PREPARING CAVOSURFACE BEVELS WITH 7404 AND 7406 BURS
Throughout this Handbook the 7406 and 7404 Friction Grip high speed turbine burs will be used to impart bevels to enamel margins, meeting Enamel Axioms #3,#4, and #5
Bevels are placed after the outline form has been generated by conventional carbide burs. Diamond burs are not recommended in this Handbook see Carbides vs Diamonds in Cavity Preparation
This is an unusual application for these carbides, which are primarily designed for finishing composite and shaping occlusal surfaces. Why are they specified?
WHY ARE THEY USED TO PRODUCE BEVELS?
These burs are
- larger diameter than cavity prep burs, so that they naturally produce smooth, flowing cavosurfaces rather than choppy and irregular margins. As well, they generate larger radiuses at line angles
- the rake angle of the teeth on the burs is lower so that they cut less aggressively, appropriate to their role in planing margins to a fine finish.
- they have 12 blades rather than 6 making a finer cut , again,appropriate to the delicacy desired
- the bullet shape of the nose of the bur naturally tapers, so that when inserted into a cavity preparation, at right angles to the enamel, a small-angled bevel ensues. If a greater bevel is desired, the handpiece can be tilted proportionately, allowing the operator to impart a bevel that is proportionate to that required in the prep, see Enamel axiom #5, always proportionate bevels
- The nose of the bur is not effective at end-cutting, so the tendency to deepen preparations axially or pulpally is slight, making them inherently conservative and minimizing iatrogenic events
WHICH BURS TO PURCHASE?
This writer has used “Jet” brand gold shank 7406 burs, a bulletform bur, for many years. However, the 7404 bur chosen is made by Brasseler; a steel-shank, football shaped bur,with a red stripe.
The reason for two different manufacturers is for the difference in appearance. Dental assistants often cannot distinguish between two very similar sizes of gold shank burs so that the set up of burs is often faulty.
This leads to interruption in the preparation phase while hunting for the correct bur. The shape difference, i.e., bulletform vs football, is not significant because only the apical half of the bur, which is similar for both burs, is used in most applications. The difference in diameter, however, is valuable for preparations that differ in size.
Cusp tip preparation
Class V occlusal margin, and also to plane the gingival margin and radius the line angles
Class III extensive preps, lingual surface margins in particular, where the lingual concavity makes it sometimes difficult to follow enamel axiom #4,always bevel from dentin.