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Journal of Oral Science, Vol. 63, No. 2, 191-194, 2021

Original article Effect of dentifrice slurry abrasivity and erosive challenge on simulated non-carious cervical lesions development in vitro Hani M. Nassar1) and Anderson T. Hara2)

1) Department of Restorative Dentistry, Faculty of Dentistry, King Abdulaziz University, Jeddah, Saudi Arabia 2) Department of Cariology, Operative Dentistry, and Dental Public Health, School of Dentistry, Indiana University, Oral Health Research Institute, Indianapolis, IN, USA

(Received September 15, 2020; Accepted February 23, 2021)

Abstract tion [17,18]. Prevention of NCCLs is important because they may lead to pain, loss Purpose: To investigate the effect of slurry abrasive levels and acidic chal- of form and function as well as esthetic concerns when advanced lenges on the development of non-carious cervical lesions. stages are reached [19]. In such circumstances, restorative therapy may Methods: Ninety-six extracted upper premolars were affixed in pairs to be indicated, but this may not necessarily stop lesion progression. Fur- acrylic blocks and had their root surfaces covered by acrylic resin except thermore, application of restorative materials in the cervical area can be for 2 mm from the cemento-enamel junction. The specimens were distrib- challenging [20] due to difficulty with isolation and the limited amount of uted into six groups (n = 8 pairs) based on two experimental factors: (1) enamel available for bonding. slurry abrasivity level [low/medium/high] and (2) citric acid challenge Investigation of the pathogenesis, management, and prevention of [yes/no]. Specimens were brushed for 5,000, 15,000, 35,000, and 65,000 NCCLs is important because the prevalence of these lesions is increas- strokes. Volume loss (VL) was determined based on optical profilometry ing as the retention rate of dentition increases [21]. Accordingly, the scans of specimens impressions at the baseline and at subsequent brushing development of predictable models for the study of cervical lesions in levels. Data were analyzed using repeated measures analysis of variance controlled environments is required. Several studies have reported a and Bonferroni pairwise comparison (α = 0.05). direct relationship between volume loss (VL) and abrasivity Results: Higher VL values were associated with high-abrasivity slurries [7,22,23]. This process can be further complicated by acidic challenges relative to low- and medium-abrasivity slurries (P < 0.001). Increasing the that can demineralize the surface, consequently lowering its resistance to slurry abrasivity level increased the VL regardless of the acidic challenge, [24]. Exposure to extrinsic or intrinsic acids can cause dissolu- which did not have a significant effect (P = 0.184). After 65,000 strokes, tion of inorganic components of tooth structure, leading to softening of the significant VL was recorded in all groups relative to preceding brushing surface layers of enamel and dentin [25,26]. When this process is followed levels (P < 0.001). by toothbrushing, accelerated surface loss might occur, especially if the Conclusion: Higher values of time-dependent surface loss were associ- dentifrice slurry contains highly abrasive components [27]. Although the ated with increased dentifrice slurry abrasivity, regardless of the citric acid frequency of dietary acidic challenges can be considered one of the most challenge. important risk factors for defects [28,29], the interplay between acidic erosion and toothbrushing abrasion is not clearly understood [30]. Keywords; abrasion, cervical lesions, erosion toothbrushing This necessitates the utilization of a mixed model in which erosive chal- lenges are supplemented with brushing abrasion to simulate the interaction between the two processes. Against this background, the main objective Introduction of the present study was to investigate the influence of different levels of slurry abrasivity, with or without an acidic challenge, on the development Non-carious cervical lesions (NCCLs) is a term used to refer to pathologi- of NCCLs in vitro. cal loss of structure in the cervical area of the tooth unrelated to caries [1]. These types of lesions may result from dental abrasion, and/or erosion at Materials and Methods different points during lesion progression. Dental abrasion is the patho- logical wearing of dental hard tissue resulting from abnormal mechanical Experimental design processes involving foreign objects or substances [2]. Dental erosion This study employed a 3 × 2 factorial design involving two main factors: differs from dental abrasion by the involvement of acid and/or chelating slurry abrasivity level (low, medium, or high) and presence of an acidic agents from non-bacterial sources that chemically etch away dental hard challenge (yes, no). The null hypothesis was that there would be no effect tissues [2,3]. However, it is not uncommon for the two processes to occur of the abrasivity level and acidic challenge on the development of NCCLs. simultaneously [1,4,5]. Premolar pairs were affixed to acrylic stages and reference points were Abrasive forces are probably the most relevant forces affecting the protected by denture-base resin and plastic sheets. Impressions of the cre- cervical area of the tooth [6]. The relationship between brushing abrasion ated lesions were taken after 5,000, 15,000, 35,000, and 65,000 brushing and tooth wear has long been established [7,8]. There are multiple param- stokes. Subtraction analysis of the impressions was done using an optical eters during the toothbrushing process that can affect the development of profilometer in order to determine VL values. Table 1 contains information NCCLs, including characteristics, brushing load, brushing fre- on the materials used for specimen and slurry preparation. quency, toothpaste abrasiveness, and the presence or absence of an erosive agent [9-16]. Although NCCLs can involve both enamel and root dentin, Specimen preparation higher rates of progression can be observed on root dentin, due mainly to Ninety-six permanent upper first premolars were selected and mounted, its lower mechanical resistance and higher susceptibility to demineraliza- in pairs, on acrylic blocks that allowed their positioning on an automated brushing machine. The use of extracted human teeth was approved by the local Institutional Review Board (# NS0911-07). The root portions of all Correspondence to Dr. Hani M. Nassar, Department of Restorative Dentistry, Faculty of Dentistry, teeth were covered by denture-base acrylic resin material (Triad, VLC King Abdulaziz University, P.O.Box 80209, Jeddah 21589, Saudi Arabia Fax: +966-12-6403316 E-mail: [email protected] Material, Dentsply Int., York, PA, USA) with the exception of a 2-mm area Color figures can be viewed in the online issue at J-STAGE. just below the cemento-enamel junction (CEJ; Fig. 1a). This was followed doi.org/10.2334/josnusd.20-0478 by application of a rubber-base material (Hydrophilic Vinyl Polysiloxan, DN/JST.JSTAGE/josnusd/20-0478 Examix NDS Injection Type; GC America, Alsip, IL, USA) to protect the 192

