Australian Dental Journal, December, 1975
355
DECEMBER, 1975
Volume 20 :: Number 6
An adhesive paint-on restorative for tooth surface defects Henry L. Lee, Ph.D. Jan A. Orlowski, Ph.D. AND
Enayda M. Enabe, D.D.S.
Asmcr-The physical properties of an adhesive paint-on restorative material have been examined and it is shown that the adhesion to enamel is good, its compressive strength about equal to silicate, hardness is less than for composite resins, and stain resistance is good. (Received for publication July, 1974. Revised June, 1975)
lntroductlon
A rapid and simple treatment of enamel surface defects, as opposed to cavity restorations, has not been available to the dental profession until the recent introduction of a new kind of paint-on restorative’ to which the term “adhesive restorative dentistry” is being applied. Since sound tooth structure is not sacrificed the method is conservative and a superior substitute for the classical methods for Class V restorations, and in the treatment of buccal surface defects. The defects that would be aided by such a restorative include cervical erosion, fluorosis, tetracycline
* Enamelite. 91733.
Lee Pharmaceuticals, South El Monte, Calif.
stains and hypocalcifications. The classical treatment of anterior stains due to fluorosis, tetracycline administration, or hypocalcification involves the use of porcelain jacket crowns, an expensive method which entails the loss of sound tooth structure. Bleaching of stains by means of potent combinations of peroxides and acids does not in general afford relief over the long term. The disadvantages of standard methods of treatment together with the limitations in the physical and aesthetic qualities of some of the materials used encouraged search for a better procedure, preferably one using a minimum of tooth preparation.
3 56
A paint-on restorative, applied after a minimum of surface treatment, would be painless, rapid and easy to apply, would conserve sound tooth structure and could halt the erosion. Three papers dealing with research in such treatment of enamel defects were found in the literature. Sanjol investigated an adhesive restorative material based on a polymethyl methacrylate. He found that when abraded, eroded, or fractured surfaces were immediately filled without preparation, 70 per cent failures occurred within one month. When the soft dentin was removed with a spoon excavator before restoration, 47 of 55 restorations were intact after five months. When the areas were given a V-shape without retentive features, only 6 of 109 failed at the end of 7 months. It is significant that these results were obtained without pretreatment of the enamel with acids, and with a resin system that has high polymerization shrinkage. In 1971 Laswell, Welk and Regenosz concluded on the basis of clinical observations over an 11 year period on the use of polymethyl methacrylate for the restoration of incisal fractures and developmental enamel defects, that the procedure offered a conservative approach to these problems. Also in 1971, Groper3 reported, without clinical details the use of an acid etch to eliminate the need, in selected cases, for conventional mechanical retention when using the newer composite restoratives. All of these restoratives can be used only in bulk, since they were not designed for use as a thin film that can adhere to the enamel surface and to satisfy other stringent performance requirements. Special requirements of paint-on material
An ideal dental restorative that is to be used as a thin film to cover enamel defects by a conservative treatment, in addition to being nonirritating and non-toxic, would need to have the following properties: (i) Adhesiveness. A thin film must be able to adhere to tooth structure. Currently, good adhesion can be obtained to enamel, but it would also be desirable if this material would show
studies on a cold-curing dental resin with a tri-n-butylborane catalyst. J. D . Res.. 5 0 1 , 60-65 (Jan.-Feb.) 1971. 2 Laswell. H., Welk. D. A.. and Regenos, 1. W.-Attachment of resin restorations to acid pretreated enamel. J.A.D.A., 82:). 558-563 (March) 1971. 3 Groper, J.-The etch technique for anterior tooth restoration. J. South Calif. State D.A.. 39:9, 756758 (Sept.) 1971. 1 Sanjo D.-Clinical
Australian Dental Journal, December, 1975 adhesion to dentin. Areas of deep erosion in dentin can be covered with this material in such a way that a film extends over the adjoining enamel. If insufficient enamel is present, a few retentive areas should be made. Zisman' explained the importance of surface wetting by the adhesive for adhesion to occur. The wetting ability of a liquid may be measured by the contact angle it makes with a surface. In turn, the ability of a surface to be wetted is measured by the critical contact angle which is determined by extrapolating a plot of the cosine of the contact angle vs. the surface tension of a series of homologous liquids to a value of contact angle of zero. A liquid will wet a surface if its surface tension is less than the critical surface tension of the surface. The increase in adhesion because of etching is considered to be partly due to mechanical interlocking of the roughened surface with the cured resin, but mostly due to modifications in the surface properties of the enamel, ie., an increase not only in the surface area but also in the critical surface tension of wetting. Mechanical interlocking was indicated by the experiments by Buonocore, Matsui and Gwinnetts, Gwinnett and Matsuis and also by Sharp and Grenoble?, which showed tag-like extensions protruding into the enamel up to about 25 microns. The increase in critical surface tension is shown by the increase in the wettability of enamel by water, which makes a contact angle of 70" before etching, but spreads spontaneously after etching. Another increase in adhesion is obtained by the removal of surface moisture, e.g., Uy and Changu have shown that the critical surface tension of enamel increases from 31.5 to 38.5-40 dynes/cm when the relative humidity is decreased from 100 to 50 per cent.
