Brushing force of manual and sonictoothbrushes affects dental hard tissue abrasion
Annette Wiegand &John Patrik Matthias Burkhard &FlorinEggmann & ThomasAttin
Received: 24 February 2012/Accepted: 5 July 2012 /Published online: 13 July 2012 # Springer-Verlag 2012
ObjectivesThis studyaimed to determine the brushing forcesapplied during in vivo toothbrushing with manual and sonic toothbrushes and to analysethe effect of these brushingforces on abrasion of sound and eroded enamel and dentin in vitro.
Materialsand methods Brushingforces of a manual and twosonic toothbrushes (low and high frequency mode) were measured in 27 adults before andafter instruction of the respectivebrushing technique and statistically analysed by repeated measures analysis ofvariance (ANOVA). In the in vitroexperiment, sound and eroded enamel and dentin specimens (each subgroup n012) were brushed in an automaticbrushing machine with the respective brushing forces using a fluoridated toothpasteslurry. Abrasion was determined by profilometry and statistically analysed byone-way ANOVA.
ResultsAveragebrushing force of the manual toothbrush (1.6±0.3N) was significantly higher than for the sonic toothbrushes (0.9±0.2 N), whichwere not significantly differentfrom each other. Brushing force prior and after instruction of the brushingtechnique was not significantly different.The manual toothbrush caused highest abrasion of sound and eroded dentin, butlowest on sound enamel. No significantdifferences were detected on eroded enamel.
Conclusion Brushingforces of manual and sonic toothbrushes are different and affect their abrasivecapacity. Clinical significance Patients with severe tooth wearand exposed and/oreroded dentin surfaces should use sonic toothbrushesto reduce abrasion, while patients without tooth wear or with erosivelesions confining only to enamel donot benefit from sonic toothbrushes with regard to
Keywords Abrasion. Dentin . Enamel . Erosion . Toothbrushing force . Tooth wear
Toothbrushing abrasion is onefactor in the multifactorial process of tooth wear. Whiletoothbrushing is considered to beof minor importance for abrasion of sound enamel and dentin , it was shownto be a significant risk factor for the aetiology of erosive lesions [2–5]. Especially oneroded enamel anddentin, toothbrushing abrasion is determined by the abrasivity [6, 7] andconcentration of the toothpaste, but also modified by the kind of toothbrush [9, 10] and thebrushing force [11–13].
A large number of differentelectric toothbrushes (oscillating–rotating, sonic and ultrasonic)and many different brandsare currently available on the market. Recent systematic reviews suggest thatpower toothbrushes do not demonstratea clinically relevant damage potential to soft and hard tissues when comparedwith manual toothbrushes [14, 15]. Among others,this observation was related to the assumption that powertoothbrushes were usually applied withsignificantly lower brushing forces compared with manual toothbrushes. Thus, invitro studies showed that powertoothbrushes produced less dentin wear than manual toothbrushes when applied at alower brushing force [16,17]. In contrast,oscillating–rotating,sonic and ultrasonic toothbrushesexhibited a higher abrasive potential compared
with manual toothbrushes whenthey are applied at the same brushingforce [9,10].However, a comparison of the abrasivity of different types of toothbrushesremains difficult,not least as studiesinvestigating the brushing forces of power toothbrushes date back several yearsand exhibited
a high variation from 0.5 to 6 N [18–21].
No systematic evaluation of brushing forces in different sextants and sites,and of the effect of instruction on brushing technique has so far beenperformed. This issue is of particular interest as increasing brushing forcesare associated with the development of wedge-shaped lesions  and with a higher abrasion potentialon enamel and dentin [12, 13]. Therefore, this study aimed (1) todetermine and compare the brushing forces applied during in vivo toothbrushingwith manual and sonic toothbrushes and (2) to analyse the effect of these brushingforces on abrasion of sound and eroded enamel and dentin in vitro. The nullhypotheses were that (1) the brushing forces of manual and sonic toothbrushesare not significantly different and, thus, that (2) the abrasion of sound anderoded enamel or dentin is not significantly different between the toothbrushesat their specific brushing force.
Materials and methods
The study was divided in two experiments: (1) clinical assessment of brushingforces applied during toothbrushing with one manual and two sonic toothbrushes(crossover design) and (2) in vitro analysis of the effect of these brushingforces on abrasion of sound and eroded enamel and dentin.
