Effect of dental bleaching on pulp oxygen saturation in maxillary central incisors - a randomized clinical trial

Abstract Objective To assess pulp oxygen saturation levels (SaO2) in maxillary central incisors after dental bleaching. Materials and Methods 80 participants (160 teeth) were randomly allocated to four groups: G1 In-office bleaching with two applications of 35% hydrogen peroxide (HP) (20 minutes), followed by at-home bleaching with 10% carbamide peroxide (CP) (2 hours/day for 16 days); G2 - Same protocol as G1, plus desensitizing toothpaste; G3 - In-office bleaching with 35% HP and one application of placebo gel (20 minutes), followed by at-home bleaching with 10% CP (2 hours/day for 16 days); and G4 - Same protocol as G3, plus desensitizing toothpaste. Pulp SaO2 levels were measured before (T0) and immediately after (T1) in-office bleaching; on the 5th (T2), 8th (T3), 12th (T4), and 16th days of at-home bleaching (T5); and on the 7th (T6) and 30th (T7) days. Mean (SD) pulp SaO2 levels were compared within groups by generalized estimating equations (GEE) and Student’s t-test (P<0.05). Results Mean pulp SaO2 at T0 was 84.29% in G1, 84.38% in G2, 84.79% in G3, and 85.83% in G4. At T1, these values decreased to 81.96%, 82.06%, 82.19%, and 81.15% in G1, G2, G3, and G4 respectively, with significant difference in G4 (P<0.05). During home bleaching, pulp SaO2 levels varied in all groups, with 86.55%, 86.60%, 85.71%, and 87.15% means at T7 for G1, G2, G3, and G4, respectively; G2 presented significant difference (P<0.05). Conclusions Pulp SaO2 level in maxillary central incisors was similar at baseline, reducing immediately after in-office bleaching, regardless of using desensitizing toothpaste and increasing at 30 days after dental bleaching.


Introduction
The search for a smile that conveys health and beauty is a common reason for seeking dental care.
Interpersonal relationships and self-esteem also seem to be associated with the search for dental bleaching.
The preference for whitening is justified because it is considered a conservative, safe, and effective treatment. 1 There are two types of dentist-supervised bleaching techniques: at-home or in-office bleaching, 2 and both techniques usually employ products based on hydrogen peroxide or carbamide peroxide. 3 However, some authors have proposed a combined bleaching technique 1 to combine the benefits of both techniques, such as the minimal adverse effects of at-home bleaching 4 and the faster whitening potential of inoffice bleaching. 2 Despite the aesthetic benefits, tooth sensitivity (TS) is a very common side effect 5 of dental bleaching, causing discomfort in two-thirds of patients who undergo dental bleaching. 6 A recent multivariable logistic regression analysis reported a 51% probability of developing TS after home bleaching and 62.9% after in-office bleaching. 1 Given this context, some authors have proposed changes in bleaching techniques, such as the reduction of contact time of bleaching gels in in-office procedures, 7,8 the addition of substances to bleaching agents, and the use of desensitizing agents or dentifrices. 3,9 Studies have discussed the penetration of bleaching agents into the pulp chamber. 10,11 Hydrogen peroxide and its products have the ability to rapidly diffuse through the mineralized tissues of the tooth, which is attributed to the low molecular weight of these substances and to the permeability of enamel and dentin. 10,11 The volume of bleaching agent that enters the pulp chamber depends on the peroxide concentration, 8,10 the duration of contact with the tooth structure, 8,12 the enamel and dentin thickness of whitened teeth, 13 and the presence of restorations. 11 Hydrogen peroxide after penetrating the pulp chamber comes into contact with the dental pulp, crosses cell membranes and dissociates into free radicals in the cytoplasm, resulting in the establishment of an oxidative stress state. 8,14 The adverse effects of oxidative stress correlate negatively with the enamel/dentin thickness of the whitened tooth.
Therefore, small teeth such as mandibular incisors are more susceptible to oxidative damage, being more amenable to diffusion of the bleaching agent into the pulp chamber. 13 A histopathological study demonstrated pulpitis and superficial necrosis in incisors of young patients after in-office bleaching. In premolars, changes were similar to those observed in unbleached teeth. 13 High macrophage density, collagen degradation, and inflammatory infiltration of the dental pulp were observed in molars 7 days after completion of in-office bleaching procedures. 15 The clinical diagnosis of pulp status remains highly challenging. Due to being located within a closed cavity, the dental pulp is inaccessible to direct inspection. Thermal, electrical, and cavity tests are usually employed to assess its clinical condition. The limitations of these pulp sensitivity tests include the possibility of false-positive and false-negative results. 16 Various electronic modalities have assessed the utility of determining pulp status, including spectrophotometry, laser Doppler flowmetry, and pulse oximetry. 16,17 Pulse oximetry has been widely used in medical practice. In dentistry, it is considered a promising resource for measuring oxygen saturation levels (SaO 2 ) in dental pulps. 16,17 Oxygen is carried in the body bound to hemoglobin, an iron-containing protein present in red blood cells. Each molecule of hemoglobin can carry up to four molecules of oxygen, this state is described as "saturated" with oxygen (100%). A healthy individual, with healthy lungs, breathing ambient air, shows arterial SaO 2 levels between 95% and 100%. 19   Unblinding was performed at the end of the study.
All clinical interventions were performed by the same practitioner, a specialist in restorative dentistry who was blinded to group allocation. All materials for the bleaching procedure were prepared by another researcher so the intervention product and placebo were indistinguishable. Measurement of pulp oxygen saturation level before, during, and after bleaching procedures was performed by another blinded researcher (a endodontics specialist).

