Arch

oral Biol. Vol. 22. pp 215 to 220.

Pergamon Press1977 Prmted in GreatBntain

A COMPARISON OF LASER- AND ACID-ETCHED HUMAN ENAMEL USING SCANNING ELECTRON MICROSCOPY B. D.

GOODMAN

and A. J.

GWINNETT

Department of Oral Biology and Pathology, School of Dental Medicine, S.U.N.Y. at Stony Brook, New York 11794, U.S.A. Summary--A comparison was made between human enamel surfaces treated with 50 per cent phosphoric acid and those subjected to an argon-ion laser beam. Preferential loss of rod structure and the creation of inter-rod and inter-crystal porosity were characteristic of acid etching. Pits, cracks, craters and blisters were common topological features in laser enamel. The globular, frequently coalesced enamel rods and crystals differed from the spicular nature of these structures exposed by acid pretreatment. The lack of porosity and the presence of particulate surface debris following lasing did not allow resin penetration into enamel. In contrast, long resin tags were found following acid pretreatment.

INTRODUCTION

Dental adhesives have added a new dimension to dentistry through the introduction of more rapid, simple, effective and conservative clinical procedures. Physical and chemical forces of adhesion are difficult to obtain and sustain between current dental materials and tooth tissues under oral conditions. However, Buonocore (1955) demonstrated a significant increase in the retention period of an unfilled resin to enamel which had been treated with orthophosphoric acid. A durable, mechanical bond is established by means into tissue pores created or of resin penetrating enlarged by acid trea.tment (Gwinnett and Buonocore, 1965; Gwinnett and Matsui, 1967). Some investigators have intimated that the laser could assist in bonding materials to teeth. However, attempts to obtund pits and fissures by the fusion of hydroxyapatite to sound enamel (Lobene, Bhussary and Fine, 1968) and to fuse various restorative materials to tooth struclure (Winkler, Flyn and Miller, 1973) have failed. Goodman et al. (1976) noted with scanning microscopy certain topologic similarities and differences between acid-etched and laser-etched enamel. An argon-ion laser was used. Simultaneously, Winkler et al. (1976) reported a comparison between phosphoric acid-treated enamel surfaces and those etched using a neodymium laser with a two millisecond pulse of 12.1 joules/cm’. Using optical microscopy and crystal violet dye, they found no significant differences between the two types of etched surfaces. However, the force required to dislodge resin from the acidetched enamel was much greater than for laser-etched enamel. The purpose of our study was to investigate further using the superior resolution of the scanning electron microscope. MATER1 ALS AND METHODS

Small, flat areas of 10 thoroughly cleaned human molar and incisor teeth were exposed to whole-band argon-ion laser (Model CR-5, Coherent Radiation,

Palo Alto, California) radiation with an output power of 5 W. The continual wave beam was chopped by mechanical shuttering for 0.5 s and guided to the target by a system of plane mirrors. The beam was focused by a long focal length convex lens. A photometer was used to check output wattage before each pulse was allowed to impact, thus ensuring reasonable energy density consistency (approx. 300 joules/cm*) at the enamel surface. It should be noted that, although the output wattage was kept constant, slight variations in spot size did occur due to manual focusing. Although this variation might result in energy density differences, they would be minimal because of the use of a long focal length lens. All treated surfaces were thoroughly washed with water and airdried. A preliminary study showed that, despite some chalky surface debris vigorous washing, remained. Approximately one centimetre from the lased sites, the enamel was treated for one minute with a 50 per cent phosphoric acid gel, washed for 30s and dried in air. The samples were mounted and coated with gold-palladium for examination in an A.M.R. 1000 scanning electron microscope (S.E.M.) operating at 20 kV. A preliminary experiment was also conducted to determine the extent of resin penetration (Delton, Johnson and Johnson, East Windsor, New Jersey) into both acid- and laser-etched enamel. Tags, indicative of resin penetration, were sought by an S.E.M examination of the fitting surface of the resin which was recovered following demineralization of the enamel in 10 per cent hydrochloric acid. RESULTS

Cursory visual examination of the laser impact sites revealed a chalky centre portion circumscribed by a narrow, brownish, peripheral zone. The irregularly circular sites were approximately 1.0 mm in diameter. Low magnification scanning electron microscopy confirmed the irregular outline of the impact site and the extent to which the enamel was either altered or