Brushing forces were determined in 27 volunteers (5 male, 22 female; 18–55years) before and after instruction of the respective brushing technique.Inclusion criteria were adult age and dentition with a minimum of 24 teeth.Exclusion criteria were removable dentures or orthodontic appliances, less than24 teeth and physical disabilities with the potential to influence manualskills. Ethical approval for the
study was granted by the local Ethics Committee (No 2011- 0211/4). Thevolunteers were given oral and written information that the purpose of thestudy will be explained only at the end of the investigation in order to avoidbias in brushing force. All participants gave their written consent.
Mean brushing forces determined in vivo were used for the invitro experiment, where sound and eroded enamel and dentin specimens (eachsubgroup n012) were brushed with the toothbrushes in an automatic brushingmachine at the respective brushing forces. The amount of abrasion was determinedby profilometry.
One manual (Candida Fresh Family X-Change, Migros, Switzerland) and two sonictoothbrushes (Sensonic Professional SR-1000 E, Waterpik; Sonic complete DLX, OralB) were used in this crossover study. The sonic toothbrushes were used at bothlow and high frequency mode.Detailed information about the toothbrushes isgiven in Table 1.
Clinical assessment of brushing force
Brushing forces before and after the instruction of the respective brushingtechnique were determined with a 2-week training period in between. Thesequence of toothbrushes was randomly assigned to the volunteers. The studycomprised a total of six visits to the department for each volunteer.
In the first session, the volunteers were asked to perform theirhabitual brushing technique and to brush the vestibular, occlusal/incisal andlingual/palatal sites (each 20 s) sextantwise (total, 6 min). As the first andlast 5 s of each 20-s brushing interval had to be discarded due to alterationsof the brushing forces by moving the toothbrush from one sextant or site to theother, the brushing time used for statistical analysis (10 s) of each sextantand site was considered to be clinically relevant. After the measurement ofbrushing force, volunteers received verbal information and training of the respectivebrushing technique. The brushing technique recommended for the manualtoothbrush was the modified Bass technique. According to the manufacturers, thesonic toothbrushes were recommended to be used with the bristles angled towardsthe gumline, brush head gently moved in a slightly circular motion (Soniccomplete DLX) and with the brush angled towards the gumline at a 45° angle,slow movement of the brush (Sensonic Professional SR 1000-E). The volunteers wereprovided with the respective toothbrush and were asked to practise the brushingtechnique at each toothbrushing within the 2-week training period. Two weeks afterthe instruction, brushing forces were measured again as described above. In bothsessions, brushing was performed with a commercially available toothpaste(Elmex Sensitive Plus, GABA International, Switzerland).
Brushing force measurement was achieved by mounting two strain gauges to thestem of the sonic toothbrushes or to a metal cantilever bridge betweentoothbrush holder and toothbrush head of the manual toothbrush, respectively.Flexing of the brush head was translated through a strain amplifier into differentvoltages, which were then monitored by a special software (PicoScope 6.0, PicoTechnology, Cambridgeshire, UK, 50 measurements per second) and transferred toMicrosoft Excel software. Prior to each brushing session, the set-up wascalibrated for each toothbrush with standardized weights.
In vitro abrasion experiment
Cylindric enamel and dentin specimens (3 mm in diameter, in total 120 enameland 120 dentin specimens) were obtained from the crowns or roots, respectively,of freshly extracted, non-damaged bovine incisors.
Table 1 Characteristics of the manual and sonic toothbrushes
Both sonic toothbrushes do not have a feedback system forcontrolled pressure
The specimens were embedded in acrylic resin (Paladur, HeraeusKulzer, Hanau, Germany), and surfaces were ground flat and polished withwater-cooled carborundum discs (1,200, 2,400 and 4,000 grit, Waterproof SiliconCarbide Paper, Stuers, Erkrat, Germany). Approximately 200 μm of the outermost layerwere removed as verified with a micrometer (Digimatic, Mitutoyo, Tokyo, Japan).The enamel and dentin specimens were distributed randomly to ten groups of n=12specimens each. They were fixed in custom-made resin appliances (Eracetal,Angst + Pfister, Zürich, Switzerland) allowing exact repositioning of thespecimens in both the brushing machine and the profilometer.
