Bleaching of Non-vital Discolored Teeth: Keys of Success

In our daily dental practice, we observe the increasing demand for esthetics in all fields of dentistry. Esthetic procedures can vary from restorative techniques such as conventional crowns, veneers, or bonding to bleaching treatments. 26 Despite different methods that can improve esthetic

Abstract

Teeth discoloration is a fundamental aestheticproblem. It has a social impact on children,adolescents and adults. It is necessary for dentiststo identify the etiology of teeth discoloration to establish an accurate and proper diagnosis.
There are different options to treat discolored teeth such as restorative procedures, veneers, crowns
Actually, bleaching of non-vital teeth is a routine conservative approach to have a suitable esthetic result for endodontically treated teeth.

Various methods to bleach non-vital teeth have been proposed. Three techniques were describedthrough the literature: the walking bleachtechnique, the inside/outside bleach technique and the in-office bleaching procedure. The most recommended one is the walking bleachtechnique since it is simple, safe, with lower risks and suitable for patients and dentists.

Different bleaching agents were used such as sodium perborate, hydrogen peroxide andcarbamide peroxide with various concentrations.
Some side effects were reported like external root resorption, diminution of tooth resistance(fracture), morphological alteration of dental hard tissues and alteration of the characteristicsof dental materials.

The objectives of this article:

To describe the causes of discolored non-vital teeth,
To explain the protocol of the different bleaching procedures and the agents used for,
To report different clinical cases of discolored non-vital teeth treated in our service of Dental Medicine with internal bleaching procedures.

Key words

Non-vital teeth, Discoloration, Bleaching, Carbamideperoxide, Hydrogen peroxide, Sodium perborate, Cervical root resorption.

Introduction

In our daily dental practice, we observe theincreasing demand for esthetics in all fieldsof dentistry. Esthetic procedures can vary fromrestorative techniques such as conventionalcrowns, veneers, or bonding to bleachingtreatments.26Despite different methods that can improve esthetics; bleaching procedures are safe, minimally invasive, conservative, low cost andeffective to treat discolored teeth.11

Referring to Zarow et al. 2014, even the esthetictreatment plan is based on conservative orprosthodontic procedure; dentist should start with teeth bleaching.

Referring to Rona et al. 2009, the discolored teeth can have an incidence on the self-confidence,self-image, attractiveness and employability ofevery one.6

Actually, bleaching of non-vital teeth is consideredas a popular procedure because of the continuous need for white teeth and good looks.1

The internal tooth-bleaching is used to lighten adiscolored tooth that was endodontically treated and the procedure consists of the placement of a chemical oxidizing agent within the cavity access chamber to remove discoloration.25

The first reports concerning bleaching of discolored non-vital teeth were published in the middle of the 19th century.2, 3

Different chemical agents were used to bleach discolored teeth such as chlorinated lime, oxalic acid, acetic acid, chlorinecompounds and solutions, sodium peroxide, sodiumhypochlorite, different concentrations of hydrogen peroxide, carbamide peroxide and sodium perborate.1, 2, 3, 4, 23

The mechanism of action of the different bleachingproducts is the same. The hydrogen peroxide will decompose intooxygen and water. Then, the oxygen will cause the oxidation andreduction of the organic pigments that are mainly concentrated
in the dentin structure. As a result, we obtain the bleachingimpact.7Referring to literature, we can use light, heat and even electric currents to activate the bleaching agents and to have a rapid result.2

Clinical examination and radiographic exploration are important before establishing the treatment plan of bleaching.
In case of discolored tooth, a periapical radiograph is necessary to evaluate the quality of endodontic filling, the periapex region and a possible alteration of a resorption process.24Dentist must define the etiology of the dental discoloration that will have a deep impact on the success of the bleaching treatment.3

Various procedures are available for non-vital tooth bleaching: In-office bleach technique, Inside/outside bleach technique and Walking bleach technique. Some side effects have been reported in non-vital tooth bleaching including external root resorption, alteration of morphology of dental tissues, modification of the properties of restorative materials and reduction of tooth resistance and adhesion.7