Table 1 Information on the materials used for specimen and toothbrushing slurry preparation

Material Type Main composition Manufacturer details Use in the model Triad VLC denture-base light cure polymethylmethacrylate, sodium potassium Dentsply Int., York, PA, cover the root portion of the specimen acrylic resin material aluminum silicate USA except for 2 mm below the cemento- enamel junction Examix NDS injection type hydrophilic vinyl amorphous silicon dioxide, polyethylene GC America, Alsip, IL, protect the crowns from acidic polysiloxan rubber-base glycol derivative, methylhydrogen USA challenge and create negative replicas impression material dimethylpolysiloxane of the specimens Thermal Forming Material, thermomolded bleaching ethylene-vinyl acetate Henry Schein Inc., protect the reference points on the clear temporary splint plastic material Gillingham, UK crowns and the denture-base material from brushing abrasion Zeodent abrasives precipitated silica silicon dioxide J.M. Huber Corp., Atlanta, toothbrushing slurry component particles GA, USA Carboxymethylcellulose cellulose derivative sodium carboxymethyl cellulose Ashland Inc., Covington, toothbrushing slurry component Blanose 7MF OH, USA

Table 2 Study protocol showing different group treatments including the composition of abrasive slurries, acidic challenge, and brushing strokes

Slurry Session 1 Session 2 Session 3 Session 4 Total

Group Acid Acid Acid Acid Acid Abrasives1 RDA2 Strokes exposure Strokes exposure Strokes exposure Strokes exposure Strokes sessions (min)3 (min) (min) (min) (min) low: 3 g A 69.2 ± 2.6 5,000 0 (0) 10,000 0 (0) 20,000 0 (0) 30,000 0 (0) 65,000 0 (0) Zeodent 113 medium: 6 g B 146.9 ± 3.5 5,000 0 (0) 10,000 0 (0) 20,000 0 (0) 30,000 0 (0) 65,000 0 (0) Zeodent 124 high: 9 g C 208.0 ± 9.4 5,000 0 (0) 10,000 0 (0) 20,000 0 (0) 30,000 0 (0) 65,000 0 (0) Zeodent 103 low: 3 g D 69.2 ± 2.6 5,000 1 (5) 10,000 3 (15) 20,000 5 (25) 30,000 7 (35) 65,000 16 (80) Zeodent 113 medium: 6 g E 146.9 ± 3.5 5,000 1 (5) 10,000 3 (15) 20,000 5 (25) 30,000 7 (35) 65,000 16 (80) Zeodent 124 high: 9 g F 208.0 ± 9.4 5,000 1 (5) 10,000 3 (15) 20,000 5 (25) 30,000 7 (35) 65,000 16 (80) Zeodent 103 1All slurries contained 2.5 g carboxymethylcellulose (Blanose 7MF, Ashland Inc., Covington, OH, USA), 5.0 g glycerol and were made up to 60.0 g with deionized water. Zeodent abrasives are precipitated silica particles (J.M. Huber Corp., Atlanta, GA, USA). 2Relative dentin abrasivity (mean ± standard deviation). 3Acid challenge with 0.3% citric acid at pH = 2.48 (natural level; Sigma-Aldrich GmbH, St. Louis, MO, USA), deionized water was used as a control for groups not exposed to acid challenge.

