INTRODUCTION

Surgical orthodontic treatment of Class III patients includes presurgical orthodontic decompensation of malocclusion, followed by surgical correction of the skeletal discrepancy and postsurgical detailing and finishing of the occlusion. Typically, the orthodontic decompensation of a Class III malocclusion requires uprighting the proclined maxillary incisors and retroclining the mandibular incisors to more normal axial inclinations.

For the purpose of dental decompensation in presurgical orthodontic treatment, the alveolar bone around the incisors should be considered. Fenestration of the alveolar bone and stripping of the gingiva become increasingly likely as mandibular incisors are proclined.1 From clinical observation, it appears that in patients with mandibular prognathism, the occurrence of alveolar bone loss or fenestration is more common in the lower anterior teeth. Previous studies have shown that more proclined teeth compared with less proclined teeth or untreated teeth, along with movement of the incisors out of the osseous envelope of the alveolar process, might be associated with a higher tendency to develop gingival recessions.2–4 However, few studies on alveolar bone change in mandibular prognathism patients undergoing orthognathic surgery are available. Wehrbein et al.5 evaluated the alveolar bone and symphysis complex of a deceased patient who had undergone orthodontic treatment and found severe bone loss on the labial and lingual cortical plates. With cone-beam computed tomography (CBCT) technology, it is now possible to acquire accurate radiographic images that allow clinicians and researchers to quantitatively evaluate bone changes in three dimensions with minimal distortion and lower radiation dosages. Indeed, CBCT is a useful and more practical clinical tool than digital subtraction radiography for assessment of changes in periodontal bone over time.6,7 Using CBCT data, Kim et al.8 studied alveolar bone loss around the maxillary and mandibular incisors in surgically treated skeletal Class III malocclusion patients. They did not report, however, on alveolar bone change around the mandibular incisors before and after the surgical orthodontic treatment.

The purpose of the present study, therefore, was to quantitatively evaluate and compare alveolar bone change around the lower incisors during surgical orthodontic treatment in individuals with mandibular prognathism.

MATERIALS AND METHODS

This is a retrospective study. The subjects consisted of 25 patients (13 men, 12 women; mean age 26.3 ± 2.7 years) who presented with jaw deformities diagnosed as skeletal Class III mandibular prognathism. All of the subjects underwent presurgical and postsurgical orthodontic treatment and mandibular setback sagittal split ramus osteotomy with rigid internal fixation, and none of them had the genioplasty during the surgery. This study was reviewed and approved by the Ethics Committee at Pusan National University Hospital.

Inclusion criteria were as follows: bilateral Class III molar relationship, ANB degree of less than 0°, lack of severe facial asymmetry (less than 3 mm of chin point deviation from the facial midline),9 crowding in the lower arch of less than 3 mm, bracket prescription with 0.022-inch straight-wire appliance of Roth setup and fully bonded to the second molars, final archwire with 0.019 × 0.025-inch stainless-steel wire, and no use of intrusion archwire in the lower arch.

Exclusion criteria were cleft lip/palate or other craniofacial syndrome patients, missing teeth (except for the third molars), spacing, or tooth size anomaly.

Definition of Reference Points and Measurements of Lower Incisors on CBCT Image

Reference points and measurements are indicated and described in Figure 1 and Tables 1 and 2. The measurements were modifications of those obtained in the report by Handelman10 and Beckmann et al.11 The alveolar crest was defined as the most coronal level of the alveolar bone.12 The distances between the alveolar crest and the cementoenamel junction were measured at the labial and lingual surfaces of the left lower central incisor and lateral incisor, parallel to the long axis of the tooth. This represented the extent of vertical alveolar bone loss. Measurements of alveolar bone thickness at the midroot and root apex were made from the midroot and root apex to the limit of the alveolar cortex, respectively, perpendicular to the long axis of the tooth, and these were recorded as horizontal bone thickness at midroot and horizontal bone thickness at root apex. In addition, the root length and the incisor inclination to the mandibular plane were assessed. To evaluate the incisor inclinations on the same mandibular plane before treatment (T0), after presurgical orthodontic treatment (T1), and after debonding (T2), the CBCT data were superimposed by the new superimposition method.

View Figure

• Download as Powerpoint
• Download Figure

Table 1 Definitions of Reference Points Used in This Study

View Fullscreen

Table 2 Definitions of Measurements Used in This Study

View Fullscreen

The new superimposition method applied the mutual information theory to automatic voxel-by-voxel registration, with subvoxel accuracy.13 The mandibular symphysis region was used as the rigid registration area for evaluation of the inclination on the same mandibular plane. With these measurements, the ratio of the vertical bone level to the root length was calculated (Figure 2).

View Figure

• Download as Powerpoint
• Download Figure

RESULTS

The systematic intra-examiner error, evaluated at P < .05, was found to be statistically insignificant. The ICC measurement, showing a mean of 0.926 (ICC  =  .84–.95), indicated excellent reliability. Table 3 shows the means and standard deviations for cephalometric measurements at T0, T1, and T2 for all subjects. Significant differences were found in all measurements. The means and standard deviations for the alveolar bone change of the lower central incisor and lateral incisor at T0, T1, and T2 are shown in Tables 4 and 5.

