Air abrasion: the wrong pressure on zirconia creates microcracks instead of retention

Air abrasion is the most used surface treatment in prosthodontics and the one with the widest gap between current practice and documented parameters. The same pressure is used for materials requiring opposite treatments.

The principle is simple: abrasive particles propelled by air create microscopic roughness, which increases the surface available for bonding and improves wettability.

On zirconia the treatment is indispensable because hydrofluoric acid has no effect: with no glassy phase to dissolve, etching produces no retention whatsoever. Air abrasion is the only way to condition the surface mechanically.

But zirconia requires specific parameters, and here the problem arises. High pressures induce phase transformation from tetragonal to monoclinic at the surface, and can generate microcracks that become initiation sites.

Recommendations converge on low pressures — around 1 to 2 bar — with 50-micrometre particles, at about 10 millimetres distance and for a few seconds. Blasting at 4 bar because that is what the gauge is set to produces the opposite of the intended effect.

On metal the situation differs: alloys tolerate higher pressures and coarser grains, and air abrasion also serves to remove oxides and investment residues from casting.

On abutments intraorally, before adhesive cementation, air abrasion is performed with chairside units at low pressure and fine particles. It removes temporary cement residue better than any mechanical cleaning.

The distinction between air abrasion and etching must be kept clear because the two treatments are not interchangeable. Glass ceramics — feldspathic, disilicate — are etched with hydrofluoric acid, which selectively dissolves the glassy phase leaving a retentive structure.

Air-abrading a thin glass ceramic is counterproductive: it removes material uncontrollably and can fracture restorations half a millimetre thick.

After air abrasion the surface must be cleaned, and this step is often skipped. Aluminium oxide particles remain embedded in the surface, and should partly be removed with ultrasonics in alcohol or distilled water.

Touching the abraded surface with the fingers defeats the treatment: the skin's lipid film reduces the surface energy that air abrasion had raised. It is handled with tweezers from air abrasion onward.

The interval between abrasion and cementation should be as short as possible, because the activated surface becomes contaminated by airborne dust within minutes.

In summary: low pressure and fine particles on zirconia, never hydrofluoric acid on it; etching rather than air abrasion on glass ceramics; ultrasonic cleaning after treatment and no finger contact until cementation.