- Email: [email protected]
Surface texture and composition of titanium brushed with toothpaste slurries of different pHs
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 186–192
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Surface texture and composition of titanium brushed with toothpaste slurries of different pHs Awlad Hossain, Seigo Okawa ∗ , Osamu Miyakawa Division of Biomaterial Science, Graduate School of Medical and Dental Sciences, Niigata University, Gakkoucho-dori 2-5274, Niigata 951-8514, Japan
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. This in vitro study characterized the surface texture and composition of titanium
Received 9 August 2005
brushed with toothpaste slurries of different pHs, and thereby elucidated mechanochemical
Accepted 9 January 2006
interactions between the metal and abrasive material in dentifrice. Methods. Two fluoride-free toothpastes, which contained crystalline CaHPO4 ·2H2 O and amorphous SiO2 particles as abrasive, were mixed with acidic buffers to provide slurries of pH
Keywords:
6.8 and 4.8. Specimens were cast from CP Ti, mirror-polished, and then toothbrushed at
Toothbrushing
120 strokes/min for 350,400 strokes under a load of 2.45 N. Specimen surfaces were char-
Dentifrice
acterized by means of SPM and EPMA. The obtained data were compared with the already
Titanium
reported results of water-diluted alkaline slurries. SPM data of each paste were analyzed
Surface texture
using one-way ANOVA, followed by post hoc Tukey test.
Surface composition
Results. Irrespective of toothpaste, neutral slurries, as with alkaline slurries, yielded a chem-
Acidity
ically altered surface with rough texture, whereas acidic slurries formed a chemically clean surface with relatively smooth texture. Mechanochemical polishing effect might be mainly responsible for the cleanness and smoothness. Significance. Acidic slurry-induced smooth surface may minimize plaque formation. However, the augmentation of released titanium ions may be adverse to the human body. For evaluation of toothpaste abrasion effects on titanium, paste slurry pH should be taken into account. © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Titanium is regarded as one of the most biocompatible metals. With the improvement of casting and finishing techniques, titanium has been used for fabrication of crowns, fixed and removable partial dentures, and implant frameworks [1,2]. Orthodontic brackets made up of pure titanium have been well accepted [3,4], especially for patients with allergies to nickel and other specific substances [5]. The distinguished biocompatibility of titanium is mainly attributed to its surface oxide film that has an excellent resistance to corrosion. However, titanium tends to alter its ∗
surface properties in biological environments; hence, this alteration has been one of the crucial aspects in biomaterial research. In the oral cavity, titanium restorations and prostheses are attacked by toothbrush and toothpaste daily. Therefore, several investigations have focused on changes in surface texture due to these adverse conditions [6–9]. Toothbrush bristle rather than additive fluoride in toothpaste was reported to cause alterations in surface texture [10]. Thomson-Neal et al. [11] have demonstrated that a personal dental hygiene procedure with a toothbrush produced superficial grooves on the titanium implant abutments.
Corresponding author. Tel.: +81 25 227 2852; fax: +81 25 227 2854. E-mail address: [email protected] (S. Okawa). 0109-5641/$ – see front matter © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2006.01.010
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 186–192
Recently, an in vitro brushing test of titanium has been performed using water-diluted alkaline slurries of two fluoridefree toothpastes [12]. Abrasives in the dentifrices were reported to substantially alter the surface composition as well as texture. The clinical significances of such surface disturbance and resultant chemical alteration were discussed. On the other hand, metals in the oral cavity are exposed to fluctuations in pH, frequently due to acidic food or drink and on occasion due to bacterial plaque. For instance, Quirynen and Bollen [13] and Quirynen et al. [14] have found out that the titanium surface promoted the formation of bacterial plaque because of its high surface free energy. Dental plaque has been reported to produce acids after the consumption of acidic foods [15,16]. Accordingly, a titanium surface brushed in acidic environments also need to be characterized. This in vitro study was designed to investigate the effects of toothpaste slurry pH on a titanium surface. The texture and composition of brushed surfaces were characterized by means of scanning probe microscopy (SPM) and electron probe microanalysis (EPMA). The results obtained were discussed from the viewpoints of slurry viscosity, abrasive’s acid-resistance, and mechanochemical polishing effect.
2.
Materials and methods
2.1.
Preparation of specimens and toothpaste slurries
Specimens were cast plates prepared from CP Ti ingots (T-alloy M, GC, Japan), equivalent to Grade 2 classified in the Japan Industry Standards. A previous study [12] was referred to for the detailed preparation procedures, including casting, grinding, and finishing. Two fluoride-free toothpastes (New Salt A, Sunstar, Japan and Etiquette Lion, Lion, Japan) were selected. The respective pastes (denoted by NSA and ETL) contained crystalline CaHPO4 ·2H2 O and amorphous SiO2 particles as abrasives. The distilled water-diluted slurries (30 mL/15 g) showed pHs of 7.8 and 9.8, respectively. At the same liquid/paste ratio, pastes were also mixed with acidic buffers, which contained lactic acid and sodium lactate, so that slurry pH would be 6.8 and 4.8. The prepared slurries were denoted as listed in Table 1.
2.2.
following simple equation [17]: (s /s )/(w /w ) = ts /tw ,
2.3.
