Introduction
The primary objective of the study was to quantitatively analyze the width and height of the mandibular buccal shelf area (MBS) at 3 different potential locations for mini-implant placement. In addition, we aimed to compare and contrast the bone parameters of the MBS to study the correlation between different growth status (growing or nongrowing), facial types (hypodivergent, normodivergent, and hyperdivergent), and sex differences (male or female).
Methods
In this retrospective cone-beam computed tomography study, 678 subjects were included. They were divided into groups according to growth status, facial type, and sex. Scans were imported into the reconstruction program and were aligned in 3 different steps. Measurements were made at 6 different coronal sections: mandibular first molar distal root, second molar mesial root, and second molar distal root (bilaterally). The roots of mandibular molars were used as a reference to measure the width and the roof of the inferior alveolar canal to measure the height of the buccal shelf area. Intraobserver reliability was assessed by measuring the width and height of MBS in 20 randomly selected subjects.
Results
No significant difference ( P > 0.05) was found in the width of MBS between males and females. MBS width increased, and height decreased ( P < 0.0001) as moved distally from the first molar distal root to the second molar distal root in all 3 facial types irrespective of age or sex. The hypodivergent facial type had significantly greater bone width than the hyperdivergent facial type at all the 3 locations in both males and females. The hypodivergent facial type had significantly less ( P < 0.0001) bone height than the hyperdivergent group at all the 3 locations irrespective of age or sex.
Conclusions
The optimal site for MBS mini-implant is the buccal region of the distal root of mandibular second molars. Hypodivergent patients have more width and less height of MBS compared with hyperdivergent patients. MBS mini-implants are not advised for growing patients because of proximity to developing roots.
Highlights
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Mandibular buccal shelf mini-implant placement was investigated in patient groups.
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The optimal site was the buccal region, distal root, of mandibular second molars.
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Buccal bone width has a negative correlation with the age of the subjects.
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Buccal bone height has a positive correlation with the age of the subjects.
Anchorage control is the key to successful orthodontic treatment. The use of mini-implant to obtain absolute anchorage has been demonstrated in clinical orthodontics and has had promising results. During the past decade, mini-implant has been successfully used to obtain absolute anchorage in varied malocclusion. However, a large body of evidence shows a failure rate of mini-implants between 14% and 20%, and the success rate of mini-implant has been positively correlated with the anatomy of the insertion site. In addition, it has been shown that the mini-implants placed in the palate have a higher success rate compared with interradicular mini-implants placed in the maxilla and the mandible. ,
Because of ease of placement and application of direct orthodontic force, maxillary and mandibular interradicular sites are still the preferred locations for the placement of mini-implants. However, the interradicular mini-implants in the posterior zone (behind the first premolar) of the mandible has the highest failure rate of about 20%-29%. Furthermore, these interradicular mini-implants may damage the roots of the teeth at the site of placement and may also interfere with the desired orthodontic tooth movement (eg, distalization of the mandibular dentition).
To overcome these potential limitations, orthodontists have started placing mini-implants in the palatal bone of the maxilla and the mandibular buccal shelf (MBS) in the mandible. The bone quality and quantity in the palatal area of the maxilla have been extensively studied in both the sexes to decrease the chances of failure of the mini-implant. However, there is a limited number of studies with a very small sample size looking at the bone quality and quantity of the MBS location.
The major determinant for the success of the mini-implant is the bone stock surrounding it; therefore, it is extremely important to study the bone parameters (bone width and bone height) at the MBS location. The primary objective of this study was to quantitatively analyze the width and height of the buccal shelf area at 3 different potential locations for mini-implant placement in relation to mandibular first and second molars. In addition, we aimed to compare and contrast the bone parameters (bone width and bone height) of the MBS in males and females in 3 different facial types (hypodivergent, normodivergent, and hyperdivergent). Our null hypothesis is that the bone parameters (bone width and bone height) at the MBS locations is not different between males and females in all the 3 different facial types.
Material and methods
An institutional review board exemption was obtained for evaluating cone-beam computed tomography (CBCT) volumes, archived in the Department of Oral and Maxillofacial Radiology. This retrospective study reviewed 678 CBCT scans of patients who were referred for orthodontic treatment. All CBCT scanned images were deidentified for Protected Health Information by authorized personnel from the Department of Oral and Maxillofacial Radiology before using them as a part of the study subject. CBCT scans were acquired using the i-CAT Next Generation (Imaging Sciences International, Hatfield, Pa) CBCT unit. A standardized protocol of the i-CAT for the extended (17 × 23 cm) field of view with 0.3-mm slice thickness and 26.9-second acquisition time was used. All scans were saved in the Digital Imaging and Communications in Medicine–3 format and were evaluated using a third party CBCT reconstruction software InVivo5 (version 5.0; Anatomage, San Jose, Calif).
