Influence of Dental Rotary Instruments

80 Volume 102 Issue 2

The Journal of Prosthetic Dentistry Ayad et alPigozzi et al

8. Cune M, van Kampen F, van der Bilt A, Bosman F. Patient satisfaction and prefer-ence with magnet, bar-clip, and ball-socket retained mandibular implant overdentures: a cross-over clinical trial. Int J Prosthodont 2005;18:99-105.

9. Epstein DD, Epstein PL, Cohen BI, Pagnillo MK. Comparison of the retentive proper-ties of six prefabricated post overdenture attachment systems. J Prosthet Dent 1999;82:579-84.

10.Attard NJ, Laporte A, Locker D, Zarb GA. A prospective study on immediate loading of implants with mandibular overdentures: patient-mediated and economic outcomes. Int J Prosthodont 2006;19:67-73.

11.Mensor MC Jr. Attachment fixation for overdentures. Part I. J Prosthet Dent 1977;37:366-73.

12.Burns DR, Unger JW, Elswick RK Jr, Giglio JA. Prospective clinical evaluation of mandibular implant overdentures: Part II--Patient satisfaction and preference. J Prosthet Dent 1995;73:364-9.

13.Besimo CE, Guarneri A. In vitro retention force changes of prefabricated attach-ments for overdentures. J Oral Rehabil 2003;30:671-8.

14.Setz I, Lee SH, Engel E. Retention of prefabricated attachments for implant stabilized overdentures in the edentulous mandible: an in vitro study. J Prosthet Dent 1998;80:323-9.

15.Petropoulos VC, Smith W. Maximum dis-lodging forces of implant overdenture stud attachments. Int J Oral Maxillofac Implants 2002;17:526-35.

16.Gamborena JI, Hazelton LR, NaBadalung D, Brudvik J. Retention of ERA direct overdenture attachments before and after fatigue loading. Int J Prosthodont 1997;10:123-30.

17.Williams BH, Ochiai KT, Hojo S, Nishimura R, Caputo AA. Retention of maxillary im-plant overdenture bars of different designs. J Prosthet Dent 2001;86:603-7.

18.van Kampen F, Cune M, van der Bilt A, Bosman F. Retention and postinsertion maintenance of bar-clip, ball and mag-net attachments in mandibular implant overdenture treatment: an in vivo compari-son after 3 months of function. Clin Oral Implants Res 2003;14:720-6.

19.Payne AG, Solomons YF. Mandibular implant-supported overdentures: a prospective evaluation of the burden of prosthodontic maintenance with 3 differ-ent attachment systems. Int J Prosthodont 2000;13:246-53.

20.Porter JA Jr, Petropoulos VC, Brunski JB. Comparison of load distribution for implant overdenture attachments. Int J Oral Maxillofac Implants 2002;17:651-62.

Corresponding author:Dr Monica Nogueira PigozzoRua Orlando Matielo, 57Jaú, São Paulo 17207-750 BRAZILFax: +5514 3625 5220E-mail: [email protected] or [email protected]

Copyright © 2009 by the Editorial Council for The Journal of Prosthetic Dentistry.

Clinical ImplicationsThe best wetting is obtained when preparations are completed with finishing burs, allowing high precision castings. However, the smooth surface generated may reduce the frictional reten-tion of traditional, nonadhesive luting agents.

Statement of problem. Although it is known that different dental rotary instrument types produce distinct surface roughness of the axial walls which may contribute to the clinical performance of a cast restoration, the ideal rough-ness that produces optimum wetting at the adhesive interface is unclear.

Purpose. The purpose of this study was to characterize the surface roughness and its influence on wettability of teeth prepared for complete veneer crowns with different dental rotary instruments.

Material and methods. Sixty standardized tooth preparations for complete crowns were completed using a modified milling machine on extracted human teeth with diamond, tungsten carbide finishing, and cross-cut carbide rotary in-struments of similar shape (n=20). Morphological changes obtained were investigated with a surface texture analyzer. Two additional specimens from each group were treated and prepared for scanning electron microscopy. The contact angle of a drop of distilled water on each of the prepared surfaces was used as the measure of wettability with a go-niometer microscope. The data were analyzed with 1-way ANOVA and Tukey’s Honestly Significant Difference (HSD) test (α=.05).

