Introduction
Mini-implants are widely used for predictable tooth movements, but insertion is often restricted by anatomic structures. The aims of this study were to investigate the incidence of penetration of mini-implants into the sinus and the relationship between penetration depth and sinus tissue.
Methods
Data from 32 patients who received mini-implants in the infrazygomatic crest were collected from a data base. The success rate of mini-implants was determined by clinical retrospective analysis. The incidence of penetration, penetration depth, and sinus configuration were investigated and compared between cone-beam computed tomography scans obtained immediately after insertion and before mini-implant removal.
Results
The overall success rate of mini-implants in the infrazygomatic crest was 96.7%, and 78.3% penetrated into the sinus. In the group in which penetration exceeded 1 mm, the incidence of membrane thickening was 88.2%, and the mean value of thickening was 1.0 mm; however, the variable values of penetration in the 1-mm group were only 37.5% and 0.2 mm, respectively ( P <0.05).
Conclusions
The incidence of penetration of infrazygomatic crest mini-implants into the sinus may be high. Penetration through double cortical bone plates with limitation of the penetration depth within 1 mm is recommended for infrazygomatic crest mini-implant anchorage.
Highlights
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Penetration of the infrazygomatic crest mini-implant into the sinus is common.
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Double cortical plates may be beneficial for the success rate of the mini-implant.
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A penetration depth within 1.0 mm is recommended to reduce sinus irritation.
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Full analysis of the infrazygomatic crest is essential before mini-implant insertion.
The adoption of the mini-implant as the “absolute” stable skeletal anchorage has become an effective treatment strategy for orthodontic patients, enabling precise control of tooth movement. One frequently-selected insertion site for an orthodontic mini-implant is the infrazygomatic crest region, a bony ridge running along the curvature between the alveolar and zygomatic processes of the maxilla. Due to the relatively long distance from the root region, an infrazygomatic crest mini-implant will not interfere with tooth movement, and the risk of contact with the natural tooth root may be reduced. In the clinic, the infrazygomatic crest anchorage system has been used successfully for space closure, anterior retraction, posterior intrusion, and molar and even maxillary dental arch distalization.
However, placement of a mini-implant in the infrazygomatic crest is often limited by vital anatomic structures, especially the maxillary sinus. Although the approximate mean bone thickness in the infrazygomatic crest site was found in a previous study to vary between 5 and 8 mm, which could be considered adequate for mini-implant insertion, a major factor associated with primary stability and placement torque of a mini-implant is the quantity of cortical bone. Farnsworth et al reported that the average cortical thickness of the infrazygomatic crest is only 1.44 to 1.58 mm. As generally accepted, cortical bone thickness of more than 1 mm is required for good stability and a high success rate with orthodontic mini-implants. That means that, to obtain adequate primary stability, the mini-implant may have to penetrate through double cortical bone plates with the potential risk of invading the maxillary sinus. However, whether the cortical plate of the maxillary sinus floor is perforated during mini-implant insertion into the infrazygomatic crest has not been widely recognized until now.
The impact of dental implant penetration of the maxillary sinus has been investigated. It was reported that sinus infection and implant failure might be potential complications in cases with large perforations. However, few studies have provided information regarding orthodontic mini-implants inserted in the infrazygomatic crest site. Considering the common application of infrazygomatic crest mini-implants and their close proximity to the maxillary sinus, the incidence of mini-implant penetration into the sinus should be determined. In addition, the relationship between penetration status and sinus tissue reaction should also be investigated.
Cone-beam computed tomography (CBCT) can provide accurate 3-dimensional and high-resolution images of hard and soft tissues in the infrazygomatic crest and maxillary sinus, with a relatively low radiation dose and low cost. The aims of this study were to determine with CBCT the incidence of the infrazygomatic crest mini-implant penetration into the maxillary sinus in clinical practice and to investigate the irritation caused by penetration of the sinus tissue.
Material and methods
This retrospective study was registered and approved by the biomedical ethics committee (approval ID: 2016-P2-089-01) of Capital Medical University, Beijing, PR China. All preexisting clinical data and CBCT scans performed from January 2014 to November 2016 in the department of orthodontics of Beijing Friendship Hospital were screened for further evaluation. Appropriate methodology and sample size were determined by a pilot study. The sample size was calculated based on an alpha of 0.05, a sample rate of 87%, and an allowable error of 10 percent of the sample rate. It was determined that a sample of 60 mini-implants was needed to represent a reasonable overall incidence of mini-implant penetration into the maxillary sinus.
Subjects selected for this study had to fulfill the following inclusion criteria: (1) Chinese patients with mini-implants inserted in the infrazygomatic crest as anchorage for distalization of the maxillary dental arch, (2) completion of the fixed orthodontic procedure, (3) CBCT performed just before removal of the mini-implant anchorage (with or without CBCT scans immediately after mini-implant insertion), and (4) images of the infrazygomatic crest and maxillary sinus floor complete and clear. Mini-implants in contact with a tooth root after insertion were excluded.
A total of 60 mini-implants placed in the infrazygomatic crests in 32 subjects (10 men, 22 women) were available for this study, with a mean age of 28 ± 6 years. The mean length of the mini-implant was 14 mm, and the mean embedded angulation was 29.6°. The interobserver and intraobserver agreements were 0.852 and 0.898, respectively ( P >0.05).
