# Comparative Study of the Expansive Behaviour of Different Internal Bleaching Agents
## 1 Introduction  
Over the last few decades, there has been a growing concern about dental aesthetics and smile appearance among the population [1, 2]. Since tooth colour is one of the main factors determining the appearance of a smile, the darkening or alteration of the colour can be of real concern for patients, and even more so when only one tooth in the anterior area undergoes colour change and no longer matches the rest [3, 4, 5, 6, 7]. Patients with this type of problem tend to be more worried about appearance when the darkening also becomes more pronounced over time. Therapeutic procedures aimed at treating and resolving this type of problem are thus relatively common in the dental clinic. Normally, this type of colour alteration is due to intrinsic staining, although it is necessary to study and individualise each case in order to prescribe the appropriate treatment [8]. Colour changes of intrinsic origin are more noticeable and more difficult to treat due to their colour distribution over the entire tooth structure. The most common causes of intrinsic tooth staining are diverse: pulp haemorrhage usually caused due to trauma in anterior teeth, pulp necrosis, necrotic tissue remaining in the pulp chamber after endodontic treatment, pulp calcification, infections and regenerative endodontic protocols using triple antibiotic pastes containing minocycline and mineral trioxide aggregate (MTA) as an intracanal barrier over a blood clot induced at the cementoenamel junction (CE) [9, 10, 11]. Such staining has also been associated with atitrogenic factors (application of various irrigants used for the chemical treatment of canals, drugs and endodontic sealants) [10, 11]. In these cases, the simplest and mostconservative treatment option for restoring the original colour of the darkened tooth is internal bleaching [12]. Endodontically treated teeth can be bleached externally, internally or through a combination of both [13]. There are several internal bleaching techniques: intracoronal bleaching using the thermocatalytic technique, the Walking Bleach technique or a combination of both [9; 14; 15]. Walking Bleach is currently the intracoronal bleaching method of choice for whitening darkened teeth, because it is considered safer for the patient and has a high success rate [6]. Different concentrations of hydrogen peroxide (HP) are used in tooth whitening, which can be applied directly or can also be generated from sodium perborate (SP) or carbamide peroxide (CP) [10; 12]. Generally, HP is used at higher concentrations for nonvital teeth whitening than for vital teeth whitening. However, it also depends on whether the whitening is performed in the dentist's office or at home. Similar concentrations are applied for both vital and nonvital teeth in the dental office under professional supervision, but in lower concentrations in home bleaching for safety reasons [11]. Hydrogen peroxide acts as an active agent in an oxidative process that degrades tooth-darkening compounds and larger stains. These bleaching agents are introduced into the pulp chamber of the endodontically treated tooth between 5 and 14 days, always after checking that the endodontic filling is perfect and without problems in the apex with X-rays, sealing the cavity with temporary restorative material [16; 17]. If the desired shade has not been achieved after this time, the procedure can be repeated. The HP present in all whitening products penetrates through tooth structures and produces reactive oxygen species, free radicals and HP anions, which oxidatively rupture the double bonds of the chromogens deposited on the tooth and which are responsible for tooth darkening, thereby resulting in whiter teeth [15; 18]. Internal bleaching is also sometimes performed with the aim of lightening the tooth substrate as a step prior to the placement of a subsequent aesthetic prosthetic restoration [19]. However, some aspects of the behaviour of bleaching agents remain to be elucidated.