Results. Surface preparation resulted in significant differences for surface topography and wettability (P<.001). The carbide burs had significantly higher mean roughness readings (SD) for all parameters: Ra (8.6 (1.9) µm), Rq (10.9 (2.2) µm), and Ry (41.1 (2.1) µm), except for the mean Rz (12.1 (3.1) µm) value, which was recorded for diamonds. The other roughness parameters for the diamond rotary instruments were intermediate. Teeth completed with carbide finishing burs produced a smoother surface Ra (1.2 (0.5) µm), Rq (1.6 (0.6) µm), Ry (6.4 (2.6) µm), and Rz (1.9 (0.6) µm). The highest mean contact angle (SD), 81.5 (9.1) degrees, was associated with diamond rotary instru-ments, and the lowest (75.3 (5.9) degrees) was recorded for finishing burs. The contact angle for teeth prepared with tungsten carbide burs was intermediate: 76.1 (5.9) degrees.

Conclusions. The selection of dental rotary instruments for tooth preparation significantly affects surface roughness and wettability of dentin, which may influence the retention of cast restorations. (J Prosthet Dent 2009;102:81-88)

Influence of dental rotary instruments on the roughness and wettability of human dentin surfaces

Mohamed F. Ayad, BDS, MScD, PhD,a William M. Johnston, PhD,b and Stephen F. Rosenstiel, BDS, MSDc

College of Dentistry, University of Tanta, Tanta, Egypt; The Ohio State University, Columbus, Ohio

aAssistant Professor, Section of Restorative Dentistry, Prosthodontics, and Endodontics, University of Tanta.bProfessor, Division of Restorative Dentistry and Prosthodontics, The Ohio State University.cProfessor and Chairman, Division of Restorative Dentistry and Prosthodontics, The Ohio State University.

Long-term clinical performance of cast dental restorations requires prolonged retention to the prepared tooth structure.1 This retention de-pends on many variables, including

the geometry of the tooth prepara-tion,2,3 the fit of the restoration,4,5 and the type of luting agent.6,7 Previ-ous studies have investigated many of these variables, and authors have rec-

ommended different instrumentation to prepare teeth appropriately.8-10

The mechanism by which fluted burs remove tooth structure differs from the abrading action of a dia-

Correction

The article by Kinsel et al entitled “Retrospective analysis of porcelain failures of metal ceramic crowns and fixed partial dentures supported by 729 implants in 152 patients: Patient-specific and implant-specific predictors of ceramic fail-ure,” published in the June 2009 issue of the Journal (J Prosthet Dent 2009;101:388-394), contained an error. Table IV, referenced on page 392 of the Journal, was omitted from the published article. The omitted table appears below.

Table IV. Odds ratio (OR) estimates (95% confidence interval (CI)) for comparison of op-posing dental condition, bruxism, and occlusal device use with respect to ceramic failures

Opposing dentition MC-Implant vs. Tooth MC-Tooth vs. Tooth

Bruxism Yes vs. No

Occlusal device Without vs. With

*Partial proportional odds model using generalized estimating equation (GEE) binomial method with P<.05.

13.95 (2.25, 86.41)*4.59 (0.77, 24.94)

5.60 (1.88, 16.66)*

1.92 (0.62, 5.99)

Major Ceramic Failure(n=58)

OR (95% CI)

7.06 (2.57, 19.37)*1.90 (0.91, 4.52)

7.23 (3.86, 13.54)*

1.92 (1.01, 3.67)*

Major and Minor Ceramic Failure(n=94)

OR (95% CI)


The Journal of Prosthetic Dentistry

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Ayad et alAyad et al

mond rotary cutting instrument.10 As burs rotate, the flutes undermine den-tal tissue and the amount removed is determined by the flute angle of at-tack, a basic feature of bur design. In the case of diamond rotary cutting instruments, the abrasive particles pass across the tooth surface and plough troughs in the substrate sur-face. Tooth structure is ejected ahead of abrading particles and the surface is transformed into a series of ridges running parallel to the direction of the moving particles.7 The resultant axial wall roughness may influence the wettability and bonding quality of adhesive luting agents.11

The development of an adhesive bond requires establishing intimate contact between the liquid adhesive and the solid adherent, minimizing the stress concentration at the inter-face, and reducing the influence of en-vironmental factors on the interface integrity.12 The degree of spreading of a liquid on a surface is a measure of the wettability of the surface by a liq-uid, and it can be quantified by con-tact angle determination.13 If the ad-hesive has high wettability, there will be an intimate contact, and adhesive efficiency will be improved.12

Factors that influence the wetting of a solid by a liquid include the rela-tive surface energy of the solid and the surface tension of the liquid, the viscosity of the liquid, which is a mea-sure of the resistance to flow, and the surface topography of the adherend.11 It is presumed that surface roughness promotes wettability by producing increased surface area and that the bond between the adherend surface and the adhesive will be subsequently stronger. However, excessive rough-ness may hinder even flow of the liq-uid adhesive and result in air pockets being entrapped at the interface.