216

B. D. Goodman and A. J. Gwinnett

removed. The topology of the floor of the impact site varied; cracks, pits, fractures and craters were most common (Figs. 1 and 2). Close examination of the peripheral portions of the impact sites indicated apparent coalescence of enamel punctuated by small rounded elevations (Fig. 3). The central portions of these blisters were frequently fissured, probably due to bursting either at the time of lasing or subsequent specimen handling. Sub-surface enamel was exposed by cratering amidst the coalesced enamel surface (Fig. 4). The topography of these craters was irregular (Fig. 5) and shapes similar to preferentially exposed enamel rods could be seen. Small globules dominated the surface (Fig. 6). In some regions there was extensive coalescence of these globules with continuity mediated by a homogeneous matrix. Although an obvious increase in surface area was created, porosity appeared minimal, the surface being cluttered by scattered or contiguous, irregular particulate debris (Fig. 7). In contrast, acid-etched enamel surfaces were relatively uniform. Preferential loss of rod substance resulted in an increase in surface area and marked porosity (Fig. 8). The crystal structure of the rods was resolved (Fig. 9). Both the laser- and acid-etched enamel differed from normal enamel (Fig. 10). The resin surfaces related to acid-etched enamel showed extensive tags (Fig. 11) whereas these were absent in relation to lased surfaces. Resin related to lased surfaces showed irregular, shallow elevations, small spheroidal depressions and occasional irregular debris (Fig. 12). DISCUSSION

Laser-induced chalky enamel areas have been reported by other investigators. Lobene, Bhussary and Fine (1968) and Kantola, Laine and Tarna (1973) showed them to be due to the formation of cc-calcium orthophosphate. As a temperature of at least 1400°C (Van Wazer, 1958) is required to mediate this conversion and as our 5 W argon laser only produced an absorbance of between 5-10 per cent in enamel, this temperature probably would not be generated. It was concluded that the chalky material in our samples was principally disrupted, dessicated enamel. This conclusion was supported by X-ray diffraction data (Goodman and Kaufman, 1977) in which no forms other than apatite were found in scrapings from lased teeth and lased enamel powder using the same argonion laser. The blistered, crusted portions of contrasting brownish colouration probably represent inorganic as well as charred organic integument not removed during prophylaxis. Along with the particles of variable size and geometry, a “sugar deposit” effect was produced despite vigorous washing. This globular, coalesced nature of the elements comprising the enamel rods contrasted markedly with the discrete, spicular nature of these components exposed by acid etching. A significant difference was noted in the amount of surface porosity. Although an increase in surface area was produced by both types of surface treatment, heat generated by lasing apparently melted and coalesced some of the exposed crystals producing a relatively non-porous surface. A

similar observation was reported by Stern, Vahl and Sognnaes (1972) using a carbon dioxide laser. The consequence of acid attack produced extensive intercrystal and inter-rod porosity by dissolution of organic and inorganic components. Porosity was also present in the tissue. This was manifest by the production of numerous, detailed tags of resin. These were absent following laser treatment in which the resin simply replicated topologic characteristics similar to those observed by direct examination of lased enamel. This observation would explain the findings of Winkler et al. (1976) in which bond strengths for acidetched enamel were markedly superior to those for lased enamel. Penetration into enamel is a key to the resin bond strength (Gwinnett and Buonocore 1965). Furthermore, surface contamination with loosely attenuated debris would not be conducive to good bonding. Lasers at present appear to have limitations. The beam spot size is kept relatively small to minimize thermal histologic damage, a property of considerable significance clinically (Stern, Reagen and Howell, 1969). A threshold dose of approximately 2,000 joules/ cm’ (i = 6943 A) from a 1.3 msec pulse has been established for the tooth pulps of dogs (Adrian, Bernier and Sprague, 1971). The preparation of a multitude of impact sites dictated by the small beam would be tedious and, if localized bonding foci were established, this would probably lead to strain and resin failure. Current dental resin materials, with few exceptions. can only be bonded by tissue penetration. These considerations lead us to conclude that laser etching, in particular argon-ion laser etching, is neither practical, desirable nor an economically sound means by which available resins can be bonded to enamel surfaces. Acknowledgrments-We wish to express thanks to Drs H. Metcalf and L. Wilcox for their assistance and cooperation in the use of the laser housed in the Department of Physics at Stony Brook. REFERENCES