Abrasion was performed in an automatic brushing machine, where the toothbrusheswere applied at the respective brushing force determined in vivo (overall meanbrushing force: manual, 1.6 N; sonic, 0.9 N). The toothbrushes were fixed inthe holder of the brushing machine allowing alignment of the toothbrushing headparallel to the surface of the samples. The right and left sides of thespecimens were
covered with a stainless steel foil (0.1-mm thick) leaving a 2-mm wide area inthe middle of each specimen exposed for brushing.
As the toothbrush head of the manual toothbrush was about1.28-fold longer than the sonic toothbrushes (Table 1), the brushing time of the sonic toothbrushes was increased accordinglyto ensure that the product of contact area (millimetres) and application time(minutes or seconds, respectively) remained constant for all brushes. Linearbrushing motion was set at 100 brushing strokes per minute for the manual andat 20 brushing strokes per minute for the sonic toothbrushes [9, 10].
Sound specimens were brushed for 100 min with the manual toothbrush and for 128min with the sonic toothbrushes at low and high frequency mode. For abrasion oferoded enamel and dentin, specimens were subjected to a cyclic erosion–abrasion experiment. In each cycle, specimenswere eroded (30 s, citric acid, pH 2.6, 1.5 ml/ specimen), stored in artificialsaliva (composition as given by Klimek et al. , 15 min), brushed and again stored in artificial saliva (30min). Brushing with the manual toothbrush was performed with eight linearbrushing strokes (each third cycle: nine linear brushing strokes); brushing withthe sonic toothbrushes at high and low frequency was performed with two linearbrushing strokes (tenth cycle: three; 20th cycle: four; 30th cycle: threelinear
brushing strokes). Thus, mean contact time of the brushes in each cycleamounted to 5 s (manual) and 6.4 s (sonic) to compensate for the differentlength of the toothbrush heads.
Brushing was performed with a toothpaste slurry containing fluoridatedtoothpaste (Elmex, GABA International, Switzerland; relative dentin abrasivity,35) and water in a ratio of 1:3 (3 ml). The slurry was renewed after each 5 minbrushing (sound specimens) or after each brushing cycle (eroded specimens),respectively.
Substance loss was analysed with a stylus profilometer (PerthometerS2, Mahr, Göttingen, Germany). The device was equipped with a custom-made jigfor repositioning the
appliances with the samples for successive measurements. Dentin specimens weremeasured under wet conditions. Identification marks (scratches) on the acrylicresin surface of the embedded specimen were used for exact superimposition ofthe profiles . These scratches were covered by themetal foil during brushing. Substance loss was calculated based on thedifferences between pre- and post-brushing profiles with a custom-designedsoftware (4D Client, University Zurich, Zurich, Switzerland). Five profileswere performed on each specimen via scanning from the reference surface to thetreated surface. Abrasion of sound specimens was measured after 100 or 128 minbrushing, respectively. Abrasion of eroded enamel and dentin was analysed after10, 20 and 30 cycles of the erosion–abrasion experiment. An average ofthese five readings (micrometres) was obtained and used for data analysis.
For each volunteer, brushing force of each sextant and each site was averagedfor the mean brushing force. As values might be altered by moving thetoothbrush from one sextant or site to the other, the values of the first andlast 5 s of the 20-s brushing interval were discarded. Based on the fact that thebrushing force was measured with a frequency of 50 measurements per second, atotal of 500 single values were averaged to the mean brushing force of eachinterval.
Statistical analysis was performed by repeated measures analysisof variance (ANOVA) and Bonferroni post-hoc tests (p<0.05) to analysedifferences between the toothbrushes, between the brushing forces before andafter instruction of the brushing technique, and between different sited andsextants. Due to the number of volunteers, differences in brushing forces withrespect to gender and handedness were not statistically analysed.
Mean enamel and dentin losses of sound and eroded specimens were computed. Theassumption of the normal distribution was analyzed by means of Kolmogorov–Smirnovand Shapiro–Wilk's tests. One-way ANOVA followed by the Scheffé post-hoc testwas used to investigate if there are differences in mean enamel or dentin lossbetween the different groups of toothbrushes (p<0.05).
Mean brushing forces of different sextants and sites are presented in Tables 2 (upper jaw) and 3 (lower jaw). Considering all sextantsand sites, the manual toothbrush was applied with a significant higher force(mean overall,1.6 N) compared with the sonic toothbrushes (mean overall, 0.9N), while comparisons between high and low frequency modes of the sonictoothbrushes revealed no significant differences.
