The removal of phosphoric acid and calcium phosphate precipitates: an analysis of rinse time.

The acid-etch technique is used extensively in dentistry. Variations in rinse time are found throughout the dental literature. Even though controversies concerning the appropriate rinse time exist, all investigators agree that copious water lavage is important. This study compared different rinse times using both liquid and gel etchants labeled with 32Phosphorus. Each etchant type was evaluated on the time necessary to remove the residual soluble and insoluble radiolabel from the human enamel surfaces. The radioIabeIed etchants were removed in much shorter rinse times than is currently suggested by dental manufacturers and investigators. The acid-etch technique is used extensively in dentistry to facilitate bonding to enamel. This procedure involves etching the outermost layer of enamel with phosphoric acid to form a porous retentive surface. Buonocore (1955) was the first to demonstrate the use acid to etch enamel surfaces. Since then, controversies concerning the type and concentration of etchant, length of etch, and rinse time have developed. Until recently, the recommended etchant was 50% w/w (weight-to-weight) orthophosphoric acid buffered with 7% zinc oxide (Williams and von Fraunhofer 1977). Silverstone (1974) found that phosphoric acid concentrations of 30-40% with a 60sec etch time produced the most retentive enamel surface topography. Most commonly used today is 37% phosphoric acid with an application time of 60 sec, followed by copious rinsing with water for 15-60 sec. The effect of maintaining constant acid concentrations and varying both etch and rinse times is not well documented. Williams and von Fraunhofer (1977) studied bond strength in which the acid concentration was constant (62% w/w orthophosphoric acid) and the etch and rinse times varied from 10 to 60 sec. They demonstrated in short etch times that bond strength increased as rinse time increased. For long etch times, the bond strength decreased as the rinse time increased. This research demonstrated that a 60-sec etch time with a 10-sec rinse time produced the best bond strength. Other reports based on clinical observations suggest rinse times ranging from 5 to 60 sec.’ Even though controversies exist on the appropriate length of rinse time, all investigators agree that copious water lavage is necessary. Etching enamel with phosphoric acid results in deposition of calcium phosphates which must be removed to give optimal bonding (Williams and von Fraunhofer 1977; Soetopo and Hardwick 1978). Therefore, rinse time must be adequate for removal of both the etching medium and precipitated calcium phosphates. In pediatric dental patients, prolonged rinsing time may be impractical due to the ino creased likelihood of salivary contamination. The purpose of this study was to evaluate the efficacy of rinse time on removing residual etchant and calcium phosphate salts from the etched enamel surface. Methods and Materials Experimental Etch and Rinse Procedure Sixty-four extracted permanent posterior teeth were used in this study. The teeth were polished with pumice, the roots removed with a separating disk, and the teeth paired based on morphology. Using an adhesive dot on a nonfissured, smooth surface, an area of 0.32 cm2 was covered. The teeth were coated in melted baseplate wax and the dot was removed exposing a standardized area of enamel. To ensure that no adhesive material remained on the enamel surface, a cotton-tipped applicator dipped in acetone was used to dissolve any remaining adhesive. Each matched pair was assigned randomly to one of eight rinse times which ranged from 0 to 60 sec (Marks 1982). Each tooth pair then was assigned to receive an etching ~Horn 1981; Jordan et al. 1981; Simonsen 1981. 208 RINsE-TIME ANALYSIS OFETCH PROCEDURe: TURNER TAL. treatment with either 37% gel" or liquid" etchant. A modified vacuum filter apparatus was used to contain radioactivity associated with the etching procedure. The filter paper was removed from this apparatus and a hole placed in the center of the lid (Fig 1). A small piece of wax was placed on the exposed grating to stabilize the teeth during the etch and rinse procedures. A 1:500 dilution of P-labeled orthophosphoric acid (specific activity 9120 Ci/mmol, concentration = 2.0 mG/ml) was prepared in both liquid and gel etchants. Twentyfive (il of either gel or liquid etchant was placed on the prepared area of each tooth for 60 sec. Each tooth then was washed within the containment apparatus for the assigned rinse time using a standard dental air/water syringe. Both air and water controls were fully depressed during this rinse procedure. Scintillation Counting After rinsing, the teeth were placed in 5-ml scintillation vials containing 3 ml of scintillation fluid" and vortexed once each hour for 4 hr. The control groups included triplicate scintillation vials containing: (1) scintillation fluid only; (2) 25 [il of 1:500 dilution of liquid etchant in scintillation fluid; and (3) 25 ul of 1:500 dilution of gel etchant in scintillation fluid. All samples were evaluated on a liquid scintillation counter using theP channel to determine counts per minute (CFM).