- Email: [email protected]
Lubricating coolants for high-speed dental handpieces
JA B A RESEARCH
REPORTS
C utting studies were performed on tooth enamel, dental amalgam, and composite resin through use o f carbide burs and diam ond stones w ith three irrigants. A m ixture o f water, alcohol, and glycerol significantly increased the rate o f material removal when enamel was cut w ith diam onds and when dental am algam was cut with diam onds and carbide burs. In contrast, the rate o f material removal fo r composite resin was significantly faster w ith use o f carbide burs than diam ond stones, and the irrigant improved the cutting action for carbide burs. The studies indicate that chemomechanical effects operate with dental substrates, but the effect varies w ith the material being cut and the cutting tool used. The chemomechanical effects observed here significantly improve the rate o f material removal w ith diam ond stones and carbide burs.
Lubricating coolants for high-speed dental handpieces J. A. von Fraunhofer, MSc, PhD C. D. Givens, DMD T. J. Overmyer, DMD
igh-speed handpieces and burs are essential to modern restor ative dentistry. Water supplied by a h ig h -p re ssu re delivery system th ro u g h the h andpiece to the c u ttin g in stru m e n t is the accepted irrig a tio n m edium for clinical procedures. Water cooling prevents thermal damage to the p u lp , rem oves debris d u rin g c u ttin g , and minimizes clogging of the cutting instrument. If the cutting blades are kept clean and unclogged, m aterial w ill be rem oved by c h ip p in g ra th e r th a n by abrasive wear, w hich enhances cu ttin g efficiency. There is no evidence that water acts as a lubricating agent and it is not known to act as a lubricant under any circumstance. Heat generated during any cutting procedure is caused by friction between the c u ttin g in s tru m e n t an d the w ork surface.1 An irrigating solution that can also function as a lubricant and reduce friction between the cutting tool and
H
work surface will reduce heat. In effect, the use of lubricating coolants for dental h an d p ieces sh o u ld reduce the risk of thermal damage to teeth. Because friction and instrum ent wear are primary causes of reduced cutting efficiency and short ened tool life, a lu b ric a tin g irrig a n t should p rolong the life of dental burs by red u cin g frictio n betw een the bur and work surface. Reduced bur wear and suppressed thermal effects perm it faster cutting rates for longer periods without risk of d am age to to o th stru c tu re or restorative material. Recently, it was found that the addition of a p roprietary oral rinse co n taining glycerol and alcohol to the water supply of a high-speed handpiece produced a sm o o th e r surface fin is h th a n w a ter.2 C lin ical im pressions indicate the rate of enamel removal during normal restor ative procedures is greatly enhanced.2 T h e current study shows the cutting rate of enamel improved with glycerol and
alcohol additions to the w ater supply of the h andpiece. T h e effect of these irrig a tin g solu tio n s on the c u ttin g of dental amalgam and composite restorative materials in vitro was also determined. Methods and materials C utting of enamel and restorative mate rials was performed with three irrigating solutions. Solution A (test solution) was a mixture of glycerol, ethanol, and water (2:1:2), s o lu tio n B was a w ater-based oral rinse (Scope, Procter and Gamble, 10:1), and water was used as the control cutting medium. Solutions A and B were m ixed w ith the w ater su p p ly of the handpiece as described.3,4 T h e supply of irrigating solution from the handpiece approxim ated clinical norms at 15 mL per minute. Cutting efficiency of enamel enhanced by the a d d itio n of oral rin se an d the test solution to the water supply was JA DA, Vol. 119, A ugust 1989 a 291
RESEARCH
REPORTS
resin were used as the cutting surfaces. Cylindrical specimens, 4 mm in diameter, were prepared from a unicompositional high-copper dental amalgam (Contour, Kerr USA) in accordance with American Dental Association specification no. I.5 A tria l study in d ica ted th a t 4-mm diam eter specimens of com posite resin were too thick to cut w ithout clogging the d ental burs. C onsequently, hem icylindrical specimens of anterior filling
assessed w ith the use of two freshly extracted hum an molars. T he teeth were mounted individually on a flat car with fre e -ru n n in g w heels p u lle d by a 70-g w eight under a ro ta tin g b u t unloaded h ig h -sp eed d ia m o n d stone. T h e tim e required for a 4-mm horizontal cut to be produced was determined. Because it was d iffic u lt to achieve re p ro d u c ib ility in c u ttin g studies on enamel, dental am algam and a composite
Table 1 ■ Cutting times. Tooth enamel (sec) Tooth A T o o th B Composite resin (ms) Bur no. 56 556 Coarse diam ond Fine diam ond Dental amalgam (ms) Bur no. 56 556 Coarse diam ond Fine diam ond
Solution A
Solution B
Water
R atiof
42.5 28
56 45
134 82
0.32 0.34
Solution A
Solution B
Water
R atiof
70 ± 27* 47 ± 2 1 327 ± 47
150 ± 20 73 ± 6 383 ± 35
153 ± 15 77 ± 6 353 ± 15
0.46 0.61 0.93
410 ± 53
473 ± 35
537 ± 29
0.76
Solution A
Solution B
Water
463 ± 15 287 ± 21 420 ± 17
607 ± 32 480 ± 40 600 ± 17
853 ± 15 670 ± 160 673 ± 45
0.54 0.43 0.62
533 ± 15
753 ± 29
833 ± 60
0.64
* M ean a n d sta n d a rd deviation. t R a tio of c u ttin g tim es u n d e r so lu tio n A a n d w ater irrig a tio n .
