“Safe Zones” for miniscrew implant placement in different dentoskeletal patterns

Recently, the use of miniscrew implants has become an accepted and reliable method for providing orthodontic anchorage.1–3 The placement of miniscrews in the interradicular bone has been frequently recommended by the specialized literature for allowing simple placement and removal procedures, and for allowing the application of relatively simple force systems.4,5 However, concerns about damaging dental roots, allied with the limited interradicular space, still represent a barrier for the clinical application of these miniscrews.6–8 

Several studies have been performed to assess the safe locations in the interradicular spaces for miniscrew placement, the so-called “safe zones.”9–15 A minimal clearance of 1 mm of alveolar bone around the screw has been recommended to preserve the periodontal health.10 Therefore, when the diameter of the miniscrew and the minimum clearance of alveolar bone are considered, interradicular space larger than 3 mm is needed for safe miniscrew placement.9,10 

However, in these studies, the assessments of the safe zones were performed in samples with minor or no malocclusions. Moreover, subjects with skeletal discrepancies were not included in the assessment of availability of interradicular spaces.

Dentoalveolar compensation is a common adaptive feature observed in subjects with different skeletal patterns.16,17 Excessive proclined mandibular incisors and retroclined maxillary incisors are typically observed in subjects with skeletal Class II discrepancies, while retroclination of the lower incisors combined with proclination of the maxillary incisors are observed in subjects with skeletal Class III discrepancies.16,17 Therefore, we hypothesized that such characteristic dentoalveolar compensation observed in different skeletal discrepancies might affect the availability of interradicular spaces.

The purposes of this study were to assess the influence of different dentoskeletal patterns on the availability of interradicular spaces and to determine the safe zones between the dental roots of the posterior teeth for miniscrew placement in different dentoskeletal patterns.

Pretreatment lateral cephalograms and periapical radiographs, made using the paralleling technique, of Thai subjects (both male and female; age range: 15–30 years) were selected from the database of the Department of Orthodontics and Pediatric Dentistry, Faculty of Dentistry, Chiang Mai University. Selection criteria included acceptable radiographic quality, a full complement of erupted teeth from second molar to second molar, and no history of previous orthodontic or prosthodontic treatment. Dental arches with severe crowding or rotation in the posterior region, missing teeth, or radiographic signs of periodontal disease were excluded.10 In the experimental group, 60 subjects were selected and divided into Class I (n  =  20), Class II (n  =  20), and Class III (n  =  20) skeletal patterns on the basis of the ANB angle measured on the lateral cephalograms.

In the control group, 60 subjects with natural optimal occlusion and normal facial profile (appropriate for race) were selected. The selection criteria for optimal occlusion samples were as follows18:

• Class I molar and canine relationships with normal occlusal interdigitation;

• 2.0–4.0 mm overjet and overbite; and

• Minimal crowding (less than 3.0 mm) and spacing (less than 1.0 mm).

Lateral cephalometric measurements were obtained using Smart'n Ceph Researcher V 9.0 software (Y&B Products, Chiang Mai, Thailand) that was designed for this study. The cephalometric measurements were as follows (Figure 1):

• SNA angle: angle formed by the SN line and the NA line19;

• SNB angle: angle formed by the SN line and the NB line19;

• ANB angle: angle formed by the NA line and the NB line19;

• U1–PP: angle formed by the maxillary central incisor axis and the palatal plane (ANS-PNS)19; and

• L1–MP: angle formed by the mandibular central incisor axis and the mandibular plane (Me-Go′).19 

A total of 12 posterior tooth interradicular sites were examined in each experimental subject (Figure 2). All periapical radiographs were photographed as digital images at fixed magnification with a resolution of 600 DPI and then transferred to the computer. Each measurement was performed with custom-made software, Smart'n Ceph V 15.0 software (Y&B Products).

Linear and area measurements were as follows (Figure 3):

• Interradicular distances (mesiodistal dimension): the horizontal measurements between the lamina dura of adjacent tooth roots, at 3, 5, 7, 9, and 11-mm depths from the alveolar crest. This measurement was made perpendicularly to a vertical line extended from the alveolar crest.