Brushing test
Specimens were brushed with a toothbrush (Dent EX 33 soft, Lion, Japan) at room temperature, at 120 strokes/min for 350,400 strokes, and under a load of 2.45 N. The other test conditions were mentioned in a previous study [12].
2.4.
SPM
SPM analysis was conducted in tapping mode to evaluate surface roughness (JSPM-4210, JEOL, Japan). The non-brushed surface was used as control. Three specimens were prepared for each condition. As described in a previous study [12], centerline average roughness (Ra ) values were computed along 15 lines randomly chosen on each specimen. Since the variation of Ra with position was much greater than that with specimen, the mean and standard deviation were calculated from the 45 (3 × 15) measurements. The results of each paste group were tested by one-way ANOVA. Post hoc Tukey’s test was conducted to compare the means.
2.5.
EPMA
Morphological comparison of brushed surfaces was made by SE image observation (EPMA-8705-HII, Shimadzu, Japan). The areas corresponding to SE images were analyzed for distributions of calcium, phosphorus, and silicon. The analysis conditions were listed in a previous study [12]. A certain volume of post-brushing slurry was repeatedly diluted with distilled water, and thereby soluble salts were removed from the slurry. Collected abrasive particles were morphologically observed through SE image. Subsequently, element analysis was conducted with a focus on the attachment of titanium to abrasive particle surface.
Viscosity coefficients of prepared slurries were determined by using an Ostwarld viscometer. The capillary tube was cleaned with acetone, dried, and then loaded with a specific volume of slurry. Slurry was allowed to fall in the tube under atmospheric pressure and its falling time was recorded. When the falling rate is not high, viscosity is calculated according to the
Table 1 – Denotation of prepared slurries Slurry pH
NSA ETL
9.8
7.8
6.8
4.8
– E98
N78 –
N68 E68
N48 E48
(1)
where , , and t are viscosity, density, and falling time, respectively. Subscripts s and w mean slurry and distilled water at 25 ◦ C, respectively. Fifteen measurements were done for each condition.
Measurement of slurry viscosity
Paste
187
Fig. 1 – Influences of toothpaste slurry pH on surface roughness.
188
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 186–192
3.
Results
3.1.
Surface roughness
All brushed surfaces were much rougher than the control (Fig. 1). No significant difference was found between N78 and N68 surfaces, or between E98 and E68 surfaces. In contrast, acidic slurries significantly lowered Ra value (p < 0.01 for N48 and p < 0.05 for E48).
3.2.
Surface texture and composition
N78 and N68 surfaces showed sharp hairline scratches along the brushing direction, while on the N48 surface, there was a slightly visible scratching pattern of noticeably dulled lines
(Fig. 2a). A number of dimples, as well as slightly visible groves, were dispersed on E98 and E68 surfaces (Fig. 2b). In contrast, E48 slurry yielded a relatively smooth surface without dimples or grooves. Line profiles of characteristic X-ray intensities were depicted along the horizontal midline on the respective SE images shown in Fig. 2. Ca K␣ and P K␣ intensities from the N78 surface varied widely with a correspondence to the hairline pattern (Fig. 3a). Si K␣ intensity from the E98 surface varied, corresponding to the dimple pattern (Fig. 3b). These variations might partly result from surface texture, as the directions normal to abraded slopes were different from point to point with respect to the location of each element detector. As pH value decreased, these intensities became uniform, approaching the respective background levels.
Fig. 2 – Representative SE images of brushed surfaces.
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 186–192
189
Fig. 3 – Variations of Ca K␣, P K␣, and Si K␣ intensities from brushed surfaces.
3.3.
Viscosity of paste slurries
The viscosity of NSA paste slurries decreased as pH value decreased (Fig. 4). Approximately 20% reduction was found between N78 and N48 slurries. ETL paste slurries were much more viscous than NSA paste slurries, and the difference was roughly in one order of magnitude. The viscosity of the E48 slurry was lower by approximately 30% than those of E98 and/or E68 slurries.
3.4. Morphology of abrasive particles and attachment of titanium to abrasive surface Post-brushing the N78 and N68 slurries still included large irregular particles with sharp edges, whereas the N48 slurry, had noticeably rounded particles (Fig. 5a). On the other hand, most abrasive particles in ETL paste originally looked like a coagulated body of small round particulates, and no morphological change was found after brushing (Fig. 5b). The same areas as shown in Fig. 5b were analyzed for silicon and titanium. Contour lines of SiO2 particles were depicted from the silicon maps and superimposed with the corresponding titanium maps (Fig. 6). Most titanium-rich particulates in the isolated state were coloring agent (TiO2 ) in
Fig. 4 – Viscosity vs. pH of toothpaste slurries.
ETL paste. Irrespective of slurry pH, titanium was attached to the surface of the coagulated bodies. Minute chips attached to abrasive particles were observed in NSA paste slurries (not shown).
4.
