Management of Non-vital Permanent Teeth with Incomplete Root Formation

Introduction

Management of non-vital teeth with incomplete root formation (immature tooth) in children and adolescents presents a clinical challenge in paediatric dentistry and endodontics (Nazzal and Duggal, 2017). When the immature tooth becomes non-vital, it halts the root development and results in the unfavourable crown to root ratio, and thin root dentine walls with the open apex (Nazzal and Duggal, 2017). This makes endodontic treatment difficult to get a proper apical seal with obturation of root canal and moreover the thin dentine walls are unable to withstand physiological forces of mastication which result in high fracture rate with poor prognosis in medium and long-term (Andreasen et al., 2002). Teeth become non-vital mainly due to caries, dental trauma and dental anomalies (dens invaginatus and dens evaginatus) (Flanagan, 2014). Traditionally the management of non-vital immature permanent tooth includes apexification with calcium hydroxide, and with apical barrier formation with mineral trioxide aggregate (MTA) (Velmurugan N, 2016). However, both these techniques do not result in continuous root development and ultimately results in root fracture (Nazzal and Duggal, 2017).    In recent years, there is a paradigm shift in management approach with the aim of inducing further root development and thickening of root dentine wall with regenerative endodontic techniques (Nazzal and Duggal, 2017; Velmurugan N, 2016).

Causes of non-vitality of immature permanent teeth

Dental caries is the most common cause of loss of vitality to teeth (Paulindraraj S et al., 2015). Whereas, dental trauma is the main cause of loss of vitality to anterior teeth followed by dental caries (Paulindraraj S et al., 2015). The deep carious lesions may insult the pulp with bacterial by-products that will cause reversible pulpitis. If not treated, it will progress to irreversible pulpitis and finally pulp necrosis (AAPD, 2014).

Dental trauma to children is more common. The teeth have a natural defence mechanism against trauma. Subsequent to dental trauma, the periodontal ligament acts as a cushion and absorbs the impact of the trauma (Nazzal and Duggal, 2017). However, if the trauma force exceeds the absorptive capacity of the periodontal ligament, the injury to tooth supporting tissues, which include neurovascular tissue, Hertwigs epithelial root sheet (HERS), bone, tooth fractures, etc., may be observed. (Nazzal and Duggal, 2017).

Any disruption of the neurovascular tissue at the apex of the tooth mainly leads to pulp necrosis. The risk of pulp necrosis varies widely depending on the type of dental trauma. In 2001, Borum and associates conducted a study on dental trauma and reported that the likelihood of pulp necrosis varies with the type of dental trauma as follows: intrusion 94%, avulsion 92%, lateral luxation 58%, extrusion 26%, root fractures 20-44%, concussion 3%, and enamel infraction 0% (Borum et al,. 2001). Moreover, the incidence of pulp necrosis was found higher in teeth with combined crown fracture and luxation injury, compared to the luxation injury alone (Diangelis et al., 2012).  However, the immature permanent tooth has considerable capacity for healing after traumatic pulp exposures, luxation injury or fractures, because of the wide open apex and high vasculature (Diangelis et al., 2012).

The other conditions that cause non vitality of tooth are developmental anomalies which include den’s invaginatus and den’s evaginatus (Trope, 2010). In dens invaginatus, a thin layer of dental hard tissue protects the pulp, and pulp infection and necrosis occur soon after the tooth eruption in many cases. Whereas for dens evaginatus, the tubercle fracture results in pulp infection and necrosis (Flanagan, 2014).