Toothbrushing and acid challenge protocol The 48 specimen pairs were randomly distributed among six experimental groups (n = 8 pairs) based on the interaction of two factors: 1. toothbrush- ing slurry abrasivity (low: relative dentin abrasivity (RDA) = 69.2 ± 2.6, medium: RDA = 146.9 ± 3.5, high; RDA = 208.0 ± 9.4) [18,31] and 2. exposure to 5-minute challenges of 0.3% citric acid (pH = 2.48) every 2,500 strokes (Table 2). Slurries were prepared by mixing precipitated silica par- ticles of different abrasivity levels (Zeodent Abrasives, J.M. Huber Corp., Atlanta, GA, USA), 2.5 g carboxymethylcellulose (Blanose 7MF, Ashland Inc., Covington, OH, USA) and 5.0 g glycerol, and were made up to 60 g with deionized water. The specimens were placed in a V-8 custom-made toothbrushing machine, based on ISO11609 (Dentifrice—Requirements, test methods and marking), with their long axis perpendicular to that of the . Specimen pairs were subjected to reciprocal brushing strokes under a 200-g load using soft toothbrushes (Oral-B 40, Procter & Gamble, Cincinnati, OH, USA). A standard brushing protocol of 5,000, 15,000, 35,000, and 65,000 strokes (simulating toothbrushing for 1, 3, 7, and 13 years, respectively) was undertaken using the allocated toothpaste Fig. 1 Specimen setup and steps of volume loss analysis. a) two premolars are mounted on acrylic slurries. Half of the groups were exposed to citric acidic challenge steps blocks and covered by self-curing denture base material exposing 2 mm below the cemento-enamel after which the brushing was resumed for the allocated number of strokes. junction (marked area) with reference points of the crowns and on the acrylic denture-base material, b) rubber-based impression material prints after removal of the specimen block, c) optical scanning Surface loss measurement of the surface of the impression print inversed using dedicated software showing the reference areas Impressions from each experimental group were taken at the baseline and after each assigned brushing period (Fig. 1b) with the utilization of petri crowns from acidic challenge. In order to protect the reference areas on the dishes as reported previously [31]. An area of 20 mm × 25 mm of each crown and denture base, a sheet of thermomolded bleaching plastic mate- impression was scanned with an optical profilometer (Proscan 2000, Scant- rial was fabricated. A thin plastic sheet (1 mm thickness; Thermal Forming ron, Taunton, UK). A 10-mm S65/10a sensor was used with 100 repetitions Material, clear temporary splint, Henry Schein Inc., Gillingham, UK) was in the X-axis and 125 repetitions in the Y-axis with a step size set at 0.2 adapted to each specimen pair using a vacuum machine (ECONO-VAC, mm. The reference areas on the crowns and denture-base materials were Buffalo Dental Mfg Co., Syosset, NY, USA). A 2-mm window was cut in identified after inversion of the impression scans using dedicated software the plastic sheet exposing the area of interest on the root surface just below (Proform, Scantron). Three-dimensional subtraction analysis was done the CEJ on both teeth leaving the reference area on the crowns and the to achieve VL values after 5,000, 15,000, 35,000, and 65,000 brushing denture-base material protected from toothbrushing abrasion. strokes in comparison to baseline images within each group (Fig. 1c). 193