Table 2 Mean (± standard deviation) brushing forces (Newton) inthe upper jaw before and after the instruction of the respective brushing technique
In each row, significant differences between the toothbrushesbefore or after the instruction of the brushing technique, respectively, weremarked with different small letters Within each column, significant differencesin brushing force at the different sites of each sextant were marked by differentcapital letters. Within each site, significant differences between the first tothird sextants were marked by different Greek letters Within one toothbrush,differences between brushing forces before and after instruction of thebrushing technique were not significant independently of the site and sextant
Table 3 Mean (± standard deviation) brushing forces (Newton) inthe lower jaw before and after the instruction of the respective brushing technique
In each row, significant differences between the toothbrushesbefore or after the instruction of the brushing technique, respectively, weremarked with different small letters Within each column, significant differencesin brushing force at the different sites of each sextant were marked by differentcapital letters. Within each site, significant differences between the fourthto sixth sextants were marked by different Greek letters Within one toothbrush,differences between brushing forces before and after instruction of thebrushing technique were not significant independently of the site and sextant
Summarizing the multiple comparisons between sextants and sites,brushing forces in the premolar/molar region (first, third, fourth and sixthsextants) were slightly lower on the vestibular site than on the occlusal andpalatal/lingual sites. The incisors and canines (second and fifth sextants) werebrushed with higher brushing forces at the vestibular and palatal/lingual sitesthan on the incisal site. Overall, brushing forces within the vestibular,occlusal or palatal/ lingual sites were relatively consistent and in a narrow range.
Brushing forces before and after instruction of the respective brushingtechnique were not significantly different.
Substance loss of sound and eroded specimens is presented in Figs. 1 and 2.
Abrasion of sound enamel was greatest for specimens brushed withthe sonic toothbrush Sensonic Professional SR-1000 E, followed by the sonictoothbrush Sonic complete DLX and the manual toothbrush. Abrasive potential of thedifferent toothbrushes on eroded enamel was only slightly, but mostly notsignificantly different. The manual toothbrush (1.6 N brushing force) causedsignificantly higher abrasion of sound and eroded dentin than the sonictoothbrushes (0.9 N brushing force).
Thisstudy showed that the manual toothbrush was applied with higher brushing forcesthan sonic toothbrushes, independently of the sextant, the site and whether thebrushing technique was instructed or not. Applying these higher brushing forcesresulted in an increased abrasion of sound and eroded dentin, but not of soundand eroded enamel as compared with the sonic toothbrushes. Therefore, the firstnull hypothesis that the brushing forces of manual and sonic toothbrushes arenot significantly different was rejected. The second null hypothesis that theabrasion of sound and
Fig. 1 Enamel (a) and dentin (b) loss (mean ± standarddeviation, micrometres) of sound specimens after brushing with the different toothbrushes.Significant differences are marked by different letters.
Fig. 2 Enamel (a) and dentin (b) loss (mean ± standarddeviation, micrometres) of eroded specimens after 10, 20 and 30 cycles of the erosion–abrasionexperiment. Significant differences after 30 cycles are marked by differentletters
eroded dental hard tissue is not significantly different betweenthe toothbrushes at their specific brushing force was accepted for erodedenamel and rejected for dentin and sound enamel.
For the first time, the brushing forces of sonic and manualtoothbrushes were systematically analysed for different sextants and sites and—incase of the sonic toothbrushes— for different frequencies. Generally, brushingwith the manual toothbrush demonstrated significantly higher brushing forces onall sites compared with the sonic toothbrushes at both low and high frequencymodes. The mean brushing force of the manual toothbrush was slightly lower thanbrushing forces reported earlier [21, 25].Although the purpose of the study was disclosed only at the end of the study, itcan not be excluded that toothbrushing habits of the volunteers were affectedby the awareness of being monitored and the fact that they were asked toperform the brushing systematically in 20-s intervals.
Toothbrushing forces in the premolar/molar region were slightly lower onvestibular than on occlusal or palatal/ lingual sites. A similar trend wasshown by van der Weijden et al. ,where significantly higher brushing forces on lingual than on buccal toothsurfaces were observed. As oral sites might be more difficult to access than vestibulartooth surfaces, brushing forces might be increased unconsciously. However,brushing forces within vestibular or palatal/lingual sites were relativelyconstant among the different sextants, confirming that the mean brushing forcesof uninstructed adults do not differ significantly between different quadrants.
