INTRODUCTION

Anchorage is the cornerstone of the orthodontic force system. Various techniques to reinforce anchorage have been devised and used in orthodontic practice. For over a decade, orthodontists have been able to achieve absolute anchorage with bony anchorage techniques, which are now widely used. Compared to other methods of bony anchorage, orthodontic micro-implants (OMIs) have been hailed as particularly efficient for orthodontic treatment, having advantages such as a wide variety of application areas, reduced need for patient cooperation, and fewer side effects on teeth. As the frequency of OMI use increases, clinicians are becoming more concerned about the factors affecting its success rate.

The success rate of OMIs has been reported very differently due to several variables.1–10 In addition, many studies have recently been performed to determine factors affecting the success rate. Among them, root proximity to the orthodontic implant and cortical bone thickness have been reported frequently.11–17

Kuroda et al.11 concluded that the proximity of a miniscrew to the root was a major risk factor for failure of the screw. In contrast, Kim et al.12 reported that the amount of root contact area of a mini-implant was more important for its stability than root proximity. Park and Cho13 suggested that the interradicular spaces between the second premolar and the first molar in the maxillary buccal alveolar bone were adequate and safe locations for OMI placements.

Baumgaertel and Hans14 suggested that knowledge of the pattern of variation in cortical bone thickness and its mean values could aid in mini-implant site selection and preparation, but future studies would be needed to determine the exact relationship between cortical bone thickness, the method of implant site preparation, and success rates. Deguchi et al.15 suggested that angling the implant at approximately 30° would increase the contact with the cortical bone by as much as 1.5 times more than placing implants perpendicular to the long axis of the teeth. Lim et al.16 also suggested that placing mini-implants at 30° or 45° would increase the contact with the cortical bone.

Many studies have also investigated factors affecting the success rate of OMIs using three-dimensional (3D) computed tomography (CT) imaging in addition to 2D radiography. Recently, cone-beam CT (CBCT) has been widely used in the field of orthodontics to reduce the shortcomings of conventional CT such as radiation dosage, cost, and device size. However, there are few clinical reports of CBCT imaging analysis of OMIs implanted in patients for orthodontic anchorage. Therefore, in this study, root proximity and cortical bone thickness were measured using CBCT imaging, and the correlation between the measurements and OMI success rates was evaluated.

MATERIALS AND METHODS

Before this study, a power analysis with G*Power 3.1.3 (Franz Faul, Christian-Albrechts-Universität, Kiel, Germany) was performed to estimate the sample size. The sample size for the study was calculated based on a significance level of .05 and a power of 80%. The power analysis showed that 88 patients were required. Institutional Review Board approval was granted by the Wonkwang University Dental Hospital (IRB WKD IRB 20101201) to conduct this study.

The sample consisted of CBCT images of all 94 consecutive patients (33 men, 61 women; mean age, 19.36 years; SD, 5.66 years) who received OMIs into the maxillary buccal alveolar bone for orthodontic anchorage from a single operator at the Department of Orthodontics, Wonkwang University Daejeon Hospital, in Daejeon, Korea, from January 2008 to November 2009 (Table 1). Before implantation, the patients were informed of the advantages and disadvantages of this procedure, and informed consent was obtained from all patients. We also evaluated the range of bone availability at interradicular locations using panoramic radiographs. The patients agreed to have CBCT scans after placement of OMIs into the maxillary buccal alveolar bone. The images were taken with a CBCT device (PSR 9000N, Asahi Alphard Vega, Kyoto, Japan) with dosimetry parameters of 12 mA, 80 kV, and a 17-second scan time on a dental mode (D mode, scan size 51 × 51 mm, voxel size 0.1 mm).

Table 1 Patient Characteristics (n  =  94)

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The CBCT data were saved as digital imaging and communications in medicine (DICOM) files, and Simplant CMF software (version 13, Materialise NV, Leuven, Belgium) was used for analysis of the DICOM data to generate quantitative measurements.

We examined 172 OMIs (1.2–1.3 mm in diameter; 8 mm in length; implant type: AbsoAnchor SH1312-08 [self-drilling style], Dentos, Taegu, Korea) implanted into the maxillary buccal alveolar bone. All OMIs were placed directly with a hand driver, according to the self-drilling procedure, by a single operator at the Department of Orthodontics, with patients under local anesthesia. The OMIs were inserted into the attached gingiva just adjacent to the mucogingival junction, and immediately loaded with orthodontic forces of about 50 to 200 g with elastic chains (Figure 1).

