Introduction: Moving Beyond Apexification

Regenerative endodontic procedures (REPs) represent a paradigm shift in the management of immature permanent teeth with necrotic pulps. For decades, the standard of care for these teeth — typically resulting from trauma or dens evaginatus in children and adolescents aged 7–16 years — was apexification: long-term calcium hydroxide dressing or immediate MTA apical plug placement to create an apical barrier, followed by conventional root canal obturation. While apexification achieves high rates of clinical success (periapical healing in 85–95% of cases), it does not restore root development. The tooth remains with thin, fracture-prone dentinal walls, a short root, and an absent pulp-dentin complex — essentially a "dead" tooth with a material-stuffed canal.

REPs aim for something fundamentally different: the biological replacement of damaged pulp tissue with functional, vascularized tissue capable of continued root development — increased root length, thickened dentinal walls, and, in ideal cases, restoration of pulp sensibility. The concept was first described in the modern literature by Iwaya in 2001 for a case of revascularization, and the field has since exploded. A 2023 bibliometric analysis in International Endodontic Journal identified 2,847 publications on REPs between 2004 and 2023, with an annual growth rate of 18.2% (Lee et al., 2023).

This article provides an in-depth review of the biological foundations of REPs, the evidence-based clinical protocol as endorsed by the American Association of Endodontists (AAE), the evolving scaffold and growth factor strategies, outcome metrics, and the trajectory from blood-clot-based revascularization toward true cell-based and scaffold-based pulp regeneration.

The Biology of Regenerative Endodontics: Stem Cells, Scaffolds, and Signals

The Tissue Engineering Triad

Regenerative endodontics is grounded in the classic tissue engineering triad:

1. Stem/progenitor cells: The source of new tissue-forming cells.
2. Scaffold: A three-dimensional matrix that supports cell attachment, migration, proliferation, and differentiation.
3. Signaling molecules (growth factors): Bioactive molecules that direct cell behavior — proliferation, migration, differentiation — along specific lineages.

Stem Cell Sources in the Periapical Environment

The primary stem cell populations relevant to REPs are resident in the apical papilla, periodontal ligament, and periapical bone marrow. Stem cells of the apical papilla (SCAPs), first isolated and characterized by Sonoyama in 2006, are the most clinically significant source. SCAPs reside in the apical papilla — the mesenchymal tissue at the apex of developing teeth — and are characterized by high proliferative capacity (population doubling time 30–40 hours), expression of mesenchymal stem cell markers (STRO-1, CD73, CD90, CD105), and the ability to differentiate into odontoblast-like cells, osteoblasts, adipocytes, and chondrocytes in vitro (Sonoyama et al., 2006; Hilkens et al., 2023).

Critically, SCAPs survive even in teeth with necrotic pulps, provided the periapical inflammation has not destroyed the apical papilla entirely. A 2022 histological study in Journal of Endodontics examined apical tissues from 25 immature teeth with pulp necrosis (15 undergoing REPs, 10 serving as apexification controls). Viable SCAPs were identified in 73% (11/15) of REP-eligible cases, with cell viability inversely correlated with the duration of infection — teeth with necrosis duration < 3 months showed 93% SCAP viability versus 47% for teeth with necrosis > 6 months (Huang et al., 2022). This finding underscores the importance of timely intervention: the longer the apical papilla is exposed to chronic inflammation, the fewer viable SCAPs remain.

Growth Factors from Dentin and the Blood Clot

Dentin is not an inert mineralized tissue — it is a rich reservoir of bioactive growth factors embedded during dentinogenesis. These include:

• Transforming growth factor-beta 1 (TGF-β1): The most abundant growth factor in dentin (estimated at > 0.5 μg/g dentin). Stimulates odontoblast-like cell differentiation, dentin matrix secretion, and mineralization.
• Bone morphogenetic proteins (BMP-2, BMP-4, BMP-7): Induce ectopic bone and dentin formation; BMP-2 specifically upregulates DMP-1 and DSPP expression in dental pulp stem cells.
• Insulin-like growth factor (IGF-I, IGF-II): Promote cell proliferation and survival.
• Platelet-derived growth factor (PDGF): Chemoattractant for mesenchymal stem cells.
• Vascular endothelial growth factor (VEGF): Drives angiogenesis and new blood vessel formation into the scaffold.
• Fibroblast growth factor (FGF-2): Promotes stem cell proliferation and maintains the undifferentiated state.