Different causes of intrinsic discolorations

It is important that clinicians understand the etiology of tooth discoloration to establish the appropriate diagnosis and to define which treatment to apply.2, 3

Referring to Zimmerli et al. 2010, the intrinsic discolorationis a discoloration that originates from the pulp chamber.
Dental trauma or removal of the pulp tissue are the mostcommon causes of intrinsic discoloration.19

The optical properties and interaction with light of both enamel and dentin help to define the intrinsic color.2

The modifications of enamel or dentine structures and theinduction of different chromatogenic materials into the dentaltissues during odontogenesis or in post eruption lead to theintrinsic tooth discoloration.2, 5

The intrinsic discoloration can be brown, grey, red oryellow.5The regular prophylactic techniques cannot remove theintrinsic discoloration and the best treatment is bleaching using various penetrating agents into enamel and dentin to oxidizethe chromogens.8

Pulp necrosis:

Pulp necrosis can be caused by mechanical, bacterial or chemical irritations. Chromogenic degradation products such as protein degradation products of the pulp tissues can be generated by pulp necrosis leading to tooth discoloration.1

The degree and the severity of the intrinsic discoloration isproportional to the period that pulp tissues has been necrotic.2, 3
The treatment of choice for this discoloration is the intracoronal bleaching.2, 3

Intrapulpal hemorrhage:

After dental trauma, the intrapulpal bleeding is the most
common etiology of tooth discoloration.1After rupture of blood vessels, the blood will penetrate the tubules of dentin and its consecutive break down will lead to different chromogenic
degradation products like hematoidin, hemine, hemosiderin and hematin.1, 5Added to that, pulp extirpation can generate hemorrhage in the pulp chamber.1, 2, 3, 26

At first, a pink coloration of the crown can be noticed.2, 3The iron (Fe) formed by the combination of heme and the
putrefying tissue of the pulp will be turned into dark colored iron sulfates by hydrogen sulfates causing a grey staining of the tooth2Referring to Umar et al. 2014, the modification of tooth color is related to the degree of disintegrated hemoglobin.

Calcification of the pulp:

In case of calcification of the pulp, there is an obliteration of the dentinal tubules and accumulation of reactionary tertiary dentin.1

Referring to Fearon et al. 2007, we observe a yellow discoloration in case of accumulation of tertiary dentin in the canal and the pulp chamber. This yellow discoloration is not only due to the important volume of additional dentin in the pulp chamber but also due to the translucent enamel.

Generally, the treatment of calcified and discolored tooth isinvasive and not conserving the vitality of the pulp (direct orindirect veneers, or ceramic crowns). However, according to Ramos et al. 2013, when respecting the concepts of minimally invasive dentistry and preserving tooth vitality, performing an in- office external bleaching combined with supervised home bleaching is the recommended treatment especially when there is no indication for endodontic treatment.

Endodontic materials:

The incomplete removal of filling materials, sealer residuesand medicaments with tetracycline in endodontically treated tooth can cause coronal discoloration.2, 3 , 26

Referring to Zimmerli et al. 2010, the combination of someirrigants like sodium hypochlorite and chlorhexidine will cause brownish-red precipitates. That is why a separate rinsebetween the two different solutions with distilled water orRingers solution is recommended. Despite intracoronalbleaching is the best treatment; the prognosis is related to the type of materials and the contact period.2, 3

Incomplete removal of pulp tissue:

The incomplete removal of the pulp tissue especially in the pulp horn area during endodontic treatment can cause discoloration by the persistent residual tissue.1, 5, 26

The residual tissues will disintegrate progressively, and blood components will infiltrate into the dentinal tubules, causing
discoloration.2, 3The elimination of the remaining tissue and the intra-coronal bleaching is the recommended treatment.3

Coronal restorative materials:

Ancient composite restorations can lead to dark discoloration of margins and even discoloration of dental tissues due to the
microleakage formed in resin composite.2, 3