Treatment options

In immature permanent tooth, the pulp is essential for root development (AAPD, 2014). Therefore the primary goal of endodontic treatment is to preserve the vitality of pulp affected with caries or dental trauma and pulp affected with dental anomalies (AAPD, 2014).   However, a tooth with non-vital pulp can remain functional (AAPD, 2014). Pulp therapy is dependent on whether the pulp is vital or non-vital after the insult. A proper diagnosis is achieved by thorough medical and dental histories, clinical examination with sensibility tests (cold and electric pulp tests), and radiographic evaluations (AAPD, 2014). Pulp sensibility tests such as electric pulp test and thermal tests are of limited value, because of the varied responses, as roots are immature. In addition, invalid data might be obtained as a result of the often unreliable responses from children because of fear, management problems, and inability to understand or communicate accurately (Klein, 1978). Consequently, the diagnosis is made by clinical history and symptoms, and radiographic evidence of pathosis.

If the pulp is vital with reversible pulpitis then vital pulp therapy techniques are useful in apexogenesis. Depending on the type and the cause of the insult to the pulp, the vital pulp therapy includes indirect pulp capping or direct pulp capping or partial pulpotomy or Cevk pulpotomy (AAPD, 2014). But in the tooth with non-vital or necrotic pulp (irreversible pulpitis), the treatment options include conventional root canal treatment in the mature tooth, whereas in the immature teeth, apexification with calcium hydroxide or MTA or pulp regeneration are the options (Trope, 2010).

Management of non-vital immature permanent teeth

Currently there are three treatment options for management of non-vital immature permanent teeth, which includes apexification with calcium hydroxide or apical plug barrier with MTA and pulp regeneration (Trope, 2010).

Apexification

Apexification is a technique of inducing root end closure in an immature non vital permanent tooth by removing the coronal and radicular tissue and placing a suitable biocompatible agent (AAPD, 2014). The primary goals of apexification is to create an apical barrier which prevents the passage of toxins and bacteria from the root canal into periapical tissues, and to allow the compaction of the root filling material (Paulindraraj S et al., 2015).

Previously the immature teeth with necrotic pulp were treated by placing a custom sized gutta percha cone and cement at the apex or by carrying out periapical surgery (Gaitonde et al., 2007). However, this technique did not gain popularity because of lack of apical barrier for obturation and leakage resulting in reinfection (Gaitonde et al., 2007). Later studies focused on apical closure with biocompatible antibiotic and antiseptic pastes like calcium hydroxide.

Calcium hydroxide apexification

The traditional method of treating non vital permanent immature tooth is with apexification using calcium hydroxide dressings. Kaiser in 1964 first reported the use of calcium hydroxide in apexification and also specified that it would induce the formation of calcific barrier at the apex (Paulindraraj et al., 2015).

Calcium hydroxide has antimicrobial properties by discharging the hydroxyl ions, which results in destroying the cellular components of bacteria. High pH of calcium hydroxide (pH 12.5 -12.8) may contribute to osteoinductive property (Pitt Ford, 2002).The calcified barrier around the apical foramen is formed by cementoid or osteoid material (Trope, 2010). The normal time required for calcific barrier formation ranges from 6-24 months (Trope, 2010).  Many factors that influence the time taken for calcific apical barrier formation include: the size of the apical foramen at the beginning of the treatment; age of the patient (for narrow apex tooth, less calcified material is required when compared to wide open apex); presence of periapical radiolucency may increase the calcific barrier formation time and; patient compliance; and apical healing (Trope M, 2010).

Clinical procedure

(Trope, 2010)

1. Under local anaesthesia, rubber dam isolation and access cavity opening, the root canal is irrigated with low concentration sodium chloride (NaOCl). The working length of the root canal is estimated radiographically by placing an endodontic instrument in the canal.
2. The root canal length cannot bedetermined by using apex locator as it is not reliable in tooth with open apex.
3. Low concentration of NaOCl is used as there is a risk of extruding into the periapical region which can be toxic to cells.
4. Root canal dried with paper points and creamy calcium hydroxide mix is placed into the canal with lentulospiral.
5. The calcium hydroxide is left as a minimum of one week to be effective in achieving disinfection.
6. At the second visit, after reopening the access cavity and irrigation, a thick paste of calcium hydroxide is packed into the canal by using lentulospiral or by pulgger.
7. The normal time required to attain calcific barrier apexification is 6-24 months.
8. The calcific barrier formation should be review radiographically.
9. The root canal will be obturated after achieving calcific barrier.
10. In the dental literature, reports vary on how often the canal should be refilled with calcium hydroxide paste to produce calcific barrier at apex, and the decision appears to be empirical (Paulindraraj S et al., 2015).

