This study investigated previous hypotheses that the tongue can abrade acid softened/eroded enamel surfaces.
Twelve upper removable appliances each retaining 2 anterior and 2 posterior human enamel specimens were constructed. Each specimen was exposed to acid on both surfaces, but only one surface was allowed contact with the tongue. Therefore, 96 surfaces were assessed. Appliances were worn from 9.30 to 17.00 Monday to Friday for 22 days. Acid eroded lesions were created by immersing the specimens for 5 min in 50 ml orange juice three times daily. Enamel loss was measured using Quantitative light- induced fluorescence (QLF) and Non- contact profilometry (NCLP) and the differences (D) between tongue (D t ) and palate facing (D P ) surfaces determined.
%ΔF D(t-p) from the two anterior specimens were greater than from those placed posteriorly with mean values of 15.9% (±9.1) and 14.4% (±8.4), 5.6% (±8.7) and 4.5% (±6.6) respectively. Similarly, NCLP data showed anterior specimens had greater differences for mean step height (MSH) between tongue – facing and the palate – facing (control) surfaces than posterior specimens. MSH D(t-p) values were 59.4 μm (±30.3) for anterior tongue facing surfaces and 55.5 μm (±29.4) for posterior palate facing surfaces. For the posterior specimens MSH was 48.1 μm (±26.1) and 51.7 μm (±30.4) respectively (p < 0.05).
The greater enamel surface loss of the anterior specimens demonstrates that abrasion by the tongue on acid softened/eroded enamel in situ is likely.
Dental erosion is the irreversible loss of dental hard tissues by acids without bacterial involvement . As a result, the enamel surface will be characterised by partial demineralisation (softening) followed by a complete loss of the outermost enamel layer. If not allowed to re – mineralise, the remaining softened enamel is susceptible to be removed by mechanical forces such as toothbrushing .
Friction from oral soft tissues, particularly the tongue, has also been reported as responsible for tissue loss from pre – eroded surfaces .
The interaction between erosion and abrasion, in many in vitro and in situ models, has been studied separately; eroded lesions are first created on enamel surfaces which are then subjected to abrasion. Such models are applicable when wear has been caused by brushing of teeth after the consumption of acidic food or beverages. However, it is possible for erosion and abrasion to occur simultaneously, for instance, during the consumption of acidic food or drinks.
Despite the growing research interest in the effect of pressure from the tongue on previously eroded enamel surfaces, related studies are still scarce with only one tongue simulation study and one, in vitro , tongue licking study . Greater wear occurred after erosion with attrition or tongue rubbing followed by remineralisation in artificial saliva in vitro than in the other regimens which included simulated tooth brush abrasion, tongue rubbing and attrition without the erosion in citric acid . Another in vitro study also found greater tooth substance loss on enamel and dentine specimens eroded for 10 min and then licked by the tongue for one minute although ultrasonication had a significant synergistic effect . Both studies were carried out in vitro using relatively harsh regimens.
Prevalence of enamel erosion on different enamel surfaces in healthy individuals has been investigated previously . Despite wide agreement that palatal surfaces are more susceptible to erosion , some studies showed that labial enamel was mostly affected while others could not find differences between the two surfaces .
The aim of this study was to investigate the effect of tongue abrasion, in vivo , on pre-eroded enamel specimens using a novel in situ appliance and to compare that effect on different tooth surfaces (labial vs . palatal). It was hypothesised that: (1) enamel specimens previously exposed to acid erosion in vitro followed by tongue abrasion would show more wear than if eroded only and (2) that there would be no difference in susceptibility to erosion/abrasion between labial and palatal enamel surfaces.
Materials and methods
The protocol was approved by the North-West 2 Research Ethics Committee – Liverpool Central, UK (reference number: 09/H1005/69).Twelve medically and dentally healthy volunteers of both genders, aged between 20 and 60 years were recruited from the staff of Liverpool University Dental Hospital. Subjects were given verbal and written information concerning the study and provided written consent to participate. All subjects had their un-stimulated and stimulated salivary flow rates and their corresponding pH measured.
In order to be eligible for inclusion in the study, subjects were required to be/have: (i) aged between 20–60 years, (ii) non – smokers with good general and oral health, (iii) a full upper dentition, (iv) a good salivary function with 0.3 ml/min and 1.0 ml/min flow rates for un- stimulated and stimulated saliva.
Using the spitting method, un-stimulated saliva was collected by asking subjects to spit into a pre- weighed container over a period of 5 min. They were then asked to chew on a piece of paraffin gum and at the same time spit into a separate container over a period of 5 min. Flow rates were calculated and recorded in ml/min.
The crowns of twelve human upper central incisors were gently abraded and pumiced using 1200 – grit sandpaper (Water Sandpaper, 151 Products Limited, Manchester, UK) to remove extrinsic stains and other residues and to obtain a homogenous surface. Each crown was sectioned in a labio- palatal direction (n = 12 labial, 12 palatal specimens) using water – cooled diamond wire (Well type 2400, Walter EBNER, Le Locle, Switzerland). The labial and palatal surfaces were further sectioned mesio- distally to produce 2 specimens from each surface (n = 24 labial, 24 palatal specimens).
