INTRODUCTION

Orthodontic treatment is achieved by moving the teeth into the planned position; efficient tooth movement and stable position cannot be ensured without adequate alveolar bone support. Even with carefully planned orthodontic treatment and supportive periodontal therapy, periodontal problems such as bony dehiscence and gingival recession1 are sometimes observed. Because adult and elderly orthodontic patients are increasing in number, orthodontists should pay more attention to potential periodontal problems that may develop or may be exacerbated during orthodontic treatment.2–4 Therefore, the patient group that is more vulnerable to alveolar bone loss should be diagnosed thoroughly, and this matter should be addressed before treatment.

Our previous study proved that skeletal Class III patients with anterior open bite, who were examined 1 month before orthognathic surgery during presurgical orthodontic treatment, had greater bone loss in the mandibular incisors than in the maxillary incisors, especially at the lingual cortical plate.5 However, it was not known whether this was due to presurgical orthodontic treatment alone, or to preexisting bone loss before orthodontic treatment with developmental origin in these typical malocclusion groups, or to the combination of the two. The prevailing notion is that in skeletal Class III patients with a thin symphysis, excessive labioversion of the mandibular incisors may increase alveolar bone loss.6–8

Alveolar bone morphology was recently measured with the introduction of three-dimensional (3D) cone beam computed tomography (CBCT).9,10 CBCT has great value in that it is more accurate11,12 for assessing bony architecture or quantifying bone volume than traditional radiographic images such as periapical or panoramic views.13–16

To our knowledge, no study has scientifically assessed the alveolar bone status of normal occlusion samples with no noticeable periodontal disease, and few studies have reported that certain malocclusion groups had greater bone loss around specific teeth than was seen in normal occlusion samples.5 Therefore, the purpose of this study was to compare the amount of alveolar bone loss and thickness around incisors of Korean normal occlusion samples and adult skeletal Class III patients with the use of 3D CBCT images. The null hypothesis is that there is no difference between the two groups.

MATERIALS AND METHODS

Class I normal occlusion (group 1) and skeletal Class III malocclusion subjects (group 2) were selected for this study. Group 1 consisted of 20 subjects (10 male, 10 female; mean age, 22.1 years) selected from the Normal Occlusion Sample Data in the Department of Orthodontics, The Catholic University of Korea. Subjects were selected from 480 Korean students (mean age, 24.3 years; range, 19.1–34.6 years) at WonKwang University, Iksan, Korea, according to the selection criteria. Exclusion criteria included (1) missing or decayed teeth; (2) prosthetic crowns; (3) crowding more than 3 mm or spacing more than 1 mm; (4) facial asymmetry with crossbite; and (5) noticeable periodontal disease. All subjects had fully developed permanent dentitions with normal overbite and overjet between 1 and 3 mm.

Group 2 consisted of 20 patients (10 male, 10 female; mean age, 22.4 years) who visited the Department of Orthodontics, Seoul St. Mary's Hospital, Seoul, Korea, and were diagnosed as having skeletal Class III malocclusion and were indicated as in need of orthognathic surgery. All patients had anterior open bite before treatment. Patients with missing anterior teeth, more than 3 mm of crowding or more than 1 mm of spacing, and noticeable periodontal disease, and with other craniofacial anomalies, were excluded. Experimental protocols were approved by the Institutional Review Board of The Catholic University of Korea (KC11EISI0244, KC09EISI0146). This study was exempted from patient consent by the Institutional Review Board of the Seoul St. Mary's Hospital, The Catholic University of Korea.

CBCT images of group 1 were acquired with VEGA (Asahi Roentgen Ind Co, Ltd, Kyoto, Japan) with a 200 × 179-mm field of view, 80 kVp, and 50 mA, resulting in 0.39-voxel resolution. CBCT images of group 2 were obtained with an iCAT scanner (Imaging Science International, Hatfield, Pa) with a 200 × 400-mm field of view, 120 kVp, and 47.7 mA, resulting in a voxel size of 0.4 mm.

The data obtained were exported in DICOM format into InVivo Dental software (Anatomage, San Jose, Calif), and 3D reconstructions were done. Sagittal slices were evaluated where the maxillary and mandibular right central incisor was widest labio-lingually in the axial view, respectively. Additional cephalograms and panoramic radiographs were taken. All measurements were made by the same operators.

