Introduction: Proper tooth brushing is a complicated process for children. Therefore, the aim of this study was to investigate the effect of differential learning to improve tooth brushing in children. Methods: In this prospective, controlled, single-blinded, randomized clinical trial, 58 children between 3 and 8 years of age (mean: 5.7 ± 1.5 years; 29 female) were randomly assigned to test or control group through the child’s self-drawing of an unlabeled envelope from a box. All children received oral hygiene instructions and information in these sealed envelopes and were asked to follow the corresponding instructions at home for 28 days. Children in the test group received instructions with exercises using the differential learning method, whereas the children in the control group received the usual tooth brushing instructions. Results: At baseline and planned follow-ups after 4 and 12 weeks, plaque and gingival indices (QHI, PBI) were recorded in both groups by 2 calibrated and blinded investigators. At baseline, there were no significant differences between the test and control groups regarding plaque and gingival indices (QHI: 4.1 ± 0.5 vs. 4.1 ± 0.4; p = 0.7; PBI: 0.6 ± 0.3 vs. 0.6 ± 0.3; p = 0.7). At the 1st and 2nd follow-up, both groups showed improved oral health indices, but there was an overall better improvement in the test group. While the difference in gingival indices was statistically significant in the 1st recall (PBI/test: 0.1 ± 0.2 vs. control: 0.3 ± 0.2; p < 0.001), the difference in plaque indices was not (QHI/test: 2.1 ± 0.9; control: 2.6 ± 0.9; p = 0.07). At the 2nd recall (mean week = 19.5 weeks), the test group showed statistically significant and clinically relevant better oral health indices than the control group (2nd recall, QHI/test: 2.1 ± 0.9 vs. control: 3.2 ± 1; p < 0.001; PBI/test: 0.1 ± 0.2 vs. control: 0.5 ± 0.2; p < 0.001). Conclusion: In conclusion, differential learning leads to oral hygiene improvement in children with high caries risk and initially poor oral hygiene, which was superior to the conventional learning method through repetition in the medium term.

Although dental caries is a preventable disease, it is still one of the most prevalent chronic diseases in children [1]. According to the World Health Organization [2], approximately 514 million children suffer from deciduous teeth decay. This enormous number demonstrates the need to start caries prevention at the earliest age possible. Therefore, efficient oral hygiene is essential and should be maintained in the long run [3, 4]. It should be noted that tooth brushing behavior develops concurrently with the child’s development and duration of tooth brushing. Thus, tooth brushing skills of children under 10 years need to be improved and monitored [5]. Trying to modify and improve brushing the teeth in the traditional way through instructions and demonstrations may not be enough to achieve effective tooth brushing [6, 7]. A useful method in this regard could be differential learning. Differential learning refers to an approach that assists individuals in finding their own optimal motor performance patterns for given complex tasks, such as tooth brushing [8]. This approach has been shown to be effective in sports [9], in training schoolchildren to brush their teeth [10] and in dental education [11]. However, more research is needed to prove its effectiveness, especially in its application at home.

The traditional tooth brushing method relies on repetition and correction to perfect specific movements [12, 13]. Differential learning is based on as many variations of the movement to be trained as possible rather than on the movement repetition or correction as the basis of learning [9]. Hence, to enable learners to achieve an optimal result, the learning process is promoted by adding different movements or tasks to the targeted movement [8, 14]. Therefore, the aim of this randomized, controlled, blinded, clinical trial was to investigate the effects of differential learning at home versus habitual tooth brushing via the assessment of the changes in plaque levels and gingivitis in children aged 3–8 years.

This two-arm, single-blinded, randomized, and controlled clinical trial was conducted after ethical approval by the Ethics Committee of the University of Greifswald (Reg. No. BB 031/21) at the Department of Preventive and Pediatric Dentistry, University of Greifswald. It was also registered before the enrollment at the NCT registry (NCT04905784). Furthermore, the study was conducted in accordance with the Declaration of Helsinki [15] and followed the CONSORT guidelines (Fig. 1). In the study conception phase, the CONSORT checklist and risk of bias tool RoB-2 was considered to assure methodological quality and minimize possible bias [16, 17].

Fig. 1.

Flowchart according to CONSORT for this study.

Fig. 1.

Flowchart according to CONSORT for this study.

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Patient’s Consent

Patients were fully informed about the aim of the study, the study procedure, and the potential benefits or risks of this study. Parents or caregivers were given time and opportunity to inquire about the details of the study and to decide whether or not to participate. As a mandatory requirement for participation in this study, uncoerced written consent was obtained from the accompanying caregiver of each child.