• Interradicular area: the area between the lamina dura of adjacent tooth roots was calculated using the reference landmarks at the alveolar crest and at 3, 5, 7, 9, and 11-mm depths from the alveolar crest.

The statistical analyses were performed using the SPSS program (SPSS Inc, Chicago, Ill). Mean and standard deviation of the measurements were calculated. One-factor analysis of variance (ANOVA) was used to compare means of measurements between different skeletal patterns. Post hoc multiple comparisons were also performed with Tukey test when ANOVA yielded significant results indicating that there was a difference. Results were considered statistically significant at P < .05.

Comparisons between lateral cephalometric measurements of the control group and the different skeletal patterns are presented in Table 1.

The ANB angles were significantly different in all skeletal patterns (P < .01). Significant differences in the SNB angle were observed between the control group and the subjects with skeletal Class II and III patterns (P < .01).

The maxillary incisors of the subjects with skeletal Class III patterns were significantly more proclined than were those of the control group or of the subjects with skeletal Class II patterns (P < .01 and P < .05, respectively). In contrast, the mandibular incisors of the subjects with skeletal Class III patterns were significantly retroclined when compared with those of the control group or of the subjects with skeletal Class I or II patterns (P < .01).

Table 2 shows the interradicular distance measurements and comparisons between different skeletal patterns.

In the maxilla, the greatest interradicular distances were between the second premolar and the first molar, at 11-mm depth (Class I  =  3.9 ± 1.7 mm; Class II  =  4.0 ± 1.8 mm; Class III  =  3.8 ± 1.8 mm); the least was between the first and second molars.

In the mandible, the greatest interradicular distances were between the first and second molars, at 11-mm depth (Class I  =  5.3 ± 1.8 mm; Class II  =  6.0 ± 1.6 mm; Class III  =  5.5 ± 1.7 mm), followed by between the first and second premolars. Significant differences in interradicular distances between different dentoskeletal patterns were observed. The interradicular distances between the maxillary first and second molars, at 3-mm depth, in the subjects with skeletal Class II patterns were significantly greater than those in the subjects with skeletal Class III patterns (P < .05). In contrast, in the mandible, the interradicular distances between the first and second premolars, at all depths of measurement, were greater in the subjects with skeletal Class III patterns than those of the subjects with skeletal Class II patterns (P < .05). The same result was observed between the second premolar and the first molar, at 7-, 9- and 11-mm depths. However, the interradicular distances between the first and second molars, at 3- and 5-mm depths, in the subjects with skeletal Class II patterns were greater than those in the subjects with skeletal Class I patterns (P < .01 and P < .05, respectively).

Table 3 shows the interradicular area measurements and comparisons between different dentoskeletal patterns.

In the maxilla, the greatest interradicular areas were also between the second premolar and the first molar (Class I  =  34.2 ± 10.6 mm²; Class II  =  38.7 ± 13.2 mm²; Class III  =  33.3 ± 12.7 mm²); the least was between the first and second molars.

In the mandible, the greatest interradicular areas were between the first and second premolars (Class I  =  53.2 ± 15.5 mm²; Class II  =  45.6 ± 17.8 mm²; Class III  =  57.9 ± 13.6 mm²). Significant differences in interradicular areas between different dentoskeletal patterns were also observed. In the maxilla, the interradicular areas between the first and second molars in the subjects with skeletal Class III patterns were significantly less than those in the subjects with skeletal Class II patterns (P < .05).

In contrast, in the mandible, the interradicular areas between the first and second premolars and between the second premolar and the first molar in the subjects with skeletal Class III patterns were significantly larger than those in the subjects with skeletal Class II patterns (P < .01 and P < .05, respectively).

However, the subjects with skeletal Class II patterns presented significantly more interradicular areas between the mandibular first and second molars than did the subjects with skeletal Class III patterns (P < .05).