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OMIs that were maintained in the bone and acted as an appropriate anchorage unit for over 1 year under orthodontic force during treatment were considered successful. Success rates according to age, sex, and side of placement (right or left) and the overall success rate were calculated (Table 2). To assess the effect of age on the success rate, the patients were divided into two groups according to age: young patients (less than 20 years of age) with 93 OMIs and adult patients (20 years of age or older) with 79 OMIs (Table 2).

Table 2 Success Rates and Number of Orthodontic Micro-implants (OMIs) According to Sex, Age, and Side of Placement

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Root proximity to the OMI was determined by measuring the shortest distance between the root surface and the long axis or center of the OMI on both the axial and sagittal views of the CBCT image (Figures 2 and 3). The thickness of the cortical bone was measured on the coronal view of the CBCT image as the depth of the cortical bone contacting the OMI. We selected an average of two measurements of cortical bone thickness in contact with the OMI (Figure 4).

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RESULTS

The overall success rate was 90.7% (156 of 172 OMIs). All 172 OMIs were inserted into the maxillary buccal alveolar bone; 90 were placed on the right side and 82 on the left side. Placement side was related to success rate: right side success rate was 88.9% (80 of 90 OMIs), and left side success rate was 92.7% (76 of 82 OMIs). Age was also related to success rate: in young patients (<20 years), the success rate was 89.2% (83 of 93 OMIs), and in adult patients (≥20 years), the success rate was 92.4% (73 of 79 OMIs). Finally, sex was related to success rate. The success rate in men was 89.6% (43 of 48 OMIs) and that in women was 91.1% (113 of 124 OMIs). Thus, the success rates were higher in the left side than in the right side, higher in adult patients than in young patients, and higher in female than in male patients, although these trends were not statistically significant (Tables 2 and 3).

Table 3 Logistic Regression Analysis Results According to Sex, Age, and Side of Placement^a

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Table 4 shows the averages and standard deviations of the root proximity to the OMI (the shortest distance between the root surface and the long axis or center of the OMI) on both axial and sagittal views of the CBCT image, and also the cortical bone thickness on the coronal view of the CBCT image. Table 5 and Figure 5 show the values and distribution of the root proximity and cortical bone thickness according to success and failure of OMIs. The mean values of root proximity were 1.33 (0.71) mm and 0.66 (0.04) mm in the success and failure groups, respectively. In parenthesis is the value minus the mean radius of the OMI (0.625 mm) from the shortest distance between the root surface and axis the implant. Sixteen OMIs were contacted with the root surface (16/172, 9.3%) and 11 of 16 OMIs were failed, which showed a failure rate of 68.8%. And, the mean values of cortical bone thickness were 1.23 and 1.06 mm in the success and failure groups, respectively (Table 5).

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Table 4 Descriptive Statistics for Various Factors in Cone-beam Computed Tomography^a (n  =  172)

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Table 5 Descriptive Statistics for Root Proximity and Cortical Bone Thickness According to Success or Failure of Orthodontic Micro-implants (OMIs)^a

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The results of the logistic regression analysis are shown in Table 6. Since exp(6.6546)  =  696.448 in the results of root proximity, a 0.1-mm increase in the distance between the root surface and OMI increased the success rate 69.6448 times. Thus, the success rate increased as the shortest distance between the root surface and OMI increased, showing a statistically significant association of success rate with root proximity (P < .05). Since exp(1.297)  =  3.660 in the results of cortical bone thickness, a 0.1-mm increase in the cortical bone thickness increased the success rate 0.3660 times. Thus, the success rate also increased as the cortical bone thickness increased, although this association was not significant (P > .05). Wald was 19.510 and 1.796 in the results of the root proximity and cortical bone thickness, respectively. Therefore, the success rate of OMIs was affected more significantly by the root proximity than the cortical bone thickness.

Table 6 Logistic Regression Analysis Results According to Root Proximity and Cortical Bone Thickness^a

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DISCUSSION

Although various studies have reported the use of OMIs as anchorage for orthodontic tooth movement for several decades, their clinical performance and the crucial factors affecting their success are not well known. The quality of orthodontic treatment has been increasingly affected by the success or failure of OMIs as clinicians have increased the frequency of their use. Therefore, they have become more concerned about the factors affecting the success rate of OMIs.

