Introduction: In this study, the relationship between the spinnbarkeit, i.e., the stretchability of saliva, and dental caries was investigated. Methods: Dentistry students were divided into a group with more than 2 decayed, missed, and filled teeth (DMFT ≥2, n = 30) and caries-free group (DMFT = 0, n = 36). Results: Unstimulated saliva flow rate, pH, and spinnbarkeit were determined. Salivary spinnbarkeit was significantly lower in the caries-prone group compared to the caries-free group (5.4 ± 3.9 mm vs. 13.5 ± 7.6 mm, respectively, p < 0.001). Conclusion: This suggests that saliva with high spinnbarkeit protects better against dental caries.

Dental caries is one of the most common oral diseases [1]. Essentially, for the development of caries a cariogenic dental plaque and fast fermentable carbohydrates are needed. When fermentable carbohydrates are available, cariogenic bacteria use them as a nutrient resulting in the formation of lactic acid and a decrease in pH [2]. In case of frequent and prolonged periods of low pH in dental plaque, the selection of acidophilic bacteria like Streptococcus mutans and Lactobacillus species is favoured. These bacteria are more acidogenic than healthy dental plaque bacteria, which results in lower pH after sugar consumption. Chronic and repeated low pH conditions lead to demineralization of the dental hard tissues and cavities [3, 4].

In situ, saliva plays an essential role in the prevention of dental caries [5, 6]. Notably, saliva is crucial for oral health in general as it plays a role in various processes including digestion, microbial homeostasis, taste perception, wound healing, regulation of the water balance, moisturizing, and lubrication of the oral surfaces [7‒10]. As a consequence, hyposalivation increases the risk of a broad range of oral diseases and complaints. Patients with impaired salivary flow experience dry mouth, oral discomfort, pain, difficulty in speaking, taste alterations, and difficulty in swallowing [11, 12]. Conditions related to hyposalivation include burning mouth syndrome, gingivitis, periodontitis, cracked lips, glossitis with fissuring of the tongue, halitosis, tooth wear, dental erosion, candidiasis, and mucositis [13‒15]. In addition, reduced salivary function results in an increased risk for dental caries [5, 16].

Saliva protects against dental caries in several ways, such as clearance of food debris and sugar, aggregation and killing of cariogenic bacteria, acid neutralization, maintaining supersaturation with respect to tooth mineral, participation in formation of the acquired pellicle, and antimicrobial defence [11]. So, due to reduced antimicrobial, buffering, and cleansing properties of saliva in patients suffering from hyposalivation the incidence of dental caries increases drastically [6]. To exemplify, a 75% higher plaque index in patients suffering from hyposalivation has been reported [5, 17].

Spinnbarkeit, also known as fibrosity or stretchability, refers to the stretchy properties of saliva [18, 19]. Saliva with high spinnbarkeit gives a better protection against dehydration and xerostomia [19, 20]. Spinnbarkeit correlates positively with calcium concentrations in saliva and mucins MUC5B and MUC7 [21, 22]. MUC5B is a large, multimeric salivary glycoprotein with a molecular mass of >104 kDa composed of a protein backbone and a high number of hydrophilic carbohydrate side chains forming filamentous watery polymer brushes [23, 24]. MUC5B is part of the dental pellicle important for properties such as moistening, lubrication, viscoelasticity, and spinnbarkeit [20, 25]. MUC7 is a smaller monomeric mucin (∼150 kDa) with short sialylated carbohydrate side chains. MUC5B levels and salivary spinnbarkeit were lower in patients suffering from hyposalivation and xerostomia after radiotherapy [26] whereas lower spinnbarkeit and less sialylation of MUC7 were found in saliva of Sjögren’s patients [27].

Saliva with high spinnbarkeit protects against dry mouth, but little is known about the relation between dental caries and salivary spinnbarkeit. Therefore, the aim of this pilot study was to explore a possible relationship between salivary spinnbarkeit and caries. For this purpose, unstimulated saliva was collected in a number of healthy dentistry students of which flow rate, pH, and spinnbarkeit were measured and related to their decayed, missed, and filled teeth (DMFT) index.

Participants

This case-control study was conducted at the Academic Centre for Dentistry Amsterdam (ACTA). Participants were recruited by social media among ACTA dentistry students. Participants were over 18 years old and had a self-reported ASA score of 1 according to the physical status classification system of the American Society of Anesthesiologists [28]. Exclusion criteria were complaints of dry mouth and ASA scores of II, III, IV.