Fig. 3 Line graphs showing means of volume loss in the different groups. Error bars indicate standard deviations. Same letters indicate non-significant differences (P > 0.05) between values at different brushing levels within the same group.

Fig. 2 Photographs showing top and side views of samples in the groups exposed to Fig. 4 Bar graph showing means of volume loss at different brushing levels. Error bars indicate citric acid challenge after 65,000 brushing strokes. a) low-, b) medium-, and c) high- standard deviations. Same letters indicate non-significant differences (P > 0.05) between groups at a abrasivity slurry specific brushing level.

Statistical analysis the objective of the present study was to investigate the effect of slurry Normality and homoscedasticity tests were conducted using the Shapiro- abrasivity levels and the introduction of acidic challenges in order to Wilk and Levene tests, respectively. A repeated-measures analysis of develop a simplified, yet valid, in vitro model for studies of toothbrushing variance (ANOVA) mixed model was used to test the effects of slurry abrasion. abrasivity (low, medium, or high), the effect of acid challenge (yes or no), The recommended brushing time needed to ensure adequate plaque and the number of strokes (5,000, 15,000, 35,000, and 65,000 strokes) on removal is 2 min [32], which translates into 20 s per sextant or approxi- the development of NCCLs. Pairwise comparisons among the treatment mately 6-7 s per surface per brushing session [32,33]. As described combinations were made using the Bonferroni method to control the over- previously [31], the numbers of brushing strokes of 5,000, 15,000, 35,000, all significance level at %5 . and 65,000 used in this study might translate into approximately 1, 3, 7, and 13 years, respectively. This model allowed accelerated simulated abra- Results sion mimicking what usually happens in the oral cavity. VL values were reported quantitatively using optical profilometry allowing for more valid Photographs of samples from the experimental groups are included in Fig. interpretation of the results than that for qualitative presentations [32,34]. 2. Normality and homoscedasticity assumptions were satisfied (P > 0.05), The shapes of the resulting lesions were in accordance with a previous necessitating parametric analysis. The effects of different dentifrice slurry report in which more wedge-shaped lesions were associated with tooth- abrasivities (P < 0.001) as well as the number of brushing strokes were brushing using highly abrasive slurries [31]. Even though there was no significant (P < 0.001). However, there was no significant effect of acid detailed analysis of the shapes of the resulting lesions in the study groups, challenge (P = 0.184). The interaction term between the study factors was flatter cervical lesions were observed in groups brushed with low-abrasiv- not statistically significant (P = 0.311). ity slurry compared to the other groups (Fig. 2). Cup- and wedge-shaped Means and standard deviations of surface loss in the six tested groups lesions were seen in groups brushed with medium- and high-abrasivity with the results of repeated measures ANOVA of the effect of brush- slurries, respectively. The majority of teeth in the latter group had lesions ing strokes are included in Fig. 3. Increasing the slurry abrasivity level reaching the pulp cavity. increased the abrasion regardless of the presence of an acidic challenge. Abrasives in toothpaste formulations are required to achieve adequate A significant increase in VL value became evident at 15,000 strokes in cleaning during toothbrushing [35]. As reported previously, slurries with groups exposed to highly abrasive slurries and at 35,000 strokes for other high RDA values lead to more VL with natural tooth structure [18,22,23]. groups. At 65,000 strokes, all groups had significantly higher VL values Toothpaste abrasivity level has