Discussion
The present study conducted continuous brushing for 48 h. The number of strokes (350,400) was equivalent to 2 min brushing per session, twice a day for 2 years and 8 months, according to the brushing scheme of Wataha et al. [18]. The condition of pH 4.8 was selected on the basis of the facts that plaque pH dropped below this level after consumption of acidic food, and that the return process to neutrality was relatively slow around pH 4.8 [15,16]. A value 6.8 was considered to be the pH of resting plaque. These extreme conditions might not occur in the same fashion in the oral cavity. For instance, plaque pH changes due to the uptake of drinking water. Brushing is performed for 2–3 min in a single session, followed by a long period of recovery. Accordingly, it would be problematic to apply the entire present results to clinical practice, but worth understanding that interactions between titanium surface and abrasive change depend on pH conditions. The brushing effects with alkaline slurries have been reported in detail in a previous study [12]. Each abrasive mechanically roughened and chemically altered the titanium surface. Differences in surface texture, roughness, and composition between N78 and E98 surfaces primarily resulted from the morphological and compositional differences between the abrasive agents. Based on the results of X-ray photoelectron spectroscopy, it was concluded that, as far as titanium was concerned, embedding of abrasive particles into the surface did not play a major role in the alteration of surface composition. Probably, the reaction of titanium with abrasive material left compounds in the oxide film regenerated immediately after brushing.
190
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 186–192
Fig. 5 – SE images of abrasive particles in post-brushing slurries.
Fig. 6 – Ti attached to abrasive particles in ETL paste slurries.
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 186–192
Compared with these results, individually both neutral slurries had similar effects on the titanium surface (Figs. 1–3). Irrespective of paste, no statistical difference in Ra value was found between alkaline and neutral slurries. On the other hand, acidic slurries yielded a chemically clean (at EPMA level) surface with relatively smooth texture. Accordingly, extrapolation of the abovementioned hypothesis was not applied to brushing with acidic slurry. Three mechanisms by which acidic slurry yielded smooth and clean surface were assumed as follows. First, slurry with lower viscosity reduces the abrasiveness of dentifrice, since toothbrush filaments become less retentive of abrasive particles [19]. In ETL paste, changes in slurry viscosity corresponded fairly to those in Ra value, whereas in NSA paste, no clear correspondence was found (Figs. 1 and 4). Accordingly, slurry viscosity may be one of the significant factors for ETL paste, but properties of abrasive may be rather crucial for NSA paste. Second, the solubility of CaHPO4 ·2H2 O increases with decreasing pH value [20]. It was most likely that CaHPO4 ·2H2 O particles were gradually dissolved in N48 slurry, and hence their abrasiveness was reduced with time. Actually, extremely rounded particles were observed in N48 slurry (Fig. 5a). This mechanism could account for a significant difference in Ra value between N78 and/or N68 and N48 slurries (Fig. 1). However, since SiO2 is excellently acid resistant, this mechanism will not apply to E48 slurry. Third, irrespective of pH, titanium was densely present on SiO2 particle surface (Fig. 6), thus suggesting active mechanical interactions between titanium and abrasive particles even in an acidic environment. However, suggested interactions were not consistent with the smoothness and cleanness of the E48 surface. Therefore, another hypothesis was introduced. Toothbrushing with acidic slurry was similar to mechanochemical polishing [21]. Generally, a non-fluoride acidic environment, even at pH 4, should cause no corrosion of titanium [6], which is protected by the passive oxide film. During brushing, however, the oxide film is destroyed by abrasive agent, instantly followed by the spontaneous recovery of oxide film through the re-oxidation procedure. However, it takes a time for the regenerated film to attain to a stable state [22,23]. In the meantime, a number of titanium ions are released into the environment. More importantly, the acidic environment promoted this element release, which served to remove silicon-containing contaminants as well as minute irregularities on the E48 surface. The same mechanism might operate in the formation of the N48 surface. The effects of morphological and chemical surface alterations on the biocompatibility of titanium should be investigated in detail in vivo. Nonetheless, a rough surface has been reported to harbor 25 times more bacteria than a smooth surface [24]. In addition, a titanium surface tends to accumulate more plaque because of its high surface free energy [13,14]. Therefore, scratches and grooves on a brushed titanium surface may promote the formation, retention, and maturation of dental plaque. The presence of surface contaminants may lead to dissolution of the protective oxide film [25]. As one of the toothbrushing effects, the influence of altered surface composition on element release should be investigated.
191
On the other hand, an acidic slurry-induced smooth surface may minimize plaque formation and therefore latently reduce the occurrence of caries and periodontitis. However, the augmentation of released titanium ions may be adverse to the human body. As pointed out in a previous paper [12], in vivo behavior of released ions, titanium attached to abrasive particulates, and minute chips need to be investigated, especially in relation to metallic allergy. Like this, paste slurry acidity, as with morphology and composition of abrasive agent, had a remarkable influences on the surface texture and composition of titanium. For the evaluation of toothpaste abrasion effects on titanium, the pH of paste slurry should be taken into account.
5.
Conclusions
Within the limitations of this study, the following conclusions were made:
1. Toothbrushing with neutral slurries, as with alkaline slurries, substantially altered the surface texture and composition of titanium surface. 2. Acidic slurries yielded a chemically clean surface with a relatively smooth texture, despite active mechanical interactions between titanium and abrasive particles. Like mechanochemical polishing, acidic dissolution of a mechanically and chemically altered surface might cause the smoothness and cleanness.
