More molar distal movement than pretreatment cone-beam computed tomography posterior space available at the root level in mandibular dentition distalization with microimplants

Molar distalization is one of the ways to treat patients with mild crowding and/or slight lip protrusion without premolar extraction. Three decades ago, microimplants began being used for distalization of the maxillary and mandibular dentition and showed effective molar distal movement.^1–3 

As en masse distal movement using microimplants became popular, more attention was paid to the posterior anatomical limit and achievable extent of tooth movement. In previous studies using two-dimensional images, the anterior ramus border was considered the mandibular posterior limit for molar distalization.^4,5  However, considering the three-dimensional position of the bony structures and mandibular second molar, the mandibular lingual cortical plate distal to the molar root is more greatly associated with the anatomical limit of the mandibular dentition rather than the anterior ramus border.^6–8 

The posterior space available in patients with Class I and III malocclusions has been observed by measuring the shortest distance from the distal root of the second molar to the lingual cortical plate.^7,8  However, in some Class III camouflage treatments, the mandibular molars need to be distalized up to approximately 5 mm to improve facial esthetics and achieve a Class I occlusal relationship.^6,9  This distal movement can be much greater than the posterior space available as measured before treatment, which may lead to lingual bone dehiscence caused by considerable molar movement, as shown in some case reports.^6,10  This raises the question of whether the mandibular lingual cortical plate is the limit of distalization for the mandibular molars. However, no study has compared the posterior space available measured before treatment with the distalization distance achieved after treatment using cone-beam computed tomography (CBCT).

Therefore, the purpose of this study was to compare the pretreatment space available distal to the mandibular second molar at the root level measured on CBCT with the distalization distance achieved after whole dentition distalization to determine whether the lingual cortical plate is the anatomical limit for distalization. Thinning of the lingual cortical bone close to the second molar distal roots, root resorption on the distalized molars, and the relationship between thinning of the cortical bone and root resorption were also evaluated. The null hypothesis was that no difference would be found between the posterior space available on CBCT and the distalization distance achieved.

This retrospective study was approved by the Institutional Review Board of Kyungpook National University Dental Hospital (KNUDH 2023-12-05-00).

G*power version 3.1.9.7 (Heinrich Heine University Düsseldorf, Düsseldorf, Germany) was used to determine the sample size for this study based on a previous CBCT study on the retromolar space.⁷  A power analysis with a test power of 0.90, an effect size of 0.41, and a two-tailed significance level of .05 indicated a required sample size of 66 patients.

CBCT images were collected from patients who were diagnosed and had nonextraction treatment with molar distalization in the Department of Orthodontics, Kyungpook National University Dental Hospital, Daegu, Korea, between January 2011 and December 2022. The inclusion criteria were (1) adult patients (>18 years old); (2) skeletal Class I or III malocclusion; (3) no missing or extracted teeth except the third molars; (4) bodily distal movement of the mandibular second molars greater than 2.5 mm with microimplants; and (5) available high-quality CBCT images before and after treatment. The exclusion criteria were patients with previous orthodontic treatment or orthognathic surgery, severe facial asymmetry, or dental prostheses or implants. Finally, a total of 66 patients (27 with Class I malocclusion, 39 with Class III malocclusion) with a mean age of 24.46 ± 4.89 years were included (Table 1).

Mandibular third molars were extracted before treatment. Straight-wire orthodontic brackets (0.022-inch slot) were bonded, and the mandibular dentition was distalized by applying a 150–200-gm force from microimplants that were 1.4 mm in diameter and 7 mm in length (AbsoAnchor, Dentos Co. Ltd, Daegu, Korea) placed bilaterally in the mandibular posterior buccal area to hooks crimped on stainless steel archwire between the lateral incisors and canines.^1–3  Forty patients underwent microimplant placement into the alveolar bone between the first and second molars, while 26 patients had microimplants placed in the bone distobuccal to the second molars. The average total treatment duration was 31.21 ± 13.61 months.