Previous studies have demon-strated that a smear layer is created by grinding debris whenever rotary instruments are used for tooth prep-aration.10,14 The smear layer covers the dentin surface and occludes the dentin tubules.15 The thickness of the

smear layer is affected by the type of rotary instrument.16 Diamond rotary instruments produce a thicker, un-even smear layer, compared to car-bide burs, which produce a thin and unevenly distributed layer.17,18 Differ-ences in the smear layer have been re-ported to affect the bond strength of adhesive resin cement.17,19

While significant progress has been made in the area of roughness and adhesion,20-31 no scientific data are available to gauge the specific amount of roughness created during tooth preparation and how the choice of rotary instrument influences wet-ting. This investigation used a surface texture analyzer and scanning elec-tron microscopy to describe the mor-phologic features of dentin surfaces prepared by common rotary instru-ments, including diamond, tungsten carbide, and tungsten carbide finish-ing burs of similar shapes, and to de-termine their effects on the wettability of dentin surfaces for complete cast crowns. The null hypothesis was that surface texture differences after den-tin surface preparation with different rotary instruments would have no in-fluence on wettability.

MATERIAL AND METHODS

Sixty extracted, intact human mo-lar teeth of similar size were collected and stored in distilled water at room temperature. The teeth were prepared to simulate clinical preparations for complete crowns. Each tooth was aligned vertically in an individual polymeric tube and embedded in ep-oxy resin (Epoxide Resin; Leco Corp,

St. Joseph, Mich) to within 2 mm of the cemento-enamel junction. A den-tal surveyor (Ney Surveyor; Dentsply Ceramco, Burlington, NJ) was used to position the long axis of each tooth parallel to the tube. Mounted teeth were stored in an atmosphere of 100% humidity. The teeth were assigned to 3 groups of 20 each, according to the rotary instruments used (Table I). The 3 rotary instruments chosen for the tooth preparations had similar con-figurations in terms of taper, diam-eter, and tip shape (Fig. 1).

The teeth were prepared to receive complete cast crowns by flattening the occlusal surface to the depth of the central groove to expose dentin. The reduced occlusal surface was ex-amined with a 19-power stereoscopic microscope (SMZ-1; Nikon, Inc, Mel-ville, NY). Additional reductions were accomplished as necessary to remove any remaining enamel. Occlusal re-duction was oriented perpendicular to the axis of the polymeric tube. Axial reduction was standardized by using a milling machine (model F1; Evonik Degussa GmbH, Essen, Germany), modified to produce replicas guided by a stylized metal master model of a tooth prepared for a complete crown. An aluminum fixture was used to at-tach the stylus to the superior part of the machine. The stylus taper, di-ameter, and tip shape were machined to the same dimensions as the rotary instruments and fixed at a known distance from the cutting tools. A movable X-Y table on the milling ma-chine supported another fixture that secured embedded specimens at the same distance from the master tooth

Diamond*

Finishing*

Tungsten carbide*

*Brasseler USA, Savannah, Ga

Round-end taper, coarse grit

12-fluted round-end taper

Cross-cut round-end fissure

Description

6856 L-016

H375 R-016

H33 R-016

Catalog No.Rotary Instrument

Table I. Rotary instruments for tooth preparations of complete crowns (n=20)

1 Rotary instruments for tooth preparations. 2 Milling machine used for complete crown tooth prepa-rations.

preparation model (Fig. 2). The apparatus operated similarly

to a hardware store key cutter. The teeth were prepared by moving the ta-ble and tooth assembly past the fixed portion of the machine after the tool and stylus tips were centered above the occlusal surfaces of both the tooth and the master die. All tooth prepa-rations were initially prepared with a diamond instrument and then refined with the assigned instruments. The depth of tooth preparation was lim-ited by a track of the stylus along the master die. The length of tooth prepa-ration was controlled by the working height setting of the milling machine. A chamfer margin was formed as the negative image of the round-ended tapered rotary instruments. A new rotary instrument was used for each tooth, and a continuous water jet was directed at the rotary instruments. The surface roughness of the 20 spec-imens per instrumentation group was measured with a surface profilometer (Surfanalyzer 4000; Mahr Federal, Inc, Providence, RI). Each traverse of the profilometer stylus was made from the occlusal direction toward the gingival, approximately paral-lel to the long axis of the tooth. Ra (roughness average), the mathemati-cal average height (absolute value) of roughness irregularities measured from a mean line within the sampling length, which is the most commonly reported roughness parameter in the dental literature, was recorded.19,20