In 1981, Tronstad and associates reported that every 3-6 months calcium hydroxide root canal dressing is favoured for creating calcified barrier at the apex (Tronstad et al., 1981). Other reports favour refilling only if there is any evidence of resorption of calcium hydroxide paste radiographically (Cohen & Burns 2002). But a study reported that there is no need of refilling monthly or 3 months for at least 6 months as there is no gain in refilling with calcium hydroxide frequently (Chosack & Cleaton, 1997).

The limitations of using calcium hydroxide apexification includes:

• Long time span for formation of calcific barrier around the apical foramen. Patients may be required to come for every three months for assessing whether the calcium hydroxide has dissolved in tissue fluids or calcific barrier is complete and sufficient to provide endodontic filling, and this requires patient compliance (Trope, 2010).
• Even after the calcific barrier formation at the root apex and completion of treatment, the root canal walls still remain thin and immature with no root elongation and maturation (Nosrat A et al., 2012).
• The absence of root development may lead to increased risk of root fracture and crown to root ratio remain future prosthetic and periodontal sequlae (Mctique D et al., 2013).
• Long term calcium hydroxide dressings make the root dentine brittle because of its hygroscopic and proteolytic action on dentine and increase the risk of root fracture (Chen X et al., 2013).
• It was hypothesised that calcium hydroxide’s high pH could destroy any cell with regenerative potential for further root maturation and root development (Mohammadi and Dummer, 2011).

Because of these limitations, calcium hydroxide is no longer the choice of material for apexification and has been out-dated by MTA to fill the apical end

MTA apexification

Mineral trioxide aggregate (MTA) was the first bioceramic material developed from Portland cement in Loma Linda University, California in early 1990’s by Torabinejad and associates for using it as a retrograde filling material in endodontics (Jitaru S et al., 2016). MTA is available in two versions as grey and white. MTA is a mixture of tricalcium silicate, dicalcium silicate, tricalcium aluminate, gypsum, tetracalcium aluminoferrate. In addition, it contains radio opaque compound bismuth oxide (Jitaru S et al., 2016). MTA is biocompatible to tissues and have good sealing ability preventing micro leakage and also has good compressive strength of 70 Mpa (Jitaru S et al., 2016). It has pH of 12.5 and has antibacterial property (Jitaru S et al., 2016).

The advantages of MTA in apexification includes are: less treatment time compared to calcium hydroxide leading to better patient compliance; good biocompatibility with periapical tissues allows periapical healing; and can set in moist environment with good sealing ability (Trope, 2010).

Clinical Procedure

1. After gaining local anaesthesia to tooth, rubber dam isolation, and access cavity preparation, the root canal should be disinfected by irrigating with low concentration of sodium hypochlorite (NaOCl) (Trope, 2010).
2. The root canal length should be determined radiographically by placing an endodontic file in the root canal (Trope, 2010).
3. After drying the root canal with paper points, to further disinfect the canal system, a calcium hydroxide paste should be placed in the root canal for at least one week and coronally sealed with temporary restoration like Cavit (Trope, 2010).
4. In the second visit after local anaesthesia, rubber dam isolation and access cavity reopening, the calcium hydroxide in the root canal should be irrigated with NaOCl and later with 17% EDTA and canal dried with sterile paper points (Trope, 2010).
5. A thick mixture of MTA, made from MTA powder and sterile water should be placed in the canal with specific MTA carrier and then condensed to the apical end with the appropriate plugger or with back end of paper point. A 3-4 mm MTA apical plug should be created and its position, density and extension should be determined radiographically (Trope, 2010). In case of inadequate apical MTA plug, the whole procedure should be repeated (Mohammadi Z, 2011).
6. Moist cotton pellet should be placed in the canal over MTA plug and coronally sealed with Cavit for at least 3-4 hours for MTA setting.
7. Root canals are reopened and obturated and coronally restored with composite (Trope M, 2010).

