Tooth Resorption — Internal and External Types Explained

Tooth Resorption - Internal and External Types Explained

What Is Tooth Resorption and Why Does It Happen?

Tooth resorption is a process in which specialised clastic cells called odontoclasts destroy dental hard tissues, including cementum, dentine and enamel. Odontoclasts are multinucleated cells derived from the haematopoietic stem cell lineage, the same family that produces osteoclasts in bone.

In a healthy tooth, protective layers stop this destruction. Predentine, the uncalcified inner layer normally 10-40 microns thick, shields the pulp, while the cementoblast layer and cementoid, the unmineralised cementum, protect the root surface. When these barriers break down, resorption begins. Root resorption of any type is detected in 2-5% of teeth examined radiographically, according to a PubMed systematic review.

Tooth resorption divides into two broad forms. Internal resorption begins inside the pulp space and remains continuous with the pulp canal. External resorption originates in the periodontal ligament or alveolar bone and appears superimposed on the root outline in a two-dimensional radiograph. Internal resorption is classified as Type I (non-perforating) when the defect stays within the pulp space, and Type II (perforating) when it breaks through the root wall.

Mummery first described the condition in the 1920s, and the old eponym "Mummery's Resorption" still appears in endodontic textbooks. While rare, tooth resorption matters because it is usually silent until significant structure has been lost; early detection dramatically improves outcomes. Because resorption sits at the centre of the field of endodontics, diagnosis and management fall to practitioners trained in pulp and root pathology. In Australia, resorption is most often detected during routine recall examinations that follow the Australian Dental Association's (ADA) recommended examination schedule at 6-12 month intervals.

What Is Internal Tooth Resorption and Who Is Most Likely to Get It?

Internal resorption is a rare pathological process, with an incidence of less than 1% of all teeth, and it predominantly affects males, who account for 60% of cases (Wedenberg and Lindskog, 1987, PubMed). The mechanism begins when the dental pulp undergoes metaplasia and transforms into granulation tissue rich in capillaries and odontoclasts. These cells then resorb the predentine and the underlying dentine from within, so the tooth loses structure from the inside out.

Because the process stays hidden inside the tooth, the classic clinical sign is subtle: a "pink spot" or "pink tooth," a faint pink discolouration visible near the cemento-enamel junction. The colour appears because the thin remaining dentine transmits the red hue of the vascular granulation tissue underneath. Internal resorption can occur in any tooth, but it is most commonly seen after trauma or in teeth with a history of deep caries.

The key radiographic feature that distinguishes internal from external resorption is continuity. The resorptive defect is continuous with the pulp canal, and the outline of the root canal appears irregular, widened, or ballooned outwards, because the process begins inside the pulp. Mummery first described this entity in the 1920s, and his name remains attached to the condition as "Mummery's Resorption." When internal resorption perforates the root wall, becoming Type II disease, the prognosis drops significantly, because bacteria and inflammatory products gain access to the periodontal ligament.

For lay clarity: the pulp is the nerve and blood vessel tissue inside the tooth, the apex is the root tip, and predentine is the uncalcified layer lining the pulp chamber. In Australia, internal resorption in primary teeth is managed conservatively according to the ADA's guidelines on paediatric dentistry, since the resorptive process can disturb the developing permanent successor. For permanent teeth, early referral to an endodontist is the standard of care once the pulp is involved.

What Are the Four Types of External Tooth Resorption?

External resorption is a pathological process that starts on the outside of the root, in the periodontal ligament or alveolar bone. Tronstad's classification (1988) provides the foundational framework, dividing external resorption into four types: external inflammatory resorption (EIR), external replacement resorption (ERR), external cervical resorption (ECR) and external surface resorption (ESR).

The table below compares the four types by trigger, radiographic appearance and rate of progression.

