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This article introduces a new approach for restoring endodontically treated posterior teeth. Significantly reduced natural tooth structure often remains not only because of previous restorative measures but also because of endodontic procedures. Cast restorations for these teeth are almost universally recommended. The exception to this is the rare instance in which only conservative endodontic access openings exist in teeth presenting with no former existing restorations. Typically, multiple clinical appointments are required to complete the final cast restoration. This article presents Computer-Aided Design and Computer-Assisted Manufacturing/Computer-Aided Design and Computer-Integrated Manufacturing applications for restorative dentistry that provide the necessary care in an expeditious manner.
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This article introduces a new approach for restoring endodontically treated posterior teeth. Significantly reduced natural tooth structure often remains not only because of previous restorative measures but also because of endodontic procedures. Cast restorations for these teeth are almost universally recommended. The exception to this is the rare instance in which only conservative endodontic access openings exist in teeth presenting with no former existing restorations. Typically, multiple clinical appointments are required to complete the final cast restoration. This article presents ComputerAided Design and Computer-Assisted Manufacturing/Computer-Aided Design and Computer-Integrated Manufacturing applications for restorative dentistry that provide the necessary care in an expeditious manner.
Introduction
The 1990s have been a decade of downsizing, frequent deployments, and fewer practitioners within the military dental care system addressing the needs of the active duty population. This, along with the primary emphasis on dental readiness, has demanded a reexamination of our traditional methods of providing care and definitive treatment in an expeditious manner. This article describes a new commercially available modality that can manufacture in one appointment cost-effective multisurface and full-coverage ceramic restorations that demonstrate convincing clinical success.
Despite a multitude of concepts for restoring endodontically treated posterior teeth, the proper management of nonvital teeth continues to be a subject of extensive research and controversy. Numerous articles have been published focusing on the most appropriate methods for restoring these teeth. 1-6 Gutmann, in a review of anatomic and physiologic changes resulting from endodontic treatment, demonstrated clearly that endodontically treated teeth present with unique structural characteristics exceeding the restorative requirements of vital teeth.7 Posterior teeth with limited endodontic access openings may fare well with a conservation occlusal restoration. Shillingburg et al. recommend that this option be limited to occlusal access openings with totally intact marginal ridges, including buccal and lingual cusp ridges.8 Advances in adhesive dentistry have resulted in bonding values at or near original tooth strengths, lending credence to this treatment approach.
Teeth presenting with sufficiently compromised coronal structure after endodontic and previous restorative procedures are predisposed to fracture during occlusal loading (Fig. 1). Most commonly recommended is the placement of cast restorations on posterior endodontically treated teeth to prevent fracture of the remaining tooth structure. 1,3,4
Goodacre and Spolnik, in a series of three articles, summarized 25 years of studies concerning treatment concepts, maintenance of the apical seal, and tooth preparation considerations.4-6 The timeliness of definitively restoring the endodontically treated tooth was among the specific issues reviewed. Multiple dental appointments are typically required to provide coronal coverage. Frequently, a significant delay occurs between the completion of endodontic therapy and the placement of a suitable cast. These teeth often fracture before they can be restored appropriately. This delay creates an additional problem, that of maintaining the apical seal. Magurra et al. showed that interim restorative material provisional restorations in anterior endodontically treated teeth demonstrated significant leakage between 1 and 3 months after completion of root canal therapy.9 Endodontic retreatment of teeth restored provisionally with zinc oxide and eugenol for prolonged periods of time was recommended to ensure proper apical seal prior to placement of the final restoration. Because of Magurra's findings, this protocol is now considered to be the standard of care.
This article introduces an advanced treatment option to the military dental care system that combines computer-generated restorations and adhesive bonding capabilities. Access to care can be improved markedly by reducing the number of appointments required to restore endodontically treated posterior teeth with this technology, compared with the increased clinical visits needed to provide conventional cast restorations. The effects on expeditious accomplishment of the dental readiness mission are equally obvious.
The CAD-CAM/CAD-CIM Systems
By means of Computer-Aided Design and Computer-Assisted Manufacturing (CAD-CAM) and Computer-Aided Design and Computer-Integrated Manufacturing (CAD-CIM), various types of ceramic restorations can be produced during a single appointment.10-12 Recent advances in CAD-CAM/CAD-CIM technology and adhesive bonding agents provide the distinct advantage of enabling restoration of an endodontically treated posterior tooth during a single patient visit. The computer allows electronic design of a ceramic restoration from an optical scan taken of the prepared tooth. The design data are then transmitted to a computer-guided milling machine, which produces a well-fitting restoration from a prefabricated block of high-performance ceramic. Adjustment and placement of the ceramic restoration by adhesive resin cementation techniques are completed chairside in just one treatment session.
Development of CAD/CAM applications for the dental profession began in the 1970s with Francois Duret in France, Bruce Altschuler in the United States, and Werner Mormann and Marco Brandestini in Switzerland.11,12 The first system to become commercially available and currently in use worldwide was the CEREC (CEramic REConstruction), developed by Mormann and Brandestini and acquired by the Siemens Company of Germany in 1986.11-13 The original system has been replaced with an improved CAD-CIM unit, the CEREC II Fig. 2).
This computer-generated technology for dental application is a subject of continued research and development, and numerous systems have been described in the literature.l0-14 CAD-CAM systems include CEREC I (Siemens), Duret/Sopha, the Rekow/Erdman/"Minnesota," Procera (Nobel Biocare), Precident DCS, Microdenta, and the University of Showa/Nissan system. One system that incorporates the CAD-CIM technology is the CEREC II system. 15 Machined restorations made possible by copy-milling technology also include the Ceramatic automatic system and the Celay Vitadent) manually operated systems.