Measurement

Reference points, lines, and measurement variables used are described in the previous report5 and are presented in Figure 1 and in Tables 1 and 2. The alveolar crest (AC) was defined as the most coronal level of alveolar bone.13 Distances between the AC and the cementoenamel junction (CEJ) represented the amount of vertical alveolar bone loss17 and were designated as UABL, UPBL, LABP, and LPBL, according to position. Measurements of alveolar bone thickness at the root apex were made from the root apex to the limit of the alveolar cortex, perpendicular to the long axis of the tooth, and were indicated as UA, UP, LA, and LP, respectively. Measurement was made using InVivo Dental software (Anatomage). Cephalometric measurements for the samples were made with V-Ceph software (CyberMed, Seoul, Korea).

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Table 1 Definitions of Reference Points and Lines Used in This Study

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Table 2 Definitions of Measurements Used

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RESULTS

Systematic intraexaminer error was evaluated at P < .05 and was found to be statistically insignificant. The ICC indicated good reliability with a mean ICC of .822 (ICC < .76–.88). In all measurements, no significant difference was found between male and female subjects (Mann-Whitney test, data not shown), and combined data were used in the following. Cephalometric characteristics of the samples described in Table 3 showed statistically significant differences between group 1 and group 2 in measurement of sagittal discrepancies (P < .05). Also, group 2 showed a better vertical growth pattern with labioversion of upper incisors and linguoversion of lower incisors (P < .05, P < .001).

Table 3 Cephalometric Characteristics of the Sample

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Amount of Alveolar Bone Loss

The mean value for vertical bone loss showed statistically significant differences between groups 1 and 2 (P < .05; Figure 2 and Tables 4 and 5). In group 2, vertical bone loss was significantly greater than in group 1, especially at the lingual alveolar plate. In both groups, vertical bone loss was greater on the lingual side than on the labial side, and it was greater in the mandibular incisors than in the maxillary incisors (P < .05, P < .001).

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Table 4 Mean Values of Alveolar Bone Loss of Maxillary Right Central Incisors

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Table 5 Mean Values of Alveolar Bone Loss of Mandibular Right Central Incisors

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Alveolar Bone Thickness at Apex

On the maxillary labial side, the mean value of alveolar bone thickness at the tooth apex showed no statistically significant differences between groups (Figure 3 and Tables 4 and 5). However, on the lingual side, group 1 showed wider bone thickness in both maxillary and mandibular incisors (P < .05, P < .001). In both groups, lingual side bone thickness was greater than labial side bone thickness in the maxillary incisors and the mandibular incisors, and in maxillary incisors, the difference was significant.

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Percentage of Alveolar Bone Loss to Root Length

The percentage of alveolar bone loss to root length showed that group 2 had a significantly greater percentage of bone loss in all areas of measurement (P < .05, P < .001; Figure 4 and Tables 4 and 5). The value was greatest at the mandibular lingual side, followed by mandibular labial, maxillary lingual, and maxillary labial sides (%LPBL> %LABL > %UPBL > %UABL) in both groups, indicating that mandibular incisors had greater bone loss than maxillary incisors relative to their respective root length, regardless of the groups. The most severe percentage of bone loss was %LPBL in group 2 at 32.03% of the root length.

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Percentage of Alveolar Bone Thickness at Apex to CEJ Width

The percentage of alveolar bone thickness at the apex of the incisors to CEJ width showed statistically significant differences between groups (P < .001; Figure 5 and Tables 4 and 5), and group 2 had thinner bone. The percentages were 138.47% (group 1) and 112.18% (group 2) in the maxillary incisors, and 126.90% (group 1) and 81.33% (group 2) in the mandibular incisors. Overall, alveolar bone thickness at the apex was greater than the CEJ width, except for the mandibular incisors in group 2.

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DISCUSSION

Numerous classic studies have been performed with normal occlusion samples; these studies have focused mainly on cephalometric values and occlusion.18–20 Without any malocclusion such as crowding or excessive overbite, a normal occlusion sample with healthy periodontium may not have abnormal alveolar bone structure. How much alveolar bone height and width of the buccal or lingual plate is necessary to maintain teeth in their position, and to ensure safe orthodontic treatment in the long term, has not been evaluated well. The study of normal occlusion samples may give us some insight regarding this issue.