In recent years, authors recommend one – visit apexification technique with MTA, which is more advantageous than calcium hydroxide apexification (Paulindraraj S et al., 2015). In this technique after disinfecting the root canal with irritants and drying, MTA is placed as an apical plug and immediate obturation, and coronally restored with composite (Paulindraraj S et al., 2015).

Inferior results were observed when MTA retruded into periapical region when compared to MTA placement at apical foramen (Paulindraraj S et al., 2015). To prevent MTA extrusion in wide open apices, several materials were recommended to use as matrix before placement of MTA to avoid extrusion, leakage and allows favourable periapical healing (Goyal et al., 2016). Those include resorbable collagen, platelet rich fibrin, absorbable suture material, and calcium sulphate. However, their position cannot be adjusted after their placement (Goyal et al., 2016). MTA is reported to be biocompatible and better outcomes are more predictable in apical closer when compared with calcium hydroxide (Paulindraraj S et al., 2015).

The limitations of MTA apexification includes, it does not allow continued root development or maturation and does not reinforce thin root walls. As a result, there is an increased risk of root fracture. However, MTA apexification is less fracture prone than calcium hydroxide apexification (Trope, 2010). MTA handling and high cost is also a limiting factor (Flanagan, 2014).

Biodentine apexification

Biodentin is a calcium silicate cement which was introduced in 2009 by Septodent (Vidal et al., 2016). It consists of tricalcium silicate, calcium corbonate, zirconium oxide, calcium chloride, polycarboxylate and water (Vidal et al., 2016). It comes with powder and liquid form. Biodentine can be used in apexification for formation of apical barrier, because it has antimicrobial properties (because of high alkalinity), superior mechanical properties,  biocompatibility, less setting time (12min), high pH, adequate handling characteristics and no discoloration of tooth when compared to MTA (Vidal et al., 2016). However it has possible disadvantage of low radioopacity and less evidence in dental literature in long term success when used in apexification. When aesthetics are of concern Biodentine is an alternative material to MTA in apexification (Vidal et al., 2016).

The apexification techniques mentioned above have basic problems in that although they allow root canal obturation, they do not contribute to any root development in increasing root dimensions qualitatively and quantitatively, and eventually suffering root fractures leaving the child with a treatment burden for the rest of their lives (Nazzal and Duggal, 2017).

Pulp regeneration

Recently there has been a paradigm shift in the treatment approach in management of non-vital immature permanent tooth with pulp regeneration by regenerative endodontics for achieving qualitative and quantitative root dimensions (Nazzal and Duggal, 2017).  The term regenerative endodontics is defined as “biologically based procedure designed to predictably replace structures, including dentine, root structure and cells of the pulp dentine complex” (Garcia-Godoy and Murray,2012). The term regenerative endodontics also includes revascularisation/revitalisation to describe the treatment of non-vital immature permanent teeth (AAE, 2016).

Revascularisation procedure of a non-vital immature permanent tooth was described in 2001 by Iwaya and associates (Iwaya et al., 2001). Since then the revascularisation has become a treatment option for non-vital immature permanent teeth (Iwaya et al., 2001; Diogenes et al., 2013). This procedure result in disinfecting the root canal and resolving clinical symptoms, and in some cases radiographic evidence of further root development with thickening of root canal dentinal walls and increased root length and positive pulp response to sensibility tests (Nazzal and Duggal, 2017). This favourable outcomes of revascularisation results in strengthening the root and increase in the crown-root ratio.  Even though there is lack of randomised prospective studies in this perceptive, the bestavailable evidence by case reports and case series in dental literature indicate that regenerative endodontics therapy with revascularisation is a feasible treatment option for non-vital immature permanent teeth (Kontakiotis et al., 2014).

