Individual susceptibility is considered a major factor in determining root resorption potential with or without orthodontic treatment. This potential exists in the deciduous and permanent roots of all persons, in varying degrees in different teeth. Histologically, individual differences in tissue response and activity irrespective of age were noted. According to Rygh, the root resorption process seems to vary among persons and within the same person at different times. Metabolic signals that generate changes in the relationship between osteoblastic and osteoclastic activity include hormones, body type, and metabolic rate. These may modify specific cell metabolism and the person's reaction pattern to disease, trauma, and aging. It is reasonable to assume that disturbances or peculiarities in this interplay may explain the individual tendency to marked root resorption. Genetics. Several studies strongly suggest a genetic component for shortened roots. Although no definite genetic conclusion was found, autosomal dominant, autosomal recessive, and polygenic modes of inheritance are possible. Systemic factors. According to Becks, endocrine problems including hypothyroidism, hypopituitarism, hyperpituitarism, and other diseases are related to root resorption. This hypothesis, based on basal metabolic rates, has not been examined by updated blood analyses. Hyperparathyroidism, hypophosphatemia, and Paget disease have been linked to root resorption in a few anecdotal case reports. It has been suggested that hormonal imbalance does not cause but influences the phenomenon. Recently, a controlled animal study did not support the hypothesis that secondary hyperparathyroidism is primarily responsible for increased root resorption. A further study suggested that the parathyroid hormone plays a major role in bone metabolism, but that low calcium levels are necessary for root resorption to occur. Calcium ions are reputed to play an important role in mediating the effects of external stimuli (force, hormones) on their target cells. Nutrition. Marshall advocated that malnutrition can cause root resorption. Becks demonstrated root resorption in animals deprived of dietary calcium and vitamin D. It was later suggested that nutritional imbalance is not a major factor in root resorption during orthodontic treatment. Controversial results were reported when a low calcium diet was fed to rats undergoing active orthodontic treatment. Chronologic age. All tissues involved in the root resorption process show changes with age. The periodontal membrane becomes less vascular, aplastic, and narrow, the bone more dense, avascular, and aplastic, and the cementum wider. These changes are reflected by a higher susceptibility to root resorption seen in adults.14 The relationship between root resorption, orthodontic treatment and patient age was investigated. A positive relationship between these parameters was often reported, showing that root resorption is more prevalent in adults. Only a few studies showed no relationship between apical root loss and patient age at orthodontic treatment. Massler and Malone claimed that even without orthodontic treatment, the incidence of root resorption increases with age. Factors such as periodontal ligament characteristics and muscular adaptation to occlusal changes may be more favorable in young patients. Many resorbed lacunae and fewer repair zones were reported in an adult population. These were attributed to increased susceptibility to root resorption and longer aplastic gaps between resorption and apposition. Dental age. Root development can be affected by tooth movement: dilacertion, decreased expected root length, and root resorption. Dilaceration and stunting may be the final result of deflection of the Hertwig epithelial sheath in the developing root during tooth movement. Partially formed roots appear to develop normally during the course of orthodontic treatment that coincides with fast growth periods. Some teeth may be stunted if treated vigorously during later childhood. Rosenberg reported that teeth with incompletely formed roots showed less root resorption than those with completely formed roots. Teeth with incompletely formed roots did, however, reach their normal root length. Treatment increased the dilaceration incidence from 25% before treatment to 33%. This incidence is higher in canines than in premolars. Linge and Linge found an average loss of 0.5 mm root length in developed teeth that were orthodontically treated. Gender. Treated and untreated random samples showed no correlation between gender and root resorption. According to other studies, females are more susceptible to root resorption. The idiopathic root resorption ratio was 3.7:1 females to males, respectively. Apical root material loss was greater in treated females (0.73 mm) compared with treated males (0.67 mm). Dougherty speculated that this may reflect the difference in root maturity as the male is chronologically less mature than the female, and the male roots are less susceptible to the traumatic effects of orthodontic stress. Hormonal differences were not advocated by researchers who found an existing disparity between male and female. In only one study, males were found to be more sensitive. The presence of root resorption before orthodontic treatment. Root resorption of an untreated population was 0% to 90.5% of the examined teeth, or 0% to 100% of the examined patients. This reported wide range is due to different methods. There is a high correlation between the amount and the severity of root resorption present before treatment to the root resorption discovered when the orthodontic appliance is removed. One study reports that the incidence of root resorption increased from 4% before orthodontic treatment to 77% after treatment. Habits. Nail-biting, tongue thrust associated with open bite, and increased tongue pressure have been statistically related to increased root resorption. Tooth structure. Deviating root form is more susceptible to postorthodontic root resorption. Convergent apical root canal is considered to be an indicative of high root resorption potential. The degree of root resorption in teeth