The incisal and cervical thirds of each specimen were discarded and the remaining surface was reshaped. Therefore, each incisor was sectioned both labio- palatally and mesio-distally to give four 4 × 4 × 1.5 mm specimens per tooth ( Fig. 1 ). The dimensions of all specimens were standardized using Vernier calliper then they were all checked under a USB microscopic light (Veho™, Discovery, UK) to ensure dentine removal.
Specimen allocation and insertion
A total of 24 labial and 24 palatal enamel specimens were randomly allocated to a 12 upper removable appliances. This was performed in such a way that 2 labial and 2 palatal enamel specimens were randomly allocated to their corresponding sites on each appliance (n = 4) so that one labial and one palatal enamel specimen were positioned in the anterior and another 2 in the posterior region ( Fig. 2 ).
Fig. 2 shows the surface and position of each enamel specimen on the appliance. Each appliance had 4 specimens but each specimen was subjected to either an erosion only regimen (palate- facing side) or an erosion and abrasion regimen (tongue – facing side). Therefore, a total of 96 surfaces were assessed.
Design of the removable appliance
Twelve upper removable acrylic base plates, each had 2 retention clasps on both maxillary first and second premolars were constructed ( Fig. 1 ). Four 1.0 × 0.7 cm slots; 2 opposite the central incisors and 2 next to the first molar teeth were cut through each base plate to accommodate 4 jigs of the same size. On one side of each slot, a hinge joint made of [- shaped 0.5 mm orthodontic wire was embedded in the acrylic resin into which an acrylic jig holding 1 enamel specimen was attached. In order to easily identify and quickly assign each specimen to its corresponding slot on the appliance, each acrylic jig had a groove on the side representing the number of each specimen e.g. specimen 1 had one groove; specimen 2 had two grooves, etc. The hinge type joint facilitated the removal and reinsertion of each specimen in its corresponding slot on each appliance.
During manufacture of the acrylic base plates a single layer of wax was placed in the slots against the palate to standardise the gap and in effect countersink the palate facing surfaces of each enamel specimen. Specimen surfaces facing the palatal mucosa were thus slightly recessed, by approx. 1 mm inside the appliance, to avoid any possible friction and thus abrasion by the opposing palatal mucosa. The surfaces were, however, still accessible to the erosive challenge allowing each specimen to act as its own control ( Fig. 3 ). Each appliance then had 4 enamel specimens; 2 from labial and 2 from palatal enamel.
The 12 upper removable appliances with the jigs inserted were sterilised by exposing them to Gamma irradiation using a cobalt – 60 source with particle energy of 0.315 MeV and irradiation cycle of 3.4 Gy / min at a source – to – specimen distance of 100 cm and field size of 15 × 15 cm for 2 days making a total overall dose of approximately 25 kGy .
In situ study
On each day of the study, volunteers were asked to wear the intra- oral appliance and its retained enamel specimens between 09.30 and 1700 h except for 1 h over lunch- time and two 30 min coffee breaks. When not in place inside the mouth, appliances were taken from the volunteers, jigs containing the specimens detached and immersed, for 5 min, in 50 ml of orange juice (Tesco value from concentrate, Tesco, UK) before each re – insertion. The mean pH of the orange juice was 3.85. The Adams cribs and collets around the teeth ensured seating in the same position after each re-insertion.
The first 90 min intraoral insertion of the appliances, however, was performed without exposure to the erosive challenge to allow the formation of a salivary pellicle layer, mimicking the natural situation found inside the mouth.
Volunteers were not allowed to consume food products nor beverages or practice any oral hygiene measure while wearing the appliance. Overnight and at weekends, the appliances and their specimens were stored in humid containers at room temperature until next use.
Erosion/erosion and abrasion lesion measurement
Quantitative light – induced fluorescence images (QLF, version 2.00c, Inspektor Research Systems BV, Amsterdam, Netherlands) were taken and analysed using an external reference composite jig to which the fluorescing intensity of tooth surfaces could be referenced ( Fig. 6 ). QLF measurements at 5% threshold were used. Non-contact light profilometry scans (NCLP, Proscan 2000, Scantron, Taunton, UK) were performed by marking three fixed points on each acrylic jig and using each as a reference ( Fig. 9 ).
QLF images and NCLP scans were taken for the entire tongue- facing and palatal- facing surfaces of each specimen, at baseline and at the end of each week throughout the 4-week study period. Measurements were made, however, on the centre of each specimen. Data reported were ΔF (% fluorescence loss of enamel lesion created compared to sound enamel). NCLP mean step height was systematically measured across four mid – points on the sides of each specimen to give four measures of step height (loss of depth). A mean value for the step height for each specimen was then calculated.
Means were calculated for saliva pH and flow rate. The data obtained were statistically analysed using SPSS V20 (SPSS Inc., Chicago, USA). Changes within each specimen were recorded from both labial (erosion and abrasion) and palatal (erosion only) surfaces at baseline and at the end of each week. As each specimen acted as its own control, the analysis of QLF and NCLP data was performed on the mean differences between the two different lesions rather than on absolute values. Those differences, for all measurements, were calculated by subtracting the mean fluorescence loss and mean step height values on the tongue – facing surfaces from those on the palate – facing surfaces. A one – way ANOVA and paired t -test were used for comparisons between mean differences throughout the study period. A p- value < 0.05 was considered statistically significant.