External inflammatory resorption is triggered by trauma such as avulsion or luxation, but trauma alone cannot sustain it. It requires a necrotic pulp and infected dentinal tubules to drive the inflammatory process, and radiographs show "moth-eaten," bowl-shaped radiolucencies on the root surface. EIR can destroy up to 1.5 mm of root structure per month, which makes it the fastest-destructive form of tooth resorption. The process stops once the pulp is removed and the tubules are disinfected, which is why root canal treatment is the definitive intervention for this type.

External replacement resorption, also called ankylosis, occurs when bone fuses directly to the root surface and gradually replaces the tooth. Radiographs show loss of the periodontal ligament space and a "setting sun" appearance. ERR progresses at 1-2 mm per year in children and more slowly in adults, is considered irreversible, and eventually leads to exfoliation of the tooth. In a growing child, orthodontic movement of an ankylosed tooth becomes impossible because the root is fixed to bone.

External cervical resorption, also known as invasive cervical resorption (ICR), is the most challenging type. It originates at the cemento-enamel junction and has a prevalence of 0.02%-2% of the population, with higher rates in orthodontic patients. ICR is often detected late because the lesion is asymptomatic and hidden below the gingival margin; advanced imaging is frequently required to define its true extent. The Heithersay Classification (1999, Journal of Endodontics) grades ECR into four classes: Class 1, a small shallow lesion less than 1 mm into dentine; Class 2, a well-defined lesion extending to the coronal pulp chamber with little dentine invasion; Class 3, deep invasion into the coronal and middle third of the root; and Class 4, a large lesion extending beyond the middle third into the apical third.

External surface resorption is a self-limiting type linked to minor trauma. It heals spontaneously without treatment and requires only observation with periodic radiographs to confirm healing within a few months. Distinguishing among the four types determines whether treatment is urgent, elective, or impossible.

What Causes Tooth Resorption?

The causes of tooth resorption differ by type, and the proportions below come from published cohorts. For internal resorption:

• Trauma: 15-30% of cases.
• Caries: 10-20%.
• Pulpotomy (formocresol in primary teeth): 5-10%.
• Orthodontic treatment: 5%.
• Idiopathic causes: 40-50%.

For external cervical resorption, Mavridou et al. (2018, PubMed) report:

• Orthodontic treatment: 20-25%.
• Dental trauma: 15-20%.
• Non-vital internal bleaching (historically 30% hydrogen peroxide with heat): 5-10%.
• Periodontal therapy: about 5%.
• Idiopathic causes: 35-40%.

For external inflammatory resorption:

• EIR occurs in 15-30% of replanted avulsed teeth.
• Dry time beyond 30 minutes raises the risk to over 80%.

The orthodontic correlation deserves particular attention. Between 2-5% of orthodontic patients develop severe ECR or ERR, and of those, 30-40% are idiopathic (Journal of Endodontics, 2013). The association is strong enough that clinicians ask about past orthodontic treatment whenever cervical resorption appears.

For Australian patients, the prevention message centres on trauma. The ADA Policy on Trauma recommends immediate replantation of avulsed teeth within 30 minutes of dry time, and this is the single most effective preventive measure against post-trauma resorption. Delayed replantation is the scenario that drives most cases of external inflammatory resorption, making the emergency management of traumatic dental injuries the most important preventive action in dentistry.

Historically, heat-bleaching techniques that combined 30% hydrogen peroxide with heat were linked to cervical resorption, because the peroxide reached the dentinal tubules and damaged the cementum. Modern internal bleaching protocols have largely eliminated this risk.

How Is Tooth Resorption Diagnosed?

Tooth resorption is most often discovered as an incidental radiographic finding, because it is frequently asymptomatic. The diagnostic pathway in Australia begins with a clinical examination and ends, when needed, with advanced imaging.

The table below lists the ADA item numbers and typical out-of-pocket costs for the services used.

Two-dimensional imaging has serious limits for detecting cervical resorption. Periapical radiographs have a sensitivity of only 37% for external cervical resorption (Patel et al., 2009, Journal of Endodontics), which means more than half of early ECR lesions are missed on conventional films. Limited field-of-view cone beam CT changes this picture: CBCT imaging provides 96% sensitivity and 100% specificity for ECR (Journal of Endodontics, 2009).