with blunt- or pipette-shaped roots was significantly higher than in teeth with normal root form. The pipette-shaped root was shown to be the most susceptible root form to root resorption. Previously traumatized teeth. Traumatized teeth can exhibit external root resorption without orthodontic treatment. Orthodontically moved traumatized teeth with previous root resorption are more sensitive to further loss of root material. The average root loss for trauma patients after orthodontic therapy was 1.07 mm compared with 0.64 mm for untraumatized teeth. Traumatized teeth without signs of resorption are not resorbed more than nontraumatized teeth. Endodontically treated teeth. A higher frequency and severity of root resorption of endodontically treated teeth during orthodontic treatment was reported. However, it has been suggested that endodontically treated teeth are more resistant to root resorption because of an increased dentin hardness and density. Alveolar bone density. Controversial reports on root resorption and alveolar bone density appear in the literature. Becks and Tager related increased root resorption to bone architecture resulting from hormonal and nutritional imbalance during growth. Several investigators found that the more dense the alveolar bone, the more root resorption occurred during orthodontic treatment. In a less dense alveolar bone, there are more marrow spaces. Tooth movement, as a result of bone resorption, is facilitated by the formation of active resorptive cells, the number of which increases according to the number of marrow spaces. According to Reitan, a strong continuous force on less dense alveolar bone causes the same root resorption as a mild continuous force on highly dense alveolar bone. Lamellar bone is more difficult to resorb with orthodontic pressure than bundle bone. Direct contact between roots and cortical bone can precipitate root resorption, especially during the second stage of Begg treatment as a result of high stress pattern at the apex. Wainwright reported that bone density affects the tooth movement rate but has no relation to the extent of the root resorption. Recently, two controlled studies reported controversial results. Lactation, coupled with a calcium deficiency in rats, produced a decrease in bone density concomitant with an increase in parathyroid hormone secretion. The observed low levels of root surface resorption and enhanced tooth movement in these hypocalcemic animals suggest that increased bone resorption and decreased bone density facilitate remodeling of alveolar bone in preference to the root. Engström et al., in their histologic study on hypocalcemic rats, found that root resorption was connected to the degradation process occurring in proximity to the hyaline zone, and that in hypocalcemic situations, the increase in root resorption was related to an enhanced alveolar bone resorption because of the increased alveolar bone turnover. Classification of malocclusion. VonderAhe found no correlation between root resorption and malocclusion classification. Specific tooth vulnerability to root resorption. Different teeth have different tendencies to root resorption. All examined teeth after orthodontic treatment showed evidence of root resorption. Most studies report that maxillary teeth are more sensitive than mandibular teeth. Converse incidences were shown in conflicting reports. The maxillary incisors are the teeth most affected by root resorption. The extent of movement in these teeth is usually greater than in others because of malocclusion, function, and esthetics. Their root structure and relationship to bone and periodontal membrane tend to transfer the forces mainly to the apex. Others have found the mandibular incisors to be more affected. It is believed that if there is no apical root resorption seen in the maxillary and mandibular incisors, then significant apical resorption in other teeth is less likely to occur. The most frequently affected teeth, according to severity, are the maxillary laterals, maxillary centrals, mandibular incisors, distal root of mandibular first molars, mandibular second premolars, and maxillary second premolars.
Appliances. It is often stated that the degree of root damage is a function of the appliance used. Fixed versus removable: Only one study compared root resorption resulting from fixed and removable appliances, concluding that the use of fixed appliances is more detrimental to the roots. Ketcham claimed that normal function is disturbed by the splinting effect of orthodontic fixed appliances over a long period that can cause root resorption. Stuteville, on the other hand, suggested that the jiggling forces caused by removable appliances are more harmful to the roots. Begg versus edgewise: It is often stated that the light wire Begg technique causes less root resorption than edgewise, although maxillary incisor root resorption during the Begg third stage has been documented. Malmgren et al. suggested that there is no difference between these techniques, but found that the frequency of root resorption was signficantly higher (48%) in traumatized maxillary incisors when intruded by the Begg technique compared with edgewise technique (43%). Magnets: It is suggested that the increase in force as space closes with time (attraction) can stimulate a more physiologic tissue response, and thus decrease the potential for root resorption. Intermaxillary elastics: Linge and Linge found significantly more root resorption on the side where elastics were used and suggested that jiggling forces the result of function combined with elastics are responsible for the incisors' root resorption. It was reported that Class III elastics used for anchorage preparation increased mandibular first molar distal root resorption. Extraction versus nonextraction: McFadden and VonderAhe found no difference in the extent of root resorption in patients treated with or without extractions. Serial extractions: Serial extractions without complementary orthodontic treatment gave the least root resorption compared with serial extractions with orthodontic treatment or to four premolar extractions followed by fixed appliance treatment. Other appliances: Rapid maxillary expansion, with cervical traction, has been reported to cause severe root resorption of the first maxillary molars. Orthodontic movement type. It seems that there is no safe tooth movement. Intrusion is probably the most detrimental to the roots involved, but tipping, torque, bodily movement, and palatal expansion can also be implicated. 