R atiof
material (Concise, 3-M Co) were prepared w ith a 4-mm diam eter steel m old cut along its midsection to provide a hem i spherical cross-section. T h e resto rativ e m a teria l specim ens were held in a stand and clamp and cut p e rp e n d ic u la r to their long axis by a high-speed handpiece mounted on a free ly rotating arm with 147-g load attached to the head of the handpiece (Fig 1). A lthough the handpiece was m ounted on a 360° rotational arm, with the radius of curvature being the distance from the tubing attachment to the bur, the work surfaces were p o sitio n e d so th a t the cutting arc was limited to 2° of the circle described by the head of the handpiece. T h u s, any significant intro d u ctio n of weight caused by downward movement of the head d u rin g c u ttin g was e lim in ated , an d a c o n sta n t c u ttin g load comparable to that exerted by experienced clinicians in practice, as m easured in a trial study, was provided. Four different types of dental burs were used to cut the specimens, namely a no. 56 straig h t fissure, a no. 556 crosscut fissure, a no. 767.9C coarse diam ond, and a no. 255.8F fine diam ond. Fresh burs were used for each cut, and after each cut, the head was repositioned along the long axis of the specimen. All cutting studies were perform ed at least three times. The burs were irrigated by a highpressure delivery system th ro u g h the handpiece with the irrigating solutions contained in a vacuum canister.3,4 The
Fig 1,2 ■ Fig 1, left, schematic dia gram of handpiece mounting for lab oratory cutting studies of compos ite resin and dental am algam . Fig 2, lower right, spec im ens bein g cut perpendicular to their long axis.
292 ■ JA DA, Vol. 119, A ugust 1989
RESEARCH
time required for the burs to cut com pletely through the am algam or com posite specimens was measured to 0.01 seconds with an electronic stopwatch. C utting times were com pared by one factor analysis of variance, and individual differences were identified by the S tu dent’s i-test.
Seconds 140
120 100Fig 3 ■ Cutting times for extracted molars with diamond stones.
80
Results T he time to produce a 4-mm horizontal cut in two molars using the three irrigants is show n in T a b le 1 a n d F ig u re 3. S olutions A an d B (test so lu tio n and Scope oral rin se m ix tu re ) achieved significantly lower cu ttin g times (P < .01 and P < .001) for both teeth compared with water, although differences in the c u ttin g rates existed betw een the two teeth. The ratio of cutting times under w ater and the test so lu tio n , however, were virtually identical for the two teeth (Table 1). The cutting times for composite resin and dental am algam w ith the four dental burs and three irrigant solutions are given in T able 1 an d Figures 4 and 5. T h e ratios of cu ttin g tim es under solution A and water are also presented in Table 1. For composite resin, increased cutting rates were noted w ith the test solution (A), for both the no. 56 and no. 556 burs, com pared w ith so lu tio n B an d w ater (P < .001), but there were no differences (P > .05) in c u ttin g tim es betw een solution B and water. Statistically sig nificant decreases in cutting time (P < .05) were noted for the fine diam o n d stone when used w ith solutions A and B compared with water, but there were no differences in the cutting rates achieved w ith the two test irrig a n ts (P > .05). It should be noted that the differences in cutting rates for the three solutions were statistically significant for the fine d iam ond stone b u t did n o t ap p ro a c h the nearly 50% decrease in cutting times found for the no. 56 and no. 556 burs. No differences in cutting rates were found for the coarse diam ond stone w ith any of the irrigants (P > .05). The cutting speed of dental amalgam was improved by solution A (P < .001) w ith a ll fo u r b u rs, com p ared w ith solution B and water. T he cutting times for solution A were about half as long as those for w ater irrig a tio n w ith all four burs. S o lu tio n B h a d increased cutting speeds com pared w ith water as an irrigant for the no. 56 bur (P < .001), the no. 556 bur (P < .05), and the fine diamond stone (P < .05), but the decrease
REPORTS
60 H 40
20 0 Tooth A I
Tooth B
I Test solution
I
I Scope
I
I Water
600 600 400
Fig 4 m Cutting times for composite resin.
300
200 100
No. 56 bur
No. 556 bur
Diamond (C)
Diamond (F)
Cutting instrument I
1Test solution
I
I Scope
I
I Water
1000 Y 800 600 -
Fig 5 ■ Cutting times for dental amalgam.
400-
200¿7
0 No. 56 bur
No. 556 bur
Diamond (C)
Diamond (F)
Cutting instrument 1
I Test solution
I
I Scope
was far less than that found with solution A. No statistically significant difference in c u ttin g tim es was fo u n d between s o lu tio n B and w ater w ith the coarse diam ond stone. T he cutting times of dental amalgam w ith the four burs under the three types of irrigant were comparable. In contrast, the diam ond stones cut composite resin far more slowly than did the fissure burs under all irrigants.
!