more influence on the development of relative to the preceding brushing levels. cervical lesions than the toothbrush used [7]. Even though the use of a Figure 4 illustrates the effects of abrasivity level among the groups. toothbrush alone has an insignificant effect on sound enamel [11], it is still Groups exposed to high abrasive slurry had significantly higher VL values advisable to use soft rather than hard toothbrushes in order to avoid injury than the groups with low or medium abrasivity starting at 5,000 strokes. to soft tissues [36]. High-abrasivity slurries were associated with more VL The difference in VL values between the low and medium abrasivity of the exposed root surface in the present study, corroborating other reports groups was clear after 35,000 brushing strokes. that found more VL associated with high-RDA slurries [7,22,23,31]. The effect of high abrasivity was significant starting at 5,000 strokes, which Discussion is equivalent to one year of brushing. In general, it is not advisable to brush exposed root dentin with highly abrasive , such as some Studying NCCLs using a predictable model is necessary in order to under- whitening formulations. Sabrah and collaborators reported accelerated stand their progression and possible methods for prevention. To this end, development and progression of wedge-shaped abrasive lesions when a 194 whitening toothpaste was used [31]. In the same study, the use of low- 3. Eccles JD (1982) Tooth surface loss from abrasion, attrition and erosion. Dent Update 9, abrasivity formulations led to flat lesions with low VL vales. A similar 373-374, 376-378, 380-381. 4. Wiegand A, Attin T (2011) Design of erosion/abrasion studies--insights and rational con- trend was recorded in the present investigation, where VL remained low cepts. Caries Res 45 Suppl 1, 53-59. with low abrasivity over 35,000 strokes (7 years; Fig. 3). 5. Grippo JO, Simring M, Coleman TA (2012) Abfraction, abrasion, biocorrosion, and the The introduction of the citric acid challenge was used to simulate enigma of noncarious cervical lesions: a 20-year perspective. J Esthet Restor Dent 24, 10-23. the effect of dietary acidic beverages, since dental erosion and abrasion 6. Davis WB, Winter PJ (1976) Measurement in vitro of enamel abrasion by dentifrice. J Dent can occur concurrently [1,4]. Furthermore, an acidic challenge has been Res 55, 970-975. reported to cause superficial demineralization of the surface, thus accel- 7. Litonjua LA, Andreana S, Bush PJ, Tobias TS, Cohen RE (2004) Wedged cervical lesions produced by toothbrushing. Am J Dent 17, 237-240. erating surface wear [24]. In the present study, specimens were exposed 8. Alvarez-Arenal A, Alvarez-Menendez L, Gonzalez-Gonzalez I, Alvarez-Riesgo JA, to citric acid for 5 min after every 2,500 brushing strokes. Even though Brizuela-Velasco A, deLlanos-Lanchares H (2019) Non-carious cervical lesions and risk VL values were slightly increased when citric acid was used in conjunc- factors: a case-control study. J Oral Rehabil 46, 65-75. 9. Mannerberg F (1961) Changes in the enamel surface in cases of erosion. A replica study. tion with medium- and high-abrasivity slurry groups, the effect was not Arch Oral Biol 4, 59-62. statistically significant. One reason for this could be that the erosive 10. Sangnes G (1976) Traumatization of teeth and gingiva related to habitual tooth cleaning challenge was not severe enough to have a significant effect on the acceler- procedures. J Clin Periodontol 3, 94-103. ated surface abrasion. Nevertheless, this project was the initial proof of 11. Bergström J, Lavstedt S (1979) An epidemiologic approach to toothbrushing and dental abrasion. Community Dent Oral Epidemiol 7, 57-64. concept. Consequently, additional episodes with longer durations of acidic 12. Johnson GK, Sivers JE (1987) Attrition, abrasion and erosion: diagnosis and therapy. Clin challenge could be incorporated in future studies to better resemble the Prev Dent 9, 12-16. clinical situation in which extended exposure to acidic beverages could 13. Bishop K, Kelleher M, Briggs P, Joshi R (1997) Wear now? An update on the etiology of tooth wear. Quintessence Int 28, 305-313. be frequent. 