2.1

Preparation and evaluation of specimens

120 freshly extracted fully intact human third molars were stored for a maximum of 30 days in aqueous 0.5% chloramine T-solution. The teeth were cleaned and evaluated for defects under a stereomicroscope (50×). The number and total length of visual enamel cracks was noticed (stereomicroscope SV 11, Zeiss, Jena/WinMes 2.0 software).

Standardized cavities were prepared (MOD, oro-vestibular isthmus width 4 mm, 3 mm depth occlusally, 2 mm mesio-distal width of the proximal box). The proximal box ended mesially 1–2 mm above the cemento-enamel junction and distally 1–2 mm below the amelocemental junction in cementum. Preparation was performed using a diamond bur under sufficient water cooling (80 μm diamond, Two-Striper ^® Prep-Set, Premier, St. Paul, USA), and finally with finishing diamonds (25 μm, Inlay Prep-Set, Intensiv, Viganello-Lugano, Switzerland). Internal cavity transitions were rounded, the enamel margins were not beveled.

2.2

Cerec scan

The teeth were powdered (Vita-Cerec-Powder, Vita Zahnfabrik, Bad Säckingen, Germany) and the optical impression was computed by a Cerec scanner (Cerec 2, Sirona, Bensheim). Vitablocs Mark II (Vita Zahnfabrik) were used as ceramic blocks. After the scan process the powder was removed with air–water spray for 30 s and the teeth were finally dried.

2.3

Impressions, temporary restorations and loading

Single-step impressions were taken from the cavities (Provil Novo, Heraeus Kulzer, Hanau, Germany) and impressions were filled with epoxy-resin die (Alpha Die MF, Schütz Dental, Rosbach, Germany). IPS Empress inlays were fabricated within one week. While the chairside-fabricated inlays were inserted immediately, the cavities of the labside-fabricated inlays were temporarily restored with either Luxatemp temporary inlays luted with temporary cement (TempBond NE) or Systemp.inlay inserted without temporary cement. All labside-fabricated inlay groups underwent a thermo-mechanical loading in order to simulate loading of the temporary restorations (1000 cycles á 50 N + 25 thermocycles between 5 °C and 55 °C + storage in distilled water at 37 °C, for 7 days). Thereafter, the temporary restorations were removed and cavities were cleaned with pumice powder. A repeated determination of enamel cracks and a control of the cavity margins for marginal fractures was carried out.

2.4

Fitting and adhesive luting

After fabrication of all inlays, their fit was checked and adjusted with finishing diamonds under water cooling in order to obtain a luting gap of 500 ± 50 μm for Cerec inlays and 200 ± 20 μm for IPS Empress inlays. At the bottom of the cervical boxes the luting gap width was determined prior to the luting process (100× magnification, SV 11, Zeiss, Jena, Germany).

The prepared and cleaned specimens were randomly assigned to 15 groups ( n = 8) ( Table 1 ). The bottom sides of the inlays were etched with hydrofluoric acid for 45 s (Ceramic Etch, Vita Zahnfabrik, Bad Säckingen), rinsed for 45 s with water spray and cleaned in an ultrasonically activated ethanol bath for 5 min. The inlays were silanated with Monobond S (Ivoclar Vivadent), dried after 60 s, coated with Heliobond which was air-thinned. Then the inlays were covered with a light shield (VivaPad, Ivoclar Vivadent, Schaan, Principality of Liechtenstein). Adhesive luting was performed with Syntac and Variolink or with RelyX Unicem according to the manufacturers’ recommendations or experimentally modified ( Tables 1 and 2 ). These modifications were a separate polymerization of Heliobond for 40 s and selective phosphoric acid etching of enamel margins when RelyX Unicem was used. After removal of excess luting composite, the luting gaps were covered with glycerol gel (Airblock, Dentsply DeTrey, Konstanz, Germany) in order to prevent the formation of an oxygen inhibition layer.