I Water
Discussion An earlier study2 revealed an improved surface fin ish of dental enam el w hen an oral rinse was added to the irrigant of a d e n tal h an d p iece. H ere, stu d ies dem onstrate that an irrigant composed of water, alcohol, and glycerol increases the rate of material removal for enamel, dental am algam , and com posite resin. Irrigation with solution A—the 40:20:40
Von F raunhofer-G ivens-O verm yer : L U B R IC A T IN G C O O L A N T S F O R H A N D P IE C E S ■ 293
RESEARCH
REPORTS
mixture of glycerol, ethanol, and water— enhanced cutting rates more effectively than solution B (the oral rinse irrigant) of the earlier study.2 The decrease in cutting time with solution A compared with water was greater for the carbide burs than the diamond stones on dental amalgam and composite resin. The increased cutting rate by carbide burs and diamond stones of enamel and dental amalgam under solution A, compared with cutting rates for water and solution B, was paralleled by the increased cutting rate by carbide burs of composite resin under solution A. In contrast, the relative increase in the cutting rate of composite resin with the fine diamond stone was small, and there was no difference in the cutting rates with the coarse diamond stone (Table 1). The accelerated cutting rate achieved with an irrigant composed of water, ethanol, and glycerol could be caused by a lubricating action of the additives, but is unlikely because the reduction in surface tension by the additives is slight; although ethanol has little effect on the viscosity of water, glycerol additions significantly increase viscosity. Thus, improved cutting efficiency likely results from interactions between the irrigant and the work surface, and possibly interactions between the irrigant and the cutting tool. The cutting rates of dental amalgam with the four burs under the three irrigants were comparable, but the diamond stones cut composite resin far more slowly than did the carbide fissure burs under all three irrigants. These differences in cutting rates of dental amalgam and composite resin with the carbide burs and diamond stones suggest that cutting mechanisms differ for the two classes of cutting instruments when used on filled polymer matrix (composite resin) surfaces. Interactive effects, whereby adsorbed species affect the flow and fracture behavior of solids, known as chemomechanical effects (CMEs), were discovered more than 40 years ago. The best known of these is the Rehbinder effect,6"8 named after its discoverer who found the hard ness of solids was influenced by absorbed surface-active species, and that the plastic flow of metals in nonpolar hydrocarbons was substantially facilitated in the presence of a surfactant. It was also noted that the hardness of metals in aqueous electrolytes passed through a maximum at the point of zero charge with a 294 ■ JADA, Vol. 119, August 1989
corresponding minimum in the creep rate at the point of zero charge. Rehbinder effects (adsorption-induced changes in the cohesive strength of interatomic bonds by surface-active species) are thought to be caused by a reduction in surface free energy. Chemically or adsorption-induced changes in the flow, fracture behavior, and hardness characteristics have also been observed with nonmetallic solids. Rehbinder and coworkers7’8 reported increased rates of rock drilling in aqueous solutions of various salts and surfactants. These latter effects are a subset of Rehbinder effects, can occur in envir onments that do not significantly affect surface free energy, and their magnitude depend on a number of factors. Hardness, for example, is maximized when the surface potential of the solid (the £ potential) is at or close to zero, an effect known as the £ correlation, which applies to crystalline and noncrystalline inor ganic nonmetals. Hardness changes of up to 30% that persist to depths of 10 jum have been observed.7,8 The underlying theory of chemomechanical effects is still not completely understood. However, they appear to arise from either adsorption-induced changes in the near surface electrical field or from charge transfer between the adsorbate and the internal structure of the solid in near surface regions. It follows that the improved rate of material removal in the studies reported here results from chemomechanical interactions between the irrigant media and the substrates, assuming material removal with carbide burs and diamond stones is by drilling (that is, chipping) rather than by abrasion. However, the mechanism of cutting differs for the two types of tools because of the different near-surface flow processes during chip formation. Carbide bit penetration is greatest for media that minimize hardness, an inverse £ correlation. As the carbide bit advances, plastic deformation results in the flow zone immediately ahead of the cutting edge (comparable to the effect occurring in metal cutting). Material in the flow zone cannot escape around the edge of the bit so that strain occurs, dislocations accumulate, and cracks nucleate, which grow rapidly and propagate and interact to form chips. This process involves the movement of dislocations in the work surface and is environment-sensitive. Media that maximize dislocation mobility (and minimize hardness) will promote