Roughness was determined by using a mathematical filter to remove the other surface components. However, Ra alone may be insufficient to distin-guish among surfaces. Under certain conditions, other parameters (Rq, Ry, or Rz) may be more critical and should be used in addition to Ra if the surface finish is to be properly speci-fied.32 Rq (root mean square) is the geometric average of roughness com-ponent irregularities measured from the mean line within the sampling length. Therefore, Rq is the root mean square of the roughness profile and is more sensitive to occasional peaks. Ry is the maximal peak-to-valley height measured parallel to the traverse, and Rz (low-point height) is the av-erage distance between the 5 highest peaks and 5 deepest valleys within the sampling length, measured from the constant line that runs parallel to, but does not cross, the roughness profile (Fig. 3). The precision of the surface profilometer meets the requirements for the measuring accuracy as defined by ANSI (American National Stan-dard Institute) for surface roughness not in excess of 1%. The average of 3 measurements recorded in different areas of the 60 specimens was deter-mined. One investigator prepared all specimens and measured the surface roughness.

A drop of filtered, purified wa-ter with an approximate diameter of 2.0 mm was carefully deposited with a micropipette (Eppendorf, Ham-

burg, Germany) on each slightly dried prepared surface. The contact angles were measured on 2 sides of each drop using a contact angle go-niometer (Model NRL 100-00; Ramé-Hart Instrument Co, Netcong, NJ) controlled by a single investigator. Each specimen was mounted on the traveling stage of the measuring mi-croscope so that the long axis of the specimen was parallel to the objec-tive lens. The microscope magnifica-tion was adjusted so that the tooth preparation occupied the entire field of the measuring microscope to fa-cilitate accurate measurements. The microscope was adjusted to a large aperture to produce the narrowest depth of field and create the bright-est image. The mean of the 6 contact angle measurements was used as the contact angle for that specimen.

Two additional specimens from each group were treated and pre-pared for scanning electron micros-copy by mounting them on alumi-num stubs and sputter coating with gold palladium (Denton Vacuu, Inc, Moorestown, NJ); the specimens were then observed under a scanning electron microscope (JEOL JSM-820; JEOL Ltd, Tokyo, Japan). Each speci-men was photographed at x100 mag-nification. Mean values of the rough-ness parameters and contact angles for each group were calculated, and differences between these means within each property were tested for statistical significance by use of 1-way



85August 2009


analysis of variance (ANOVA) and Tukey Honestly Significant Difference (HSD) test at α=.05.

RESULTS

The 1-way ANOVA of each rough-ness parameter revealed a significant difference among rotary instruments (P<.001), as shown in Table II. The magnitude of the roughness param-eters depends on the rotary instru-ment. Figure 4 shows the mean and standard deviation measurements for all roughness parameters. The Tukey HSD test disclosed a significant dif-ference between each pair of rotary instruments for each parameter of roughness, as shown in Figure 4.

Tooth preparations completed with finishing burs had the lowest surface roughness, with roughness param-eters ranging from 11.6% to 18.8% compared to the other rotary instru-ments. Tooth preparations complet-ed with carbide burs had significantly higher roughness readings for all pa-rameters, except for Rz, which was recorded for diamond instruments. The other roughness parameters for the diamond rotary instruments were intermediate. One-way ANOVA of the results of contact angle to prepared tooth surfaces revealed that there were significant differences among rotary instruments (df, 2; Error df, 57; F, 4.4; P<.05).

The Tukey HSD test disclosed no

significant difference in mean contact angle measurement between carbide and finishing burs; however, diamond rotary instruments were significantly different, as shown in Figure 5. Tooth preparations completed with finish-ing burs had the lowest contact an-gle, with measurements ranging from 1.1% to 8.2% compared to the other rotary instruments. Tooth prepara-tions completed with diamond rotary instruments had significantly higher contact angles with no difference between finishing and carbide burs. Analysis of the linear relationship be-tween the mean contact angle and the mean for each roughness param-eter revealed no significant relation-ships (Ra (P=.735), Rq (P=.724), Ry (P=.689), and Rz (P=.407)).

Scanning electron microscopic (SEM) evaluation of the prepared tooth surfaces revealed several types of morphological changes. Figure 6 illustrates the differences in topo-graphic detail after instrumentation. Compared with a finishing bur, cross-cut carbide and diamond instruments created a more undulating surface; the troughs ran perpendicular to the path of the handpiece. Grooves dem-onstrated a mean depth of 25 µm and a distance between cutting ridges of nearly 100 µm for the diamonds, 150 µm for carbide, and 10 µm for finish-ing burs. Fine grooves were observed that ran perpendicular to undulations

Table II. One-way ANOVA for each roughness parameter among burs

3 Surfanalyzer profilometer: Ra, mean value of roughness irregulari-ties measured from mean line within sampling length (L); Rq, root mean square of roughness profile; Ry, maximum peak-to-valley height measured parallel to mean line; Rz, average distance between 5 height-peaks (R1, R3, R5, R7, R9) and 5 deepest valleys (R2, R4, R6, R8, R10) within sampling length measured from line parallel to mean line and not crossing roughness profile.

