1

Introduction

Over the last 20 years, material-related advances have led to a significant increase in the use of all-ceramic restorations. These improvements are in biocompatibility, mechanical properties and aesthetics. The procedure for cementation of glass-ceramics has become standardized, involving: (i) etching the restoration with HF, then (ii) silanization to ensure a durable bond between restoration and composite cement via chemical and micromechanical mechanisms. However, in contrast to glass ceramics, there is no uniform “gold standard” for cementation of oxide ceramics such as zirconium oxide, for which zinc phosphate, glass ionomer and self-adhesive composite cements can be used. But these lack the advantageous properties (increased fracture resistance and improved retention) of composite cements which adhere to etched and silanized glass ceramics [ ].

This leads to low retention between zirconia restorations, luting cement and tooth structure. Since zirconia does not contain silicates, it is neither etchable nor can an effective bond be achieved with silanes between restoration and luting cement [ , ]. This means that conventional luting protocols of glass ceramics cannot be applied to oxide ceramics [ ]. Another problem is the low wettability of the zirconia surface, which is hydrophobic [ ]. However, successful cementation of oxide ceramic restorations is an important factor for long-term clinical success [ , , ]. But stronger micromechanical retention can be achieved via a rougher, larger or more energetic surface that changes the wetting capacity and facilitates the flow of cement into the roughened surface [ ]. There have been many attempts over the last 2 decades to increase the roughness of zirconia surface to improve cement bonding [ , ]. Proposed methods include grit blasting with different materials and grain sizes, so-called “tribochemical coating”, laser treatments [ ], grinding, primer and acid treatment [ , ], “plasma spraying” or silanization [ ]. Alternative strategies include establishing a glass layer on the zirconia surface, known as “Internal Coating”, “Selective Infiltration Etching” or “Glaze-on Techniques” [ , ]. Whereas, in “Selective Infiltration Etching”, a thin glass layer is applied to the surface by means of a conditioning agent and then completely dissolved [ ], in the “Internal Coating” and “Glaze-on” technique the glass or porcelain layer is preserved as the “inner lining” of the restoration. In the latter two methods [ , ] HF is applied to the fired glass layer of the oxide ceramics to roughen the surface. The silicates contained in the glass layer enable bonding with silanes. The aforementioned “glaze-on” technique has led to significantly increased bonding in some studies, which were also considered in some systematic reviews and meta-analyses [ , , ]. In the “glaze-on” studies examined in these reviews this technique was only tested in combination with metals or composites, but not with dentin.

Another possibility for surface treatment is air abrasion (grit blasting) with small particles of silica (SiO ₂ ) and/or alumina (Al ₂ O ₃ ) particles. Specifically in the Rocatec™, Rocatec™ Plus and Rocatec™ Soft systems (3M), particles of aluminium oxide with a silica coating are blasted at high speed onto a ceramic workpiece. According to the manufacturer during this process components of the particles melt on the ceramic surface and microdefects are created, resulting in surface enlargement. The extent of these surface changes depends on particle size, jet pressure, angle of incidence, duration and working distance [ ]. Multiple studies have established the value of grit blasting [ , ]. Largely unexplored – or insufficiently well documented – are comparisons of different grit blasting protocols with regard to the resultant bond strength. Likewise, the combined effect of grit blasting grain size and pressure is still largely unexplored. Furthermore, there is insufficient data on whether primer addition can further improve the SBS.

In order to achieve the aim of this study, to optimize the fitting surface preparation of zirconia restorations for bonding to dentin, the following objectives were investigated:

Part 1:

• a)

  Coating of zirconia with glass ceramic using Zenostar Magic Glaze Spray (ZM), IPS e.max Ceram Glaze Spray (IPS) or Hotbond zirconnect Spray (HB) followed by etching with HF and silanization.

• b)

  Pretreatment of zirconia with Rocatec™ Soft followed by the application of a ceramic primer versus an universal primer.

Both issues were investigated using zirconia discs bonded via RCC to bovine dentin substrates.

Part 2:

• a)

  Coating of zirconia with glass ceramic (HB) followed by etching with HF and silanization.

• b)

  Pretreatment of zirconia substrates with air abrasion using different protocols followed by application of two universal primers with and without MDP monomer and two RCCs with and without MDP.

Both issues were investigated using stainless steel discs bonded via RCC to zirconia substrates. The logic of using stainless steel (SS) discs is that adhesive failure will occur between RCC/zirconia, rather than at the SS/RCC interface.

The following null-hypotheses were formulated:

• (i)

  Coating of zirconia surfaces with glass-ceramic does not improve bonding of composite cements to zirconia compared to the control (untreated zirconia surfaces).

• (ii)

  Air abrasion of zirconia surfaces does not increase bonding via composite cements, regardless of grain size and pressure.

• (iii)

  Primers do not increase the bonding of composite cements to zirconia.

• (iv)

  Self-adhesive RCCs, with or without MDP, do not show similar bond strenths for the bovine dentin/RCC/zirconia interfaces (part 1) versus the RCC/zirconia interface (part 2).

2

Materials and methods

Materials are listed in Tables 1A , 1B , 1C . Study variables are presented in Tables 2A , 2B and Fig. 1 A, 1B.

Table 2A

Surface preparation of zirconia substrates and cementation.

	Part 1		Part 2
Zirconia cylinders or zirconia cuboids	480 cylinders of Prettau® Zirconia (6 mm diameter / 3 mm height).		480 cuboids of Y-TZP Cercon ht® (12 mm edge length / 2 mm thickness)
	Glazing	Air abrasion	Glazing	Air abrasion
Air abrasion	Pure Al 2 O 3 : Particle size 110 μm Pressure 2 bar Duration 15 s Distance 10 mm	Rocatec SOFT (R-TEC): Particle size 30 μm Pressure 2 bar Duration 15 s Distance 10 mm	Pure Al 2 O 3 : Particle size 110 μm Pressure 2 bar Duration 5 sec Distance 10 mm, Angle 45°	Pure Al 2 O 3 : Group 1: Particle size 110 μm, pressure 3.5 bar Group 2: Particle size 110 μm, pressure 2 bar Group 3: Particle size 50 μm, pressure 2 bar Group 4: Particle size 50 μm, pressure 0.5 bar For all four subgroups: Distance 10 mm, angle 45°, duration 5−7 sec
Glazing	HB, ZM or IPS: Sprayed in circular movement, angle 90°, distance 10 cm		HB: Distance 10 cm
Air abrasion after glazing	HB and ZM: Pure Al 2 O 3 Particle size 110 μm Pressure 1 bar Duration 5 s. Distance 10 mm IPS: Pure Al 2 O 3, Particle size 50 μm Pressure 0.75 bar Duration 5 s. Distance 10 mm		Pure Al 2 O 3 Particle size 110 μm, Pressure 0.1 MPa Distance 10 mm Angle 45°
HF-Treatment	Hydrofluoric acid 9% 60 s, rinsed for 30 s		Hydrofluoric acid 10% 60 s, rinsed for 60 s
Silanization/Priming	Monobond Plus or Clearfill Ceramic Primer 60 s	Monobond Plus or Clearfill Ceramic Primer 60 s	Calibra Silane 60s or Clearfil CCP plus 60 s	P&B active or Clearfil CCP plus 20 s For half of the specimens
Bonding			P&B active or Clearfil universal bond for half of the specimens
Self-adhesive composite cements	Maxcem Elite or RelyX Unicem 2 Automix	Maxcem Elite or RelyX Unicem 2 Automix	Calibra Universal or Panavia SA Cement Plus	Calibra Universal or Panavia SA Cement Plus

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