INTRODUCTION

Orthodontic miniscrews have been widely used for a variety of orthodontic and orthopedic treatments.¹ They are used to enhance anchorage without patient cooperation and orthodontic tooth movement without side effects.² Recently, the use of miniscrews for intermaxillary fixation (IMF) in orthognathic surgery has become popular because of its extensive benefits. It can reduce the extrusive load to teeth and establish stable occlusion without relapse after maxillofacial surgery.³

For the orthodontist, common concerns about orthodontic miniscrews are failure rates, risk factors for failure, and dental root damage after placement. From their meta-analysis, Schätzle et al.⁴ assessed that the success rate was about 83.6%. Shinohara et al.⁵ concluded that the success rate was about 95% after the recommended placement technique. Higher success rates prevent treatment delays and obtain improved outcomes. To improve success rates, several studies have analyzed factors affecting miniscrew failure, such as overloading,⁶ cortical bone thickness,⁷ and proximity of the miniscrew to the adjacent teeth.⁸ Since miniscrews are placed into small spaces between the roots of adjacent teeth, root proximity is often considered the major factor affecting miniscrew failure.⁹

Cone-beam computed tomography (CBCT) provides many advantages with respect to orthodontic treatment. CBCT can be used to assess the root proximity of miniscrews after placement. However, the relatively high radiation dose and high cost have restricted its use for clinicians. Most clinicians use only panoramic radiographs (PRs) or periapical films to estimate the gap between roots of adjacent teeth and assess root proximity of miniscrews after placement. PRs often provide distorted and overlapping images of miniscrews and dental roots affected by several factors, including the distance between the patient and the film¹⁰ and the position of the object measured.¹¹ Therefore, the actual position of miniscrews placed by a clinician using PRs needs to be evaluated by three-dimensional images such as CBCT.

Most research has reported the root proximity after miniscrew placement only between the second premolar and the first molar.^12–14 Recently, other buccal sites have been commonly used for various orthodontic tooth movements.¹⁵ There has been no previous study on the root proximity after miniscrew placement at a variety of maxillary and mandibular buccal sites.

The aim of this study was to assess the root proximity and the insertion angles of miniscrews after miniscrew placements at a variety of maxillary and mandibular buccal sites and to determine the differences in detection of root proximity between CBCT and PR.

MATERIALS AND METHODS

Data Acquisition

All miniscrews (diameter, 2.0 mm; length, 8 mm; Dual Top Anchor System, Jeil Medical, Seoul, Korea) were placed directly with a hand driver by one of three oral and maxillofacial surgeons, each of whom more than 10 years of clinical experience, using the self-drilling method under general anesthesia. PRs were used to estimate the gap between the roots of the adjacent teeth before placement. Twelve miniscrews were placed in the buccal alveolar bone of each patient: at sites between the central incisor and lateral incisor (SII), sites between the canine and first premolar (SCP), and sites between the second premolar and the first molar (SPM) on the right and left sides of the mandible and maxilla (Figure 1). All patients had postoperative CBCT and PRs taken 3 days after orthognathic surgery. The CBCT (DCT pro, Vatech Co, Seoul, Korea) settings were as follows: voxel size, 0.3 mm; field of view, 200 × 190 mm. CBCT data were converted and saved as DICOM files, and Ez3D pro (Vatech Co) was used for analyzing the DICOM data. The multiplanar reformation images of Ez3D pro were viewed using a window width of 3000 HU and a window level of 1000 HU. The root proximity was categorized into five groups using a modification of the method of Watanabe and Shigeeda^13,14 (Figure 2). The horizontal and vertical inclinations of the miniscrews were measured using Shinohara's method.⁵ A tomographic view was fixed parallel to the occlusal plane. The angle of the distal side between the bone surface and the long axis of each miniscrew was measured as the horizontal placement angle in the transverse view. The angle between the long axis of the miniscrew and the long axis of the neighborhood teeth was measured as the vertical placement angle in the coronal view. For comparison of CBCT and PR, paired PRs (Planmeca, Helsinki, Finland) were also categorized into five groups (Figure 3). All images were acquired by the same radiologist.

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RESULTS

Insertion Angle

There was a statistically significant difference in the vertical placement angles (P = .000; Table 1) and the horizontal placement angles (P = .000; Table 2) among the 12 placement sites. The mean vertical placement angles were 88.57° ± 9.67° in the maxilla and 93.85° ± 8.51° in the mandible, which was significantly different (P = .000); 91.59° ± 9.65° for the right side and 90.83° ± 9.30° for the left side, which was not statistically significant (P = .325); and 90.50° ± 10.28° in the SII, 92.94° ± 8.52° in the SCP, and 90.20° ± 9.35° in the SPM, which were significantly different (P = .006). The mean horizontal placement angles were 96.93° ± 8.16° in the maxilla and 93.75° ± 7.48° in the mandible, which were significantly different (P = .000); 94.84° ± 8.05° for the right side and 95.83° ± 7.89° for the left side, which were not statistically significantly different (P = .131); and 92.78° ± 5.15° in the SII, 94.51° ± 8.82° in the SCP, and 98.72° ± 8.28° in the SPM, which were significantly different (P = .000).