On the basis of the growth status, the scans were divided into 2 primary groups: (1) growing, and (2) nongrowing. Cervical Vertebral Maturity Index (CVMI) was measured to evaluate the skeletal age of the study subjects. Lateral cephalograms generated from CBCT volumes using the SuperCeph module of InVivo5 software were used for CVMI and cephalometric measurements. Group-specific CVMI is reported in Figure 1 . The scans were further divided into 3 groups: (1) hyperdivergent, (2) hypodivergent, and (3) normodivergent. Angular and linear cephalometric measurements were taken, and categories were determined using the following cephalometric measurement parameters: (1) facial height index: the ratio of the posterior facial height to the anterior facial height using the measurements of sella (S) to gonion (Go) divided by the distance of nasion (N) to menton (Me); (2) mandibular plane angle: the angle between the anterior cranial base (sella to nasion) and the mandibular plane (formed by menton to gonion) and; (3) Frankfort mandibular plane angle: the angle between Frankfort horizontal (porion to orbitale) and the mandibular plane (menton to gonion). For the facial height index, a ratio of <61%, 61% to 69%, and >69% indicated increased, normal, and decreased facial heights, respectively. Concerning the mandibular plane angle, angles of <21°, 21° to 29°, and >29° indicated decreased, normal, and increased facial heights, respectively. For Frankfort mandibular plane angle, angles of <27°, 27° to 37°, and >37° indicated decreased, normal, and increased facial heights, respectively. If at least 2 of the 3 measurements were not in agreement regarding the category of decreased, normal, or increased facial height, then those scans were excluded from the study. These groups were again subdivided into 2 subgroups on the basis of sex (1 female, 2 male) ( Table I ; Fig 1 ). The exclusion criteria were set as follows, cause these might have affected the buccal shelf area thickness and height: (1) patients with the extraction of either mandibular first or second molars; (2) patients with implant or pontics replacing either first or second mandibular molars; (3) CBCT scans showing supernumerary teeth, enlarged cystic follicle, or any other pathology; (4) CBCT scans showing impacted teeth in the area of interest; and (5) patients have periodontal disease, orthognathic surgery, or any genetic syndromes. These inclusion and exclusion criteria were based on the article published by Nucera et al.
| Groups | Hyperdivergent | Hypodivergent | Normodivergent |
|---|---|---|---|
| Growing female | |||
| Mean | 14 y 7 mo | 14 y 5 mo | 14 y 4 mo |
| SD | 1 y 5 mo | 1 y 6 mo | 1 y 3 mo |
| Growing male | |||
| Mean | 14 y 9 mo | 14 y 5 mo | 14 y 2 mo |
| SD | 1 y 4 mo | 1 y 2 mo | 1 y 1 mo |
| Nongrowing female | |||
| Mean | 28 y 7 mo | 28 y 10 mo | 27 y 1 mo |
| SD | 8 y 3 mo | 8 y 4 mo | 8 y 6 mo |
| Nongrowing male | |||
| Mean | 28 y 8 mo | 27 y 3 mo | 28 y 1 mo |
| SD | 8 y 7 mo | 6 y 8 mo | 9 y 10 mo |
All CBCT volumes were imported into InVivo5 (version 5.3) software, and a single examiner (V.G.) reviewed all the scans independently. The investigator (V.G.) reviewed the images on a split-screen dual display monitor (HP Compaq LA2205wg; HP, Inc, Palo Alto, Calif) under standardized conditions of ambient light and sound. The investigator had the full capability to evaluate the volumes in all the 3 orthogonal planes and manipulate contrast and histogram ( Fig 2 ).
Once the scans were imported into the reconstruction program, all scans were aligned using 3 steps ( Fig 2 ). Step 1: scans were aligned in the field of view on axial sections at the level of the mandibular first molar furcation area. Step 2: as our primary objective was to evaluate the safe zone for mini-implant placement, we considered mandibular molar roots as a reference to calculate the width and an inferior alveolar canal (IAC) for the height to measure the buccal shelf area. Thus, all measurements were taken after aligning the sagittal section to get the specific molar root (a distal root of the first molar or second molar roots) in an upright position. Furthermore, an IAC was traced using a software tool on an orthopantomographic image ( Fig 3 ). The roof of the IAC was used as the reference point to measure the height of buccal self-area in this study. Step 3: all the measurements were made at 6 different coronal sections as follows: mandibular left first molar distal root, mandibular left second molar mesial root, mandibular left second molar distal root, and same for the opposite side. Once the center was established on all the 3 planes (axial, sagittal, and coronal) using the toggled crosshairs in the program, buccal shelf area width and height were measured in a coronal plane at 3 different levels bilaterally as described above.