Light-curing components were light-cured with a polymerization lamp (Translux CL, Heraeus Kulzer, Hanau, Germany). The intensity of the light source was checked after each inlay insertion with a radiometer (Demetron Research Corp., Danbury, CT, USA) in order to ensure that 600 mW/cm ² were exceeded. Heliobond was light-cured for 40 s if advised by the protocol, otherwise a combined polymerization of adhesive and luting composite for a total of 240 s was performed, starting from proximal aspects.

Visible overhangs were removed with a scaler (A8 S204S, Hu-Friedy, Leimen, Germany). The finish of the margins was performed with finishing diamonds (Two-Striper, Premier) and polishing disks (SofLex Pop-on, 3 M ESPE, Seefeld, Germany). High gloss polishing was performed with felt polishing cones (Dia-Finish E Filzscheiben, Renfert, Hilzingen, Germany) and diamond polishing paste (Brinell, Renfert, Hilzingen). Luting gap width of each inlay was again measured at the bottom of the cervical box.

2.5

Thermo-mechanical loading and SEM analysis

After repeated measurement of enamel cracks, all specimens were stored for 21 days in distilled water at 37 °C. Impressions of proximal enamel and dentin margins were taken and a first set of replicas for SEM evaluation of the marginal quality was made.

To simulate clinical service, specimens were fixed in a chewing simulator, with one tooth per chamber that was adjusted in oblique contact to the antagonist (steatite, 6 mm). Thermo-mechanical loading comprised 100,000 mechanical cycles @ 50 N at 0.5 Hz and 2500 thermocycles between 5 °C and 55 °C. Subsequently, another set of replicas was made. Replicas were sputtered with gold and evaluated under a SEM microscope (Leitz ISI 50, Akashi, Tokio, Japan) at 200-fold magnification with regard to marginal quality in enamel and dentin, with the percentage of perfect/gap-free margin serving as quality criterion . SEM examination was performed by one operator having experience with quantitative margin analysis who was blinded to the restorative procedures. Marginal integrity was expressed as a percentage of the entire margin length in enamel and dentin. Marginal qualities were classified according to the criteria “continuous margin”, “gap/irregularity” and “not judgeable/artifact” according to a well-proven protocol . Afterwards the percentage “continuous margin” in relation to the individual judgeable margin was calculated as marginal integrity.

2.6

Statistical appraisal

Statistical analysis was computed with SPSS 14.0 software package for Windows (SPSS Inc., Chicago, IL, USA). The Kolmogorov–Smirnov test was performed for testing of normal distribution, non-parametric tests (Wilcoxon matched-pairs-signed-ranks-test, Mann–Whitney- U -test) were applied for comparisons on the 95% level.

2.1

Preparation and evaluation of specimens

120 freshly extracted fully intact human third molars were stored for a maximum of 30 days in aqueous 0.5% chloramine T-solution. The teeth were cleaned and evaluated for defects under a stereomicroscope (50×). The number and total length of visual enamel cracks was noticed (stereomicroscope SV 11, Zeiss, Jena/WinMes 2.0 software).

Standardized cavities were prepared (MOD, oro-vestibular isthmus width 4 mm, 3 mm depth occlusally, 2 mm mesio-distal width of the proximal box). The proximal box ended mesially 1–2 mm above the cemento-enamel junction and distally 1–2 mm below the amelocemental junction in cementum. Preparation was performed using a diamond bur under sufficient water cooling (80 μm diamond, Two-Striper ^® Prep-Set, Premier, St. Paul, USA), and finally with finishing diamonds (25 μm, Inlay Prep-Set, Intensiv, Viganello-Lugano, Switzerland). Internal cavity transitions were rounded, the enamel margins were not beveled.

