TL;DR

Key Facts

What Is It?

Root resorption is the pathological process by which the mineralised tissues of the tooth root — cementum, dentine, and in severe cases enamel — are progressively destroyed by odontoclasts, the clastic cells derived from the monocyte/macrophage lineage. Under normal circumstances, the root is protected from resorption by the intact predentine layer internally and by the cementoblast layer and intact periodontal ligament (PDL) externally. When either protective barrier is disrupted — by inflammation, infection, trauma, or pressure — odontoclasts gain access to mineralised surfaces and resorption begins.

Root resorption is classified by its anatomical origin into internal (originating from the pulp cavity) and external (originating from the external root surface), and further subclassified by aetiology and mechanism within each category. The distinction matters profoundly because the treatment, prognosis, and urgency differ significantly between types.

Why It Matters (Clinical + Exam Context)

Root resorption appears frequently on INBDE/NBDE boards because it requires integration of radiographic interpretation, knowledge of trauma sequelae, and understanding of treatment logic. Board questions typically present a radiograph description and ask candidates to identify the type of resorption and the appropriate management.

Clinical Relevance

• External inflammatory resorption is an endodontic emergency: Following tooth avulsion and replantation, external inflammatory resorption can destroy the entire root within months. Root canal treatment with calcium hydroxide must be initiated within 1–2 weeks of replantation to halt the process.
• Internal resorption stops with RCT: Removing the pulp tissue eliminates the cellular source of odontoclastic activity. RCT is curative if performed before perforation of the root wall.
• Replacement resorption has no endodontic treatment: Once the PDL is destroyed and bone directly contacts the root, no endodontic procedure can reverse the process. Management is monitoring, decoronation in growing patients, or extraction.

Internal Root Resorption

Internal root resorption (IRR) results from activation of odontoclasts within the pulp cavity, destroying dentine from the inside out. It occurs in the presence of a specific condition: partial pulp necrosis — the coronal portion of the pulp is necrotic (provides the stimulus but no vital cells to halt resorption), while the apical portion remains vital (providing the blood supply and clastic cells that perform the resorption). This is why internal resorption cannot occur in a fully vital pulp (the intact predentine barrier protects dentine) or a fully necrotic pulp (no viable cells remain to carry out resorption).

Aetiology

• Dental trauma (luxation injuries, especially concussion and subluxation)
• Carious pulp exposure
• Prolonged pulp capping / incomplete pulpotomy
• Orthodontic forces (rare)
• Idiopathic (no identifiable cause)

Clinical and Radiographic Features

• Symptoms: Usually asymptomatic; discovered as an incidental radiographic finding. Occasionally presents with mild discomfort.
• Pulp tests: Variable — may be positive (if partial vital pulp remains apically) or negative (if pulp is fully necrotic).
• Radiograph: Oval, round, or fusiform radiolucency within the root canal that is symmetrical and centrally located. The root canal walls appear to balloon outward. The internal and external surfaces of the root are distinct. Crucially: the radiolucency is continuous with the pulp canal space and does NOT change position relative to the canal when the X-ray tube angle is changed.

Pink Spot (Mummery’s Tooth)

When internal resorption occurs in the coronal pulp chamber beneath translucent enamel, the hyperplastic, vascular granulation tissue within the pulp becomes visible through the enamel as a pink or reddish discolouration of the crown. This is called the pink spot or Mummery’s tooth. It is pathognomonic for internal resorption and requires immediate radiographic investigation. It can be confused with dental caries or developmental enamel defects — radiographs and transillumination differentiate these.

Treatment of Internal Resorption

• Root canal treatment: Definitive treatment. Removing the pulp eliminates the clastic cell source and halts resorption. Standard RCT is adequate if the root wall is intact (no perforation). Calcium hydroxide interim dressing promotes healing of dentinal walls.
• Mineral trioxide aggregate (MTA) or Biodentine: Required if internal resorption has perforated the root wall. These bioactive cements seal the perforation and promote cementum deposition from the external PDL surface.
• Extraction: If perforation is extensive and the remaining root structure cannot provide a satisfactory seal, extraction is indicated.

External Root Resorption

External root resorption originates from outside the root — from the PDL, cementum surface, or alveolar bone — and progresses inward. Multiple subtypes exist based on aetiology and mechanism, each with different clinical implications.