14. Bartlett D, Phillips K, Smith B (1999) A difference in perspective--the North American and As reported by many investigations, VL is a time-dependent process European interpretations of tooth wear. Int J Prosthodont 12, 401-408. with more tooth structure loss in response to continuous toothbrushing 15. Hattab FN, Yassin OM (2000) Etiology and diagnosis of tooth wear: a literature review and presentation of selected cases. Int J Prosthodont 13, 101-107. [31,33,37,38]. A similar trend was evident in the present study in which 16. Heasman PA, Holliday R, Bryant A, Preshaw PM (2015) Evidence for the occurrence of a significant effect was recorded after 15,000 strokes for highly abrasive and non-carious cervical lesions as a consequence of traumatic tooth- slurry. However, a detectable significant effect was delayed in the low- brushing. J Clin Periodontol 42 Suppl 16, S237-255. 17. Giles A, Claydon NC, Addy M, Hughes N, Sufi F, West NX (2009) Clinical in situ study and medium-abrasivity groups until 35,000 brushing strokes, indicating a investigating abrasive effects of two commercially available toothpastes. J Oral Rehabil 36, less harmful effect on VL with these formulations, as reported previously 498-507. [22,33]. Furthermore, a distinction between low- and medium-abrasivity 18. Lippert F, Arrageg MA, Eckert GJ, Hara AT (2017) Interaction between toothpaste abra- sivity and toothbrush filament stiffness on the development of erosive/abrasive lesions in slurries became evident at the same brushing level. However, it would be vitro. Int Dent J 67, 344-350. very relevant to study the effect of different frequencies of toothbrushing, 19. Bader JD, McClure F, Scurria MS, Shugars DA, Heymann HO (1996) Case-control study as this has been reported previously to play a major role in the wear and of non-carious cervical lesions. Community Dent Oral Epidemiol 24, 286-291. abrasion of sound as well as demineralized enamel and dentin [10,38]. 20. Hayes M, Brady P, Burke FM, Allen PF (2016) Failure rates of class V restorations in the management of root caries in adults - a systematic review. Gerodontology 33, 299-307. This study had some limitations inherent to its laboratory nature. First, 21. Bartlett DW, Shah P (2006) A critical review of non-carious cervical (wear) lesions and the the aggressive toothbrushing protocol did not closely resembles the clini- role of abfraction, erosion, and abrasion. J Dent Res 85, 306-312. cal scenario in which a patient brushes twice daily for an average of 50 22. Philpotts CJ, Weader E, Joiner A (2005) The measurement in vitro of enamel and dentine wear by toothpastes of different abrasivity. Int Dent J 55, 183-187. strokes per brushing session [33]. Furthermore, the effect of other related 23. Turssi CP, Binsaleh F, Lippert F, Bottino MC, Eckert GJ, Moser EAS et al. (2019) Interplay parameters such as varying brushing loads and frequencies were not rep- between toothbrush stiffness and dentifrice abrasivity on the development of non-carious licated. However, adopting this protocol allowed us to simulate the effect cervical lesions. Clin Oral Investig 23, 3551-3556. 24. Voronets J, Lussi A (2010) Thickness of softened human enamel removed by toothbrush of prolonged exposure of root dentin to abrasion, which contributes to the abrasion: an in vitro study. Clin Oral Investig 14, 251-256. development of NCCLs in vitro. This model can be utilized to study the 25. Lussi A, Megert B, Eggenberger D, Jaeggi T (2008) Impact of different toothpastes on the progression of lesions as well as to investigate the effect on restorative prevention of erosion. Caries Res 42, 62-67. 