INTRODUCTION

Ceramic restorations are considered an excellent option to restore posterior teeth when esthetics is required and the size of the cavity preparation has exceeded the conventional indication for direct resin composites. Ceramic systems can combine esthetics with wear resistance, being considered a reliable treatment choice. Peutzfeldt¹ investigated the failure rate of different restorative materials used in posterior teeth and reported that gold inlays did not present a much lower failure rate than ceramic or composite inlays. Consequently, the author suggested that other aspects besides longevity, such as esthetics, price, and number and duration of dental appointments should also be considered when comparing treatment options. Different ceramic systems for fabricating inlay and onlay restorations are available on the market. Some of these ceramic restorations are made using feldspathic ceramic (from the conventional application of a slurry powder onto a refractory die); the CEREC computer-aided design/computer-aided manufacturing, CAD/CAM system, (Sirona Dental Systems GmbH, Bensheim, Germany); or the hot-pressed leucite-reinforced ceramic fabricated by the conventional lost-wax technique (IPS Empress, Ivoclar Vivadent, Schaan, Liechtenstein). Other systems that employ alumina or zirconia are available, but they are generally indicated for single- and multiple-unit crowns.^2,3

Clinical studies have shown that higher success rates can be achieved when ceramic systems are used in conjunction with an adhesive cementation technique.^4,5 In order to accomplish the bonding requirements, the ceramic should permit selective dissolution via etching to create micromechanical adherence to the resin-based cements.⁶ When etchable ceramics are treated with a hydrofluoric acid (HF), a volatile silicon tetrafluoride complex is first formed, and then a second reaction takes place to form a soluble complex ion, hexafluorosilicate, which will further react with the protons to form tetrafluorosilicic acid, which can be rinsed off with water.² This reaction enables the etched ceramic surface to be bonded to resin-based cements through the silane. The silane coupling agent is used to promote the chemical adhesion, functioning as a mediator between inorganic and organic substrates through dual reactivity to achieve adhesion.

Although short- and long-term clinical studies have shown low failure rates of adhesively bonded ceramic inlay and onlay restorations, some drawbacks have been reported, bulk fracture and marginal discoloration being the most commonly cited problems.^7–13 Deterioration of marginal quality has been addressed with regard to cement wear, which may be accelerated due to high differences in modulus of elasticity between ceramic and resin cement materials,^8,14 while bulk fracture has been associated with crack propagation through the ceramic, due to the brittle characteristic of the ceramic material.¹ Also, some other factors have been reported as coadjutants on the ceramic crack propagation, such as the microstructure of the ceramic material, the fabrication technique, the surface finishing, and the luting protocol.¹⁵ It has been suggested that an observation period of at least five years should be employed to evaluate the performance of all-ceramic restorations in the posterior region.^16,17

In that context, the aim of the present prospective study was to evaluate the clinical performance of adhesively bonded all-ceramic inlay and onlay restorations made with two different systems (IPS Empress and Duceram), according to United States Public Health Service (USPHS) criteria over five years. The null hypothesis tested was that there would be no difference in clinical performance between the two systems: IPS Empress and Duceram.

MATERIALS AND METHODS

This study involved 86 Class II inlay and onlay restorations fabricated with two different ceramic systems: 42 sintered ceramics (Duceram Plus and Duceram LFC, Dentsply Degussa Dental, Hanau, Germany) and 44 pressable ceramics (IPS Empress, Ivoclar Vivadent). A total of 33 onlays and 53 inlays were made in 27 premolars and 59 molars by one operator to create a standardized cavity preparation. In patients who had more than one restoration placed, the two systems were used in an attempt to achieve the same number of each ceramic system in all patients.

Thirty-five patients, including 17 women and 18 men, with a median age of 33 years (ranging from 25 to 44 years) who required inlay and onlay restorations were selected for this study. The involved teeth were in occlusal contact. The volunteers underwent a careful case history review, and bitewing and periapical radiographs were taken. Vitality of the teeth was tested with carbon dioxide snow of −26.2°C.

The following items were considered as exclusion criteria: high caries risk (presence of incipient lesions, plaque, and xerostomia), periodontal disease, the presence of a removable or fixed orthodontic appliance, signs of bruxism or clenching, the absence of more than one unit in the posterior region, and poor oral hygiene or pregnancy. All patients were treated at the Bauru Dental School, University of São Paulo, SP, Brazil. They were informed about the research methodology, risks and benefits, and their right to withdraw participation in this research at any time. A written informed consent was signed. The study was carried out according to research norms and guidelines for human beings deriving from Resolution 196 approved in October 1996 by the National Health Council and Ethics Research Committee from the Bauru Dental School, University of São Paulo, SP, Brazil.

