Cleaning potential of interdental brushes around orthodontic brackets – an in vitro investigation

Introduction

Adequate mechanical plaque control is the key in maintaining good oral health, also in patients with fixed orthodontic multibracket appliances (ATACK et al. 1996; HUSER et al. 1990). The risk for oral pathologies like caries and periodontal inflammation can thereby significantly reduced (LIU et al 2011). Especially demineralization around brackets represents a severe problem (KUKLEVA et al. 2002; BUCK et al. 2011). Various studies have demonstrated that the use of specific fluoride sources and regular instruction in cleaning methods may have a significant impact to reduce the risk of caries (NORDSTRÖM & BIRKHEAD 2010; DUANE 2012). Moreover, the periodontal health condition of patients with multibracket appliances should be carefully monitored, and a good prophylaxis program and routine diagnostics must also be strictly envisaged in this regard. Studies have highlighted the clinical efficacy in preventing or at least reducing periodontal problems in patients with fixed orthodontic appliances under the premise of adequate oral hygiene (HUBER et al. 1987). In general, it remains a principal clinical concern in orthodontics that pathologies are not promoted by iatrogenic or even elective interventions.

With regard to mechanical aids for cleaning around brackets, various oral hygiene aids are available. Of course, the manual toothbrush remains the most common and established tool. When different electric toothbrushes are compared, the sonic powered brushes seem to perform better than the oscillating rotating counterparts (SCHÄTZLE et al. 2010). However, and in general, electric toothbrushes are not significantly superior to the manual toothbrush in terms of cleaning efficacy, especially when additional aids such as monobrushes and other cleaning agents for the inter-proximal areas are used (JACKSON 1991; ROSEMA et al. 2008). Still, most brushes display some deficits and do not reach all areas that are partially inaccessible due to brackets and wires. Therefore, additional are still warranted in patients with multibracket appliances for cleaning the buccal area between the brackets and the ligated archwire.

The use of interdental brushes around the plaque-retentive areas under the wires around brackets may be a supplementary method of caring for these areas even more efficiently, which was highlighted a previous report based on the Zurich laboratory model for evaluating the accessibility and cleanability of interdental spaces in teeth with brackets (SCHMIDLIN et al. 2022). This study used a scoring system referring to six surfaces around brackets and showed that the gathered data can applied either dichotomously (plaque presenta: yes/no), with quantitative numerical values (percentage of cleaned/uncleaned areas), or respective scores. The results revealed subtle differences in terms of cleaning efficacy already between different interdental waist-shaped brushes with different filament numbers. The model could therefore also be used to compare the cleansing efficacy waist-shaped interdental brushes to their straight cylindrical counterparts. A previous study has shown that – in the interdental area - waist-shaped brushes lead to significantly lower plaque scores, especially in difficult to access buccal and lingual line angles (CHONGCHAROEN et al. 2012).

Based on this previous work, the aim of the present study was to establish a new in vitro model and to test the brushing efficacy of three commercially available interdental brushes (IDB) with comparable diameters (two cylindric ones at diameters of 5 an 9 mm, respectively, and on waist shaped IDB, 9-5-9 mm) around orthodontic brackets in vitro. The null hypothesis was that all IDB would clean equally with identical pressures applied.

Materials and Methods

Interdental brushes

Three interdental brushes with comparable filament size and diameter (Top Caredent, Zurich, Switzerland) were tested: A was waist-shaped IDB with diameters of 9 mm at both ends and 5 mm in the middle (B1), 2) a cylindrical brush with a diameter of 9 mm (B2) and an one with a diameter of 5 mm (B3; Fig. 1). The brush diameters were selected based on pre-tests on the models. The choice of the set of corresponding brushes was defined by a clinically acceptable patency in our models. To test the cleaning performance and the associated force for insertion into the interdental spaces, anatomic tooth models and a brushing machine were custom-made representing the maxillary anterior region from tooth 13 to 23.  