increased drilling rates. Thus for carbide drills, the greatest chemomechanical effect is obtained when the medium enhances dislocation mobility because dislocation motion is the first step in chip formation.8 For diamond drill bits, the drilling rate is greatest at maximum work surface hardness; for example, the drilling rate for glass is 20 times greater in heptyl alcohol than in water.8 Each irregularly shaped diamond on a diamond drill bit functions as a cutting tool with a large negative rake angle traveling in grooves. Dislocation movement adjacent to the tool causes an outward radial flow of material towards the groove edges, which limits the amount of plastic strain. However, passage of the tool generates high-tensile stresses in the near-surface region, and causes cracks to form that coalesce and interact to form chips. Any medium that reduces dislocation mobility, increases hardness, and has a positive ^-correlation (greater pene tration rates being achieved at £ potentials at or close to zero) will accelerate the drilling rate. In other words, material removal depends on localized brittle fracture of the work surface. The mechanism of material removal differs in grinding with diamond (and presumably other) abrasives; there is a dependence on both viscosity and £ potential, whereas in drilling there is no dependence on viscosity. In grinding, material removal involves flow processes with the high-shear force between abra sive particles and the work surface, resulting in plastic deformation and subsequent brittle fractures adjacent to the furrows. Because the least amount of near-surface plastic flow occurs at the point of zero charge, the rate of material removal will be lowest at £ potential values at or close to zero. Theoretical considerations of che momechanical effects suggest that enhanced cutting rates of dental amalgam for carbide burs and diamond stones arise from Rehbinder effects; that is, the irrigant reduces the free surface energy, and material removal is facilitated, which accounts for the comparable improvement in cutting rates for burs and stones with this work surface. Cutting effects observed with the composite resin indicate that a chemome chanical effect occurs with this material. The improved cutting action of carbide burs suggests that solution A (water, alcohol, glycerol) maximizes dislocation mobility and minimizes hardness in the composite filler; that is, a negative £
RESEARCH
correlation. Because o p tim u m cu ttin g w ith d ia m o n d stones occurs w ith a positive £ co rrelatio n (m in im al d islo cation mobility and m aximum hardness), little change w ill occur in the cutting action w ith d iam ond stones com pared with water for composite materials. T ooth studies indicate that solutions A an d B im p ro v e the c u ttin g ra te of enamel, compared with water, when used with a diam ond stone. Again, improved rates of material removal may be ascribed to a chem om echanical effect th a t has a po sitiv e £ c o rre la tio n , because the cutting action of diam ond was enhanced. Therefore, the irrigants appear to reduce dislocation m obility and increase hard ness. Because the test so lu tio n (water, alcohol, glycerol) was more effective than the rinse when used on enamel, findings here suggest that other irrigants m ight enhance the cutting action of diam ond stones even more. Conclusions
T h is study in d icates th a t an irrig a n t composed of water, alcohol, and glycerol improves the cutting action of diam ond stones on dental enamel and the action of carbide burs on com posite resin. In co n trast, th is irrig a n t fa c ilita te d the cutting of dental amalgam with carbide burs and diam ond stones. Findings indicate that different che momechanical effects occur, depending
on the substrate being cut. In the case of dental am algam , a Rehbinder effect occurs; the irrigant reduces the surface free energy an d cohesive stre n g th of in te ra to m ic bonds, w h ich facilitates c u ttin g . T h e irrig a n t also exerts a chemomechanical effect on dental enamel and, based o n the c u ttin g ac tio n of d iam onds, ap pears to reduce the dis location mobility, increase hardness, and thereby facilitate m aterial rem oval by diam ond stones. In contrast, the chemomechanical effect on composite resin appears to result from an increase in dislocation mobility and reduced hardness, w hich facilitates cut ting by carbide burs. While the detailed mechanism of chemomechanical effects is uncertain, it appears that these con siderations could profoundly affect cur re n t th o u g h t on to o th an d m aterial c u ttin g procedures. C areful selection of irrigants m ight facilitate the rate of m aterial rem oval and im prove surface finish of restorative materials and teeth by optim izing the cutting action of burs and stones.
------------- J ' AO A -------------
Inform ation about the manufacturers of the products mentioned in this article may be available from the authors. Neither the authors nor the American Dental Association has any commercial interest in the products mentioned.
REPORTS
Dr. von Fraunhofer is professor of biomaterials science, department of primary patient care, School of Dentistry, Health Sciences Center, University of Louisville, L ouisville, 40292. Dr. Givens is a general practitioner in Nashville, TN. Dr. Overmyer is a general practitioner in Danville, KY. Address requests for reprints to Dr. von Fraunhofer.
1. Eirich FR. The role of friction and abrasion in the d rillin g of teeth. In: T h e cu ttin g edge (Interfacial dynam ics of cu ttin g and grinding). W ashington, DC: US Department of H ealth, Education, and Welfare, 1976; DHEW publication no. 76-670. 2. von Fraunhofer JA; Overmyer TJ; and Johnson AA. Improved cutting of tooth enamel with dental burs. Quintessence Int 1987;18:383-5. 3. Overmyer TJ. Liquid admixing apparatus for dental water injection systems. US Patent no. 4,668,190, 1986. 4. Overmyer TJ. Method of cooling and lubricating human hard tissue during power tool cutting. US Patent no. 4,695,255, 1987. 5. American Dental Association. Revised ADA specification no. 1 for alloy for dental amalgam. JADA 1977;95:1-4. 6. Johnson AA; Johnson DN; and von Fraunhofer JA. Milestones in the history of physical metallurgy: Rehbinder effect. In: Guide to materials engineering data and inform ation. Metals Park, OH: ASM International: 1986. 7. W estwood ARC, Ahearn JS. Adsorptionsensitive flow and fracture of solids. In: Physical chemistry of the solid state: applications to metals and their compounds. Amsterdam: Elsevier Science, 1984. 8. Westwood ARC. Chemical effects in the cutting of nonm etals. In: The cu ttin g edge (Interfacial dynamics of cutting and grinding). Washington, DC: US Department of H ealth, Education, and Welfare, 1976; DHEW publication no. 76-670.
