In Vivo Actual Abrasiveness of Three Dentifrices against Acrylic Surfaces of Veneer Crowns

Abstract

Many in vitro methods and technics have been used to investigate the comparative abrasiveness of dentifrices. Several excellent publications re­ view these procedures . 1 -3 In general, abrasion is achieved in a relatively short time by exposing the material to be abraded to a large number of brushing strokes with the dentifrice under investi­ gation; specially constructed laboratory toothbrushing devices are used for this purpose. The comparative abrasiveness of a particular denti­ frice has been estimated by the use of such tech­ nics as: determination of weight loss,4 measure­ ment of the cross-sectional area of loss with shad­ owgraphs and a planimeter, 3 and determination of loss by use of radioactive isotopes.<! Since den­ tal enamel is a relatively difficult substance to abrade, dentin has been the material most frequent­ ly used in these procedures. Unfortunately, it is difficult to correlate results obtained from these in vitro technics with the ac­ tual abrasiveness of these dentifrices on similar structures in the oral cavity. Two factors that make this correlation difficult are: the difference between the normal conditions of dentifrice use and use of the accelerated toothbrushing apparatus; and the effect of the oral environment (saliva, pellicle, plaque, diet, and so forth) on a particu­ lar tooth surface. Therefore, it appeared desirable to perform an in vivo study of dentifrice abrasion. Although the two tooth structures of greatest interest are enamel and dentin, the former is not a suitable in vivo study material because its great hardness would make the effects of dentifrice abrasion too small to measure with any degree of accuracy. Available areas of dentin are usually located in the cervical region of the teeth and this makes it difficult to evaluate changes in surface character­ istics properly. Dental acrylic resin was selected as the best material for the following reasons: ■ It has been used in several in vitro abrasion studies;7 -1 2 ■ It possesses a degree of hardness only slight­ ly greater than that of dentin ; 13 ■ Since it is widely used on the facial aspects of full veneer crowns and fixed bridge pontics, it is generally available and in a suitable location for careful evaluation; ■ The abrasive effect of dentifrices on acrylic facings is of interest to the clinician. The purpose of this double-blind clinical study was to determine the in vivo actual abrasiveness of three dentifrices on acrylic surfaces of fixed oral prostheses. To achieve this purpose, we de­ veloped a new technic that permitted a detailed microscopic evaluation of the acrylic surfaces un­ der investigation. The acrylic surfaces were scored with indentation lines of reasonably uni­ form depth and length. These surfaces were repli­ cated with a highly accurate dental impression material and subsequently studied with a scanning electron microscope before and after use of the three dentifrices. Dentifrice abrasion was deter­ mined as a function of the subsequent change in depth of the indentation lines as a result of using a specific dentifrice for a specific period. The replication technic (to obtain an accurate reproduction of the acrylic surface under investi­ gation) was an adaptation of a similar procedure used to study human skin . 14 This “pressureless” replica procedure, although originally developed to study soft tissues such as skin, was readily adapted for the replication of hard tissues such as tooth surfaces or acrylic resin. These replicas were then processed for observation under high magnification. Both the light-section microscope and the interference microscope were initially used to study the processed replicas. Both instruments, however, had specific in­ adequacies. The resolution of the light-section microscope was not great enough to permit accu-