|Year : 2013 | Volume
| Issue : 3 | Page : 75-79
External root resorption: Different etiologies explained from the composition of the human root-close periodontal membrane
Department of Odontology, Orthodontic Section, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen N, Denmark
|Date of Web Publication||8-Aug-2013|
Department of Odontology, Orthodonti c Secti on, Faculty of Health and Medical Sciences, University of Copenhagen, 20 Nørre Allé, DK-2200 Copenhagen N
Introduction: This paper summarizes different conditions, which have a well-known influence on the resorption of tooth roots. It also highlights factors important for individual susceptibility to root resorption. Furthermore, the paper focuses on idiopathic root resorption where the provoking factor is not known. The Hypothesis: The several different disturbances causing root resorption can be either orthodontically provoked or acquired by trauma, virus or congenital diseases. It is presumed that all these conditions lead to inflammatory processes in the three main tissue layers, comprising the peri-root sheet. Evaluation of the Hypothesis: This paper explains how different etiologies behind root resorption and how different phenotypic traits in root resorption can be understood from immunohistochemical studies of the human periodontal membrane close to the root and thus, gain a new understanding of the phenomenon of root resorption.
Keywords: Diseases, innervation, periodontium, resorption, susceptibility for resorption
|How to cite this article:|
Kjaer I. External root resorption: Different etiologies explained from the composition of the human root-close periodontal membrane. Dent Hypotheses 2013;4:75-9
| Introduction|| |
Under normal conditions, permanent teeth do not undergo root resorption. In the present review, focus will be given to the etiology behind root resorption of permanent teeth.
Root resorption can be observed in the tooth root in cases with known and in cases with unknown etiology.
Research on root resorption has focused on two aspects. First, the cellular and biological processes that occur on the root surface during the resorption. This aspect is mainly studied experimentally on animals. The second aspect concerns causes behind root resorption and individual susceptibility for root resorption. This aspect is studied on human material.
This review on root resorption focuses mainly on the second aspect.
| Root Resorption with Known Etiology|| |
The known etiologies are presented in an overview article by Gunraj.  One of these is external resorption after trauma. In such cases, resorption is caused by inflammation in the pulp and/or in the periodontal tissue. Permanent tooth roots can also be affected by resorption caused by pressure in the periodontal ligament, seen for example, in connection with tooth eruption, orthodontic tooth movement or tumors. It is presumed that these pressure changes create an inflammatory condition in the periodontal membrane that induces the root resorption process.
General diseases can also cause root resorption, but information in the literature is extremely sparse. One general disease with root resorption as a known complication is osteitis deformans (Paget's disease of bone).  In this condition, root resorption begins at the collum and plumb roots are observed. Another general disease in which root resorption is a complication is tuberous sclerosis, a dominant hereditary disease that affects chromosome 9q34 or 16p13. In this general disease, cervical resorption occurs as a first symptom [Figure 1], seemingly not previously reported.
|Figure 1: Photographs and radiographs of a patient with tuberous sclerosis. Upper left: Skin affection of the forehead. Upper right: The dentition after several teeth has been extracted. Lower left: Right mandibular and premolar region, demonstrating collum resorption in the first molar and second premolar. Lower right: Three mandibular incisors, demonstrating internal and external root resorption|
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Ectodermal dysplasia is another general condition where root resorption is observed. 
The spectrum of resorption types with known etiology is diverse and characterized by local and general conditions with different physiological and pathological backgrounds. As a conclusion, research in the etiology behind root resorption has focused also on inflammatory reactions and resorption conditions caused by general diseases affecting bone tissue and/or ectodermal tissue.
A recent study of two cases with aggressive resorption has demonstrated that also peripheral nerves play a role in root resorption.  The resorptions were first categorized as idiopathic. After follow-up and treatment over a course of 6 years, the first case revealed a whole new explanation of innervation-induced resorption limited to jaw fields where innervation changes occurred after whooping cough.  The second case was a patient diagnosed with aggressive/idiopathic resorption in a frontonasal incisor field. In that case, resorption was caused by meningitis virus. The virus attack had disturbed the peripheral nerves to the incisors that were affected by resorption.  With these two examples of collum resorptions the idiopathic classification of these conditions was changed. In both cases, the resorption types were innervation-induced and occurred in limited fields defined by the innervation of different branches of the peripheral nerves.
In summary, known causes other than inflammatory reactions are general affection of the dentition (ectoderm and ectomesenchyme) and local affection (ectomesenchyme and innervation) [Figure 2].
|Figure 2: Panoramic radiographs with marking of tissue components, not visible radiographically. Panoramic radiograph of a girl 10 years old. Red line drawings indicate areas where ectodermal mucosa is important for tooth development. The red lines mark the site of the inner enamel epithelium between enamel and dentin and the epithelial lining of Malassez along the root contour. Furthermore, the lines indicate the ectoderm of the crown follicle in non-erupted teeth, b: Panoramic radiograph of an adult female. The figure demonstrates the fields in the jaws with different innervation and different ectomesenchyme. In the maxilla the red area marks innervations and ectomesenchyme in the fronto-nasal field, the green area marks the canine/premolar fi eld (maxillary neural crest cells) and the blue area marks the molar fi eld. In the mandible, the red area marks innervation and ectomesenchyme to the incisors, the green fi eld to the canine/premolars and the blue field to the molars|
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| Root Resorption with Unknown Etiology|| |
In cases of resorption with unknown etiology, the problem is just as complex. These resorption types are called idiopathic resorptions. They normally occur at different sites in the dentition and often with the same phenotypic appearance : s0 hort premolar roots, short and pointed incisor roots and short distal roots of the first molar in the mandible.
