Anatomic assessment of the mandibular buccal shelf for miniscrew insertion in white patients

Introduction

Cortical bone thickness, bone width, insertion depth, and proximity to nerves are important factors when planning and placing orthodontic miniscrews. The objective of this study was to anatomically assess the mandibular buccal shelf in a white patient population as the insertion site for orthodontic miniscrews by investigating these 4 variables.

Methods

Measurements were made on cone-beam computed tomography scans of 30 white patients (18 girls, 12 boys; mean age, 14.5 ± 2 years). All measurements were taken adjacent to the distobuccal cusp of the first molar, and the mesiobuccal and distobuccal cusps of the second molar. Additionally, bone depth was measured at 2 height levels, 4 and 8 mm from the cementoenamel junction. Stereolithographic models of patients were superimposed on the cone-beam computed tomography volumes to virtually create an outline of the soft tissue on the cone-beam computed tomography image to allow identification of the purchase point height (mucogingival junction). The inferior alveolar nerve was digitally traced. Miniscrews (1.6 × 10 mm) were virtually placed at the buccal shelf, and their insertion depths and relationships to the nerve were assessed. Analysis of variance with post hoc analysis was used for data analysis.

Results

Insertion sites and measurement levels had significant impacts on both cortical bone thickness and bone width. Cortical bone thickness was typically greatest at the distobuccal cusp of the second molar. Bone width was also greatest at the distobuccal cusp of the second molar 8 mm from the cementoenamel junction. The greatest insertion depth was found again at the distobuccal cusp to the second molar, whereas the miniscrews had the greatest proximity to the nerve at this site also.

Conclusions

The distobuccal cusp level of the mandibular second molar is the most appropriate site for miniscrew insertion at the buccal shelf in white patients.

Highlights

  • Anatomy of the buccal shelf in white orthodontic patients was examined.

  • Suitability as an insertion site for orthodontic miniscrews was assessed.

  • Buccal shelf at the height of the second molar distobuccal cusp was best for miniscrew insertion.

One of the most important factors when placing orthodontic miniscrews is the presence of sufficient bone at the insertion site. Miniscrews are placed in many anatomic sites depending on the biomechanics used. The most popular anatomic sites appear to be the palate, lingual aspect of the maxillary alveolar process, retromolar area, and maxillary and mandibular buccal alveolar processes. Several studies have used cone-beam computed tomography (CBCT) to assess cortical bone thickness and overall bone depth to determine the most favorable anatomic insertion sites and to evaluate the structures at risk at various sites. Recently, the mandibular buccal shelf has been used as an insertion site for orthodontic miniscrews. Indications for the buccal shelf as the insertion site are plentiful, but this site seems to be most useful for the correction of Class III malocclusions. However, despite reports of numerous treated patients, there was inconsistency in selecting the exact placement site in the mandibular buccal shelf; recommendations included adjacent to the first molar, between the first and second molars, and adjacent to the second molar. This wide range of recommendations may be due to strong local anatomic variations at the buccal shelf or the lack of studies that investigated the local anatomy. The purposes of this study were to remedy this lack of anatomic information by evaluating cortical bone thickness and bone width of the mandibular buccal shelf at different potential insertion sites and to assess the relationship between the miniscrews and the inferior alveolar nerve as the only sensitive anatomic structure in this area.

Material and methods

This study was approved by the Institutional Review Board of Case Western Reserve University in Cleveland, Ohio. The sample consisted of 30 CBCT scans of untreated orthodontic patients (18 girls, 12 boys; average age, 14.5 ± 2 years) from the Department of Orthodontics at Case Western Reserve University who had CBCT imaging prescribed as part of their initial records. No CBCT image was taken for research purposes only. Inclusion criteria consisted of white patients seeking orthodontic treatment and full permanent dentition with fully erupted mandibular second molars and no craniofacial pathology or developmental abnormality. All CBCT images were taken with a low-dose scanner, CB MercuRay (Hitachi Medical Systems of America, Twinsburg, Ohio), using 2 mA, 120 kV(p), resulting in a voxel size of 0.37 mm. All images were analyzed with Dolphin 3D (version 11.9; Dolphin Imaging and Management Solutions, Chatsworth, Calif).

After proper orientation, cortical bone thickness and buccal shelf bone width were surveyed at 3 sites on each side: buccal to the distobuccal cusp of the mandibular first molar (6D), and buccal to the mesiobuccal (7M) and distobuccal (7D) cusps of the mandibular second molar. Cortical bone thickness was defined as the dimension of the cortical bone measured from the midpoint of the osseous ledge buccal to the mandibular first and second molars (buccal shelf), parallel to the contour of the buccal root surfaces of the first or second molar ( Fig 1 ).

Fig 1
Coronal slice at the distal root of the second molar showing measurement of the cortical bone.

Buccal shelf bone width was defined as the total amount of bone available in the buccolingual direction from the most buccal point of the alveolar bone to the root of the mandibular molars at 4 and 8 mm from the cementoenamel junction (CEJ), parallel to the occlusal plane. Again, measurements were taken at the same 3 sites: buccal to the distobuccal cusp of the mandibular first molar (6D4, 6D8), buccal to the mesiobuccal cusp of the mandibular second molar (7M4, 7M8), and buccal to the distobuccal cusp of the mandibular second molar (7D4, 7D8) ( Fig 2 ).

Fig 2
Coronal slice at the distal root of the second molar showing measurement of the overall bone width at: A , 4 mm from the CEJ and B , 8 mm from the CEJ.

