Adhesive bonding: Corundum blasting vs. hydrofluoric acid etching

The adhesive bonding of all-ceramic restorations plays an important role in the success of prosthetic restorations. When processing silicate-based ceramics, conditioning the restoration surface with hydrofluoric acid is the gold standard. However, handling it also involves risks. A study group at the LMU investigated whether corundum blasting in combination with universal primer could be an equivalent or even superior alternative to hydrofluoric acid etching.

Hydrofluoric acid (HF) is used in prosthetic dentistry to etch silicate-based ceramics, enabling the bonding of crowns, bridges, and veneers to the tooth structure. The etching process selectively dissolves the glassy (SiO₂ ₎ matrix, creating a rough surface that provides micromechanical retention. However, improper handling of HF can lead to severe burns to the skin and eyes, posing a significant risk to dental staff and patients. Therefore, alternative surface treatments such as corundum blasting are gaining prominence as a less toxic option.

Adhesive Fixation: Micromechanics and Chemical Adhesion

Zirconia restorations (oxide ceramics), which exhibit high crystallinity and no glassy phase, cannot be etched with hydrofluoric acid. Instead, the restoration is sandblasted with aluminum oxide (Al₂O₃ ) _{to create micromechanical retention. The effectiveness of this method depends on factors such as particle size (25–50 µm), pressure, distance, and application time. Incorrect parameters ,} however, can lead to surface damage and inaccuracies in the restoration's fit. In addition to micromechanical retention, chemical adhesion is crucial. Universal primers with functional monomers such as MDP promote bonding to zirconia surfaces, while functional silanes enable adhesion to silicate-based ceramics.

The materials science research group at LMU investigated whether silicate-based ceramics can be effectively bonded without HF etching using corundum blasting with aluminum oxide in combination with a universal primer [1]. In addition, the sandblasting parameters for zirconium oxide were evaluated.

material and methods

In this in vitro study, three different CAD/CAM ceramics were investigated:

• Feldspar ceramic (VITA-BLOCS Mark II, VITA Zahnfabrik, Bad Säckingen, Germany),
• Lithium silicate ceramic (IPS e.max CAD, Ivoclar, Schaan, Liechtenstein),
• Zirconia (3Y-TZP) (Lava Plus, Solventum, Seefeld, Germany).

The ceramic blocks were cut into 280 substrates (7,5 × 7,5 × 2 mm³) and crystallized or sintered according to the manufacturer's specifications. After polishing, the substrates were divided into four pretreatment groups and one control group.

• Group 1: 25 µm Al₂O₃particles and 0,05 MPa jet pressure
• Group 2: 50 µm Al₂O₃particles and 0,05 MPa jet pressure
• Group 3: 25 µm Al₂O₃particles and 0,1 MPa jet pressure
• Group 4: 50 µm Al₂O₃particles and 0,1 MPa jet pressure
• Control group: Feldspar and lithium silicate ceramic substrates were etched with 9% HF gel.

Subsequently, the surface energy and surface roughness were determined. Before adhesively bonding the substrates with a dual-curing luting composite (Variolink Esthetic, Ivoclar), the blasted or etched substrate surface was treated with a universal primer (Monobond Plus, Ivoclar). Half of the test specimens underwent thermocyclic aging (10.000 cycles between 5 °C and 55 °C) to obtain results as clinically relevant as possible regarding the bond strength. The tensile bond strength [MPa] was tested in a universal testing machine. The fracture patterns were microscopically analyzed to determine whether the fracture occurred adhesively, cohesively in the substrate, or in the luting composite (Figure).

Results

Zirconia substrates exhibited a lower surface energy compared to silicate ceramics (p < 0.001). The highest surface roughness values ​​were recorded after blasting with 50 µm Al₂O₃ particles _{and a} blasting pressure of 0,1 MPa (p < 0.001).

Zirconia substrates (Figure 1) exhibited higher bond strength values ​​(30–45 MPa) than feldspar (16–26 MPa) and lithium silicate (10–21 MPa). While high-frequency etching initially resulted in the highest bond strength values ​​(p < 0.030), artificial aging affected the etched substrates, reducing the bond strength compared to corundum blasting (p < 0.039). For zirconia, the use of 50 µm Al₂O₃ particles _{increased the} bond strength regardless of the blasting pressure (p < 0.002).

Conclusion

With regard to adhesive bonding, the study was able to show that corundum blasting (25 and 50 µm particle size) with 0,1 MPa blasting pressure could be an effective pretreatment method for bonding silicate-based ceramics such as feldspar and lithium disilicate ceramics and could well replace etching with the health-hazardous hydrofluoric acid. However, the evaluation of the fracture patterns showed an increased number of cohesive fractures in the feldspar ceramics after corundum blasting, especially at a pressure of 0,1 MPa, compared to HF etching. This could indicate that corundum blasting can cause microcracks that weaken the substrate. In order to be able to make definitive clinical recommendations, future studies should include flexural strength measurements after corundum blasting to assess possible structural weakening.

For zirconia, blasting with 50 µm Al ₂ O ₃ particle size is still recommended, with the applied pressure showing no significant influence on the bond strength.