INTRODUCTION

The characteristics of periodontal tissue surrounding teeth are critical for periodontal health during and after orthodontic treatment. The anatomic integrity of periodontal tissue includes that of both soft and hard tissue structures. Soft tissue characteristics of the periodontium are usually defined by gingival biotype.¹ When the gingival biotype is thin, orthodontic tooth movements may be considered unfavorable, increasing the risk of gingival recession.² Melsen and Allais³ found significant correlations between keratinized gingival width, gingival biotype, and the development of or increase in gingival recession.

Alveolar bone is the hard tissue part of periodontium, which is a major remodeling site during orthodontic tooth movement. It has been suggested that a thin alveolar plate is an anatomic risk factor for the development of a periodontal problem, and tooth movement beyond the alveolar plate may cause vertical alveolar bone loss and subsequent gingival recession.^4–7

Depending on the vertical and sagittal skeletal relationship, there is a characteristic periodontal condition that is closely related with dental compensation. The occurrence of gingival recession might be more common, especially in the mandibular incisors, in patients with skeletal Class III malocclusion and mandibular prognathism because the teeth are inclined lingually and then are moved forward on the narrow alveolus during presurgical orthodontic treatment.^8–11

As three-dimensional cone-beam computed tomography (CBCT) is widely available, comprehensive integration of soft and hard tissue parameters of the periodontium is essential for optimized treatment planning. Unfortunately, it is not known which hard tissue factors are good predictors of soft tissue characteristics and vice versa. Recently, several studies have reported correlations between gingival biotype and labial plate thickness in individual teeth. However they have focused mainly on the buccal plate of maxillary anterior teeth.^2,12–16 To the best of the authors' knowledge, no studies have investigated the relationships between soft tissue biotypes and the buccolingual alveolar structures, particularly in patients with skeletal Class III malocclusion.

The aim of this study was to investigate associations between the periodontal soft tissue, alveolar bone, and dental parameters surrounding incisors at baseline in patients with skeletal Class III malocclusion.

MATERIALS AND METHODS

Periodontal Soft Tissue Examination

The condition of the maxillary and mandibular incisor periodontal soft tissues was evaluated using a periodontal probe. Gingival biotype was classified as either thin or thick based on the visibility of the underlying periodontal probe through the gingival tissue (Figure 1).¹⁷ Soft tissue measurements included gingival recession, probing pocket depth, keratinized gingival width and thickness (Table 1).

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Table 1. Definitions of Measurement

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CBCT Orientation and Periodontal Hard Tissue Measurement

CBCT scanning was performed via an Alphard Vega instrument (Asahi Roentgen Ind Co, Ltd, Kyoto, Japan). The following parameters were used: 80 kV, 5 mA, 0.39-mm voxel resolution, 17.0-second scan time, and a 199.68 × 199.68 mm field of view. CBCT images were reconstructed using InVivo-Dental software (version 5.3, Anatomage, San Jose, Calif).

To examine the morphologic features of the alveolar bone, each CBCT image was oriented along the long axis of the root and the sagittal plane running transversely through the midpoint of the incisal edge–root apex. The measurement variables used are described in Table 1 and Figure 2. Root length was defined as the distance from the CEJ to the root apex and was divided into four sections of equal length (where level 0 was the CEJ and level 4 was at the root apex), and the thickness of the alveolar plate was measured at four locations (levels 1, 2, 3, and 4). In addition, alveolar crest height and thickness, alveolar plate area, and tooth inclination relative to the basal bone or alveolar ridge were evaluated.

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RESULTS

Descriptive Data Assessment

The initial cephalometric data showed a skeletal Class III relationship (ANB –3.96° ± 3.66°), normal to hyperdivergent vertical pattern (MPA 30.81° ± 4.42°), and dental compensations (U1 to FH 121.92° ± 5.63°; IMPA 80.31° ± 7.04°).

