Zirconia can't be etched. Therefore, can't be truly chemically bonded with an agent. Any zirconia maryland is reliant upon the prep design for retention in relation to the function force applied. Worst case scenario , bridge pops off..... the separation will be between the adhesive and the zirconia.
To boost true bond interface, and if the prep allows, we'll cut back the zirconia as if doing a porcelain margin and press the ceramic for the whole bridge. Etch the porcelain internally and then advise the Dr to Bond with a separate etch, bond, and luting agent. We haven't had any failures......even with posterior onlay bridges.
Be sure to slow cool any of your Zr restorations 15 minutes from the high temp or your units will be prone to chipping and fracture in anyway.
, James