In vitro tooth models

The model teeth were cast from black-colored acrylic resin and fitted into a pink-colored gingival mask to emulate the in vivo anatomy of four clinical situations as closely as possible (Fig. 2).

For this purpose, model teeth (Kilgore International, Chicago, USA) were used as templates for the teeth and respective molds were fabricated using a duplicating silicone material (Siladent Dr. Böhme & Schöps GmbH, Goslar, Germany). The teeth were then cast from liquid acrylic with a black color (Siladent Dr. Böhme & Schöps GmbH, Goslar, Germany).

For the production of the gingival masks, anatomic models (Kilgore International, Chicago, USA) were used as a template for the production of the soft tissue mold. These were adapted by hand to simulate misaligned teeth with crowding and an aligned dentition. For this purpose, anatomic models were individually fabricated i and supplemented with wax (Belladi Ruscher modeling wax "Superior" pink, Shop Belladi Ruscher Schleusser GmbH, Amriswil, Switzerland) if needed. Replicas were then made (Siladent Dr. Böhme & Schöps GmbH, Goslar, Germany) and blocks were cast from liquid acrylic (1:1 A+B SilaPoly components A: Base with B: Catalysat reddish Lot: 49011542, RER: 24002 from Siladent, Dr. Böhme & Schöps GmbH, Goslar, Germany), which were dyed pink (Silapoly Color Lot: 2231548, REF 24002, Siladent Dr. Böhme & Schöps GmbH, Goslar, Germany). While the misaligned models were intended to simulate the situation at the beginning of orthodontic treatment, models with straight teeth aimed to simulate the situation after the orthodontic leveling phase.

Orthodontic brackets were placed on the buccal surfaces on all teeth as follows: Brackets (American Orthodontics, Washington, USA) were attached to the central buccal surfaces of the teeth vertically aligned to the respective tooth axis. In order to fix the brackets, small round notches were milled in the areas to be bonded, each 0.5 mm wide and deep (MAILLEFER rose drill D0023: RA medium, 28 mm, ISO 018, Densply, Charlotte, USA). A primer was the applied (SR Connect, Ivoclar Vivadent AG, Schaan, Lichtenstein) and cured for 20 seconds (Bluephase G2, ivoclar vivadent, Schaan, Liechtenstein). The brackets were then bonded dual-curing material (Variolink Esthetic, Ivoclar Vivadent AG, Schaan, Lichtenstein), which was also light-cured for 20 seconds. A round 014 steel wire (Ormco Corportation, California, USA) with elastic rubber ligatures (G&H Orthodontics, Franklin, USA) was ligated into the brackets.

Brushing device, cleaning and force evaluation process

For reproducible testing, a brushing device was custom-made to test the cleaning efficacy of the different interdental brushes (Fig. 3). The test apparatus was made of Kanya frames (Kanya AG, Rüti, Switzerland). The applied motion sequences were performed manually. With the aid of this appliance, the IDB was inserted into the labial surface between the brackets and the wire.  The models were fixed in such a way that the tooth axes were horizontally positioned. The positioning of the models in the appliance was achieved by means of a cross table with an attached adapter. This cross table allowed the models to be aligned in the X and Y directions. In this setup, the cleaning efficacy and the pressure of the individual brushes could be determined in predefined sites (Fig. 4).

Before brushing, the black teeth were evenly coated with a titanium oxide suspension (suspension of titanium oxide in ethanol 26% by volume in a ratio of 1:3), which was carefully fixed using a hair dryer at a distance of 30 cm for 60 s.

The coated models were then placed in the appliance. The spaces between teeth 13 to 23 of each model were tested. This resulted actually in five interdental spaces. The IDBs were applied five times. Each of the sixteen models was cleaned once with each of the three test brushes. Subsequently, the tested models were digitally photographed with alignment to the respective interdental spaces. This resulted in five images per model. The resulting 2D images were evaluated with the aid of a planimeter. The tooth surfaces of the evaluated teeth were divided into eight regions around the brackets. Each surface was evaluated individually and the percentage of cleaned area was calculated. Tooth areas that were free of the white coating after the brushing test and appeared black or gray again were considered to be potentially cleaned (IMFELD et al. 2000).