26. Ganss C, Lussi A, Schlueter N (2014) The histological features and physical properties of materials in future investigations. Also, no exposure to natural or artificial eroded dental hard tissues. Monogr Oral Sci 25, 99-107. saliva was included, which could have acted as a lubricating and/or rem- 27. Ganss C, Marten J, Hara AT, Schlueter N (2016) Toothpastes and enamel erosion/abrasion ineralizing agent as well as a protective layer [39]. This was to keep the - Impact of active ingredients and the particulate fraction. J Dent 54, 62-67. 28. Moazzez R, Smith BG, Bartlett DW (2000) Oral pH and drinking habit during ingestion of model simple, allowing for easier replication in future studies. a carbonated drink in a group of adolescents with dental erosion. J Dent 28, 395-397. In conclusion, the simulated NCCLs developed and progressed more 29. Bartlett DW, Fares J, Shirodaria S, Chiu K, Ahmad N, Sherriff M (2011) The association of rapidly when highly abrasive slurries were used. The addition of a citric tooth wear, diet and dietary habits in adults aged 18-30 years old. J Dent 39, 811-816. acid challenge did not affect surface loss in the present study. However, 30. Bartlett DW, Lussi A, West NX, Bouchard P, Sanz M, Bourgeois D (2013) Prevalence of tooth wear on buccal and lingual surfaces and possible risk factors in young European this model has the potential to create abrasive lesions with different depths adults. J Dent 41, 1007-1013. and morphologies depending on the duration of the brushing protocol. 31. Sabrah AH, Turssi CP, Lippert F, Eckert GJ, Kelly AB, Hara AT (2018) 3D-Image analysis of the impact of toothpaste abrasivity on the progression of simulated non-carious cervical lesions. J Dent 73, 14-18. Acknowledgments 32. Van der Weijden GA, Timmerman MF, Nijboer A, Lie MA, Van der Velden U (1993) A The authors would like to thank Mr. Adam B. Kelly for his contribution comparative study of electric toothbrushes for the effectiveness of plaque removal in rela- to the project. This research was funded by the Deanship of Scientific tion to toothbrushing duration. Timerstudy. J Clin Periodontol 20, 476-481. 33. Nassar HM, Lippert F, Eckert GJ, Hara AT (2014) Dentifrice and abrasivity inter- Research (DSR), King Abdulaziz University, Jeddah, Saudi Arabia, under play on artificial caries lesions. Caries Res 48, 557-565. grant No. DF-048-165-1441. The authors gratefully acknowledge DSR 34. Sawlani K, Lawson NC, Burgess JO, Lemons JE, Kinderknecht KE, Givan DA et al. (2016) technical and financial support. Factors influencing the progression of noncarious cervical lesions: a 5-year prospective clinical evaluation. J Prosthet Dent 115, 571-577. 35. Stookey GK, Burkhard TA, Schemehorn BR (1982) In vitro removal of stain with denti- Conflict of interest frices. J Dent Res 61, 1236-1239. The authors of this paper have no conflicts of interest to declare. 36. Dyer D, Addy M, Newcombe RG (2000) Studies in vitro of abrasion by different manual toothbrush heads and a standard toothpaste. J Clin Periodontol 27, 99-103. 37. Addy M, Hunter ML (2003) Can tooth brushing damage your health? Effects on oral and References dental tissues. Int Dent J 53 Suppl 3, 177-186. 38. Nassar HM, Lippert F, Eckert GJ, Hara AT (2018) Impact of toothbrushing frequency and 1. Litonjua LA, Andreana S, Bush PJ, Cohen RE (2003) Tooth wear: attrition, erosion, and toothpaste fluoride/abrasivity levels on incipient artificial caries lesion abrasion. J Dent 76, abrasion. Quintessence Int 34, 435-446. 89-92. 2. Schlueter N, Amaechi BT, Bartlett D, Buzalaf MAR, Carvalho TS, Ganss C et al. (2020) 39. West NX, Maxwell A, Hughes JA, Parker DM, Newcombe RG, Addy M (1998) A method Terminology of erosive tooth wear: consensus report of a workshop organized by the to measure clinical erosion: the effect of orange juice consumption on erosion of enamel. J ORCA and the Cariology Research Group of the IADR. Caries Res 54, 2-6. Dent 26, 329-335.