restoration is fabricated using a neutral shade ceramic ingot and then pigmented color is added and glazed to achieve matching color characteristics.

The leucite crystals act as barriers that prevent tensile strength buildup and fracture. All-ceramic restorations show surface microporosities generated during the sintering process, which predispose the restorations to fracture. Through the injection-moulding process under the application of heat and pressure, the leucite crystals are added to the restoration, which improves the restoration’s fracture resistance. The application of heat and pressure during casting reduces the amount of ceramic shrinkage and provides higher flexural strength to the restoration.

One study showed leucite-reinforced castable glass ceramic inlays and onlays exhibited satisfactory clinical outcomes over a 12-year period.¹⁶ Restorations luted with dual-cured resin composites revealed significantly fewer bulk fractures.¹⁶

CONCLUSION

Tooth-colored intracoronal restorations are an excellent alternative to silver amalgam and cast gold restorations. These restorations offer enhanced esthetics and have proven rates of success. Porcelain restorations have a low incidence of failure caused by caries and marginal leakage. These restorations are also patient-friendly due to their increased esthetics. To achieve successful results, dental professionals must carefully examine the indications and contraindications of each type of restoration for each patient. No one restoration can be considered better than the other, but rather the individual patient’s clinical evaluation by the practitioner should be the deciding factor. Tooth-colored porcelain restorations are an excellent option for patients with good oral hygiene, low caries risk, and no signs of any parafunctional habits.

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From Dimensions of Dental Hygiene. November 2010; 8(11): 54-59.