Inlays are the smallest of the high-end metallic or ceramic restorations. As per definition (as it was with metallic inlays) the inlay should not exceed half the distance between the cusp tips at the isthmus (the shortest distance in bucco-lingual direction). This has functional reasons. This rule is not relevant anymore for all ceramic systems, as there should be no chewing force elements integrated into the restoration.
Due to the continuous functional improvements of the composite (plastic) restorations in small molar defects, inlays become significantly less common as initial restoration. Either the defects are that small that they can be successfully treated with the lot less invasive composite filling, or they are already so large, that the areas with chewing forces are being replaced, too. Here the puzzle pieces are described as partial crowns and not as in inlays anymore. It is important to point out that there are no definite preparation forms indicated, especially with ceramic restorations, but that it is important, that the margins of the individual restoration shapes are smooth. Thatâ€™s why we prefer to speak of puzzle pieces, regardless of the form or if the volume is larger or smaller.