Early root resorption was detected when the alleged tooth was extracted (or lost) and visually inspected ( Fig. 102.1 ). It could also be associated with prior pain or increased mobility. Root resorption in permanent teeth, initially described as ‘absorption defects’ was first reported in 1829. Bell described the physical appearance of the hard tissue damage of the root surfaces associated with tooth loss following a disease state. This ‘root absorption’ process was further documented in 1856 and 1932. The breaking down process of the cementum layers until it stopped short of the pulpal cavity was described. The pulpal tissues apparently had the greatest resistant to this process and further observations proposed that resorption perhaps correlated to the degree of mineralisation of the dental tissues.

Tomes’ description of the dental pulp remaining perfectly encased in a tube of dentin.

Source: Reprinted from Tomes J. A System of Dental Surgery. Philadelphia: Lindsay and Blakiston, 1859; Gutmann JL. Some historical musings on tooth/root resorption. J Istanb Univ Fac Dent. 2017 Dec 2;51(3 Suppl 1):S1–S9. doi: 10.17096/jiufd.14022. PMID: 29354305; PMCID: PMC5750824.

Radiographic detection of root resorption was first reported in 1927, and was associated with orthodontic treatment in the 1930s. ^, It was noted that following orthodontic treatment, there will be inevitable damage in the cementum, periodontal ligaments, alveolar bone and pulpal tissues.

Clinical detection of OIRR is usually through radiographic findings. Routine ortho-pantomogram (OPG) and lateral cephalogram radiographs are taken prior to the commencement of orthodontic treatment. This procedure is required for the assessment of general dental health, including root form and the presence or absence of any underlying disease, and to show the position and number of developing teeth. Intra oral peri-apical radiographs are recommended to supplement these radiographs when there is a clinical indication, such as suspected underlying pathology or any other developmental anomaly.

The naturally occurring mesial migration of teeth contributes to cementum cratering (early external root resorption) and they are likely to be undetected. Surface root resorption occurs in as much as 85% of normal teeth, and much more around the apical region. Mild root resorption such as microscopic cratering of the root cementum cannot be detected via routine radiographic methods. Hence the literature documenting OIRR’s incidences may be inadequate, and under reported.

Nevertheless, the degree of OIRR has been reported to increase from a 15% pre-treatment to as high as 73% after 12 months of fixed appliance therapy. Other reports have indicated that up to 80% of all orthodontically treated patients will exhibit some degree of OIRR. However, despite most orthodontically treated patients showing some extent of OIRR, they do not exhibit further risk of tooth loss post-treatment. ^, Most mild to moderate OIRR cases have the process of resorption halted, or even repaired when orthodontic forces ceased or at the completion of orthodontic treatment.

When root resorption could not be directly implicated by orthodontic nor environmental factors, researchers postulated a genetic connection to this clinical phenomenon. Newman was the first to formally propose a genetic basis for OIRR. The literature further supports this genetic link. Although autosomal dominant, autosomal recessive and polygenic modes of inheritance are conceivable, there remains no firm conclusion.

Approximately half of OIRR, and almost 66% of OIRR in maxillary central incisors specifically, is genetically linked. A retrospective twin study examining orthodontic root resorption has identified evidence supporting the influence of both genetic and environmental factors on this phenomenon. In addition, studies in a panel of different inbred mice also supported a genetic component involving multiple genes in histological root resorption.

Fundamentally, inflammatory responses and the activation of osteoclasts are the key pathways inducing OIRR. A number molecular pathways such as ATP/P2XR7/IL-1β, RANK/RANKL/OPG (receptor activator of nuclear factor-κβ, receptor activator of nuclear factor-κβ ligand, osteoprotegerin—a cytokine receptor, respectively), Notch, Rho kinase and Wnt signalling pathways are suggested to modulate these two processes. ^, Hence, genes regulating these processes are thought to participate in inducing OIRR.

Although another cytokine, TNF-α has also been shown to induce osteoclastogenesis, its function depends on the exposure of bone marrow macrophages with adequate amounts of RANKL to convert these macrophages to functional resorbing osteoclasts.