Von Fraunhofer-Givens-Overmyer : LUBRICATING COOLANTS FOR HANDPIECES ■ 295
REPORTS
C utting studies were performed on tooth enamel, dental amalgam, and composite resin through use o f carbide burs and diam ond stones w ith three irrigants. A m ixture o f water, alcohol, and glycerol significantly increased the rate o f material removal when enamel was cut w ith diam onds and when dental am algam was cut with diam onds and carbide burs. In contrast, the rate o f material removal fo r composite resin was significantly faster w ith use o f carbide burs than diam ond stones, and the irrigant improved the cutting action for carbide burs. The studies indicate that chemomechanical effects operate with dental substrates, but the effect varies w ith the material being cut and the cutting tool used. The chemomechanical effects observed here significantly improve the rate o f material removal w ith diam ond stones and carbide burs.
Lubricating coolants for high-speed dental handpieces J. A. von Fraunhofer, MSc, PhD C. D. Givens, DMD T. J. Overmyer, DMD
igh-speed handpieces and burs are essential to modern restor ative dentistry. Water supplied by a h ig h -p re ssu re delivery system th ro u g h the h andpiece to the c u ttin g in stru m e n t is the accepted irrig a tio n m edium for clinical procedures. Water cooling prevents thermal damage to the p u lp , rem oves debris d u rin g c u ttin g , and minimizes clogging of the cutting instrument. If the cutting blades are kept clean and unclogged, m aterial w ill be rem oved by c h ip p in g ra th e r th a n by abrasive wear, w hich enhances cu ttin g efficiency. There is no evidence that water acts as a lubricating agent and it is not known to act as a lubricant under any circumstance. Heat generated during any cutting procedure is caused by friction between the c u ttin g in s tru m e n t an d the w ork surface.1 An irrigating solution that can also function as a lubricant and reduce friction between the cutting tool and
H
work surface will reduce heat. In effect, the use of lubricating coolants for dental h an d p ieces sh o u ld reduce the risk of thermal damage to teeth. Because friction and instrum ent wear are primary causes of reduced cutting efficiency and short ened tool life, a lu b ric a tin g irrig a n t should p rolong the life of dental burs by red u cin g frictio n betw een the bur and work surface. Reduced bur wear and suppressed thermal effects perm it faster cutting rates for longer periods without risk of d am age to to o th stru c tu re or restorative material. Recently, it was found that the addition of a p roprietary oral rinse co n taining glycerol and alcohol to the water supply of a high-speed handpiece produced a sm o o th e r surface fin is h th a n w a ter.2 C lin ical im pressions indicate the rate of enamel removal during normal restor ative procedures is greatly enhanced.2 T h e current study shows the cutting rate of enamel improved with glycerol and
alcohol additions to the w ater supply of the h andpiece. T h e effect of these irrig a tin g solu tio n s on the c u ttin g of dental amalgam and composite restorative materials in vitro was also determined. Methods and materials C utting of enamel and restorative mate rials was performed with three irrigating solutions. Solution A (test solution) was a mixture of glycerol, ethanol, and water (2:1:2), s o lu tio n B was a w ater-based oral rinse (Scope, Procter and Gamble, 10:1), and water was used as the control cutting medium. Solutions A and B were m ixed w ith the w ater su p p ly of the handpiece as described.3,4 T h e supply of irrigating solution from the handpiece approxim ated clinical norms at 15 mL per minute. Cutting efficiency of enamel enhanced by the a d d itio n of oral rin se an d the test solution to the water supply was JA DA, Vol. 119, A ugust 1989 a 291
RESEARCH
REPORTS
resin were used as the cutting surfaces. Cylindrical specimens, 4 mm in diameter, were prepared from a unicompositional high-copper dental amalgam (Contour, Kerr USA) in accordance with American Dental Association specification no. I.5 A tria l study in d ica ted th a t 4-mm diam eter specimens of com posite resin were too thick to cut w ithout clogging the d ental burs. C onsequently, hem icylindrical specimens of anterior filling
assessed w ith the use of two freshly extracted hum an molars. T he teeth were mounted individually on a flat car with fre e -ru n n in g w heels p u lle d by a 70-g w eight under a ro ta tin g b u t unloaded h ig h -sp eed d ia m o n d stone. T h e tim e required for a 4-mm horizontal cut to be produced was determined. Because it was d iffic u lt to achieve re p ro d u c ib ility in c u ttin g studies on enamel, dental am algam and a composite
Table 1 ■ Cutting times. Tooth enamel (sec) Tooth A T o o th B Composite resin (ms) Bur no. 56 556 Coarse diam ond Fine diam ond Dental amalgam (ms) Bur no. 56 556 Coarse diam ond Fine diam ond
Solution A
Solution B
Water
R atiof
42.5 28
56 45
134 82
0.32 0.34
Solution A
Solution B
Water
R atiof
70 ± 27* 47 ± 2 1 327 ± 47
150 ± 20 73 ± 6 383 ± 35
153 ± 15 77 ± 6 353 ± 15
0.46 0.61 0.93
410 ± 53
473 ± 35
537 ± 29
0.76
Solution A
Solution B
Water
463 ± 15 287 ± 21 420 ± 17
607 ± 32 480 ± 40 600 ± 17
853 ± 15 670 ± 160 673 ± 45
0.54 0.43 0.62
533 ± 15
753 ± 29
833 ± 60
0.64
* M ean a n d sta n d a rd deviation. t R a tio of c u ttin g tim es u n d e r so lu tio n A a n d w ater irrig a tio n .
