Diamond burs: the right grit changes more than the brand

Diamond burs are the consumable most often economised on and the one that determines margin quality. A preparation with an irregular margin cannot be corrected in the laboratory, and its cause is almost always the wrong or a worn instrument.

Grit is coded by a coloured ring on the shank, under a widely shared international standard. Black indicates super-coarse, green coarse, blue medium, red fine, yellow extra-fine, white ultra-fine.

The correct sequence runs from coarse to fine, and each step must remove the grooves left by the previous one. Skipping a step leaves striations that persist in the finished preparation and are reproduced in the impression.

The commonest error is finishing the margin with a medium-grit bur because that is what is already mounted. A margin finished with medium grit carries irregularities the impression material copies faithfully, and which the technician reads as the intended form.

The roughness of the prepared surface has opposite effects depending on what will be cemented over it. For a conventionally cemented restoration, some roughness increases frictional retention.

For an adhesive restoration on enamel, the surface should instead be smooth: microfractures produced by coarse grit weaken the enamel exactly where the bond will form. This is why veneers are finished with fine grits.

Bur shape must match the intended margin, and that correspondence is closer than commonly supposed. A round-ended bur produces a chamfer; a flat-ended one with rounded shoulder produces a shoulder; a flame-shaped bur produces no defined margin at all.

Tip diameter determines margin depth: a 1.2-millimetre bur produces a chamfer of roughly 0.6 millimetres, that is half the diameter. It is the simplest way to control thickness without measuring it.

Wear is not visible to the naked eye and manifests as a change in behaviour. A worn bur does not cut: it skids, demands more pressure and generates heat.

Heat is the consequence that matters, because pulpal damage from preparation depends more on temperature than on depth. An efficient bur with adequate irrigation heats less than a worn one used with force.

Irrigation should be verified as a procedure, not assumed. A partially blocked nozzle reduces the spray without interrupting it, and the drop passes unnoticed until measured.

Single-use burs offer an advantage beyond hygiene: they always cut as new. Unit cost is higher, but the comparison should be with a reusable bur at its tenth use, not its first.

In summary: sequence from coarse to fine without skipping steps, fine grit at the margin and on surfaces that will receive adhesive, bur shape matching the intended margin, and replacement as soon as the bur begins to skid rather than cut.