The amalgam can produce a dark grey coloration of dentin when it is used after endodontic treatment because of the blackmetallic compounds.2, 3The use of metal posts for realizing a core can cause discoloration of the crown because of the metallic ion release and transparent enamel.2, 3

Bleaching agents of Root-filled Teeth

Referring to Carey et al. 2014, bleaching procedure is determined as the chemical degradation of the chromogens. The most used bleaching agents are hydrogen peroxide, carbamide peroxide and sodium perborate. Other ingredients may be present in the different bleaching products like thickening agents, surfactant, carrier and pigment dispersant, preservative, and flavoring.8

Hydrogen peroxide HP (H2O2):2, 4, 9, 26

Nowadays, it is the active and most commonly used ingredient in whitening products. It is used directly or resulting from a reaction involving sodium perborate or cabamide peroxide. Referring to Bahuguna et al. 2013, the commonly used bleachingchemical agents are oxidizers since they degrade the organic structures of the tooth into shorter lighter in color molecules.

Carbamide peroxide CP (CH4N2O H2O2):1, 2, 9, 19, 25

It is also called urea peroxide. An organic white crystallinecompound formed by hydrogen peroxide and urea. Bleaching agents with 10% carbamide peroxide willrelease 3.5% hydrogen. The stable complex will decompose
in contact with water to release hydrogen peroxide.
Most of the time, carbamide peroxide is used in a 37%concentration for internal bleaching.

Referring to Douglas et al. 2016, the use of carbamide peroxide for non-vital bleaching is a safe and secure alternative since the diffusion of the hydrogen peroxide to the external root surface is lower and the pH in the external root surface is higher. Added to that, the use of 37% carbamide peroxide generates less structural weakening of the bleached structure compared with sodium perborate.

Sodium perborate:1, 2, 25, 26

It is also called perboric acid. Different forms: mono-, tri- (NaBO2 H2O2 3H2O) or tetrahydrate. It is an oxidizing agentpresented as a powder.

Sodium perborate will decompose to form sodium metaborate, hydrogen peroxide and nascent oxygen when it is in contact with water, acid or even warm air.

It is often used for internal bleaching since it has excellent results and it respects the periodontal tissues.

Different authors recommend the use of a mixture of sodium perborate and distilled water. However, Kwon et al. 2011suggested that when we use 30% of hydrogen peroxide mixed with sodium perborate (ratio 2:1 g/mL), we obtain an alkalinepH and the bleaching agent will be more effective.

Importance of the Initial Examination26, 27

The initial examination is very important in the treatment plan. It includes the evaluation of the color of teeth and the adjacent gingiva.

Added to that, the vitality of teeth must be testedand a radiographic examination is recommended.
Dentists must question patients that are presenting adiscolored tooth about any history of traumatic injury.

Referring to Van B. Haywood et al. 2010, pulpal problemscan happen from 1 to 20 years after dental trauma. Dentists should evaluate the color of the gingival tissues, their thickness and their level. In addition, the color of the root must be taken into consideration.

The discolorations of the gingival tissues may lead to a tooth with a not harmonious color match.
Referring to Kwon et al. 2011, the root dentin isdifferent from the dentin in the crown and does not bleach well if at all, nevertheless internal or external bleaching is done.
In case that the patient suffers from a gummy smile orhyperactive lip, these problems will be more evident.

The different bleaching techniques

The walking bleach technique:

In this therapy, clinician applies a bleaching agent into the empty pulp chamber of an endodontically treated and discolored tooth.5

This technique was first described using a mixture of sodiumperborate and distilled water. Later, some authors suggested replacing water by 30% hydrogen peroxide to ameliorate the bleaching efficiency.1, 2

However, referring to Zimmerli et al. 2010, there is nodifference in efficiency between the two methods and that usingcarbamide peroxide in concentrations of 10% or 35% mixed with sodium perborate will improve the bleaching effectiveness.

Treatment steps referring to Kwon et al, 2011:

Make a radiographic exam to evaluate the presenceor not of a root canal filling and to assess the quality of theendodontic filling.

Isolate the tooth by means of a rubber dam.

