Periodontal health is of great concern for the orthodontists and periodontists, especially in the inter-disciplinary management of bimaxillary protrusion patients. Since protrusive lips is one chief complaint of patients seeking orthodontic treatment in the Eastern Asian community, four bicuspid extraction was commonly utilized to correct dental and lip protrusion. The principal finding of the present research was that mass retraction of maxillary incisors leaded to a significant reduction in the ABA on the palatal side, and apex displacement is the major factor that contributed to decreased palatal ABA. Moreover, multiple line regression indicated an equation to potentially predicate alveolar bone resorption on the palatal side by integrating apex displacement and changes in the inclination of incisors. Therefore, the null hypotheses were rejected.
Dentists have been seeking personalized approaches to improve treatment outcome and avoid deleterious sequelae for a long time. The new era of precision medicine requires a personalized, or individualized treatment plan and case management. The use of digital orthodontic setups has grown quickly to aid individualized plan, mimic tooth movement and predict treatment outcome. Three-dimensional CBCT imaging have exponentially enhanced the capability to evaluate regional anatomy in the alveolar process and assess periodontal risks during tooth movement.
Alveolar bone, which originates from the dental follicle during embryogenesis, is unique in its dynamic remodeling capacity during tooth eruption and OTM. It has been long accepted that OTM is a dynamic process whereby the application of orthodontic force induces bone resorption on the pressure side and bone apposition on the tension side, which maintains structural integrity of the alveolar bone. Although the alveolar bone might be dynamically remodeled to house moving teeth in growing adolescents, OTM is limited by the cortical plates of the alveolus at the level of the incisor apices, which can be regarded as “orthodontic walls” , in non-growing adults. Moving teeth out of these boundaries may lead to occurrence of severe iatrogenic sequelae of alveolar bone resorption, and this is especially disastrous in adult patients[5, 22]. The width of the anterior palate at the level of the apex remained unaltered despite long-term incisor retraction in adult patients. Indeed, we observed significant decrease in the height, thickness and area of alveolar bone on the palatal side.
The acumen of CBCT may help orthodontists and periodontists assess periodontal bone status and remodeling in OTM. Stages of alignment, bite opening and space closing are integrated processes in extraction cases. It has been demonstrated that alignment of anterior teeth by tipping leaded to significant vertical and horizontal loss of alveolar bone in non-extraction cases[24-26]. In addition, Yodthong et. al. investigated the effect of incisor retraction on changes of alveolar bone. CBCT was taken before retraction and after 6 months of retraction. They reported an increase of 0.4 mm in ABT at the crestal level on the labial side, 0.2 mm and 0.6 mm decrease at crestal and apical level on the palatal side respectively for the central incisors, indicating that incisor retraction is a risk of alveolar bone resorption in bimaxillary protrusion patients. However, the total thickness of the alveolar bone even increased an average of 0.6 mm at the apical level. Sarikaya et. al. reported that no changes in the labial side were observed, while decrease in ABT in the maxillary arch was observed in four premolar extraction cases with 0.7 mm and 1.2 mm at the crestal and mid-root level. Our results were consistent with Ahn et. al. that remarkable bone absorption can be found in all three levels on the palatal side, while bone thickness on the labial side increased in the middle third, by 0.27 mm for upper central incisors and by 0.65 mm for lateral incisors, with statistically significant differences.
In the process of assessing the periodontal risk, dentists should not only evaluate anatomic features of alveolar bone before treatment, but also the remodeling potential and 3-dimensional position of targeted teeth in the alveolus. 3D imaging techniques may help orthodontic-periodontal interdisciplinary coordination in managing periodontal iatrogenic effects. Consistent absorption of alveolar bone on the palatal side in our study and in Ahn et. al. indicates that retraction actually moves incisors ‘‘through-the-bone’’. Despite the increase in alveolar bone thickness and area on the buccal side, decrease in the total thickness and area in the incisor region suggests that orthodontic treatment may lead to increased risks for periodontal sequelae.
Digital setups have been widely used for diagnosis, treatment planning, indirect bonding, simulating treatment, and designing and producing orthodontic appliances, especially in the aligner techniques[21, 30, 31]. Much emphasis has been laid on crown position rather than root position because spatial position of roots is not available until the emergence of CBCT imaging and roots are usually not directly related to esthetics and occlusal contact. However, if the alveolar bone is not taken into consideration, such digital setup may push the tooth out of the bone for considerable distance. Our present data suggest that without taking the alveolar bone remodeling into consideration, 3D Digital setups, especially in aligners, may push the teeth out of the bone boundary.
Both the height and thickness of alveolar bone are critical factors to protect the teeth from plaque-induced (i.e., periodontitis) and non-plaque-induced gingival lesions. Thinner alveolar wall after incisor retraction may lead to dehiscences or fenestrations that compromise alveolar bone support. In order to investigate overall effect of incisor retraction on alveolar remodeling, we made a preliminary attempt to describe changes in alveolar bone area, which reflected changes in ABL and ABT; we found that apical displacement and alterations in inclination were significantly related to ABA on the palatal side; moreover, we developed an equation to describe changes in ABA, which described that changes of palatal ABA (T1-T0) is equal to (-3.258- 0.139× changes of inclination (T1-T0) + 2.533×apex displacement (T1-T0)).