The American Association of Endodontists suggests that endodontic regeneration by revascularisation procedure is indicated to non-vital immature tooth, when pulp space is not needed for post/core, and compliant patient not allergic to medicaments (AAE, 2016). There is no standardised protocol for revascularisation of non-vital immature permanent teeth till date.

The American Academy of Endodontics suggests the treatment protocol in two or more appointments as:

First appointment

After clinical and radiographic diagnosis and consent taken from parent and child.

1. Under local anaesthesia to tooth, isolating with rubber dam, and access cavity preparation, the root canal should be irrigated with 20ml of 1.5% NaOCl for five minutes and later with 17% EDTA for five minutes.  The root length should be measured radiographically.
2. Low concentration of NaOCl is used to minimise cytotoxicity to stem cells and is performed by placing the irrigation needle 1 mm short of root apex and EDTA is a chelating agent which has low toxicity to stem cells and also helps in releasing growth factors like dentinal sailophosphoprotein (DSPP) from root dentinal walls, which help in differentiating stem cells into odontoblast cells (Goncalves et al., 2016). Mechanical instrumentation should be avoided or kept to a very minimal level as it weakens the thin root canal walls and also destroys the stem cells (Velmurugan, 2016).
3. Dry the root canal with paper points, later the root canal is dressed with disinfecting paste like calcium hydroxide or low concentration of triple antibiotic paste (mix 1:1:1 ciprofloxacin 100mg : metronidazole 100mg: minocycline 100mg) or double antibiotic paste (ciprofloxacin and metronidazole without minocycline) or substitution  of minocycline with other antibiotic (amoxicillin or cefaclor or clindamycin).
4. If triple antibiotic paste is used then it should be placed below cemento enamel junction to minimise tooth discolouration and coronally restored with Cavit or IRM or Glass ionomer cement (GIC).

Second appointment

1. The second appointment should be in one to four weeks after first appointment. If the tooth clinical signs and symptoms are resolved then under local anaesthesia to the tooth without vasoconstrictor like adrenaline (vasoconstrictor reduces blood flow around tooth) and rubber dam isolation the access cavity is reopened and irrigated with 20 ml of 17% EDTA.
2. Dry the root canal with paper points. Induce bleeding into root canal by over instrumentation 2 mm past the apical foramen with endo file. Allow the blood to fill and clot to the level of cemento enamel junction. Platelet rich fibrin or platelet rich plasma or autologous fibrin matrix can also be used to create blood clot.
3. Place MTA or Biodentine over the clot as capping material and restore with 3-4 mm layer of GIC and composite.
4. Biodentine should be considered in teeth where there is aesthetic concern.
5. Periapical radiographs should be taken as baseline record and treated tooth should be reviewed radiographically every 6 months to evaluate the root development.
6. If any clinical signs and symptoms noticed after first appointment, alternative antibiotic dressing should be considered and some authors prefer using antibiotic sensitivity test in selecting intra canal dressing (Namour and Theys, 2014).

During follow up appointments, the periapical radiolucency will be reduced at 6-12 months, thickening of root dentinal walls and root elongation are often observed in 12-24 months and later positive response to pulp vitality tests (AAE, 2016).

In 2013, McTigue and associates reported a case series of 32 non-vital immature permanent teeth treated with revascularisation procedure in 28 children with follow up for 1 -4 years. They stated that apical healing was observed in 31 teeth, closure of root apex occurred in 23 teeth, the root walls thickened in 22 teeth, and root length increased in 21 teeth. One tooth that did not heal was reinjured with coronal restoration fracture (McTigue et al., 2013). The authors suggested that revascularisation procedure should be considered as a feasible treatment option in non-vital immature permanent teeth.

