Journal of Oral Rehabilitation, 1975, Volume 2, pages 75-96
A comparative study of three etching solutions: effects on enamel surface and adhesive-enamel interface
D. H. R E T I E F Dental Research Unit of the University of the Witwatersrand and the South African Medical Research Council, Johannesburg
Summary
The chemical treatment of enamel surfaces is an approach in obtaining increased bonding of dental materials to tooth surfaces. Three etching solutions commonly used at present are 50% H3PO4, 50% H3PO4 attenuated with 7% zinc oxide and 50% citric acid. The effects of these three conditioning solutions on normal and polished enamel surfaces and on the interfaces between untreated enamel and etched enamel surfaces were studied by scanning electron microscopy. The variable etching pattern observed with each of the acids made a comparative study difficult. This was further complicated by the dependence of the etching action on prism orientation on polished enamel surfaces. Additional factors that have to be considered are the presence of prismless enamel and perikymata. Surface profile recordings of polished and acid-etched polished enamel surfaces provided some quantitative measure of the etching action of the conditioning solutions. The 50% citric acid solution had the mildest etching action while 50% H3HO4 and 50% H3PO4 attenuated with 7% zinc oxide elicited comparable responses. Introduction
The chemical treatment of enamel surfaces is an approach in obtaining increased bonding of dental materials to tooth surfaces. Buonocore (1955) was the first to demonstrate that the bonding of acrylic restorative materials was substantially increased by conditioning the enamel surface with 85% phosphoric acid (H3PO4). Subsequently, the efficacy of a great number of organic and inorganic acids and chelating agents such as ethylenediaminetetra-acetic acid (EDTA) at different concentrations, has been evaluated in an effort to obtain increased bond strengths of dental materials to tooth structure. The results of these laboratory evaluations have shown that etching enamel surfaces with 50% H3PO4 (Newman, 1965; Lee, Phillips & Swartz, 1971), 50% H3PO4 attenuated with 7% zinc oxide by weight (Gwinnett & Buonocore, 1965; Wright & Beck, 1973) and 50% citric acid (Lee & Swartz, 1971; Cadwell & Johannessen, 1971) produced a marked increase in the bonding of a great variety of dental materials to etched enamel. Correspondence: Professor D. H. Retief, Dental Research Unit, School of Dentistry, University of the Witwatersrand, Milner Park, 2001 Johannesburg, Republic of South Africa.
75
76
D. H. Retief
As a result of these findings, conditioning of the enamel surface is now an accepted and widely applied procedure to improve bonding to enamel in restorative dentistry (Hinding, 1973; Buonocore & Davilla, 1973), in preventive dentistry (Buonocore, 1970; Gwinnett, 1971c) and for the direct bonding of orthodontic attachments (Retief, Dreyer & Gavron, 1970; Newman, 1971). In a previous study the effects of these conditioning solutions on the contact angle of an uncured epoxy resin and the bond strength of an epoxy resin formulation to polished etched enamel surfaces were determined and compared with results obtained on polished, unconditioned enamel surfaces. In addition the rates of etching of enamel surfaces by these acids were studied (Retief, 1974). The object of the present investigation was to extend this comparative study. The effects of the three etching solutions on normal and polished enamel surfaces were compared by examining the etched surfaces by scanning electron microscopy (SEM). The interfaces between untreated enamel and enamel surfaces etched with the three conditioning solutions and an epoxy adhesive formulation developed for the direct bonding of orthodontic attachments (Retief & Dreyer, 1967) were studied by SEM. An attempt was made to quantitate the etching action of the three acid solutions on polished enamel surfaces by registration of the surface profiles of the etched enamel surfaces by means of a surface roughness measuring apparatus. Materials and methods Freshly extracted, sound human central and lateral maxillary incisors were used in this investigation. The teeth were cleaned and stored at — 4°C until required. (1) 77?^ effect of the etching solutions on normal and polished enamel surfaces The crowns of the incisors were lightly polished with pumice and a soft bristle brush. The labial surfaces of half the number of selected teeth were polished on wet 600 grit silicon carbide paper on a Kent MK 2 polishing machine.* Normal and polished labial enamel surfaces were etched with 50% H3PO4 for 1 min, 50% H3PO4 attenuated with 7 % zinc oxide for 1 min and 50 % citric acid for 2 min respectively. These etching solutions were made up to contain 50 g H3PO4 in 100 g solution, 7 g zinc oxide in 93 g of a 50% H3PO4 solution and 50 g citric acid in 100 g solution. Immediately after etching, the conditioning solutions were removed by thorough rinsing of the teeth in running tap water. The etched surfaces were dried with a chip syringe and the teeth stored in a desiccator overnight. The roots of the teeth were removed with a diamond disc and the etched crowns mounted on aluminium stubs using an alcoholic colloidal graphite solution.! The specimens were coated with silver in an Edward Coating Unit Model E12E4| and viewed in a Cambridge Stereoscan S4§ SEM operated at 15 kV. The beam-specimen angle was varied to obtain the best surface projection. (2) Effect of the etching solutions on the adhesive-enamel interface Two methods were used to study the effects of the three etching solutions on the adhesive-enamel interface. * Engis Ltd., Maidstone, England. t DAG Dispersion 580, Acheson Colloids Ltd., England. X Edwards Ltd., Cramley, Kent, England. § Cambridge Scientific Instruments Ltd., Cambridge, England.