[Figure 3] and [Figure 4] show two examples of idiopathic resorption in permanent dentitions that have never been treated orthodontically. The phenotypic appearance of the roots is in many ways similar to the root changes that are seen after unsuccessful orthodontic treatments. [Figure 5] shows a dental film from a dentition affected by progressive idiopathic resorption. Idiopathic resorption types occur presumably both in single teeth/regions and generally in all teeth in a dentition.
|Figure 3: Panoramic radiograph of a female (aged 24) with open bite who has never received orthodontic treatment. Note general deviations in the dentition, such as short premolar roots, short maxillary incisor roots and taurodontic second maxillary molars|
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|Figure 4: Panoramic radiograph of a male (aged 20) who has never received orthodontic treatment. Note short distal roots of the mandibular first molars and secondary arrested eruption of the left maxillary first molar and taurodontic second maxillary molars|
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|Figure 5: Idiopathic aggressive root resorption in a male (aged 29) who has never received orthodontic treatment. The maxillary left premolar roots and molar roots are extremely short|
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| Root Resorption - Individual susceptibility|| |
A distinction between dentitions affected by root resorption with known and unknown etiology is important for a number of reasons, especially when evaluating whether resorption is orthodontically provoked or not. If an orthodontic appliance is inserted in a dentition with milder signs of idiopathic root resorption, severe progression of the resorption process can be expected and the responsibility will be with the treating specialist in orthodontics. Therefore, careful diagnostics of the dentition is important before the introduction of orthodontic appliance.
Another argument for a distinction between idiopathic and orthodontically induced resorption is evident in cases of complaints.
A national study involving panoramic and profile radiographs from 107 patients who had developed excessive root resorption during the orthodontic treatment (more than 1/3 of one or more roots affected) identified two characteristic phenotypes of dentitions in 102 of the patients.  Before treatment was initiated approximately 90 of the patients had more than three of the following morphological signs:
The remaining 12 patients showed plumb roots, possibly combined with resorptive changes in the condyle and with open bite.
- Short roots
- Deviant root morphology
- Collum resorptions
- Crown malformations
- Ectopia and agenesis.
When comparing the national study,  and the previous description of known causes behind root resorption, obvious similarities are seen. It appears that the 102 patients with deviant morphology and deviant occlusion have dentitions with less severe traits of ectodermal deviations (approximately 90 patients) and of mesodermal deviations (approximately 12 patients).
| Can the Root-close Periodontal Membrane Explains the Different Etiologies?|| |
In order to understand how causes behind root resorption can be assigned to mainly three tissue types, a significant mapping process was initiated of the tissue elements close to the roots of permanent teeth. ,, The tissue components close to the root are shown in [Figure 6], previously published. 
|Figure 6: Demonstration of the tissue layers in the human root-close peri-root sheet cover. D: Dentine. C: Cementum. Yellow dots: Peripheral nerves. Green crosses: Area of fiber-dense ectomesenchymatic layer. Red structures: Malassez epithelium. The horizontal arrows indicate the tissue layers affected in different diseases (mentioned by examples). Root resorption with known etiology is caused by inflammatory reactions in these tissue layers (trauma, pressure, orthodontic treatment or inflammation due to disturbances in a single tissue layers caused by different general diseases, affecting different tissue components in the peri-root sheet|
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Different immunohistochemical examinations of tissue/cells close to the roots showed that particularly nerves (closest to the root), ectomesenchyme (dense fiber layer) and epithelium of Malassez (farthest from the root) comprised a peri-root sheet layer comparable to a periost. , How these tissue types are diagnosed on panoramic radiographs is demonstrated in Fig 2. 
It seems logical that a disruption of the innervation by virus attack can provoke an inflammatory condition in the periodontal layer close to the root. Similarly, it can be presumed that changes in congenital diseases that involve the ectodermal-Malassez layer can create an imbalance in this layer and provoke resorption. Particularly the Malassez layer has been in focus in order to understand the etiology behind resorption,  and the process behind orthodontic tooth movement.  Furthermore, the mesodermal intermediate fiber layer seems to be responsible for changes in osteitis deformans and for the plumb roots and the open bite.
| Hypothesis|| |
The hypothesis is that deviations (congenital or acquired) in the three tissue layers provoke an inflammatory process in the root-close periodontal membrane (peri-root sheet). It seems to be exactly these three cell/tissue layers in the membrane that are decisive for a successful transplantation of a tooth from one site in the mouth to another. It is well-known that treatment is only successful when the tissue attached to the tooth root is transplanted with the tooth from the original site to the site of transplantation. In a recent study, it was demonstrated that transplantation of a premolar has a poor prognosis when the tooth is transplanted to a region where a primary molar has been arrested in eruption.  This shows how the eruption process, the alveolar bone growth and the resorption process depend on a healthy and functioning peri-root sheet layer.
| Evoluation of The Hypothesis|| |
This paper explains how different etiologies behind root resorption and how different phenotypic traits in root resorption can be understood from immunohistochemical studies of the human periodontal membrane close to the root and thus gain a new understanding of the phenomenon of root resorption.
This paper is the first attempt to explain how different etiologies behind root resorption and how different phenotypic traits in root resorption can be coordinated through immunohistochemical studies of the root-close periodontal membrane and thus, gain a new understanding of the phenomenon of root resorption.
This paper invites future research on idiopathic root resorption and it is the goal to explain these idiopathic root resorptions step by step. The overall aim is to improve diagnostics of the dentition before initiating orthodontic treatment.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]