In dentistry, an inlay is usually an indirect restoration (filling) consisting of a solid substance (as gold, porcelain or less often a cured composite resin) fitted to a cavity in a tooth and cemented into place.
This technique involves fabricating the restoration outside of the mouth (by a dental technician) using the dental impressions of the prepared tooth, rather than placing a soft filling into the prepared tooth before the material sets hard.
An onlay is the same as an inlay, except that it incorporates a replacement for a tooth cusp by covering the area where the missing cusp would be.
Crowns are onlays which completely cover all surfaces of a tooth. See below...
Comparison of inlays and direct fillings
When an inlay is used, the tooth-to-restoration margin may be finished and polished to a very fine line of contact to minimize recurrent decay.
Opposed to this, direct composite filling pastes shrink a few percent in volume during hardening. This can lead to shrinkage stress and rarely to marginal gaps and failure.
Although improvements of the composite resins could be achieved in the last years, solid inlays do exclude this problem.
Another advantage of inlays over direct fillings is that there is almost no limitations in the choice of material.
While inlays might be ten times the price of direct restorations, it is often expected that inlays are superior in terms of resistance to occlusal forces, protection against recurrent decay, precision of fabrication, marginal integrity, proper contouring for gingival (tissue) health, and ease of cleansing offers.
However, this might be only the case for gold. While short term studies come to inconsistent conclusions, a respectable number of long-term studies detect no significantly lower failure rates of ceramic or composite inlays compared to composite direct fillings.
Another study detected an increased survival time of composite resin inlays but it was rated to not necessarily justify their bigger effort and price.