Adrian J. C., Bernier J. L. and Sprague W. G. 1971. Laser and the dental pulp. J. Am. dent. Ass. 83, 113-I 17. Buonocore M. G. 1955. A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces. J. dent. Res. 34, 849-853. Goodman B. D., Kaufman H. W., Gwinnett A. J. and Metcalf H. 1976. Laser induced mechanisms for caries nrevention. Intemat. Ass. for Dent. Res. Preprinted abstracts. 54th General Meeting. Abstract 780. Goodman B. D. and Kaufman H. W. 1977. Effects of an argon laser on the crystalline properties and rate of dissolution in acid of tooth enamel in the presence of sodium fluoride. JI. dent. Rrs. In press. Gwinnett A. J. and Buonocore M. G. 1965. Adhesives and caries prevention. Br. dent. J. 119, 77-80. Gwinnett A. J. and Matsui’ A. 1967. A study of enamel adhesives. The physical relationship between enamel and adhesive. Arch oral Biol. 12, 1615-1620. Kantola S., Laine E. and Tarna T. 1973. Laser induced effects on tooth structure. VI. X-ray diffraction study of dental enamel exposed to a CO* laser. Actu odont. stand. 31, 369-379. Lobene R. R.. Bhussary B. R. and Fine S. 1968. Interaction of carbon dioxide laser radiation with enamel and dentin. J. dent. Res. 47, 311-317.

SEM of laser and acid-etched enamel Stern R. H., Reagen H. L. and Howell F. V. 1969. Laser effect on vital dental pulps. Br. dent. J. 127, 2628. Stern R. H., Vahl J. and Sognnaes R. F. 1972. Lased enamel. Ultrastructural observations of pulsed carbon dioxide laser effects. J. dent. Res. 51, 455460. Van Wazer J. R. 1958. Phosphorus and its Compounds. Vol. 1. pp. 526527. Interscience, New York.

211

Winkler S., Flyn K. L. and Miller W. A. 1973. Neodymium laser fusion of restorative materials to tooth structure. N.Y. State dent. J. 39, 614618. Winkler S., Patregnani A. M., Carter M. J., Hill R. M. and Miller W. A. 1976. Laser etching of human enamel. Internat. Ass. for Dent. Res. Preprinted abstracts. 54th Genera1 Meeting, Abstract 331.

Plates 1 and 2 overleaf

B. D. Goodman

218

and A. J. Gwinnett

Plate Figs.

1 and 2. Impact

Fig. 3. Peripheral

sites on enamel

portion

Fig. 4. An area in which

Fig. 5. The floor

in which

of an impact

a crater

of a crater

Fig. 6. The rod-like

1

the laser produced x 60

site showing small craters.

numerous x 500

blisters

of exposed sub-surface enamel enamel. x 200

in which

structures

the topology resembles loss of structure. x 2000 are composed

craters,

blisters,

(arrows)

is surrounded

enamel

of numerous

rods

globules

cracks

amidst

and

pits.

which

are

by coalesced,

exposed

(arrow).

pitted

by preferential

x 5000

Plate 2 Fig. 7. Coalesced

Fig. 8. Etching

globular

enamel

structures

character

Fig.

and grooves

12. Rounded

elevations

debris

preferential x 2000

(arrow)

dominating

an otherwise its penetration

smooth

untreated

into acid-etched

can be seen.

loss of rod structure

of the enamel rods and the inter-rod and inter-crystal teristic feature of acid etching. x 5000

Fig. 11. Long taps of resin manifesting Fig.

and contiguous

with 50 per cent phosphoric acid produced with the appearance of typical arcades.

Fig. 9. The spicular

10. Micropits

upon which scattered x 5000

porosity;

enamel

surface.

enamel.

(arrows) and small spheroidal depressions suggesting replicated the lased enamel surface. x 2000

a charac-

that

x 5000

x 2000 resin simply

SEM of laser and acid-etched

Plate 1.

enamel

219

220

B. D. Goodman

and A. J. Gwinnctt

Plate 2.