A comparative study of three etching solutions
11
In the first technique, the epoxy adhesive formulation was mixed as previously described (Retief e/ al.., 1970) and placed on unconditioned labial surfaces of maxillary incisors and crown surfaces etched with the three etching solutions as described earlier. The adhesive was allowed to set for 15 min and the teeth immersed in water at 37°C overnight. Transverse sections, approximately 2 mm thick, were cut through the adhesive-enamel interfaces and one of the cut surfaces polished with wet 600 grit silicon carbide paper on the automatic lapping and polishing machine. The sections were washed well to remove debris, dried, mounted on aluminium stubs with the polished surface facing upwards before coating and viewing in the SEM. By means of the second technique, exposed adhesive surfaces formed on unconditioned and etched enamel surfaces were obtained. The adhesive was similarly placed on unetched enamel surfaces and surfaces conditioned with the three etching solutions respectively. Oblique, transverse sections were cut through the adhesiveenamel interfaces with a diamond disc. The enamel on the mesial, lateral and distal aspects of the sections were covered with dental sticky wax but the enamel on the cut surfaces left exposed. The sections were placed in 10% hydrochloric acid for 10 min, rinsed-well with water, dried and the sticky wax removed. The specimens were prepared for viewing in the SEM. The adhesive surfaces which had previously contacted the unetched and conditioned enamel surfaces were exposed and examined by rotating the specimen stage of the SEM. (3) Comparison of surface profile recordings on polished and conditioned polished enamel surfaces The following surfaces were prepared for surface profile recordings; labial surfaces of maxillary central incisors polished with 600 grit silicon carbide paper on the polishing machine and polished enamel surfaces etched with 50 % H3PO4 for 1 min, 50 % H3PO4 containing 7 % zinc oxide for 1 min and 50 % citric acid for 2 min. The surface profiles of these specimens were recorded with a Taylor-Hobson Talysurf Model 3 surface roughness testing machine.* The instrument was set on external datum and a stylus pressure of 100 mg employed. The diameter of the stylus used was
A comparative study of three etching solutions: effects on enamel surface and adhesive-enamel interface
D. H. R E T I E F Dental Research Unit of the University of the Witwatersrand and the South African Medical Research Council, Johannesburg
Summary
The chemical treatment of enamel surfaces is an approach in obtaining increased bonding of dental materials to tooth surfaces. Three etching solutions commonly used at present are 50% H3PO4, 50% H3PO4 attenuated with 7% zinc oxide and 50% citric acid. The effects of these three conditioning solutions on normal and polished enamel surfaces and on the interfaces between untreated enamel and etched enamel surfaces were studied by scanning electron microscopy. The variable etching pattern observed with each of the acids made a comparative study difficult. This was further complicated by the dependence of the etching action on prism orientation on polished enamel surfaces. Additional factors that have to be considered are the presence of prismless enamel and perikymata. Surface profile recordings of polished and acid-etched polished enamel surfaces provided some quantitative measure of the etching action of the conditioning solutions. The 50% citric acid solution had the mildest etching action while 50% H3HO4 and 50% H3PO4 attenuated with 7% zinc oxide elicited comparable responses. Introduction
The chemical treatment of enamel surfaces is an approach in obtaining increased bonding of dental materials to tooth surfaces. Buonocore (1955) was the first to demonstrate that the bonding of acrylic restorative materials was substantially increased by conditioning the enamel surface with 85% phosphoric acid (H3PO4). Subsequently, the efficacy of a great number of organic and inorganic acids and chelating agents such as ethylenediaminetetra-acetic acid (EDTA) at different concentrations, has been evaluated in an effort to obtain increased bond strengths of dental materials to tooth structure. The results of these laboratory evaluations have shown that etching enamel surfaces with 50% H3PO4 (Newman, 1965; Lee, Phillips & Swartz, 1971), 50% H3PO4 attenuated with 7% zinc oxide by weight (Gwinnett & Buonocore, 1965; Wright & Beck, 1973) and 50% citric acid (Lee & Swartz, 1971; Cadwell & Johannessen, 1971) produced a marked increase in the bonding of a great variety of dental materials to etched enamel. Correspondence: Professor D. H. Retief, Dental Research Unit, School of Dentistry, University of the Witwatersrand, Milner Park, 2001 Johannesburg, Republic of South Africa.