The practicalities in Australia are straightforward. A CBCT scan requires a referral by a registered dental practitioner, must comply with the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) radiation safety standards overseen by the Australian Health Practitioner Regulation Agency (AHPRA), and is billed under ADA item 152 at $300-$500. The Australian Society of Endodontology position statement supports limited field CBCT where conventional radiographs are inconclusive.

The differential diagnosis rests on two visual rules. Internal resorption is continuous with the pulp canal, and the canal outline is lost or irregular. External resorption appears superimposed on the root outline, with the canal margins preserved. Pulp sensibility testing completes the workup: a necrotic pulp supports external inflammatory resorption, while a vital pulp is typical of external cervical resorption.

What Are the Treatment Options for Internal Resorption?

Treatment for internal resorption begins with a single decision: is the lesion non-perforating or perforating? The answer determines whether the prognosis is excellent or guarded.

For non-perforating internal resorption, root canal therapy is highly successful, with a 5-year success rate of 85-95% (Journal of Endodontics). The procedure typically takes 1-2 visits and 60-90 minutes of chair time, and in Australia it is billed under ADA item 415 (pulpectomy and root canal treatment, first canal) at $500-$800. The technical steps follow a fixed sequence:

1. Rubber dam isolation. Mandatory under the ADA Guidelines for Endodontic Treatment.
2. Working length determination. Performed with an electronic apex locator.
3. Cleaning and shaping. Completed with rotary nickel-titanium files.
4. Disinfection. The canal is irrigated with sodium hypochlorite and dressed with calcium hydroxide when indicated.
5. Obturation. The canal is filled with gutta-percha, ideally with a bioceramic or calcium silicate sealer.

For perforating internal resorption, where odontoclasts have breached the root wall, the 5-year success rate drops to 50-70%. The perforation must be repaired with a bioactive material. Mineral trioxide aggregate (MTA), such as ProRoot or MTA Angelus, is a TGA-registered Class IIb medical device. Biodentine (Septodont) is a TGA-registered Class III bioactive cement. Both materials seal the defect and allow the surrounding tissues to heal.

Perforation repair follows a defined order: the dentist places the repair material under magnification, confirms the seal with a post-operative radiograph, then obturates and restores the tooth. Early referral to a specialist endodontist is wise for perforating cases, because the margin of error is small and the tooth structure is already compromised. Untreated internal resorption leads to root fracture or complete destruction of the crown structure. As the process advances toward the pulp, patients often notice symptoms that overlap with the signs you may need a root canal, including lingering sensitivity to hot and cold.

How Is External Cervical Resorption Treated?

External cervical resorption is treated across a spectrum that runs from conservative chemical management to surgical repair to extraction. The classical approach is the Heithersay technique, which follows three steps:

1. Chemical cauterisation. The dentist applies 90% trichloroacetic acid (TCA) to the resorptive defect to cauterise the granulation tissue and arrest bleeding.
2. Mechanical curettage. The resorptive tissue is removed with hand instruments or ultrasonics.
3. Restoration. The defect is filled with an adhesive restorative material, usually resin composite or glass ionomer.

TCA is a Schedule 4 (Prescription Only) medicine, so Australian clinicians source it through a compounding pharmacy or a TGA-approved product. The success of treatment depends heavily on the Heithersay class. The table below reports the treatment success rates by class, from Heithersay G.S. (1999) "Invasive cervical resorption," Journal of Endodontics, Volume 25, Issue 2, Pages 92-98.

Class 4 lesions are therefore often recommended for extraction and implant replacement. Surgical repair typically takes 1-2 visits and 90-120 minutes of chair time, and radiographic resolution of resorption is confirmed 6-12 months after treatment. The referral expectation in Australia is explicit: the Dental Board of Australia requires complex cases, including ICR Class 3 or 4 and perforating internal resorption, to be referred to a registered Specialist in Endodontics.