Evaluation Procedures

One week following placement, the restorations were assessed according to the modified USPHS criteria²⁰ (Table 1) by two independent investigators calibrated in the use of the system using only mirrors and probes. The investigators did not participate in the clinical procedures and did not know which system was used on the teeth they were evaluating. The same procedures performed at the baseline were performed at one, two, three, and five years. Statistical analyses were carried out with Fisher and McNemar tests at a 0.05 level of significance.

Table 1:  Modified United States Public Health Service Criteria for the Clinical Evaluation of Ceramic Inlays and Onlays Used in This Study

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RESULTS

Table 2 summarizes the results of Alpha ratings obtained for both ceramic materials at baseline and at one-, two-, three-, and five-year recalls, according to the USPHS criteria.

Table 2:  Alpha Results of IPS Empress (IPS) and Duceram (D) Ceramics According to Modified United States Public Health Service Criteria at Baseline, and at One, Two, Three, and Five Years (n = number of restorations evaluated)

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DISCUSSION

The null hypothesis that there would be no difference in clinical performance between the two ceramic systems was supported. At five-year evaluation of adhesively luted ceramic inlay and onlay restorations, no significant difference between Duceram and IPS Empress ceramic systems was found for any aspect evaluated at different recall appointments (p>0.05). This result is in accordance with a previous study that reported no significant difference between the conventional porcelain (Vitadur Alpha) and the leucite-reinforced porcelain (IPS Empress) at five-year evaluation.²¹ Regarding the conventional ceramic systems, it has been reported that the main disadvantage of fired ceramic restorations is the degree of microporosities and inhomogeneities between the ceramic particles due to the fabrication technique used to process these restorations.²² On the other hand, the IPS Empress system has been considered a more homogeneous ceramic through the use of precerammed ingots and has also shown superior flexure strength in laboratory tests when compared to less reinforced ceramics.²³ However, the clinical relevance of such a difference between the two systems could not be upheld in this study at five-year evaluation. Both systems presented satisfactory results with a success rate of 87% for IPS Empress and 93.3% for Duceram.

Other studies with a similar time frame of evaluation have reported overall success rates of between 90% and 95% of ceramic restorations investigated.^7,9,21,24,25 In these previous studies, the main cause of failure was associated with bulk fracture of the ceramic restoration and secondary caries.

In the present study, 67 restorations were evaluated at five years. The sample size may not be large, but it is comparable to previous studies^7,9,11,13,25 in which one operator carried out all of the restorative treatments, rather than multiple operators carrying out treatments at multiple centers. The dropout rate with regard to the number of restorations was 22% (19 restorations), and 26% with respect to the number of patients (nine patients). Of the nine patients who could not participate in the five-year recall, four had moved to another city, three could not be reached by telephone nor e-mail, and two did not want to participate. At five-year recall, eight restorations needed to be replaced (11.9%). Four IPS Empress restorations suffered cohesive bulk fractures (6%), two restorations presented secondary caries (one from IPS and one from Duceram) and two restorations showed unacceptable defects at the restoration margin needing replacement (one restoration from each ceramic system). Bulk fracture is still considered one common problem reported in clinical trials.²⁶ The failure rate associated with bulk fracture in the present study is in accordance with other studies that performed their evaluations in the same time frame (Arnelund and others,²¹ 8% after five years; Naeselius and others,²⁷ 7.3% after four years; Krämer and others,⁹ 5.5% after four years). Molin and Karlsson⁷ investigated the five-year performance of three inlay ceramic systems (CAD-CAM, Vita Cerec-Siemens; a conventional porcelain buildup sintering technique, Mirage, Myron; and a glass ceramic casting high-pressure technique, IPS Empress, Ivoclar Vivadent) and reported that among the 60 ceramic inlays placed, five inlays (8%) fractured within the five-year follow-up period, four being from the IPS Empress system and one from the Cerec system.

Unlike metals, resin composites, and dental tissues, ceramic materials are unable to endure elastic deformation to the same level. Ceramic materials possess a high modulus of elasticity and low flexural strength, which is a limiting property of brittle materials.²⁸

Fischer and others²⁹ evaluated the long-term failure probability of different ceramic materials using a computational method and reported a high tendency for failure of the IPS Empress system (2.6% after one year, 4.6% after five years, and 6.0% after 10 years, respectively). Previous clinical studies that have investigated the IPS Empress system have reported different failure rates due to fracture in different periods of evaluation (Studer and others,²⁶ 2.3% after three years; Galiatsatos and Bergou,¹¹ 3.1% after six years; Frankenberger and others,¹⁰ 16% after 12 years). However, it is relevant to take in account that factors other than the ceramic material may have an influence on the restoration survival. According to Martin and Jedynakiewicz,³⁰ the most common reasons for all-ceramic restoration failures are fracture of the ceramic, fracture of the supporting tooth, postoperative hypersensitivity, and wear of the resin luting agent. In their study, the main reason for fracture was usually related to either excessive occlusal loads or insufficient ceramic thickness. The recommended minimum thickness of 1.5 mm should always be observed in order to improve strength. According to van Dijken and others³¹ a cuspal overlay should permit a material thickness of ideally 2 mm. Also, the presence of cracks on the ceramic restoration can lead to fracture of the ceramic material by crack propagation under excessive tensile stresses. These cracks can be produced by finishing and polishing procedures, by processing, or by intrinsic defects in the structure of the material.³² So, a careful finishing procedure should be employed when adjusting the occlusal surface of the restoration, since it has been reported that cracks as small as 25 μm can lead to fracture of the ceramic restoration under function.²⁸

