Wear of Artificial Denture Teeth by Use of Toothbrushes. Part 1: Abrasive Wear of Anterior Teeth

Transcription

1 J. Nihon Univ. Sch. Dent., Vol. 32, , 1990 Key words: Wear of Artificial Denture Teeth by Use of Toothbrushes Part 1: Abrasive Wear of Anterior Teeth Yoshinori SATOH 1, Kenji OHTANI 1, Kengo MAEJIMA 1, Masao MORIKAWA 1, Masamichi MATSUZU 1, Eiichi NAGAI 1, Hitoshi TOYOMA, Masaaki OHWA 1, Katsuzo OHKI 1, Masahiro KAKETANI 2 and Minoru NISHIYAMA 2 (Received 22 May and accepted 13 July 1990) high-strength denture teeth, plastic denture teeth, toothbrush abrasion, abrasive wear resistance Abstract High-strength denture teeth (HS teeth) were developed in order to improve the hardness and wear resistance of conventional plastic denture teeth (PL teeth), while retaining their feature of easy occlusal adjustment. The objective of this study was to evaluate the abrasive wear resistance of HS teeth. We conducted wear tests and measured surface roughness using six types of anterior artificial teeth, i.e., three types of HS teeth and three types of PL teeth, the latter serving as the control. The results of the toothbrush abrasion test revealed that the HS teeth had about 5 times greater wear resistance than the PL teeth. It was also found that the type of artificial teeth and the number of abrasive wear-testing strokes had a significant (P<0.05) influence on the surface roughness of artificial teeth. Introduction High-strength denture teeth have been used in clinical practice as artificial teeth, showing hardness and wear resistance superior to those of conventionally used plastic denture teeth[1]. Findings of basic studies on these high-strength denture teeth have been reported, including impact resistance[2], bonding strength with denture base resin[1,3,4] and hardness. On the other hand, findings of clinical studies have been reported with regard to the wear of posterior artificial teeth[5]. However, there seem to have been no studies on the abrasive wear resistance of anterior artificial teeth, a factor which affects the esthetic appearance of denture prostheses. Therefore we conducted abrasive wear tests in order to evaluate the abrasive Departments of Partial Denture Prosthodontics 1 and Dental Materials 2 To whom all correspondence should be addressed: Dr. Yoshinori Satoh, Department of Partial Denture Prosthodontics, Nihon University School of Dentistry, Kanda Surugadai, Chiyada-ku, Tokyo 101, Japan

2 248 wear resistance of high-strength denture teeth, using a total of 6 sets of anterior artificial teeth; 3 types of high-strength denture teeth and 3 types of plastic denture teeth as the control. We also measured surface roughness, and observed the surfaces of these artificial teeth by scanning electron microscopy (SEM) for comparative evaluation of abrasive wear resistance of high-strength denture teeth and conventional plastic denture teeth. Materials and Methods 1. Materials Six types of artificial teeth were used in total (Table 1). Three types of high-strength denture teeth (HS teeth), Vivosite (Ivoclar), Endura (Shofu) and Duradent (GC), were used as the test materials. Three types of conventional plastic denture teeth (PL teeth), Bioblend (Dentsplay), Real crown (Shofu) and Acrylic teeth (GC) were used as the control materials. The number of specimens was 24 in total, including 4 lateral incisors of each of the above-mentioned types. The form and size of the specimens were matched as closely as possible. Table 1 Materials 2. Methods 1) Wear test A stroke-type abrasive wear testing machine (Tokyo Seimitsu Co. Ltd.) was used in our study, and is shown in Fig. 1. The specimen holder of the test machine was able to retain 24 teeth at one time, so that all the specimens could be tested simultaneously under the same conditions. Forty-eight Butler Ex-hard toothbrushes (Butler) were fixed to the toothbrush holder of this test machine. Fig. 1 Abrasive wear testing machine

