Introduction
The objective of this research was to study the factors associated with the alveolar bone depth mesial to the mandibular third molars (M8) after the mandibular second (M7) and third molars were protracted into the space of the mandibular first molars (M6), which were newly extracted for orthodontic treatment or extracted more than 1 year before treatment.
Methods
This retrospective study included 57 adult patients (mean age 23.40 ± 4.40 years) in whom M6 were newly extracted for orthodontic treatment or extracted more than 1 year before treatment. The alveolar bone depth mesial to M8 was measured on posttreatment panoramic radiographs. The vertical, horizontal, and angular changes of M8 were measured on both pre- and posttreatment panoramic radiographs. Linear correlation and regression analyses were conducted to explore the factors associated with the alveolar bone depth mesial to M8.
Results
The alveolar bone conditions of M6 ( R = −0.391, P <0.001) and the vertical movement directions of M8 ( R = −0.433, P <0.001) were significant factors associated with the alveolar bone depth mesial to M8 after orthodontic protraction.
Conclusions
Without considering the pretreatment periodontal status of M8, patients with M6 extracted exceeding 1 year before treatment and with M8 extruded after orthodontic protraction may exhibit deeper alveolar bone depth mesial to M8.
Highlights
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Extrusion of M8 and alveolar bone conditions of M6 were closely related to mesial alveolar depth of M8 after treatment.
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Protraction of M7 and M8 is an option for patients with extracted M6.
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Periodontal condition of M8 should be considered when treatment planning.
The mandibular first molars are the earliest permanent teeth that erupt out into the mouth. They are susceptible to some dental diseases caused by poor oral hygiene and some other reasons. Edentulous space caused by mandibular first molar loss is a common problem for clinicians. Treatment options involve movable or fixed dentures, dental implants, and orthodontic treatments. However, prosthetic treatment can sometimes be problematic because of supra-eruption of the opposing teeth or tipping of the adjacent molars. Furthermore, the longevity of the prosthetic restorations or autotransplanted teeth is questionable. By contrast, orthodontic treatment can overcome the above shortcomings and deal with other malocclusion issues at the same time; thus, it may be the priority choice among the alternative options.
However, orthodontic treatments that involve protracting the second and third molars into the edentulous space of the first molar may be associated with some adverse effects. The potential effects include external apical root resorption or resorption of the alveolar bone. , Baik et al studied the space closure of missing mandibular first molars or second premolars by orthodontic protraction in 51 young patients and concluded that mandibular second molars may exhibit alveolar bone resorption in the distal root, while the mesial alveolar bone level of the third molars was within normal ranges. The findings of this article prompted the following questions: what are the alveolar bone levels mesial to the mandibular third molars after orthodontic protraction in adults? What are the related factors that are associated with them?
Therefore, the objective of this study was to evaluate the factors associated with the depth of the alveolar bone mesial to the mandibular third molars after orthodontic protraction of the second and third molars into the extracted first molar space.
Material and methods
The study sample included patients who were treated at the Department of Orthodontics, Hospital of Stomatology from July 2009 to July 2019. All patients were treated with fixed appliances using the straight wire arch technology. Fully erupted M8 were bonded early in the treatment and moved forward, while partially erupted M8 were later bonded when the crowns were ready for bracket bonding. The wire was gradually replaced from 0.012 nickel titanium wire to 0.019 × 0.025 nickel titanium wire. The edentulous space was closed by intramaxillary and intermaxillary elastic traction on the whole fully aligned dentition. The study was conducted in accordance with the Declaration of Helsinki. All patients signed the informed consent form. The inclusion criteria were as follows: (1) patients were older than 18 years with no congenital missing teeth or supernumerary teeth; (2) M6 were newly extracted for orthodontic treatment or extracted exceeding 1 year before treatment; (3) M7 and M8 were present on the same side of the M6 edentulous space; (4) M8 were partial or fully erupted with the same inclinations and fully developed roots regardless of the root numbers; (5) the patients had good general condition and no active periodontal disease; and (6) there were clear pre- and posttreatment panoramic radiographs with no obvious distortions.
The exclusion criteria were as follows: (1) malformation of M8; (2) another tooth extracted in addition to M6 on the same side; (3) spontaneous space closure in the M6 area owing to a long time of absence; (4) failure to fully close the M6 space after treatment; (5) failure to align M8 into a normal occlusion after treatment; (6) previous fixed orthodontic treatment.