Acknowledgements Part of this research was supported by Grant-in-Aid for Scientific Research (# 14571842) from the Ministry of Education, Science, Sports and Culture of Japan. The authors are grateful to Mr. M. Kobayashi, Niigata University Center for Instrumental Analysis, for his helpful suggestions and assistance in element analysis.
references
[1] Ida K, Tani Y, Tsutsumi S, Togaya T, Nambu T, Suese K, et al. Clinical application of pure titanium crowns. Dent Mater J 1985;4:191–5. [2] Wang RR, Fenton A. Titanium for prosthetic application: a review of the literature. Quintessence Int 1996;27:401–8. [3] Kusy RP, Whitley JQ, Ambrose WW, Newman JG. Evaluation of titanium brackets for orthodonctic treatment. Part I. The passive configuration. Am J Orthod Dentofacial Orthop 1998;114:558–72. [4] Kusy RP, O’Grady PW. Evaluation of titanium brackets for orthodonctic treatment. Part II. The active configuration. Am J Orthod Dentofacial Orthop 2000;118:675–84. [5] Harzer W, Schroter A, Gedrange T, Muschter F. Sensitivity of titanium brackets to the corrosive influence of fluoride-containing toothpaste and tea. Angle Orthodontist 2001;71:318–23. ˝ ¨ [6] Probster L, Lin W, Huttemann H. Effect of fluoride prophylactic agents on titanium surfaces. Int J Oral Maxillofac Implants 1992;7:390–4.
192
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 186–192
[7] Toumelin-Chemla F, Rouelle F, Burdairon G. Corrosive properties of fluoride-containing odontologic gels against titanium. J Dent 1996;24:109–15. ´ H. Surface changes on [8] Johannsen G, Redmalm G, Ryden dental materials. The influence of two different dentifrices on surface roughness measured by laser reflexion and profilometer techniques. Swed Dent J 1989;13:267–76. ´ H. Surface changes on [9] Johannsen G, Redmalm G, Ryden dental materials. II. The influence of two different dentifrices on surface roughness measured by laser reflexion and profilometer techniques. Swed Dent J 1992;16:13–20. ¨ onen ¨ [10] Siirila¨ HS, Kon M. The effect of oral topical fluorides on the surface of commercially pure titanium. Int J Oral Maxillofac Implants 1991;6:50–4. [11] Thomson-Neal D, Evans GH, Meffert RM. Effects of various prophylactic treatments on titanium, sapphire, and hydroxyapatite-coated implants. Int J Periodont Rest Dent 1989;9:301–11. [12] Hossain A, Okawa S, Miyakawa O. Effect of toothbrushing on titanium surface: an approach to understanding surface properties of brushed titanium. Dent Mater 2006;22, in press. [13] Quirynen M, Bollen CML. The influence of surface roughness and surface-free energy on supra- and subgingival plaque formation in man. A review of the literature. J Clin Periodontol 1995;22:1–14. [14] Quirynen M, Mei HCVD, Bollen CML. The influence of surface-free energy on supara- and subgingival plaque microbiology. An in vivo study on implants. J Peridontol 1994;65:162–7. [15] Aamdal-Scheie A, Luan W-M, Dahlen G, Fejerskov O. Plaque pH and microflora of dental plaque on sound and carious root surfaces. J Dent Res 1996;75:1901–8.
[16] Macpherson LMD, Dawes C. An in vitro simulation of the effects of chewing sugar-free and sugar-containing chewing gums on pH changes in dental plaque. J Dent Res 1993;72:1391–7. [17] Barrow GM. Physical Chemistry. 3rd ed. Tokyo: McGraw-Hill Kogakusya; 1973. pp. 532–536. [18] Wataha JC, Lockwood PE, Noda M, Nelson SK, Mettenburg DJ. Effect of toothbrushing on the toxicity of casting alloys. J Prosthet Dent 2002;87:94–8. [19] Redmalm G. Dentifrice abrasivity. The use of laser light for determination of the abrasive properties of different silicas. An in vitro study. Swed Dent J 1986;10:243–50. [20] Gregory TM, Moreno EC, Brown WE. Solubility of CaHPO4 ·2H2 O in the system Ca(OH)2 –H3 PO4 –H2 O at 5, 15, 25 and 37.5 ◦ C. J Res Natl Bur Stand A Phys Chem 1970;74A:461–75. [21] Okawa S, Nakano S, Hossain A, Miyakawa O, Kobayashi M. Effect of polishing pressure and slurry pH on the properties of titanium surface. J Dent Res 2003;82:61 [Special Issue B]. [22] Hanawa T, Asami K, Asaoka K. Repassivation of titanium and surface oxide film regenerated in stimulated bioliquid. J Biomed Mater Res 1998;40:530–8. [23] Sittig C, Textor M, Spencer ND, Wieland M, Vallotton PH. Surface characterization of implant materials c.p. Ti, Ti–6Al–7Nb and Ti–6Al–4V with different pretreatments. J Mater Sci Mater Med 1999;10:35–46. [24] Quirynen M, Mei HCVD, Bollen CML, Schotte A, Marechal M, Doornbusch GI, et al. An in vivo study of the influence of the surface roughness of implants on the microbiology of supraand subgingival plaque. J Dent Res 1993;72:1304–9. [25] Kasemo B. Biocompatibility of titanium implants: surface science. J Prosthet Dent 1983;49:832–7.