Key factors in pulp regeneration

The pulp regeneration mainly depends on four key factors of tissue engineering (Nazzal and Duggal, 2017). They include sterile environment, availability of stem cells, suitable scaffolds, favourable signalling molecules for stem cell to regenerate, whereas, majority of pulp regeneration techniques reported in dental literature used these principles inadequately (Nazzal and Duggal, 2017).

Sterilisation of the root canal

Sterilisation of the root canal is the only factor under clinician’s control. Sterile environment in the root canal in non-vital immature teeth can be achieved by disinfecting the root canal with antimicrobial irrigants, intra canal medicament dressing, and proper coronal seal (Nazzal and Duggal, 2017).

Sodium hypochlorite is used as an Irrigant in 1.5% concentrations or in combination with other irrigant like 17% EDTA in many reported studies for disinfecting the root canal. NaOCl has antimicrobial and organic matter dissolving properties.

In 2014, Marin and associates evaluated the effect of different concentrations of NaOCl (6,3,1.5 and 0.5) followed by 17% EDTA or normal saline to evaluate the effect of various concentrations of NaOCl on stem cells of apical papilla (SCAPs) and dentin sailophosphoprotein (DSPP) expression when used as intracanal irrigant on extracted teeth. This study reported that the use of 1.5% NaOCl followed by 17% EDTA would increase survival rate of stem cells and greater DSPP expression (Martin et al., 2014).

Irrigating the root canal itself is not sufficient in eliminating the bacteria. The use of triple antibiotic paste (ciprofloxacin, metronidazole, and minocycline) as intra canal dressing material has shown to be an effective antibacterial agent to eliminate bacteria from the root canal dentinal walls (Nazzal and Duggal, 2017). However, minocycline in triple antibiotic paste (TAP) can potentially discolour the teeth and as a result many authors suggest elimination of minocycline from the TAP and suggest using double antibiotic paste (DAP) instead, which consists of ciprofloxacin and metronidazole. Moreover, recent study have reported that DAP has similar efficacy as TAP (Namour and Theys, 2014).

In 2010, Kim and associates evaluated the tooth discolouration by TAP. In their study they used TAP as intra canal medicament on extracted teeth. They reported that minocycline in triple antibiotic paste caused the tooth discolouration and suggested using DAP in teeth where aesthetics are of concern (Kim et al., 2010a).

Achieving coronal seal is also important in maintaining a sterile root canal environment (Nazzal and Duggal, 2017). The use of MTA along with GIC and composite results in good coronal seal, however the downside of MTA results in tooth decolourisation (Nazzal and Duggal, 2017). The MTA consists of radiopaque agent called bismuth oxide which causes tooth decolourisation (Nazzal and Duggal, 2017).  American Association of Endodontics recommended the use of Biodentine as an alternative to MTA when aesthetics are of concern (AAE, 2016).

Stem cells

The induction of bleeding in to root canal from periapical area result in populating undifferentiated, multipotent mesenchymal stem cells in to the root canal space (Hargreaves et al., 2013). This induced blood has 400-600 fold greater concentration of mesenchymal stem cell markers CD73 and CD105 as compared to cells in systemic circulation (Lovelace et al., 2011). The source of stem cells are mainly from SCAPs, other possible stem cell sources are from periodontal ligament, dental follicle, dental pulp, alveolar bone, and systemic blood (Lovelace et al., 2011).

Scaffold

Scaffolds provide three dimensional support for differentiating and proliferation of stem cells into odontoblasts (Nazzal and Duggal, 2017). Most of the studies on pulp regeneration depend on induced blood into root canal and successive clot formation as a scaffold. The blood clot induced scaffold is promoting the tooth root development with pulp dentine like tissue, but it is not regenerating the desired tissues (Yang et al., 2016). Further research in this area is needed for more reliable and consistent results (Nazzal and Duggal, 2017).

