PREPARING CAVOSURFACE BEVELS WITH 7404 AND 7406 BURS
To qualify as a “preparation”, the cut plan of the case must meet many criteria.
- enamel and dentin axioms. see enamel axioms and dentin axioms.
- smooth and flowing outline form
- crisp margins
- no internal stress risers see dentin axioms
- no residual caries
- no iatrogenic damage to adjacent teeth
- no occlusal stops on cavosurface margins
- cusp shoes of adequate reduction
The cavosurface of most preparations resembles a hybrid of GV Black amalgam preparations and those of classic cast gold, especially those of the eminent late Dr. Richard Tucker. As such, they have family resemblances but readers should be ready to reconfigure their thinking and methods.
One key factor to understand is that diamonds generate cavosurface chipping and crazing. Carbides produce a crisper finish line that resists contraction forces better because the enamel rods have not been shattered like diamonds do. Resin adapts to a finely cut margin more readily at the microscopic level. See Carbides vs Diamonds in Cavity Preparation
After the outline form has been generated by conventional carbide burs or in some cases diamond burs, they are refined to make a smooth and flowing cavosurface using 7406 or 7404 burs with light pressure, slower speeds, and a two-handed grip to triangulate the turbine and gain exquisite control, in cases of:
- occlusal margin of class V buccal and lingual margins
- shoed cusps buccal and lingual
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 the choppy and irregular margins characteristic of small diameter burs such as the 330 bur. At line angles of Class V preparations they generate larger radiuses which are easier to adapt paste resin to than sharp radii, restore better, look more attractive
- 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
And for radiusing Class V outline form at the line angles.
Class V occlusal margin, and also to plane the gingival margin and radius the line angles
Class II Shoed-cusp molars and bicuspids
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.