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A fundamental rethinking of the desirable outline form for Class III margins can be developed by examining the axioms for optimum bonding to enamel, see Enamel Axioms
These guidelines mandate an outline form that is comprised of rod ends not rod sides; these margins will have twice the bond strength, will resist contractile damage of polymerization shrinkage that leads to white line margins, or brown line margins which indicate marginal leakage and subsequent stain from adhesive de-bond or cohesive margin construction. The reasons for these claims are found at the above link.
New thinking to improve Lingual Class III margins:
Creating lingual rod- end margins is often just a matter of bur inclination as one finalizes the margin.Small -diameter operative burs tend to produce an irregular outline, which is improved by final finishing with a 7404 or 7406 bur to generate a smooth outline and, a the same time,a shallow-angle bevel corresponding to that of the bur’s football or bullet shape. A smooth margin is easier to locate and finish than a choppy one one created by a smaller-diameter bur. The football shape and 12-bladed fine cutting tip of these burs imparts the shallow bevel to resist contraction damage and stain, over the long run.
New thinking to improve Facial Class III margins:
In conservative Class IIIs one wants minimal facial encroachment. The fine tip of a 7902 gold-shank finishing bur is small enough to plane and bevel-from the facial, to achieve a marginal bevel with minimal increase in facial footprint.
Intrinsic Bevels vs Cosmetic Afterthought Bevels:
Crucial to bevel design is this concept: a bevel begins in dentin and continues to the cavosurface. Throughout this journey the bevel transects enamel rods and exposes a consistent rod-end internal surface. It is not an afterthought to a rod-side margin that changes direction somewhere as it nears the cavosurface. Very shallow bevels extend the facial surface minimally. Old concepts of 45 degree bevels are hugely over-destructive. 6 degrees is often all that is necessary.
While “cosmetic beveling” can improve blending of resin to tooth, it is utterly dependent on using a resin with the right degree of transparency. If the bevel originates at the dentin, you simplify the esthetic challenge while at the same time as producing structurally superior outcomes.