3 249 The toothpaste (Clinica, Lion Co. Ltd.) was diluted 2-fold with purified water, in which the specimens and the toothbrushes were immersed. A contact stress of 300 g/tooth was constantly loaded at 60 strokes/minute using a stroke length of 100 mm. The toothbrush glided over each specimen 50,000 times in the direction of the specimen's long axis, and the toothbrush was replaced with a new one every 10,000 strokes. 2) Surface roughness A Surflyzer Surfcom 2000 A (Tokyo Seimitsu Co. Ltd.) surface roughness measuring machine was used. It had a probe diameter of 5pm, a measuring length of 1 mm, a contact stress of 0.5 g and a cut-off value of 0.25 mm. Measurement was made at a site 1 mm cervical from the incisal edge with a mesio-distal width of 1 mm. The average height of the central line (Ra) and the profile of surface roughness were recorded. The times at which measurements were taken were at 0 toothbrush stroke (i.e. before the wear test), and at completion of 5,000, 10,000, 20,000, 30,000, 40,000 and 50,000 toothbrush strokes, respectively. 3) SEM observation of the surface of artificial teeth For SEM observation, the surface of each specimen was photographed using a scanning electron microscope (Horiba Co. Ltd., EMAX 8700) after completion of 50,000 wear strokes. 4) Knoop hardness test The Knoop hardness of each specimen was computed from the length of the diamond shape indented on the surface of the specimen, using a Knoop hardness measuring device (Akashi Co. Ltd., MVK-C) (Fig. 2). Fig. 2 Knoop test device (Akashi Co. Ltd., MVK-C) The load used for the Knoop hardness test was 50 g for HS teeth and 25 g for PL teeth. Measurement was made after loading for 30 at 3 points per tooth, and the mean value was taken as the Knoop hardness.

4 250 Results 1. Surface roughness Table 2 shows the mean value and standard deviation of surface roughness of each specimen at 0 stroke, and at completion of 5,000, 10,000, 20,000, 30,000, 40,000, and 50,000 toothbrush strokes. The results of variance analysis of toothbrush strokes for the 6 types of artificial teeth are shown in Tables 3-8, respectively. Table 2 Surface roughness of artificial teeth (Ra) Unit:ƒÊm Table 3 Analysis of variance for Vivosite Table 4 Analysis of variance for Endura

5 251 Table 5 Analysis of variance for Duradent Table 6 Analysis of variance for Bioblend Table 7 Analysis of variance for Real crown Table 8 Analysis of variance for Acrylic teeth

6 252 The type of artificial teeth and the number of toothbrush strokes were studied as factors which could affect the surface roughness. The results of two-way analysis of variance tests are shown in Table Roughness curve Figure 3 shows the recorded values for 3 types of HS teeth, while Fig. 4 shows the values for 3 types of PL teeth measured at 0 and 50,000 toothbrush strokes, respectively. 3. SEM images Figure 5 shows typical SEM images of the 3 HS teeth, while Fig. 6 shows those of the 3 PL teeth, each photographed at 50,000 toothbrush strokes. 4. Knoop hardness Table 10 shows the Knoop hardness of each of the 6 artificial teeth, while Table 11 shows the result of the variance analysis of the Knoop hardness. Table 9 ANOVA table for surface roughness of artificial teeth Table 10 Knoop hardness of artificial teeth Table 11 Analysis of variance for Knoop hardness

7 253 Fig. 3 Profile of surface roughness (HS teeth) Fig. 4 Profile of surface roughness (PL teeth)

8 254 Fig. 5 Typical SEM images of HS tooth surface after brushing for 50,000 strokes Fig. 6 Typical SEM images of PL tooth surface after brushing for 50,000 strokes

