Introduction

External root resorption is a loss of cementum and dentin from the dental roots that starts in the periodontal ligament. This resorption is produced by multiple factors that can affect the dental root, and it is not easy to detect since it is usually asymptomatic [1]. It can be considered a physiological process in the primary dentition, corresponding to a normal evolution of exfoliation of a primary tooth, and a pathological process in the permanent dentition, in which the tooth structure is regressively altered [1,2].

Diagnosis is mainly based on radiographic methods, but a complete clinical history and adequate examination are essential to confirm the diagnosis. Such diagnosis is often challenging because it requires radiographic techniques to see the tooth in question and the adjacent bony structures [2,3]. In addition, the two-dimensional nature of conventional radiographs shows an inaccurate image when locating the site, extent and/or type of resorption [2]. Takeshita [3] indicates that, among all root anomalies, external root resorption is the most difficult to detect with two-dimensional imaging methods. D'addazio [4] showed that in periapical radiographs, it was possible to recognize external root resorption in all cases; however, the exact location and type of resorption could not be identified.

External root resorptions usually occur in response to stimuli such as orthodontic movements, which activate specific molecular pathways that produce a non-physiological stimulation of odontoclasts. Activated odontoclasts attach to the mineral matrix, where they fuse with the cell membrane. Consequently, they release hydrogen ions and proteolytic enzymes, generating an acidic environment, and as a result, dissolution of calcified hard tissue occurs [5]. If there is injury or irritation to the tooth tissues, clastic cells will be attracted to the affected area of the root surface, and resorption occurs as part of the normal scavenging function of the cells [6]. Several factors could cause root resorption, including dental trauma, bacteria, orthodontic treatment, periodontal disease, tooth whitening, viruses, or idiopathic. Some predisposing systemic factors have also been associated, such as endocrine alterations and vitamin and mineral deficiency [1].

External root resorption can occur in different ways, which has led to various proposals for its classification in the literature. Andreasen’s [7] is fundamental and the most widely spread. Later, Benitez et al. [8] classified root resorptions according to their location and etiology. However, the most recent, and the one currently used, was described by Patel and Saberi [9]. In it, the authors consider the location and pathogenesis of resorption, classifying them as follows:

1. Inflammatory external resorption.

2. External replacement resorption.

3. Cervical external resorption.

4. Superficial external resorption.

5. External resorption of transient apical collapse.

A quick and accurate diagnosis would allow adequate initial management, minimizing the sequelae of this pathology [10]. Meanwhile, misdiagnosis will worsen the prognosis of the tooth [2]. On the other hand, radiographs do not indicate whether root resorption is active or inactive. Considering the limitations of radiography, more sensitive, safe, and specific diagnostic and prognostic methods are required to detect this pathology in the early stages [11].

Because of the difficulty in diagnosing external root resorptions, this review aims to identify studies that describe the clinical, radiographic, and other diagnostic methods that should be considered for the correct diagnosis of this pathology. With this, we seek to establish guidelines that facilitate its recognition, providing tools for timely treatment and a better prognosis.

Methods

A narrative review was performed [12,13]. Studies in Spanish and English, literature reviews, randomized clinical trials, case-control, and cohort studies conducted in adult permanent teeth from 2010 to 2022 were included. Case report studies, expert opinion, editorials, studies in young primary or permanent teeth, and studies in patients with systemic disease were excluded. A search was carried out in the MEDLINE/PubMed, Cochrane Library, and Google Scholar databases under the term external root resorption, carried out between June and July 2022, in which 257 articles were found. Two individual authors read the titles and abstracts of the publications, discarding the articles irrelevant to the study, leaving 65. The complete texts were read of the selected articles, and 17 were included by consensus. The categorization was defined into clinical characteristics, radiographic characteristics, and treatment of external root resorptions. Then, the clinical and radiographic characteristics and the treatment of each type of resorption were tabulated in Excel tables.

Results

Seventeen articles were finally included in the review. Of the total number of articles, five refer to the clinical characteristics described in Table 1. Eight deal with the radiographic characteristics described in Table 2, and four deal with the main treatments of external root resorptions, presented in Table 3.