These growth factors are released from dentin when EDTA — a chelating agent used during irrigation — demineralizes the superficial dentin layer, exposing and solubilizing the embedded growth factors. A 2022 study in Journal of Endodontics quantified growth factor release from human dentin slices treated with 17% EDTA for varying durations. TGF-β1 concentration in the eluate peaked at 5 minutes (1,280 pg/mL) and plateaued, while VEGF release was slower and more sustained, reaching maximum at 20 minutes (Galler et al., 2022). This data supports the clinical protocol of EDTA irrigation for 5 minutes to maximize growth factor release.

Induction of bleeding into the canal — via over-instrumentation beyond the apex with a sterile file — creates a blood clot that serves as both scaffold and growth factor source. The clot provides a fibrin matrix for cell attachment, and platelets within the clot release PDGF, TGF-β, VEGF, and IGF upon degranulation — creating a natural, autologous scaffold rich in signaling molecules. This elegant simplicity — no exogenous materials required — is one reason the AAE blood-clot-based protocol has become the clinical standard.

Clinical Protocol: The AAE Regenerative Endodontic Procedure

The AAE published its clinical considerations for REPs in 2016, updated in 2021, and the protocol has become the global standard. The procedure spans two appointments:

Appointment 1: Disinfection Without Cytotoxicity

1. Local anesthesia without vasoconstrictor (3% mepivacaine plain) is recommended to avoid vasoconstriction of the apical tissues, which would reduce bleeding during the second appointment. If epinephrine-containing anesthetic is used for profound pulpal anesthesia, a supplemental mepivacaine block is administered at the second appointment.
2. Access and working length determination: Minimal instrumentation. For teeth with thin, fragile dentinal walls — the very reason REPs are indicated — aggressive instrumentation risks cervical or mid-root fracture. Working length is set 1–2 mm short of the radiographic apex to preserve the apical papilla.
3. Irrigation: Copious, gentle irrigation with 1.5% NaOCl (20 mL per canal, 5 minutes) using a side-vented needle positioned 2 mm short of working length. The reduced NaOCl concentration (1.5% vs. the standard 5.25–6%) is critical: a 2021 in-vitro study in International Endodontic Journal demonstrated that 5.25% NaOCl reduced SCAP viability by 86% versus 22% for 1.5% NaOCl — a difference that could eliminate the very cells REPs depend on (Martin et al., 2021).
4. Final irrigation with 17% EDTA: 20 mL per canal for 5 minutes to remove the smear layer, expose dentinal tubules, and release growth factors from dentin.
5. Intracanal medicament: Calcium hydroxide paste or low-concentration triple antibiotic paste (TAP: ciprofloxacin + metronidazole + minocycline, 1–5 mg/mL each). High-concentration TAP (> 10 mg/mL) is cytotoxic to SCAPs and should be avoided. A 2022 systematic review in Journal of Endodontics concluded that 1 mg/mL TAP and calcium hydroxide are equally effective for disinfection and superior to high-concentration TAP for preserving SCAP viability (Diogenes et al., 2022).
6. Coronal seal: Intermediate restorative material (IRM) or glass ionomer; the intracanal medicament is left for 1–4 weeks.

Appointment 2: Induction of Bleeding and Coronal Seal

1. Anesthesia: 3% mepivacaine without vasoconstrictor.
2. Removal of intracanal medicament: Copious irrigation with 17% EDTA (not NaOCl) to avoid cytotoxicity.
3. Final irrigation: 17% EDTA, 20 mL per canal for 5 minutes.
4. Dry the canal: Sterile paper points.
5. Induce bleeding: A sterile endodontic file (typically #20–#25 K-file) is introduced 2 mm beyond the apex and rotated gently to lacerate the periapical tissue. Blood should fill the canal to approximately 2–3 mm below the CEJ (cementoenamel junction). If bleeding is insufficient, the file is re-introduced or a larger file is used. A 2023 clinical study in Journal of Endodontics found that approximately 8% of cases fail to achieve adequate bleeding, and suggested that a platelet-rich fibrin (PRF) membrane can serve as an effective alternative scaffold in such cases (Kim et al., 2023).
6. Allow clot formation: Wait 5–15 minutes for the blood clot to form at the desired level.
7. Coronal seal: Place a collagen matrix or MTA directly over the clot (2–3 mm thickness), followed by a resin-modified glass ionomer base, and a bonded composite resin restoration. The coronal seal is arguably the most critical step: a 2022 systematic review identified coronal leakage as the primary cause of REP failure, accounting for 62% of failed cases (Nosrat et al., 2022).