Panoramic radiographs were obtained using the same machine (Orthophos; Dentsply Sirona, Sirona, Germany) and collected from each patient at the start of treatment (T1) and on removal of the whole full-fixed appliance (T2). The panoramic radiographs at T1 and T2 were measured and calculated by a single investigator using Dolphin Imaging software (version 11.5; Dolphin Imaging and Management Solutions, Chatsworth, Calif). The landmarks and reference lines used in our study are shown in Figures 1 and 2 . The measurements are as follows: (1) mesial alveolar bone depth of M8 after treatment (MABD): distance between the cementoenamel junction (CEJ) and the most apical point of the mesial alveolar bone of M8 (P) on the posttreatment panoramic radiograph. The MABD indicates the degree of vertical resorption of the mesial alveolar bone of M8. The larger the value, the more heavily the alveolar bone was absorbed; (2) vertical changes in M8 (ΔV = V1−V2): distance between the CEJ and the intersection point of a line that is perpendicular to line 2 and passes through the CEJ (E) on the pretreatment (V1) panoramic radiograph minus that on the posttreatment (V2) panoramic radiograph. Positive values indicated extrusion movement of M8, while negative values indicated intrusion movement; (3) angular changes in M8 (Δα = α2−α1): angle of M8 on the posttreatment (α2) panoramic radiograph minus that on the pretreatment (α1) panoramic radiograph. Positive values indicated that M8 was upright after treatment, while negative values indicated that M8 leaned forward; (4) horizontal changes in M8 (ΔH = H2−H1): distance between center of the rectangle (D) and E on the posttreatment (H2) panoramic radiograph minus that on the pretreatment (H1) panoramic radiograph; (5) crown width of M7 (W7): distance between the proximal and distal contact points of the mandibular second molar.
The crown width standardizing method described in a previous study was used for calculating the magnification and distortion rate on the pre- and posttreatment radiographs. The correction factor was the ratio of the mesiodistal width of the mandibular second molar (M7) on dental casts to those on the pre- or posttreatment panoramic radiographs of each patient. The width of M7 on dental casts was measured with a digital caliper (Tricle Brand; Jinye, Taizhou, China; precision: 0.02 mm). All measurements were performed twice in a 2-week interval by the same investigator. The average of 2 measurements for each item was corrected by correction factor for statistical analysis.
Statistical analysis
The distribution types of all variables were explored by the Kolmogorov-Smirnov test. Normally distributed data ( P >0.05) are described by means and standard deviations (x ± s), and nonnormally distributed data are described by medians and quartile intervals (M ± Q). Mann-Whitney U tests were used to test the difference in age and treatment duration between sexes. Variables were classified into categorical and continuous types. Differences in various categorical variables and the mesial alveolar bone depth of M8 were evaluated with Mann-Whitney U tests and Student’s t test. Associations of different continuous variables and the mesial alveolar bone depth of M8 were evaluated with Spearman correlations. Finally, all possible related factors were incorporated into a multiple linear regression model. The alveolar bone depth mesial to the mandibular third molar was the dependent factor. Statistical analysis was carried out by using SPSS software (version 20.0; SPSS, Chicago, Ill). P values below 0.05 were considered statistically significant.
Results
A typical case in our study is shown in Figure 3 . A total of 71 pairs of mandibular second and third molars in 57 patients were protracted into the edentulous space. The patients ranged in age from 18.1 to 32.5 years, with an average of 23.4 ± 4.4 years. The treatment duration was 27.0 ± 9.0 months. No significant differences were found between the 2 sexes in terms of age and treatment duration ( Table I ).
| Characteristic | Male (n = 14) | Female (n = 57) | Total (n = 71) | P value |
|---|---|---|---|---|
| Pretreatment age, y | 22.15 ± 5.90 | 23.40 ± 4.50 | 23.40 ± 4.40 | 0.301 ∗ |
| Treatment duration, m | 26.00 ± 11.75 | 27.00 ± 9.50 | 27.00 ± 9.00 | 0.567 ∗ |
The differences in the MABD among various categorical and continuous variables are displayed in Tables II and III . Significant differences existed regarding the alveolar bone conditions of M6, the directions of vertical movement and the angular changes in M8 ( P >0.05). The results of Spearman correlations showed that the amounts of vertical movement ( r = 0.698, P <0.001) and angular change ( r = 0.281, P <0.05) were positively related to the MABD. The angle of M8 at the start of treatment was negatively related to the MABD ( r = −0.306, P <0.05).