Inclusion and Exclusion Criteria

Inclusion criteria were children with poor oral hygiene (determined by high plaque accumulation noticed at baseline examination session using plaque disclosing agent) aged 3–8 years, needing assistance with tooth brushing by parents and their willingness to attend follow-up appointments after approximately 4 and 12 weeks. Exclusion criteria were patients with acute pain requiring immediate treatment, patients with serious systemic diseases requiring special attention during their dental treatment and parents or children who refused to participate in the study.

Randomization and Allocation

This study was single-blinded, meaning that the investigators of the oral hygiene indices were unaware of the participant group (test vs. control group). Children were randomly enrolled by placing information and instructions for both groups in white unlabeled envelopes that were prepared by one of the investigators (L.L.); of which half contained instructions for each group. These envelopes were randomly shuffled and placed in a box. Therefore, randomization was not in the hands of the investigators or the study supervisors. During the baseline examination, the investigator asked the patients to randomly select an envelope from the box. Patients were asked not to open it until they got home. They were also asked to bring the envelope closed back to the 1st follow-up appointment. The envelopes remained closed and were collected in a separate box and were only opened after the end of the study period.

Primary and Secondary Outcomes of the Study

The examinations at baseline and at the 1st and 2nd recall visits included common oral hygiene indices as a primary improvement indicators and were: the papillary bleeding index (PBI) according to Mühlemann and Saxer [18] and the plaque index according to Quigley-Hein Index (QHI), modified according to Turesky et al. [19]. The plaque score (QHI) was measured after staining the teeth with a plaque disclosing solution (Mira-2-Ton, Hager & Werken, Duisburg, Germany). Plaque accumulation was measured on the buccal surfaces of all teeth on a six-point scale: score 0 (no plaque) to score 5 (plaque extends to the coronal third).

The gingival inflammation was evaluated by using a periodontal probe for this purpose. The PBI ranges from score 0 (no bleeding/inflammation-free gingiva) to score 4 (heavy bleeding/severe inflammation). As a secondary outcome, dental caries indices (DMFT, dmft) according to the WHO criteria [20] were recorded during the baseline examination.

Study Procedures

At first visit, both groups received a baseline examination and the standard preventive program offered within the German National Health System consisting of explaining the etiology of caries and preventive approaches. In addition, the first examination of oral health indices (QHI, PBI) was performed. Participants were then asked to randomly select one unlabeled envelope and open it at home, and follow the corresponding brushing instructions for the next 28 days. At 1st and 2nd follow-up, plaque and bleeding scores were measured again.

For the control group, the usual instructions for tooth brushing were placed in their unlabeled sealed envelopes. Participants were instructed in a defined brushing sequence (COI technique: chewing surfaces, outside surfaces, inside surfaces) and in horizontal angle, with the tooth brush placed perpendicular to the tooth surfaces and using a back-and-forth motion. Children and parents were asked to repeat the tooth brushing sequence and technique daily.

In the differential learning group, participants also received printed instructions in their unlabeled sealed envelope and were asked therein to perform tooth brushing at home with different exercises (each exercise for 3 days, then change to the next exercise). These exercises were explicitly shown using figures included in the instructions sheet regarding brushing teeth as follows:

  • 1.

    While lying down.

  • 2.

    With one eye covered (i.e., with the hand).

  • 3.

    With large gloves.

  • 4.

    With the non-dominant hand (i.e., left for right-handers).

  • 5.

    While grasping the tooth brush with both hands.

  • 6.

    With a different order (i.e., starting with brushing the inner surfaces first instead of the chewing surfaces).

  • 7.

    With closed eyes.

  • 8.

    With an obstacle at the elbow of the dominant hand (e.g., wrap a piece of cloth/scarf around the elbow for this purpose).

  • 9.

    While watching a 3–5 min video (e.g., on YouTube).

Bias Preventing Strategy

Due to the selected age range and the variability in tooth change, the highest possible range and thus internal validity of the study could be ensured. Moreover, the age of the children and the type of the tooth brush (manual or electric) was considered in the analysis as a potential influencing factor.

To motivate the children and at the same time to ensure their adherence to the study protocol, both groups received a sticker sheet in the envelope and a table sheet on which the patients were asked to place a sticker on each day they brushed their teeth according to the instructions. To assess potential bias parents in the test group were asked about difficulties in performing the differential learning exercises in the follow-up questionnaire.

Also, to ensure that patients attend the recall sessions, a follow-up appointment was set immediately after the completion of the first examination session, and patients were given an additional sheet containing the details of the new appointment. In the event that the participants did not show up to either follow-up appointment, attempts were made to contact them by telephone to set a new appointment as soon as possible. If they denied attending the follow-up appointment, they were asked to reveal their group.