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Surface texture and composition of titanium brushed with toothpaste slurries of different pHs Awlad Hossain, Seigo Okawa ∗ , Osamu Miyakawa Division of Biomaterial Science, Graduate School of Medical and Dental Sciences, Niigata University, Gakkoucho-dori 2-5274, Niigata 951-8514, Japan
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. This in vitro study characterized the surface texture and composition of titanium
Received 9 August 2005
brushed with toothpaste slurries of different pHs, and thereby elucidated mechanochemical
Accepted 9 January 2006
interactions between the metal and abrasive material in dentifrice. Methods. Two fluoride-free toothpastes, which contained crystalline CaHPO4 ·2H2 O and amorphous SiO2 particles as abrasive, were mixed with acidic buffers to provide slurries of pH
Keywords:
6.8 and 4.8. Specimens were cast from CP Ti, mirror-polished, and then toothbrushed at
Toothbrushing
120 strokes/min for 350,400 strokes under a load of 2.45 N. Specimen surfaces were char-
Dentifrice
acterized by means of SPM and EPMA. The obtained data were compared with the already
Titanium
reported results of water-diluted alkaline slurries. SPM data of each paste were analyzed
Surface texture
using one-way ANOVA, followed by post hoc Tukey test.
Surface composition
Results. Irrespective of toothpaste, neutral slurries, as with alkaline slurries, yielded a chem-
Acidity
ically altered surface with rough texture, whereas acidic slurries formed a chemically clean surface with relatively smooth texture. Mechanochemical polishing effect might be mainly responsible for the cleanness and smoothness. Significance. Acidic slurry-induced smooth surface may minimize plaque formation. However, the augmentation of released titanium ions may be adverse to the human body. For evaluation of toothpaste abrasion effects on titanium, paste slurry pH should be taken into account. © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Titanium is regarded as one of the most biocompatible metals. With the improvement of casting and finishing techniques, titanium has been used for fabrication of crowns, fixed and removable partial dentures, and implant frameworks [1,2]. Orthodontic brackets made up of pure titanium have been well accepted [3,4], especially for patients with allergies to nickel and other specific substances [5]. The distinguished biocompatibility of titanium is mainly attributed to its surface oxide film that has an excellent resistance to corrosion. However, titanium tends to alter its ∗
surface properties in biological environments; hence, this alteration has been one of the crucial aspects in biomaterial research. In the oral cavity, titanium restorations and prostheses are attacked by toothbrush and toothpaste daily. Therefore, several investigations have focused on changes in surface texture due to these adverse conditions [6–9]. Toothbrush bristle rather than additive fluoride in toothpaste was reported to cause alterations in surface texture [10]. Thomson-Neal et al. [11] have demonstrated that a personal dental hygiene procedure with a toothbrush produced superficial grooves on the titanium implant abutments.
Corresponding author. Tel.: +81 25 227 2852; fax: +81 25 227 2854. E-mail address: [email protected] (S. Okawa). 0109-5641/$ – see front matter © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2006.01.010
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 186–192
Recently, an in vitro brushing test of titanium has been performed using water-diluted alkaline slurries of two fluoridefree toothpastes [12]. Abrasives in the dentifrices were reported to substantially alter the surface composition as well as texture. The clinical significances of such surface disturbance and resultant chemical alteration were discussed. On the other hand, metals in the oral cavity are exposed to fluctuations in pH, frequently due to acidic food or drink and on occasion due to bacterial plaque. For instance, Quirynen and Bollen [13] and Quirynen et al. [14] have found out that the titanium surface promoted the formation of bacterial plaque because of its high surface free energy. Dental plaque has been reported to produce acids after the consumption of acidic foods [15,16]. Accordingly, a titanium surface brushed in acidic environments also need to be characterized. This in vitro study was designed to investigate the effects of toothpaste slurry pH on a titanium surface. The texture and composition of brushed surfaces were characterized by means of scanning probe microscopy (SPM) and electron probe microanalysis (EPMA). The results obtained were discussed from the viewpoints of slurry viscosity, abrasive’s acid-resistance, and mechanochemical polishing effect.
2.
Materials and methods
2.1.
Preparation of specimens and toothpaste slurries
Specimens were cast plates prepared from CP Ti ingots (T-alloy M, GC, Japan), equivalent to Grade 2 classified in the Japan Industry Standards. A previous study [12] was referred to for the detailed preparation procedures, including casting, grinding, and finishing. Two fluoride-free toothpastes (New Salt A, Sunstar, Japan and Etiquette Lion, Lion, Japan) were selected. The respective pastes (denoted by NSA and ETL) contained crystalline CaHPO4 ·2H2 O and amorphous SiO2 particles as abrasives. The distilled water-diluted slurries (30 mL/15 g) showed pHs of 7.8 and 9.8, respectively. At the same liquid/paste ratio, pastes were also mixed with acidic buffers, which contained lactic acid and sodium lactate, so that slurry pH would be 6.8 and 4.8. The prepared slurries were denoted as listed in Table 1.
2.2.
following simple equation [17]: (s /s )/(w /w ) = ts /tw ,
2.3.