CBCT images were obtained using CB MercuRay™ (Hitachi, Osaka, Japan) under the following conditions: 120 kV, 5 mA, 19-cm field of view, 0.377-mm voxel size, and 9.6-second scanning time. InvivoDental software program (Version 5.4.6, InVivo, Anatomage Corp, San Jose, CA) was used for all measurements.

The CBCT images were reoriented using the mandibular occlusal plane, connecting the midpoint of the mandibular incisal edge and the mesiobuccal cusp tips of the left and right mandibular first molars, as the horizontal reference plane.

The pretreatment mandibular posterior space available was measured on axial CBCT images.⁷  The long axis of the mandibular second molars, passing through the occlusal central fossa and root furcation, was used to define vertical levels. The zero level was set at the furcation of the second molar. The additional three planes were located 2, 4, and 6 mm apical to the furcation (Figure 1). The cuspal line was constructed by connecting the mesiobuccal cusps of the mandibular first and second molars and was considered the distalization direction (Figure 2). The space available for molar distalization was defined as the shortest distance from the distolingual point of the distal root of the second molar to the inner lingual cortical plate when measured parallel to the cuspal line.

As presented in Figure 3, to calculate the amount of the second molar distal movement, the distance from the distal surface of the second molar to the line perpendicular to the mandibular plane and tangent to the most posterior point of the genial tubercle was measured before and after treatment at each root level. The amount of molar distalization was calculated by subtracting the measured distances before and after treatment.

During molar distalization, the roots of the mandibular second molars were moved into the mandibular lingual cortex, which led to thinning of the cortical plate. Therefore, the amount of cortical thinning was measured at the root apex level by subtracting the lingual cortical thickness before and after treatment (Figure 4). To ensure high reliability of the measurements, the pretreatment cortical thickness was measured at the corresponding site of the posttreatment thinned cortical plate. In addition, the amount of root resorption was determined by calculating the change in root length of the second molar after treatment (Figure 5).

IBM SPSS software (version 24.0, IBM Corp, NY, USA) was used for the statistical analysis. All variables were measured twice by the same examiner at a 4-week interval. A paired t-test showed no significant difference between the two measurements. Dahlberg’s formula was used to evaluate methodological error and showed a mean value of 0.1 mm for the linear measurements.

To assess normality of the data, the Kolmogorov-Smirnov test was used. To compare the measurements before and after treatment, a paired t-test was performed for normally distributed data, and a Wilcoxon signed-rank test was performed for nonnormally distributed data. Spearman correlation was used to analyze the correlation between thinning of the lingual cortex and root resorption of the second molar.

The posterior space available and distance of distalization achieved for the mandibular molars are illustrated in Table 2. Pretreatment, the posterior space available in the Class III group was 2.03, 1.76, 1.45, and 1.21 mm at levels 0, 2, 4, and 6, respectively, showing a gradual decrease toward the root apex. After treatment, the distance of distalization achieved was significantly greater than the pretreatment posterior space available at all levels (P < .001), and the difference at each level was approximately 0.8 mm, suggesting that more distal movement of the molars occurred compared with the posterior space available. Similarly, the Class I group demonstrated a distance of distalization achieved that was significantly greater than the posterior space available by approximately 0.8 mm at all levels (P < .001). The ratio of distance of distalization achieved to posterior space available was 140% to 173% in the Class I and III groups.

In addition, a significant decrease was found in the lingual cortical thickness after treatment in the Class I and III groups (P < .001; Table 3), indicating thinning of the cortical plate after molar distalization. The amount of cortical thinning ranged from 0.60 mm to 0.86 mm and increased toward the root apex. Significant decreases in root length were observed in the mesial and distal roots after treatment in the Class I and III groups and in the total sample (Table 4). Mean root resorption of the distal root was 1.0 mm, which was significantly greater than the 0.5 mm observed for the mesial root (P < .001).

Significant positive correlations were found between lingual cortical bone thinning and root resorption at all levels, meaning that thinning of the lingual cortex was closely associated with distal root resorption (Table 5).

The posterior space available gradually decreased from level 0 to 6 due to the V-shaped and downward divergence of the lingual cortical mandibular body bone, indicating that the amount of distalization is restricted by the lingual cortical plate at the root apex level.^6,7  Thus, the space available at the root apex can determine the actual space available for molar distalization.

