Speciai Report

A scanning electron microscopic investigation on the acid-etchedcervical margin of Class 11 cavitiesGary S. R Cheung*

The use of composite resin as a restorative nutlcriai for Class ! and If cavities is be-coming more conimon. The tnatginal leakage at the proximal cervical margin of Class IIcomposite resin restorations retnaitis a mafor limitation to the widespread aeeeptance ofthi.'! material. Although somewhat controversial, beveling of the cavosurfcice line anglehas been suggested as a wcty to enhance the bond al the cervical mat gin that finisheson etiamel. The acid-etched appearance of beveled and nonbeveled enamel at the proxi-mal cervical tnargin of Class II cavities was studied wider the scannitig electron micro-scope. The results appear to support the efficacy of the beveling of this margin forClass II composite resin restorations. (Quintessence Int 1990:21:299-302.)

Introduction

Since its introduction in the 1950s by Buonocore,' theacid-etching technique has been tbe most effectivemeans of bonding restorative resins to dental enamel.The bond achieved is a form of micromechanical re-tention by the penetration ofthe resin into the micro-pores, hence the "resin tags" in the treated enamelsurface.- This technique is widely employed and formsan integral part of modern restorative procedures forcomposite resin restorations.

The primitive type of cavity preparation for directresin restorations was a copy of that for silver amal-gam, with a 90-degree butt joint at the cavosurfaceline angle.''' Although it has been sbown that etchingand applying an unfilled resin to the enamel on thecavity wall reduces the microieakage around the res-toration.^"'' some investigators recommend moditlca-tions of this butt joint marginal finish for compositeresins.''^ Beveling of the cavosurface angle, which isgenerally advocated currently, was one of tbe proposedmodifications. It was reasoned that a cavosurface be-vel would give an increased surface area and a betterorientation of enamel rods for acid etehing, and hencea better bond and marginal adaptation.*"'" In a num-

• Department of Conservative Dentislry, The Prinze Philip DéniaiHospitai, 34 Hospital Road, Sai Ying Pun, Hong Kong

ber of microieakage studies, researchers were unableto demonstrate a significant improvement in the mar-ginal leakage pattern around composite resin resto-rations with a ehange in the cavosurface design." '''Allrestorations in these experiments had their marginsfinished entirely in enamel. This prompted the ques-tion of whether beveling of the cavosurface line anglewould provide any significant improvement in thebond acbieved with the acid-etching technique.

Because an increasing number of composite resinmaterials are being marketed for use in posterior ClassI and II restorations, it was the purpose of this studyto examine the effect of placement of a bevel at theproximal cervical margin of Class II cavities on themorphology of the enamei surface after aeid etching.

Method and material.s

Ten extracted teeth, five premoiars and five molars,were stored in 10% neutral formahn. The teeth weregently scaled to remove any gingival tissue and plaquethat were attached.

A box cavity was prepared in one of the proximalsurfaces of each tooth using a high-speed tungstencarbide bur. The box measured approximately 3 mmin buccolingual width and 1,5 to 2,0 mm in depth. Thecervical margin of the cavity was finished about 1 mmabove the cementoenamel junction with a sharp handinstrument (Wedelstaedt chisel) to a butt joint. Halfthe length of this margin was then beveled with a fine-grit diamond at high speed to give a 45-degree cavo-

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Fig 1 Box cavity preparation used in this siudy Half thelengtti a\ the cervical margin was beveled with a fine-gritdiamond running at iiigti speed under air-water spray cool-ant.

Fig 3 Severe smearing of ttie surtace at the butt joint mar-gin finished with hand instrument.

Fig 2 Scanning electron microscopic appearance of thebeveled and butt joint margins atter preparation.

Fig 4 Part of the margin beveled with tine-grit diamond.Shallow grooves are evident and numerous in the preparedsurtace.

surface angle (Fig 1), The cavity margin was etchedwith a 35% phosphoric acid gel for 1 minute, rinsedwith wateT for 30 seconds, and blown dry.

Impressions were taken of the cervical cavity mar-gin, before and after the acid-etching procedure, witha rubber-based elastomeric material. The positive re-plicas, poured in epoxy material and sputtered withgold particles,'^ were examined under a scanning elec-tron microscope.

Results and discussion

Typical scanning electron micrographie appearancesof the cavity margin with the two different finishingprocedures are shown in Figs 2 to 4,

Hand instruments invariably produced severesmearing ofthe prepared surface'* (Fig 3). Under theair-water spray cooiant, the fine-gnt diamond bur gavea fairly clean surface, although scratches were evidenton the surface (Fig 4), These findings were similar tothose observed by Leidal and Tronstad," Barnes,'^ andCrawford and Wittaker,'"

The treatment of the prepared surface with dilutedphosphoric acid completely removed the smear layer.In addition, the acid dissolved the enamel rods differ-entially, producing an irregular, tnicroporous enamelsurface, the so-called etching pattern,-" The appear-ance of this pattern depends oti the orientation of theenamel rods prior to acid treatment," In the presentstudy, all bittt joint finishes demonstrated a fairlysmooth and homogenous appearance on most of their

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Figs 5a to 5c Etching pattern obtained on butt joint cavity tinisti.

Fig 5a Typical etched appearance of longitudinally sec-tioned enamel rods fotjnd on most ot ttie enamel surfaceof ttie butt joint margin.

Fig 5b Weil-delineö etching pattern found in part of a buttjoint margin.