9 Discussion PL teeth used in denture prostheses can be subject to various problems such as deterioration of esthetic appearance in the anterior region, decrease of masticatory efficiency in the posterior region, decrease of vertical dimension and displacement of mandibular position[2,3]. Because of their easy occlusal adjustment and low cost, however, many clinicians often use plastic artificial teeth for denture prostheses in many cases, despite their insufficient wear resistance[1]. HS teeth were recently developed in Japan for use as artificial dentures showing hardness and wear resistance superior to those of PL teeth, while maintaining some of the advantages of PL teeth, such as easy workability and low cost. HS teeth usually consist of polyfunctional methacrylate monomers as the base material[2,4,6], to which inorganic or organic composite fillers are added. Thus their polymer has a three-dimensional lattice structure], reinforcing their mechanical properties and making them superior to PL teeth. On the other hand, problems such as dislocation from the resin denture base and fracture have been reported for HS teeth[1,3], so that the technical procedures associated with them should be further refined. Since the clinical use of HS teeth with the above-mentioned advantages is expected to increase in the future, the authors conducted the present experiments in order to evaluate the abrasive wear resistance of anterior artificial teeth, which affect the esthetic appearance of the base denture. 1. Abrasive wear test With regard to the number of wear strokes, which is one of the factors171 associated with wear resistance, the results of wear tests have differed among researchers. The reported numbers of strokes used for posterior artificial teeth have been 2, and 50,000191, whereas 45,000[10] and 200,000[11] strokes have been reported for resin crowns, and 20,000[12] and 30, strokes for composite resin. In this study, 50,000 strokes were used. Based on the assumption that toothbrush cleaning is performed 3 times/day after each meal[14], totaling 100 strokes/ day and 36,500 strokes/year, 50,000 strokes is considered to be comparable to toothbrush cleaning of a denture prosthesis for about one year and a half. Another factor associated with abrasive wear is the contact stress given to artificial teeth171. A contact stress of 300 g/tooth was given to specimens in this study. This was determined by considering conditions used in other abrasive wear tests, including a 1,000-g load reported by Jmnu1151, a g load on composite resin teeth by SHINKAI et al.1131, a 300-g load by SMALLEY and NicHoLLs[16], a 500- g load in the study of WILSON et al.1171, and abrasive wear test conditions reported by KumAsAKA et al. [7] from the viewpoint of dental material engineering, in which a loading of 200 g or more was advocated. 2. Surface roughness As shown in Table 2, the surface roughness values of 6 artificial teeth were all in the range of pm at the time of 0 stroke. On the other hand, the 3 types

10 256 of HS teeth showed surface roughness values in the range of ƒÊm at completion of 50,000 strokes, while the surface roughness of PL teeth was in the range of ƒÊm, i. e. 5 times greater than that of the HS teeth. Furthermore, as shown in Figs. 3-8, it was obvious from the results of variance analysis of toothbrush strokes for the 6 artificial teeth that there was no significant difference in the surface roughness after 30,000 toothbrush strokes in the case of HS teeth. However, in the PL teeth there was a tendency for the surface roughness to be significantly increased according to the increase of toothbrush strokes. As it is obvious from the values measured at 0 and 50,000 strokes on the roughness curve shown in Fig. 3, there was no significant difference in the roughness between 0 and 50,000 strokes in the case of HS teeth. On the other hand, the conventional PL teeth showed a significant difference in the values at 0 and 50,000 strokes on the roughness curve shown in Fig. 4. As shown in the typical SEM images at 50,000 strokes (Figs. 5, 6), no grooves attributable to toothbrush abrasion were observed in the case of HS teeth (Fig. 5), whereas many deep grooves due to toothbrush abrasion were observed on PL teeth (Fig. 6). Therefore we conducted two-way analysis of variance tests concerning two factors which affect the surface roughness of denture teeth, that is, the type of denture teeth and the number of abrasive wear strokes. The results revealed that both factors significantly affected the surface roughness (P C 0.05), as shown in Table 9. As the number of abrasive wear strokes increases, the surface roughness of the PL teeth increased significantly in comparison with HS teeth. This was attributable to the difference in the Knoop hardness between the two types of teeth. The Knoop hardness of HS teeth is about 26-36[1,3], whereas that of PL teeth is about 20[1,4]. Therefore it is considered that the Knoop hardness affects wear resistance. The results of our present tests also revealed a significant difference in Knoop hardness between HS teeth and PL teeth, the value being about 1.5 times greater in the former than in the latter, as shown in Tables 10, 11 and 12. Furthermore, the inorganic or organic composite fillers incorporated in HS teeth[2,4,6] are considered to contribute to improved hardness and abrasive wear resistance. From these findings, it is suggested that denture prostheses with anterior plastic teeth have an inferior esthetic appearance due to the formation of many deep grooves on the tooth surface as the number of toothbrush strokes increases, resulting in discoloration of the dentures[18.19]. Conclusion An abrasive wear test was conducted using toothbrushes in order to evaluate the abrasive wear resistance of high-strength denture teeth. Three types of the high-strength denture teeth were used as the test materials, and 3 types of conventional plastic denture teeth were used as control material. The following findings were obtained from measurement of the surface roughness and SEM observations:

11 The surface roughness values of the 6 specimens were all within the range of ƒÊm before the abrasive wear test. 2. The surface roughness at completion of 50,000 toothbrush strokes was in the range of ƒÊm in the 3 high-strength denture teeth. On the other hand, the 3 types of conventional plastic teeth showed the surface roughness values in the range of ƒÊm at 50,000 strokes, which was about 5 times higher than the value for high-strength denture teeth. 3. The type of artificial teeth and the number of wear strokes were the factors which significantly affected the surface roughness (P<0.05). 4. The Knoop hardness of the 3 types of high-strength denture teeth was significantly greater than that of the 3 types of conventional plastic teeth (P<0.05). References [1] SUZUKI, S., KUNESHITA, H. and SHIBA, A.: The evaluation of adhesive bonding of the denture base resin to the hard resin teeth, J. Jpn. Prosthodont. Soc., 32, 37-42, 1988 (in Japanese) [2] SUZUKi, S., SAKOH, M. and SHIBA, A.: The evaluation of the impact resistance of crosslinked plastic denture teeth, J. Jpn. Prosthodont. Soc., 32, , 1988 (in Japanese) [3] YAMAUCHI, M., OKAMURA, K., SAKAI, M., ANDO, M., KAWANO, J. and YOKOI, K.: Bonding strength of the hardened plastic teeth to the denture base acrylic resin, J. Jpn. Prosthodont. Soc., 32, , 1988 (in Japanese) [4] HASEGAWA, J.: Hard plastic teeth, Dental Diamond, 3, , 1987 (in Japanese) [5] YOKOYAMA, M.: The evaluation of posterior plastic teeth, J. Pract. Prosthodontics, 19, , 1986, (in Japanese) [6] OHASHI, M., NAKAI, H., HASHIMOTO, K., HORIE, K., HORIBE, T., MORIWAKI, Y. and YAMAKI, M.: Dental Materials, , Gakken Shoin Co., Tokyo, Japan 1987 (in Japanese) [7] KUMASAKA, A., KOSHINAKA, Y. and NOGUCHI, H.: Abrasion test conditions for dental materials, J. J. Dent. Mater., 2, , 1983 (in Japanese) [8] FRAUNHOFER, J.A., RAZAVI, R. and KAHN, Z.: Wear characteristics of high-strength denture teeth, J. Prosthet. Dent., 59, , 1988 [9] TSUJI, Y.: Fundamental study on the occlusal wear of artificial posterior teeth, J. Jpn. Prosthodont. Soc., 26, 63-70, 1982 (in Japanese) [10] FUJII, K., ARIKAWA, H., INOUE, K., ONIZUKA, T. and Jim', T.: Fundamental studies on abrasion resistance of dental resins, J. Jpn. Prosthodont. Soc., 27, , 1983 (in Japanese) [11] SUZUKI, S., SHIBA, A., YASUDA, N., Ai, M., TOMODA, K. and NAKABAYASHI, N.: A clinical study of new dental resin, J. Jpn. Prosthodont. Soc., 30, , 1986 (in Japanese) [12] HERRINGTON, E., JONES, P.A., FISHER, S. and WILSON, H.J.: Toothbrush-dentifrice abrasion, Br. Dent. J., 153, , 1982 [13] SHINKAI, K., KAMIGA, H., OKUBO, T., NAGASAKI, T., SATOH, M., HASEGAWA, K., HASEGAWA, H., TAKAHASHI, H. and UESUGI, A.: A study on toothbrush abrasion of restorative materials, J. Japan J. Conserv. Dent., 30, , 1987 (in Japanese) [14] HEBEL, K.S. and GRASER, G.N.: Abrasion of enamel and composite resin by removable partial denture clasps, J. Prosthet. Dent., 52, , 1984 [15] JIBKI, H.: Studies of abrasion resistance of various materials used for artificial teeth and crowns, J. Stomatol. Soc. Jpn., 41, , 1974 (in Japanese) [16] SMALLEY, W.M. and NICHOLLS, J. I.: In vitro two-body wear of polymeric veneering materials, J. Prosthet. Dent., 56, , 1986 [17] WILSON, G.S., DAVIE, E.H. and FRAUNHOFER, J.A.: Abrasive wear characteristics of anterior restorative materials, Br. Dent. J., 151, , 1981 [18] UEDA, K., TAKEDA, T., ICHIWATA, T., KOBAYASHI, I., NAKAMURA, T., MORITA, E., GOTOH, H., UMI, T., SATOH, Y. and OHKI, K.: Studies on the coloration of denture base, J. Jpn.

12 258 Prosthodont. Soc., 29, 85-93, 1985 (in Japanese) [19] NAKAMURA, A.: Experimental studies on coloration of denture base resin, Nihon Univ. Dent. J., 63, , 1989 (in Japanese)