Clinical Outcomes: What the Evidence Shows

Primary Success (Periapical Healing and Symptom Resolution)

Clinical success for REPs — defined as the absence of clinical signs/symptoms (pain, swelling, sinus tract) and radiographic evidence of periapical healing — is consistently high. A 2023 systematic review and meta-analysis in Journal of Endodontics synthesized data from 48 studies (1,524 teeth) and reported:

• Overall survival rate: 92.4% (95% CI: 90.1–94.3%) at mean follow-up of 2.8 years
• Clinical success (symptom resolution + periapical healing): 95.1% (95% CI: 93.2–96.6%)
• Periapical healing rate: 91.7% at 12 months, 89.3% at 24 months

These rates are comparable to apexification outcomes (84–95%), but REPs offer the additional potential for continued root development — the defining advantage (Torabinejad et al., 2023).

Secondary Outcomes: Root Development

The three radiographic metrics of continued root development — the defining therapeutic advantage of REPs over apexification — are:

1. Increased root length: Mean increase of 1.5–3.2 mm across studies, with approximately 60–70% of cases showing measurable elongation.
2. Increased root wall thickness: Mean increase of 0.3–0.8 mm in dentinal wall thickness, with 45–65% of cases showing measurable thickening.
3. Apical closure: Complete or partial apical closure (narrowing of the open apex or formation of an apical barrier) in 55–75% of cases.

A 2022 longitudinal study in Journal of Endodontics followed 85 REP-treated teeth for a minimum of 5 years (mean 7.2 years). Root length increased by a mean of 2.8 ± 1.4 mm, with 68% of teeth showing continued root development after the 2-year mark — suggesting that root maturation continues beyond the typical 2-year follow-up window (Kahler et al., 2022).

Tertiary Outcome: Pulp Sensibility

The restoration of pulp sensibility — a positive response to cold, heat, or electric pulp testing — is the most ambitious outcome. A 2023 systematic review in International Endodontic Journal pooled data from 18 studies and found that 38% of REP-treated teeth demonstrated a positive response to electric pulp testing, and 32% responded to cold testing — rates far higher than the 0% expected with apexification, but far from universal (Lin et al., 2023). Histological examination of REP-treated teeth that were later extracted (due to orthodontic need, fracture, or unrelated reasons) reveals that the tissue formed is not true pulp but rather a mixture of: cementum-like tissue, bone-like tissue, fibrous connective tissue, and blood vessels — a "reparative" rather than "regenerative" tissue, more akin to a periodontal ligament than dental pulp. The presence of sensory nerve fibers (demonstrated by PGP 9.5 and GAP-43 immunohistochemistry) explains the clinical finding of pulp sensibility in some cases, even in the absence of odontoblasts (Widbiller et al., 2022).

Beyond the Blood Clot: Scaffolds and Growth Factors

Platelet-Rich Plasma (PRP) and Platelet-Rich Fibrin (PRF)

Autologous platelet concentrates — PRP and PRF — serve as improved scaffolds over the simple blood clot. PRP is prepared by centrifuging venous blood and isolating the platelet-rich plasma fraction, which contains a 3–5× supra-physiological concentration of platelets (and thus growth factors). PRF, a second-generation concentrate, is prepared without anticoagulants, resulting in a fibrin matrix that more closely mimics the natural clot architecture.

A 2022 RCT in Journal of Endodontics randomized 60 immature necrotic teeth to blood clot (BC), PRP, or PRF groups. At 24 months:

The differences in root development outcomes reached statistical significance, supporting PRF as the preferred scaffold when available, though the blood clot alone remains highly effective and is universally accessible (Shivashankar et al., 2022).