4 Zisman, W. A.-Constitutional
effects on adhesion and abhesion. In, Adhesive restorative dental materials. Proceedings of NIDR Workshop. Washington, U S . Dept. H.E.W.. Public Health Service, 1961. 5 Buonocore, M. G.. Matsui. A.. and Gwinnett. A. 1.Penetration of resin dental materials into enamel surfaces with reference to bonding. Arch. Oral Biol., 13:l. 61-70 (Jan.) 1968. GGwinnett, A. 1. and Matsui, A.-A study of enamel adhesives. The physical relation between enamel and adhesive. Arch. Oral Bbl.. 12:12, 1615-1620 (Dec.) 1967. 'ISharp, E. C., and Grenoble, D . E.-Dental resin penetration into acid etched subsurface enamel. 1. South. Calif. D . A . , 39:Y. 741-746 (Sept.) 1971. 8Uy. K. C., and Chang. R.-Approaches to achieve adhesion. In, Adhesive restoration dental materials 11.. Proceedings of NIDR Workshop, Washington, U.S. Dept. H.E.W.. Public Health Service. 1965.
357
Australian Dental Journal, December, 1975 These facts illustrated the importance of etching and drying the enamel surface prior to application of the paint-on adhesive. (ii) Consistency. Since the restorative is to be applied by a paint-on or flow technique. the material must flow in a controlled manner over the surface only where it is intended to be used. Hence, the viscosity and flow characteristics are of great importance. (iii) Mechanical properties. The restorative must possess sufficient mechanical strength to withstand mastication and to resist wear. The compressive and tensile strengths of composites are increased by the incorporation of fillers, but this increase must be balanced against the stringent flow requirements of the uncured material. (iv) Optical properties. In order to cover the defects, the restorative should have adequate hiding power, but at the same time, the thin coating should have the appearance of tooth enamel in either reflected or transmitted light. The restorative must have good colour stability under ultra violet radiation, and good stain resistance against colouring matter from materials in the oral environment. (v) Polymerization shrinkage. The shrinkage on curing of the resin should be as low as possible since a change in the dimensions of the coating can affect the adhesive bonds to the tooth structure and could open up the margins. (vi) Thermal properties. The thermal expansion should be close to that of the tooth; otherwise the bonds between the restorative and tooth structure could be broken, leading to marginal leakage and in extreme cases, to loss of the coating. The thermal conductivity and thermal diffusivity should be low to insulate the pulp from thermal shock and to protect margin integrity. (vii) Water absorption and solubility. Absorp tion of water can affect other properties, such as mechanical properties, dimensional integrity, wear resistance, solubility, colour, stain resistance and adhesion. Hence, the extent of water absorption should be as small as possible. Low water solubility is required not only to preserve the margins, but also to retain the filler particles in the resin matrix. Loss of filler would deteriorate the wear of the surface resulting in an eventual change in the contour of the applied coating. The loss of minor but vital additives could change certain properties, e.g., the loss of ultra violet absorbers could affect colour stability. (viii) Finishing and polishing. It should be possible to complete the finishing and polishing
soon after the coating has set. In addition, the polishing operation should not result in a rough surface, a problem with many composite restorations 0.10, 11. Materials and methods
The objectives of this paper are 1) to compare the physical properties of this coating material to those of an unfilled resin* which u p to now has also been used for the conservative treatment of enamel defects, and to those of three restoratives which require cavity preparations, i.e., filled composite pastest, silicate cements, and amalgam, and 2) to analyze and explain the importance of specific properties on performance. The materials were tested for adhesion to enamel, gel and set times, polymerization shrinkage, thermal expansion, thermal conductivity (by means of a Cenco-Fitch Thermal Conductivity apparatus), compressive and tensile strengths, hardness, water absorption and solubility, chemical resistance, colour stability, resistance to staining, hiding power, wear resistance, and finishing and polishing. Hiding power was determined as follows: by means of a Boston Bradley Adjustable Doctor Blade, films 76 mm wide were drawn down over a Morest Hiding Power Chart Form 09. The reflectance of each film was measured with a Gardner XGlO Colour Difference Meter, first over the black portion of the chart and then over the white portion. The hiding power is the ratio of the reflectance over black to the reflectance over white. For finishing and polishing a sample composite was allowed to set under a pulled mylar matrix strip and polished with a diamond bur in one set of teeth. In another set, a 12-fluted carbide bur was used. The teeth were finished using pumice powder and a rubber cup. Examinations of the surface were made visually and with a scanning electron microscope. Specimens were prepared as described and cured for one week in a water bath at 37°C. All tests were conducted at 25°C unless otherwise noted.
*
t
Sevriton. Amalgamated Dental Trade Distributors, Ltd. Adaptic, Johnson and Johnson, Inc.
H. H., Bowen, R. L and Paffenbarger, G. C.Mcthoh for finishing comsosimte mtorative materials. J.A.D.A. 83:2. 344-348 (Aug.) 1971. 10Dewison. J. B., and Craig, R. G.-Physical properties and finished surface texture of composite restorative resins. JA..D.A.. 8 5 2 , 101-108 (Aug.) 1972. 11 Johnson. L. N.. Jordan R. E., and Lynn, J. A.-Effects of various finishing dlvices o n resin surfaces. J.A.D.A.. 83:2, 321-331 (Aug.) 1971.