Brushing test
Specimens were brushed with a toothbrush (Dent EX 33 soft, Lion, Japan) at room temperature, at 120 strokes/min for 350,400 strokes, and under a load of 2.45 N. The other test conditions were mentioned in a previous study [12].
2.4.
SPM
SPM analysis was conducted in tapping mode to evaluate surface roughness (JSPM-4210, JEOL, Japan). The non-brushed surface was used as control. Three specimens were prepared for each condition. As described in a previous study [12], centerline average roughness (Ra ) values were computed along 15 lines randomly chosen on each specimen. Since the variation of Ra with position was much greater than that with specimen, the mean and standard deviation were calculated from the 45 (3 × 15) measurements. The results of each paste group were tested by one-way ANOVA. Post hoc Tukey’s test was conducted to compare the means.
2.5.
EPMA
Morphological comparison of brushed surfaces was made by SE image observation (EPMA-8705-HII, Shimadzu, Japan). The areas corresponding to SE images were analyzed for distributions of calcium, phosphorus, and silicon. The analysis conditions were listed in a previous study [12]. A certain volume of post-brushing slurry was repeatedly diluted with distilled water, and thereby soluble salts were removed from the slurry. Collected abrasive particles were morphologically observed through SE image. Subsequently, element analysis was conducted with a focus on the attachment of titanium to abrasive particle surface.
Viscosity coefficients of prepared slurries were determined by using an Ostwarld viscometer. The capillary tube was cleaned with acetone, dried, and then loaded with a specific volume of slurry. Slurry was allowed to fall in the tube under atmospheric pressure and its falling time was recorded. When the falling rate is not high, viscosity is calculated according to the
Table 1 – Denotation of prepared slurries Slurry pH
NSA ETL
9.8
7.8
6.8
4.8
– E98
N78 –
N68 E68
N48 E48
(1)
where , , and t are viscosity, density, and falling time, respectively. Subscripts s and w mean slurry and distilled water at 25 ◦ C, respectively. Fifteen measurements were done for each condition.
Measurement of slurry viscosity
Paste
187
Fig. 1 – Influences of toothpaste slurry pH on surface roughness.
188
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 186–192
3.
Results
3.1.
Surface roughness
All brushed surfaces were much rougher than the control (Fig. 1). No significant difference was found between N78 and N68 surfaces, or between E98 and E68 surfaces. In contrast, acidic slurries significantly lowered Ra value (p < 0.01 for N48 and p < 0.05 for E48).
3.2.
Surface texture and composition
N78 and N68 surfaces showed sharp hairline scratches along the brushing direction, while on the N48 surface, there was a slightly visible scratching pattern of noticeably dulled lines
(Fig. 2a). A number of dimples, as well as slightly visible groves, were dispersed on E98 and E68 surfaces (Fig. 2b). In contrast, E48 slurry yielded a relatively smooth surface without dimples or grooves. Line profiles of characteristic X-ray intensities were depicted along the horizontal midline on the respective SE images shown in Fig. 2. Ca K␣ and P K␣ intensities from the N78 surface varied widely with a correspondence to the hairline pattern (Fig. 3a). Si K␣ intensity from the E98 surface varied, corresponding to the dimple pattern (Fig. 3b). These variations might partly result from surface texture, as the directions normal to abraded slopes were different from point to point with respect to the location of each element detector. As pH value decreased, these intensities became uniform, approaching the respective background levels.
Fig. 2 – Representative SE images of brushed surfaces.
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 186–192
189
Fig. 3 – Variations of Ca K␣, P K␣, and Si K␣ intensities from brushed surfaces.
3.3.
Viscosity of paste slurries
The viscosity of NSA paste slurries decreased as pH value decreased (Fig. 4). Approximately 20% reduction was found between N78 and N48 slurries. ETL paste slurries were much more viscous than NSA paste slurries, and the difference was roughly in one order of magnitude. The viscosity of the E48 slurry was lower by approximately 30% than those of E98 and/or E68 slurries.
3.4. Morphology of abrasive particles and attachment of titanium to abrasive surface Post-brushing the N78 and N68 slurries still included large irregular particles with sharp edges, whereas the N48 slurry, had noticeably rounded particles (Fig. 5a). On the other hand, most abrasive particles in ETL paste originally looked like a coagulated body of small round particulates, and no morphological change was found after brushing (Fig. 5b). The same areas as shown in Fig. 5b were analyzed for silicon and titanium. Contour lines of SiO2 particles were depicted from the silicon maps and superimposed with the corresponding titanium maps (Fig. 6). Most titanium-rich particulates in the isolated state were coloring agent (TiO2 ) in
Fig. 4 – Viscosity vs. pH of toothpaste slurries.
ETL paste. Irrespective of slurry pH, titanium was attached to the surface of the coagulated bodies. Minute chips attached to abrasive particles were observed in NSA paste slurries (not shown).
4.
