Influence of skin cold sensation threshold in the occurrence of dental sensitivity during dental bleaching: a placebo controlled clinical trial

Abstract Objective This study verified the occurrence of dental sensitivity in patients submitted to a 35% hydrogen peroxide based product (Whiteness HP Maxx 35% – FGM), skin cold sensation threshold (SCST) and its influence on dental sensitivity. Material and Methods Sixty volunteers were divided into 4 groups (n = 15), according to SCST (low: GI and GIII, and high: GII and IV) and bleaching treatment (hydrogen peroxide: GI and GII, and placebo: GIII and GIV). SCST was determined in the inner forearm for 6 different times using a neurosensory analyzer, the TSA II (Medoc Advanced Medical Systems, Ramat Yishai, Northern District, Israel). Dental sensitivity measurements were performed 10 different times using a thermal stimulus and an intraoral device attached to TSA II, positioned in the buccal surface of the upper right central incisor. Spontaneous dental sensitivity was also determined using the Visual Analogue Scale (VAS). Data were submitted to Student's t-test and Pearson's Correlation Test (α=0.05). SCST remained the same during bleaching treatment. Results Distinct responses of dental sensitivity were found in patients with low and high SCST during the first and third bleaching session (p≤0.05). The teeth submitted to the bleaching treatment became more sensitive to cold than those treated with placebo. Moreover, data obtained with TSA and VAS presented moderate correlation. Conclusions Bleaching treatment increased dental sensitivity and skin cold sensation threshold might represent a determining factor in this occurrence, since low and high SCST patients had different responses to the thermal stimulus in the teeth.


Introduction
Tooth bleaching is one of the most popular esthetic procedures requested by patients and a conservative approach with efficient results 7 . The in-office technique using highly concentrated hydrogen peroxide products has become an excellent alternative for both professionals and patients 11 .
Despite whitening efficacy, recent studies showed that patients submitted to dental bleaching reported different intensities of dental sensitivity 11,15,19,24,26 . This symptom is a concern for dentists and patients as a limitation for treatment evolution and satisfaction.
It has been established that the mechanisms of the bleaching agent action are based on the presence of reactive forms of oxygen, which are extremely unstable and promote oxidation of pigments embedded in dental tissues, giving them a lighter appearance 24 .
On the other hand, upon penetrating the dental tissues to oxidize the pigmenting agents, the reactive forms of oxygen diffuse quickly in the dental tissues reaching the chemosensitive ion channel (TRPA1), activating the intradental nerve and causing discomfort 14,15 .
Postbleaching sensitivity has also been related to the morphological changes that presumably alter permeability, resulting in temporary sensitivity after the bleaching procedure 6,23 .
Some clinical trials about dental sensitivity showed that pain is different among individuals and it is usually stronger at the final phase of treatment 3,4 .
This variation may be associated with the sensation threshold of each patient, which is classified as low when cold sensation is easily detected and high in the opposite situation 22 . Some studies related to pain and even dental anxiety, state that these differences in pain and sensitivity threshold present a challenge to the diagnosis and consequent treatment of patients, since the subjectivity of sensitivity makes the response to several treatments very peculiar, beyond the dental scope 21,29 .
Visual Analogue Scales (VAS) are commonly used in clinical trials to determine spontaneous dental sensitivity occurrence during bleaching treatments 11 without evidence of being the most appropriate method. Nowadays, the pain industry developed some new devices that enable computerized neurosensory analysis, called Quantitative Sensory Testing (QST) 13,17,25 . TSA II (Medoc Advanced Medical Systems, Ramat Yishai, Northern District, Israel) represents this modern technology for the study of pain and is effectively used to evaluate and quantify the neurosensory response regarding major and minor nervous fibers commonly found in teeth, using thermal, mechanical, or electrical stimuli 13,15,18,25 .
Thus, test for cold and heat sensation can be conducted using accurate devices that transfer temperature changes to several body structures under predetermined speed 13,25 .
In addition, a simultaneous study for skin sensation threshold and dental sensitivity experienced during whitening would provide clinical safety levels to prevent and treat this uncomfortable side effect 29 , such as the use of different bleaching agents and individual protocol of treatment aiming to control dental sensitivity.
In this way, the aim of this study was to evaluate the correlation between the dental sensitivity data obtained using QST and VAS and to verify the influence of skin cold sensation threshold in dental sensitivity using the neurosensory analysis to quantify thermal sensitivity during dental bleaching.
The null hypothesis assumed that: -there is no difference in skin cold sensation threshold during bleaching with 35% hydrogen peroxide products; -skin cold sensation threshold of each patient does not influence dental sensitivity during bleaching; -there is no difference in dental sensitivity at different periods during bleaching with 35% hydrogen peroxide products; -there is no correlation between dental sensitivity obtained using QST and VAS. It is noteworthy that patients were also instructed not to use analgesic or anti-inflammatory medications and desensitizing toothpastes during 30 days after the bleaching treatment to avoid data misreading.