9 Chandler
TABLE 1
Summary of comparative data on paint-on adhesive and other restoratives Property
Painton
silica-5Ile-l
=-tic/ aliphatic diacrylates
Adhesion to enamel, psi Gel time Set time Polymerization shrinkage, %
1100-1300 110-150 sec 150-170 wc 1.22
Coefficient of thermal expansion, ppm/"C Thermal conductivity, cal/T/cm/sec x 10-4 Thermal dfisivity, cmZ/sec x 10-4 Compressive stiength, psi Tensile strength, psi Hardness Water absorption, mg/cmZ
90-92 (H) 0.46
Quartzrfillcd al.OtMtiC
diacrylatg aliphatic monoacrylaks
300-800 120-140 sec 160-180 S ~ C 0.5
UdiUcp aliphahc acrylatef polyacrylate",
920
-
5 min 2.00
Amalgam-,
1J
Silicate",
=
=
none
none
3 min 20 min*
1-2 min 3-8 min* 4 . 3 to 0.5
expands upn settmg
30
28
22
22.1-28
7.6
12.6
-
5
550
18
3.2 24,000 6,000
30,000 6,250
12 12,000
920 57,000
23-2 23,000
8,500 90 (KHN)
440-1000 70 (KHN)
none
dissolves a t 1-370 per week dissolves at 1-3% per week
97-98 (H) 0.43
too soft to measure 46 (HI 3.00
Water absorption, wt %
0.75
0.80
5.85
none
Water solubility, mg/cm2 Water solubility, wt % Chemical resistance, wt loss % 5% acetic acid, 120 hrs. 5% citric acid, 120 hrs. 45% ethanol, 72 hrs. Color stability, NBS units Stain resistance; color change, visual Wear resistance, gm/rnz/hr Wear resistance, p / h r
0.27 0.50
0.13 0.20
0.68 0.85
none none
0.6 0.03 0.2 3.3
1.4 0.02 0.4 3.5
0.8
moderate
marked
slight
2.8 1.8
10.60 7.55
16.5 21.60
Final physical properties not attained at this time.
11.5
1.10 2.00
Australian Dental Journal, December, 1975 Resulh and discussion A comparison of the properties of this paint-on adhesive with those of four other restoratives is shown in Table 1. It should be borne in mind that this adhesive is to be applied as a thin film and hence some of its properties will differ from those of the usual restoratives that are used in bulk. The adhesive strength of the coating, when used with the adhesion booster, is higher than that of an unfilled acrylate and of a quartz-filled acrylate. Silicates and amalgams do not adhere and hence require special tooth preparations for mechanical retention. The paint-on restorative has a gel time of 110-150 sec, and a set time of 150-170 sec, which are similar to the values for the filling composite restoratives, but shorter than those for amalgams and silicate cements. The introduction of filler greatly reduces the shrinkage of composites. In the paint-on restorative, the filler/resin weight ratio is low relative to other filled dental composites (1.3:l vs. an average of 3.5:l) in order to enhance its flow characteristics, but due to the thin film application of this coating material, shrinkage would be from the free film surface and thus higher shrinkages than in composites used in bulk can be tolerated without affecting either adhesion or marginal seal. The value of the thermal coefficient of linear expansion for the paint-on adhesive is very close to that for the quartz-filled acrylic and to that of amalgam. It is very high for unfilled acrylic resin, and very low for silicateslz. The data for the thermal conductivity and thermal diffusivity listed on Table 1 may be compared to the corresponding values for enamel, namely 22 x 1 V cal/"C cm sec and 40 x 1V cm2/sec13. The fairly close match of the values for the composites and silicates indicate that use of these materials will not subject the pulp to unduly high thermal shock, and will not create high thermal stresses at the margins. However, the very high values for amalgam14 show that temperature fluctuations in the oral environment will be transmitted to the tooth structure much more efficiently than by the other restoratives.
to Dental Materials and Devices. Chicago, American Dental Association, 7th ed.. 1974 (p. 54). 13 Civjan, S.. Barone. J. J.. Reinke, P. E.. and Selting. W. J.-Tbermal properties of aomatrllrc restorative mluterials. J.D. Res.. 51:4, 1030-1037 (July-Aup.) 19R.
12Guide
.
I4 Souder W
and Paffenbarger, 0 C.-Physical properties of d i n k ' materials. Washingtdn, National Bureau of Standards, Circular No. ( 3 3 3 , 1942.