Signalling molecules

Signalling molecules (growth factor/cytokines) play a major role in differentiating stem cell in pulp regeneration procedures. Currently usage of 17% EDTA as intracanal irrigant result in dentinal sailophosphoprotein (DSPP) release from dentinal walls of root canal. These DSPP molecules promote stem cells to differentiate into odontoblasts (Nazzal and Duggal, 2017). At present, there is limited evidence in the availability of relevant signalling molecules for angiogenesis, neurogenesis and promoting odontoblast cells in scaffolds produced by blood clot in revascularisation procedure (Nazzal and Duggal, 2017). Some of the growth factors required for pulp regeneration are: for angiogenesis, platelet derived growth factor (PDGF) and vascular endothelial growth factor (VEGF); for chemotaxis, stroma cell derived factor 1 (SDF1) and platelet derived growth factor (PDGF); for neurogenesis, nerve growth factors (NGF); and for odontoblast differentiation and mineralisation, bone morphogenic protein 7 (BMP7) and DSPP are required (Yang et al., 2016). In vitro studies showed a promising results in regeneration of pulp tissue by incorporating these growth factors in synthetic scaffolds (Yang et al., 2016).

The histological studies on animal teeth reported that the hard tissue formed in the root canal after the revascularisation was likely from periodontal ligament and contains of cementum, bone and dentine like material rather than pulp tissue and that continued root thickening was due to apical cementum deposition (Flanagan, 2014). This indicates that there is a reparative tissue formation rather than regeneration with the specific tissue (Nazzal and Duggal, 2017). Such reparative process with bone like tissue may result internal ankyloses of tooth over several years (Nazzal and Duggal, 2017).

Limitations in regenerative endodontics (revascularisation)

Although revascularisation procedure in non-vital immature teeth can lead to healing of periapical tissues and further root development, there is significant controversy with regard to the nature of tissue formed inside the root canal (Velmurugan, 2016). In cases where loss of vitality resulted from dental trauma, after revascularisation procedure the periodontal healing is predictable, whereas root development and root wall thickening is highly unpredictable because the root development and root wall thickening depend on stem cells and HERS (Nazzal and Duggal, 2017). While root development and dentinal wall thickness is predictable after revascularisation procedure in the tooth with loss of tooth vitality as a result of developmental anomaly, as the degree of damage to HERS is possibly minimal (Nazzal and Duggal, 2017). Discolouration of teeth is commonly noticed where TAP or MTA used in regenerative endodontic procedure (Nosrat et al., 2012). Some studies reported the failure to induce intra canal bleeding which is essential in regeneration of pulp tissue (Nazzal and Duggal, 2017).

Even though true pulp dentin complex is not formed after revascularisation procedure with current evidence, the root development that occurred after this treatment allows long term survival of the teeth (Velmurugan, 2016). Henceforth, revascularisation procedure should be considered as a feasible treatment option in non-vital immature permanent teeth (Velmurugan, 2016).

Translational research in pulp regeneration

At present the evidence in dental literature shows more cementum/bone like tissue formation in the root canal after revascularisation procedure, but many laboratory based researches have proven pulp/dentin regeneration is possible (Velmurugan, 2016). However, this research still requires years of development to translate into clinical practice.

Presently there are two pulp regeneration approaches which are being tried out at laboratory level. They are:

1. Cell based approach
2. Cell free or cell homing approach

Cell based approach

In cell based pulp/dentin regeneration approach, the exogenous stem cells are transplanted in to the root canals of the host to allow regeneration (Huang et al., 2013). The transplanted cells are either derived from the host (autologous) or from other individual (allogenic), and they are grown in cultures with minimal processing (Huang et al., 2013).

In 2011, Iohara and associates conducted a study to regenerate pulp/dentin tissue on animal model. In this study, the autologous pulp stem cells with CD 105⁺ marker and stromal cell derived factor 1 (SDF 1) were transplanted in to a dog tooth after pulpectomy. In three weeks they observed synthesis of neurovasculerised tissue along with dentine along the root dentinal walls in the root canal (Iohara et al., 2011).