Inclusion Criteria
Healthy and vital maxillary teeth   The values obtained during the first test were discarded, and the mean of the following tests was used as the temperature for cold sensation threshold testing in skin.
The mean temperature was considered the skin cold sensation threshold and used to classify patients

Evaluation Period Tooth Skin
Period 00 Baseline (24 hours before treatment) Baseline (24 hours before treatment)

Period 01
Immediately before treatment Immediately before treatment

Period 02
Immediately after the 1 st treatment session Immediately after the 1 st treatment session Period 03 24 hours after the 1 st treatment session -----

Period 04
Immediately after the 2 nd treatment session Immediately after the 2 nd treatment session Period 05 24 hours after the 2 nd treatment session -----Period 06 Immediately after the 3 rd treatment session Immediately after the 3 rd treatment session Period 07 24 hours after the 3 rd treatment session -----Period 08 7 days after treatment 7 days after treatment Period 09 30 days after treatment -----    Catarina, Brazil), the bleaching agent was inserted into a graduated syringe, and 0.06 ml of the bleaching product was applied on the buccal surface of the tooth for 15 minutes. After the first application, the teeth were cleaned and dried with gauze, and the procedure was repeated twice, totalizing 45 minutes of contact between the bleaching product and enamel in each session. After 7 and 14 days, the same procedure was performed. Although color was not the subject of our study, to analyze dental sensitivity occurred during the whitening treatment, it was necessary to perform three sessions, the normal duration in a real dental office.
In Groups III and IV, a placebo agent, identical to the original bleaching agent presentation, purchased from the same manufacturer, was applied to the dental surface the same way as reported for the bleaching agent ( Figure 7). The evaluator and the operator were the same person, thus, patients and the operator were not informed of the group to which they belonged ( Figure 7).

Statistical analysis
Temperature variations (delta) were used to perform the statistical analysis. They were obtained by subtracting the initial temperatures/baseline from those found in the other periods of study.
Data were submitted to Student's t-test at a 5% level of significance and also to Pearson's Correlation Test (α=0.05) to determine whether there was correlation between the dental sensitivity obtained using VAS and QST. Pacotico 5.1 statistics software was used.

Results
The analysis of skin cold sensation threshold during dental bleaching showed no statistical difference between the periods (p>0.05), revealing no alteration in skin cold sensation threshold regardless of the initial classification as low or high threshold ( comparison of temperature change (delta) for cold sensation in the maxillary central incisor of experimental groups with low and high threshold. Statistically significant differences were observed at P 02, P 03, P 06, P 07, and P 08 (p≤0.05) ( Table 2).
As for the values of dental cold sensation, the bleached groups (G I and II) were statistically different from the initial values (p≤0.05). In groups treated with placebo (G III and IV), cold sensation remained the same during the study (p>0.05), except for P 02 in group III (p≤0.05) and P 03 in group IV (Table 3).
We also observed that temperature variations for  Table 4-Temperature variations for skin cold sensation (°C) in different evaluation periods, after the beginning of dental bleaching   Analysis using VAS also showed different spontaneous dental sensitivity intensities in the different periods of study for bleached groups (G I and II). Generally, GI showed more incidence of spontaneous dental sensitivity than GII. Patients with low SCST presented higher incidence of moderate and intense magnitudes of spontaneous dental sensitivity during the bleaching treatment than patients with high SCST, who presented lighter magnitudes of spontaneous dental sensitivity during the treatment (Figures 8 and 9).