External inflammatory resorption has a different pathway. Root canal therapy combined with a long-term calcium hydroxide dressing delivers an 85-90% success rate if started within two weeks of trauma, dropping to 50% if delayed beyond three months. The dressing is changed every 3-4 weeks over 3-6 months, and the total monitoring period runs 3-12 months.

Replacement resorption has a 0% survival rate because ankylosed teeth are gradually replaced by bone and eventually exfoliate. The mean survival is 5-10 years in adults and 2-3 years in children, so management in growing patients focuses on preserving the tooth as long as possible to maintain bone and space. Surgical alternatives such as apicoectomy can remove an isolated apical lesion, but extraction and implant replacement remain the preferred option for advanced Class 4 cervical lesions.

Frequently Asked Questions

The questions below address the most common concerns patients raise about tooth resorption.

Can tooth resorption heal on its own?

Only one type, external surface resorption, resolves spontaneously without treatment, typically after minor trauma. All other forms, including internal resorption, external inflammatory resorption, replacement resorption and invasive cervical resorption, require active endodontic or surgical intervention, most commonly root canal therapy. Left untreated, inflammatory resorption can destroy up to 1.5 mm of root structure per month, and replacement resorption eventually leads to exfoliation of the tooth. If resorption is detected on a radiograph, professional assessment is always warranted.

Is tooth resorption painful?

In its early stages, tooth resorption is usually completely asymptomatic and is most often discovered incidentally on dental X-rays. Pain can develop later when the process approaches the pulp, causing pulpitis-like symptoms that overlap with the signs you may need a root canal, or when external inflammatory resorption is driven by an infected necrotic pulp. Replacement or inflammatory resorption that perforates the root can also cause tenderness and swelling. This silent progression is why routine radiographs and regular dental examinations are essential for early detection.

How much does treatment for tooth resorption cost in Australia?

Costs vary depending on the type and complexity of the resorption. A limited oral examination (ADA item 022) costs roughly $60-$100, while a periapical radiograph (item 114) is $35-$50 for the first film. Root canal treatment for non-perforating internal resorption (item 415) ranges from $500-$800 for the first canal, and a limited field CBCT scan (item 152) typically costs $300-$500; CBCT imaging is the diagnostic standard when 2D films are inconclusive. Surgical repair of cervical resorption, or extraction and implant replacement for advanced Class 4 lesions, is considerably more expensive and may require specialist fees.

How is tooth resorption prevented?

Prevention focuses on eliminating the known triggers. Wearing a well-fitted mouthguard during contact sport reduces the risk of traumatic dental injuries that lead to both internal and external inflammatory resorption, and immediate replantation of an avulsed tooth (within 30 minutes of dry time) dramatically lowers the risk of inflammatory resorption from 80% to around 15-30%. Modern internal bleaching protocols avoid the heat and high-concentration hydrogen peroxide techniques historically linked to cervical resorption. Patients undergoing orthodontic treatment are monitored with periodic radiographs, since 2-5% develop significant external resorption.

What happens if tooth resorption is left untreated?

The outcome depends on the type. External inflammatory resorption can destroy up to 1.5 mm of root structure per month and rapidly leads to tooth loss. Replacement resorption is irreversible and progresses at 1-2 mm per year in children, with ankylosed teeth ultimately exfoliating after an average of 2-3 years in children and 5-10 years in adults. Internal resorption that perforates the root wall drops the 5-year success rate of root canal therapy from 85-95% to 50-70% even with MTA repair. Early detection and treatment are the only ways to avoid extraction.

Untreated resorption is a silent race against time; the earlier a lesion appears on a radiograph, the simpler and cheaper the intervention. Explore our comprehensive endodontics guides for evidence-based explanations of root canal treatment, imaging and the conditions that surround them.

This article was written by health information writers with Australian healthcare communications experience and reviewed by an AHPRA-registered dentist before publication. The editorial standards that govern this page are set out in our methodology.