For measurements, a reference point (point A) was marked on the buccal root surface at the junction of coronal 2/3 and apical 1/3. The horizontal distance from point A to the buccal most point (point B) of the buccal cortex was measured. This AB line represents the width of the buccal shelf area in the study. Furthermore, a horizontal reference plane (CD) was drawn from the roof of the IAC traced earlier. Then, a perpendicular was dropped from the AB plane to this CD plane, and this distance represents the height of the buccal shelf area ( Fig 4 ). The same procedure was repeated for all 6 coronal positions of each scanned image ( Fig 5 ). In addition, for a situation like a severely lingual tilted tooth with roots angulated more buccally or buccally curved roots, A′ point was taken for reference instead of point A. Point A′ is the projection of point A at the same horizontal level, on a vertical line drawn from the buccal most point of a root. A′B distance is dictated as the width of the buccal shelf area in those kinds of situations. To test the intrarater reliability, we measured 20 randomly selected scans 4 weeks later by the same person, both for the height and width of the buccal shelf area.
Statistical analysis
Simple descriptive statistics were used to summarize the data. Mean, standard deviation, and percentile distributions were computed for bone width (first molar distal root, second molar mesial root, and second molar distal root) and bone height (first molar distal root, second molar mesial root, and second molar distal root) for (1) hyperdivergent, hypodivergent, and normodivergent; (2) growing and nongrowing; and (3) males and females. For all the outcomes, interexaminer reliability was computed by Cronbach alpha values (intraclass correlation coefficients). A 1-sample Kolmogorov-Smirnov test was used to examine the normality of bone width distribution and bone height at different locations. All the measurements were normally distributed. For bone width and bone height at different locations, analysis of variance, and Tukey multiple comparisons were used. All statistical tests were 2-sided, and a P value of <0.05 was deemed to be statistically significant. Statistical analysis was computed using GraphPad Software (La Jolla, Calif).
Results
A total of 678 patients were included in the study. Group-specific mean and standard deviation of the study subjects’ age are shown in Table I ( Fig 1 ). Cohen kappa was 0.91 for intraexaminer reliability.
Distribution of bone width is summarized in Table II for growing (CVMI, 3.64 ± 0.75) and nongrowing (CVMI, 5.82 ± 0.39) female and male patients. The sex comparison showed no significant difference ( P > 0.05) in the bone width between females and males. Bone width significantly increases ( P < 0.0001) as we move distally from the first molar distal root to the second molar distal root in all the 3 different facial types in both females (growing and nongrowing) and males (growing and nongrowing) ( Figs 6-9 ; Table II ). The growing female (CVMI, 4.09 ± 0.51) and growing male (CVMI, 3.23 ± 0.71) had significantly more bone width ( P < 0.002) than the nongrowing female (CVMI, 5.86 ± 0.35) and nongrowing male (CVMI, 5.77 ± 0.43), respectively, at the first molar distal root location in different facial types ( Table II ). The hypodivergent facial type has significantly more bone width than the hyperdivergent facial type at all the 3 locations in both males and females ( Figs 6-9 ; Table II ).
