The present study was aimed to investigate the influence of dentifrice abrasivity and toothbrush stiffness on the development of NCCLs. Three questions were raised:
- Does the abrasive level have an influence on the initiation and progression of NCCLs?
- Does the stiffness of the toothbrush have an impact on the initiation and progression of NCCLs?
- Does the association between the abrasive level and toothbrush stiffness have an effect on the initiation and progression of NCCLs?
To investigate the chosen study questions, the prepared specimens were brushed in a V8 cross-brushing machine developed by Sabri Dental Enterprises Inc. Simulated toothpaste slurries of varying abrasivity and toothbrushes of varying stiffness were used in two different cycles, 35,000 and 65,000. These cycles correlated to three and a half years and six and a half years. Since every 5,000 cycle approximately represents the period of six months of individual brushing (Kanter et al., 1982). Such a decision is made to see and measure the effects of different toothbrushes, slurry, and frequency of tooth brushing on the long term in the development of NCCLs. These cycles were chosen because, in the pilot study of the low cycle, the non-contact profilometry was not sensitive enough to detect the small differences between the study groups, especially in order to control and low abrasive slurry combined with a soft toothbrush.
In research, the specimens were subjected to a 200-g load. We wanted to simulate the brushing force that could be generated by patients. In their study, Wiegand et al. (2013) reported that the average brushing force of manual toothbrushes was 1.6 ± 0.3 which was equivalent to 163 to 200 grams.
In the study, we did not use commercial toothpaste but prepared our own slurry made of a solution of 5% of CMC (Carboxymethyl Cellulose) and abrasive. The choices were made to simulate toothpaste. To prepare them, we used a specific lab protocol in regards to the manufacture instructions developed by J.M. Huber Corporation. Such manipulations helped to control variability among toothpaste slurries.
In the study, the elastomeric impression materials (Hydrophilic Vinyl Polysiloxane, Examix, GC America, Inc.) were made at baseline and after each brushing period. Three impressions were made for each group. Two reasons for recording the polyvinyl siloxane impression of the sample and scanning by the laser profilometer could be given. First, it might help to prevent the dehydration that could occur to the teeth since each tooth could take seven and a half minutes to be scanned (15 min/block). Second, it has been suggested that the color and transparency of dental hard tissue could affect the result of non-contact profilometry (Rodriguez et al., 2008; McBride and Maul, 2004). Therefore, it is possible to eliminate the translucency and color problem by scanning polyvinyl siloxane impressions.
Non-contact surface profilometry was used in the study because it indirectly measured the surface by laser beam deflection to overcome the possibility of surface damage when using contact surface profilometry (Heurich et al., 2010). It also provides an accurate surface topography to quantify the surface loss.
The Effect of Slurry At 35,000 Cycles
In the study, we used toothpaste abrasivity at four levels: high abrasive (Z103), medium abrasive (Z124), low abrasive (Z113), and non-abrasive slurry (as a negative control). They were categorized according to their RDA-value:
- High-Zeodent (103) had 208 RDA-value;
- Medium-Zeodent (124) had 146 RDA-value;
- Low-Zeodent (113) had 69.24 RDA-value.
We tried to simulate stain-removal/whitening toothpaste with a high abrasive (Z103), regular toothpaste with medium abrasive (Z124), and gentle (hypersensitivity, anti-erosion) toothpaste with the low abrasive (Z113).
In research, a surface loss could be more produced by the high abrasive slurries than the lower abrasive regardless of the type of the toothbrush. The earlier studies results turned out to be in agreement with our findings. The researchers found the higher RDA-value of the toothpaste slurry resulted in a higher surface loss (Dyer et al., 2000; Hooper et al., 2003; Wiegand et al., 2009).
Brushing with water caused frequent tooth loss because of the tooth-brushing abrasion, since there was an increase of the mean values at 65k, suggesting the progression of the loss. However, the possibility of the mistakes in the chosen method (a subtraction analysis) cannot be completely ruled out.
Toothbrush Effect at 35,000
Toothbrush Stiffness (soft, medium, hard) is not constant between the brands of toothbrushes (Harte & Manly, 1976). Regarding this fact, we used only one brand of a toothbrush of three different types of toothbrushes (soft, medium, hard) with nylon bristle and ended with a rounded head in our experiment.
The current study found that the type of toothbrush was irrelevant when brushing with water or low abrasive. However, a more surface tooth loss was observed for the medium and hard brushes when medium and high abrasive slurries were used. These results contradict the previous studies reported by Dyer et all. (2000), who proved that softer brushes could be more detrimental to a tooth as they were more flexible than a hard toothbrush and had the ability to carry more abrasive/slurry to a larger area during the tooth-brushing motion. The current study also contradicts the study of Wiegand et al. (2009) where dentin exposed to toothbrushes with a different diameter, 0.15mm, 0.20mm, and 0.25mm. The wear of dentin decreased along with the increased diameter of toothbrushes (Wiegand et al., 2009). In that study, they used bovine teeth and acrylic sample. In the current model, we used natural teeth and tried an actual simulation of the tooth-brushing process instead of the theoretical analysis and studies done on acrylic specimens.
Several studies showed the similar results. Tellefsen et al. (2011) stated that when brushing with water, a hard toothbrush would cause more surface loss than the softer toothbrushes. Harte and Manly (1976) found that a hard toothbrush caused more abrasion to the tooth surface than a soft toothbrush. Furthermore, the clinical evaluation done by Brandini et al. (2011) found that hard toothbrushes caused considerable abrasion.
Although the present study showed no differences between medium and hard toothbrushes, there was a numerical trend (non-significant) indicating that hard toothbrush could cause more damage when associated to the highly abrasive slurry. Such situation has to be further investigated. The previous investigation by Arrageg et al. (2015) showed that when high abrasive slurry was used, hard toothbrush caused more tooth loss than the medium toothbrush in a simple in vitro simulation model could.
Overall, a significant high tooth that wears all patterns of toothbrushes (soft, medium, hard) and abrasive level (low, medium, high) was seen at 65,000 cycles and then at 35,000 cycles. Such results were not surprising regarding the material offered by Addy and Hunter (2003) about the process of abrasion and its time dependence.
The results for comparison slurry vs. toothbrush were the same as 35,000 cycles, except, there was no difference between soft and medium toothbrush when associated with medium slurry.
Reference areas to the brushing apical and occlusal surfaces were protected from the brushing abrasion by the fabrication of a protective custom tray, leaving the CEJ and adjacent 2 mm root dentin surface exposed. We can speculate that the force was distributed to a larger area at the beginning of brushing, but once the indentation or lesion occurred, a more concentration stress generated from the area of the lesion that could cause a lesion to progress faster. Progression of the lesions on that model did not show the possibility to reach a plateau within the brushing periods studied.