Marginal quality of ceramic inlays after three different instrumental cavity preparation methods of the proximal boxes

Tooth collection was approved by the ethical committee of Witten/Herdecke University (permission 116/2013). For this study, a total of 60 extracted human molar teeth were collected. They were caries-free, lacked dental calculus, and had completed root growth, comparable sizes, and absolute integrity. Immediately after extraction, the teeth were stored in 0.9% NaCl containing 0.1% thymol at room temperature (maximum 1–3 months) until use. Tissue remnants on the teeth surface were removed with a universal scaler. The teeth were randomly distributed into three groups (Fig. 1).

For the realistic simulation of tooth preparation, a specially designed original lower jaw model was constructed for precise positioning and stable fixation of the experimental teeth (Fig. 2). Initially, the roots of the experimental teeth were coated with condensation-K-Silikon (HLW Dental, Wernberg-Köblitz, Germany). Then, the experimental teeth were placed in the U-profile (the row) of the lower jaw between teeth 45 and 47 (Frasaco GmbH, Tettnang, Germany) and aligned along the occlusion. Then, teeth 45 and 47 were moved until the proximal contacts from both sides of the experimental tooth were reached. Thereafter, the original lower jaw model with the experimental tooth was placed and fixed in the head model (Frasaco GmbH, Tettnang, Germany) with the face mask P-6 GM (Frasaco GmbH, Tettnang, Germany) and the opposing upper jaw ANKA-4 (Frasaco GmbH, Tettnang, Germany).

All specimen preparations were performed by the same operator and calibrated prior to the study by one of the authors to avoid inter-examiner differences.

For the preparation, the AG ceramics guidelines of Ahlers et al. 2009 [27] for a ceramic inlay with a mod-cavity were followed. The following geometrical parameters for the mod-cavity were kept: a diverging angle of approximately 6°, a minimal depth of the occlusal box in the deepest area of the fissure of approximately 1.5 mm, a minimal width of the isthmus of approximately 1.5 mm, a cavity surface angle of approximately 90°, and a planar occlusal and proximal cavity floor. The cervical margin preparation was located in enamel with a maximum dimension of 1.5 mm above the enamel-cementum junction.

The dimensions of the mod-cavities were maintained with the preparation instruments with defined conicity and laser depth markings and were finally controlled with the CP-15 UNC periodontometer (Hu-Friedy Mfg. Co., LLC, Tuttlingen, Germany). The transitions within the mod-cavity were rounded off, and the proximal contacts to the adjacent tooth were completely separated.

The mod-cavity preparation was performed in three steps: (1) initial whole mod-cavity shaping in all groups (n = 60), (2) mod-cavity proximal box finishing within every three groups (n = 20), and (3) rounding and smoothing all sharp edges inside of the mod-cavity in all groups (n = 60). Initial whole cavity shaping for 60 teeth was performed in all groups using the same method with the Synea Vision WK-99 LT handpiece (W & H Germany GmbH, Laufen, Germany) under ca. 80% power (ratio 1:5, engine speed 32,000 rpm, speed of the instrument 160,000 rpm) and diamond instruments. Then, a purely occlusal limited cavity along the main fissure was produced with the 959KRD 314,018 non-ringed diamond conical grinder (Komet, Lemgo, Germany) (grain size ca. 100 μm, laser marking at 2 and 4 mm, conicity ca. 2°). After this, the proximal box to the dissolution of the basal contact point was created with a 6847KRD 314 016 green ringed conical diamond grinder (Komet, Lemgo, Germany) (grain size ca. 125 μm, laser marking at 2 and 4 mm, conicity ca. 2°). This was done with the protection of the adjacent tooth with steel-band matrices, which were fixed with a wooden wedge. The occlusal box was smoothed with the red-ringed conical diamond instrument 8959KR 314 018 (Komet, Lemgo, Germany) (grain size ca. 30–45 μm, conicity ca. 2°). After the initial whole mod-cavity shaping of five teeth, the used instruments were replaced with new ones. This rule was also applied to subsequent proximal box finishing within groups.

Proximal box finishing was performed with three different methods and instruments in the three groups.

The prepared teeth were imprinted with Impregum Penta H Duosoft and Impregum Garant L Duosoft (3M, Seefeld, Germany) using a double mixing technique at room temperature.

Then, the ceramic inlays were conventionally modeled, pressed, adjusted, and completed according to standard laboratory procedures.

