A STUDY OF THE CUTTING EFFICIENCY OF DENTAL BURS FOR THE STRAIGHT HANDPIECE

¹ School of Dentistry, University of Michigan

This study has been limited to a comparison of the efficiency of the fissure and inverted cone type of dental burs when cutting a material with a hardness similar to dentin. This permitted a comparison of bur types on a basis of design only and did not consider the endurance of the bur, or the tendency to become dull which would result if hard materials like glass or enamel were cut. The effect of bur size and composition as related to cutting efficiency were also studied to a limited extent. The physical factors of speed of rotation between the limits of 1000 and 8000 r.p.m., and force applied to the cutting instrument from 200 grams (0.44 pounds) to 750 grams (1.65 pounds) were studied. An effort was made to integrate these variables as far as practicable.

It was observed that the tungsten carbide burs examined cut at essentially the same rate as the steel burs when operated under comparable conditions, but the carbide burs retain their cutting edge longer. The carbide burs were more brittle and susceptible to breakage than steel burs when subjected to side pressure and torque due to being embedded in the material cut.

The No. 557 fissure type steel bur with spiral axial grooves removed more material per unit time than burs with straight grooves when operated under the same conditions. It was observed that the spiral type bur showed a little greater tendency to become clogged with cut material than did the straight bur.

A pronounced difference in the amount of material removed per unit time by the inverted cone type bur as compared with the fissure type bur was observed. Although the No. 37 bur has only about one-third as much active area as the No. 557 fissure bur, it is capable of removing approximately five times the amount of material per unit time as the fissure bur. On a basis of unit active area this would indicate a ratio of approximately 15 to 1 in favor of the inverted cone type.

When using the No. 557 steel bur it was found that for each speed increase of 2000 r.p.m., between the limits of 2000 and 8000 r.p.m., the amount of material removed per unit time was approximately doubled, while the applied pressure remained constant. Accordingly, the rate of removal increases rapidly at speeds greater than 4000 r.p.m., which is near the upper limit of most present day engines.

When using a No. 557 bur with a constant energy input, a comparatively large increase in material removed is achieved when the pressure is raised from 200 to 500 grams. With an increase in pressure from 500 to 750 grams, the amount of material removed is increased, but the relative increase is less pronounced. With an inverted cone bur, however, this trend is reversed so that with increased pressure there is an increased rate of removal of material.

The results reported indicate that an increase in size of bur for a given type will result in greater amounts of material removed when other factors remain constant.