Surface Resorption

Surface resorption (external surface resorption) is a self-limiting, transient process occurring as a response to minor trauma to the root surface. Small cemental defects are created by odontoclastic activity but are rapidly repaired by cementoblasts when the stimulus is resolved. It is not detectable clinically or radiographically in most cases and requires no treatment. It is considered a physiological repair process and is only of academic significance in endodontics.

External Inflammatory Resorption (EIR)

External inflammatory resorption is one of the most clinically significant and destructive forms of root resorption. It requires two simultaneous conditions:

1. Infected dentinal tubules: Bacteria (or their byproducts) in the dentinal tubules act as the stimulus for clastic activity from within the dentine.
2. Intact, inflamed PDL: The PDL must be present but inflamed — it provides the vascular supply and clastic precursor cells that carry out the resorption on the root surface.

EIR is most commonly seen following avulsion and replantation. The dry storage period (extra-oral dry time) desiccates and kills PDL cells and cementoblasts. Bacteria from the oral flora infect the exposed dentinal tubules. When the tooth is replanted, the inflamed PDL (from adjacent bone) recognises the infected dentine as foreign and mounts an osteoclastic response. The combination of infected dentinal stimulus and PDL-derived clastic activity produces a devastating, rapidly progressive resorption — the tooth can be completely lost within months.

• Radiograph: Bowl-shaped or moth-eaten radiolucencies on the external root surface, often with concurrent periapical bone loss.
• Treatment: Immediate root canal treatment (removes the infected dentinal tubule contents = eliminates the stimulus) + long-term calcium hydroxide dressing (raises intratubular pH, kills residual bacteria, inhibits clastic enzyme activity). CaOH must be renewed every 3–6 months for 12–24 months before obturation. Failure to treat rapidly results in total root destruction within weeks to months.

Replacement Resorption (Ankylosis)

Replacement resorption occurs when the PDL is destroyed and direct bone-to-root contact occurs. Without the PDL acting as a biological barrier, osteoclasts remodel the root as if it were bone — progressively replacing it with bone tissue in a process called ankylosis. The root is eventually completely resorbed and replaced by alveolar bone.

• Most common cause: Avulsion with extended dry extra-oral time (>60 minutes) or storage in inappropriate medium; severe luxation injuries destroying the PDL; intrusion injuries.
• Clinical features: Infraocclusion (the tooth appears to “sink” as the jaw grows around it, since the ankylosed tooth cannot erupt with the alveolar ridge); metallic/high percussion sound (“ankylotic sound”) on tapping — compared to the dull sound of a normal PDL-supported tooth.
• Radiograph: Loss of PDL space (continuous bone across the root surface); normal radiographic appearance may persist initially until significant bone replacement has occurred; radiopaque bone texture within the root outline in advanced cases.
• Treatment: No RCT or other endodontic procedure can reverse replacement resorption — the PDL is absent. In growing patients: decoronation (removal of crown, leaving root to resorb naturally without preventing alveolar bone development) preserves the alveolar ridge for later implant placement. In mature patients: monitor; extraction when resorption is extensive. Orthodontic movement of ankylosed teeth is not possible.

Invasive Cervical Resorption (External Cervical Resorption — ECR)

Invasive cervical resorption (also called external cervical resorption — ECR, or Heithersay resorption) begins at or just below the cervical margin of the tooth, apical to the epithelial attachment. It is initiated at the cervical region and invades the dentine and root canal, often leaving the pulp encased by a thin shell of tissue for a prolonged period before the pulp is directly involved.

• Associated risk factors: Internal tooth bleaching (especially thermocatalytic bleaching; less commonly walking bleach technique), dental trauma, orthodontic treatment, periodontal surgery, and surgical procedures in the cervical region. Bleaching-related ECR is thought to arise from hydrogen peroxide diffusing through the cement-enamel junction to stimulate osteoclastic activity.
• Radiograph: Ill-defined, mottled radiolucency in the cervical region of the root, often appearing to eat into the dentine. The root canal may appear intact for a long time (the pulp is protected by a fibrous “pseudomembrane”). The lesion moves with the X-ray tube (SLOB rule) confirming external rather than internal origin.
• Heithersay Classification (Class 1–4): Class 1 = small, limited to cervical area; Class 2 = extends to coronal pulp chamber but not root; Class 3 = extends to coronal third of root; Class 4 = extends beyond coronal third. Classes 1–3 are potentially treatable; Class 4 usually requires extraction.
• Treatment: Surgical curettage of the resorptive tissue + chemical cauterisation with 90% trichloroacetic acid (TCA) applied to the resorptive tissue to devitalise clastic cells + restoration of the defect. RCT indicated if the pulp is involved. MTA or Biodentine for internal aspects of the defect.