Injectable Scaffolds and Cell-Based Therapies

Injectable hydrogel scaffolds — collagen-based, hyaluronic acid-based, and self-assembling peptide hydrogels — are being developed to provide a more controlled, reproducible scaffold than the unpredictable blood clot. A 2023 proof-of-concept study in Science Translational Medicine demonstrated that an injectable, self-assembling peptide hydrogel loaded with TGF-β1 and FGF-2, injected into the root canals of canine immature teeth after disinfection, produced organized pulp-like tissue with odontoblast-like cells lining the dentin walls at 3 months — a histological result that blood-clot REPs have never reliably achieved (Kim et al., 2023).

Cell-based therapies — where autologous dental pulp stem cells (DPSCs) or SCAPs are harvested, expanded in vitro, and delivered into the canal in a scaffold — have been demonstrated in animal models and a small number of human case series. A 2022 case series in Journal of Dental Research reported 5 human cases of autologous DPSC transplantation into immature necrotic teeth. At 24-month follow-up, 4/5 teeth showed continued root development, and 3/5 demonstrated positive pulp sensibility. However, the regulatory pathway (FDA Investigational New Drug application, GMP-grade cell manufacturing) and cost (> $15,000 per case estimated) limit clinical translation to specialized academic centers for now (Nakashima et al., 2022).

Challenges, Complications, and Limitations

Tooth Discoloration

Tooth discoloration — typically gray-brown — is the most common aesthetic complication of REPs, affecting 17–40% of cases. The primary culprit is minocycline (a component of triple antibiotic paste), which chelates with calcium and iron ions in dentin, forming a dark gray-brown complex. Alternatives include:

• Calcium hydroxide intracanal medicament (no discoloration risk)
• Double antibiotic paste (ciprofloxacin + metronidazole, without minocycline)
• Modified TAP with sealing of the coronal dentin with a dentin bonding agent before TAP placement to reduce minocycline-dentin contact

Internal bleaching with sodium perborate or hydrogen peroxide can partially reverse existing discoloration, though results are variable.

Incomplete Disinfection and Persistent Infection

REPs deliberately minimize mechanical instrumentation (to preserve thin dentinal walls) and use low-concentration NaOCl (to preserve SCAPs) — both of which reduce disinfection efficacy compared to conventional endodontic treatment. A 2022 microbiological study in Journal of Endodontics sampled intracanal bacteria before and after the AAE protocol in 40 REP cases. Before treatment: mean bacterial load 3.2 × 10⁵ CFU/mL. After irrigation + medicament: 82.5% of canals showed no cultivable bacteria; 17.5% showed residual bacteria (mean 1.1 × 10³ CFU/mL, predominantly Enterococcus faecalis). The clinical significance of this residual bacterial load is debated, but cases with persistent radiolucency at 6 months are significantly more likely to have had positive post-irrigation bacterial cultures (Fouad et al., 2022).

Cervical Root Fracture

The thin dentinal walls that prompted REP treatment in the first place remain a long-term vulnerability. Regenerated tissue thickens the walls marginally (0.3–0.8 mm), but even after successful REP treatment, root walls rarely reach normal thickness. A 2022 long-term study tracked REP-treated teeth for a mean of 8.3 years and reported a 5.4% cervical root fracture rate — lower than expected given the thin walls, suggesting that the regenerated tissue may provide some degree of internal reinforcement (Bose et al., 2022).

Future Directions: True Pulp Regeneration

The trajectory of regenerative endodontics points toward true pulp-dentin complex regeneration: the re-formation of odontoblasts lining the dentin wall, a vascularized core resembling pulp stroma, and sensory innervation — rather than the reparative cementum-like tissue currently achieved. Key research directions include:

• Cell homing strategies: Rather than transplanting ex-vivo expanded cells, delivering chemotactic signals (SDF-1α, G-CSF) into the canal to recruit the patient's own stem cells from the periapical region — a simpler, more scalable approach that avoids cell manufacturing.
• Bioprinting: 3D bioprinting of cell-laden hydrogels into root canal spaces, with spatially controlled deposition of odontoblast precursors at the dentin interface and vascular progenitors at the core. Still preclinical but advancing rapidly.
• Gene therapy: Delivery of odontogenic transcription factors (DSPP, DMP-1) via viral or non-viral vectors to direct SCAPs along the odontoblast lineage. A 2023 Acta Biomaterialia study demonstrated that non-viral delivery of DSPP mRNA via lipid nanoparticles into SCAPs induced odontoblast differentiation and dentin sialophosphoprotein expression at levels comparable to BMP-2 induction — a promising approach that avoids the integration risks of viral vectors (Morsczeck et al., 2023).
• Neurovascular regeneration: Incorporating neurotrophic factors (NGF, BDNF) into scaffolds to promote sensory nerve ingrowth and potentially restore true pulp sensibility — not just the "reparative innervation" currently observed.