Genetic studies found two types of polymorphisms: single nucleotide polymorphisms (SNPs) located within cytokine-coding genes and SNPs found within genes that play a role in bone remodelling. The rs1800587 and rs1143634 in IL-1α and IL-1β genes have been the mostly assessed SNPs in different populations. Yet, the results of investigations in different populations are not consistent.

• 1.

  The association between specific genetic variations—SNPs, and OIRR has been studied in various populations. In a Caucasian population, the rs1718119 SNP in the P2XR7 gene, which plays a role in bone remodelling and inflammatory responses, showed only a minor influence on OIRR variability. Similarly, the IL-1α and IL-1β SNPs were examined in different populations, with IL-1α genotype frequencies showing a significant difference between OIRR cases and controls. In Brazilian patients, the IL-1β SNP allele 1 was identified as a risk allele for OIRR.

  • In Iranian patients, the A allele of the rs1143634 SNP was associated with susceptibility to OIRR. However, in American families, the IL-1β gene was identified as a critical predisposing factor for OIRR, explaining 15% of the variation in maxillary incisor resorption. Additionally, linkage between OIRR and TNFRSF11A gene loci was found in some families, suggesting a potential genetic predisposition.

  • Despite the lack of association between certain SNPs and inflammatory external root resorption following tooth replantation, factors such as the timing of treatment and patient age at the time of injury were identified as risk factors for severe resorption. These studies highlighted the importance of genetic analysis in predicting susceptibility to OIRR and related dental conditions.

• 2.

  An Australian study observed that RANKL and OPG levels were seen to increase in the environment during root resorption with the application of heavy forces in a rat study. They might play a significant role during root resorption processes during orthodontic tooth movement. A study in Brazil found that certain genetic variations in the RANKL, RANK and OPG genes are linked to OIRR in patients with Class II Division 1 malocclusion undergoing orthodontic treatment. Factors such as root length and patient age were associated with the resorption, while specific SNPs in the RANK and OPG genes were found to increase the risk for OIRR. Meanwhile, research in Spain revealed that common alleles in the osteopontin gene protect against OIRR, while specific genotypes increase the risk of OIRR. Additionally, a study in the Czech Republic found that certain haplotypes in the P2RX7 gene alter the risk of OIRR. In another study, the C allele of the TaqI vitamin D receptor polymorphism was found to offer some protection against OIRR in orthodontic patients compared to untreated individuals.

It has been reported that Hispanic patients are more vulnerable than White and Asian patients to OIRR. White patients tended to have less OIRR than Asians, although this was not statistically significant. The actual genetic transmission presently remains unclear and further genetic study is required.

No correlations between gender and root resorption were observed in treated and untreated random samples. ^{, ,} Some studies have implicated a higher prevalence for females in susceptibility to OIRR. ^{, , ,} A female to male ratio susceptibility to resorption of 3.7:1 was proposed as early as 1975. Another study that supported this gender bias reported OIRR greater in treated females (0.73 mm) compared with treated males (0.67 mm). It could be postulated that as the males are less mature than females chronologically, the male roots appear to be less susceptible to orthodontic stress and hence less OIRR seen. ^, However, despite other studies implicating a higher resorption rate in males, ^, the association between gender and root resorption remains inconclusive.

Orthodontic tooth movement may affect the development of the dental roots. Dilacerations, ^, decreased root length ^, and root resorptions were reported during treatment in clinical reports. During root formation, the Hertwig epithelial root sheath may be deflected resulting in these phenomena. During the patients’ growth peak, partially formed roots appear to develop normally. On the other hand, if heavy forces were applied during treatment, root formation may be stunted. Incompletely formed dental roots demonstrated less root resorption then completely formed ones. Teeth with incompletely formed roots did, however, reach their normal root length as treatment progressed. Orthodontic treatment resulted in increased dilaceration incidence from 25% before treatment to 33%. This incidence is higher in canines than in premolars. There was also an average loss of 0.5 mm root length in developed teeth that were orthodontically treated.