Caries Experience

Caries experience was expressed as the number of DMFT score which students had scored as part of their dentistry education just before their participation to the study. If an element had a caries lesion, the element was noted as “D.” Elements that were extracted due to caries were noted as “M.” If it appeared that an element had a (permanent/temporary) restauration due to caries, it was noted as “F.” Supernumerary elements, unerupted and congenitally missing elements, elements extracted for reasons other than caries, elements from the primary dentition and wisdom teeth were not counted in. For group-wise comparison, participants were divided into a group with two or more caries (DMFT ≥2, n = 30) and a group without caries (DMFT = 0, n = 36), which is clearly below and above the average score in Western Europe (1.0 ± 0.2) [29]. For correlation analysis also, participants with DMFT = 1 were included (n = 5).

Saliva Collection, Secretion Rate, pH, and Spinnbarkeit

Saliva was collected according to the spitting method [30, 31]. Participants were instructed to refrain from food and drinking with the exception of water 1 h before donating saliva. Then, unstimulated whole saliva was collected by expectoration into a tube every 30 s for 5 min. Samples were immediately analysed for weight, whereafter the secretion rate could be calculated assuming a saliva density of 1.0 g/ml. pH was directly measured with a pH/ion-meter (PHM240, Radiometer Analytical, Copenhagen, DK). Simultaneously, the samples were analysed for spinnbarkeit. The spinnbarkeit of each sample was measured using a New Equipment for Viscosity Assessment (NEVA) meter (IMI-011, Ishikawa Ironworks Co., Ltd, Kitakyushu, Japan) as described previously [19].

Statistical Analysis

Data were analysed using Statistical Package for Social Sciences (SPSS) for Windows version 26 (IBM, Armonk, USA). The results are numerical and unpaired. Unpaired t tests or Mann-Whitney U tests were used to determine differences in parameters between the caries-prone and caries-free group. Spearman ρ was used for determining a correlation between dental caries and spinnbarkeit. Significance level was set at p ≤ 0.05.

A total of 85 participants were recruited. Four participants were excluded for subjective complaints of a dry mouth and 10 participants were excluded for an ASA score >1. Five participants with DMFT = 1 were excluded from either the caries-prone or caries-free group but were included for the correlation analysis. The “caries-free” group (DMFT = 0) consisted of 36 participants, the “caries-prone” group (DMFT ≥2) of 30 participants. The general characteristics of the study population are shown in Table 1. No statistical difference was found between the groups in age, sex, and smoking. The caries-prone group showed a significantly lower mean spinnbarkeit compared to the caries-free group (p < 0.001), whereas saliva flow rate and pH were not significantly different (Table 2).

Table 1.

Characteristics of the caries-prone group (DMFT ≥2) and caries-free group (DMFT = 0)

DMFT ≥2 (N = 30)DMFT = 0 (N = 36)p value
Age 23.2±2.1 22.9±2.2 0.524 
Sex 
 Male 8 (38.1) 13 (61.9) 0.417 
 Female 22 (48.9) 23 (51.1)  
Smoking 
 Yes 2 (50) 2 (50) 0.855 
 No 28 (45.2) 34 (54.8)  
DMFT ≥2 (N = 30)DMFT = 0 (N = 36)p value
Age 23.2±2.1 22.9±2.2 0.524 
Sex 
 Male 8 (38.1) 13 (61.9) 0.417 
 Female 22 (48.9) 23 (51.1)  
Smoking 
 Yes 2 (50) 2 (50) 0.855 
 No 28 (45.2) 34 (54.8)  

Values represent means ± standard deviation or numbers (%).

Table 2.

Comparison of salivary spinnbarkeit, flow rate, and pH between the caries-prone and caries-free group (mean ± SD)

DMFT ≥2 (n = 30)DMFT = 0 (n = 36)p value
Spinnbarkeit, mm 5.4±3.9 13.5±7.6 <0.001 
UWS flow rate, mL/min 0.7±0.4 0.8±0.3 0.260 
pH 7.3±0.3 7.2±0.3 0.520 
DMFT ≥2 (n = 30)DMFT = 0 (n = 36)p value
Spinnbarkeit, mm 5.4±3.9 13.5±7.6 <0.001 
UWS flow rate, mL/min 0.7±0.4 0.8±0.3 0.260 
pH 7.3±0.3 7.2±0.3 0.520 

UWS, unstimulated whole saliva.

Moreover, a correlation coefficient of −0.73 (p < 0.001) was reported between the reported DMFT score and measured spinnbarkeit (shown in Fig. 1). This suggests that higher DMFT scores are associated with decreased spinnbarkeit.

Fig. 1.