| Site by group | Hyperdivergent | Hypodivergent | Normodivergent | ||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Mean | SD | Min | Max | Percentiles | 95% CI | Mean | SD | Min | Max | Percentiles | 95% CI | Mean | SD | Min | Max | Percentiles | 95% CI | ||||||||||
| 25 | 50 | 75 | Lower | Upper | 25 | 50 | 75 | Lower | Upper | 25 | 50 | 75 | Lower | Upper | |||||||||||||
| Growing female | |||||||||||||||||||||||||||
| Md6D | 3.9 | 1.5 | 1.0 | 8.9 | 2.8 | 3.7 | 4.6 | 3.6 | 4.1 | 4.9 | 1.4 | 2.7 | 8.3 | 4.0 | 4.5 | 5.7 | 4.5 | 5.2 | 4.5 | 1.6 | 1.3 | 8.4 | 3.4 | 4.2 | 5.7 | 4.1 | 4.9 |
| Md7M | 5.9 | 1.6 | 2.0 | 10.4 | 4.9 | 5.9 | 7.2 | 5.7 | 6.2 | 6.6 | 1.5 | 1.8 | 9.6 | 5.4 | 6.6 | 7.7 | 6.2 | 6.9 | 6.6 | 1.3 | 4.1 | 9.8 | 5.6 | 6.6 | 7.5 | 6.3 | 6.9 |
| Md7D | 7.2 | 1.7 | 1.2 | 10.9 | 6.0 | 7.3 | 8.4 | 6.9 | 7.5 | 7.9 | 1.6 | 4.5 | 12.2 | 6.8 | 7.9 | 8.9 | 7.5 | 8.3 | 7.4 | 1.5 | 4.0 | 10.6 | 6.2 | 7.3 | 8.5 | 7.0 | 7.7 |
| Growing male | |||||||||||||||||||||||||||
| Md6D | 4.3 | 1.9 | 0.8 | 10.3 | 3.0 | 4.2 | 5.4 | 4.0 | 4.6 | 5.8 | 1.7 | 2.2 | 9.7 | 4.7 | 6.3 | 6.9 | 5.5 | 6.2 | 4.7 | 1.6 | 1.9 | 8.2 | 3.5 | 4.5 | 6.1 | 4.4 | 5.1 |
| Md7M | 6.2 | 1.9 | 1.2 | 10.6 | 5.0 | 6.3 | 7.6 | 5.9 | 6.5 | 6.8 | 1.7 | 3.3 | 11.8 | 5.5 | 6.6 | 7.8 | 6.4 | 7.2 | 6.3 | 1.9 | 1.2 | 11.0 | 4.9 | 6.4 | 7.7 | 5.9 | 6.8 |
| Md7D | 7.0 | 1.9 | 2.2 | 11.4 | 5.8 | 7.4 | 8.5 | 6.7 | 7.3 | 7.9 | 1.6 | 4.1 | 11.8 | 6.8 | 8.0 | 8.8 | 7.5 | 8.2 | 7.2 | 1.9 | 3.1 | 12.2 | 6.1 | 7.2 | 8.4 | 6.7 | 7.6 |
| Nongrowing female | |||||||||||||||||||||||||||
| Md6D | 3.3 | 2.0 | 0.3 | 12.0 | 1.8 | 3.1 | 4.3 | 3.0 | 3.6 | 4.0 | 1.6 | 0.8 | 8.2 | 3.0 | 4.0 | 5.1 | 3.8 | 4.3 | 3.6 | 1.5 | 0.7 | 7.6 | 2.7 | 3.4 | 4.4 | 3.2 | 3.9 |
| Md7M | 5.8 | 2.0 | 1.8 | 12.0 | 4.2 | 5.8 | 7.0 | 5.5 | 6.0 | 6.7 | 1.8 | 2.3 | 11.4 | 5.3 | 6.6 | 8.0 | 6.4 | 6.9 | 6.0 | 1.7 | 1.8 | 9.0 | 5.1 | 6.2 | 7.3 | 5.6 | 6.4 |
| Md7D | 6.6 | 1.8 | 1.1 | 11.1 | 5.6 | 6.6 | 8.0 | 6.4 | 6.9 | 7.2 | 1.8 | 1.8 | 11.6 | 6.0 | 7.1 | 8.5 | 6.9 | 7.5 | 7.2 | 1.9 | 2.8 | 11.8 | 6.2 | 7.4 | 8.6 | 6.8 | 7.7 |
| Nongrowing male | |||||||||||||||||||||||||||
| Md6D | 3.2 | 1.8 | 0.4 | 8.5 | 1.9 | 3.1 | 4.2 | 3.0 | 3.5 | 3.9 | 1.7 | 0.7 | 9.0 | 2.6 | 4.0 | 4.8 | 3.6 | 4.3 | 3.7 | 1.8 | 0.7 | 10.4 | 2.6 | 3.4 | 4.4 | 3.3 | 4.1 |
| Md7M | 5.9 | 2.3 | 1.2 | 12.8 | 4.2 | 5.8 | 7.4 | 5.6 | 6.3 | 6.7 | 2.0 | 3.2 | 13.6 | 5.3 | 6.7 | 8.0 | 6.3 | 7.2 | 6.2 | 2.3 | 1.1 | 13.4 | 4.8 | 6.0 | 7.2 | 5.7 | 6.8 |
| Md7D | 7.0 | 2.1 | 1.1 | 13.5 | 5.7 | 6.9 | 8.3 | 6.7 | 7.3 | 7.7 | 2.0 | 3.3 | 13.4 | 6.3 | 7.3 | 9.2 | 7.2 | 8.2 | 7.0 | 2.0 | 3.2 | 13.4 | 5.9 | 6.7 | 8.1 | 6.6 | 7.5 |
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