Clean the pulp chamber and the pulp horns of any debrisor pulpal remnants to have the entire pulpal cavity visibleand cleanable.

Eliminate the gutta percha with a heated instrument or alow-speed small round bur to 2 mm below the cementoenamel junction CEJ. We can also use Gates-Glidden or Largo burs. This step can be controlled with a periodontal probe.

Place a cervical barrier of 2mm thickness with a glass-ionomer cement or a flowable resin to seal the endodontic filling materialfrom the pulp chamber. It is important that the shape of thecervical barrier should be like to the external anatomic landmarks replicating the CEJ level and the interproximal bone level.

Mix sodium perborate with water or hydrogen peroxide in a ratio of 2:1 (g/mL) to a thick mix.

Insert the mixture into the pulp chamber with an amalgamcarrier or an applicator.

Use a damp cotton pellet to eliminate the excess.

Seal the access cavity with a provisional filling material likeCavit, glass-ionomer cement or IRM.

Referring to Fearon et al 2007, the bleaching agent should be covered with cotton pellet and sealed with an adhesivetemporary restoration.5

The provisional adhesive restoration is recommended because it ameliorates the seal against the penetration of bacteria.1

In the next appointment (after 3 to 5 days), the bleachingresult is evaluated and, if necessary, the walking bleachprocedure is repeated (3 to 5 times) until the color becomessimilar to the adjacent teeth.26

The final step after the last walking bleach technique is therestoration of the access cavity with resin composite. Thisprocedure is achieved 2 to 3 weeks later when the desired color change is obtained and the bond strength to the tooth has returned to normal.26The resin composite used for finalrestoration should have a light color.5

The walking bleach technique is not recommended only for adults but also for children with colored non-vital tooth. A case report of a 12-year-old female patient treated with internal bleaching and with 6 years follow-up showed good prognosis and no reversal of tooth discoloration.21

The walking bleach technique using sodium perborate mixed with distilled water is recommended as a secure alternative in whitening non-vital primary teeth with intrinsic discoloration.28

The inside/outside bleach technique:

The principle of this technique is to apply the bleaching agent in the same time on the facial enamel (outside) and the sealedcavity access (inside) of the discolored endodonticallytreated tooth.1, 5

For this technique, we use a vacuum-processed plasticmouthguard that is cut to the facial and lingual margins after
taking an alginate impression of the whole arch.5The preparationof the cavity access and the achievement of the cervicalbarrier were performed as previously described for the walkingbleach technique.

The bleaching agent used is 10% to 20% carbamide peroxide gel (syringe) that the patient inserts in the access cavity and the marked tooth of the vacuum-drawn splint.1, 5, 26, 27

Some instructions to give to the patient: To eliminate the excess of the bleaching agent after inserting the splint with a fingertip or a cotton swab and to place a cotton wool plug in the cavity access to prevent accumulation of food particles.1, 5

In the control visits, when the non-vital tooth has been bleached, the steps followed are as previouslydescribed for the walking bleach technique. The majordrawback of this technique is the lack of bacterial controlaffecting the result and the endodontic treatment longevity.1

The risk of this technique is stopping the bleaching by the patient himself and not returning to the dental office in time to seal the cavity access.27

In-office bleaching technique:

For this technique, a high concentration of hydrogenperoxide 35% is used like in-office bleaching of vital teeth.26
The hydrogen peroxide will be applied in the external and/or the internal surface of the discolored tooth and it can beactivated with light and/or heat to increase the chemicalreaction.27

The major risk of this technique is resorption and loss of the tooth because of the history of trauma, the high concentration of hydrogen peroxide used, the high heat used to activate the bleaching agent, the absence of seal over the gutta-percha and the absence of connection between enamel and cementum.27
For this technique, it is recommended to isolate the tooth with a rubber-dam. [1, 26]

The application time of the bleaching agent is 1520 minutes. Then, the gel is eliminated and rinsed off. The application is repeated until the desired shade is obtained. [1]

Side effects of bleaching of non-vital teeth

Cervical root resorption:

Referring to Bahuguna et al. 2013, it is defined as an inflammatory mediated external resorption affecting the root, occurring after trauma and after internal bleaching. The bleaching agent will diffuse via dentinal tubules and reaches the cervical cementum and the periodontal ligament initiating an inflammatory reaction.4, 6

The use of an important concentration of hydrogen peroxide activated with heating can lead to cervical root resorption.4, 5, 6

This complication of bleaching can develop when a thermo-catalytic bleaching technique is employed in teeth presenting cervical defects of the cementum.4

The use of 30% hydrogen peroxide alone or mixed to sodium perborate is more toxic and dangerous for the periodontal ligament cells than the mixture of sodium perborate and water.5

Most of the times, the diagnosis of the cervical root resorption is done after several years.4

This complication is observed through routine radiographs. However, in some cases we can note the presence of papillary swelling and tenderness to percussion. [5] Referring to Leith et al. 2009, there is no evidence that cervical resorption can occur after using a low concentration of hydrogen peroxide associated with heat like in inside/outside bleaching procedure.

A clinical case treated by Kim et al. 2012 showed an invasive cervical resorption. The explanation of the authors was the damage of the cervical cementum resulting from avulsion (after trauma) and from intra-coronal walking bleach technique (sodium perborate and distilled water).

Effects on enamel:

Bleaching of non-vital teeth can lead to substantial alteration of the enamel surface topography. The amount of alteration depends on concentration of peroxide used and the period of exposure.5
The considerable alterations affecting the enamel after bleaching described in literature are:8

Augmentation of porosity of the superficial enamel structure.

Demineralization.

Reduction of protein concentration.

Degradation of organic matrix.

Changing in the calcium: phosphate ratio, calcium loss.

Other morphological alterations that can affect enamel surface are shallow depressions and slight erosion.8

After bleaching, the surface roughness will be increased and the bleached teeth will be more vulnerable to extrinsic discoloration.5

Added to that, the use of 35% hydrogen peroxide increase the diminution of hardness and the histomorphologic changes of the enamel surfaces exposing the tooth to risk of caries.8Reducing the contact time of 35% hydrogen peroxide or its concentrationwill produce a progressive color change of the tooth and willminimize the diffusion of hydrogen peroxide through both enamel and dentin.12

The diminution of bond strength of enamel and dentin treated with HP is caused by residual oxygen existent in enamel and
dentin pores after bleaching treatment.10

Effects on dentin:

Referring to Berger et al. 2014, the presence of calcium in the composition of bleaching products will influence the flexural strength of bovine dentin after bleaching procedure (office bleaching 35% HP and home bleaching 7.5% HP).

A study of Pessarello et al. 2012 that evaluated the bond strength of adhesive systems and composite resin to bleached dentin
concluded that the association with fluoridated adhesive and flowable resin ameliorates the shear bond strength of dentin submitted to internal bleaching.

Effects on restorative materials:

The oxygen released into the tooth structure after bleaching will inhibit the polymerization of resin.5

The use of 1016% carbamide peroxide will lead to an increase in surface roughness and numbers of porosities of microfilled and hybrid composite resins.8

Referring to Bahannan et al. 2015, the influence of bleaching agents on surface roughness can be considered concentration dependent.

Carbamide peroxide with different concentrations did notproduce a prejudicial effect on the fracture toughness andflexural strength of composite resin. Added to that, in-office bleaching agents (35% CP and HP) did notaffect the tensile strength of composite resin.13Themicrohardness of nano composite resin and resin modifiedglass ionomer is reduced significantly with 20% and 35%carbamide peroxide.11

Clinicians have to inform their patients concerning possiblemodifications that may affect their existing restorations by bleaching agents and the possibility of replacing them after the treatment due to color difference and surface or subsurface
alteration.8, 13Concerning the glass ionomer cement, the setting of cement is inhibited by the residual oxygen.10

It is necessary to wait at least 2 weeks before placement anyadhesive restorative material after completion of bleaching.13

Referring to Vohra et al. 2013, bleaching procedure using 10% carbamide peroxide reduce importantly the microtensile bond strength of resin composite to enamel. Moreover, it is
recommended to use 10% sodium ascorbate when resincomposite restoration will be placed instantly after bleaching.