R1

RQ

RY

RA

R3 R9

R7R5

R2 R4 R6 R8 R10

L

Model – Ra

Error

Model – Rq

Error

Model – Ry

Error

Model – Rz

Error

2

57

2

57

2

57

2

57

df

298.6

1.5

484.5

2.2

6874.5

12.7

1083.7

5.2

Mean Square

1194

224

543

209

FSource

<.001

<.001

<.001

<.001

P

4 Mean and standard deviation measurements for all roughness parameters. Ra, mean value of roughness ir-regularities; Rq, root mean square of roughness profile; Ry, maximum peak-to-valley height; Rz, average distance between 5 height-peaks and 5 deepest valleys.

5 Mean contact angle measurements for each rotary instrument. Horizontal lines connect values that are not significantly different (P>.05).

6 SEM of prepared teeth. A, Carbide bur. Galling pattern on dentinal surface. Grooves demon-strate mean depth of approximately 25 µm and 150 µm between ridges. B, Diamond rotary instru-ment. Fine grooves running with deeper grooves have mean depth of 15 µm and distance between ridges of nearly 100 µm. C, Finishing bur. Smooth surface is evident. Original magnification x100.

50

μ

Roughness Parameters

30

40

20

10

0Ra Rq Ry Rz

CarbideRotary Instrument

DiamondFinishing

Roughness Instrument

60

80

40

20

0

FinishingCarbideDiamond

A B

C



85August 2009


analysis of variance (ANOVA) and Tukey Honestly Significant Difference (HSD) test at α=.05.

RESULTS

The 1-way ANOVA of each rough-ness parameter revealed a significant difference among rotary instruments (P<.001), as shown in Table II. The magnitude of the roughness param-eters depends on the rotary instru-ment. Figure 4 shows the mean and standard deviation measurements for all roughness parameters. The Tukey HSD test disclosed a significant dif-ference between each pair of rotary instruments for each parameter of roughness, as shown in Figure 4.

Tooth preparations completed with finishing burs had the lowest surface roughness, with roughness param-eters ranging from 11.6% to 18.8% compared to the other rotary instru-ments. Tooth preparations complet-ed with carbide burs had significantly higher roughness readings for all pa-rameters, except for Rz, which was recorded for diamond instruments. The other roughness parameters for the diamond rotary instruments were intermediate. One-way ANOVA of the results of contact angle to prepared tooth surfaces revealed that there were significant differences among rotary instruments (df, 2; Error df, 57; F, 4.4; P<.05).

The Tukey HSD test disclosed no

significant difference in mean contact angle measurement between carbide and finishing burs; however, diamond rotary instruments were significantly different, as shown in Figure 5. Tooth preparations completed with finish-ing burs had the lowest contact an-gle, with measurements ranging from 1.1% to 8.2% compared to the other rotary instruments. Tooth prepara-tions completed with diamond rotary instruments had significantly higher contact angles with no difference between finishing and carbide burs. Analysis of the linear relationship be-tween the mean contact angle and the mean for each roughness param-eter revealed no significant relation-ships (Ra (P=.735), Rq (P=.724), Ry (P=.689), and Rz (P=.407)).

Scanning electron microscopic (SEM) evaluation of the prepared tooth surfaces revealed several types of morphological changes. Figure 6 illustrates the differences in topo-graphic detail after instrumentation. Compared with a finishing bur, cross-cut carbide and diamond instruments created a more undulating surface; the troughs ran perpendicular to the path of the handpiece. Grooves dem-onstrated a mean depth of 25 µm and a distance between cutting ridges of nearly 100 µm for the diamonds, 150 µm for carbide, and 10 µm for finish-ing burs. Fine grooves were observed that ran perpendicular to undulations

Table II. One-way ANOVA for each roughness parameter among burs

3 Surfanalyzer profilometer: Ra, mean value of roughness irregulari-ties measured from mean line within sampling length (L); Rq, root mean square of roughness profile; Ry, maximum peak-to-valley height measured parallel to mean line; Rz, average distance between 5 height-peaks (R1, R3, R5, R7, R9) and 5 deepest valleys (R2, R4, R6, R8, R10) within sampling length measured from line parallel to mean line and not crossing roughness profile.