Toothbrushing forces at baseline and 2 weeks after instruction of the correctbrushing technique were not significantly different. Heasman et al.  observed that the brushing forces ofoscillating–rotating but not of manual toothbrushes decreased slightly within a6-week training period, which was attributed to the controlled pressure systemof the toothbrushes, which provided feedback by a click if a certain thresholdwas reached. However, the sonic toothbrushes tested in the present study didnot exhibit any feedback system for control of brushing force. Therefore, it isassumed that brushing force—at least at the certain level found in the presentstudy—is not affected by the brushing technique. Moreover, it has to be takeninto account that the correct brushing technique can be hardly adopted even in highlymotivated patients [27, 28]. Although the volunteers were asked to brush their teeth in a specificsequence with the correct brushing technique at the second visit, it is speculatedthat the brushing technique was not fully adopted during the 2-week trainingperiod at home.
In the in vitro experiment, the mean brushing forces determined in vivo weretransferred to the automatic brushing machine that controls all relevant keyparameters in a standardized manner . In both abrasion experiments, the brushing time with thesonic toothbrushes was increased by a factor of 1.28 to account for thedifferent length of the toothbrush heads. As electric toothbrushes are usuallygently moved from one tooth to the next, the sonic toothbrushes were appliedwith fewer linear brushing strokes than the manual toothbrush [9, 10]. However, the brushing speed was not shown to have anysignificant impact on enamel and dentin abrasion .
Abrasion of sound specimens was intentionally exaggerated to a level whereenamel loss exceeds the detection limit of the profilometer (lower limit ofquantification, 0.105 μm) .The parameters of the erosion–abrasion experiment, especially the shortduration of erosion and abrasion treatment, followed recent recommendationsaiming to simulate clinical conditions as closely as possible . The use of
bovine enamel and dentin instead of human dental hard tissue inerosion/abrasion experiments is widely accepted, especially as relative tissueloss rather than absolute values are of interest and only slight differencesbetween human and bovine substrates exist [29, 30].
The present results confirmed previous studies showing thatdentin abrasion is indeed higher for manual than for power toothbrushes [16, 17]. The lower abrasivity of the sonic toothbrushes is most likelydue to the lower brushing force. While it was demonstrated that dentin abrasiondue to brushing with manual toothbrushes at 2.5 N and power toothbrushes at 1.5N brushing force was not significantly different, decreasing the brushingforces of power toothbrushes to 0.9 N approximately halved dentin loss . Differences in filament stiffness,bristle design and particular movement of the toothbrushes might also affectthe results. The high frequency mode of the sonic toothbrushes caused higherwear than the low frequency mode, and the Sensonic Professional SR-1000 Etoothbrush caused higher abrasion than the Sonic complete DLX. However,overall, these differences are suggested to be less relevant when compared tothe brushing force, as it was shown that the abrasivity of various ultrasonic,sonic, oscillating–rotating and manual toothbrushes varied only slightly whenthe brushes were applied on the same force . The higher wear of eroded dentin in specimens brushed withthe manual toothbrush is also attributed to the higher brushing force as Gansset al.  demonstrated that the exposedorganic layer of eroded dentin is compressed with increasing brushing forces.
However, the lower abrasivity of sonic toothbrushes was notconfirmed on enamel specimens. On sound enamel, both sonic toothbrushes causedgreater loss than the manual toothbrush, although it has to be noticed that thesusceptibility against abrasion was distinctly lower for enamel than fordentin. Abrasion of eroded enamel was not significantly different among thedifferent toothbrush groups. These results are in accordance with a previousstudy  which found no significant impact ofthe brushing force on enamel abrasion when brushing forces were below 4.5 N(sound enamel) or 3.5 N (eroded enamel), respectively. Even at the samebrushing force, the abrasion potential of sonic and
manual toothbrushes was not significantly different , indicating that under the conditions of the present study, thesoftened enamel layer is removed completely irrespective of the kind oftoothbrush.
From the results of the present study, it can be recommended that patients withsevere tooth wear and exposed (eroded) dentin surfaces should use sonictoothbrushes to reduce abrasion, while patients without tooth wear or with erosivelesions confined only to enamel do not benefit from sonic toothbrushes withregard to abrasion.
Acknowledgements The authors wish to thank all volunteers fortheir participationin the study. We also thank the companies BiomedAG, Switzerland and Procter & Gamble GmbH, Germany, forproviding the toothbrushes.Conflictsof interest The authorsdeclare that they have no conflicts
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