This study, which was carried out to evaluate the influence of hygiene, brushing technique and acid consumption factors on the initiation and progression of NCCLs, shows that brushing teeth twice or more times a day does not significantly influence the appearance of lesions of this type. Although brushing the teeth is a basic rule of dental hygiene to prevent tooth decay and periodontal disease, the greater or lesser frequency with which it is performed does not appear to be a preventive factor for NCCLs. Studies that make a reference to this variable show mixed results, with clinical studies and meta-analyses that report a significant association between a frequency of brushing of two or more times a day and NCCLs [7, 25, 36] and others which do not support this relationship [2, 5, 8, 16, 27, 28]. Nevertheless, the frequency of brushing can become an important factor associated with the vertical or horizontal brushing technique [1, 5, 36]. In addition, the parts played by this last factor and the hardness of bristles are also controversial. Similar to the data in this study, previous studies have not reported a significant relationship of these factors in the origin and progression of NCCLs [16, 24, 29, 37]. However, recent cross-sectional studies with and without logistic regression analysis [26–28] and in vitro studies [38] underline the influence of bristle hardness on the onset progression of NCCLs. Also, unlike the findings of this study, the brushing technique, especially horizontal [1, 2, 5, 25, 26] is another factor associated with NCCLs. Irrespective of the frequency, brushing technique and bristle hardness, tooth brushing force is shown in this study as an NCCL risk factor, with NCCLs being 1.7 times more frequent in those subjects who brush their teeth vigorously compared to those who do not. Although with differences in the evaluation of tooth-brushing force, this result concurs with previous studies that have indicated a significant association between brushing force and the presence of NCCLs [1, 3, 8, 20, 27]. However, a 5-year prospective clinical study carried out in 2016 [16] does not find this association, evaluating the brushing force with the aid of videos that record the participants brushing their teeth for a minute. Nor was brushing with excessive force a risk factor in the 185 patients and 5,180 teeth examined in the study by Teixeira et al, 2018 [30].
Although the idea of a model formulated with variables related to tooth brushing (toothbrushing frequency, toothbrushing technique, bristle hardness, duration of tooth-brushing, toothbrusing force and toothpaste) may be attractive for explaining the etiology, progression or aggravation of NCCLs, available scientific evidence does not support this. With differences in the research design, sample selection and size and in the methodology of evaluation and recording of the variables, the results of the various studies for each and every brushing factor differ and are therefore not conclusive. Brushing factors may be necessary but not sufficient for the development of NCCLs. Variables, other than brushing ones, are needed to explain the occurrence of NCCLs in populations that do not brush their teeth or have poor dental hygiene [22–24] or the appearance of NCCLs on the lingual surfaces of the teeth where the brush hardly reaches.
The current study finds that the Community Periodontal Index of Treatment Needs (CPITN) behaves as an important risk factor in the onset of NCCLs, with odds ratio values increasing progressively as the CPITN rises. This data is similar to the results provided by a cross-sectional study that reported a significant association of NCCLs with oral hygiene [30] and also with another study performed with people between 20 and 29 years of age that reported loss of attachment in most teeth with NCCLs. [5]. Likewise, a significant increase in the number of cervical wear lesions in relation to calculus index and the frequency of periodontal pockets has been reported [24], as the simultaneous presence of cervical wear with calculus, plaque or periodontitis [39]. Notwithstanding, the CPITN data of the present study, considered in isolation, must be viewed with caution. Under normal conditions, considering the result of the inflammatory activity of the periodontal microbiota in the different stages of the CPITN, it is unlikely that a significant change occurs in the pH or acid-base environment in the crevicular fluid at the level of the amelocementary union and that dental wear is initiated. At the same time, it is well known that the gingival sulcus has a slightly alkaline pH that oscillates towards greater alkalinity with an increase in the periodontal conditions [40] or even towards slight acidity in chronic periodontitis [41], though not approaching the value of 5.5, considered to be the critical pH level for enamel demineralization to occur. Therefore, the concurrence of some other factor is required. Therefore, a previous study in older patients (59.3 years average) found no significant association between bacterial plaque accumulation and pocket depth [11]. According to data from the present logistic regression model, CPITN may be considered as a predisposing or additional factor that, together with vigorous brushing and acidic food/beverage consumption, could favor the progression of NCCLs. The action of brushing factors in concert with dental erosion in the development of NCCLs is suggested or reported in different clinical and laboratory studies [1, 3, 11, 14, 30, 42]. Exposure an acidic diet might weaken the enamel/dentin, making it more susceptible to wear through the action of brushing.
