JADA Continuing Education
The effect of multiple uses of disposable diamond burs on restoration leakage
JOSEPH A. von FRAUNHOFER, M.Sc., Ph.D., F.A.D.M., F.R.S.C.,
TROY A. SMITH, B.S. and
KYLE R. MARSHALL, B.S.
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ABSTRACT
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Background. The authors tested the clinical longevity of disposable diamond burs. They cut a series of five preparations and assessed the leakage after restoring the tooth.
Methods. The authors prepared 10 teeth for Class V restorations, and used a new disposable diamond bur for each tooth. The burs were used to cut preparations in extra teeth before being used to prepare a second series of 10 teeth (third use). The authors then cut preparations in extra teeth before preparing a third set of 10 teeth (fifth use). They removed existing restorations in a second group of 30 teeth and extended the preparations using the same regimen of one, three and five bur uses. All preparations were etched and conditioned, and the teeth were restored with resin-based composite. Using a 20-volt direct-current power source and a stainless-steel counter electrode, the authors measured the leakage electrochemically in 1.0 percent sodium chloride for 30 days.
Results. Freshly prepared and restored teeth leaked less than reprepared teeth. Leakage was similar for the first and third uses of the bur, but was far greater for the fifth use (P < .01). The previously restored teeth that were cut with the first- and third-use burs behaved the same, but the third-use bur caused more leakage than the fifth-use bur (P < .01).
Conclusions. Reuse of disposable burs can affect leakage behavior. With new preparations, use of a disposable bur to cut more than three preparations increased leakage. For teeth that were reprepared and restored, greater leakage occurred than it did with new preparations, although repeated use of a bur may reduce leakage.
Clinical Implications. Disposable diamond burs may cut preparations in up to three teeth before adversely affecting leakage behavior. Restoration removal and repreparation of teeth results in greater leakage than that with freshly prepared teeth.
Key Words: Cutting; diamond burs; restoration leakage
Dentists in the United States commonly use diamond burs for extracoronal cutting and carbide burs for intracoronal cutting, and dental schools typically recommend diamond burs for gross tooth reduction and finishing of axial walls of crowns and fixed partial dentures.1 Clearly, both the cutting efficiency and service longevity of diamond burs are important.
Disposable diamond burs may cut preparations in up to three teeth before adversely affecting leakage behavior.
Although disposable (single-use) diamond burs are available only in a limited range of shapes and grit sizes, their clinical use has grown in recent years. This may be partly in response to recommendations from the Centers for Disease Control and Prevention and the American Dental Association that organized dentistry should minimize cross-contamination risks, because use of disposable burs absolutely prevents transfer of microorganisms between patients.
Another element may be the marked cost differential between disposable and conventional (multiuse) diamond burs, combined with study findings that suggest comparable cutting efficacies.2–4 Anecdotal reports suggest that dentists use disposable burs to cut several cavity preparations in each patient.
We undertook this study to assess the cutting efficiency of disposable diamond burs with repeated use (that is, when the same bur was used to cut several cavity preparations).
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MATERIALS AND METHODS
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We conducted this study on sound anterior and premolar teeth extracted for periodontal or orthodontic reasons. Two of us (T.S., K.M.) removed the apical 2 millimeters of root from each tooth, and removed the pulpal tissue to reamer size 90.
The passage of current denoted the onset of leakage and the magnitude of the current indicated the amount of leakage.
Group 1.
The senior dental students prepared 30 teeth for a 3 x 2 x 2-mm Class V restoration (one-half in enamel and one-half in cementum). They used medium-grit, flat, end-tapered disposable diamond burs (Patriot PA 25M, SDS Kerr, Orange, Calif.) in a high-speed handpiece under water cooling to prepare the first 10 teeth; they used a new bur for each of these teeth. The burs then were used to cut preparations in extra teeth before being used to prepare a second set of 10 teeth (that is, the third use of the bur). The dental students then used the burs to cut preparations in extra teeth before preparing a third set of 10 teeth (that is, the fifth use of each bur).
Group 2.
The students removed the existing restorations in a second group of 30 teeth (previously used in leakage studies) and minimally refined the one-half enamel and one-half cementum preparations using the same regimen of one, three and five uses of each bur as that used for the freshly prepared teeth.
After cutting the cavity preparations, the dental students etched them for 15 seconds with a 37 percent phosphoric acid solution, conditioned the preparations (Prime & Bond NT, DENTSPLY Caulk, Milford, Del.) and cured the conditioner for 20 seconds using a curing light (Optilux 400, Demetron Research, Danbury, Conn.). They then restored the teeth with resin-based composite (Prisma TPH, DENTSPLY Caulk) by placing 2-mm increments with a plastic filling instrument, and curing each increment with the curing light for 30 seconds before placing the next increment. The restorations were not polished. The dental students then inserted bare-ended polyvinyl chloride–insulated wires into the root canal apexes to establish firm contact with the pulp chamber roof, and sealed the root surfaces with sticky wax and three layers of nail varnish.
Positive controls consisted of two teeth with one-half enamel and one-half cementum preparations, but without conditioning of the dentin or restoration placement. The negative controls consisted of two conditioned and restored teeth, but they received three layers of nail varnish over the entire tooth surface and tooth-wire interface to ensure an impermeable barrier.
We then immersed the teeth in a 1.0 percent sodium chloride solution, and also placed a stainless steel strip in the container. Each tooth functioned as one electrode in the test circuit, while the stainless steel strip acted as the other electrode. We measured the amount of leakage by connecting each test tooth to one output of a 20-volt direct-current voltage supply, while the other voltage output was connected continuously via a 100
resistor in series with the stainless steel strip (Figure 1
). The passage of current denoted the onset of leakage. The magnitude of the current, calculated by Ohm’s law from the voltage measured across the resistor (that is, current = voltage ÷ resistance), indicated the amount of leakage. We recorded the current readings on alternate days for a minimum of 30 days.