Discussion
The Quantitative Sensory Testing (QST) used in this study to measure dental cold sensation during whitening has been used in several fields for pain On the other hand, it plays an essential role in dental sensitivity since those individuals with high and low skin cold sensation thresholds showed different behavior at the initial phase of treatment (during the 1 st and 3 rd sessions). In those cases, the skin threshold influenced clinical tooth response, representing different tolerance levels against thermal stimuli on tooth. Thus, the second null hypothesis was rejected.
The third null hypothesis was also rejected since the temperature for dental cold sensation was different during treatment. Previous studies evaluating dental sensitivity through self-reports and questionnaires 1,12 with Analogue Visual Scale are in accordance with our results 3,11 . However, the dental cold sensation threshold remained unaltered in the groups treated with placebo (III and IV), which justifies its use in the study. The alterations in those groups were observed at P 02 (low threshold) and P 03 (high threshold) and may be associated to stress and placebo response linked to physiological mechanisms. Evidence suggests that the expectations of a patient can markedly affect the outcome of treatment, since the placebo effect causes the brain to respond by releasing proper endogenous neurotransmissions 8,16 . It is noteworthy that dental cold sensation remained unaltered 30 days after finishing the treatment. It has been demonstrated that bleaching agents cause histomorphological alterations in enamel 2,28 and our results suggested that some of those modification persisted over time. The product pH and its action on enamel proteins may have increased the diffusion channels and tissue permeability, influencing the response to thermal stimulus 2,27 . In addition, penetration of peroxide into dental pulp may promote the activation of ion channel TRPA1, present in some of the intradental nerves, causing structural damage and inflammation with more inflammatory cells and interruption of odontoblasts layers as a result of reversible pulpitis 10,15,27 . This event has been also implicated in the mediation of dental pain induced by cold 15 .
In this study, patients with low and high SCST who underwent bleaching treatment presented dental sensitivity during all treatment periods in different intensities. These data corroborate with the literature that reports that a sizable proportion of the patients experienced this symptom during and after dental bleaching 3,15 . Moreover, moderate-to-intense level was reported more frequently in group I, with low SCST.
This information allows us to state that patients with low SCST are more susceptible to dental sensitivity occurrence during the bleaching treatment.
Moreover, the moderate correlation found between spontaneous (VAS) and induced (QST) sensitivity allowed us to reject the fourth hypothesis of this study and also certified the accuracy of the data obtained in our study. In scientific literature, there are no papers reporting the correlation between the intensity of dental sensitivity obtained with Visual Analogue Scale and using Quantitative Sensory Testing. Other methods using cooling agents are used to test cold sensation, but they were performed in a constant temperature (-50°C) 9 . However, they are not able to quantify the thermal response. In these cases, a normal or healthy pulp presents with absence of symptomatology, producing a slight transient painful response to the cold stimulus.
In general, the neurosensory analysis with TSA II in tooth is beneficial for Dentistry as knowledge about bleaching side effects is relevant for development of safe and comfortable protocols, since taking into consideration the subjectivity of pain 21 , the use of individualized therapies, such as different peroxide concentrations or number of applications and dental whitening sessions, could prevent or even avoid dental sensitivity, a factor highly responsible for discouraging patients to undergo whitening treatment.
Otherwise, further investigations are necessary to improve this method of quantitative analysis of cold sensation in tooth, which is a hard and inelastic structure.

Conclusion
Based on these results and despite all the abovementioned limitations of the method, skin cold sensation threshold did not change during bleaching treatment using 35% hydrogen peroxide. Skin cold sensation threshold might represent a determining factor in the occurrence of dental sensitivity.
Temperature for dental cold sensation did not remain the same during bleaching treatment and the data obtained with induced (QST) and spontaneous sensitivity (VAS) presented moderate correlation.