The results obtained are summarized in Table 4. For external inflammatory resorption, according to the evidence collected, the following clinical characteristics are described: discoloration of the crown; sensitivity to percussion or palpation; symptoms of pulpitis or apical periodontitis; mobility; periodontal defects and negative pulp vitality [9,29,30]. Regarding its radiographic characteristics, it is described as a radiolucent area in the form of an undercut in the periapical area of the tooth, an intact root canal at the beginning, and in advanced stages, areas of resorption can be observed inside the canal [9,20]. Regarding managing this resorption, endodontics is the indicated treatment to limit the process, and intracanal medication with calcium hydroxide is recommended [20,27,30].

According to the evidence collected, external replacement resorption presents the following clinical characteristics: metallic percussion sound, absence of mobility, crown without alterations, and preserving pulp vitality. Regarding its radiographic characteristics, periodontal ligament space and bone apposition loss are observed [23,26]. In these cases, periodic control of the lesion is chosen. However, when intervention is necessary, exodontia may be chosen [28].

For cervical external resorption, the clinical characteristics we can find are pink crown coloration, normal pulp vitality or sensitivity to vitality tests, and probing with a "sponge" texture [31]. Radiographically, this resorption can be seen as a radiolucent area at the cervical root level [30]. Various treatment options, from control to exodontia, are described [11].

According to the evidence collected, superficial external resorption describes similar clinical characteristics: a healthy tooth with pulp vitality [9,11]. Radiographically, a loss of root tissue is observed, and the root canal remains intact [9]. When the stimulation ceases, the resorption cavities will heal without incident [10].

Finally, clinical features such as transient tooth discoloration and delayed or absent response to sensitivity testing are described for transient apical collapse resorption. Its radiographic manifestations are initially seen as widening the periodontal ligament and a blurred appearance of the apical lamina dura [29]. Its management will depend on the disappearance of signs and symptoms, which should resolve within one year [32].

Discussion

The diagnosis of root resorption has been somewhat controversial, as a broad spectrum of features makes it difficult to reach. Early detection and subsequent intervention remain a challenge for dentists. This study aims to promptly identify clinical and radiological characteristics to evaluate the presence of these lesions, with clear guidelines that allow us to identify external root resorption.

Inflammatory external radicular resorption

Inflammatory external root resorption is a pathology associated with prolonged stimuli, resulting in a localized inflammatory response.

Its treatment includes the use of intracanal medication. According to the evidence collected, calcium hydroxide is recommended because it is the most widely used and has numerous properties [20,27]. Other temporary medications have been mentioned in the literature, all comparable to calcium hydroxide, such as antibiotic pastes, acetazalamide, and Ledermix. The latter is a commercial water-soluble paste containing 1% triamcinolone, a corticosteroid, and 3% demeclocycline, a tetracycline [27].

Recent studies indicate that, in addition, an apical sealant can be used to supplement therapy. Adding mineral trioxide has resulted in easier management of root resorption [30]. Biodentine has also been successful in endodontic treatment. Patel [28] and Esteves [32] emphasize that using mineral trioxide would be the most effective treatment, presenting better periapical healing of the tooth to be treated. At the same time, Marâo [33] indicates that mineral trioxide can be considered a viable option for root canal obturation in cases of external root resorption.

External replacement root resorption

Replacement root resorption is a lesion that occurs due to severe luxation or avulsion injuries [1]. In these cases, periodic control of the lesion is chosen. However, when intervention is necessary, tooth exodontia and repositioning can be performed with the application of Emdogain in the affected root area. If it progresses to resorption with ankylosis, a decoronation and submersion technique is recommended, which consists of removing the crown and leaving the root in the socket for subsequent replacement with bone [15].