External Apical Root Resorption (EARR)

External apical root resorption (EARR) is resorption at the root apex, most commonly associated with orthodontic tooth movement. It affects up to 90% of orthodontically treated teeth to some degree, though clinically significant resorption (>4 mm root shortening) is less common. Risk factors include intrusion (highest risk), torque movements, dilacerated/blunt/pipette-shaped root apices, genetic predisposition, and excessive orthodontic force.

• Radiograph: Blunting or rounding of the root apex; shortened root length compared to pre-treatment records. Monitor with radiographs every 6 months during active orthodontic treatment.
• Treatment: If significant EARR detected: rest period of 2–3 months (allows partial repair), consider reducing forces, or complete treatment without further torquing/intrusion. Severe EARR may require modification of the treatment plan. The pulp is usually vital and no endodontic treatment is indicated unless secondary pathology develops.

Radiographic Differentiation: Internal vs External

The most clinically critical skill is distinguishing internal from external resorption on a periapical radiograph. Three principles guide this assessment:

Practical Radiographic Rules

• Canal continuity: If the radiolucency can be traced as a direct, continuous widening of the root canal = internal resorption. If the root canal outline appears intact and separate from the lesion = external resorption.
• Symmetry: Internal resorption is typically symmetric (oval/round/fusiform) because it expands equally from the canal centre. External resorption is often asymmetric and moth-eaten.
• Location: Internal resorption is centrally located within the root. External resorption may appear to be superimposed on or adjacent to the root from different angles.
• CBCT: Cone-beam CT eliminates superimposition and is the gold standard for differentiating internal from external resorption and for assessing three-dimensional extent before treatment planning.

Comparison: Resorption Types at a Glance

Clinical Considerations

• Avulsion management timeline is critical: Extra-oral dry time is the single most important predictor of resorption type after avulsion. <60 minutes dry time: viable PDL cells, replantation prognosis good, risk of inflammatory resorption. >60 minutes dry time: PDL cells non-viable, replacement resorption/ankylosis likely. Storage medium matters: Hank’s balanced salt solution (HBSS) or milk best; saline acceptable; dry = worst. Initiate RCT with CaOH dressing 7–14 days post-replantation for teeth with extended dry time.
• Internal resorption perforation prognosis: If internal resorption perforates the root wall, the prognosis changes significantly. Perforations in the crestal one-third of the root carry a poorer prognosis than mid-root or apical perforations. MTA sealed from within the canal, or surgical curettage and MTA from outside, may salvage the tooth, but success rates are significantly lower than non-perforated cases.
• Pink spot must not be confused with caries: A pink/reddish spot on the crown of a tooth — especially under an intact restoration — should trigger suspicion for internal resorption. Transillumination shows uniform pink discolouration rather than the patchy opacity of caries. Periapical radiograph is diagnostic.
• Infraocclusion as a clinical sign of ankylosis: In growing patients, an ankylosed tooth will appear progressively shorter than adjacent erupting teeth as the alveolar ridge develops. The tooth appears to “sink” into the gingiva. This is infraocclusion, not intrusion — the tooth has not moved; the surrounding teeth and bone have grown vertically while the ankylosed tooth remained stationary. Early decoronation is preferred to preserve alveolar bone height for future implant placement.
• CBCT is definitive for resorption characterisation: When 2D periapical radiographs are ambiguous — especially for cervical resorption or complex internal/external differentiation — CBCT provides three-dimensional localisation of the defect, assessment of root wall integrity, and measurement of extent. It is the imaging of choice before surgical treatment of cervical resorption and before any decision about restorability.