The means and standard deviations of all variables are shown in Table 2. The difference between buccal and lingual vertical bone loss was only statistically significant in the maxillary incisors (P < .01; Figure 3). The buccal plate of the maxillary incisors was thinner than the lingual plate at all positions except at the alveolar crest level (all P < .001, except level 1, P <.05). In the mandibular incisors, the thickness of the buccal plate was greater than that of the lingual plate at the alveolar crest level and level 1 (P < .01 and P < .05, respectively), whereas the lingual alveolar plate was thicker than the buccal alveolar plate at levels 2, 3, and 4 (P < .01, P < .001, and P < .001, respectively).

Table 2. Descriptive Data for Measured Variables

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Factor Analysis of CBCT Measurements

Using factor analysis, three hard tissue factors were derived based on eigenvalue greater than one, explaining 74.71% of the variance of the original 20 variables (Table 3). Each factor was a combination of the original variables representing the vertical position of the alveolar crest and the thickness of the alveolar plate on the buccal and lingual aspects. Correlations can be summarized as follows (Table 4):

• Factor 1 (lingual plate factor): positively correlated with lingual alveolar plate thickness and total alveolar ridge thickness at all heights and negatively correlated with vertical bone loss of the lingual plate.

• Factor 2 (coronal-buccal plate factor): positively correlated with buccal alveolar crest thickness and buccal alveolar plate thickness (levels 1 and 2) and tooth inclination to basal bone and negatively correlated with vertical alveolar bone loss at the buccal surface.

• Factor 3 (apical-buccal plate factor): positively correlated with buccal plate thickness (levels 3 and 4) and tooth inclination to the alveolar ridge.

Table 3. Variance Explained by the Three Extracted Factors

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Table 4. Factors Extracted Using Varimax Rotation

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Correlations Between Soft Tissue Parameters and Hard Tissue Factors

Correlations between soft tissue parameters and hard tissue factors are shown in Table 5. Coronal buccal plate factor was positively correlated with keratinized gingival width (P < .001) and keratinized gingival thickness (P < .01). Conversely, apical buccal plate factor was negatively correlated with keratinized gingival width (P < .05) and keratinized gingival thickness (P < .001). Lingual plate factor was not significantly correlated with any soft tissue parameters. Gingival recession and probing pocket depth at baseline were not significantly associated with any hard tissue factors.

Table 5. Correlation Analysis Between Soft Tissue Parameters and Hard Tissue Factors

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Comparisons of Hard Tissue Parameters between Thick and Thin Gingival Biotypes

Differences in hard tissue parameters between thick and thin biotypes are shown in Table 6. No statistically significant differences in any of the variables in the maxilla were detected between the two biotypes. In the mandibular incisors, the thin biotype was associated with greater thickness of the buccal plate at the apex and third quarter of the root level, (P < .05 and P < .01, respectively) and on the lingual plate at the alveolar crest and first quarter of the root level (all P < .05). Mandibular incisors were more proclined to the alveolar ridge and basal bone in the thin biotype (P < .01 and P < .001, respectively).

Table 6. Comparisons of Hard Tissue Parameters Between Thick and Thin Gingival Biotype, Mean ± Standard Deviation

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DISCUSSION

In this study, the correlation between soft tissue characteristics and three hard tissue factors extracted from factor analysis of the periodontium in patients with Class III malocclusion was reported. Because of the thin buccal plate overlying the root surface, the variables defining total alveolar ridge thickness and lingual plate thickness were grouped together as a single factor, referred to as the “lingual plate factor.” However, no significant association was found between the lingual plate factor and buccal soft tissue parameters.