The inferior alveolar nerve canal was digitally traced using a tool in the software ( Fig 3 ). Stereolithographic models of the patients were superimposed on the CBCT volumes to virtually create an outline of the soft tissues ( Fig 4 ). Miniscrews (1.6 mm diameter × 10 mm shank length) were then virtually placed at the designated sites. Based on the recommendations of Chang et al, the insertion was initiated (purchase point) at the mucogingival junction as identified on the stereolithographic model. The insertion took place perpendicular to the occlusal plane and was considered complete with the screw head 5 mm above the level of the soft tissue ( Fig 5 ). The insertion depth of the miniscrew and the relationship to the digitally traced inferior alveolar nerve were also assessed at the 3 sites (D6, M7, D7) ( Fig 6 ).

Fig 3
Three-dimensional reconstructions of CBCT image showing the traced nerve canal in different views: A, coronal slice; B, sagittal slice.

Fig 4
Lower stereolithographic model superimposition on volume renderings in different views.

Fig 5
Stereolithographic model after virtual placement of the mini implant showing the screw head 5 mm above the level of the soft tissue.

Fig 6
Tomographic slice with traced inferior alveolar nerve, virtually placed miniscrew, and measurements of insertion depth and distance to the nerve.

Statistical analysis

Software (version 17.0; SPSS, Chicago, Ill) was used for all statistical analyses, and significance levels for all tests were set at P ≤0.05. An a priori power calculation suggested that a minimum sample size of 24 participants would be required. Preliminary data analysis suggested a normal frequency distribution (Shapiro-Wilk test). Reliability of the measurement method was assessed by repeating all measurements twice, 2 months apart on 10 slices of 10 CBCTs each. Intraclass correlation showed good reliability ( r = 0.9). A paired Student t test was used for additional preliminary data analysis to test for differences between the left and right sides. No statistically significant differences were found, so the data were pooled. Analysis of variance was used to evaluate the influence of the site and measurement level on cortical bone thickness, buccal shelf bone width, amount of implant-bone engagement, and distance from the implant tip to the inferior alveolar nerve, with the Tukey post hoc test evaluating further intereactions of the different variables.

Results

Buccal shelf bone width ( Table I ) was generally thinnest at the distobuccal cusp level of the mandibular first molar when measured at 4 mm from the CEJ (6D4; 3.48 ± 1.29 mm) and thickest at the distobuccal cusp of the mandibular second molar when measured at 8 mm from the CEJ (7D8; 8.13 ± 1.97 mm) ( Table I ). Multiple post hoc group comparisons showed a significant statistical difference when comparing 6D4 with all other sites. 6D8 (5.10 ± 1.35 mm) showed no significant difference compared with 7M4 (5.64 ± 1.18 mm), and 7M4 only showed a significant difference to 6D8. 7M8 (6.93 ± 1.07 mm) showed no significant difference compared with 7D4 (7.46 ± 1.21 mm), and 7D4 showed no difference compared with 7M8 and 7D8 (8.13 ± 1.97 mm), which showed a significant difference to all other sites.

Table I
Bone depth and cortical bone thickness plus standard deviations at different insertion sites in the buccal shelf
Insertion site P value
6D4 6D8 7M4 7M8 7D4 7D8
Bone depth (mm) 3.48 ± 1.29 5.10 ± 1.35 5.64 ± 1.18 6.93 ± 1.07 7.46 ± 1.21 8.13 ± 1.97 0.000

The findings of cortical bone thickness ( Table II ) showed the least thickness at 6D (2.0 ± 0.71 mm) and the most thickness at 7D (3.96 ± 0.57 mm). Multiple post hoc group comparisons showed a significant statistical difference when comparing the cortical bone thickness in the 3 sites ( Tables III and IV ). The insertion depth was found to be the least at 6D (4.98 ± 0.84 mm) and the greatest at 7D (8.40 ± 1.23 mm). The screws had the greatest proximity to the nerve at this latter site also (5.46 ± 1.63 mm). Statistical analysis showed significant main effects of both site and measurement level on buccal shelf bone width and cortical bone thickness. Also, the sites had a significant effect on the insertion depth and distance from the implant tip to the inferior alveolar nerve.

Table II
Insertion depth and distance to the inferior alveolar nerve plus standard deviations at different insertion sites in the buccal shelf
Insertion site P value
6D 7M 7D
Insertion depth (mm) 4.98 ± 0.84 6.72 ± 0.98 8.40 ± 1.23 0.000
Distance to nerve (mm) 7.70 ± 1.10 6.30 ± 1.20 5.46 ± 1.63 0.000
Cortical bone thickness 2.0 ± 0.71 3.52 ± 0.54 3.96 ± 0.57 0.000

Table III
Post hoc analysis results for bone depth
Bone depth Location Significance
6D4 6D8 0.000
7M4 0.000
7M8 0.000
7D4 0.000
7D8 0.000
6D8 6D4 0.000
7M4 0.160
7M8 0.000
7D4 0.000
7D8 0.000
7M4 6D4 0.000
6D8 0.160
7M8 0.000
7D4 0.000
7D8 0.000
7M8 6D4 0.000
6D8 0.000
7M4 0.000
7D4 0.160
7D4 6D4 0.000
6D8 0.000
7M4 0.000
7M8 0.160
7D8 0.035
7D8 6D4 0.000
6D8 0.000
7M4 0.000
7M8 0.001
7D4 0.035
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Dec 12, 2018 | Posted by in Orthodontics | Comments Off on Anatomic assessment of the mandibular buccal shelf for miniscrew insertion in white patients
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