359 The compressive strength of composites depends on the kind of filler particles, loading ratio, type resin binder and bonding forces between filler and resin matrix. In the paint-,on adhesive, the filler content is limited by the stringent flow characteristics of the uncured composite. Hence, its compressive strength (24,000 psi) is lower than that of the quartz-filled acrylate, but higher than that of the unfilled acrylic resin at 12,000 psi. The value for the paint-on restorative is completely adequate for the purpose for which the coating is intended, namely for application to the surfaces of teeth which are not subjected to the same compressive stresses as the occlusal or proximal surfaces. The ultimate tensile strength, determined at 25"C, of the paint-on restorative after exposure in 37°C for either 24 hours or 30 days was 6,000 psi. These and other data show the rapidity with which this restorative attains its maximum tensile strength. The obvious advantage of reaching maximum strength quickly is that the danger of mechanical failure is greatly lessened. The Rockwell hardness (H) of the paint-on restorative after 1 hour at 25' and after 24 hours at 37°C are 90-92 and 94-96, respectively. Restoratives with a higher filler content have a slightly higher hardness, but the unfilled acrylate is much softer at 46. Water absorption values for all the filled composite restoratives tested are quite low; in fact, insignificant when compared with the silicate cements, which dissolve at the rate of 1 to 3 per cent per week so that absorption measurements are not possible. The unfilled acrylate absorbs 5.85 wt per cent, or approximately eight times as much as the paint-on restorative. The results show that the paint-on adhesive has one of the lower solubilities of restorative materials. The high resistance of the paint-on adhesive is indicative of its durability in the oral environment. The quartz-filled acrylate showed higher weight losses in the acetic acid and ethanol solutions and approximately the same in citric acid. After one hour in boiling water, the hardness of the paint-on restorative and the filled acrylate was essentially unchanged. In both cases, the compressive strength of samples cured for one hour at 37°C increased, from 1500 to 1680 kg/cms for the paint-on restorative and from 2520 to 2650 kg/cmz for the filled acrylate. Discolourations observed visually were judged very slight for the paint-on restorative and slight for the filled acrylate. Visual inspection of the composites after staining showed the paint-on
360
Australian Dental Journal, December, 1975
composite and the acrylic resin experienced slight to moderate colour changes, but the filled acrylic composite underwent marked changes. The hiding power, or contrast ratio, is given in the last column of Table 2. Inspection of these data show that the hiding power increases with increasing film thickness. TABLE 2
Reflectance measurements of the paint-on restorative ~
Film thickness before cm
Reflectance over black
Reflectance over white
Contrast ratio
0.008 0.015 0.023 0.030 0.038 0.05 1
2.5 4.0 4.6 9.4 10.8 10.8
79.5 76.0 76.8 72.0 76.0 67.5
0.03 1 0.052 0.060 0.130
0.142 0.160
The unfilled acrylic resins wear at a much faster rate than the filled composites. Examination of the brushed samples using the scanning electron microscope show that filler hardness has an influence on wear resistance15. The SEM photographs show that in the case of the acrylate filled with the very hard filler, quartz, the polymer matrix wears away, exposing filler particles which are pulled from the surface, exposing a rough polymer surface, which can then be worn more rapidly. The paint-on restorative contains a somewhat softer filler and consequently wears more evenly and hence more slowly.
The results of Eames and O’NeallB, who subjected a number of restorations to a reciprocating brushing action for 448,000 strokes, showed that the abrasion volume loss (CC x 103) was 78.00 for the paint-on restorative, 174.60 for the acrylic resin, and 2.20 for the filled acrylate. The composites abraded more rapidly than amalgam using impact or reciprocating hard heads, but amalgam abraded more under the reciprocal brushing action. Some wear is expected in the paint-on restorative used for idiopathic erosion, as would be for any restorative material in that location. The service life may well be determined by the thickness of the coating. Finishing and polishing Visual examination of surfaces in which a mylar strip was used to hold the paint-on restorative in place, showed that this surface was very smooth and glossy. This material showed better polishing properties compared with the other composites and the unfilled diacrylate. A marked difference was noted between the surfaces finished with the paint-on adhesive and with the filled acrylate. The surface of the latter was very rough due to the hard quartz particles which had been abraded, a point which has been discussed previously. The 12-fluted carbide bur, as shown from the SEM results, gives a better finish to composites than the use of the diamond bur. Conclusion
15 Lce, H. L.. Orlowski, J. A.. Glace, R. W., Kidd, P. D..
and Enabe. E.-Evaluation of wear resistance of dental restorative materials. Z , Advances in polymer friction and wear, Edit. Lee, L. New York. Plenum Publishing Co., Vol. 5B. 1974 @p. 705-723). 16 Eames, W. B., and O’Neal, S. J.-A comparison of three abrasion test methods. IADR Abstracts. Abstract 28. J. D. Res., 52:Special issue, 65 (Fcb.) 1973. 1T Retief, D . H.. Cleaton-Jones. P. E., and Austin, J. C.Pulpal response to a new composite dental restorative material, J. Oral Path., 2 4 , 215,221, 1973. 18 I b m , R. L.-Non-operative treatment of gingival erosion. Dcnt. Survey, 48:3, 22-24 (March) 1972. 19 Portnoy. L. L.-Retention studies of composite coating for gingival erosion. Dent. Survey, 5011. 28-30 (Nov.) 1974. 20 Stuart, R.-Treatment of anterior teeth for aesthetic problems. Quinteswnz Internat. In press.
The need for a paint-on restorative has been shown for tooth surfaces. The paint-on restorative described in this paper is suitable for use in coating of stained enamel and repair of certain types of cervical erosion by an adhesive technique employing 50 per cent phosphoric acid without the need for mechanical preparation of retentive forms. It is formulated especially for application by a flow-on technique. Numerous published clinical reports attest to its satisfactory in vivo properties 1% 18, 19, m. Lee Pharmaceuticals, P.O. Box 3836, South El Monte, Calif., 91733, U.S.A.