Discussion
The present study conducted continuous brushing for 48 h. The number of strokes (350,400) was equivalent to 2 min brushing per session, twice a day for 2 years and 8 months, according to the brushing scheme of Wataha et al. [18]. The condition of pH 4.8 was selected on the basis of the facts that plaque pH dropped below this level after consumption of acidic food, and that the return process to neutrality was relatively slow around pH 4.8 [15,16]. A value 6.8 was considered to be the pH of resting plaque. These extreme conditions might not occur in the same fashion in the oral cavity. For instance, plaque pH changes due to the uptake of drinking water. Brushing is performed for 2–3 min in a single session, followed by a long period of recovery. Accordingly, it would be problematic to apply the entire present results to clinical practice, but worth understanding that interactions between titanium surface and abrasive change depend on pH conditions. The brushing effects with alkaline slurries have been reported in detail in a previous study [12]. Each abrasive mechanically roughened and chemically altered the titanium surface. Differences in surface texture, roughness, and composition between N78 and E98 surfaces primarily resulted from the morphological and compositional differences between the abrasive agents. Based on the results of X-ray photoelectron spectroscopy, it was concluded that, as far as titanium was concerned, embedding of abrasive particles into the surface did not play a major role in the alteration of surface composition. Probably, the reaction of titanium with abrasive material left compounds in the oxide film regenerated immediately after brushing.
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Fig. 5 – SE images of abrasive particles in post-brushing slurries.
Fig. 6 – Ti attached to abrasive particles in ETL paste slurries.
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Compared with these results, individually both neutral slurries had similar effects on the titanium surface (Figs. 1–3). Irrespective of paste, no statistical difference in Ra value was found between alkaline and neutral slurries. On the other hand, acidic slurries yielded a chemically clean (at EPMA level) surface with relatively smooth texture. Accordingly, extrapolation of the abovementioned hypothesis was not applied to brushing with acidic slurry. Three mechanisms by which acidic slurry yielded smooth and clean surface were assumed as follows. First, slurry with lower viscosity reduces the abrasiveness of dentifrice, since toothbrush filaments become less retentive of abrasive particles [19]. In ETL paste, changes in slurry viscosity corresponded fairly to those in Ra value, whereas in NSA paste, no clear correspondence was found (Figs. 1 and 4). Accordingly, slurry viscosity may be one of the significant factors for ETL paste, but properties of abrasive may be rather crucial for NSA paste. Second, the solubility of CaHPO4 ·2H2 O increases with decreasing pH value [20]. It was most likely that CaHPO4 ·2H2 O particles were gradually dissolved in N48 slurry, and hence their abrasiveness was reduced with time. Actually, extremely rounded particles were observed in N48 slurry (Fig. 5a). This mechanism could account for a significant difference in Ra value between N78 and/or N68 and N48 slurries (Fig. 1). However, since SiO2 is excellently acid resistant, this mechanism will not apply to E48 slurry. Third, irrespective of pH, titanium was densely present on SiO2 particle surface (Fig. 6), thus suggesting active mechanical interactions between titanium and abrasive particles even in an acidic environment. However, suggested interactions were not consistent with the smoothness and cleanness of the E48 surface. Therefore, another hypothesis was introduced. Toothbrushing with acidic slurry was similar to mechanochemical polishing [21]. Generally, a non-fluoride acidic environment, even at pH 4, should cause no corrosion of titanium [6], which is protected by the passive oxide film. During brushing, however, the oxide film is destroyed by abrasive agent, instantly followed by the spontaneous recovery of oxide film through the re-oxidation procedure. However, it takes a time for the regenerated film to attain to a stable state [22,23]. In the meantime, a number of titanium ions are released into the environment. More importantly, the acidic environment promoted this element release, which served to remove silicon-containing contaminants as well as minute irregularities on the E48 surface. The same mechanism might operate in the formation of the N48 surface. The effects of morphological and chemical surface alterations on the biocompatibility of titanium should be investigated in detail in vivo. Nonetheless, a rough surface has been reported to harbor 25 times more bacteria than a smooth surface [24]. In addition, a titanium surface tends to accumulate more plaque because of its high surface free energy [13,14]. Therefore, scratches and grooves on a brushed titanium surface may promote the formation, retention, and maturation of dental plaque. The presence of surface contaminants may lead to dissolution of the protective oxide film [25]. As one of the toothbrushing effects, the influence of altered surface composition on element release should be investigated.
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On the other hand, an acidic slurry-induced smooth surface may minimize plaque formation and therefore latently reduce the occurrence of caries and periodontitis. However, the augmentation of released titanium ions may be adverse to the human body. As pointed out in a previous paper [12], in vivo behavior of released ions, titanium attached to abrasive particulates, and minute chips need to be investigated, especially in relation to metallic allergy. Like this, paste slurry acidity, as with morphology and composition of abrasive agent, had a remarkable influences on the surface texture and composition of titanium. For the evaluation of toothpaste abrasion effects on titanium, the pH of paste slurry should be taken into account.
5.
Conclusions
Within the limitations of this study, the following conclusions were made:
1. Toothbrushing with neutral slurries, as with alkaline slurries, substantially altered the surface texture and composition of titanium surface. 2. Acidic slurries yielded a chemically clean surface with a relatively smooth texture, despite active mechanical interactions between titanium and abrasive particles. Like mechanochemical polishing, acidic dissolution of a mechanically and chemically altered surface might cause the smoothness and cleanness.
