Optimal Onlay Material for Molar-gold, All Ceramic, EMax, PFM, Zirconia, or CEREC?

For a combination of strength and aesthetics, nothing compares to Emax.  I use it daily in my practice for onlay restorations and love this material.  On rare occasions, in severe grinders I will use Zirconia crowns when being placed on second molars. Good luck.   

Much to consider here.  First molar, second molar or wisdom tooth?

In general the only time I would use gold today is on second molars or wisdom teeth.  Yes, gold can be made thinner (less drilling, less tooth preparation) than PFMs, Zirconia, Empress and Cerec.The other alternative for second molars or wisdom teeth might be Emax, which is strong and can be made thinner than the other types just mentioned.

I wouldn't be too concerned about the galvanizing effects of gold vs a PFM, as the porcelain is an insulator and the metal underneath the PFM would not be touching the gold of the adjacent tooth.  However, consider the abrasive potential when a porcelain crown (Emax,PFM, Cerec, Zirconia, etc.) opposes a gold crown.  These restorations are more abrasive than gold, and with bruxers (teeth grinders) over time one can often expect the porcelain restoration grinding away the surface of the gold restoration. Thus, it is often preferable to have like restorations opposing one another, gold vs. gold, or porcelain vs. porcelain restorations.

Saying all of that, my preference most of the time for molars is Emax.  For patients who are insurance driven, and insurance often only pays for the least costly treatments, we will frequently use PFMs.  Although I do not consider most PFMs the state of the art, they have a long track record in dentistry and esthetics looks better than gold, but not quite as natural looking as Emax.

The best material depends on the individual case - CEREC is a process, not a material. Unless you use gold, it gives the dentist a choice of medium strength materials. Gold is the best for wear and won't fracture. For least tooth material reduced, good strength and low wear, CEREC using eMax would be my choice. But again, choose a dentist that you know has all the materials and techniques in his skill set.