75
76
D. H. Retief
As a result of these findings, conditioning of the enamel surface is now an accepted and widely applied procedure to improve bonding to enamel in restorative dentistry (Hinding, 1973; Buonocore & Davilla, 1973), in preventive dentistry (Buonocore, 1970; Gwinnett, 1971c) and for the direct bonding of orthodontic attachments (Retief, Dreyer & Gavron, 1970; Newman, 1971). In a previous study the effects of these conditioning solutions on the contact angle of an uncured epoxy resin and the bond strength of an epoxy resin formulation to polished etched enamel surfaces were determined and compared with results obtained on polished, unconditioned enamel surfaces. In addition the rates of etching of enamel surfaces by these acids were studied (Retief, 1974). The object of the present investigation was to extend this comparative study. The effects of the three etching solutions on normal and polished enamel surfaces were compared by examining the etched surfaces by scanning electron microscopy (SEM). The interfaces between untreated enamel and enamel surfaces etched with the three conditioning solutions and an epoxy adhesive formulation developed for the direct bonding of orthodontic attachments (Retief & Dreyer, 1967) were studied by SEM. An attempt was made to quantitate the etching action of the three acid solutions on polished enamel surfaces by registration of the surface profiles of the etched enamel surfaces by means of a surface roughness measuring apparatus. Materials and methods Freshly extracted, sound human central and lateral maxillary incisors were used in this investigation. The teeth were cleaned and stored at — 4°C until required. (1) 77?^ effect of the etching solutions on normal and polished enamel surfaces The crowns of the incisors were lightly polished with pumice and a soft bristle brush. The labial surfaces of half the number of selected teeth were polished on wet 600 grit silicon carbide paper on a Kent MK 2 polishing machine.* Normal and polished labial enamel surfaces were etched with 50% H3PO4 for 1 min, 50% H3PO4 attenuated with 7 % zinc oxide for 1 min and 50 % citric acid for 2 min respectively. These etching solutions were made up to contain 50 g H3PO4 in 100 g solution, 7 g zinc oxide in 93 g of a 50% H3PO4 solution and 50 g citric acid in 100 g solution. Immediately after etching, the conditioning solutions were removed by thorough rinsing of the teeth in running tap water. The etched surfaces were dried with a chip syringe and the teeth stored in a desiccator overnight. The roots of the teeth were removed with a diamond disc and the etched crowns mounted on aluminium stubs using an alcoholic colloidal graphite solution.! The specimens were coated with silver in an Edward Coating Unit Model E12E4| and viewed in a Cambridge Stereoscan S4§ SEM operated at 15 kV. The beam-specimen angle was varied to obtain the best surface projection. (2) Effect of the etching solutions on the adhesive-enamel interface Two methods were used to study the effects of the three etching solutions on the adhesive-enamel interface. * Engis Ltd., Maidstone, England. t DAG Dispersion 580, Acheson Colloids Ltd., England. X Edwards Ltd., Cramley, Kent, England. § Cambridge Scientific Instruments Ltd., Cambridge, England.
A comparative study of three etching solutions
11
In the first technique, the epoxy adhesive formulation was mixed as previously described (Retief e/ al.., 1970) and placed on unconditioned labial surfaces of maxillary incisors and crown surfaces etched with the three etching solutions as described earlier. The adhesive was allowed to set for 15 min and the teeth immersed in water at 37°C overnight. Transverse sections, approximately 2 mm thick, were cut through the adhesive-enamel interfaces and one of the cut surfaces polished with wet 600 grit silicon carbide paper on the automatic lapping and polishing machine. The sections were washed well to remove debris, dried, mounted on aluminium stubs with the polished surface facing upwards before coating and viewing in the SEM. By means of the second technique, exposed adhesive surfaces formed on unconditioned and etched enamel surfaces were obtained. The adhesive was similarly placed on unetched enamel surfaces and surfaces conditioned with the three etching solutions respectively. Oblique, transverse sections were cut through the adhesiveenamel interfaces with a diamond disc. The enamel on the mesial, lateral and distal aspects of the sections were covered with dental sticky wax but the enamel on the cut surfaces left exposed. The sections were placed in 10% hydrochloric acid for 10 min, rinsed-well with water, dried and the sticky wax removed. The specimens were prepared for viewing in the SEM. The adhesive surfaces which had previously contacted the unetched and conditioned enamel surfaces were exposed and examined by rotating the specimen stage of the SEM. (3) Comparison of surface profile recordings on polished and conditioned polished enamel surfaces The following surfaces were prepared for surface profile recordings; labial surfaces of maxillary central incisors polished with 600 grit silicon carbide paper on the polishing machine and polished enamel surfaces etched with 50 % H3PO4 for 1 min, 50 % H3PO4 containing 7 % zinc oxide for 1 min and 50 % citric acid for 2 min. The surface profiles of these specimens were recorded with a Taylor-Hobson Talysurf Model 3 surface roughness testing machine.* The instrument was set on external datum and a stylus pressure of 100 mg employed. The diameter of the stylus used was