Prior to cementing, two independent examiners visually inspected the ceramic inlays under a magnifying glass (× 2.5). The entire restoration margin was explicitly controlled in the mesial and distal proximal range. Only clinically acceptable inlays were added to the study. Clinically unacceptable inlays were newly manufactured and controlled again.

The basal surface of the ceramic inlay was etched for 20 s with ceramic etching gel that contained 5% hydrofluoric acid IPS (Ivoclar Vivadent AG, Schaan, Liechtenstein), then rinsed for 20 s with the multifunctional syringe, dried and covered for 60 s with the single-component adhesion primer Monobond Plus (Ivoclar Vivadent AG, Schaan, Liechtenstein). Enamel and dentine in the mod-cavity were etched for 30 and 15 s, respectively, with 37% phosphoric acid (Ivoclar Vivadent AG, Schaan, Liechtenstein), then rinsed for 30 s with the multifunctional syringe, dried and covered for 20 s with Universal Adhesive (Ivoclar Vivadent AG, Schaan, Liechtenstein). Adhesive was blown into a uniformly thin film and cured for 10 s with the Satelec Mini Led SP dental curing light (Acteon Germany GmbH, Mettmann, Germany) at a distance of approximately 1 mm. Then, the basal surface of the inlay was covered with the dual-curing luting composite Variolink esthetic DC (Ivoclar Vivadent AG, Schaan, Liechtenstein) and inserted into the mod-cavity with uniform pressure. The excess composite was removed with a foam pellet. Then, the ceramic inlay was kept in the correct position with a Heidemann-spatula under gentle pressure. The inlay was cured for 10 s with the Satelec Mini Led SP dental curing light (Acteon Germany GmbH, Mettmann, Germany) from the occlusal-mesial and occlusal-distal sides at an angle of approximately 45° to the occlusal plane, at a distance of approximately 1 mm from the tooth. To avoid an oxygen-inhibiting layer, the gap between the inlay and the tooth-hard substance before polymerization was covered with a glyceringel liquid strip (Ivoclar Vivadent AG, Schaan, Liechtenstein).

After removal from the thermocycler, the teeth were rinsed with distilled water and dried with a paper towel. The dry teeth were sealed with two layers of Pattex Mini Trio superglue (Henkel AG & Co. KGaA, Dusseldorf, Germany). The basal proximal preparation margin and 0.5 mm circular remained unsealed. After drying for 30 min, the teeth were immersed in AgNO₃ (56.62 g/1 L H₂O) for 6 h. Subsequently, the teeth were exposed to four illuminants of 100 W for 4 h. After exposure, AgNO₃ was developed in liquid Periomat Intra developer (Dürr Dental, Bietigheim-Bissingen, Germany). The depth of silver precipitation was measured in the sections (Fig. 3).

All specimens were embedded in Technovit 9100 (Heraeus Kulzer, Werheim, Germany). Then, the specimens were cut in the middle of the sagittal plane through the ceramic inlay with a Leica SP1600 Saw Microtome (Leica Biosystems GmbH, Wetzlar, Germany).

Prior to statistical calculations, the normality of the data was checked using the Kolmogorov-Smirnov test. As the data were not normally distributed, non-parametric tests were applied. For the comparison of group differences, the Mann-Whitney U test was applied. As multiple comparisons were calculated, the Bonferroni correction of the alpha error resulted in a p value of 0.016. Correlations were calculated with the non-parametric Spearman-Rho test. The Statistical Package for Social Sciences program (SPSS, IBM, Amronk, NJ, USA) Vers. 23 was used.

Microscopically qualitative differences in the surface texture were found after preparation with the different instrumental methods (Fig. 5). A significant increase in the surface roughness of the proximal box floor was found in samples that underwent rotating preparation and sonic preparation and ultrasonic preparation (p < 0.001). No significant differences were found in the surface roughness of samples subjected to sonic and ultrasonic preparation (p = 0.016) (Fig. 6).

No statistically significant differences were found between any of the groups regarding the ceramic inlay proximal microleakage (Fig. 7).

No statistically significant differences were found between any of the groups regarding proximal marginal gap (Fig. 8).

No statistically significant differences were found between any of the groups regarding absolute marginal gap (Fig. 9).

No statistically significant correlations were found between microleakage and marginal gap (r = − 0.103; p = 0.704), microleakage and absolute marginal gap (r = − 0.024; p = 0.931), or microleakage and roughness (r = − 0.103; p = 0.704) in all three groups.

The distribution of the defined qualities at the ceramic inlay proximal margin was similar in all three groups. Qualities A1 and A2 were most common, and qualities A3 and B were the least common (Fig. 10).