Correlation between spinnbarkeit and DMFT (Spearman’s ρ = −0.724, *p < 0.001, n = 71).

Fig. 1.

Correlation between spinnbarkeit and DMFT (Spearman’s ρ = −0.724, *p < 0.001, n = 71).

Close modal

This study revealed that saliva spinnbarkeit was approximately 2.5 times higher in a group of caries-free participants compared to a group of caries-prone participants, with a negative DMFT-spinnbarkeit correlation. Spinnbarkeit, i.e., stretchability, represents one of saliva’s rheological properties. Historically however, saliva’s rheological properties have generally been described in terms of viscosity, denoting its resistance to flow, and elasticity, the property to restore to its shape after an external load is removed [32].

Viscosity and spinnbarkeit are different properties. In saliva, viscosity and spinnbarkeit exhibit a positive correlation, whereas in bronchial sputum they display a negative correlation [18, 33]. Viscosity contributes to various functions including, e.g., lubrication, moistening, and bolus formation [34], whereas spinnbarkeit is important for wetting the mucosal surfaces and protection against dryness [20]. These properties allow saliva to be spread and retained on the (vertical) surface of the oral mucosa. Interestingly, for viscosity and spinnbarkeit different salivary components play key roles. For both aspects, MUC5B and MUC7 mucins are important, but their contributions differ. Viscosity increased with the levels of MUC5B and the spinnbarkeit was more dependent on the levels of MUC7. Both mucins contribute to saliva’s microbial defence.

Decreased levels of salivary MUC5B and MUC7 were associated with increased caries susceptibility [35]. In another study, caries-free children had increased levels of MUC5B but decreased levels of MUC7 [36]. Though both mucins play a role in defence against bacteria, MUC7 achieves this by binding to oral pathogens, while MUC5B forms a bacteria-repellent layer on oral surfaces [37, 38]. In addition, an antimicrobial 15-mer peptide may be released upon proteolytic degradation of MUC7 [39].

Some studies demonstrate that increased salivary viscosity is a risk factor for dental caries [40]. When saliva viscosity in rats was enhanced by intubation with tung oil, they developed more caries than rats intubated with water or corn oil [41]. Saliva with high viscosity might be cleared less effectively from the oral cavity resulting in extended exposure of the teeth to microorganisms and sugar. This is somewhat similar to the situation in cystic fibrosis where highly viscous bronchial mucus was less effectively cleared from the lungs [42]. In line with this result, it was shown that MUC5B levels were higher in children or adolescents with dental caries [43]. However, increased viscosity is strongly associated with decreased salivary flow rate, which is a risk factor for dental caries [44]. In our study, both the caries-free and caries-prone group had similar flow rates. In addition, other studies found no difference in saliva viscosity between caries-free and caries-prone subjects [45] or even a higher viscosity in subjects with low dental caries [46]. We have some difficulties to explain these apparent contradicting findings but feel it tempting to hypothesize that this relates to the methodology of measurements. In our experience, reliable and reproductive spinnbarkeit measurements depend strongly on direct analysis after saliva collection with a minimum of potentially disturbing experimental handlings such as pipetting, aliquoting, mixing, freeze-thaw cycles, etc.

Calcium levels in saliva positively correlated with spinnbarkeit and viscosity by cross-linking MUC5B [21, 22, 47]. Calcium is also important in the defence against dental caries as calcium levels were significantly higher in caries-free subjects compared to caries-prone subjects [48].

It has to be noted that the population of this study comprised dentistry students. Dentistry students are expected to have good oral health, are of high socioeconomic level, and have access to dental care. Therefore, the cut-off value for caries-prone subjects was set at 2 damaged surfaces, which is low compared to other studies [43, 49]. It can be stated that the group we studied is not representative for the total population. In conclusion, this study demonstrates that the spinnbarkeit in the caries-active group was significantly lower than in the caries-free group and that these variables significantly correlated with each other.

We thank Prof. Dr. Marja Laine from the department of Periodontology for letting us use the Neva Meter.

The study was approved by the ACTA Ethical Committee and registered under number 202002. All participants gave their written informed consent.

The authors have no conflict of interest that could have influenced the outcome of this study.

This study was financially supported by the Academic Centre for Dentistry Amsterdam (ACTA, grant 1127).

Mouri Faruque did the writing and supervised the experiments, Kawtar Taidouch designed and did the experiments, Floris Bikker did the conceptualization, and Antoon Ligtenberg contributed to the experimental design and did extensive editing.

All data generated or analysed during this study are included in this article. Further enquiries can be directed to the corresponding author.

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