Acknowledgements Part of this research was supported by Grant-in-Aid for Scientific Research (# 14571842) from the Ministry of Education, Science, Sports and Culture of Japan. The authors are grateful to Mr. M. Kobayashi, Niigata University Center for Instrumental Analysis, for his helpful suggestions and assistance in element analysis.
references
[1] Ida K, Tani Y, Tsutsumi S, Togaya T, Nambu T, Suese K, et al. Clinical application of pure titanium crowns. Dent Mater J 1985;4:191–5. [2] Wang RR, Fenton A. Titanium for prosthetic application: a review of the literature. Quintessence Int 1996;27:401–8. [3] Kusy RP, Whitley JQ, Ambrose WW, Newman JG. Evaluation of titanium brackets for orthodonctic treatment. Part I. The passive configuration. Am J Orthod Dentofacial Orthop 1998;114:558–72. [4] Kusy RP, O’Grady PW. Evaluation of titanium brackets for orthodonctic treatment. Part II. The active configuration. Am J Orthod Dentofacial Orthop 2000;118:675–84. [5] Harzer W, Schroter A, Gedrange T, Muschter F. Sensitivity of titanium brackets to the corrosive influence of fluoride-containing toothpaste and tea. Angle Orthodontist 2001;71:318–23. ˝ ¨ [6] Probster L, Lin W, Huttemann H. Effect of fluoride prophylactic agents on titanium surfaces. Int J Oral Maxillofac Implants 1992;7:390–4.
192
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 186–192
[7] Toumelin-Chemla F, Rouelle F, Burdairon G. Corrosive properties of fluoride-containing odontologic gels against titanium. J Dent 1996;24:109–15. ´ H. Surface changes on [8] Johannsen G, Redmalm G, Ryden dental materials. The influence of two different dentifrices on surface roughness measured by laser reflexion and profilometer techniques. Swed Dent J 1989;13:267–76. ´ H. Surface changes on [9] Johannsen G, Redmalm G, Ryden dental materials. II. The influence of two different dentifrices on surface roughness measured by laser reflexion and profilometer techniques. Swed Dent J 1992;16:13–20. ¨ onen ¨ [10] Siirila¨ HS, Kon M. The effect of oral topical fluorides on the surface of commercially pure titanium. Int J Oral Maxillofac Implants 1991;6:50–4. [11] Thomson-Neal D, Evans GH, Meffert RM. Effects of various prophylactic treatments on titanium, sapphire, and hydroxyapatite-coated implants. Int J Periodont Rest Dent 1989;9:301–11. [12] Hossain A, Okawa S, Miyakawa O. Effect of toothbrushing on titanium surface: an approach to understanding surface properties of brushed titanium. Dent Mater 2006;22, in press. [13] Quirynen M, Bollen CML. The influence of surface roughness and surface-free energy on supra- and subgingival plaque formation in man. A review of the literature. J Clin Periodontol 1995;22:1–14. [14] Quirynen M, Mei HCVD, Bollen CML. The influence of surface-free energy on supara- and subgingival plaque microbiology. An in vivo study on implants. J Peridontol 1994;65:162–7. [15] Aamdal-Scheie A, Luan W-M, Dahlen G, Fejerskov O. Plaque pH and microflora of dental plaque on sound and carious root surfaces. J Dent Res 1996;75:1901–8.
[16] Macpherson LMD, Dawes C. An in vitro simulation of the effects of chewing sugar-free and sugar-containing chewing gums on pH changes in dental plaque. J Dent Res 1993;72:1391–7. [17] Barrow GM. Physical Chemistry. 3rd ed. Tokyo: McGraw-Hill Kogakusya; 1973. pp. 532–536. [18] Wataha JC, Lockwood PE, Noda M, Nelson SK, Mettenburg DJ. Effect of toothbrushing on the toxicity of casting alloys. J Prosthet Dent 2002;87:94–8. [19] Redmalm G. Dentifrice abrasivity. The use of laser light for determination of the abrasive properties of different silicas. An in vitro study. Swed Dent J 1986;10:243–50. [20] Gregory TM, Moreno EC, Brown WE. Solubility of CaHPO4 ·2H2 O in the system Ca(OH)2 –H3 PO4 –H2 O at 5, 15, 25 and 37.5 ◦ C. J Res Natl Bur Stand A Phys Chem 1970;74A:461–75. [21] Okawa S, Nakano S, Hossain A, Miyakawa O, Kobayashi M. Effect of polishing pressure and slurry pH on the properties of titanium surface. J Dent Res 2003;82:61 [Special Issue B]. [22] Hanawa T, Asami K, Asaoka K. Repassivation of titanium and surface oxide film regenerated in stimulated bioliquid. J Biomed Mater Res 1998;40:530–8. [23] Sittig C, Textor M, Spencer ND, Wieland M, Vallotton PH. Surface characterization of implant materials c.p. Ti, Ti–6Al–7Nb and Ti–6Al–4V with different pretreatments. J Mater Sci Mater Med 1999;10:35–46. [24] Quirynen M, Mei HCVD, Bollen CML, Schotte A, Marechal M, Doornbusch GI, et al. An in vivo study of the influence of the surface roughness of implants on the microbiology of supraand subgingival plaque. J Dent Res 1993;72:1304–9. [25] Kasemo B. Biocompatibility of titanium implants: surface science. J Prosthet Dent 1983;49:832–7.