Examiners Validation

At different time points (baseline, week 4, and week 12), plaque and bleeding scores were collected by two blinded investigators who were intensively trained and calibrated to assess plaque and papillary bleeding scores in advance. For validation purposes, inter-examiner reliability (between L.L. and M.K.) and intra-examiner reliability were assessed by evaluating 30 photographs in advance that represented different plaque and papillary bleeding scores, respectively. The inter-examiner weighted kappa was 0.8 (PBI) and 0.9 (QHI), and the intra-examiner weighted kappa was 0.9 (PBI) and 0.9 (QHI), representing very good agreement.

Sample Size Calculation and Statistical Analyses

Sample size was calculated in advance using the program G*power version 3.1 (Franz Faul, University of Kiel, Germany), according to previous similar oral hygiene studies [10, 21]. Assuming a difference of 0.5 in QHI as the primary outcome variable between groups during follow-ups, α = 5% and power (1 - β) = 0.9 was set, resulting in 22 children for each group. To compensate for dropouts at follow-ups (assuming approximately 30% dropouts), the final sample included 29 children in each of the test and control groups (total n = 58).

Data were documented and analyzed using Microsoft Excel (Version 2010; Microsoft, Redmond, WA, USA) for Windows. The significance threshold was set at a value of p < 0.05. Means, standard deviation, absolute numbers, and percentages were calculated for descriptive analysis. Comparisons between the two study groups were made using the independent samples t test for quantitative variables (age, dmft/DMFT, PBI, and QHI) and using the χ2 test for qualitative variables (gender).

A total of 58 children were recruited for the study between May 2021 and January 2022. The final statistical analysis included 46 participants (22 control group, 24 test group; mean age 5.7 ± 1.5 years). The reason for dropouts was missing the follow-up appointments (dropouts test group: n = 5; control group: n = 7, Fig. 1). The baseline characteristics of the study sample and the baseline characteristics of the dropouts (Table 1) indicated neither any systematic bias with respect to the distribution in test or control group, respectively, nor with respect to other characteristics of the dropouts.

Table 1.

Baseline characteristics of the study sample

Baseline variablesControl groupTest groupDropoutsp value (control/test)
Participants n = 29 n = 29 n = 12  
Gender Female, n = 17 (59%) Female, n = 12 (41%) Female, n = 7 (58%) 0.2 
Average age ± SD, years, min.–max. 5.5±1.5, 3–8 (age ≤5; n = 10) 5.9±1.5, 3–8 (age ≤5; n = 7) 6.0±1.3, 3–7 (age ≤5; n = 3) 0.3 
Mean dmft ± SD, min.–max. 8.3±3.9, 1–16 (dmft <4; n = 3) 9.0±4.5, 0–20 (dmft <4; n = 3) 8.6±5, 2–20 (dmft <4; n = 3) 0.5 
Mean DMFT ± SD 1.1±1.3 1.7±2.1 1.4±1.7 0.3 
Mean QHI ± SD, min.–max. 4.1±0.4, 3.5–4.9 4.1±0.5, 3.1–5 4.1±0.4, 3.6–4.7 0.7 
Mean PBI ± SD, min.–max. 0.6±0.3, 0–1.2 0.6±0.3, 0–1.5 0.5±0.3, 0–0.9 0.7 
Baseline variablesControl groupTest groupDropoutsp value (control/test)
Participants n = 29 n = 29 n = 12  
Gender Female, n = 17 (59%) Female, n = 12 (41%) Female, n = 7 (58%) 0.2 
Average age ± SD, years, min.–max. 5.5±1.5, 3–8 (age ≤5; n = 10) 5.9±1.5, 3–8 (age ≤5; n = 7) 6.0±1.3, 3–7 (age ≤5; n = 3) 0.3 
Mean dmft ± SD, min.–max. 8.3±3.9, 1–16 (dmft <4; n = 3) 9.0±4.5, 0–20 (dmft <4; n = 3) 8.6±5, 2–20 (dmft <4; n = 3) 0.5 
Mean DMFT ± SD 1.1±1.3 1.7±2.1 1.4±1.7 0.3 
Mean QHI ± SD, min.–max. 4.1±0.4, 3.5–4.9 4.1±0.5, 3.1–5 4.1±0.4, 3.6–4.7 0.7 
Mean PBI ± SD, min.–max. 0.6±0.3, 0–1.2 0.6±0.3, 0–1.5 0.5±0.3, 0–0.9 0.7 

Dropouts until 2nd follow-up from both control and test groups; characteristics also included in baseline description of control and test group.