To determine factors affecting the success rate of OMIs, various studies have been performed using radiography in clinical examinations. Dental radiographs are commonly used to investigate hard tissues in dentoalveolar regions. However, these radiographs may be subject to the methodologic errors typical of 2D evaluations. Kuroda et al.11 evaluated the use of 3D CT to examine root proximity, and confirmed that root contact with the screw was not observed by 2D radiography, although the radiograph did show the proximity of the screw to the root. Therefore, evaluation by 3D CT is useful to determine the actual final position of OMIs and to identify risk factors such as root proximity and cortical bone thickness that could not be seen on conventional 2D radiographs.12 CBCT has become increasingly popular in many specialties of dentistry because of reduced radiation exposure, high accessibility, and favorable cost-benefit analysis. Suomalainen et al.18 demonstrated that the error in linear measurements was even smaller with CBCT than with multislice CT.

In this study, after placement of orthodontic implants into the maxillary buccal alveolar bone, images were taken with a CBCT device. Root proximity to the OMI was then measured on the axial and sagittal views of the CBCT image, and cortical bone thickness was measured on the coronal view (Figures 2 through 4).

Joo19 used interlamina dura distances to measure the inter-root distance, whereas Kim et al.17 used the root surface as the standard to improve consistency because the root surface was seen more clearly in the CT image. Therefore, we used the root surface for the measurement of root proximity to OMIs to improve accuracy and reproducibility.

Kuroda et al.11 reported that the proximity of a miniscrew to the root was a major risk factor for failure of screw anchorage. They based this finding on measurements of the proximity between the lamina dura and the screw. However, the surface boundary of the lamina dura is difficult to identify. Therefore, in this study, we measured the shortest distance between the root surface and the OMI (Figures 2 and 3).

Due to the metallic blurring that occurs in CBCT imaging, it is difficult to identify the surface boundary of the OMIs from the axial and sagittal views. Therefore, we used the long axis or center of the OMI as the reference line or point and measured the shortest distance between the long axis or center of the OMI and the root surface (Figures 2 and 3).

The more variables we had in the logistic regression analysis, the less clear were the statistics. Therefore, to simplify the variables, we used a single measurement, namely the shortest distance between the OMI and the root surface in the axial and sagittal views, for further analysis (Table 7). The correlation coefficient indicator to the variables was largely positive, which affirmed the positive correlation between them. We also found that the success rate increased as the shortest distance between the root surface and the OMI increased, and there was a statistically significant association of success rate with root proximity (P < .05).

Table 7 Correlation Coefficient Between Axial and Sagittal Values in Root Proximity (n  =  172)

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Some OMIs seemed to contact with the root surface because minimum distances between the screw axis and root surface were less than the radius of the screw. Sixteen OMIs were contacted with the root surface and 11 of 16 OMIs were failed, which showed the failure rate of 68.8%. In the success group, the mean value of root proximity was 1.33 (0.71) mm and the maximum value of root proximity in the failure group was 1.25 (0.63) mm (Table 5). Therefore, clinical guideline of clearance from the root surface for success of OMIs is considered more than about 1.33 (0.71) mm (Table 5 and Figure 5).

Several studies have reported that the evaluation of cortical bone thickness is important because it is the major determinant of the initial stability of an OMI.13–16 Deguchi et al.15 suggested that in terms of cortical bone thickness, the best available location for a miniscrew is mesial or distal to the first molar, and the best angulation is 30° from the long axis of the tooth. Lim et al.16 reported a statistically significant increase in the cortical bone thickness when the placement angle was increased. However, Petrey et al.20 concluded that placement of temporary anchorage devices at 90° to the synthetic cortical plate gave the best retention. However, there have been few studies on the success rate of OMIs in relation to cortical bone thickness. In this study, the success rate increased as the cortical bone thickness increased, although this association was not statistically significant (P > .05). More studies are needed to determine the exact relationship between cortical bone thickness, insertion angle, and success rate.

A commonly preferred site for OMI placement is between the maxillary second premolars and first molars because of the large space and easy accessibility for various orthodontic mechanics. In this study, we examined only OMIs placed into the maxillary buccal alveolar bone, although further studies are planned to investigate OMIs placed in other sites, especially the mandibular buccal alveolar bone.

From our evaluation of factors affecting the success rate of OMIs using CBCT images, increasing the distance from the OMI to the root surface was found to be significantly important for improved success rates when inserting OMIs. But, in some cases of this study, there were no correlations between the success rate and root proximity. Therefore, to maximize the success of OMIs, clinicians should attempt to determine additional factors affecting the success rate.

CONCLUSIONS

• Root proximity to the OMI is significantly related to the success rate of OMIs.

• To increase the success rate, attempt to increase the distance from the root surface to the OMI.