New experimental setup for the measurement of cleaning efficacy and force of interdental aids in 3D-reproduced interdental areas

Beside a lot of efforts developing better toothbrushes, up to date their bristles do not reach the interproximal surfaces of teeth efficiently [1]. This seems important as interdental sites present the highest risk of plaque accumulation and the highest prevalence of caries and infrabony pockets in an adult population [2]. Therefore, additional devices are necessary to penetrate between adjacent teeth [3]. Moreover, current systematic reviews indicate that additionally to tooth brushing, cleaning with different interdental brushes versus flossing is superior for prevention and treatment of gingivitis than tooth brushing only [4, 5]. Although, interdental cleaning with wood sticks can significantly reduce bleeding on probing and gingivitis, they do not reduce plaque parameters [5, 6]. More controversially discussed are the recently developed interdental rubber picks, which seem to be able to reduce plaque but data for gingivitis are inconsistent [7, 8]. Hence it’s not surprising that a currently published Cochrane review did not find any difference by very low-certainty evidence for interdental brushes or flossing versus interdental rubber picks [6]. According to the knowledge of the authors so far, no studies have been published comparing clinical efficacy of differently designed rubber picks. The authors of this review concluded [6] that the available evidence for interdental rubber picks is limited and inconsistent.

In addition, the absolute validity and reliability in vivo regarding the interdental cleaning efficacy is hampered by the accuracy with which the tested parameters may be determined [9]; e.g. residual interdental plaque is not directly measurable resulting in a lack of precision and consistency of the data. On the other side, the majority of in vitro studies used standardized isosceles triangle interdental space, keeping the force used to penetrate the interdental space standardized or to measure experimental cleaning efficacy (ECE) in a reproduceable manner. However, by doing so, they neglect more clinically relevant morphologies, such as convex or concave shapes of the proximal root surfaces [10].

Results for ECE and force measurements are summarized for both groups of different interdental rubber picks in Table 1. In 79 out of 675 tests performed, the analysis of the ECE was not possible and the data could not be used in the final assessment (n = 596). In only three out of 675 tests performed, the data for force measurements were missing and a total of 672 data sets could be analyzed.

With the help of the newly developed in vitro procedure it could be demonstrated, that interdental rubber picks (IRP) with small elastomeric fingers and higher taper (IRP-F) had a significantly better cleaning efficacy as interdental rubber picks with lower elastics slats and lower taper (IRP-S). The longer fingers might be able to adapt better to the tooth surfaces compared to flatter less elastic slats. A further reason for the better results for ECE of IRP-F might be that the artificial interdental area sizes (e.g. 1.0, 1.1, 1.3 mm) were based on the available IRP-F and, hence, the IRP-S fitted a little bit less. Therefore, the size of the IRP needs to be exactly chosen to the present interdental area, as it is established for interdental brushes. Our sub-analysis of different morphologies of artificial interdental areas demonstrated a difference between the IRP designs only for convex and concave, but not for isosceles triangle shaped interdental areas. Additionally, looking at larger gap size of the artificial interdental space, the cleaning efficacy did improve for both tested designs, whereas significant better results were only found for IRP-F. As a lot of factors affect the cleaning efficacy of interdental brushes, e.g. design, material or length and diameter [16], the relative dimension of size of the interdental brushes in relation to the artificial interdental space seems to be very important [17]. Unfortunately, most studies do not mention the size of interdental brushes used [17]. From a technical point of view, the efficacy of cleaning will improve with the contact area between the interdental brushes and the tooth surface, which correlates to an increasing application force as indicted by our results and could be another explanation for the better results for IRP-F. The insertion force of round interdental brushes remains constant in a more parallel-walled interdental area, whereas in an equilateral triangular shaped interdental area, the necessary force will increase more with greater interdental brush dimensions. In contrast, the ECE decreased with smaller sized interdental brushes [11]. In analogy, the design of the tested IRP-F compared with IRP-S could be the reason for the better performance in our study, due to the higher contact area between the more elastic rubber fingers and the tooth surface as well as the higher taper. Maybe, the ECE could further enhance if the interdental rubber picks will be used in an angulated direction toward the occlusal plane and/or from both sides of the artificial interdental space. The main effort of this experimental setup was to enable the most uniform testing conditions as possible. Angulated movements are more demanding and need a greater effort (e.g. both tooth surfaces must be cleaned separately), which was reported by a lower scoring of dental floss in term of applicability compared to IRP-F [11].