The cleaning efficacy of the different brushes was visualized arbitrarily on the basis of the cleaning efficacy as follows:

• Between zero to 24.9% (black)
• Between 25 to 49.9% (red)
• Between 50 to 74.9% (orange)
• Between 75 to 100% (green)

For the measurement of the applied forces, the device was modified with a force gauge, which was additionally attached and allowed for measurements in the range between 0 cN to 1000 cN. Depending on the design of the interdental brush and the available space, more or less pressure was required. The forces were also visualized arbitrarily on the basis of the cleaning efficacy: Green stood for little applied pressures and therefore easy handling at respective pressures ranging from zero to 200 cN was defined. Ranged between 200 to 500 cN indicated an elevated required pressure and more difficulty to apply and clean (orange), whereas the color red stood for pressures at > 500 cN, i.e. . the wire of the brush started bending or the brush could not even be inserted (> 1000 cN).

Data presentation and statistics

Statistical analysis was performed using the computer software R (R Core Team 2021) with the aid of the packages dplyr packages (WICKHAM et al., 2022), ggplot2 (WICKHAM et al., 2016), and xtable (DAHL et al., 2019).

The effect of brush and model on expected cleaning performance was examined using an analysis of variance (ANOVA). Categorization of the cleaning performance was only for visualization and not for subsequent analyses. A significance level of alpha = 0.05 was used for all statements made here. Categorization was only for visualization and not for the subsequent analyses. A significance level of alpha = 0.01 was used for all statements made here.

Results

The results of the cleaning potential are numerically provided in Table 2 and graphically elucidated in Figure 5. The latter clearly shows distinct cleaning features and differences between the different brushes with better cleaning tendencies favoring the cylindrical brushes, slightly preferring those with the larger diameter. The waist-shaped IDB showed clear deficits in the cervical and sometimes middle area. A clear cleaning pattern between the different models and tooth alignment conditions was not evident. In general, mean cleaning values over 75% were rarely observed and mostly observed - if – for the thick cylindrical brush in the incisal and middle areas. A clear difference between mesial and distal areas was not evident.

The lowest pressures were clearly observed within all models treated with the thin IDB, followed by the waist-shaped IDN, which also showed low pressure profiles for the models representing misaligned teeth and therefore interproximal steps and increased spaces. The aligned models showed medium pressure forces toward the middle of the tooth arch (Table 3 and Figure 6). The IDB with the thicker diameter evidently showed the highest pressures.

Overall, the results showed that the interaction of both influencing variables, the anatomic model type and the type of the brush, had no significant effect on the cleaning performance. Thus, it can be concluded that the effectiveness of the brushes does not depend on the anatomic model used. The type of the brush used as well as the anatomic model have a significant effect on cleaning performance (Table 3).

Discussion

This study applied a novel in-vitro set-up for screening the cleaning potential of interdental brushes (IDB) around orthodontic brackets in standardized tooth models mimicking selected clinical situations, i.e. aligned and misaligned teeth with and without attachment loss, respectively. For this purpose, a brushing machine was modified for reproducible and standardized manual brushing cycles allowing for the measurement cleaning ability and applied forces.