Clinical intervention
One week before starting the bleaching procedure, participants were advised on oral hygiene and dietary habits. During this visit, impressions of both dental arches were obtained with type II alginate (Plastalgin, Zhermack, NJ, USA), disinfected, and cast with type III dental stone (Asfer Indústria, SP, Brazil). Oxygen saturation levels were also evaluated in 10 endodontically treated teeth, which served as negative controls.
Cold thermal pulp testing Pulp sensitivity testing was performed during the first visit as described above, and repeated 30 days (T7) after completion of the bleaching procedures, using the same protocol. The significance level was set at 5%.

Results
Of the 80 subjects selected for this study, 21.25% withdrew during the clinical intervention period, and 2.5% were excluded due to severe TS. Pulp SaO 2 levels were evaluated in 60 subjects (120 maxillary central incisors) before, during, and after combined dental bleaching ( Figure 2).
Demographic and clinical characteristics of the subjects are shown in Table 1 Table 2.
In all groups, the mean pulp SaO 2 level decreased from T0 to T1, varied somewhat from T1 to T6, and increased from T6 to T7. Table 2 shows that, in G2, there was a significant increase in this value from T0 to T7 (P<0.05), while in G4, there was a significant decrease from T0 to T1 (P<0.05).      On the 30 th day after the end of the bleaching sessions (T7), mean pulp SaO 2 was significantly higher (86.52%) than at baseline (84.76%). Soares, et al. 32 (2015) demonstrated that pulp cells exposed to bleaching protocols and suffering from reduced oxidative stress were able to proliferate significantly over time, with a three-to fourfold recovery of viability 3 days after bleaching procedures. Cartagena, et al. 28 (2015), using laser Doppler flowmetry, showed an increase in pulp blood flow to levels above baseline 7 days after the end of bleaching procedures in healthy central incisors.
Physiological blood flow in the dental pulp is approximately 0.4-0.5 mL/min/g, a rate similar to that of the brain and lower than those of the heart or kidney. 31  interfere with readings, 28 can be mitigated by obtaining measurements in temperature-controlled rooms, with the lights off, in duplicate, and for a 30-second period. 24 Dental morphology aspects involved in variability in measurements can be controlled by stratifying analyses by tooth type and age range.
All of these factors were carefully considered in our study. Some factors, however, are beyond the reach of the clinician's control, such as individual variations in blood flow and neurovascular response, differences in the optical properties of the dental structure, and the presence of surrounding tissues.
In addition, using pulse oximeters for pulp SaO 2 measurement requires the fabrication of special adapters for fingertip sensors since there are no commercial models specifically produced for teeth, which makes it difficult to keep the emitter and receiver diodes parallel. 23, 33 Currently, the distance between the emitting and receiving diodes represents Effect of dental bleaching on pulp oxygen saturation in maxillary central incisors -a randomized clinical trial J Appl Oral Sci. 2019;27:e20180442 9/10 the greatest disadvantage in pulse oximetry equipment because of its substantial influence on the signal-tonoise ratio. 28,33 Oxygen is essential for the aerobic production of cellular energy sources such as adenosine triphosphate (ATP) for mitochondria, and cell activities during tissue repair are directly related to tissue oxygen levels. In the brain, when blood flow decreases to levels under 40% of control values and it is not restored within three minutes, aerobic metabolism causes irreversible tissue damage due to lack of energy. 40 The dental pulp is surrounded by hard tissue and only has access to vasculature through small openings at the root apex, which makes it susceptible to hypoxia. Decreased pulpal blood flow has been reported to cause severe pulp damage. 41 According to Ueno, et al. 42 (2006), when pulp cells were exposed to hypoxia for periods of 24 hours there was interruption of cell growth and death was detected. Under severe hypoxia conditions, the cells seem to not survive, initiating events that lead to cell death by apoptosis. 43 Therefore, monitoring pulp oxygen saturation levels before, during, and after tooth bleaching procedures is fundamental.

Conclusion
The pulp SaO 2 level in maxillary central incisors was similar at baseline, reducing immediately after inoffice bleaching, regardless of the use of desensitizing toothpaste and increasing at 30 days after dental bleaching.