Plaque

Initially, mainly high mean plaque index values were obtained, which were almost identical for both groups and thus not statistically different (QHI: test group: 4.1 ± 0.5 vs. control group: 4.1 ± 0.4, p = 0.7; Table 1). During the 1st recall, plaque levels were reduced by approximately half in both groups. Despite a small advantage in the test group, the difference between the two groups was not statistically significant (test group: QHI = 2.1 ± 0.9 vs. control group: QHI = 2.6 ± 0.9; p = 0.07). During the 2nd recall, the test group had remained stable and the control group had regressed, resulting in a clear statistically significant difference (test group: QHI = 2.1 ± 0.9 vs. control group: QHI = 3.2 ± 1; p < 0.001; Table 2; Fig. 2). Within both groups, the improvements in plaque index between baseline and recall were also statistically significant (p < 0.001).

Table 2.

Mean values for plaque indices (QHI), papillary bleeding index (PBI), and follow-up intervals at 1st and 2nd follow-up for the test and control groups

Follow-up variables1st recall control group1st recall test groupp value2nd recall control group2nd recall test groupp value
Participants n = 24 n = 26  n = 22 n = 24  
Mean follow-up intervals ± SD, weeks 6.6±3.9 4.7±1.4 0.02 19.4±7.0 20.1±8.1 0.8 
Mean QHI ± SD 2.6±0.9 2.1±0.9 0.07 3.2±1.0 2.1±0.9 <0.001 
Mean PBI ± SD 0.3±0.2 0.1±0.2 <0.001 0.5±0.2 0.1±0.2 <0.001 
Follow-up variables1st recall control group1st recall test groupp value2nd recall control group2nd recall test groupp value
Participants n = 24 n = 26  n = 22 n = 24  
Mean follow-up intervals ± SD, weeks 6.6±3.9 4.7±1.4 0.02 19.4±7.0 20.1±8.1 0.8 
Mean QHI ± SD 2.6±0.9 2.1±0.9 0.07 3.2±1.0 2.1±0.9 <0.001 
Mean PBI ± SD 0.3±0.2 0.1±0.2 <0.001 0.5±0.2 0.1±0.2 <0.001 
Fig. 2.

Development of Quigley-Hein index (QHI) in control and test groups at baseline and follow-up shown by boxplots with mean (line) and 1st–4th quartiles. *Significant difference (p < 0.001). Within both groups, the improvements in plaque index between baseline and recall were also statistically significant (p < 0.001).

Fig. 2.

Development of Quigley-Hein index (QHI) in control and test groups at baseline and follow-up shown by boxplots with mean (line) and 1st–4th quartiles. *Significant difference (p < 0.001). Within both groups, the improvements in plaque index between baseline and recall were also statistically significant (p < 0.001).

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Gingivitis

At baseline, gingival health status was almost the same in both groups (PBI: test group: 0.6 ± 0.3 vs. control group: 0.6 ± 0.3; p = 0.7; Table 1). During 1st recall, the mean values of PBI were significantly lower than baseline in both groups.

However, a statistically significant lower PBI was observed in the test group compared to the control group (PBI/test: 0.1 ± 0.2 vs. 0.3 ± 0.2; p < 0.001). During the 2nd recall, mean PBI increased again in the control group compared with the 1st recall visit but remained stably low in the test group. Therefore, the differences between the two groups were again statistically significant and also clinically relevant (Table 2; Fig. 3). In line with the overall results, no statistically significant differences in oral hygiene parameters (QHI and PBI) could be detected between younger children (3–5 years old) in comparison to the older children (6–8 years old) within the control group for baseline, first and second recall. This is similarly the case for the test group, while here we had a statistically significant better QHI in the first recall appointment in the 7 pre-school children (QHI: 1.2 ± 0.6 vs. 2.4 ± 0.9 in schoolchildren; p = 0.006).

Fig. 3.

Development of the papillary bleeding index (PBI) in control and test groups from baseline to follow-up by boxplots with mean (line) and 1st–4th quartiles. *Significant difference (p < 0.001). Within both groups, the improvements in plaque index between baseline and recall were also statistically significant (p < 0.001).

Fig. 3.

Development of the papillary bleeding index (PBI) in control and test groups from baseline to follow-up by boxplots with mean (line) and 1st–4th quartiles. *Significant difference (p < 0.001). Within both groups, the improvements in plaque index between baseline and recall were also statistically significant (p < 0.001).