R1

RQ RA

R3 R9

R7R5

R2 R4 R6 R8 R10

L

Model – Ra

Error

Model – Rq

Error

Model – Ry

Error

Model – Rz

Error

2

57

2

57

2

57

2

57

df

298.6

1.5

484.5

2.2

6874.5

12.7

1083.7

5.2

Mean Square

1194

224

543

209

FSource

<.001

<.001

<.001

<.001

P

4 Mean and standard deviation measurements for all roughness parameters. Ra, mean value of roughness ir-regularities; Rq, root mean square of roughness profile; Ry, maximum peak-to-valley height; Rz, average distance between 5 height-peaks and 5 deepest valleys.

5 Mean contact angle measurements for each rotary instrument. Horizontal lines connect values that are not significantly different (P>.05).

6 SEM of prepared teeth. A, Carbide bur. Galling pattern on dentinal surface. Grooves demon-strate mean depth of approximately 25 µm and 150 µm between ridges. B, Diamond rotary instru-ment. Fine grooves running with deeper grooves have mean depth of 15 µm and distance between ridges of nearly 100 µm. C, Finishing bur. Smooth surface is evident. Original magnification x100.

50R

ough

ness

Mea

sure

men

ts (μ

m)

Roughness Parameters

30

40

20

10

0Ra Rq Ry Rz

CarbideRotary Instrument

DiamondFinishing

100

Con

tact

Ang

le (

degr

ee)

Roughness Instrument

60

80

40

20

0

FinishingCarbideDiamond

A B

C



87August 2009

Ayad et al Ayad et al

and parallel with the direction of ro-tation of the instrument. The smear layer, an incidental consequence of tooth preparation with rotary instru-ments, was clear. SEM revealed that the smear layer was a uniform, amor-phous structure covering the dentin and occluding the dentinal tubule orifices.

DISCUSSION

The data support rejection of the null hypothesis of the study: that den-tin surface preparation with different rotary instruments has no influence on wettability. Results of the current study are in agreement with the ma-jority of reported studies that stress the importance of dentin roughness to restoration retention.3-6 Ayad et al7 found that tungsten carbide burs produced increased dentin roughness compared to the surfaces produced by finishing burs or diamond rotary instruments. The authors considered that surface texture differences after preparation may have a major influ-ence on adhesion strength. Similarly, Al-Omari et al12 showed that instru-ment selection for tooth preparation may influence the retention and adhe-sion of resin restorative materials.

The characteristic appearance of the dentinal surfaces is determined by the shape of the instrument used. The fine grooves resulting from the abrading effect of diamond rotary instruments can be related to minute facets on the cutting flutes of the tool created by wear. These facets act as abrading points that scratch the den-tin, resulting in the plastic deforma-tion of the surface as the instrument rotates.24,25 However, the carbide burs showed significant variability. Mc-Innes et al22 compared dentinal sur-faces prepared with a carbide bur to those finished with silicon carbide pa-pers and demonstrated that a 320-grit surfacing followed by a 254-carbide bur most closely resembled the fin-ish that dentists recognize after cavity preparation with a tungsten carbide bur. Specimens prepared with finish-

ing burs exhibited a smooth surface interrupted only by slight granularity compared with greater roughness of other specimens. It is theorized that a uniform thickness of cement painted over the inner surface of the crown could be efficiently flowed into the spaces between projections for den-tin surfaces, enabling vertical crown seating.1

With the use of dentin bonding systems in combination with com-posite resin restorations, the effi-ciency of wetting the dentin surface could be influenced by the use of dif-ferent rotary instruments.27 Previous studies17,26 found that coarse rotary cutting instruments and diamond rotary cutting instruments created smear layers of different thicknesses and influenced dentinal permeabil-ity. Although it was hypothesized that surface roughness may improve wet-tability by reducing the contact angle, surface roughness results of the cur-rent study demonstrated no signifi-cant influence on surface wettability. This is in agreement with the findings of a previous study.12 The smallest contact angle was measured on the smoothest dentin surface prepared with finishing burs. However, the re-sults were not statistically different from those of the dentin surface pre-pared with carbide burs. The higher contact angle observed with diamond rotary instrument preparations may be explained by the undulated sur-faces and thick layer of debris that fre-quently accumulated at the deepest part of the undulations. This finding corroborates the findings of previous studies.18,19

Contact angle measurement is considered an indicator for interfacial tension and, thus, a means of char-acterizing the wettability of the sub-strate by the fluid phase.21 The smear layer morphology and thickness may have a more prominent role than sur-face roughness in determining wet-tability. Smear layers tend to act as a physical barrier to the diffusion of oral fluids and restorative materials to the pulp.23 If the smear layer occludes the

dentinal tubules, it reduces the dentin permeability.18,26 Debris produced by rotary instruments deposited in the dentinal tubule apertures will pre-vent fluid flowing outward onto the surface. The resultant excessive dry-ness may interfere with the bonding between hydrophilic wetting agents and dentin; therefore, studies recom-mended the removal of the smear lay-er to improve bonding.28,29