The CEREC I is capable of manufacturing chairside multisurface ceramic inlays, cusp-protection onlays, three-quarter crowns, seven-eighth crowns, and veneers. The finished restoration is etched, silanated, and adhesively cemented. The occlusal anatomy and contacts are developed using high-speed rotary instruments, and the restoration is finalized using conventional porcelain-polishing procedures. The newer CEREC II system, which became available in North America in the fall of 1996, provides expanded operations, including the milling of occlusal anatomy and contacts. The CEREC II software and hardware have also been refined recently to fabricate full-coverage ceramic crowns.
Considerations for the Endodontically Treated Tooth
The effects of polymerization and thermal expansion may be more critical if larger direct composite restorations are considered for endodontically treated posterior teeth. A greater bulk of resin generates higher-polymerization shrinkage and tensile stresses at restorative interfaces, with resulting gap formation between the tooth and the composite restoration under occlusal loading.6 Indirect composite inlays remove the effect of gross polymerization shrinkage, but machinable ceramic blocks are significantly stronger and more durable than resin-based composite material. The restorative material used with the CEREC II system, CEREC Vitablocs Mark II (Viadent), is a feldspar-based ceramic.' Besides the increased strength and durability, the coefficient of thermal expansion and the elastic modulus of this material are also much closer to those of tooth structure.13 The smaller grain size of this ceramic results in decreased wear potential of the opposing dentition compared with conventional feldspathic porcelains.13 Machinable ceramics can be adjusted, finished, and polished after cementation without adversely affecting the final product. Most importantly, resin cement-dentin bonding agent combinations can eliminate interfacial contractural gaps and create a strong bond between ceramic and tooth surfaces. 16 Bonding the occlusal cusps to the ceramic unites the tooth and restoration into a single functional unit, greatly reducing the fracture potential. 18 Bond strengths up to 27 MPa are reported between the ceramic material and the tooth preparation using fourth-generation dentin-adhesive systems.19
Before the development of the CEREC II system, the CEREC I was available to fabricate intracoronal and extracoronal restorations and is still in limited use today. After completion of root canal therapy, the pulpal chamber is filled with a glass ionomerresin hybrid material, creating a flat pulpal floor (Figs. 3 and 4). The abutment tooth is prepared, conserving as much natural tooth structure as possible, and an optical impression is made. The restoration is then designed on the screen, and the milling machine generates the restoration from a ceramic block (Fig. 5). Once completed, the internal aspect of the restoration is etched, and a silane coupling agent is applied. The cavity preparation walls are etched, and an appropriate bonding agent is placed on the remaining tooth structure and base material. The etched ceramic is cemented with a dual-cured, microfiled composite resin luting agent that bonds to the remaining tooth structure and base material. Accuracy of fit of the computer-generated ceramic restoration after adhesive luting has been discussed extensively in the literature.13,20,21 Intraoral refinement of occlusal morphology, i.e., fissures and grooves, adjustment, finishing, and polishing, is done routinely after cementation. With these advances of the CEREC bonded restoration, the endodontically treated tooth receives a durable and precisely milled biocompatible restoration and multiple appointments and laboratory procedures are eliminated (Fig. 6).
CAD-CAM/CAD-CIM dental technology has been used more extensively in Europe than in North America. Data on the use of CEREC I for endodontically treated teeth are limited; however, studies demonstrate an approximately 85% success rate using CEREC I bonded inlays with up to 7 years of follow-up.20 As stated above, numerous authors recommend complete cuspal coverage when restoring posterior endodontically treated teeth. Refinements in the CEREC II system provide the capability of designing and milling full-coverage restorations. Introduction of the C.O.S. (CEREC Operating System) 5.0/Crown 1.0 software in the fall of 1997 allowed the CEREC II unit to design and fabricate full-coverage crowns while maintaining all previous capabilities (Figs. 7 and 8).
Summary
Emphasis on the dental readiness mission during an era of diminishing resources demands more efficient resource utilization. By combining CAD-CIM technology with state-of-the-art bonding methods, three distinct advantages can be readily achieved. First, fragile endodontically treated teeth can be rapidly restored with a durable, anatomically accurate, and biologically compatible restoration. This eliminates the need for interim restorations and reduces the incidence of postendodontic tooth fracture. Second, clinical treatment time is reduced by eliminating the need for impressions, provisional restorations, and an additional appointment to deliver the restoration. Finally, teeth can be restored without the services of a dental laboratory. This reduces the overall cost of the restoration and permits the restoration of teeth in remote locations where laboratory services may not be readily available. Hence, the CADCAM/CAM-CIM system is a cost-effective restorative modality that has the potential to markedly improve dental readiness and access to care by facilitating the immediate restoration of fractured and endodontically treated teeth without the services of a dental laboratory.
References
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Guarantor: COL Jeffrey G. Foerster, DC USA*
Contributors: LTC Ann S. von Gonten, DC USA*; MAJ Goeffrey H. Robert, DC USA^
*U.S. Army Dental Activity, Walter Reed Army Medical Center, Hospital Dental Clinic, Washington, DC 20307-5400.
^General Dentistry Service, Snyder Dental Clinic, U.S. Army Dental Activity, Fort Gordon, GA 30905.
This manuscript was received for review in March 1998 and was accepted for publication in May 1998.
Reprint & Copyright by Association of Military Surgeons of U.S., 1999.
Copyright Association of Military Surgeons of the U.S. Jan 1999