R atiof
material (Concise, 3-M Co) were prepared w ith a 4-mm diam eter steel m old cut along its midsection to provide a hem i spherical cross-section. T h e resto rativ e m a teria l specim ens were held in a stand and clamp and cut p e rp e n d ic u la r to their long axis by a high-speed handpiece mounted on a free ly rotating arm with 147-g load attached to the head of the handpiece (Fig 1). A lthough the handpiece was m ounted on a 360° rotational arm, with the radius of curvature being the distance from the tubing attachment to the bur, the work surfaces were p o sitio n e d so th a t the cutting arc was limited to 2° of the circle described by the head of the handpiece. T h u s, any significant intro d u ctio n of weight caused by downward movement of the head d u rin g c u ttin g was e lim in ated , an d a c o n sta n t c u ttin g load comparable to that exerted by experienced clinicians in practice, as m easured in a trial study, was provided. Four different types of dental burs were used to cut the specimens, namely a no. 56 straig h t fissure, a no. 556 crosscut fissure, a no. 767.9C coarse diam ond, and a no. 255.8F fine diam ond. Fresh burs were used for each cut, and after each cut, the head was repositioned along the long axis of the specimen. All cutting studies were perform ed at least three times. The burs were irrigated by a highpressure delivery system th ro u g h the handpiece with the irrigating solutions contained in a vacuum canister.3,4 The
Fig 1,2 ■ Fig 1, left, schematic dia gram of handpiece mounting for lab oratory cutting studies of compos ite resin and dental am algam . Fig 2, lower right, spec im ens bein g cut perpendicular to their long axis.
292 ■ JA DA, Vol. 119, A ugust 1989
RESEARCH
time required for the burs to cut com pletely through the am algam or com posite specimens was measured to 0.01 seconds with an electronic stopwatch. C utting times were com pared by one factor analysis of variance, and individual differences were identified by the S tu dent’s i-test.
Seconds 140
120 100Fig 3 ■ Cutting times for extracted molars with diamond stones.
80
Results T he time to produce a 4-mm horizontal cut in two molars using the three irrigants is show n in T a b le 1 a n d F ig u re 3. S olutions A an d B (test so lu tio n and Scope oral rin se m ix tu re ) achieved significantly lower cu ttin g times (P < .01 and P < .001) for both teeth compared with water, although differences in the c u ttin g rates existed betw een the two teeth. The ratio of cutting times under w ater and the test so lu tio n , however, were virtually identical for the two teeth (Table 1). The cutting times for composite resin and dental am algam w ith the four dental burs and three irrigant solutions are given in T able 1 an d Figures 4 and 5. T h e ratios of cu ttin g tim es under solution A and water are also presented in Table 1. For composite resin, increased cutting rates were noted w ith the test solution (A), for both the no. 56 and no. 556 burs, com pared w ith so lu tio n B an d w ater (P < .001), but there were no differences (P > .05) in c u ttin g tim es betw een solution B and water. Statistically sig nificant decreases in cutting time (P < .05) were noted for the fine diam o n d stone when used w ith solutions A and B compared with water, but there were no differences in the cutting rates achieved w ith the two test irrig a n ts (P > .05). It should be noted that the differences in cutting rates for the three solutions were statistically significant for the fine d iam ond stone b u t did n o t ap p ro a c h the nearly 50% decrease in cutting times found for the no. 56 and no. 556 burs. No differences in cutting rates were found for the coarse diam ond stone w ith any of the irrigants (P > .05). The cutting speed of dental amalgam was improved by solution A (P < .001) w ith a ll fo u r b u rs, com p ared w ith solution B and water. T he cutting times for solution A were about half as long as those for w ater irrig a tio n w ith all four burs. S o lu tio n B h a d increased cutting speeds com pared w ith water as an irrigant for the no. 56 bur (P < .001), the no. 556 bur (P < .05), and the fine diamond stone (P < .05), but the decrease
REPORTS
60 H 40
20 0 Tooth A I
Tooth B
I Test solution
I
I Scope
I
I Water
600 600 400
Fig 4 m Cutting times for composite resin.
300
200 100
No. 56 bur
No. 556 bur
Diamond (C)
Diamond (F)
Cutting instrument I
1Test solution
I
I Scope
I
I Water
1000 Y 800 600 -
Fig 5 ■ Cutting times for dental amalgam.
400-
200¿7
0 No. 56 bur
No. 556 bur
Diamond (C)
Diamond (F)
Cutting instrument 1
I Test solution
I
I Scope
was far less than that found with solution A. No statistically significant difference in c u ttin g tim es was fo u n d between s o lu tio n B and w ater w ith the coarse diam ond stone. T he cutting times of dental amalgam w ith the four burs under the three types of irrigant were comparable. In contrast, the diam ond stones cut composite resin far more slowly than did the fissure burs under all irrigants.
!