Commercial self-drilling mini-implants (A1, Penghua, Taiwan; stainless steel, 2 mm in diameter, 12-17 mm in length according to the individual anatomic variation) were inserted by a skilled orthodontist (X.H.) with more than 20 years of clinical experience, in accordance with recommended guidelines for the insertion procedure. CBCT scans were carefully observed before mini-implant placement to select the preferred insertion site and direction in the infrazygomatic crest. After local anesthesia, an incision in the buccal keratinized gingiva near the mucogingival junction of the maxillary first molar was made and limited to less than 2 mm. A hand screwdriver was used for mini-implant insertion. Favorable primary stability was achieved. Each patient was instructed to take analgesics postoperatively, but no antibiotics were prescribed. After 1 month, an orthodontic force of 400 to 500 g was applied to the mini-implant using an elastic power chain (Ormco, Glendora, Calif). The patient was instructed to clean the implant area gently and was scheduled for regular periodontal maintenance every 3 months.
All images were acquired with a CBCT machine (5G, version FP; NewTom, Verona, Italy) by experienced radiologists using standardized procedures. The imaging parameters were set at 110 kV, 5 mA, scan time of 3.6 seconds, and field of view of 18 × 16 cm. The observer filtered the CBCT images using a liquid crystal display with a resolution of 1280 × 1024 pixels under room lighting. The data were reconstructed with cross-sectional slices at an interval of 0.3 mm. Clear CBCT views were obtained by adjusting the luminance and gray scale. The midimplant cross-sectional view was chosen for the variable measurement.
The distance and angulation measurement tool in the software (NNT viewer; NewTom) was used to measure the following variables ( Fig 1 ): (1) embedded angulation, the angulation between the long axis of the mini-implant and the sagittal plane; and (2) penetration depth, the distance between the mini-implant apex and the sinus floor cortical bone plate following the long axis of the mini-implant. The value was labeled as positive if the mini-implant penetrated the interior wall of the sinus.
The configuration of the sinus tissue around the mini-implant ( Fig 1 ) included (1) membrane thickness, the maximum thickness value of the sinus membrane measured at the insertion site; (2) palatal bone thickness, the thickness of the bone plate palatal to the existing mini-implant following the direction parallel to the axis of the mini-implant; (3) palatal cortical bone thickness, the value of palatal bone thickness minus the thickness of cancellous bone palatal to the mini-implant; (4) buccal bone thickness, the thickness of the bone plate buccal to the existing mini-implant following the direction parallel to the axis of the mini-implant; and (5) buccal cortical bone thickness, the value of buccal bone thickness minus the thickness of cancellous bone buccal to the mini-implant.
All measurements were made by 2 examiners (X.J., X.C.). The interobserver and intraobserver agreements were determined by comparing the 2 repeated measurements on 10 randomly selected CBCT images taken 1 week apart.
Mini-implant success was defined as (1) no discomfort, (2) no clinically detectable mobility, and (3) stable anchorage function until the end of the maxillary dental arch distalization. Any mini-implant that did not fulfill any of these criteria was presumed to have failed.
Statistical analysis
All variable values were analyzed using the SPSS statistical package (version 19.0; IBM, Armonk, NY). The interobserver and intraobserver agreements were determined by a paired-samples t test. The incidences of mini-implant success and penetration were presented as the percentages of the number of related sites divided by the total number of sites. All measurements are presented as means and standard deviations. The chi-square test was used to compare the success rates between the penetration and nonpenetration groups, and the incidence of membrane thickening or bone resorption in relation to penetration depth and primary membrane thickness. The Mann-Whitney U-test was used to evaluate the configuration change after placement of the mini-implant. A significant difference was defined as P <0.05.
Results
Forty-seven of the 60 mini-implants penetrated into the maxillary sinus, equivalent to 78.3% of the total mini-implants. No patient complained of clinical symptoms. Two mini-implants were removed during the orthodontic procedures because of mobility, both in the penetration group. The overall success rate was 96.7%. Table I shows the results comparing the success rates. The success rate did not have a statistically significant difference in relation to penetration, side, or sex ( P >0.05).
| Group | Mini-implant (n) | Successful (n) | Success rate (%) | P |
|---|---|---|---|---|
| Penetration status | ||||
| Penetration | 47 | 45 | 95.7 | |
| Nonpenetration | 13 | 13 | 100 | 0.449 |
| Side | ||||
| Left | 28 | 27 | 96.4 | |
| Right | 32 | 31 | 96.9 | 0.923 |
| Sex | ||||
| Male | 20 | 20 | 100.0 | |
| Female | 40 | 38 | 95.0 | 0.309 |
A detailed description of the maxillary sinus tissue is presented in Table II . In 25 of the 60 mini-implants, the CBCT images obtained immediately after insertion and before mini-implant removal were available, and they were used for further evaluation of irritation of the mini-implant to the sinus tissue. The time interval between the 2 CBCT scans was 13 months on average. Twenty-two mini-implants penetrated into the maxillary sinus, with a mean depth of 2.6 mm. After placement of the mini-implant, the mean membrane thickness increased by 0.6 mm ( P = 0.001). In addition, the mean buccal bone thickness and mean palatal bone thickness values decreased by 0.1 and 0.4 mm, respectively ( P = 0.019; P = 0.002).
| Variable | After insertion | Before removal | P |
|---|---|---|---|
| PD | 1.8 ± 1.7 | 1.7 ± 1.7 | 0.060 |
| MT | 1.4 ± 0.9 | 2.0 ± 1.1 | 0.001 |
| BT | 2.3 ± 1.1 | 2.2 ± 1.1 | 0.019 |
| BCT | 2.0 ± 0.8 | 2.0 ± 0.7 | 0.267 |
| PT | 5.1 ± 1.5 | 4.7 ± 1.7 | 0.002 |
| PCT | 1.5 ± 0.5 | 1.4 ± 0.5 | 0.494 |
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