We calculated mean values and standard deviations for the three sets of 10 teeth in the two test groups. These data were subjected to analysis of variance to determine whether differences existed between them, and the differences were identified with post hoc Scheffé tests at an a priori 
= .05 significance level.
We performed scanning electron microscopy (SEM) on new (as-received) burs, as well as on burs that had been used once, three times and five times. The burs were mounted on individual aluminum blocks, with small drops of colloidal silver paint (Electron Microscopy Sciences, Hatfield, Pa.) applied to the bur shank to ensure electrical continuity. We sputter-coated all bur specimens with gold-palladium (Hummer IV Sputter System, Anatech, Union City, Calif.) for two minutes before SEM examination. Two experienced examiners (J.v.F. and Ms. Becky Wade, University of Maryland School of Dentistry, Baltimore) inspected each bur specimen under a scanning electron microscope, and selected the images that provided the most typical representation of the burs.
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RESULTS
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Figure 2 shows the mean leakage currents for group 1 teeth (new cavity preparations). Figure 3 shows the mean leakage currents for group 2 teeth (that is, with removed and replaced restorations). We noted no differences in cutting rates (that is, the time required to cut the cavity preparations) for the first, third and fifth uses of the burs with either group of teeth.
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Figure 2. Mean leakage currents for new Class V enamel and cementum resin-based composite restorations (group 1).
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Figure 3. Mean leakage currents for Class V enamel and cementum resin-based composite restorations after removal of the restoration, preparation refining and placement of a new restoration (group 2).
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The results for the freshly prepared teeth show that the leakage currents for the first and third uses of the burs were similar to each other (P > .05) and approximately 50 percent less than those for the fifth use of the bur (P < .01). We noted that leakage appeared to continue throughout the testing period, and did not achieve a maximum value (or steady state).
The group 2 (reprepared) teeth exhibited greater leakage currents with the first and third uses of the bur (P < .01) than did the group 1 specimens. There was no statistically significant difference in leakage current at 30 days for teeth prepared with the first- or third-use burs or the first- or fifth-use burs (P > .05). However, statistically, specimens prepared with the third-use burs exhibited greater leakage than did specimens prepared with the fifth-use burs (P < .01).
Figures 4 through 7 show the SEM images (original magnification x 100) of a new bur and of burs that have been used for the first time, the third time or the fifth time. We noted little difference between the images of the new bur and the first-use bur (Figures 4 and 5). The image of the third-use bur (Figure 6) shows a small accumulation of cutting debris and some chipping and loss of the diamond particles. The SEM image of the fifth-use bur (Figure 7) shows heavy debris accumulation on the bur surface, together with greater wear and faceting of the diamond particles.
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DISCUSSION
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All restorations in our study appeared to leak, but the data reported here indicate that there are differences between teeth with freshly cut cavity preparations and those in which the restoration has been removed, the preparation has been refined and conditioned, and a new restoration has been placed. Differences also arose with regard to the amount of leakage and the leakage current–versus-time behavior when successive teeth were cut with the same disposable diamond bur.
The onset of leakage was slower with freshly prepared teeth, and the least leakage current occurred with preparations cut with the first and third use of the bur. Further use of the bur (that is, when a fifth preparation was cut with the same bur) resulted in higher restoration leakage. This may be the result of greater smearing of the cavity preparation surface with prolonged use, together with some redeposition of cutting debris onto the surface from the repeatedly used bur. As a result, the cavity preparation has a more friable and less adherent surface, which, after conditioning and restoration, may result in detachment of the restoration from the dentin substrate, leading to leakage.
The results of our study show that removing the restorations and refining the cavity preparations (group 2 specimens) resulted in immediate onset of leakage and higher leakage currents than those exhibited by freshly cut preparations. In the group 2 teeth, we found no statistically significant difference in leakage current for the specimens cut with the first- or fifth-use burs or for the specimens cut with the first- or third-use burs. However, we found a significant difference (P < .01) between the leakage currents for specimens cut with the third-or fifth-use burs. This difference in leakage current may be statistically significant, but we doubt that it would be observable in clinical practice, with the possible exception of a greater degree of marginal staining. As might be anticipated, the leakage behavior with both sets of fifth-use burs (that is, new cavity preparations and refined cavity preparations) was similar (P > .05).
The reason for the greater leakage found with the second group of specimens is not clear, but it may be due to smearing of the dentin surface, which is somewhat friable and may be a poor substrate for subsequent bonding procedures. These findings suggest that after a restoration is removed from a cavity, the clinician should extend the preparation to expose fresh enamel, dentin and cementum to ensure an optimal tooth surface for subsequent restoration.
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CONCLUSION
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The results of our study indicate that disposable diamond burs should be used for limited cutting procedures, with no more than three cavity preparations being cut with a single bur. The data also suggest that when a restoration needs to be removed and replaced, the clinician should extend the cavity preparation to ensure the availability of fresh hard tissue for subsequent conditioning and restoration.
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FOOTNOTES
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Dr. von Fraunhofer is a professor and director of biomaterials research, Department of Oral and Maxillofacial Surgery, University of Maryland School of Dentistry, 666 W. Baltimore St., Baltimore, Md. 21201, e-mail "AVF001{at}dental.umaryland.edu". Address reprint requests to Dr. von Fraunhofer.
Mr. Smith is a senior dental student, Department of Restorative Dentistry, University of Maryland School of Dentistry, Baltimore.
Mr. Marshall is a senior dental student, Department of Restorative Dentistry, University of Maryland School of Dentistry, Baltimore.
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ABSTRACT
MATERIALS AND METHODS