Drawing a comparison between the data collected at the five-year recall appointments, no fracture was observed until five-year evaluation. Secondary caries were observed in one restoration from each system at three-year recall, and one more for each system at five-year recall. It is interesting to notice that the number of Alpha scores for marginal discoloration, marginal integrity, and surface texture has declined significantly for both systems (p<0.001), from the baseline to all other subsequent follow-ups. Color match showed a significantly reduced performance for the IPS Empress system (p<0.001) after five-year evaluation. The explanation may be related to the fading of the extrinsic painting used on the external surface of these IPS ceramic restorations to enhance the esthetic result. In this system, a precerammed monochromatic ingot is selected, heated, and pressed to process the ceramic restoration using the traditional lost-wax technique, while for the conventional porcelain buildup sintering technique, layers with different degrees of translucencies and opacities can be employed.

Because of esthetic reasons, there is a tendency nowadays to select ceramic materials, rather than gold, when dealing with partial restorations. Wagner and others³³ evaluated the performance of gold and ceramic onlays for about seven years and found no statistical difference between the two restorative groups. Federlin and others³⁴ have reported similar survival rates for both gold and ceramic onlays at five and a half years of evaluation; however, they verified a statistically significant decrease of Alfa ratings for anatomic form and marginal discoloration criteria on the ceramic restorations.

In relation to marginal discoloration and marginal integrity, significant differences were detected between the baseline and the five-year recall data. After five years, significant reduction on Alpha scores was observed for marginal discoloration, marginal integrity, color match, and surface texture (McNemar chi-square test, p<0.001). Several studies have reported a significant increase of marginal discoloration and decrease of marginal integrity over time.^7–13 Frankenberger and others³⁵ verified that 94% of the surviving restorations exhibited marginal deficiencies after six years. Deterioration and wear of the resin luting agent has been considered a contributory factor in the marginal deterioration of the ceramic restorations.^7,8,11,13,35 This fact was associated with a very high modulus of elasticity of the ceramic material, which during masticatory forces transmits stress to the cement, whose modulus of elasticity is lower.^8,14 Although marginal discoloration has been regarded as a common phenomenon addressed in clinical trials, it has not been considered critical for the clinical performance of the ceramic restorations.^8,11–13

Some studies^35,36 have considered postoperative sensitivity as another clinical complication; however, it was not an issue in the present study. In this study, all deep areas and undercuts were covered with a resin-modified glass ionomer (Vitremer, 3M ESPE) before final preparation to protect the exposed deep dentin and to help standardize the depth of the pulpal floor (from 1.5 to 2.0 mm). All restorations were adhesively cemented with the three-step etch and rinse Syntac adhesive and Variolink II resin cement. Krämer and others⁹ used the same cementation protocol employed in the present study and reported low sensitivity with the use of Syntac adhesive and Variolink resin cement.

The occurrence of dropped participants is inevitable in clinical trials. In the present study 35 patients, representing 86 restorations (100%) were evaluated at the two-year recall. At the five-year examination, 62 restorations in 26 patients were evaluated (74.28%), totalling 32 IPS Empress restorations (72.72%) and 30 Duceram restorations (71.42%).

Comparing inlay vs onlay restoration types and in relation to premolar and molar regions, no statistically significant differences were observed. These results are in accordance with Arnelund and others,²¹ whose clinical evaluations revealed no significant difference between ceramic inlays and onlays after five-year evaluation. Manhart and others,³⁷ however, observed significantly higher failure rates for inlays placed in molars compared with premolars. In the present study, the fractures occurred in the molar region for both inlay and onlay restorations; however, this result was not statistically significant. In addition, despite the fact that molars are usually subjected to more intense chewing forces, this negative effect did not result in significant influence at the five-year recall evaluation.

CONCLUSIONS

It can be concluded that the adhesively bonded ceramic restorations presented satisfactory results at five-year evaluation. The null hypothesis was not rejected, since no significant differences were noticed between the two ceramic systems. The advantages of these partial all-ceramic inlay and onlay restorations include less tooth destruction; avoidance of subgingival restoration margins, usually required by crowns; and good esthetic results.