In line with the better cleaning results of IRP-F, our in vitro results indicate a requirement for significantly higher pushing and pulling forces to clean the interdental area in all simulated sceneries. However, the present study did not compare the insertion forces of IRP to interdental brushes. Eventually, a thin nylon filament of a conventional interdental brushes bends with a smaller resistance than an elastomeric rubber finger of an interdental rubber pick. In addition, the rough surface of the interdental rubber picks, especially in contact with the artificial tooth surface simulated in our study (~ 25 μm versus ~ 10 μm of natural enamel [18]), will deform and create a greater resistance under usage, than the surface of the smooth nylon filaments. With up to 5 N necessary to insert the tested interdental rubber picks in a concave interdental area, interdental rubber picks do not seem to be the appropriate interdental cleaning aid for this type of interdental space. The advantage of interdental rubber picks is their wireless construction – and both tested designs, showed high primary stability without bending or fracturing of the core in our test. It can be hypothesized that with interdental rubber picks it will be easier for the patient to find the entrance in the interdental space as no discomfort in contact with the tooth surface or marginal gingiva is expected. Correspondingly, participants in all in vivo investigation’s found interdental rubber picks to be significantly more comfortable to use than interdental brushes [19‐22].

Interdental rubber picks are more and more promoted and developed and could be seen as further technological evolution of interdental brushes. Instead of metal core or nylon filaments, they have small elastomeric fingers or slats protruding perpendicularly from a plastic core [19]. Whereas, by the means of a recently published meta-review [5], the highest evidence exists stating that interdental cleaning with interdental brushes is the most effective method of interdental plaque removal, only low evidence for the newer interdental rubber picks exist to date. The few published studies found no statistically significant difference between the interdental rubber picks and conventional interdental brushes or flossing, neither for gingivitis scores nor for plaque scores [19‐21].

Contrary to these clinical investigations, our study focuses on the development of a standardized reproducible procedure to measure the cleaning efficacy and necessary cleaning force of interdental rubber picks and not on the clinical application of this newer interdental aid in comparison to conventional interdental brushes. Although the presented in vitro data cannot be directly applied to a clinical situation, the model has some advantages.

Currently, no method exists to quantitatively assess the interdental plaque in a clinical situation in vivo. Plaque removal of interdental aids assessed on the visible surface of the teeth might be masked by differences in the general oral hygiene procedures applied by the patients (e.g. tooth brushing routine and technique). Moreover, in an in vitro study, the size of interdental brushes can be chosen more appropriately according to the defect morphology as in a clinical study. However, an appropriate size is of major importance for the cleaning efficacy. Hence, inappropriately chosen interdental brushes might be a cause for the lack of statistical difference between various interdental cleaning aids [23]. Consequently, only in in vitro set-ups, like in the chosen one, a standardized measurement of cleaning efficacy is possible [16]. The reproduced 3D printed replicas [13] allowed with high precision the differentiation between the ECE of different surface morphologies such as plane, concave and convex surfaces [24].

However, the in vitro set-up has several limitations for extrapolating these data to a patient.

Moreover, further devices including different designs of interdental brushes should be tested in future studies for better comparability.

New interdental cleaning aids can be tested by the new reproducible experimental setup supported by 3D technology mimicking a natural clinical situation. The method proved to be accurate and precise due to the computerized evaluation strategy. Within the limitations of this in vitro study, both types of interdental rubber picks successfully removed around 20% of the simulated biofilm in artificial interdental areas of different size and morphology. IRP-F cleaned more effectively at higher forces compared to IRP-S. Cleaning efficacy and force correlate positively, whereas both depend significantly on interdental rubber picks’ design and size. Hence, the correct choice with regard to the design and size of the IRP seems to be important. However, these in vitro data have to be verified by clinical studies.