1

Introduction

The popularity of tooth-colored posterior restorations has increased during the last years because of a growing demand for esthetics and concern about the biocompatibility of amalgam . Applying the adhesive bonding concept, several dental ceramic systems have been used during the last decades for veneer, inlay and onlay restorations . Most ceramic materials can be bonded to the underlying conditioned tooth surface after etching with hydrofluoric acid or ammonium bifluorid, mediated by use of an enamel/dentin bonding system and a resin composite luting material. Acid etching increase surface roughness and wetting of enamel, dentin and ceramic surfaces, which promote mechanical interlocking of resin bonding systems.

Bulk fracture and loss of restoration have been reported as the main reasons of failure in short-term evaluations of inlays and onlays . Leucite-reinforced glass–ceramic inlays showed improved clinical durability compared to fired ceramics . The advantage of bonded ceramics can be expected in the extremely non-retentive coverage situation . Traditional crown preparation techniques will in these cases occasionally result in extensive preparation and/or endodontic treatment in combination with a post and core placement in order to obtain retention. The minimal preparation for the bonded ceramic is less traumatic for the tooth, and pulp vitality can be preserved . There are few preparation design standards for the ideal dentin–enamel-bonded crowns, and Burke showed in vitro no differences between varying degrees of tooth preparation to enhance protection to fracture. The durability of the bonded ceramic will depend on the strength of the bond between tooth, luting system and ceramic and on the inherent strength of the ceramic. The use of chemically cured resin composite cements have been suggested to obtain optimal conversion and decreased stress formation during polymerization .

Few studies report the longevity of dentin–enamel-bonded all-ceramic crowns also defined as “partial or full coverage restorations in which an all-ceramic is bonded to the underlying dentin and any available enamel using a resin luting material” . In a systematic review assessing the 5-year survival rates of single crowns, densily sintered aluminum crowns and reinforced glass–ceramic crowns (Empress) showed survival rates comparable to those seen for porcelain-fused-to-metal (PFM) crowns . The short-term results of the extensive adhesively luted coverages are promising but clinical long-term data are not available . The aim of this study was to investigate the long-term durability of these extensive dentin–enamel-bonded posterior ceramic coverages. In addition the effect of luting cement, bonding system and preparation type was studied. The hypothesis tested was that there was (1) no difference in durability for the ceramic coverage placed with different luting agents and different bonding agents, and (2) no difference in durability between vital and endodontic treated teeth.