355
DECEMBER, 1975
Volume 20 :: Number 6
An adhesive paint-on restorative for tooth surface defects Henry L. Lee, Ph.D. Jan A. Orlowski, Ph.D. AND
Enayda M. Enabe, D.D.S.
Asmcr-The physical properties of an adhesive paint-on restorative material have been examined and it is shown that the adhesion to enamel is good, its compressive strength about equal to silicate, hardness is less than for composite resins, and stain resistance is good. (Received for publication July, 1974. Revised June, 1975)
lntroductlon
A rapid and simple treatment of enamel surface defects, as opposed to cavity restorations, has not been available to the dental profession until the recent introduction of a new kind of paint-on restorative’ to which the term “adhesive restorative dentistry” is being applied. Since sound tooth structure is not sacrificed the method is conservative and a superior substitute for the classical methods for Class V restorations, and in the treatment of buccal surface defects. The defects that would be aided by such a restorative include cervical erosion, fluorosis, tetracycline
* Enamelite. 91733.
Lee Pharmaceuticals, South El Monte, Calif.
stains and hypocalcifications. The classical treatment of anterior stains due to fluorosis, tetracycline administration, or hypocalcification involves the use of porcelain jacket crowns, an expensive method which entails the loss of sound tooth structure. Bleaching of stains by means of potent combinations of peroxides and acids does not in general afford relief over the long term. The disadvantages of standard methods of treatment together with the limitations in the physical and aesthetic qualities of some of the materials used encouraged search for a better procedure, preferably one using a minimum of tooth preparation.
3 56
A paint-on restorative, applied after a minimum of surface treatment, would be painless, rapid and easy to apply, would conserve sound tooth structure and could halt the erosion. Three papers dealing with research in such treatment of enamel defects were found in the literature. Sanjol investigated an adhesive restorative material based on a polymethyl methacrylate. He found that when abraded, eroded, or fractured surfaces were immediately filled without preparation, 70 per cent failures occurred within one month. When the soft dentin was removed with a spoon excavator before restoration, 47 of 55 restorations were intact after five months. When the areas were given a V-shape without retentive features, only 6 of 109 failed at the end of 7 months. It is significant that these results were obtained without pretreatment of the enamel with acids, and with a resin system that has high polymerization shrinkage. In 1971 Laswell, Welk and Regenosz concluded on the basis of clinical observations over an 11 year period on the use of polymethyl methacrylate for the restoration of incisal fractures and developmental enamel defects, that the procedure offered a conservative approach to these problems. Also in 1971, Groper3 reported, without clinical details the use of an acid etch to eliminate the need, in selected cases, for conventional mechanical retention when using the newer composite restoratives. All of these restoratives can be used only in bulk, since they were not designed for use as a thin film that can adhere to the enamel surface and to satisfy other stringent performance requirements. Special requirements of paint-on material
An ideal dental restorative that is to be used as a thin film to cover enamel defects by a conservative treatment, in addition to being nonirritating and non-toxic, would need to have the following properties: (i) Adhesiveness. A thin film must be able to adhere to tooth structure. Currently, good adhesion can be obtained to enamel, but it would also be desirable if this material would show
studies on a cold-curing dental resin with a tri-n-butylborane catalyst. J. D . Res.. 5 0 1 , 60-65 (Jan.-Feb.) 1971. 2 Laswell. H., Welk. D. A.. and Regenos, 1. W.-Attachment of resin restorations to acid pretreated enamel. J.A.D.A., 82:). 558-563 (March) 1971. 3 Groper, J.-The etch technique for anterior tooth restoration. J. South Calif. State D.A.. 39:9, 756758 (Sept.) 1971. 1 Sanjo D.-Clinical
Australian Dental Journal, December, 1975 adhesion to dentin. Areas of deep erosion in dentin can be covered with this material in such a way that a film extends over the adjoining enamel. If insufficient enamel is present, a few retentive areas should be made. Zisman' explained the importance of surface wetting by the adhesive for adhesion to occur. The wetting ability of a liquid may be measured by the contact angle it makes with a surface. In turn, the ability of a surface to be wetted is measured by the critical contact angle which is determined by extrapolating a plot of the cosine of the contact angle vs. the surface tension of a series of homologous liquids to a value of contact angle of zero. A liquid will wet a surface if its surface tension is less than the critical surface tension of the surface. The increase in adhesion because of etching is considered to be partly due to mechanical interlocking of the roughened surface with the cured resin, but mostly due to modifications in the surface properties of the enamel, ie., an increase not only in the surface area but also in the critical surface tension of wetting. Mechanical interlocking was indicated by the experiments by Buonocore, Matsui and Gwinnetts, Gwinnett and Matsuis and also by Sharp and Grenoble?, which showed tag-like extensions protruding into the enamel up to about 25 microns. The increase in critical surface tension is shown by the increase in the wettability of enamel by water, which makes a contact angle of 70" before etching, but spreads spontaneously after etching. Another increase in adhesion is obtained by the removal of surface moisture, e.g., Uy and Changu have shown that the critical surface tension of enamel increases from 31.5 to 38.5-40 dynes/cm when the relative humidity is decreased from 100 to 50 per cent.
