Physical properties of dentin.

F OR some time there has been considerable interest directed toward the evalution of dental restorations from the physical property standpoint. The dental profession has long recognized that the success of a restorative appliance is dependent not only on the satisfaction of biologic criteria but, in addition, on the capabilities of the restorative materials as well as the tooth structure to resist external displacements and internal deformations. For purposes of clarity, external displacement is defined as a relative movement of the restoration with respect to the tooth, while internal deformation is defined as the dimensional change of the restoration or tooth with respect to itself. These effects are governed by the laws of mechanics and the inherent physical properties of the materials involved; either of them can result in the failure of the restorative attempt. These movements are caused by forces applied to the tooth and restoration which, in turn, may be manifestations of mastication, thermal effects within the mouth, setting change of the restorative materials, and insertion of the restoration. Present-day restorative preparations are results of years of practical experience and investigation, and have been qualitatively designed for retention or for resistance to external displacement, as well as to restore functon to the member involved. The quantitative analysis of external displacement and internal deformation, however, has not been investigated to any great extent, largely due to a lack of knowledge of the physical properties of tooth structure and restorative materials under the conditions which exist in the mouth. As an aid in solving the over-all problem of the effects of these dimensional changes, this study was directed toward the determination of the physical properties of dentin. G. V. Black' (1895) determined the crushing strength of .080 inch cubes of dentin, but because of his equipment limitations, was unable to determine the proportional limit or the modulus of elasticity. Subsequent to this date, no substantial work has been done on these properties. The difficulties connected with this investigation were quite extensive due to the many existing variables and will be borne out by the ensuing discussion. In general, the preparation of test specimens and the testing of them were further complicated by the small sizes involved and the absence of engineering testing procedures for these sizes, in addition to the heterogeneity of the tooth structure itself. As a result of these difficulties, the initial work, which is reported in this paper, was the determination of the existence of all the variables involved, so that further investigation on the singular effect of these variables could be expedited. In addition, working values of the physical properties of dentin were desired in order to establish a basis for analyzing the movements as described previously.