Journal of Oral Rehabilitation, 1979, Volume 6, pages 267-272
Tensile strength of the bond between resin to enamel etched with phosphoric acid containing fluoride
R. GRAJOWER, A. GLICK, I. GEDALIAa^JD. KOCHAVI Unit for Dental Materials and Department of Preventive Dentistry, The Hebrew University—Hadassah School of Dental Medicine, Jerusalem, Israel
The tensile bond strength of Epoxylite 9075 and Enamelite to enamel treated with NaF and SnF2, at two concentrations, in the etching acid (H3PO4) was determined. The average fracture stress of those samples in which fracture occurred in the resin only, may be considered as the tensile strength of the resin, 69-8 + 5-6 kg/cm^. The mean fracture stresses of the Epoxy 9075 systems etched with pure phosphoric acid or with acid solutions containing 0-5 % F salts were significantly higher than those treated with acid solutions containing 2 % F salts. Incorporation of NaF or SnF2 in the etching solutions highly increased the fiuoride concentration of enamel. The fluoride increase depended on the fluoride concentration of the etching solution and was greater for NaF than SnF2. Introduction
Conditioning of the enamel surface with 50 % phosphoric acid is now an accepted and widely applied technique used to improve bonding to enamel in restorative dentistry (Retief, 1975). The possibility that etching of the enamel surface with concentrated acid may predispose the enamel to the development of caries has been mentioned (Silverstone, 1973). However it has been suggested that the incorporation of 2% NaF into the etching solution (Kochavi, Gedalia & Anaise, 1975) or treatment with 8% SnF2 after the etching action (Low, von Fraunhofer & Winter, 1975) could reduce the predisposition as was shown from SEM of the treated enamel and tensile strength studies of the enamel-resin bond. In one study tensile strength was found decreased considerably after incorporation of 2-72% NaF into the etching acid (Wright & Beck, 1973). Experimental data from bond strength studies using the currently available sealant materials after etching with fluoride-containing acids are scarce. The present investigation was carried out to study the effect of two different fluoride compounds, at two concentrations, in the etching acid on the bond strength between a sealant material and human enamel. Correspondence: Professor I. Gedalia, Department of Preventive Dentistry, Hebrew University— Hadassah School of Dental Medicine, P.O. Box 1172, Jerusalem, Israel.
© 1979 Blackwell Scientific Publications 267
R. Grajower et al
Materials and methods Preparation of the samples Sound human molars were used in this investigation. After cleaning, a slab of relatively flat labial enamel was cut from each tooth. The specimens were embedded in self curing acrylic at the base of a cylindrical mould (Fig. 1). The surfaces of the embedded
BRASS CUP ENAMEL ACRYLIC
1cm Fig. 1. Test assembly with specimen.
enamel samples were ground off using carborundum paper, until a flat area with a smallest diameter of 5 mm was obtained. The prepared surfaces were then polished on felt with alumina paste. Etching was carried out by applying a cotton swab soaked with 50% phosphoric acid containing 0-5 and 2% NaF or SnF2 respectively on the prepared enamel area for 90 s. Immediately after etching the exposed surface was thoroughly rinsed with water and dried with hot air. Epoxylite 9075 and Enamelite, the latter being more viscous, were used as the sealant materials. Fluoride determination After coating the cut surface with wax, except the etched area, enamel slabs were immersed in 0-25 N HCl for 5 min. Fluoride was determined in the solutions after immersion, with a combined activity electrode. Tensile strength measurement Cylindrical brass cups with truncated conical bottoms were prepared for resin application (Fig. 1). Holes of 5 mm diameter were drilled at the apex of the cones and covered
Tensile strength of bond between resin and etched enamel
with a 0-8 mm thick layer of soft wax. Three millimetre diameter holes were made in the wax concentrically with the cone aperture. The brass cups were mounted on the enamel sample with the wax on the enatnel, and kept iti place for proper alignment with a jig. Ten drops of each of the Epoxylite 9075* components were applied alternately through the wax aperture on the enamel surface. Enamelite* was mixed according to the manufacturers instructions and applied on the enamel surface with slight pressure of a spatula. The samples were cured for 24 h at 37°C and 100% humidity. Cords were threaded through holes in the brass cups and the acrylic base samples for attachment to the hooks of the testing apparatusf. In preliminary experiments holes of 3 mm diameter were prepared in the cone apex as well as in the wax layer. This gave rise to fracture at the level of the brass aperture at lower stresses and higher standard deviations than after increasing the diameter of the brass holes to 5 mm; the lower fracture stresses could possibly be attributed to induced stresses at the brass-resin interface. When insufficient resin was applied, similar phenomena were observed due to impaired retetition and strength of the resin in the cup. We decided therefore to disregard results from fractures which occurred at the level of the brass aperture. The fracture sites were examined with a binocular microscope at a magnification of x 10. Statistical examination Student's Mest was used for the statistical evaluation of differences between average fracture stresses, and the Fisher exact probability test was used for evaluating differences between failure/success ratios. Results
Three modes of fracture were defined according to the fracture site: (1) complete fracture within the resin; (2) less than 15 % exposure ofthe enamel surface and (3) more than 15 % exposure of the enamel surface. We differentiated between the latter two modes as their average fracture stresses were significantly different (P< 0-025). A similar classification has been reported by Wright & Beck (1973). For Epoxylite 9075 the average fracture stresses and standard deviations for these three modes were, irrelevant ofthe type of the etching solution employed: 69-8 ±5-6, 57-1 ±7-7 and 47-0 ±4-2 kg/cm2 respectively. Considering a fracture in resin only as success and the other modes of fracture as failure, the addition of 0-5% fluoride salt to the etching solutions did not increase the number of failures. Two percent fluoride salt supplement, however increased the number of failures (P