The geometrical forms of dental roots can affect the distribution of the force through the alveolar bone and root during orthodontic treatment. Morphologically, the force is more concentrated on localised areas in trigonal sharp apexes than in roots with a normal shape. It has been reported that deviating root forms are more susceptible to OIRR. ^, Generally, teeth with root dilacerations are prone to root resorption, particularly maxillary lateral incisors. A convergent apical root canal, or a root canal with two or more canals that merge into a single apical foramen, has a higher OIRR potential. Various shapes and morphology of the dental root apices has been described by Levander and co-workers ( Fig. 102.2 ). Each form showing various susceptibility to OIRR, with the pipette-shaped root form at the highest risk.

Deviating root forms.

Source: From Levander E, Malmgren O. Evaluation of the risk of root resorption during orthodontic treatment: a study of upper incisors. Eur J Orthod. 1988 Feb;10(1):30–8. doi: 10.1093/ejo/10.1.30. PMID: 3162877.

Patients with an underlying chronic asthmatic condition have an increased incidence of maxillary root resorption (although not statistically significant). This is due to the likelihood of the root apex proximity to the maxillary sinus.

Hypothyroidism has also been associated with increased risks of OIRR but the results were not conclusive. Hyperparathyroidism and its relation to OIRR and bone metabolism has been reported in both human and animal studies, respectively. However, the findings were either anecdotal or inconclusive.

The prescription of drugs to control OIRR is still a rather controversial subject. Very low doses of the hormone L-thyroxine during 10 days of orthodontic tooth movement diminished the degree of OIRR by about 50% when compared to a control group. When 0.5 g of thyroxine was administered to three high-risk OIRR human patients, there were no reports of adverse effects and no worsening of their existing OIRR. Their conclusions of this study were not definite and their selection criteria were challenged at publication. It was assumed that the hormone either increased the resistance of the cementum and dentine to clastic activity or increased the rate of alveolar bone resorption (high levels of alkaline phosphatase were found). Thus, the hormone enhances tooth movement as it indirectly reduces OIRR.

Bisphosphonates are potent inhibitors of bone resorption. They have shown to inhibit OIRR in rats and demonstrated a positive dose-dependent correlation. However, other reports have indicated an increased rate of OIRR with bisphosphonate administration. They showed that injections of 1-hydroxyethylidene-1-bisphosphonate (HEBP) in rats induced a cementum surface alteration, inhibiting the formation of acellular extrinsic fibre cementum (AEFC), and delayed the formation of cellular mixed-fibre cementum. Hence, HEBP increased the vulnerability of the root surfaces to resorption during orthodontic tooth movement.

The administration of corticosteroids in doses of 15 mg/kg to rats during orthodontic treatment increases root resorption, whereas low doses of 1 mg/kg decreased root resorption.

It has also been reported that alcohol consumption in adults during orthodontic treatment tended to increase OIRR through vitamin D hydroxylation in the liver.

Orthodontic tooth movements occur in three-dimensions and there is a constant, dynamic vector of forces acting on the tooth–bone interface through the periodontal ligament (PDL). Common reactions to orthodontic forces such as bone bending, occlusal forces as well as the compression of the PDL illicit a myriad of different cellular interactions resulting in various soft and hard tissue changes. No orthodontic force can imitate the natural harmless physiologic force. While an increase in apical movement can lead to an increase in resorption, according to Philips, there was no direct relationship between root resorption and the sagittal or angular movements of the root apex.

Fundamentally, at greater areas of pressure and where the forces are intensified, dental tissues are more prone to root resorption. With intrusive movements, almost all pressure is gathered at the root apex; the risk of resorption markedly increases because of the root anatomy. Hence, intrusion is probably the most detrimental to the roots involved. Intrusive orthodontic forces of approximately 100cN causes 11 times more OIRR than controls, and 4.5 times more OIRR than extrusive forces. A micro CT study also demonstrated two- and fourfold increase in OIRR in resorption volume when comparing light and heavy intrusive forces to controls in human subjects.

Tipping, torque, bodily movement and palatal expansion have also shown to cause OIRR. Reports on resorption during bodily movement are controversial. According to Reitan, ^, the stress distribution along the roots during bodily movement is less than the stress concentration at the apex resulting from tipping. Therefore, risk of root resorption via bodily movement should be less than that of tipping forces.