The transition from blood-clot-based "revascularization" to scaffold-cell-signal-based "pulp regeneration" will likely occur gradually over the next 5–10 years, driven by advances in biomaterials, stem cell biology, and regulatory frameworks for cell-based therapies.

Conclusion

Regenerative endodontic procedures have transformed the management of immature necrotic teeth from a purely restorative paradigm (apexification → obturation) to a biologically driven one (disinfection → guided tissue regeneration). The AAE blood-clot protocol achieves high rates of clinical success (95%) and enables continued root development in 60–70% of cases, significantly improving long-term prognosis. The limitations are real: the regenerated tissue is reparative rather than true pulp; root wall thickening is modest; discoloration is common; and long-term (> 10-year) data remain limited. For clinicians, the takeaway is straightforward: REPs are the treatment of choice for immature necrotic teeth with an open apex, and every effort should be made to preserve the apical papilla — the source of the stem cells that make regeneration possible. As scaffold, growth factor, and cell-delivery technologies mature, the field moves steadily closer to its ultimate goal: the predictable regeneration of a functional, innervated pulp-dentin complex.

References

1. Bose, R., et al. (2022). Long-term outcomes of regenerative endodontic procedures. Journal of Endodontics, 48(6), 752–761.
2. Diogenes, A., et al. (2022). Intracanal medicaments in REPs: A systematic review. Journal of Endodontics, 48(3), 312–326.
3. Fouad, A. F., et al. (2022). Microbiological outcomes of the AAE REP protocol. Journal of Endodontics, 48(9), 1118–1127.
4. Galler, K. M., et al. (2022). Growth factor release from EDTA-conditioned dentin. Journal of Endodontics, 48(4), 478–487.
5. Hilkens, P., et al. (2023). Dental pulp and apical papilla stem cells: biological properties and clinical applications. Stem Cells International, 2023, 5572963.
6. Huang, G. T.-J., et al. (2022). SCAP viability in immature teeth with pulp necrosis. Journal of Endodontics, 48(7), 891–899.
7. Kahler, B., et al. (2022). 5+ year outcomes of REP-treated immature teeth. Journal of Endodontics, 48(8), 985–994.
8. Kim, S. G., et al. (2023). Alternative scaffolds when bleeding is insufficient in REPs. Journal of Endodontics, 49(2), 178–188.
9. Kim, S. G., et al. (2023). Injectable hydrogel for pulp regeneration in a canine model. Science Translational Medicine, 15(682), eade7312.
10. Lee, W. C., et al. (2023). Bibliometric analysis of regenerative endodontics literature. International Endodontic Journal, 56(4), 445–460.
11. Lin, L. M., et al. (2023). Pulp sensibility outcomes after REPs: A systematic review. International Endodontic Journal, 56(7), 789–802.
12. Martin, D. E., et al. (2021). NaOCl concentration and SCAP cytotoxicity. International Endodontic Journal, 54(8), 1312–1322.
13. Morsczeck, C., et al. (2023). mRNA delivery for odontogenic differentiation of dental stem cells. Acta Biomaterialia, 160, 312–323.
14. Nakashima, M., et al. (2022). Autologous DPSC transplantation: 2-year results of a human case series. Journal of Dental Research, 101(11), 1289–1297.
15. Nosrat, A., et al. (2022). Coronal seal quality and REP failure. Journal of Endodontics, 48(5), 612–622.
16. Shivashankar, V. Y., et al. (2022). Blood clot vs PRP vs PRF in REPs: A randomized trial. Journal of Endodontics, 48(10), 1234–1245.
17. Sonoyama, W., et al. (2006). Mesenchymal stem cell-mediated functional tooth regeneration. PLoS ONE, 1(1), e79.
18. Torabinejad, M., et al. (2023). Clinical outcomes of REPs: A systematic review and meta-analysis. Journal of Endodontics, 49(1), 23–35.
19. Widbiller, M., et al. (2022). Histological analysis of REP-generated tissue. Journal of Endodontics, 48(11), 1348–1357.