2

Materials and methods

During the period November 1992–December 1998, 262 enamel–dentin-bonded ceramic partial and complete coverages (IPS Empress, Ivoclar, Schaan, Liechtenstein) were placed by four operators in 121 patients, 75 women and 46 men with a mean age of 52 years (range 26–81). The study was randomized in a way that all patients, which did need an extensive posterior all-ceramic restoration, during this period, were included in the study after giving their agreement. No patient or patient group was excluded. The participants were treated on a regular basis at the operator’s Public Dental Health Service clinics in Umeå and Visby, Sweden or the Umeå dental school clinic. The study was approved by the local ethics committee at the University of Umeå. The socio-economic status of the participants was varying from low to high. The reasons for placement were filling- or tooth fracture, secondary caries and replacement of extensive amalgam restorations. Preparations with retention obtained as for conventional metallic crowns by parallel walls and increased height of the walls were not included. The evaluated ceramics were distributed as follows: in the lower arch 20 premolars and 120 molars, in the upper arch 44 premolars and 68 molars. All teeth were in occlusion and had at least one proximal contact. The cavities were prepared with butt-joint margins using tapered diamond burs. Sharp internal and external preparation angles were rounded to assure optimum strength of the ceramic. The occlusal thickness of the coverage was at least 1.5 mm. To increase the enamel surface of the preparations minimal shoulder or chamfer preparations were made in part of the teeth in case of enough thickness of the remaining cusp(s). The thickness of the remaining cusp was estimated by the operator to be at risk for fracture or not before a preparation was performed. In case of risk, the remaining cusp(s) were left unprepared. The different forms of preparations evaluated have been reported earlier . Preparation group 1: partial coverage with no shoulder preparation of thin remaining cusp wall (premolars, n = 4; molars, n = 25); Preparation group 2: minimal retentive partial coverage preparation, with a combination of unprepared thin remaining cusp wall(s), and cusps which have been prepared with a shoulder or chamfer (premolars (with one remaining unprepared cusp wall), n = 18; molars (with one or two remaining unprepared cusp walls), n = 70); Preparation group 3: minimal retentive full coverage crowns with shoulder or chamfer preparation of all remaining cusps (premolars, n = 21; molars, n = 63); Preparation group 4: non-retentive endodontic treated teeth, no post and core treatment (premolars, n = 13; molars, n = 14). The numbers given are excluded the drop out restorations. Undercuts were blocked with resin-modified glass ionomer cement or dentin-bonded resin composite material or removed by preparation. A full arch impression was made with a custom-made acrylic resin tray with polyvinyl-siloxane impression materials (President, Coltène, Altstätten, Switzerland; Provil, Bayer, Leverkussen, Germany; or similar materials). Temporary restorations were cemented with an eugenol-free cement (Fermit, Vivadent, Schaan, Liechtenstein; Tempbond-NE, Kerr, Karlsruhe, Germany), or phosphate cement was used to cover the non-retentive preparations. All-ceramics were processed by four experienced technicians. The internal surfaces of the ceramic coverages were etched with hydrofluoric acid in the laboratory. At the second appointment, the operation field was isolated with cotton rolls and a conventional saliva suction device or rubber dam. At the try-in of the coverages, anatomical form, marginal adaptation and color were evaluated. After this the fitting surface of the ceramic restoration was cleaned by a 2–3 s etching with 9.5% hydrofluoric acid or 20 s etching with 36% phosphoric acid to remove any possible contaminants during the try-in like saliva or crevicular fluid. The etching times were based on internal SEM observations of cleaning methods of saliva contaminated with hydrofluoric acid pre-etched ceramic surfaces. A silan coupling agent (Ultradent, Salt lake City, UT, USA; Monobond S, Vivadent) was then applied for 2 min. The teeth were etched with 36% phosphoric acid for 15 s, enamel 10 s followed by an enamel and dentin etch for another 5 s. The 15 s etching time for enamel has been proven to give good longevity in our long-term evaluations, while the 5 s dentin etch is considered to optimize penetration of the demineralized dentin and minimize the risk for nanoleakage. The cavity was thoroughly rinsed with water for 20 s, and carefully dried taking care not to dry out the surface (wet technique introduced by J. Kanca). Four different dentin bonding systems used were applied according to the manufactureŕs instructions. In 106 restorations the classic Syntac adhesive system (Vivadent) was used in combination with a dual-cured resin composite (Variolink, Vivadent) systematically by one of the operators. In the other center restorations were luted with a chemically cured resin composite (Bisfil 2B, Bisco Inc., Itasca IL, USA). Three 3-step etch and rinse systems were used systematically in different time periods by the other operators. 37 were bonded with the Gluma system (original Gluma, Bayer, Dormhagen, Germany), 57 with the Allbond 2 system (Allbond 2, Bisco Inc.), 32 with the classic Syntac system (Vivadent). Another 20 coverages were placed at the end of the study with the simplified 2-step etch and rinse system One step (Bisco Inc.) as a comparison with the 3-step system of the same manufacturer. The distribution of the adhesive systems for the different preparation groups is shown in Table 1 . The luting materials were applied to the fitting surface of the coverages. Excess cement was removed immediately after placement with probe or brush-tips and proximal floss. The dual-cured agent was light cured from all directions for 40–60 s per location (VRC 400, Dentronic, USA; 1000 mW/cm ² ).

Occlusion was checked and adjusted as necessary after placement based on a dynamic occlusion. The reconstructions were finished with diamond or carbide finishing burs, polishing stones and the Profin finishing system (Dentatus, Hägersten, Sweden) under water cooling followed by polishing stones.