For reliable adhesion, the oppos-ing surfaces should come in close proximity to allow for the formation of a bond between the surface atoms. The attraction force is a function of polarity and surface energy. Surface impurities, debris, and the smear layer act collectively to reduce surface energy and, consequently, reduce ad-hesion. This may explain the results of the current study: relatively high val-ues of contact angles were obtained that were independent of the surface roughness. Furthermore, the smear layer, in addition to the elastic defor-mation produced by the energy cre-ated during tooth preparation, may change the morphology, the surface-chemical, and the surface-physical properties of dentin. The induced changes may influence the properties of adhesion, wetting, and adsorp-tion.20 Mowery et al8 demonstrated that rough surfaces, prepared with 60-grit silicon carbide, showed sig-nificantly higher shear bond strength to dentin than surfaces finished with finer 260- or 600-grit papers. This might be explained by the increase in the total surface area available for adhesion at the bond site. Also, this can be attributed to more frictional stresses created due to greater con-tact between the fine diamond-gener-ated grit and tooth surface, as com-pared to carbide bur preparation.19 However, other investigators9,15 could not demonstrate a relationship be-tween bond strength of composite resins and tooth surfaces prepared with carbide burs. Conversely, Ta-gami et al26 revealed that the bond strength of Super-Bond C&B (Sun Medical Co, Shiga, Japan) to dentin

was higher when prepared with a dia-mond instrument under water spray than when prepared with a diamond instrument without water. The varia-tions in bond strength results report-ed by different studies are significant. However, most studies performed to investigate dentin surface treat-ment and bonding procedures use 600-grit silicon carbide (SIC) abrasive paper to prepare a standardized sur-face.12,30,31 This method produces a flat and smooth surface and does not mimic the amount of smear layer cre-ated in clinical procedures with rotary instruments. In the current study, a more clinically relevant protocol was adopted.

The current study demonstrates that the choice of a tungsten carbide bur for tooth preparations is reason-able for wetting of the dentin surface. Nevertheless, the smear layer formed as a function of bur rotation during dentin surface characterization may influence the wettability. Studies re-ported the bond strength of differ-ent adhesive systems used in combi-nation with a luting agent to tooth enamel and dentin, but not with tra-ditional luting agents.13,15-17 Little in-formation, however, is available with regard to the function of the chemical composition and polarity of the vari-ous surfaces to be bonded, which are produced by different rotary instru-ments.

Further research is required to evaluate the effect of wetting on the shear bond strength of adhesive ce-ments. There were some limitations in the present study. Although contact angle was measured, spreading time was not evaluated. Also, it remains unclear which of the following, in-strument type, grit size, or speed, is the crucial factor in bond strength determination. Another limitation of this study was that the combined ef-fects of dentin surface roughness and smear layer removal were not ob-served. Removal of the smear layer should be considered as a parameter affecting adhesion to tooth structure. Furthermore, the present study was

an in vitro investigation, and the re-sults do not necessarily reflect the in-traoral condition.

CONCLUSIONS

Within the limitations of this study, the following conclusions were drawn:

1. The rotary instrument used for tooth preparations had a significant influence on the axial wall surface characteristics of complete crown preparations. Moreover, the amount of the smear layer was affected by the type of instrument used.

2. Completion of the tooth pre-paration with a finishing bur ap-peared to be the method of choice if a smooth tooth surface and wetting quality were preferred.

3. Diamond rotary cutting instru-ments produced a greater surface area than did carbide burs.

4. No direct correlation was found between the contact angle measure-ments and surface roughness.

5. The selection of dental rotary instruments for tooth preparation may influence the retention and adhe-sion of resin restorative materials as a function of smear layer formation.

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Corresponding author:Dr Mohamed F. AyadPO Box 443Tanta 31111EGYPTFax: +20 40 3313020E-mail: [email protected]

Copyright © 2009 by the Editorial Council for The Journal of Prosthetic Dentistry.

Clinical ImplicationsThe findings of this pilot study are of diagnostic interest because they indicate that TMD is associated with increased disc thick-ness in the anterior band and the intermediate zone, and that the open positions are associated with a thicker posterior band.

Statement of problem. Studies of temporomandibular joint (TMJ) discs removed from autopsy specimens and in surgery indicate that they are often thicker in patients with temporomandibular disorders (TMDs). Disc thickness may also change when the condyle moves in and out of the joint fossa during opening and closing.