I Water
Discussion An earlier study2 revealed an improved surface fin ish of dental enam el w hen an oral rinse was added to the irrigant of a d e n tal h an d p iece. H ere, stu d ies dem onstrate that an irrigant composed of water, alcohol, and glycerol increases the rate of material removal for enamel, dental am algam , and com posite resin. Irrigation with solution A—the 40:20:40
Von F raunhofer-G ivens-O verm yer : L U B R IC A T IN G C O O L A N T S F O R H A N D P IE C E S ■ 293
RESEARCH
REPORTS
mixture of glycerol, ethanol, and water— enhanced cutting rates more effectively than solution B (the oral rinse irrigant) of the earlier study.2 The decrease in cutting time with solution A compared with water was greater for the carbide burs than the diamond stones on dental amalgam and composite resin. The increased cutting rate by carbide burs and diamond stones of enamel and dental amalgam under solution A, compared with cutting rates for water and solution B, was paralleled by the increased cutting rate by carbide burs of composite resin under solution A. In contrast, the relative increase in the cutting rate of composite resin with the fine diamond stone was small, and there was no difference in the cutting rates with the coarse diamond stone (Table 1). The accelerated cutting rate achieved with an irrigant composed of water, ethanol, and glycerol could be caused by a lubricating action of the additives, but is unlikely because the reduction in surface tension by the additives is slight; although ethanol has little effect on the viscosity of water, glycerol additions significantly increase viscosity. Thus, improved cutting efficiency likely results from interactions between the irrigant and the work surface, and possibly interactions between the irrigant and the cutting tool. The cutting rates of dental amalgam with the four burs under the three irrigants were comparable, but the diamond stones cut composite resin far more slowly than did the carbide fissure burs under all three irrigants. These differences in cutting rates of dental amalgam and composite resin with the carbide burs and diamond stones suggest that cutting mechanisms differ for the two classes of cutting instruments when used on filled polymer matrix (composite resin) surfaces. Interactive effects, whereby adsorbed species affect the flow and fracture behavior of solids, known as chemomechanical effects (CMEs), were discovered more than 40 years ago. The best known of these is the Rehbinder effect,6"8 named after its discoverer who found the hard ness of solids was influenced by absorbed surface-active species, and that the plastic flow of metals in nonpolar hydrocarbons was substantially facilitated in the presence of a surfactant. It was also noted that the hardness of metals in aqueous electrolytes passed through a maximum at the point of zero charge with a 294 ■ JADA, Vol. 119, August 1989
corresponding minimum in the creep rate at the point of zero charge. Rehbinder effects (adsorption-induced changes in the cohesive strength of interatomic bonds by surface-active species) are thought to be caused by a reduction in surface free energy. Chemically or adsorption-induced changes in the flow, fracture behavior, and hardness characteristics have also been observed with nonmetallic solids. Rehbinder and coworkers7’8 reported increased rates of rock drilling in aqueous solutions of various salts and surfactants. These latter effects are a subset of Rehbinder effects, can occur in envir onments that do not significantly affect surface free energy, and their magnitude depend on a number of factors. Hardness, for example, is maximized when the surface potential of the solid (the £ potential) is at or close to zero, an effect known as the £ correlation, which applies to crystalline and noncrystalline inor ganic nonmetals. Hardness changes of up to 30% that persist to depths of 10 jum have been observed.7,8 The underlying theory of chemomechanical effects is still not completely understood. However, they appear to arise from either adsorption-induced changes in the near surface electrical field or from charge transfer between the adsorbate and the internal structure of the solid in near surface regions. It follows that the improved rate of material removal in the studies reported here results from chemomechanical interactions between the irrigant media and the substrates, assuming material removal with carbide burs and diamond stones is by drilling (that is, chipping) rather than by abrasion. However, the mechanism of cutting differs for the two types of tools because of the different near-surface flow processes during chip formation. Carbide bit penetration is greatest for media that minimize hardness, an inverse £ correlation. As the carbide bit advances, plastic deformation results in the flow zone immediately ahead of the cutting edge (comparable to the effect occurring in metal cutting). Material in the flow zone cannot escape around the edge of the bit so that strain occurs, dislocations accumulate, and cracks nucleate, which grow rapidly and propagate and interact to form chips. This process involves the movement of dislocations in the work surface and is environment-sensitive. Media that maximize dislocation mobility (and minimize hardness) will promote