In this study, the distance of distalization achieved for the mandibular molars was greater than the pretreatment posterior space available at all levels in both the Class I and III groups (Table 2, Figures 6 and 7). Thus, the mandibular second molars moved to a greater extent distally over the inner cortex of lingual bone. The ratio of distal molar movement to the pretreatment posterior space available was 140% at level 0 and 170% at level 6. This indicated a possibility for bone dehiscence or root-to-cortex contact after molar distalization, particularly at the root apex. Authors of previous case reports^6,10  have shown lingual bone dehiscence after penetration by the mandibular second molar distal roots after distal movement. Although the distal root of the second molar came into contact with the inner cortex of the mandibular lingual cortical plate, the root could move steadily and intrude into the lingual cortex during distalization.

From a clinical viewpoint, more distal and buccal tipping of distalized molars might occur when a distal force is applied continuously even after root-to-cortex contact. Therefore, lingual crown torque for distalized molars might be important to reduce root-to-cortex contact and increase the amount of distalization. Occasionally, molar intrusion might be required to achieve counterclockwise rotation of the mandible during distalization.^1,11  Such intrusive movement can decrease the posterior space available and thus lead to earlier root-to-cortex contact.¹²  Conversely, in patients with a flat mandibular plane who require increased vertical dimension, extrusion of the mandibular molars during distalization, using microimplants placed superior to the occlusal plane or intermaxillary elastics, would increase the space available for molar distalzation.¹¹ 

Thinning of lingual cortical bone was observed in this study as the distal root of the second molar moved into the cortical bone by continuous distalization force.^6,13–15  The posterior space available was the least at level 6; therefore, cortical thinning was the greatest at that level (Figures 6 and 7). In addition, thinning was greater in the Class III group than in the Class I group due to the greater distance of distalization. As mentioned in a previous case report,¹⁰  once the distal root has contacted the inner lingual cortex, the tooth shows slower movement and continuously moves into the cortical plate. Although no remarkable radiographic bone dehiscence was found in this study, a thinned cortical plate or bone dehiscence can cause concerns for clinicians. Previous CBCT researchers have reported favorable bone recovery during retention on the thinned or fully penetrated cortical plate caused by considerable tooth movement.^10,13,16,17  Therefore, long-term follow-up of the current sample would be expected to show regenerative bone deposition around sites of the thinned lingual cortex. Another concern caused by root-to-cortex contact is root resorption.¹⁸  The amount of root resorption measured 0.5 mm and 1.0 mm at the mesial and distal roots, respectively, and was greater in the Class III group than in the Class I group. The distal roots exhibited greater resorption than the mesial roots by approximately 0.5 mm, and this value might indicate that the root resorption resulted from root-to-cortex contact. The amounts of root resorption and cortical bone thinning were positively correlated. However, this degree of root resorption might be clinically acceptable considering that the average length change of the mandibular molar root was approximately 1.5 mm.^19,20  In addition, from a total of the current 132 distal roots, only 4 distal roots had root length changes greater than 2 mm (range, 2.03–2.27 mm), and the rest showed an average change of 1.02 ± 0.46 mm. This can be considered minor apical resorption (<2 mm) according to the index score for root resorption.²¹  Treatment times were quite long, as the distal movement of the molars slowed down after cortical bone contact, and the slow bodily movement of the molars may be a reason for reduced root resorption.

In this study, we investigated the posterior space available, tooth movement achieved, and subsequent morphologic changes of the bone and tooth after mandibular molar distalization. However, the current research had some limitations. Because voxel size might affect the accuracy of linear measurement, minute values need to be interpreted with caution. In addition, this was a single-center, retrospective study and might include some biases. Therefore, prospective multicenter studies should be executed using long-term data to enhance the generalizability of the findings.

• The null hypothesis was rejected.

• The distance of mandibular molar distalization achieved was greater than the pretreatment posterior space available, as measured on CBCT.

• Thinning of the lingual cortex and second molar distal root resorption were observed after molar distalization into the lingual cortex, distal to the second molar.