Our hypothesis that all brushes clean equally well was rejected. Nevertheless, while the cylindrical IDB with larger diameter performed best, it also displayed the highest application force. In contrast, the small diameter cylindrical IDB showed less cleaning efficacy, however, the results were still better than the ones achieved with the waist-shaped counterparts. This is in contrast to the general understanding that waist-shaped IDB clean better especially in difficult to clean areas (SCHNABL et al. 2020; BAUMGARTNER et al. 2019). Nevertheless, one has to keep in mind, that this particular IDB design was originally intended to be applied for approximal spaces around teeth and implants (CHONGCHAROEN et al. 2012; CHEN et al. 2020; PAQUÉ et al. 2020). It seems that a simple parallel insertion under an archwire around brackets using a simplified straight back-and-forth movement in the axis of the tooth represents not a comparable situation. In general, factors like filament thickness, length, orientation and density influence patency, contact capacity and thus cleaning efficacy as well as the required force for insertion. Although all IDB were fabricated from the same company displaying comparable filaments and arrangement, ISO and patency numbers are different in the three tested brushes. The ISO standard also classifies IDB according to their patency or PHD (= Passage Hole Diameter). The latter thereby actually describes the smallest “interdental space” (simply represented as a round whole in a metal plate in mm), into which a respective IDB fits without bending the wire. This approach currently still represents the only reliable or reproducible method, which enables to compare IDB of different size and arrangement (SEKUNDO & STAEHLE 2020). Using ISO number only does not sufficiently differ between different sizes (STAEHLE et al. 2021). How these parameters relate to orthodontic applications need additional work.

Not surprisingly, the wide cylindrical IDB with the greatest ISO and patency number revealed the greatest forces during insertion and – in line with this – represented the best cleaning potential. This, because the filaments were wide and provided over the whole instrumentation length a good contact potential and pressure. Notably, as a critical observation during the set-up of the experiments, the wires, however, also tended to slightly bent under some instances; in a few cases, the IDB could not even be inserted. However, this also reflects the fine line between cleaning efficacy and possible damage potential. In this context, a higher risk for gingival trauma should also be considered. Given these findings, one can consider this brush – despite displaying actually the best results – not necessarily and generally recommendable for daily use due to potential handling characteristics and maybe even the risk of injury including also the risk of bracket detachment and wire displacement or bending.

The use of interdental brushes for cleaning the difficult-to-clean areas around brackets is not new. Previous evidence from systematic reviews, could, however, not show and recommend the general use of IDB for orthodontic patients with multibracket appliances (GOH et al, 2013). Clinical studies already showed that cylindrical IDB can be successfully used for cleaning and plaque reduction (BOCK et al. 2010). Especially for removing food debris, this brush type shows better results. Unfortunately, the overall availability of literature in that respect remains scarce.

The presented model and approach could be a valuable screening tool for the screening of interdental brushes and other devices in general and represent a valuable additional method to the ISO classification, which seems to have limitations when selecting proper instruments for this specific indication. But additional research is needed in this regard.

In this context, one has to clearly highlight the limitations of this simplified laboratory approach. Of course, the teeth and alignment modalities are arbitrarily and exemplarily chosen and do not reflect individual cases. Therefore, the results cannot be generalized. In addition, the movement of the IDB is very much simplified. Additional 3D adaptive cleaning motions were not considered. However, this may rather lead to an underestimation of the overall cleaning efficacy, since the number of strokes was limited and the cleaning not individually optimized by adapting the brushes. Another drawback of this study was the coating, which may not have been complete and uniform and certainly did not reflect an actual natural biofilm or debris. However, it ensured the determination of contact brushing by easy removal of the dried slurry. Notably, the model setting for a correct horizontal insertion direction in the areas of interest was individually adjusted by hand each time. This could theoretically have led to slight differences in the angulation or position. A completely automated and standardized method would be warranted. In addition, the wire, which was used in our study rather represents a rather thin and flexible wire for short-time levelling. The bending characteristics of maybe thicker wire types could additionally interfere with the evaluated parameters and respective differences have not been taken into considerations.

In conclusion, this study showed that cylindrical interdental brushes achieved a better cleaning performance than the waist-shaped IDB when applied in this set-up. Given the above-mentioned methodological considerations and shortcomings, more research is certainly needed in this context, but IDB may represent a valuable but still clinically underused tools, which merit further investigation.