increased drilling rates. Thus for carbide drills, the greatest chemomechanical effect is obtained when the medium enhances dislocation mobility because dislocation motion is the first step in chip formation.8 For diamond drill bits, the drilling rate is greatest at maximum work surface hardness; for example, the drilling rate for glass is 20 times greater in heptyl alcohol than in water.8 Each irregularly shaped diamond on a diamond drill bit functions as a cutting tool with a large negative rake angle traveling in grooves. Dislocation movement adjacent to the tool causes an outward radial flow of material towards the groove edges, which limits the amount of plastic strain. However, passage of the tool generates high-tensile stresses in the near-surface region, and causes cracks to form that coalesce and interact to form chips. Any medium that reduces dislocation mobility, increases hardness, and has a positive ^-correlation (greater pene tration rates being achieved at £ potentials at or close to zero) will accelerate the drilling rate. In other words, material removal depends on localized brittle fracture of the work surface. The mechanism of material removal differs in grinding with diamond (and presumably other) abrasives; there is a dependence on both viscosity and £ potential, whereas in drilling there is no dependence on viscosity. In grinding, material removal involves flow processes with the high-shear force between abra sive particles and the work surface, resulting in plastic deformation and subsequent brittle fractures adjacent to the furrows. Because the least amount of near-surface plastic flow occurs at the point of zero charge, the rate of material removal will be lowest at £ potential values at or close to zero. Theoretical considerations of che momechanical effects suggest that enhanced cutting rates of dental amalgam for carbide burs and diamond stones arise from Rehbinder effects; that is, the irrigant reduces the free surface energy, and material removal is facilitated, which accounts for the comparable improvement in cutting rates for burs and stones with this work surface. Cutting effects observed with the composite resin indicate that a chemome chanical effect occurs with this material. The improved cutting action of carbide burs suggests that solution A (water, alcohol, glycerol) maximizes dislocation mobility and minimizes hardness in the composite filler; that is, a negative £
RESEARCH
correlation. Because o p tim u m cu ttin g w ith d ia m o n d stones occurs w ith a positive £ co rrelatio n (m in im al d islo cation mobility and m aximum hardness), little change w ill occur in the cutting action w ith d iam ond stones com pared with water for composite materials. T ooth studies indicate that solutions A an d B im p ro v e the c u ttin g ra te of enamel, compared with water, when used with a diam ond stone. Again, improved rates of material removal may be ascribed to a chem om echanical effect th a t has a po sitiv e £ c o rre la tio n , because the cutting action of diam ond was enhanced. Therefore, the irrigants appear to reduce dislocation m obility and increase hard ness. Because the test so lu tio n (water, alcohol, glycerol) was more effective than the rinse when used on enamel, findings here suggest that other irrigants m ight enhance the cutting action of diam ond stones even more. Conclusions
T h is study in d icates th a t an irrig a n t composed of water, alcohol, and glycerol improves the cutting action of diam ond stones on dental enamel and the action of carbide burs on com posite resin. In co n trast, th is irrig a n t fa c ilita te d the cutting of dental amalgam with carbide burs and diam ond stones. Findings indicate that different che momechanical effects occur, depending
on the substrate being cut. In the case of dental am algam , a Rehbinder effect occurs; the irrigant reduces the surface free energy an d cohesive stre n g th of in te ra to m ic bonds, w h ich facilitates c u ttin g . T h e irrig a n t also exerts a chemomechanical effect on dental enamel and, based o n the c u ttin g ac tio n of d iam onds, ap pears to reduce the dis location mobility, increase hardness, and thereby facilitate m aterial rem oval by diam ond stones. In contrast, the chemomechanical effect on composite resin appears to result from an increase in dislocation mobility and reduced hardness, w hich facilitates cut ting by carbide burs. While the detailed mechanism of chemomechanical effects is uncertain, it appears that these con siderations could profoundly affect cur re n t th o u g h t on to o th an d m aterial c u ttin g procedures. C areful selection of irrigants m ight facilitate the rate of m aterial rem oval and im prove surface finish of restorative materials and teeth by optim izing the cutting action of burs and stones.
------------- J ' AO A -------------
Inform ation about the manufacturers of the products mentioned in this article may be available from the authors. Neither the authors nor the American Dental Association has any commercial interest in the products mentioned.
REPORTS
Dr. von Fraunhofer is professor of biomaterials science, department of primary patient care, School of Dentistry, Health Sciences Center, University of Louisville, L ouisville, 40292. Dr. Givens is a general practitioner in Nashville, TN. Dr. Overmyer is a general practitioner in Danville, KY. Address requests for reprints to Dr. von Fraunhofer.
1. Eirich FR. The role of friction and abrasion in the d rillin g of teeth. In: T h e cu ttin g edge (Interfacial dynam ics of cu ttin g and grinding). W ashington, DC: US Department of H ealth, Education, and Welfare, 1976; DHEW publication no. 76-670. 2. von Fraunhofer JA; Overmyer TJ; and Johnson AA. Improved cutting of tooth enamel with dental burs. Quintessence Int 1987;18:383-5. 3. Overmyer TJ. Liquid admixing apparatus for dental water injection systems. US Patent no. 4,668,190, 1986. 4. Overmyer TJ. Method of cooling and lubricating human hard tissue during power tool cutting. US Patent no. 4,695,255, 1987. 5. American Dental Association. Revised ADA specification no. 1 for alloy for dental amalgam. JADA 1977;95:1-4. 6. Johnson AA; Johnson DN; and von Fraunhofer JA. Milestones in the history of physical metallurgy: Rehbinder effect. In: Guide to materials engineering data and inform ation. Metals Park, OH: ASM International: 1986. 7. W estwood ARC, Ahearn JS. Adsorptionsensitive flow and fracture of solids. In: Physical chemistry of the solid state: applications to metals and their compounds. Amsterdam: Elsevier Science, 1984. 8. Westwood ARC. Chemical effects in the cutting of nonm etals. In: The cu ttin g edge (Interfacial dynamics of cutting and grinding). Washington, DC: US Department of H ealth, Education, and Welfare, 1976; DHEW publication no. 76-670.
Von Fraunhofer-Givens-Overmyer : LUBRICATING COOLANTS FOR HANDPIECES ■ 295