In 2010, Huang and associates conducted a study to regenerate pulp tissue by using a combination orthotopic and ectopic animal study models. In this study, the extracted human teeth fragments were emptied, root canals were widened and filled with scaffolds containing SCAPs and dentinal pulp stem cells. Then these tooth fragments were transplanted into immunocompromised mice for blood supply. They observed synthesis of new vascularised human pulp like tissue and dentin like tissue on the canal dentinal walls of tooth fragments (Huang et al., 2010).

The limitations of cell based pulp/dentine regeneration include isolation of viable cells, huge cost and possible of immune rejection (Velmurugan, 2016).

Cell free or cell homing approach

In cell free or cell homing pulp/dentin regeneration approach the endogenous stem cells are induced by chemotaxis molecules to migrate to the site of regeneration. In this approach, the scaffold is made with polycaprolactone and hydroxyapatite, embedded with chemotaxis molecules (growth factors) SDF1, BMP7, VEGF, NGF and type 1 collagen. When this scaffold is placed in to an emptied tooth root canals walls in animal models, it has resulted in synthesising neurovascular tissue and dentine layer on canal dental walls (Kim et al., 2010b). According to some authors pulp revascularisation is considered as cell homing approach (Huang et al., 2013). This technique is simple and economical when compared to cell based approach (Velmurugan, 2016).

Comparative studies on management of non-vital immature permanent teeth

In 2016, Lin and associates conducted a systematic review and meta-analysis comparing calcium hydroxide and MTA apexification. They reviewed 216 studies reported in dental literature. After applying exclusion criteria, only four studies met inclusion criteria. They reported that both the materials provide similar success rate clinically and radiographically, the shorter treatment time with MTA apexification resulting in higher overall success rate, because of patient compliance with treatment completion (Lin et al., 2016).

In 2015, Elumalai and associates conducted a study comparing MTA and Biodentine apexification on non-vital immature permanent teeth in same patient with follow up for 12 months. They reported that both MTA and Biodentine are successful in periapical healing and root end closure in clinical and radiographic review for both teeth (Elumalai et al., 2015).

In 2012, Jeeruphan and associates conducted a retrospective study comparing radiographic and survival outcomes of 61 immature teeth treated with either regenerative endodontic or apexification methods. In this study, 22 teeth were treated with calcium hydroxide apexification, 19 teeth treated with MTA apexification, and 20 teeth with revascularisation.  The authors reported that root width and root length were significantly greater in revascularisation group 28.2% and 14.9%, compared with MTA apexification group 0.0% and 6.1% , and calcium hydroxide apexification group 1.5% and 0.4%.  Furthermore, the survival rate of revascularisation group was 100%, and MTA 95%, and calcium hydroxide group 77.2% (Jeeruphan et al., 2012).  The less survival rate in apexification groups were because of root fractures that occurred subsequently.

Conclusion

The management options for treating non-vital immature permanent teeth have been changing from apexification to regenerative endodontic procedures (revascularization) (Velmurugan, 2016). The revascularization procedures allow periapical healing, root development and thickening of root walls, while apexification does not. It would be advantageous to begin revascularization procedure in cases where non vital tooth has inadequate root length and width, as this allows root development. In cases where root development doesn’t occur by revascularization procedure, then the apexification with MTA apical plug should be preferred. The patient and parent should be informed about the treatment plan prior to the commencement of the treatment. In cases where root length and width are in acceptable proportions, then apexification with MTA can also be a treatment option, as there is less chance of root fracture. At present, there are no guidelines for case selection or randomised clinical trials to support clinical outcomes. So there is a need for further clinical research in the revascularization procedures to provide clinicians to take evidence based decisions in treating the non-vital immature permanent teeth.

References

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