2

Materials and methods

During the period November 1992–December 1998, 262 enamel–dentin-bonded ceramic partial and complete coverages (IPS Empress, Ivoclar, Schaan, Liechtenstein) were placed by four operators in 121 patients, 75 women and 46 men with a mean age of 52 years (range 26–81). The study was randomized in a way that all patients, which did need an extensive posterior all-ceramic restoration, during this period, were included in the study after giving their agreement. No patient or patient group was excluded. The participants were treated on a regular basis at the operator’s Public Dental Health Service clinics in Umeå and Visby, Sweden or the Umeå dental school clinic. The study was approved by the local ethics committee at the University of Umeå. The socio-economic status of the participants was varying from low to high. The reasons for placement were filling- or tooth fracture, secondary caries and replacement of extensive amalgam restorations. Preparations with retention obtained as for conventional metallic crowns by parallel walls and increased height of the walls were not included. The evaluated ceramics were distributed as follows: in the lower arch 20 premolars and 120 molars, in the upper arch 44 premolars and 68 molars. All teeth were in occlusion and had at least one proximal contact. The cavities were prepared with butt-joint margins using tapered diamond burs. Sharp internal and external preparation angles were rounded to assure optimum strength of the ceramic. The occlusal thickness of the coverage was at least 1.5 mm. To increase the enamel surface of the preparations minimal shoulder or chamfer preparations were made in part of the teeth in case of enough thickness of the remaining cusp(s). The thickness of the remaining cusp was estimated by the operator to be at risk for fracture or not before a preparation was performed. In case of risk, the remaining cusp(s) were left unprepared. The different forms of preparations evaluated have been reported earlier . Preparation group 1: partial coverage with no shoulder preparation of thin remaining cusp wall (premolars, n = 4; molars, n = 25); Preparation group 2: minimal retentive partial coverage preparation, with a combination of unprepared thin remaining cusp wall(s), and cusps which have been prepared with a shoulder or chamfer (premolars (with one remaining unprepared cusp wall), n = 18; molars (with one or two remaining unprepared cusp walls), n = 70); Preparation group 3: minimal retentive full coverage crowns with shoulder or chamfer preparation of all remaining cusps (premolars, n = 21; molars, n = 63); Preparation group 4: non-retentive endodontic treated teeth, no post and core treatment (premolars, n = 13; molars, n = 14). The numbers given are excluded the drop out restorations. Undercuts were blocked with resin-modified glass ionomer cement or dentin-bonded resin composite material or removed by preparation. A full arch impression was made with a custom-made acrylic resin tray with polyvinyl-siloxane impression materials (President, Coltène, Altstätten, Switzerland; Provil, Bayer, Leverkussen, Germany; or similar materials). Temporary restorations were cemented with an eugenol-free cement (Fermit, Vivadent, Schaan, Liechtenstein; Tempbond-NE, Kerr, Karlsruhe, Germany), or phosphate cement was used to cover the non-retentive preparations. All-ceramics were processed by four experienced technicians. The internal surfaces of the ceramic coverages were etched with hydrofluoric acid in the laboratory. At the second appointment, the operation field was isolated with cotton rolls and a conventional saliva suction device or rubber dam. At the try-in of the coverages, anatomical form, marginal adaptation and color were evaluated. After this the fitting surface of the ceramic restoration was cleaned by a 2–3 s etching with 9.5% hydrofluoric acid or 20 s etching with 36% phosphoric acid to remove any possible contaminants during the try-in like saliva or crevicular fluid. The etching times were based on internal SEM observations of cleaning methods of saliva contaminated with hydrofluoric acid pre-etched ceramic surfaces. A silan coupling agent (Ultradent, Salt lake City, UT, USA; Monobond S, Vivadent) was then applied for 2 min. The teeth were etched with 36% phosphoric acid for 15 s, enamel 10 s followed by an enamel and dentin etch for another 5 s. The 15 s etching time for enamel has been proven to give good longevity in our long-term evaluations, while the 5 s dentin etch is considered to optimize penetration of the demineralized dentin and minimize the risk for nanoleakage. The cavity was thoroughly rinsed with water for 20 s, and carefully dried taking care not to dry out the surface (wet technique introduced by J. Kanca). Four different dentin bonding systems used were applied according to the manufactureŕs instructions. In 106 restorations the classic Syntac adhesive system (Vivadent) was used in combination with a dual-cured resin composite (Variolink, Vivadent) systematically by one of the operators. In the other center restorations were luted with a chemically cured resin composite (Bisfil 2B, Bisco Inc., Itasca IL, USA). Three 3-step etch and rinse systems were used systematically in different time periods by the other operators. 37 were bonded with the Gluma system (original Gluma, Bayer, Dormhagen, Germany), 57 with the Allbond 2 system (Allbond 2, Bisco Inc.), 32 with the classic Syntac system (Vivadent). Another 20 coverages were placed at the end of the study with the simplified 2-step etch and rinse system One step (Bisco Inc.) as a comparison with the 3-step system of the same manufacturer. The distribution of the adhesive systems for the different preparation groups is shown in Table 1 . The luting materials were applied to the fitting surface of the coverages. Excess cement was removed immediately after placement with probe or brush-tips and proximal floss. The dual-cured agent was light cured from all directions for 40–60 s per location (VRC 400, Dentronic, USA; 1000 mW/cm ² ).

Occlusion was checked and adjusted as necessary after placement based on a dynamic occlusion. The reconstructions were finished with diamond or carbide finishing burs, polishing stones and the Profin finishing system (Dentatus, Hägersten, Sweden) under water cooling followed by polishing stones.