Common Mistakes & Misconceptions

• Misconception: “Internal resorption can occur in a fully vital pulp.”
  Correction: An intact, fully vital pulp is protected from internal resorption by the predentine layer — a non-mineralised zone that prevents odontoclast attachment to dentine. Internal resorption requires partial necrosis: necrotic tissue as stimulus + vital tissue as cellular source. Fully vital OR fully necrotic = no internal resorption.
• Misconception: “RCT treats replacement resorption (ankylosis).”
  Correction: Replacement resorption/ankylosis has no endodontic treatment. The PDL is absent, so removing the pulp cannot stop bone from remodelling the root. RCT may be performed if the pulp is necrotic for other reasons, but it does not reverse or halt the ankylosis process. Decoronation (in growing patients) is the only strategy that preserves the alveolar ridge.
• Misconception: “External inflammatory resorption and replacement resorption are treated the same way.”
  Correction: These are fundamentally different processes requiring opposite management. EIR: RCT + long-term CaOH (remove bacterial stimulus, halt clastic activity — the PDL is still present and can participate in repair). Replacement resorption: no endodontic intervention stops it (PDL is absent). Confusing these leads to either overtreatment of ankylosis or dangerous under-treatment of EIR.
• Misconception: “A radiolucency within the root outline is always internal resorption.”
  Correction: External resorption can be superimposed over the root outline on a 2D periapical radiograph, mimicking internal resorption. Angled radiographs (using the SLOB rule) and CBCT are necessary to confirm the location. External resorption moves with X-ray tube angulation; internal resorption stays fixed relative to the canal.
• Misconception: “EARR only occurs with orthodontic treatment.”
  Correction: External apical root resorption can occur due to impacted teeth exerting pressure on adjacent roots, chronic periapical inflammation, systemic conditions (hyperparathyroidism, Paget’s disease), idiopathic causes, and bleaching. It is most commonly detected in an orthodontic context but is not exclusively orthodontic in aetiology.

Related Topics

Root resorption spans endodontics, periodontology, oral surgery, and orthodontics.

References & Sources

1. Andreasen JO, Andreasen FM, Andersson L (eds), 2007. Textbook and Color Atlas of Traumatic Injuries to the Teeth, 4th ed. Blackwell Munksgaard.
2. Heithersay GS, 1999. Invasive cervical resorption: an analysis of potential predisposing factors. Quintessence International, 30(2), 83–95.
3. Tronstad L, 1988. Root resorption — etiology, terminology and clinical manifestations. Endodontics and Dental Traumatology, 4(6), 241–252.
4. Berman LH, Hargreaves KM, 2011. Cohen’s Pathways of the Pulp, 10th ed. Mosby/Elsevier.
5. Andreasen JO et al., 1995. Replantation of 400 avulsed permanent incisors. Part 4: Analysis of root resorption in healing. Endodontics and Dental Traumatology, 11(2), 76–89.
6. Cohenca N, Simon JH, Roges R et al., 2007. Clinical indications for digital imaging in dento-alveolar trauma. Part 1: traumatic injuries. Dental Traumatology, 23(2), 95–104.
7. Malmgren B, 2013. Ridge preservation/decoronation. Pediatric Dentistry, 35(2), 164–169.
8. Torabinejad M, Walton RE, Fouad AF, 2015. Endodontics: Principles and Practice, 5th ed. Elsevier Saunders.

Summary

Root resorption encompasses a spectrum of pathological processes unified by odontoclastic destruction of root structure but diverging fundamentally in aetiology, location, radiographic appearance, and management. Internal resorption requires partial pulp necrosis and is treated definitively by root canal treatment. External resorption encompasses surface, inflammatory, replacement, cervical, and apical subtypes — each with distinct causes and treatment strategies. External inflammatory resorption post-avulsion is the most clinically urgent and destructive, demanding immediate RCT and long-term calcium hydroxide dressing. Replacement resorption represents irreversible ankylosis where no endodontic treatment is curative. Radiographic differentiation using the SLOB rule and CBCT imaging is the cornerstone of accurate diagnosis.

About the Author

Dr. Andries Smith

Founder, Dental Panda

Dr. Andries Smith founded Dental Panda in 2020. As an immigrant to the United States, he had to take the INBDE exam, even though he was practicing dentistry for over 10 years. This revealed an opportunity. Andries noticed that INBDE prep course companies were putting profit over students. With his expertise and experience in dentistry, he created free dental wiki resources for students and the general public to have access to.