Close modal

Motivation Sheet and Short Questionnaire Analysis

Fifty-nine percentage (n = 13) of the 22 eligible control group participants for analysis and 75% (n = 18) of the 24 eligible test group participants filled and returned the sticker sheet. All of them have stuck a sticker for all 28 days.

Most of the brushing exercises were not assessed as difficult by all parents. Less than 20% of the parents reported only for the exercises “brushing while lying down” and “brushing with the non-dominant hand” that they were difficult but still doable. There were also no statistically significant differences regarding the type of tooth brush used.

This randomized, controlled clinical trial investigating the effect of differential learning on tooth brushing education of children used two well-established and validated oral hygiene indices to assess the amount of plaque and the extent of gingival inflammation which should be reduced by improved mechanical plaque removal [22, 23].

The number of dropouts for families/children with high caries risk and, therefore, potentially low compliance was 20% (n = 12) and below the expected value (30%). It can be considered as neutral, since 67% (n = 8) of these dropouts already occurred initially between group assignment and the 1st recall (Fig. 1). The overall ratio was rather balanced with 5:7 in test and control groups, respectively. Thus, a dropout-related bias is not very likely. Randomization resulted in similar plaque and gingivitis scores at baseline of the study groups and even the dropouts.

The initially planned 1st follow-up after 4 weeks and the 2nd follow-up after 8 weeks were delayed due to the pandemic of COVID-19, resulting in mean values for the 2nd follow-up of 19.4 ± 7 weeks in the control group and 20.1 ± 8.1 weeks for the test group which actually allowed for a longer follow-up time and possibly clearer conclusions.

The very high caries experience scores (⌽ 8.7 dmft/1.5 DMFT, at an average age of 5.7 years, where ⌽ denotes mean value) were significantly above average German caries levels (6-7-year-olds: 1.7 dmft; 12-year-olds: 0.5 DMFT) [3, 24]. Initial plaque and gingivitis scores were also high compared to other studies investigating plaque control by tooth brushing in children [21, 25] completing the picture of a high caries risk group.

Interestingly, the children in both study groups showed significant improvements in oral hygiene at the 1st follow-up, probably due to the Hawthorne effect, in which participation in an observational study alone causes changes [26]. This underscores the need to conduct a randomized controlled clinical trial to neutralize such effects. Regarding the oral health parameters in the second follow-up, the results demonstrate that differential learning for tooth brushing preserved the improvements in plaque and gingival scores in contrast to the traditional training with repetition in the control group, which seemed to revert back to the baseline oral health situation. Thus, it shows in line with other studies that differential learning leads to further improvements mainly in the retention phase, while traditional training results in stagnation or even reversals [9, 27‒29].

In contrast to many studies where habitual tooth brushing patterns are difficult to overcome and improvements in oral hygiene are challenging to achieve [6, 7, 30, 31] differential learning for optimizing tooth brushing seems to be a promising approach. Interestingly the plaque levels, in both short and medium terms could be improved, which is consistent with the results of other related studies to differential learning, including those from other disciplines [10, 11, 29].

The contribution of parents in brushing the teeth of their children could also be a most important factor in these quite young caries risk children (3–8 years), but due to the high baseline plaque, gingivitis and caries levels this was probably suboptimal until the beginning of the study. Since the parents in both groups were asked to brush their children’s teeth using the same instructions, it can certainly be assumed that the effect of this method applies not only to the children but also to their parents [11].

Simple instructions implementing the differential learning method for tooth brushing at home can lead to significant greater oral hygiene improvements even in children with a high caries risk and poor oral hygiene than traditional instructions. This seems to be valid especially for the medium term retention phase making the concept of differential learning in individual prophylaxis a promising approach.

The authors declare that they have not got any support from any party.

All procedures performed in studies involving human participants were in accordance with the ethical standards of the Institutional and/or National Research Committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. This has been approved by the Local Ethics Committee of the medical faculty in Greifswald University under number BB 031/21. Also, written informed consent was obtained from children and their parents or legal guardians.

The authors have no conflicts of interest to declare.

The authors received no financial support for the research, authorship, and/or publication of this article.

Loay Leghrouz contributed to conception, design, acquisition, analysis, interpretation, and drafted manuscript. Manasi R. Khole contributed to acquisition and critically revised the manuscript. Christian H. Splieth contributed to conception, interpretation, and critically revised the manuscript. Julian Schmoeckel contributed to conception, design, analysis, interpretation, and critically revised the manuscript. All authors gave their final approval and agree to be accountable for all aspects of the work.

All data generated or analyzed during this study are included in this article. Further inquiries can be directed to the corresponding author.

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