4 Zisman, W. A.-Constitutional
effects on adhesion and abhesion. In, Adhesive restorative dental materials. Proceedings of NIDR Workshop. Washington, U S . Dept. H.E.W.. Public Health Service, 1961. 5 Buonocore, M. G.. Matsui. A.. and Gwinnett. A. 1.Penetration of resin dental materials into enamel surfaces with reference to bonding. Arch. Oral Biol., 13:l. 61-70 (Jan.) 1968. GGwinnett, A. 1. and Matsui, A.-A study of enamel adhesives. The physical relation between enamel and adhesive. Arch. Oral Bbl.. 12:12, 1615-1620 (Dec.) 1967. 'ISharp, E. C., and Grenoble, D . E.-Dental resin penetration into acid etched subsurface enamel. 1. South. Calif. D . A . , 39:Y. 741-746 (Sept.) 1971. 8Uy. K. C., and Chang. R.-Approaches to achieve adhesion. In, Adhesive restoration dental materials 11.. Proceedings of NIDR Workshop, Washington, U.S. Dept. H.E.W.. Public Health Service. 1965.
357
Australian Dental Journal, December, 1975 These facts illustrated the importance of etching and drying the enamel surface prior to application of the paint-on adhesive. (ii) Consistency. Since the restorative is to be applied by a paint-on or flow technique. the material must flow in a controlled manner over the surface only where it is intended to be used. Hence, the viscosity and flow characteristics are of great importance. (iii) Mechanical properties. The restorative must possess sufficient mechanical strength to withstand mastication and to resist wear. The compressive and tensile strengths of composites are increased by the incorporation of fillers, but this increase must be balanced against the stringent flow requirements of the uncured material. (iv) Optical properties. In order to cover the defects, the restorative should have adequate hiding power, but at the same time, the thin coating should have the appearance of tooth enamel in either reflected or transmitted light. The restorative must have good colour stability under ultra violet radiation, and good stain resistance against colouring matter from materials in the oral environment. (v) Polymerization shrinkage. The shrinkage on curing of the resin should be as low as possible since a change in the dimensions of the coating can affect the adhesive bonds to the tooth structure and could open up the margins. (vi) Thermal properties. The thermal expansion should be close to that of the tooth; otherwise the bonds between the restorative and tooth structure could be broken, leading to marginal leakage and in extreme cases, to loss of the coating. The thermal conductivity and thermal diffusivity should be low to insulate the pulp from thermal shock and to protect margin integrity. (vii) Water absorption and solubility. Absorp tion of water can affect other properties, such as mechanical properties, dimensional integrity, wear resistance, solubility, colour, stain resistance and adhesion. Hence, the extent of water absorption should be as small as possible. Low water solubility is required not only to preserve the margins, but also to retain the filler particles in the resin matrix. Loss of filler would deteriorate the wear of the surface resulting in an eventual change in the contour of the applied coating. The loss of minor but vital additives could change certain properties, e.g., the loss of ultra violet absorbers could affect colour stability. (viii) Finishing and polishing. It should be possible to complete the finishing and polishing
soon after the coating has set. In addition, the polishing operation should not result in a rough surface, a problem with many composite restorations 0.10, 11. Materials and methods
The objectives of this paper are 1) to compare the physical properties of this coating material to those of an unfilled resin* which u p to now has also been used for the conservative treatment of enamel defects, and to those of three restoratives which require cavity preparations, i.e., filled composite pastest, silicate cements, and amalgam, and 2) to analyze and explain the importance of specific properties on performance. The materials were tested for adhesion to enamel, gel and set times, polymerization shrinkage, thermal expansion, thermal conductivity (by means of a Cenco-Fitch Thermal Conductivity apparatus), compressive and tensile strengths, hardness, water absorption and solubility, chemical resistance, colour stability, resistance to staining, hiding power, wear resistance, and finishing and polishing. Hiding power was determined as follows: by means of a Boston Bradley Adjustable Doctor Blade, films 76 mm wide were drawn down over a Morest Hiding Power Chart Form 09. The reflectance of each film was measured with a Gardner XGlO Colour Difference Meter, first over the black portion of the chart and then over the white portion. The hiding power is the ratio of the reflectance over black to the reflectance over white. For finishing and polishing a sample composite was allowed to set under a pulled mylar matrix strip and polished with a diamond bur in one set of teeth. In another set, a 12-fluted carbide bur was used. The teeth were finished using pumice powder and a rubber cup. Examinations of the surface were made visually and with a scanning electron microscope. Specimens were prepared as described and cured for one week in a water bath at 37°C. All tests were conducted at 25°C unless otherwise noted.
*
t
Sevriton. Amalgamated Dental Trade Distributors, Ltd. Adaptic, Johnson and Johnson, Inc.
H. H., Bowen, R. L and Paffenbarger, G. C.Mcthoh for finishing comsosimte mtorative materials. J.A.D.A. 83:2. 344-348 (Aug.) 1971. 10Dewison. J. B., and Craig, R. G.-Physical properties and finished surface texture of composite restorative resins. JA..D.A.. 8 5 2 , 101-108 (Aug.) 1972. 11 Johnson. L. N.. Jordan R. E., and Lynn, J. A.-Effects of various finishing dlvices o n resin surfaces. J.A.D.A.. 83:2, 321-331 (Aug.) 1971.