Cervical external root resorption

Cervical external root resorption is a localized lesion of the cervical area of the root below the epithelial junction [10]. The results obtained indicate a variability of radiographic characteristics. Other authors point out that the lesion may appear radiolucent if identified in its active resorption phase, radiopaque in the repair phase, or a combination of both phases [22]. It is recommended that these lesions be diagnosed and evaluated with the aid of cone beam computed tomography before establishing a treatment plan, which varies according to the depth of the lesion, the size of the lesion, and whether pulp damage is present [34]. Endodontic treatment should be performed when the lesion is very close to the pulp, followed by intra-canal medication and filling with a thermoplastic technique [30]. Patel mentions that treatment aims to maintain the tooth in a functional state and improve esthetics when necessary. Thus, he proposes the following treatment options:

• External repair without/with endodontic treatment.

• Internal repair and endodontic treatment.

• Intentional reimplantation.

• Periodic control in case of intractable teeth.

• Exodontia [29].

Superficial external root resorption

Superficial external root resorption is a defect that occurs due to minimal stimulus on the root surface and is accompanied by a transient lesion that is repaired, and the pulp remains vital [34]. According to the evidence collected, no pathologic features are described. Singh [30] reports no signs of resorbed tooth are observed on clinical examination. Other authors indicate a loss of root tissue of approximately 2.5 millimeters [18]. When the stimulation ceases, the resorption cavities will heal without incident [10].

Transient apical collapse resorption

Transient apical collapse is a phenomenon related to a repair process in the pulp associated with trauma that returns to normal after the repair is complete [32]. It is currently poorly described in the literature, but this phenomenon is essentially an inflammatory external resorption with a short phase of resorption followed by repair [29]. Its management will depend on the disappearance of signs and symptoms, which should resolve within one year [32].

Biomarker diagnosis

All these diagnostic guidelines mentioned above are static and cannot indicate whether the root resorption process has stopped or is in progress. One of the new diagnostic methods mentioned in the literature is the detection of specific biomarkers associated with the disease [14]. Vieira [14] and Mona [16] conducted studies to find possible gingival or salivary crevicular fluid-associated root resorption biomarkers. Mah and Prasad [35] concluded that further studies are required to develop more sensitive assays and correlate measurements with clinical findings to produce a practical test for clinical use. Huang [11] detected alterations in dentin protein levels in human crevicular fluid. Mohd [36] detected six proteins (S100A9, IGJ, HSPA1A, IGHV4-34, TYMP, and VTN) that showed significant increases during the early phase of orthodontic treatments, making them candidates as potential biomarkers that can be exploited for the detection of root resorption. Recent studies indicate which biomarkers found in saliva show a significant increase in interleukin-7, interleukin-10, interleukin-12p70, interferon γ, proteins and metabolites, and a significant decrease in interleukin-4 when inflammatory external root resorption is present [17,36]. Despite finding a relationship between biomarkers in gingival crevicular fluid and the development of root resorption, more evidence about the clinical use of these biomarkers is needed [17].

Cone beam computed tomography versus conventional radiography

Regarding radiographic diagnosis, several authors say cone beam computed tomography is the best tool for diagnosing external root resorption. Takeshita [3] concludes that cone beam computed tomography was the most accurate imaging method in diagnosing external root resorption. Yi [37] suggests this tool has greater diagnostic efficacy than periapical radiography [19,21,25]. The most recent study concludes that using cone beam computed tomography has a sensitivity of 60.8% and a specificity of 60% [24].

In the case of cervical resorptions, Patel et al. [38] proposed a three-dimensional classification model to accurately document the true nature of these, which covers three main aspects of the lesion:

1. Corono-apical extension.

2. Circumferential extension.

3. Proximity to the root canal.

As the first classification system that accurately delineates the spatial structure of the lesion, it significantly improves diagnostic accuracy [39].

Conclusions

Correctly diagnosing external root resorption in the early stages significantly improves the prognosis. A complete clinical history is essential before proceeding to intraoral examination. A complete clinical examination should emphasize probing, staining, mobility, and vitality testing.

However, cone beam computed tomography and periapical radiography are the main tools to evaluate the presence of external root resorptions, the former being the most accurate method. The clinical features, radiographic characteristics, and treatment of each type of external root resorption are different. Also, their most common differential diagnoses are internal root resorption and cervical carious lesions.