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 that is due to bodily movement should be less than that of tipping. Orthodontic force. Degree of force: Harry and Sims found the distribution of resorbed lacunae was directly related to the amount of stress on the root surface and the rate of lacunae development was more rapid with increasingly applied forces. They concluded that higher stress causes more root resorption. According to Schwartz, applied force exceeding the optimal level of 20 to 26 gm/cm2 causes periodontal ischemia, which can lead to root resorption. Other studies reported on different force levels and their relationship to root resorption. Continuous versus intermittent forces: The pause in treatment with intermittent forces allows the resorbed cementum to heal and prevents further resorption. On the other hand, intermittent forces have been linked in their detrimental effects to jiggling forces. Jiggling and occlusal trauma: Jiggling forces causing occlusal trauma and implicated in root resorption can result from the use of intermaxillary elastics or active removable appliances. Occlusal forces on poorly aligned dental inclined planes can be a contributing factor in root resorption during orthodontic treatment. Newman, however, did not find a relationship between root resorption and occlusal trauma or heavy occlusal forces. The extent of tooth movement: Many believe that root resorption is directly related to the distance moved by the roots. The maxillary incisor roots are moved more often than the roots of other teeth; therefore it is not surprising to detect greater apical root loss in these teeth. However, Phillips who examined several tooth movement types and Dermaut who examined intrusion did not find any relationship between the extent of root movement and the amount of root loss.
Combined biologic and mechanical factors
Treatment duration. Most studies report that the severity of root resorption is directly related to treatment duration. Only a few studies did not support this finding. Rudolph reported that 40%, 70%, 80%, and 100% of the patients in treatment demonstrated some root resorption after 1, 2, 3, and 7 years of active treatment, respectively. Levander and Malmgren92 found that 34% of examined teeth showed root resorption after 6 to 9 months of treatment, whereas at the end of active treatment, lasting 19 months, root resorption increased to 56%. Histologically, 34% and 56% of the examined teeth showed resorbed lacunae after 15 and 20 days of tooth movement, respectively.Goldin reported that the amount of root loss during treatment is 0.9 mm/year. Root resorption detected radiographically during orthodontic treatment. Minor resorption or an irregular root contour seen after 6 to 9 months indicates an increased risk of further root resorption. No severe resorption was detected at the end of treatment in teeth without resorption after 6 to 9 months. Relapse. Reitan44 claims that forces of relapse are not strong enough to cause root resorption. However, Ten Hoeve and Mulie believe that the teeth are prone to additional root loss during relapse as a result of light muscles forces. Recently, Sharpe et al. found a higher frequency of root resorption in patients demonstrating relapse compared with patients without relapse because of the loss of overall bone support. Root resorption after appliance removal. Clinically, root resorption associated with orthodontic treatment usually ceased once the active treatment terminated. This is expected since progressive root resorption is tissue-pressure related. Remodeling of the sharp, rough edges of the resorbed root surfaces may occur. Reitan claimed that additional active resorption lasts for about a week after appliance removal followed by cementum repair that lasts 5 to 6 weeks of orthodontic inactivity. This could explain the 0.1 mm of apical root loss seen in the study of Copland and Green. Only a few publications report on active root resorption during retention. One case of active root resorption lasted 3 years after appliance removal. Teeth splinting and root canal therapy terminated the process. Root resorption after treatment is mostly related to causes other than the active treatment itself, such as occlusal traumatism, active retainers and others.
Teeth vitality. Teeth vitality and color do not change even in cases of extensive root resorption. According to Stenvik and Mjor, orthodontic movement can cause pulp blood flow disturbances, vacuolization and, rarely, pulp necrosis that are not related to root resorption. Loss of crestal bone and tooth stability. Loss of marginal attachment is more detrimental than loss of an equivalent amount of root length by apical resorption. The resulting decrease of tooth stability according to Goldin is explained by the presence of large amounts of periodontal fibers in the crestal area compared with the apical zone. During orthodontic treatment, crestal bone loss is about 0.2 to 0.5 mm. Phillips found that 2 mm of apical root loss decreases the root surface by 6% to 9%, whereas 4 and 6 mm apical root loss causes an 18% to 22% and 28% to 38% decrease, respectively. According to Kalkwarf et al., 3 mm of root resorption is approximately equivalent to 1 mm of crestal bone loss. Prediction. Can root resorption be predicted? Who is more liable to have root resorption? Why did root resorption occur in all teeth examined in one patient and no root resorption found in another patient? The most significant diagnostic aid in predicting root resorption is radiographic evidence of root resorption before orthodontic treatment. This is especially true for the maxillary incisors. Other predicting factors are root shape and the stage of root development. Pipette-shaped roots and fully developed roots are more susceptible to resorption.