The variables derived from buccal alveolar plate measurements were grouped into two collective factors: “coronal-buccal plate factor” and “apical-buccal plate factor.” Coronal-buccal plate factor was a combination of vertical and horizontal dimensions in the coronal part of the buccal plate of incisors and tooth inclination to basal bone. Apical-buccal plate factor included the thickness of the buccal plate in the apical region and tooth inclination to the respective alveolar ridge. Tooth inclination to basal bone and to the alveolar ridge was included in different hard tissue factors. Tooth inclination to basal bone reflects the characteristics of the coronal part by measuring the angle around the root apex, while tooth inclination to the alveolar ridge reflects the characteristics of the apical part by measuring the angle around the CEJ.

As expected, gingival morphology, including keratinized gingival width and thickness, was associated with the buccal alveolar plate overlying the incisors in patients with skeletal Class III malocclusion. The correlation between soft tissue and hard tissue in the mandibular incisors of patients with Class III malocclusion were opposite in the coronal and apical parts of the buccal plate. Keratinized gingival width and thickness were positively correlated with the coronal-buccal plate factor but negatively correlated with the apical-buccal plate factor. It may be advisable to evaluate the periodontal tissue before treatment by dividing the region into the coronal-buccal part and the apical-buccal part in the mandibular incisors of patients with skeletal Class III malocclusion.

On the other hand, gingival recession and probing pocket depth before orthodontic treatment were not significantly associated with hard tissue factors. In a study by Fu et al.,¹³ there was no significant association between gingival recession and bone morphology, and the conclusion was that, rather than gingival recession at baseline being strongly associated with tissue biotype or underlying bone thickness, it was more dependent on multiple factors, including age, etiology, and tooth type.

Previous CBCT studies^18–22 investigating facial bone thickness in various regions of the maxilla and mandible have found that a thin alveolar bone wall is usually present in both jaws. In the current study, at the first quarter of the root level, the thickness of the buccal and lingual alveolar plates was less than 1 mm in 81.5% and 87.0% of the lower incisors, respectively. The amount of vertical bone loss was greater on the buccal side of maxillary incisors and on the lingual side of mandibular incisors due to the dental compensation in skeletal Class III malocclusion. Substantial anteroposterior movement of the mandibular incisors during orthodontic treatment might be critical and could lead to progressive bone loss of the buccal and lingual plates.

In the current study, there were significant differences in underlying bone structure between thick and thin biotypes in the mandible but not in the maxilla. In the mandibular incisors, the apical part of the buccal plate and the coronal part of the lingual plate were thicker in the thin biotype. In addition, more proclination was found in mandibular incisors in the thin gingival biotype. In agreement with the results of previous studies,^14,23 the more proclined the mandibular incisor was, the greater the bone thickness of the labial alveolar plate at the apex level tended to be. It could be suggested that tooth inclination affects marginal bone thickness and gingival biotype in the mandibular incisor regions in patients with skeletal Class III malocclusion. Although patients with Class III malocclusion and a thick biotype have thicker marginal bone on the buccal side, the amount of decompensation during presurgical treatment would also be greater than in patients with a thin biotype. Therefore, it cannot be concluded that the risk of gingival recession is low in the thick gingival biotype during orthodontic treatment in patients with skeletal Class III malocclusion.

This study evaluated soft tissue and hard tissue structures before orthodontic treatment in Korean patients with Class III malocclusion. The 0.39-mm voxel size of the CBCT system may have limited the ability to accurately measure the thin alveolar plate due to low contrast resolution. Further studies in other populations and with various skeletal malocclusions would be necessary to apply the results beyond this sample. In addition, it would be interesting in future studies to investigate changes in the parameters studied over the course of orthodontic treatment.

CONCLUSIONS

• Hard tissue parameters of the periodontium were categorized as coronal-buccal, apical-buccal, and lingual plate factors.

• Keratinized gingival width and thickness were positively correlated with the coronal-buccal plate factor but negatively correlated with the apical-buccal plate factor.

• Morphologic differences in hard tissue between thick and thin gingival biotypes were observed in the lower incisors. In the thin biotype, mandibular incisors were more proclined, and the apical part of the buccal plate and the coronal part of lingual plate were thicker.