Table 1 Vertical Placement Angle^a

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Table 2 Horizontal Placement Angle^a

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Root Proximity

The percentages of category I, II, III, IV, and V for CBCT images were 58.6%, 8.7%, 20.7%, 11.8%, and 0.2%, respectively (Table 3). The rates of no contact were 51.7% in the maxilla and 65.6% in the mandible, which were statistically significant (P = .000); 56.3% in the right side and 61.0% in the left side, which were not statistically significant (P = .062); and 68.0% in the SII, 50.5% in the SCP, and 57.8% in the SPM, which were statistically significant (P = .000). The lowest rate of no contact was observed in the SCP on the right side of the maxilla, and the highest rate was observed in the SII on the left side of the mandible. The percentages of category I, II, III, IV, and V for PRs were 24.3%, 11.8%, 14.7%, 33.3%, and 15.8%, respectively (Table 4).

Table 3 Classification of Root Proximity on the CBCT^a

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Table 4 Classification of Root Proximity on the PR^a

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Comparison With Panoramic Films

The total concordance rate between PR and CBCT was 41.3% (Table 5). The concordance rates in the SII, SCP, and SPM were 34.0%, 33.5%, and 56.5%, respectively.

Table 5 Root Proximity Classification on the CBCT and PR

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DISCUSSION

Orthodontic miniscrew use is one of the milestones in orthodontics. As miniscrews are more widely used, clinicians become more concerned about complications, including miniscrew loosening and tooth damage. To reduce the risk of miniscrew complications, information about positioning and the inclination of the placed miniscrew is potentially useful.

IMF had been achieved with arch bars or interdental wiring. Because these methods have some disadvantages such as periodontal problems, decalcification, and extrusion of the supporting teeth, another method has been developed.³ Miniscrews can be used to achieve IMF in orthognathic surgery. The damage to tooth roots is the most important problem with this method.

The mean vertical placement angles of 12 sites ranged from 84.27° to 95.12°, which were different from those recommended in a previous study. Lim et al.¹⁶ suggested that placing miniscrews at 30° or 45° would increase the contact with cortical bone and minimize the chance of contacting the adjacent root. Miniscrews of this study were used to hold orthodontic elastics for IMF and were accordingly placed perpendicular to the bone surface.

The mean horizontal placement angles of each site ranged from 90.93° to 101.1°, which was similar to angles used in previous studies.¹⁷ This angle range resulted from the miniscrews' being relatively far from the adjacent roots. The miniscrews in the SII were placed approximately perpendicular to the bone surface, whereas the miniscrews in the SPM were inclined more mesially. This is related to the viewpoint of the practitioner. The drilling direction in the anterior sites could be identified more easily than that in the posterior sites. In addition, there was no significant difference between the right and left sides.

This study showed that, on the CBCT, the rate of no contact between the root and the miniscrew was 58.6%, which was lower than that reported in previous studies. Shigeeda¹⁴ reported that about 80% showed no root contact on the CBCT. This is possibly because placing a miniscrew perpendicular to the long axis of the teeth and using larger diameter miniscrews would increase the chance of contacting the adjacent roots. A significantly higher rate of no root contact was observed in the mandible than in the maxilla, possibly because most patients in this study had skeletal Class III malocclusion and so the protrusive mandible provided easy accessibility for placing miniscrews. The lower rates of no root contact occurred for the miniscrews placed in the SCP. This may be attributable to a property of PR that results in greater distortion in the canine/premolar regions.¹⁸ This is also due to the smaller spaces between the adjacent roots in the SCP.¹⁹

In the PRs, the rate of no contact was 24.3%. Many miniscrews were not actually in contact with the root, even though contact of the miniscrew with the root was observed on the PRs. For example, among 95 overlapping cases between the root and the miniscrew (category V) in the PRs, only one miniscrew penetrated into the root (category V) on the CBCT images. Also, the total concordance rate between CBCT and PR was 41.3%. Therefore, there are limitations for the use of PRs for evaluating the root proximity of miniscrews, and PRs for this purpose need to be used with caution.

This study had some limitations. First, the vertical placement angle of the miniscrews in this study was larger than that of commonly placed miniscrews. Second, there were metal artifacts caused by the miniscrews. These artifacts contributed to image blurring and might have caused inaccurate assessments of root proximity. Finally, the relationship between miniscrew success and root proximity was not determined because miniscrews for IMF were removed after 1 or 2 months of placement. Further research is required.

CONCLUSIONS

• Clinicians should be extremely careful during placement of miniscrews in the SCP.

• Many miniscrews were not in actual contact with the root, even though contact of the miniscrew with the root was observed on the PRs.

• There are limitations to the use of PRs for evaluating the root proximity of miniscrews.