Methods. The authors performed cutting studies on a machinable ceramic block using an established testing regimen. One-port, three-port and four-port handpieces from one manufacturer were operated at maximum torque and rotation speed under a water flow of 25 milliliters per minute. The authors made 6-millimeter long edge and groove cuts in 13-mm cross-section blocks using six medium-grit diamond burs for each handpiece. Each bur cut a total of 78 mm. The authors determined CR as the time to transect the block and analyzed the data by two-way analysis of variance with post hoc Scheffé tests.

Results. CRs varied by the type of cut and the number of spray ports. No differences were found in CRs for the three handpieces during edge cutting. The one-port handpiece cut significantly slower (P < .001) than did the three- and four-port handpieces during groove cutting.

Conclusion. The data indicate that the number of handpiece spray ports, and their positioning relative to the bur affect water supply to the cutting interface and, consequently, the CR under these study conditions.

Clinical Implications. Optimal cutting efficiency requires good coolant access, especially within restricted areas. A multiple-port handpiece may be advantageous when preparing the interproximal region for a crown or a proximal box, owing to the better water spray pattern. Dentists should consider the influence of the number of spray ports when selecting handpieces for cutting procedures.

Protecting the health and vitality of pulpal tissues through cooling the bur/tooth interface with water during a cutting procedure has been established for decades.^8–12 Although a survey conducted in 2000 showed that many dental schools recognize the importance of water cooling, most do not make specific flow-rate recommendations.¹³ Using higher coolant flow rates to enhance thermal protection of the pulp is recognized in Europe,^14,15 and another study conducted in 2000 demonstrated clearly that faster cutting rates, or CRs, were found with higher flow rates.¹⁶

High-speed handpieces have one or more spray ports that direct a stream or mist of coolant at the bur/tooth interface to provide cooling and debris removal during cutting. Dental handpieces differ in their design and construction; one obvious difference is the number and positioning of coolant spray ports. Traditionally, high-speed handpieces had only one spray port, located at the 6 o’clock position (Figure 1). Manufacturers, however, have developed high-speed handpieces with multiple spray ports, notably with three spray ports in the 2, 6 and 10 o’clock positions and four spray ports in the 3, 6, 9 and 12 o’clock positions. There is little information on the optimum number or positioning of spray ports or the effect of the design on cutting efficiency. The literature also does not indicate whether differences exist in the cooling efficiency of different handpiece designs. One handpiece manufacturer indicated that three- and four-port heads were designed to ensure that there still was sufficient coolant spray for pulpal safety if one or more ports become obstructed (William Sleinetz, B.S., KaVo America Corp., Lake Zurich, Ill., personal communication, June 30, 2000).

View larger version (44K): [in this window] [in a new window]   Figure 1. High-speed handpiece heads.

We conducted this current study to evaluate the effect of the number of spray ports and their positions on diamond bur CRs. This is an important consideration for practitioners when they are selecting new handpieces, as well as for dentists who routinely perform a variety of complex cutting procedures within restricted areas.

	METHODS AND MATERIALS

TOP ABSTRACT METHODS AND MATERIALS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
We conducted the cutting studies employing a previously established testing regimen^18,19 for diamond burs using three high-speed handpieces manufactured by KaVo America Corp.: the one-port 635B, the three-port 647B and the four-port 649B. We selected these handpieces because KaVo America Corp. produced one-, three- and four-port head handpieces that all had an automatic air pressure regulation valve that ensured a constant bur speed. The one-port handpiece was designed for pediatric use and had a smaller head size than did the three- and four-port head handpieces, but all three handpieces had similar rotation speeds, output power and torques (Table), which allowed us to compare the three hand-pieces’ CRs.

View larger version (22K): [in this window] [in a new window]   Figure 2. Test cutting assembly.

We selected this load since the literature indicates that the average applied load during restorative procedures is in the range of 50 to 150 g.^17–20 We performed all of the cutting studies with the handpieces at maximum output power, torque and bur rotation speed. Before each set of cutting studies, we measured the coolant (tap water) flow rate and ensured that it was it was 25 milliliters per minute. During cutting, we held the bur parallel to a machinable ceramic (Macor, Corning Inc., Corning, N.Y.) bar and pulled perpendicularly down onto it, simulating clinical practice.