Table 3 Cephalometric Measurements at T0, T1, and T2 for All Subjects^a

View Fullscreen

Table 4 Comparison of Alveolar Bone Change of Lower Central Incisor During Surgical Orthodontic Treatment^a

View Fullscreen

Table 5 Comparison of Alveolar Bone Change of Lower Lateral Incisor During Surgical Orthodontic Treatment^a

View Fullscreen

Relationship Between Incisor Inclination and Alveolar Bone Change

There was no statistically significant correlation between the degree of incisor inclination and the extent of alveolar bone change of the central incisor and lateral incisor (Table 6).

Table 6 Pearson and Spearman Correlation Coefficients Between Incisal Inclination and Alveolar Bone Change^a

View Fullscreen

DISCUSSION

This study focused on the changing pattern of the alveolar bone of the lower central and lateral incisors at presurgical and postsurgical orthodontic treatment. Because of decompensation of the lower incisors during presurgical orthodontic treatment, alveolar bone loss is more common in the lower anterior teeth in individuals with mandibular prognathism.

Alveolar bone support is important for periodontal health; specifically, it is essential to the stability of anterior teeth and thus to acceptable esthetics. Optimal stability is considered to be achieved when the incisors are positioned in the medullary portion of the alveolar bone and in good balance with the labial and lingual musculature.14–16 The attempt to identify an orthodontically ideal, stable and equilibrated, and esthetically pleasing incisor position that will not cause periodontal problems, future articular pathologies, or crowding relapse has entailed efforts to determine their anterior-most limit.17 The mandibular symphysis is the anatomic structure that limits the movement of those incisors, and the bony thickness of the lower anterior teeth is thin and susceptible to periodontal disease.18 This structure has to be considered in skeletal Class III patients with mandibular prognathism, as the lower incisors are inclined lingually as the result of dental compensation and move forward in the course of presurgical orthodontic treatment.

Orthodontists must keep in mind that the cortical plates of the palate and symphysis as traced from cephalometric radiographs present a two-dimensional view of a concave surface. Thus, the actual limits of the palate and symphysis at the midline might be narrower than those images indicate.10 High-definition CT permits close examination of incisors' labiolingual osseous support without the disadvantages of conventional radiography. These images are not subject to distortion or superimposition. Moreover, secondary computerized reconstructions facilitate quantitative and qualitative evaluation of bone surfaces, quantitative evaluation of the relationship between teeth and bone, and selection of the desired sections. Fuhrmann et al.19 showed that quantitative evaluation of alveolar bone plates with CT is accurate to a minimum bone thickness of 0.5 mm. Whereas conventional dental radiographs do not allow for evaluation of dehiscence sites, CBCT findings have proven to be statistically similar to histologic measurements.19 Considering this, we used CBCT measurements to more accurately evaluate alveolar bone changes.

In the present study, the presurgical orthodontic treatment period was 21.4 ± 3.2 months on average; alveolar bone loss of the lower central incisor was significantly advanced, except for one measurement: the horizontal bone thickness of the root apex on the labial side. And in the case of the lower lateral incisor, alveolar bone loss was significantly incurred, except for two measurements: the horizontal bone thickness of the root apex on the labial side and that of the midroot on the lingual side. This suggests that the movement pattern of the lower incisors during presurgical orthodontic treatment was labial tipping, not bodily movement. The postsurgical orthodontic treatment period was 7.2 ± 1.3 months on average; the alveolar bone loss was not significantly deteriorated during this period. As a result of multiple comparison tests, the vertical alveolar bone was gain in the crown third of the root of the central incisor and the crown third of the lingual side root of the lateral incisor. But the total extents of alveolar bone gain were small, less than 1.0 mm, not enough to be the original state. This should be considered, and patients should be informed, prior to surgical orthodontic treatment, especially in skeletal Class III cases.

There was no statistically significant correlation between the degree of incisor inclination and the extent of alveolar bone change. Alveolar bone change is related to biomechanical phenomena and is influenced by many factors, including the periodontal environment, the gingival type, the patient's oral habit, and others.20 Thus, it might be possible that the extent of alveolar bone change is not mathematically or directly correlated with the degree of incisor inclination. In this study, we excluded patients with severe periodontal disease and perio-susceptible patients. However, previous studies suggested that excessive labial and lingual movement of the maxillary and mandibular incisors should be avoided so as to prevent irreversible bone loss and less bone support for teeth.21–23 Several factors such as crowding, curve of Spee and the amount of tooth movement during treatment might play a role in the bone loss process. In our study, we considered the amount of incisor inclination change because the results of many factors were reflected in incisor inclination change.

The incidence of bone loss and root resorption in adult orthodontic patients is, in general, at a clinically acceptable level.24 Even so, the present study showed a significant decrease of root length, about 1.19 mm of the mean differences. We also evaluated the ratios of labial and lingual vertical alveolar bone loss (%VBL and %VBL′), which are dependent on the root length as well as the amount of bone loss.

To minimize the alveolar bone loss during the presurgical orthodontic treatment period, the lower incisors are moved or advanced within the alveolar bone housing.

With the results of this study, excessive forward movement of the lower incisors should be reconsidered according to the patient's anatomic limits and periodontal health, and patients so informed, prior to surgical orthodontic treatment, particularly in the case of skeletal Class III patients. In addition, further long-term studies at least 6 months out of retention are still necessary.

CONCLUSION

• Based on the results, excessive forward movement of lower incisors during presurgical orthodontic treatment could cause alveolar bone loss around the lower incisors; thus, special care should be considered in individuals with mandibular prognathism.