We utilized changes in the inclination of each individual incisor rather than measurements from cephalometric to generate an equation for prediction of individual palatal ABA. Such equation may customize both periodontal and orthodontic consideration in adult patients, especially determining whether bone augmentation surgery was needed to aid orthodontic therapy, commonly recognized as surgically facilitated orthodontic therapy [SFOT] or periodontally accelerated osteogenic orthodontics [PAOO]). Our present equation further supports the conception that bodily retraction would lead to proximity of root apex to palatal cortical plate and extensive alveolar bone resorption, and this jeopardy is particularly pertinent in adults as numbers and regeneration capability of osteoblasts in the periosteum reduces with aging. Therefore, clinicians should not design bodily movement of a large distance to avoid placing the teeth out of the bone boundary during treatment planning.
Many factors may influence OTM and alveolar bone remodeling in the periodontium, such as force magnitude (light or heavy), force type (intermittent, interrupted or continuous), age( adolescents or adults), distance of movement, type of OTM(tipping, controlled tipping, translation, intrusion, extrusion), speed of space closure. For example, intrusion of incisors into a wider alveolus may increase alveolar bone support, thereby compensating the palatal bone resorption; however, excessive intrusion may lead to remarkable root resorption. Cortical plates at the root apex level did not remodel in adult patients; therefore, it may be utilized as a reliable mark to reflect the position of incisors. In our present study, we did not measure the distance of retraction for individual teeth, because the distance of root apex to palatal plates and inclination of incisors actually reflected changes in the position of incisors. Another issue was that we did not include the vertical position of root apex. Only minimal intrusion of incisors can be observed during retraction of incisors in bimaxillary protrusion patients, and large quantities of incisor intrusion can only be observed in class II patients, especially division 2 type with reverse curve of Spee. Data from the 2-dimensional changes in the lateral cephalograms were most often utilized to represent overall intrusion[27, 40, 41]; therefore, we did not further investigate influence of intrusion of incisor in 2-dimensional cephalometric on ABA.
We utilized adult patients to minimize the influence of growth on alveolar bone remodeling; therefore, interpretation of data in adolescents should be cautious. A key question critical in alveolar bone remodeling during OTM is elasticity and flexibility of alveolar bone, which undergoes rapid remodeling with low mineralization and stiffness even in adults. More CBCT data, especially from growing subjects, are needed to better predict changes in the periodontium and optimize periodontal risk assessment. A pre- and post-CBCT may be taken in cases who need mass incisor retraction, in order to assess potential periodontal risks such as fenestrations and dehiscence and provide individualized suggestion for long term periodontal maintenance for each subject after treatment. Clinicians should not be blinded to the periodontal sequalae induced by mass retraction, although soft tissue may hide severe bone resorption, fenestration and dehiscence on the palatal side. Moreover, patients should be informed of their periodontal situation, since such severe bone loss may be disastrous once the patient lose the teeth due to trauma or periapical diseases.
Currently, data regarding long-term alveolar bone remodeling after retention is not available except one case report. In the case report, significant alveolar bone formation was observed on the palatal side of the maxillary incisors 10 years after retention. Long-term studies are needed to confirm the alveolar bone apposition on the palatal side, and to improve the predication of alveolar bone remodeling in our equation.
Although CBCT may reduce periodontal risks in OTM, ALARA (As Low As Reasonably Achievable) principle should be adhered to, especially in growing adolescents. Each subject should be evaluated individually based on their unique treatment needs and set of circumstances. Digital set-up provides dentists unprecedented opportunities to predict the final position of the crowns before the treatment. Our present research indicated that with the help of CBCT imaging, dentists may further predict the final root position in the alveolus in the future.
The accuracy of CBCT imaging is closely related to voxel size. A smaller voxel size such as 0.2 mm provides a superior accuracy compared to a size of 0.4 mm in assessing the alveolar bone and soft tissues . However, the patients are exposed to a higher dose of ionizing radiation under the condition of a smaller voxel size. Indeed, a FOV of 16×16 cm and a voxel size of 0.3 mm used in this study may compromise the accuracy in assessing the alveolar bone. The approximate effective dose of 80 µSv in our present study, which was similar to the study of Pauwels et. al., may reduce the radiation risks under a lower voxel size.
We utilized the predominant vision-based method to discriminate dental structures in the dental literatures. Such vision-based method has been widely adopted to study the alveolar bone, temporomandibular joint, and airway space, and it is time-efficient. However, conventional vision-based method may be less accurate and reproducible when compared to gray value–assisted method proposed.
We only included adult cases in the present study, which partially explain the small sample size. Further long-term prospective studies including follow-up observation and studies in adolescents may bring us more information on the periodontal remodeling during orthodontic treatment.