We used Macor, which is 55 percent fluorophlogopite mica and 45 percent borosilicate glass, because its hardness (Knoop hardness number of 250), elastic modulus (66.9 gigapascals) and thermal properties are comparable with those of dental enamel.^17–19 We used 13-mm cross-section rectangular Macor bars and medium-grit 856-016 diamond burs (Brasseler USA, Savannah, Ga.). We used a 6-mm length of each bur to section through the 3-mm-thick Macor bars. We made six cuts with six different burs in each handpiece, for a total cutting distance of 78 mm/bur.

We performed two sets of cutting studies on the Macor bars. In the first series, we positioned the burs one bur diameter from the edge of the bar to make edge cuts (Figure 3). This cutting method simulated tooth preparation for crowns—for example, axial wall or occlusal reduction. In the second series, we positioned the burs at least one to two bur diameters from the edge of the bar to make a groove cut (Figure 3). This cutting method simulated interproximal cutting for crowns, as well as for operative box and groove preparations.

View larger version (49K): [in this window] [in a new window]   Figure 3. Cross-section of groove and edge cutting.

	RESULTS

TOP ABSTRACT METHODS AND MATERIALS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
The cutting data are summarized in Figure 4 and Figure 5. We found no significant differences (P > .05) between the CRs for the three-port and four-port heads when they were used for edge or groove cutting. The CRs for groove cuts made with the one-port handpieces, however, were significantly lower (P < .01) than those for the three- and four-port handpieces. Further, the CRs for groove cuts made with the one-port handpiece were significantly lower (P < .01) than those for the edge cuts. We found no differences (P > .05) in CRs among the three handpieces during edge cutting.

View larger version (40K): [in this window] [in a new window]   Figure 4. Histogram of mean edge cutting rate (using three handpiece spray nozzle designs).

View larger version (39K): [in this window] [in a new window]   Figure 5. Histogram of mean groove cutting rate (using three handpiece spray nozzle designs).

	DISCUSSION

TOP ABSTRACT METHODS AND MATERIALS RESULTS DISCUSSION CONCLUSIONS REFERENCES

Since it has been shown that the CRs for teeth and other substrates are markedly affected by coolant flow rates,^14–16 clinicians seek to provide maximal coolant flow rates at operative sites. The findings from our current study indicate that CRs are determined by the number of spray ports on the handpiece and by the cutting location. We found faster CRs with edge cutting compared with groove cutting when we used a one-port handpiece, but we found no difference in CRs between cutting methods for the three-port and four-port handpieces. Although the CRs for the four-port handpiece were greater than those for the three- and one-port handpieces, the difference was not significant except for with the one-port handpiece during groove cutting.

The differences in CRs during edge and groove cutting with a one-port handpiece arise from the restricted access of the water spray during the latter cutting method. During edge cutting, access of the coolant spray at the cutting interface still is important, but it appears to be less significant than during groove cutting. It follows that during groove cutting (for example, in interproximal axial wall preparation) a multiport handpiece offers a clear advantage in terms of cutting efficiency. Poor coolant access within shielded regions deleteriously affects cutting efficiency and, possibly, may result in elevated temperatures during cutting. Further studies are necessary to determine whether cutting facilitated by multiport heads is paralleled by greater control of heat generated during cutting.

	CONCLUSIONS

TOP ABSTRACT METHODS AND MATERIALS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Previous studies have shown that the amount of coolant spray water delivered at the bur/cut surface interface markedly affects the observed CR.^14–16 In the present study, we identified the number and positioning of spray ports in a high-speed handpiece as additional factors influencing CRs. The findings showed that the number of spray ports determines the access of coolant at the cutting interface, which, in turn, affects the CR under the conditions of this study.

The findings also pointed out that multiport heads are indicated for groove cutting in clinical situations such as in restricted access areas, while the number and positioning of spray ports do not appear to affect the cutting efficiency for other types of dental cutting. Dental practitioners, therefore, should take the handpiece design into consideration when undertaking dental preparation in restricted areas, such as the interproximal region or within grooves during crown preparation or proximal box preparation. Poor coolant access within these regions will negatively affect cutting efficiency, though the effect on heat generation is not known at this time. It follows that handpiece design and coolant spray patterns should be an important factor during equipment purchase.

View larger version (105K): [in this window] [in a new window]   Dr. Siegel is an associate professor, Department of Restorative Dentistry, School of Dentistry, University of Maryland, 666 W. Baltimore St., Baltimore, Md. 21201, e-mail "SCS001{at}Dental.umaryland.edu". Address reprint requests to Dr. Siegel.

View larger version (95K): [in this window] [in a new window]   Dr. von Fraunhofer is a professor and the director, Biomaterials Science, Department of Restorative Dentistry, School of Dentistry, University of Maryland, Baltimore.