Zusammenfassung

Einleitung

Bei Patienten mit festsitzenden kieferorthopädischen Multibracket-Apparaturen ist es besonders wichtig, die mechanische Plaque-Entfernung optimal zu gewährleisten, um durch Plaque bedingte Folgeschäden wie Karies oder Gingivitis zu verhindern. Ziel dieser Studie war die Etablierung einer In-vitro-Modellreihe zur Testung der Reinigungseffizienz verschiedener Interdentalbürsten der bukkalen Flächen um kieferorthopädische Brackets einer Multibracket-Apparatur. Dafür wurden neue Zahnmodelle und eine neue Bürstmaschine verwendet.

Material und Methoden

Getestet wurden drei Interdentalbürsten (IDB): (B1) Eine taillierte IDB, welche an beiden Enden einen Durchmesser von 9 mm und in der Mitte von 5 mm aufwies sowie zwei zylindrische Bürsten mit korrespondierenden Durchmessern als Vergleich (B2 und B3). Zum Testen der Reinigungsleistung und der dazu gehörigen Kraft zum Einführen in die Interdentalräume wurden Zahnmodelle hergestellt und eine Bürstmaschine modifiziert. Die Zahnmodelle wurden der Anatomie der Oberkieferfront 13-23 nachempfunden. Modellzähne wurden aus schwarz eingefärbtem Kunststoff gegossen und in eine Gingivamaske eingepasst. Es wurden zwei Modelltypen erstellt: Ein Modell wies verschachtelte Zähne auf während das Andere eine gerade Zahnreihe repräsentierte (B). Auf den Bukkalflächen wurden kieferorthopädische Brackets angebracht und es wurde ein runder 014-er Draht einligiert. Anschliessend wurden die Zahnmodelle mit Titan(IV)-Oxid weiss eingefärbt.  Mit einer Testapparatur mit genormter Modellpositionierung konnte manuell ein reproduzierbarer Bewegungsablauf der zu testenden Interdentalbürsten parallel zu den Zahnflächen simuliert werden. Dabei wurden die Interdentalbürsten von inzisal in die Labialfläche zwischen den Brackets und dem Draht eingeführt. Für die Auswertung wurden die gereinigten Flächen planimetrisch erfasst. Die Apparatur wurde ebenfalls für eine Kraftmessung modifiziert und ein Kraftmessgerät angebracht. So konnte der Druck in cN gemessen werden, der aufgewendet werden musste, um die Interdentalbürste in den bukkalen Raum zwischen den Brackets und dem einligierten Draht einzuführen. Der Einfluss von Bürste und Modell, sowie der Interaktion zwischen Bürste und Kraft, auf die Reinigungsleistung wurden mittels einer ANOVA analysiert.

Resultate

Die mit Abstand beste Reinigungsleistung zeigte die breitere zylindrische Bürste. Zwischen den Reinigungsleistungen bei den einzelnen Zahnzwischenräumen und Modellen gab es einen signifikanten Unterschied. Auch bei der Kraftmessung gab es einen deutlich wahrnehmbaren Unterschied; hier zeigte die Bürste mit der grössten Reinigungsleistung auch die grösste Kraftanwendung; gefolgt von der taillierten und der dünneren zylindrischen IDB. Die Korrelation zwischen Reinigungsleistung und Kraft erwies sich ebenfalls als signifikant: Je höher die benötigte Kraft desto höher war die Reinigungsleistung.

Diskussion

Zusammenfassend wurde die dünnere zylindrische IDB von den drei getesteten Bürsten für den klinischen Gebrauch am besten eingestuft approximal um kieferorthopädische Brackets eingestuft, da das Verhältnis von Kraft für das Einführen und der Reinigungsleistung insgesamt das beste Resultat erzielte. Die taillierte IDB wies keine bessere Reinigungsleistung auf. Allerdings muss eingeräumt werden, dass mit bestehenden Gängigkeit der taillierten Bürsten noch grössere Durchmesser hätten verwendet und das Resultat dementsprechend theoretisch hätte verbessert werden können. In diesem Zusammenhang sind weitere vergleichende Studien notwendig und geplant.

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