2.2

Cerec scan

The teeth were powdered (Vita-Cerec-Powder, Vita Zahnfabrik, Bad Säckingen, Germany) and the optical impression was computed by a Cerec scanner (Cerec 2, Sirona, Bensheim). Vitablocs Mark II (Vita Zahnfabrik) were used as ceramic blocks. After the scan process the powder was removed with air–water spray for 30 s and the teeth were finally dried.

2.3

Impressions, temporary restorations and loading

Single-step impressions were taken from the cavities (Provil Novo, Heraeus Kulzer, Hanau, Germany) and impressions were filled with epoxy-resin die (Alpha Die MF, Schütz Dental, Rosbach, Germany). IPS Empress inlays were fabricated within one week. While the chairside-fabricated inlays were inserted immediately, the cavities of the labside-fabricated inlays were temporarily restored with either Luxatemp temporary inlays luted with temporary cement (TempBond NE) or Systemp.inlay inserted without temporary cement. All labside-fabricated inlay groups underwent a thermo-mechanical loading in order to simulate loading of the temporary restorations (1000 cycles á 50 N + 25 thermocycles between 5 °C and 55 °C + storage in distilled water at 37 °C, for 7 days). Thereafter, the temporary restorations were removed and cavities were cleaned with pumice powder. A repeated determination of enamel cracks and a control of the cavity margins for marginal fractures was carried out.

2.4

Fitting and adhesive luting

After fabrication of all inlays, their fit was checked and adjusted with finishing diamonds under water cooling in order to obtain a luting gap of 500 ± 50 μm for Cerec inlays and 200 ± 20 μm for IPS Empress inlays. At the bottom of the cervical boxes the luting gap width was determined prior to the luting process (100× magnification, SV 11, Zeiss, Jena, Germany).

The prepared and cleaned specimens were randomly assigned to 15 groups ( n = 8) ( Table 1 ). The bottom sides of the inlays were etched with hydrofluoric acid for 45 s (Ceramic Etch, Vita Zahnfabrik, Bad Säckingen), rinsed for 45 s with water spray and cleaned in an ultrasonically activated ethanol bath for 5 min. The inlays were silanated with Monobond S (Ivoclar Vivadent), dried after 60 s, coated with Heliobond which was air-thinned. Then the inlays were covered with a light shield (VivaPad, Ivoclar Vivadent, Schaan, Principality of Liechtenstein). Adhesive luting was performed with Syntac and Variolink or with RelyX Unicem according to the manufacturers’ recommendations or experimentally modified ( Tables 1 and 2 ). These modifications were a separate polymerization of Heliobond for 40 s and selective phosphoric acid etching of enamel margins when RelyX Unicem was used. After removal of excess luting composite, the luting gaps were covered with glycerol gel (Airblock, Dentsply DeTrey, Konstanz, Germany) in order to prevent the formation of an oxygen inhibition layer.

Table 1

Experimental groups.

Group	Inlay material	Temporary cement	Temporary restorative material	Luting system
1	Cerec 2	–	–	Syntac + Variolink II
2		–	–	Syntac + Variolinik II (Heliobond polymerized separately)
3		–	–	RelyX Unicem
4		Temp Bond NE	Luxatemp	Syntac + Variolink II
5		Temp Bond NE	Luxatemp	RelyX Unicem
6		–	Systemp.inlay	Syntac + Variolink II
7		–	Systemp.inlay	RelyX Unicem
8		–	Systemp.inlay	RelyX Unicem (selective enamel etching)
9	Empress	Temp Bond NE	Luxatemp	Syntac + Variolink II
10		Temp Bond NE	Luxatemp	Syntac + Variolinik II (Heliobond polymerized separately)
11		Temp Bond NE	Luxatemp	RelyX Unicem
12		–	Systemp.inlay	Syntac + Variolink II
13		–	Systemp.inlay	Syntac + Variolinik II (Heliobond polymerized separately)
14		–	Systemp.inlay	RelyX Unicem
15		–	Systemp.inlay	RelyX Unicem (selective enamel etching)

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