Erosive theory without bacterial interaction as a mechanism of dental wear or initiation and progression of NCCLs is highly appealing for clinicians. Thus, both a recent systematic review with meta-analysis [43] and several previous studies support the association with an acidic diet and beverage consumption [1–3, 10, 26, 28, 29], gastroesophageal reflux or eating disorders [1, 30–33], and even with certain professions or work in an acid environment [34, 44]. In contrast, other previous cross-sectional [45, 46] and prospective studies [16] have found no significant relationship between the acidic diet and the progression of NCCLs. This study supports the importance of the consumption of extrinsic acids, and, in particular, the frequent ingestion of salads seasoned as a risk factor for the development of NCCLs. In the univariate logistic regression analysis, NCCLs were significantly more frequent (4.57 times) in those subjects who consumed two or more salads a day (1.75 times in the multivariate analysis) and 2.14 times more frequent with the consumption of extrinsic acids of any nature. These data agree with the main conclusion of a very recent meta-analysis that reports twice the risk of dental erosion (OR = 2.40) in subjects with a vegetarian diet compared to non-vegetarians [43].
Nevertheless, the relationship between an acidic diet and NCCLs, without other risk factors being involved, is a matter for debate. In this regard, the data in this study show that aggressiveness in brushing and the CPITN are the risk factors, which, combined with the consumption of salads seasoned with vinegar or lemon, constitute a model that classifies correctly 67.14% of cases, with 57.86% sensitivity, 76.43% specificity and an area under the ROC curve of 0.723. In general, it is not an excessively convincing model, this in turn showing the need to incorporate other risk factors that increase its predictive capacity. It does not agree, therefore, with the results of previous clinical studies based on less scientific evidence (cross-sectional designs) that indicate as predictors of NCCLs only the interaction of frequently eating fresh fruit and power toothbrusing [3] or the consumption of different extrinsic acids in combination with an age of over 35 years old [28] or in combination with bruxism and gastroesophageal reflux [1]. Instead, the data more closely matches the model of the old Bader´s case-control study [47] that, in addition to diet (fruit juices) and hard brushing as risk factors, includes occlusal alterations and bruxism.
The previous exposure of the amelocementary union to an intrinsic or extrinsic acid environment that demineralizes and weakens the enamel through degradation of the hydroxyapatite favors an increase in tooth wear due to the friction of brushing with and without toothpaste. The lower mineral content, with more voluminous pores, reported in the cervical enamel near the amelocementary junction makes this area more susceptible to demineralization. This favors the formation of steps of different length and depth [48]. This, according to Rees [49], additionally facilitates the entry of erosive agents through the pores, thus weakening the enamel even further. Additionally, proteolytic enzymes (proteases) of the gingival crevicular fluid produced by bacterial plaque microorganisms may also contribute to biocorrosion-induced wear [9]. The present study supports the combination of these risk factors (tooth brushing and acidic diet) in the onset and development of NCCLs, though data analysis of the model indicates that the action of more and different factors may be necessary to increase their predictive capacity. At all events, in accordance with the results of many other previous studies [1, 3, 4, 8–14, 28, 30, 47] this case-control study supports the multifactorial etiology of NCCLs. However, taking into account the obtained results, it seems sensible to remind dentists to recommend to patients the need for control of tooth brushing force and frequent eating of salads (vegetarian diet).
On the other hand, some limitations should be mentioned. In Spain, there are no community studies that report the prevalence of NCCLs to calculate sample size. In agreement with Thompson [50], the size chosen for the assumed OR is a conservative estimate of the sample size to combine clinical and epidemiological usefulness with the cost of the study. The sample comes from a very homogeneous population, dentistry students of the last three academic years. Although the cases and controls are comparable to each other in age and gender, a matching strategy between them was not followed. All this is a limitation for the projection of results to a different population. The average age of the participants can also be a limitation, since the majority of clinical and cross-sectioned studies have reported a significant association of NCCLs with a certain age range and increased percentage of NCCLs as age increases [1–3, 5–8, 10, 13, 30, 37]. To estimate the CPITN value, only six teeth were evaluated from the two dental arches and only the value of the one with the worst periodontal condition was recorded. Therefore, the presence or absence of NCCLs in the examined tooth is not taken into account, neither is the periodontal condition of the teeth with NCCls. At the same time, the OR obtained for the highest CPITN score should be interpreted with caution or not taken into consideration due to the relatively small number of cases and controls but large differences in these numbers. With the exception of the CPITN, the values of the rest of the variables come from the answers of the participating subjects to the questions read to them. These are indirect measures and the data obtained from the questionnaires are sometimes not reliable enough. There is no visual or other verification of the variables related to the brushing factors. Nor are the exact types of beverage included since there are some differences in pH and acidity. Likewise, there is no verification of the value of the salivary or gingival sulcus pH. Nor was the saliva analysed to assess the effect of its composition and of its buffering capacity. A direct association of the consumption of acidic foods/beverages with oral acidic environment is assumed. The morphology of the NCCLs was not taken into consideration; if it had been, it could have pointed towards ana erosive or abrasive etiology, although not in a decisive way [48, 51]. All the aforementioned can be a limitation.
In addition, the heterogeneity, limited comparability and limited quality of most studies make it necessary to improve scientific evidence with high quality studies. These should include more effective and efficient designs that allow progress towards a more reliable knowledge of the risk factors involved in the onset and progression of NCCLs, e.g. longitudinal studies that include the most important variables through multivariate analysis.