Purpose. The purpose of this pilot study was to test the hypothesis that the TMJ disc thickness is greater in TMD pa-tients than in healthy subjects and is affected by the degree of jaw opening.

Material and methods. Magnetic resonance images (MRI) were made of the TMJs in young volunteers, 9 asymptom-atic subjects and 9 TMD subjects, at closed and at 10-, 20-, and 30-mm open positions. The thickness of the anterior and posterior bands of the discs and the intermediate zones was measured and compared between TMD and con-trol groups, and between closed and opened positions, using the repeated-measures method in general linear model (α=.05).

Results. The hypothesis that the disc was thicker in the TMD than in the control group was supported with respect to the anterior band and intermediate zone (P<.046), but not with respect to the posterior band. The hypothesis that the thickness is affected by the degree of jaw opening was only supported for the posterior band, where it increased dur-ing opening in both groups (P<.005).

Conclusions. The results indicate that the anterior band and the intermediate zone of the TMJ discs are thicker in TMD patients than in healthy subjects. The posterior band thickness increases with mouth opening in both asymp-tomatic and TMD subjects. (J Prosthet Dent 2009;102:89-93)

Magnetic resonance imaging on TMJ disc thickness in TMD patients: A pilot study

Meiqing Wang, DDS, MD, PhD,a Hongtao Cao, DDS, MD, PhD,b Yali Ge,c and Sven E. Widmalm, DDS, Dr Odontd

School of Stomatology, Fourth Military Medical University, Xi’an, China; School of Dentistry, University of Michigan, Ann Arbor, Mich

Supported by grant no. 30540130469 from the Nature Science Foundation of China (NSFC).

Presented at the 21st Annual Meeting of the Japanese Society for Temporomandibular Joint, Osaka, Japan, July 2008.

aProfessor and Chair, Department of Oral Anatomy and Physiology and TMD, School of Stomatology, Fourth Military Medical University.bLecturer, Department of Oral Anatomy and Physiology and TMD, School of Stomatology, Fourth Military Medical University. c Senior Technican, Department of Radiology, Xijing Hospital, Fourth Military Medical University. dAssociate Professor, Department of Biological and Materials Sciences, Division of Prosthodontics, School of Dentistry, University of Michigan; Visiting Professor, Department of Oral Anatomy and Physiology and TMD, School of Stomatology, Fourth Military Medical University.

Noteworthy Abstracts of the Current Literature

Effect of removable partial dentures on oral health-related quality of life in subjects with shortened dental arches: A 2-center cross-sectional study

Armellini DB, Heydecke G, Witter DJ, Creugers NH.Int J Prosthodont 2008;21:524-30.

Purpose: To assess the value of removable partial dentures (RPDs) in subjects with shortened dental arches (SDAs) with regard to quality of life.

Materials and Methods: Subjects from 2 university dental centers (82 men, 78 women; mean age: 54 ± 18 years) were categorized into the following groups: (1) SDA with intact anterior regions (SDA-1, n = 44); (2) SDA and interrupted anterior region (SDA-2, n = 21); (3) SDA (intact anterior region) extended with distal-extension RPD (RPD-1, n = 25); (4) subjects with interrupted SDA and interrupted anterior region treated with RPD (RPD-2, n = 32); or (5) complete dental arches (CDA, n = 38) as a control. All subjects underwent a short clinical examination and completed 2 struc-tured questionnaires: the Oral Health Impact Profile (OHIP-49) and the Short-Form Health Survey (SF-36). Recorded clinical variables included: teeth present (yes/no), replacement by RPD (yes/no), and number of occlusal units. Age-dependent outcomes were adjusted to outcomes for the age of 60 years. Linear regression models were used to assess differences between the groups.

Results: Reliability and validity were good for all subscales. For OHIP, the investigation groups had significantly higher scores (more complaints) than CDA for the subscales “functional limitation,” “psychologic discomfort,” and “physi-cal disability.” Of the 4 investigation groups, SDA-1 subjects had the lowest mean scores. SF-36 scores showed less prominent and less conclusive differences between investigation groups and CDA subjects. SDA-2 subjects showed worse health, with significantly lower scores than RPD-2 subjects for “vitality,” “social function,” and “mental health.” For pure SDA subjects (SDA-1) there was a significant positive effect for “number of occlusal units” in 5 of the OHIP subscales and 2 of the SF-36 subscales.

Conclusion: From a quality-of-life perspective, patients with SDAs perceive benefits from RPDs only if anterior teeth replacements are included. In contrast, in subjects with uninterrupted SDAs, where only posterior teeth were replaced by distal-extension RPDs, such benefits could not be demonstrated.

Reprinted with permission of Quintessence Publishing.

Influence of Dental Rotary Instruments

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