9 Chandler
TABLE 1
Summary of comparative data on paint-on adhesive and other restoratives Property
Painton
silica-5Ile-l
=-tic/ aliphatic diacrylates
Adhesion to enamel, psi Gel time Set time Polymerization shrinkage, %
1100-1300 110-150 sec 150-170 wc 1.22
Coefficient of thermal expansion, ppm/"C Thermal conductivity, cal/T/cm/sec x 10-4 Thermal dfisivity, cmZ/sec x 10-4 Compressive stiength, psi Tensile strength, psi Hardness Water absorption, mg/cmZ
90-92 (H) 0.46
Quartzrfillcd al.OtMtiC
diacrylatg aliphatic monoacrylaks
300-800 120-140 sec 160-180 S ~ C 0.5
UdiUcp aliphahc acrylatef polyacrylate",
920
-
5 min 2.00
Amalgam-,
1J
Silicate",
=
=
none
none
3 min 20 min*
1-2 min 3-8 min* 4 . 3 to 0.5
expands upn settmg
30
28
22
22.1-28
7.6
12.6
-
5
550
18
3.2 24,000 6,000
30,000 6,250
12 12,000
920 57,000
23-2 23,000
8,500 90 (KHN)
440-1000 70 (KHN)
none
dissolves a t 1-370 per week dissolves at 1-3% per week
97-98 (H) 0.43
too soft to measure 46 (HI 3.00
Water absorption, wt %
0.75
0.80
5.85
none
Water solubility, mg/cm2 Water solubility, wt % Chemical resistance, wt loss % 5% acetic acid, 120 hrs. 5% citric acid, 120 hrs. 45% ethanol, 72 hrs. Color stability, NBS units Stain resistance; color change, visual Wear resistance, gm/rnz/hr Wear resistance, p / h r
0.27 0.50
0.13 0.20
0.68 0.85
none none
0.6 0.03 0.2 3.3
1.4 0.02 0.4 3.5
0.8
moderate
marked
slight
2.8 1.8
10.60 7.55
16.5 21.60
Final physical properties not attained at this time.
11.5
1.10 2.00
Australian Dental Journal, December, 1975 Resulh and discussion A comparison of the properties of this paint-on adhesive with those of four other restoratives is shown in Table 1. It should be borne in mind that this adhesive is to be applied as a thin film and hence some of its properties will differ from those of the usual restoratives that are used in bulk. The adhesive strength of the coating, when used with the adhesion booster, is higher than that of an unfilled acrylate and of a quartz-filled acrylate. Silicates and amalgams do not adhere and hence require special tooth preparations for mechanical retention. The paint-on restorative has a gel time of 110-150 sec, and a set time of 150-170 sec, which are similar to the values for the filling composite restoratives, but shorter than those for amalgams and silicate cements. The introduction of filler greatly reduces the shrinkage of composites. In the paint-on restorative, the filler/resin weight ratio is low relative to other filled dental composites (1.3:l vs. an average of 3.5:l) in order to enhance its flow characteristics, but due to the thin film application of this coating material, shrinkage would be from the free film surface and thus higher shrinkages than in composites used in bulk can be tolerated without affecting either adhesion or marginal seal. The value of the thermal coefficient of linear expansion for the paint-on adhesive is very close to that for the quartz-filled acrylic and to that of amalgam. It is very high for unfilled acrylic resin, and very low for silicateslz. The data for the thermal conductivity and thermal diffusivity listed on Table 1 may be compared to the corresponding values for enamel, namely 22 x 1 V cal/"C cm sec and 40 x 1V cm2/sec13. The fairly close match of the values for the composites and silicates indicate that use of these materials will not subject the pulp to unduly high thermal shock, and will not create high thermal stresses at the margins. However, the very high values for amalgam14 show that temperature fluctuations in the oral environment will be transmitted to the tooth structure much more efficiently than by the other restoratives.
to Dental Materials and Devices. Chicago, American Dental Association, 7th ed.. 1974 (p. 54). 13 Civjan, S.. Barone. J. J.. Reinke, P. E.. and Selting. W. J.-Tbermal properties of aomatrllrc restorative mluterials. J.D. Res.. 51:4, 1030-1037 (July-Aup.) 19R.
12Guide
.
I4 Souder W
and Paffenbarger, 0 C.-Physical properties of d i n k ' materials. Washingtdn, National Bureau of Standards, Circular No. ( 3 3 3 , 1942.