It has been verified in many studies that heavy orthodontic forces cause more OIRR. ^, This has been demonstrated in both human and animal models. It was also noted that with either a light or heavy orthodontic force, with an increase in force application time, OIRR increases as well.

In a series of studies at the university of Sydney, Darendeliler, Kharbanda and a group of co-workers studied the root cementum of first premolars, which were subjected to light and heavy forces. The data suggested that the high-pressure zones might be more susceptible to resorption after 28 days of force application. There was more resorption by volume in the heavy force group as compared with the light group and controls.

The orthodontic literature documents various types of orthodontic movements. Continuous and intermittent, or jiggling forces are the forces mostly considered. Although it is clinically difficult to apply intermittent forces in orthodontic treatment with fixed appliances, intermittent forces are preferred instead of continuous forces in preventing serious root resorptions. ^, The key to intermittent forces is providing a chance to relieve pressure off the associated soft and hard tissues to allow regeneration and healing.

Although there is direct implication of heavy orthodontic forces causing OIRR, the repair process begins immediately after the release of the orthodontic force, or when there is a decrease in the magnitude of force. The repair is first observed around the resorption lacunae. This process shows similarity to the early cementogenesis during the development of the teeth. Resorption lacunae are recovered with the accumulation of new cementum and formation of a new periodontal ligament.

The possible repair level in resorption cavities that can be histologically observed are summarised as follows :

• 1.

  Partial repair: Part of the surface of the resorption cavity is covered with reparative cementum (cellular or acellular cementum).

• 2.

  Functional repair: The total surface of the resorption cavity is covered with reparative cementum without the re-establishment of the original root contour (cellular cementum).

• 3.

  Anatomic repair: The total surface of the resorption cavity is covered with reparative cementum to an extent such that the original root contour is re-established.

After the reduction of orthodontic forces, OIRR may still persist for up to 4 weeks. OIRR activity was notably more pronounced after the activation of heavy forces. Although there were no significant differences in the amounts of repair between light and heavy force groups, the reparative processes were different between them, with marked individual variations. After 4 weeks of force application, the light force group reached steady repair by 4 weeks of passive retention. Whereas, the heavy force group had most repair occurring only after 4 weeks of passive retention.

With different types of forces applied through their respective designs, fixed appliances and removable appliances are often compared. Most studies have noted that fixed appliances, due to a continued, higher level of forces, caused more OIRR. ^,

Historically, although there was more root shortening in a straight wire appliance group versus a conventional edgewise group, the difference was not clinically significantly. More recent studies have looked at various self-ligating bracket systems and clear aligners and their correlation to OIRR. Self-ligating bracket systems did not differ to conventional bracket systems in respect to OIRR. Clear aligners were compared to traditional fixed appliances and the OIRR observed were similar to those of fixed appliances activated with light forces.

Heavy forces during rapid maxillary expansion also induces root resorption directly on the premolars and molars which the appliance is attached to. The rapid expansion process causes OIRR in the unattached second premolar teeth as well. However, repair was observed after 6 months of retention.

Special care must be taken in cases with reported family history of root resorption. Document any family members who had suffered from OIRR from previous orthodontic treatment. It is essential to gather such information in planning the current treatment plan for the patient. What kind of appliances did the family member have, how long was the treatment for, which particular tooth/teeth were affected, how severe was the OIRR and what was the eventual outcome.

Routine radiographs taken prior to the commencement of orthodontic treatment is essential for the orthodontic diagnosis and treatment plans. However, supplementary radiographs should be considered for high-risk patients. Peri-apical radiographs, CBCT scans are indicated in these high-risk cases. High-risk cases are cases with (but not limited to) previous trauma, heavy fillings, heavy wear and attrition, occlusal trauma, increased mobility, previously root canal-treated teeth, impacted teeth, teeth in close proximity with supernumerary teeth, odontomes or any other dentigerous anomalies.

External root resorption is a three-dimensional phenomenon. Routine radiographs are two-dimensional. Therefore, unless the resorption is severe or restricted to just root shortening/blunting, they will not be easily evident. Radiographs taken beyond a 6-month period in these high-risk patients should also be repeated. Periodic updated radiographs are also essential to document any incipient resorption and their progress.