Traditional radiographic images such as cephalograms, panoramic views, and periapical radiographs had inherent inaccuracy in evaluating bony architecture13,21; this may have hindered the study of alveolar bone in orthodontics. With the advent of 3D CBCT, visualization of the bony anatomy has become possible because of the inherent accuracy of the CBCT and the clipping function, which can visualize thin alveolar bone out amidst complex overlapping craniofacial structures.9,10 With relatively low radiation compared with conventional helical computed tomography (CT),22 CBCT has enabled alveolar bony measurement with good to excellent repeatability.23 Consequently, the number of published studies undertaken to explore alveolar bone has been increasing.15,24

The clinical impression that some malocclusion groups may have the tendency toward increased periodontal problems has been prevailing. For example, patients with skeletal Class III malocclusion appear to have greater gingival recession in the mandibular incisors, and we found in the previous study that the mandibular incisors had greater bone loss than the maxillary incisors in skeletal Class III patients.5 Also, studies show a detrimental effect on the periodontium of crowded teeth, although some studies have reported no effect.25–27 In this study, using the Korean normal occlusion sample data (group 1), we aimed to assess the bony architecture around incisors and to compared it with that of skeletal Class III malocclusion patients (group 2). Results show that group 2 had significantly greater bone loss in all areas of measurement, except for posterior bone thickness of the maxillary incisors (P < .05, P < .001). In addition, alveolar bone thickness at the apex was significantly thinner in group 2 than in group 1 (P < .05), except at the maxillary incisors. Contrasting studies have investigated the overestimation23 or underestimation11,28 of alveolar bone measurement by CBCT; significant differences between the two groups in this study affirm the difference.

A CEJ-to-AC measurement of 2 mm or less is considered normal by earlier studies.29,30 Given this, in the maxillary incisors, vertical bone loss was not severe (around 2.0 mm range) in both groups, and thickness at the apex appears to be sufficient for orthodontic tooth movement in both groups. Typically, bone at the palatal side of the maxillary incisors is very thick and wide (Table 4). In contrast to this, for the mandibular incisors, group 2 showed statistically significant greater bone loss (greater than 2 mm), and bone loss was greater than 30% on the mandibular lingual side (Table 5 and Figure 4). Bone thickness at the mandibular incisor apex was only 81.33%, which is less than 100%, and it appears to present a challenge during presurgical orthodontic treatment when labioversion of the mandibular incisors is planned; special attention should be paid. This proves that group 2 had a so-called thin symphysis, and mandibular incisors were at risk if moved uncontrolled. In addition, mandibular incisors showed greater alveolar bone loss than maxillary incisors in both groups, especially on the lingual side in group 2 (P < .05); thus more attention should be paid to the mandibular incisors when a large amount of tooth movement is planned, and patients should be informed. Also, it may be prudent to modify the treatment plan to assist with orthognathic or periodontal surgery; periodic periodontal care may be essential.

The question of whether well-planned presurgical orthodontic treatment or conventional orthodontic treatment may cause bone loss was not addressed in this study. Although the amount of bone loss reported in the previous study was greater, the specific cause of this is not certain, because samples in the previous report were taken from presurgical Class III patients, regardless of the duration of orthodontic treatment, the presurgical status of the crowding, the amount of tooth movement, oral hygiene status, and so forth.5 Future studies should address the association of bone loss and orthodontic treatment with larger sample size. Studies with other malocclusion groups with potentially high risk of bone loss may also be necessary.

CONCLUSIONS

• The hypothesis is rejected. Vertical alveolar bone loss was greater in the skeletal Class III subjects than in the normal occlusion samples. In both groups, vertical bone loss was more severe in the mandibular incisors than in the maxillary incisors.

• Alveolar bone thickness at the tooth apex was significantly greater in the normal occlusion sample than in skeletal Class III patients, except for the labial side of the maxillary incisors. In the mandibular incisors of skeletal Class III patients, the buccolingual alveolar bone thickness at the apex was narrower than the CEJ width.

• Skeletal Class III patients with anterior open bite may be more prone to bone loss; special care should be paid to avoid aggravating preexisting bone loss during orthodontic tooth movement.