Jacquelin S.

The advantage of the onlay is that less sound tooth structure is trimmed away. (As compared when a crown is placed, a greater portion of your original tooth still exists.)

That's an advantage (the usual goal of dentistry is to preserve natural tooth structure to the greatest extent possible), the trade-off however is that retention (keeping the restoration in place) may be a problem. (Fully covering over a tooth [like a crown does] usually aids restoration retention.)

In your case, an eMax restoration can be cemented adhesively (bonded) or conventionally (like your PFM probably would be). Either way there's no real reason to expect that retention will be a problem. It's just that as compared to a crown there is more potential for it. The possibility of adhesive cementation makes the point of retention less of an issue for the onlay.

There could be strength concerns (fracture) with an all ceramic onlay (with a gold onlay there wouldn't be). Once again, there doesn't have to be, but as compared to a PFM crown there is more potential for it. (The link above is for all-ceramic crowns but applies to onlays too. Read points #2, #3 and #4. Note, monolithic ceramic construction is the strongest. Also notice that Zirconia is the strongest type of synthetic ceramic.)

Philosophically, assuming that conditions are correct for the onlay (no exceptionally heavy chewing forces, no teeth grinding habit, preferably monolithic construction), the onlay is probably more in line with what will be practiced as dentistry 10 years from now.

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