359 The compressive strength of composites depends on the kind of filler particles, loading ratio, type resin binder and bonding forces between filler and resin matrix. In the paint-,on adhesive, the filler content is limited by the stringent flow characteristics of the uncured composite. Hence, its compressive strength (24,000 psi) is lower than that of the quartz-filled acrylate, but higher than that of the unfilled acrylic resin at 12,000 psi. The value for the paint-on restorative is completely adequate for the purpose for which the coating is intended, namely for application to the surfaces of teeth which are not subjected to the same compressive stresses as the occlusal or proximal surfaces. The ultimate tensile strength, determined at 25"C, of the paint-on restorative after exposure in 37°C for either 24 hours or 30 days was 6,000 psi. These and other data show the rapidity with which this restorative attains its maximum tensile strength. The obvious advantage of reaching maximum strength quickly is that the danger of mechanical failure is greatly lessened. The Rockwell hardness (H) of the paint-on restorative after 1 hour at 25' and after 24 hours at 37°C are 90-92 and 94-96, respectively. Restoratives with a higher filler content have a slightly higher hardness, but the unfilled acrylate is much softer at 46. Water absorption values for all the filled composite restoratives tested are quite low; in fact, insignificant when compared with the silicate cements, which dissolve at the rate of 1 to 3 per cent per week so that absorption measurements are not possible. The unfilled acrylate absorbs 5.85 wt per cent, or approximately eight times as much as the paint-on restorative. The results show that the paint-on adhesive has one of the lower solubilities of restorative materials. The high resistance of the paint-on adhesive is indicative of its durability in the oral environment. The quartz-filled acrylate showed higher weight losses in the acetic acid and ethanol solutions and approximately the same in citric acid. After one hour in boiling water, the hardness of the paint-on restorative and the filled acrylate was essentially unchanged. In both cases, the compressive strength of samples cured for one hour at 37°C increased, from 1500 to 1680 kg/cms for the paint-on restorative and from 2520 to 2650 kg/cmz for the filled acrylate. Discolourations observed visually were judged very slight for the paint-on restorative and slight for the filled acrylate. Visual inspection of the composites after staining showed the paint-on
360
Australian Dental Journal, December, 1975
composite and the acrylic resin experienced slight to moderate colour changes, but the filled acrylic composite underwent marked changes. The hiding power, or contrast ratio, is given in the last column of Table 2. Inspection of these data show that the hiding power increases with increasing film thickness. TABLE 2
Reflectance measurements of the paint-on restorative ~
Film thickness before cm
Reflectance over black
Reflectance over white
Contrast ratio
0.008 0.015 0.023 0.030 0.038 0.05 1
2.5 4.0 4.6 9.4 10.8 10.8
79.5 76.0 76.8 72.0 76.0 67.5
0.03 1 0.052 0.060 0.130
0.142 0.160
The unfilled acrylic resins wear at a much faster rate than the filled composites. Examination of the brushed samples using the scanning electron microscope show that filler hardness has an influence on wear resistance15. The SEM photographs show that in the case of the acrylate filled with the very hard filler, quartz, the polymer matrix wears away, exposing filler particles which are pulled from the surface, exposing a rough polymer surface, which can then be worn more rapidly. The paint-on restorative contains a somewhat softer filler and consequently wears more evenly and hence more slowly.
The results of Eames and O’NeallB, who subjected a number of restorations to a reciprocating brushing action for 448,000 strokes, showed that the abrasion volume loss (CC x 103) was 78.00 for the paint-on restorative, 174.60 for the acrylic resin, and 2.20 for the filled acrylate. The composites abraded more rapidly than amalgam using impact or reciprocating hard heads, but amalgam abraded more under the reciprocal brushing action. Some wear is expected in the paint-on restorative used for idiopathic erosion, as would be for any restorative material in that location. The service life may well be determined by the thickness of the coating. Finishing and polishing Visual examination of surfaces in which a mylar strip was used to hold the paint-on restorative in place, showed that this surface was very smooth and glossy. This material showed better polishing properties compared with the other composites and the unfilled diacrylate. A marked difference was noted between the surfaces finished with the paint-on adhesive and with the filled acrylate. The surface of the latter was very rough due to the hard quartz particles which had been abraded, a point which has been discussed previously. The 12-fluted carbide bur, as shown from the SEM results, gives a better finish to composites than the use of the diamond bur. Conclusion
15 Lce, H. L.. Orlowski, J. A.. Glace, R. W., Kidd, P. D..
and Enabe. E.-Evaluation of wear resistance of dental restorative materials. Z , Advances in polymer friction and wear, Edit. Lee, L. New York. Plenum Publishing Co., Vol. 5B. 1974 @p. 705-723). 16 Eames, W. B., and O’Neal, S. J.-A comparison of three abrasion test methods. IADR Abstracts. Abstract 28. J. D. Res., 52:Special issue, 65 (Fcb.) 1973. 1T Retief, D . H.. Cleaton-Jones. P. E., and Austin, J. C.Pulpal response to a new composite dental restorative material, J. Oral Path., 2 4 , 215,221, 1973. 18 I b m , R. L.-Non-operative treatment of gingival erosion. Dcnt. Survey, 48:3, 22-24 (March) 1972. 19 Portnoy. L. L.-Retention studies of composite coating for gingival erosion. Dent. Survey, 5011. 28-30 (Nov.) 1974. 20 Stuart, R.-Treatment of anterior teeth for aesthetic problems. Quinteswnz Internat. In press.
The need for a paint-on restorative has been shown for tooth surfaces. The paint-on restorative described in this paper is suitable for use in coating of stained enamel and repair of certain types of cervical erosion by an adhesive technique employing 50 per cent phosphoric acid without the need for mechanical preparation of retentive forms. It is formulated especially for application by a flow-on technique. Numerous published clinical reports attest to its satisfactory in vivo properties 1% 18, 19, m. Lee Pharmaceuticals, P.O. Box 3836, South El Monte, Calif., 91733, U.S.A.
