The mechanical testing in the two reported studies shows that the addition of a flexible neck to the toothbrush designs significantly reduces handle stiffness in the neck and head regions compared to rigid-neck toothbrushes. This could help reduce the force applied to the oral mucosa and tooth surfaces. Moreover, this flexibility is apparent at force levels typically used in manual toothbrushing. The tests of toothbrush deflection clearly showed very different force-deflection curves, with the test toothbrush showing 2.5 times lower measured force at the head than the control toothbrush. This deflection arose as a result of the flexible neck component rather than different leverage effects or the angle of the brush heads. The presence of the flexible neck results in smaller moment arm for the test toothbrushes, resulting in lower bending moments on the tooth surface compared to the control toothbrushes. The lower force and moment produced by the test toothbrushes can help provide a more gentle brushing action and could therefore help prevent overbrushing and gum damage.
The force-deflection curve indicates that when used for cleaning, the test toothbrushes could theorectically have a more controlled force distribution of brushes on the teeth. The curve also indicates that high forces are achieved with small deformations. The force steps are smaller with the test compared to the control toothbrush, enabling improved force control and better cleaning.
The comparison of the five flexible-neck test designs with the control design consistently demonstrated improved in vitro performance in terms of cleaning efficacy over multiple different tooth surfaces when the brushing movement was either horizontal or rotating. With vertical brushing movement, however, the clear superiority of performance was less marked. This is due to the fact that how the flexible neck bends supports horizontal/rotating motion, but vertical motion limits the force transfer, and the neck handle reacts in a similar manner to a rigid toothbrush. It should be noted that the most common brushing actions are horizontal and rotating [13, 24, 25] and their efficacy is clearly supported by the current findings, even though the efficacy achieved by vertical brushing movements was at least comparable to that achieved by rigid brushing handles.
In this in vitro model, when compared to the rigid-necked control toothbrush, the flexible toothbrush neck appeared to contribute to cleaning in hidden biofilm-accumulating regions inbetween the teeth and around exposed roots. The majority of consumers worldwide, when using toothbrushes, exhibit random brushing habits with horizontal scrubbing, rotating and gliding actions. Whilst our robot model is able to replicate highly standardised human brushing techniques , the model is unable to reflect sensory feedback affecting individual random clinical brushing technique and dynamic force adaptation. Our data suggest that toothbrushes with high neck flexibility could be suited to typical human techniques than more rigid designs (Additional Files 2,3,4: Videos 1–3). To appropiately investigate if toothbrushes with greater neck flexibility confer improved cleaning efficacy in people, it will need to be investigated in clinical trials.
An alternative strategy to reduce the force transmitted to the teeth during brushing is to use softer bristles, and previous studies have investigated the relationship between bristle stiffness and abrasive dentine wear, a correlate of transmitted force [23, 26,27,28,29,30]. However, whilst some studies suggest that softer bristles are associated reduced dentine abrasion [23, 27, 28], others indicate the opposite [26, 31]. Other factors in addition to bristle stiffness, such as their density, diameter, length, orientation and arrangement, as well as toothpaste properties are also thought to influence the findings of these studies [29, 30]. As bristle configuration were not standardised across the test brushes, these factors could have influenced the results in this study, and how all of these factors interact with toothbrush neck flexibility are interesting topics for further study.
The analysis of single teeth also showed superior cleaning performance at multiple different sites by the flexible-neck designs. This cleaning was most effective for incisors, followed by wisdom teeth, canines, premolars and molars. This could be an issue of access to individual teeth and may be worth further investigation. The cleaning efficacy study advances previous work on robot typodonts, real toothbrushing, and visualisation and quantification of simulated plaque removal [13,14,15,16]. This technology is important for the accurate assessment of tooth cleaning potential and in the development of new approaches in oral hygiene.