The application of 10% sodium ascorbate for 10 min is enough to have a reversal effect.14

Referring to Feiz et al. 2017, the use of antioxidant agents,regardless of their type, form, concentration and duration ofapplication, can improve the shear bond strength after bleaching.
To dissolve the remnants of peroxide, dentists can clean thecavities with catalase or 10% sodium- ascorbate. However, their applications can be time-consuming or expensive.10

The placement of a protective varnish on the surface ofrestoration can be advantageous to reduce the adversebleaching effects on restorations.13

When the duration of application of sodium perborate hydrogen peroxide mixture is increased, the rate of microleakage of the restored access cavities will be more important. For this reason, it is recommended to use intracoronal medication of calcium hydroxide (7 days) after walking bleach technique.10

Fracture resistance of bleached teeth:

A study of Azevedo et al. 2011, evaluated the fracture resistance of teeth treated with internal bleaching and restored with various procedures. The conclusion was that bleached teeth can be restored with composite resin alone and the use of posts in these teeth did not ameliorate their resistance.

Another study of Leonardo et al. 2014, evaluated the fracture resistance of teeth exposed to several internal bleaching protocols using 35% hydrogen peroxide, 37% carbamide peroxide, 15% hydrogen peroxide with titanium dioxide nanoparticles photo activated by LED-laser or sodium perborate. The conclusion was that all these protocols reduce, in equal values, the fracture resistance of endodontically treated teeth.

Clinical cases

Case report N1:

A 17-year old patient was unsatisfied with the discoloration of the tooth 21. History: trauma. The treatment proposed was the endodontic retreatment of the 21 and walking bleach technique using Endoperox kit (100 % carbamide peroxide).

Figure 1: Non-vital tooth discoloration of the tooth 21.

Figure 2: Periapical radiograph showing bad quality of root canal filling.

Figure 3: Endodontic retreatment of the tooth 21.

Figure 4: Rubber dam isolation.

Figure 5: The distance between the CEJ and the incisal edge is measured with a periodontal probe.

Figure 6: Having recorded the measurement between the CEJ and the incisal edge, the periodontal probe assists for more precision.

Figure 7: Removing the gutta percha root canal filling material with a round ended, long shank bur below the CEJ and placing a cervical barrier of 2mm thickness with glass-ionomer cement to protect the periodontal ligaments from the diffusion of the bleaching agent.

Figure 8: Using 37% phosphoric acid gel to remove the smear layer on the dentinal surface of the pulp chamber.

Figure 9: The bleaching agent used Endoperox kit (100 % carbamide peroxide). Kit containing: 1 x 5 g bottle of powder, 1 x 5 ml flask of glycerol and 1 measuring spoon.

Figures 10, 11, 12: Making a paste by mixing two spoonfuls of powder with 1 drop of glycerol to a firm consistency then placing the mixture into the pulp chamber.

Figure 13: Clinical appearance of the tooth 21 after bleaching performed, 1 week later.

Case report N2:

A 29-year old patient was unsatisfied with the discoloration of the tooth 21. History: trauma. The treatment proposed was the endodontic treatment of the tooth 21 and walking bleach technique using Endoperox kit (100 % carbamide peroxide).

Figure 14:Pre-treatment photograph of left maxillary central incisor showing discoloration due to a necrotic pulp caused by trauma.

Figure 15: Periapical radiograph of the traumatized tooth before the root canal treatment was performed.

Figure 16: Endodontic preparation and filling of the tooth 21.

Figure 17: Endodontic filling with the Herofillsystem.

Figure 18: Clinical appearance of the tooth 21 after walking bleach technique, 1 week later.

Case report N3:

A 64-year old patient was unsatisfied with the discoloration of the tooth 11 caused by trauma. The treatment proposed was the walking bleach technique using sodium perborate with distilled water.

