An important step in bonding in an IPS e.max ( Lithium Disilicate) restoration is to prepare the intaglio surface of the glass by etching with hydrofluoric acid. The quality of the bond is dependent on the effectiveness of how the glass is etched, so understanding the manufacture recommendations is critical. Lithium Disilicate has very different etching protocols then other porcelain restorations due to it’s unique crystalline structure and chemistry.
There are multiple different protocols being discussed for this product when I attend lectures, it seems between this being a relatively new material for many practitioners, and the difference from the protocol for other porcelain products confusion has resulted. Ivoclar recommends preparing the glass with IPS ceramic Etching Gel, a 5% hydrofluoric acid, for a maximum of 20 seconds. Do not rely on the appearance of the internal surface as an indicator of adequate etching, as a frosty appearance actually indicates over-etching..
When etching porcelain the old adage of “if a little is good more is better” does not apply. Over-etching Lithium Disilicate, due to it’s filler content and crystalline matrix, can be very detrimental to its strength and the long term success of the restoration. To ensure that you are not over-etching, first confirm for a lab processed restoration whether it has been etched by the lab. If you are etching in your office, use the correct strength of hydrofluoric acid and a timer so you rinse after a twenty second exposure time. A restoration that is tried in the mouth after etching should not be exposed again to hydrofluoric acid, but cleaned prior to the application of porcelain conditioner to remove saliva and debris.