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4 lií^í.'^íSV

iFig 5c Occasional patcties of Class 1 and 2 etctiing pat-terns observed in an otherwise longitudinally sectioned en-amel rods.

Fig 6 Well-defined Class 1 and 2 etching patterns in allbeveied enamel margins.

surfaces (Fig 5a), which is typical of the etched to-pography of longitudinally sectioned enamel rods.'"Only one specimen, a molar, revealed a weli-defmedClass 1 and/or 2 etching pattern (Fig 5b) that wascomparable to that obtained on the beveled enamelsurface (Fig 6). A premolar tooth showed patches ofclass 1 and/or 2 etching pattern in an essentialiysmooth surface (Fig 5c). These results seem to agreewith the statement that the direction of enamel rodsis variable at the cervical region of the enamel nearthe cementoenamel junction of a tooth.'''

At the beveled margin, typical and well-deflnedClass 1 and 2 etching patterns were obtained in allcases (Fig 6). It appears that beveling of the marginin this area improved the orientation of enamel rodsfor acid-etching, contrary to the conclusion of Retief

et al.̂ Presumably, if an unfilled resin is applied onthe etched surface, a better bond is likely to result atthe beveled margin than at the butt joint margin, pro-vided that no other factors intervene. However crazingor fracture of enamel may still be caused by contract-ing restorative resins that bond weil to the etched en-amel wall.-'

Conclusion

The present study indicated that beveling of the cerv-ical enamel margin of the proximal box of Class IIcavities would result in an oblique orientation of en-amel rods in this area, and hence a better etchingpattern after acid conditioning. This would, presum-

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ably, result in a better bond with restorative resins thanwould occur at the butt joint margin, if all other fac-tors related to bonding were favorable.

Acknowledgment

The author would hke to Ihank his colleague, Dr A. Dimmer, forher advite in preparation of the nianuscn]:il.

References

1. Buonocore MG: A simple method of increasing the adhesionof acrylic filling materials to enamel surfaces. } DenI Res195i;34:849-853.

2. Buonocore MG, Matsui A, Gwinnetl A!. Penetration of resindental materials into enamel surfaces with referente to bonding.Arch Oral Biol t968;13:61-7Ü.

3. Retief DH, Woods E. Jamison HC. et al: Surface topographyof the enamel margms of bult and beveled Class V preparations.J Pro.uhel Dem 1982;48:lú6-170.

4. Gitmore HW, Lund MR, Bales CDJ, et al: Operative Dentistry,ed 2. St Louis, CV Mosby Co, 1973, pp 158-197.

5. Hembree JH, Andrews JT: Microleakage of several acid.-etchcomposite resin systems, a laboratory study Oper Dem1976;i:91-97

6. Barnes IE; The adaptation of cotnposite resins to tooth struc-ture. Part 2. The adaptation of composite resins to etehed en-amel. Bril Dent J t977;142:t85-191

7. Hormati AA, Chan K.C: Margmal leakage of compacted gold,composite resin, and high-copper amalgam restorations. JProslhet Dem 1980;44:4t8^22.

8. Eriksen HM, Buonocore MG: Marginal leakage wilh ditTerentcomposite restorative materials in vitro. Effect of cavity design.J Oral Rehiibil 1976:3:315-322.

9. Porle A, Lutz F, Lund MR, el al: Cavjty designs Ibr compositeresins. Oper Dent t984;9:5O-56.

IÜ. Schneider PM: The effect of enamel surtaLV ri'duulion in vitroon the bonding uf cotnposite resin to permariciil liuman enamel.J Dem Rc.t 198t:fiO;895-900.

11. Kempler D, Stark MM. Leung RL, et al: Enamel-compositeinterlace relative to cavosurtace configuration, abrasion, andbonding agents. Oper Dent 1976;t:t37-145.

12. Eiiasson ST, Hill GL: Cavosurface design and marginal leakageof composite resin restorations. Oper Dem 1977;2:55-58.

13. Retief DH, Woods E, Jamison HC: Effect of cavosurface treat-ment on marginal leakage in C!ass V composite resin restora-tions. / í'roWití Dent 1982:47:496-501.

14. Hembree JH: Microleakage of microfilled composite resin res-torations with different cavosurface design. / Pro.t,the! Dem1984;52:653-6S6.

15. Grundy JR: An intr;i-iM.il replica technique for use with thescanning electron microscope. Br Dent J 1971;130:113-117.

16. Boyde A, Knight PJ, Jones SJ Further scanning electron mi-croscope studies of the preparation of Class 11 cavities. Br DentJ 1972;t32:447-456.

17. Leidal Tl, Tronstad L: Scantling electron microscopy of cavitymargins finished with ultra-speed instruments. / DenI Res1975;54:152-I59.

18. Barnes IE: The adaptation of composite resins to tooth struc-ture. Part 4. The inlluence of cavity wail instrumentation uponthe adaptation of composite resins, and discussion. Br Dem J1977:142:319-326.

19. Crawford PJM, Wittaker DK: Etching and bonding patterns inhuman sub-surface enamel. Br Dem J 1977.143:261-266.

20. Retief DH: The mechanical bond. Im Dem J 1978;28:18-27.

21. Jorgensen K.D, Asmussen E, Shimokobe H: Enamel damagescaused by contracting restorative resins. Scand J Dem Res1975;83:120-122. D

144 pages 69 illustrations S32

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