In restoring a tooth damaged by dental caries, fracture, etc., a restorative material called “resin composite”, which is a curable paste, is in wide use for its easy operation. This curable paste is ordinarily photo-curable. In recent years, the curable paste has been highly improved in mechanical strength and bond strength to tooth and, therefore, is in use not only for restoration of anterior teeth but also for posterior teeth which receive a high occlusal pressure.
Restoration of a tooth using a restorative material therefore is conducted generally in a manner mentioned in the following 1) to 3).
1) The damaged portion of a tooth damaged by dental caries or the like (the tooth is hereinafter referred to also as “to-be-restored tooth”) is cut and removed, whereby a cavity is formed in the tooth.
2) On the inner surface of the cavity of the tooth, there is ordinarily coated an adhesive called “bonding agent”. Then, air-blowing is conducted as necessary. Thereafter, the adhesive is irradiated with a light, whereby the inner surface of the cavity of the tooth is covered with the adhesive. Next, a curable paste is heaped up on the surface of the adhesive covering the above inner surface. The curable paste is photo-cured, whereby the cavity is filled.
3) As a final step, the cured body of the curable paste (referred to also as composite filling and restorative material) filled in the cavity is subjected to shape modification and polishing.
In tooth restoration, the restorative material used need be easily handled; the cured body of the composite filling and restorative material filled in the cavity needs to have high mechanical strength, etc.; and the tooth after restoration needs to be beautiful.
Natural tooth comprises dentin and enamel. Natural tooth differs in color tone (Hue, Chroma and Value) depending upon the portions. For example, the incisal region of natural tooth contains a thin dentin layer and is constituted substantially by enamel and, therefore, has high transparency. In contrast, the cervical region of tooth contains a thick dentin layer; therefore, is opaque and, as compared with the incisal resion, has high Value (degree of lightness or darkness) and high Chroma (intensity of the shade).
That is, in natural tooth, the Chroma and Value become lower from the cervical region of tooth containing a thick dentin layer toward the incisal resion of tooth containing a thin dentin layer. Thus, the color tone of tooth differs depending upon the portions of tooth. Accordingly, in tooth restoration, it is necessary to select the color tone of the restorative material used, in such a way that this color tone fits the to-be-restored portion of tooth, in order to allow the restored tooth to have high esthetics. For allowing the restored tooth to have esthetics, it is conducted that a plurality of kinds of curable pastes each differing in color tone are prepared, there is selected, from these curable pastes, a curable paste having a color tone closest to the color tones of to-be-restored tooth and teeth adjacent thereto (these adjacent teeth are referred to also as “the periphery of to-be-restored tooth”), and this selected curable paste is used for tooth restoration (Non-patent Literature 1).
The selection of the color tone of the curable paste actually used is conducted specifically as follows. Firstly, there is prepared a shade guide (a color sample) which is a collection of samples of cured bodies of curable pastes. Then, a dentist conducting tooth restoration compares the color tones of the cured body samples of the shade guide with the color tone of the periphery of to-be-restored tooth which is confirmed by looking into the mouth. Lastly, the dentist selects, based on the comparison, a curable paste corresponding to the cured body sample considered to have the color tone closest to that of the periphery of to-be-restored tooth.
When the damage of to-be-restored tooth is large and the cavity is deep, it is difficult to achieve the above color tone adjustment only by filling of a single kind of curable paste. When a tooth has a deep cavity (for example, class IV cavity), the color tone of the tooth cannot be determined by the color tone of tooth surface (enamel portion) alone. Actually, the color tone of tooth is observed in such a state that the color tones of tooth surface portion to inner portion (dentin portion) which can be seen through from the surface portion are blended and show gradation.
In order to reproduce this gradation of color tones, there is known a method in which curable pastes are filled each in the form of layer in the restoration of cavity. In this method, a curable paste of selected color tone is filled so as to occupy a given depth of cavity and then cured; this operation is repeated each time with a curable paste of different color tone; in this way, the delicate gradation of color tones is reproduced.
Ordinarily, a plurality of kinds of curable pastes for dentin restoration, for reproduction of the color tones of dentin are filled each in the form of layer in order from the innermost portion of cavity toward the surface portion of tooth; then, a curable paste for enamel restoration is filled in the form of layer on the last surface portion (for example, Non-patent Literature 1 and Non-patent Literature 2). Incidentally, in the above filling, it is common to cure the filled curable paste after filling of each curable paste.