Figure 19: Clinical appearance of the discolored traumatized tooth 11 before treatment.

Figure 20: The panoramic radiograph.

Figure 21: Opening the cavity access and removing the gutta percha root canal filling material to 2 mm below the CEJ.

Figure 22: The bleaching agent used was a mixture of sodium perborate and distilled water. Then, Cavit was used as a temporary filling material.

Figure 23: After reaching the final result (1 month), the definitive resin composite restoration was placed.

Case report N4:

A 32-year-old patient was unsatisfied with the discoloration of the tooth 11 caused by trauma. The treatment proposed was the walking bleach technique using sodium perborate with distilled water.

Figure 24: The pre-operative appearance of the maxillary discolored right incisor.

Figure 25: The periapical radiograph showing acceptable endodontic treatment.

Figure 26: The bleaching effect of sodium perborate on the tooth 11 after two applications.

Figure 27: Final result after 1 month (Microabrasion + Resin composite restoration).

Conclusion

Bleaching of non-vital teeth can be achieved by either walking bleach technique, inside/outside bleach technique or in-office bleaching procedure. The bleaching agents used are sodium perborate, hydrogen peroxide and carbamide peroxide.

The above clinical cases highlight the effectiveness of the walking bleach technique in producing successful esthetic results and patients showed satisfaction with the outcome. Referring to literature, bleaching of non-vital teeth is an effective and safe technique to manage discoloration. However, clinicians must inform their patients about the potential risk of cervical resorption.

After termination of bleaching, it is recommended to delay resin composite restoration for at least 13 weeks.
Each year follow-up visits are advisable to control the outcome and to repeat the bleaching treatment when clinician detects a regression of the initial result to preserve the desired esthetic color.

References

1.Bleaching of Nonvital Teeth A Clinically Relevant Literature Review. Brigitte Zimmerli, Franziska Jeger, Adrian Lussi. Schweiz Monatsschr Zahnmed 120: 306313 (2010).

2.Nonvital Tooth Bleaching: A Review of the Literature and Clinical Procedures.

Gianluca Plotino, Laura Buono, Nicola M. Grande, Cornelis H. Pameijer, Francesco Somma. JOE Volume 34, Number 4, April 2008.

3.Bleaching discolored devital teeth with using of new agents. Ibrahim Umar, Hakan Kamalak. IOSR Journal of Dental and Medical Sciences (IOSR-JDMS) Volume 13, Issue 3 Ver. I. (Mar. 2014), PP 79-82.

4.Cervical root resorption and non vital bleaching. Nikhil Bahuguna
ENDODONTOLOGY Volume: 25 Issue 2 December 2013.

5.Tooth whitening: concepts and controversies. Johnny Fearon
Journal of the Irish Dental Association 2007; 53: 132 140.

6.An effective bleaching technique for non-vital, discoloured teeth in children and adolescents. Rona Leith, Abigail Moore, Anne C. OConnell Journal of the Irish Dental Association 2009; 55 (4): 184 189.

7.Preferences on Vital and Nonvital Tooth Bleaching: A Survey Among Dentists from a City of Southern Brazil. Flvio Fernando Demarco, Marcus Cristian Muniz Conde, Caroline Ely, Eliana Nascimento Torre, Jos Ricardo Souza Costa, Mara Raquel Fernndez, Sandra Beatriz Chaves Tarquinio. Brazilian Dental Journal (2013) 24(5): 527-531.

8.Tooth-bleaching procedures and their controversial effects: A literature review. Mohammed Q. Alqahtani. The Saudi Dental Journal (2014) 26, 3346

9.Tooth Whitening: What We Now Know. Carey CM. The Journal of Evidence-Based Dental Practice (2014), doi: 10.1016/j.jebdp.2014.02.006.

10.Effect of bleaching on restorative materials and restorationsa systematic review.
Thomas Attin, Christian Hannig, Annette Wiegand, Rengin Attin. Dental Materials (2004) 20, 852861.

11.Effects of different bleaching agent concentrations on surface roughn


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