In our study, only subjects with well-defined skeletal patterns (Classes I, II, and III) were included. Therefore, the influence of different skeletal patterns and their characteristic dentoalveolar compensation on the availability of interradicular spaces were analyzed.

Excessive proclination of the mandibular incisors and retroclination of the maxillary incisors were observed in the subjects with skeletal Class II discrepancies. In contrast, retroclination of the mandibular incisors combined with proclination of the maxillary incisors were observed in the subjects with skeletal Class III discrepancies. The characteristics of dentoalveolar compensation observed in this study are in agreement with those described by Ishikawa et al.16 and by Solow.17 

For all skeletal patterns, the safest zone in the interradicular space of the posterior maxilla was the space between the second premolar and the first molar. In the posterior mandible, the safest zones were located between the first and second premolars and between the first and second molars. These results are in agreement with those of previous studies that assessed the interradicular spaces in subjects with minor or no malocclusion.10–15 

However, significant differences in the interradicular spaces between different dentoskeletal patterns were observed. Subjects with Class II skeletal patterns presented significantly greater interradicular distances and larger areas in the maxilla when compared with the subjects with skeletal Class III patterns. In contrast, in the mandible, the interradicular distances and areas in the subjects with skeletal Class III patterns were greater than those in the subjects with skeletal Class II patterns.

A probable explanation for the results is the difference in dentoalveolar compensation observed between these groups. Subjects with skeletal Class II patterns presented with retrognathic mandibles and more upright maxillary incisors than did the subjects with skeletal Class III patterns; as a result, the subjects with skeletal Class II patterns presented with greater amounts of interradicular space in the maxillary arch. In contrast, subjects with skeletal Class III patterns presented with prognathic mandibles combined with excessively retroclined mandibular incisors; therefore, greater amounts of mandibular interradicular space were observed in these subjects than in the subjects with skeletal Class II patterns.

Accordingly, it is possible to conclude that the availability of interradicular space was mainly influenced by the axial inclination of teeth due to dentoalveolar compensatory changes for variations in sagittal skeletal discrepancies. Greater dental inclination presented with less interradicular space, whereas more upright teeth presented with more interradicular space. In our study, only the effect of different skeletal patterns and their characteristic dentoalveolar compensation on the availability of interradicular spaces were analyzed. Several factors that would potentially affect the availability of interradicular spaces (such as severity of crowding, tooth anatomy, larger sinuses, ethnic variability, and path of eruption of the third molars) were not addressed.

In this study, available interradicular space for miniscrew implant placement in the maxilla greater than 3 mm was found only at 9- and 11-mm depths from the alveolar crest between the first molar and second premolars, an area likely to be covered by movable mucosa. Poggio et al.10 reported that the insertion of miniscrews in the maxillary molar region above 8 mm from the alveolar crest must be avoided because of the presence of the sinus. The authors further recommended miniscrew placement in an oblique direction to the dental axis (30°–40°) to allow for miniscrew placement on the attached gingiva.10 Moreover, more available space can be obtained with angulated placement of miniscrews in an apical direction because of divergent tooth root morphology.10 

In our study, assessment of both interradicular distance and area between the adjacent tooth roots was performed. Interradicular distance has been the conventional method for assessment of interradicular space using radiographic images.10–13 Moreover, the use of the alveolar crest as a reference for measurements is relatively simple and reliable, and provides a clinical guideline for miniscrew placement.10–13 However, the simple linear measurements at defined heights from the alveolar crest do not provide complete information about each interradicular space. In order to avoid this limitation, assessment of the interradicular area was performed in order to provide overall information of the interradicular space.

In this study, comparisons of the interradicular space between different skeletal patterns were performed using both distances and areas. Similar results were observed both in areas and in distances at all sites, except for the mandibular site between the first and second molars. A probable explanation was the presence of the third molars. Fayad et al.20 showed that the eruption of maxillary third molars played an important role in the sagittal inclination of adjacent molars. Therefore, the presence of third molars influences the inclination of maxillary molars and may play an important role in the availability of the interradicular space. Further studies are necessary to investigate the influence of the third molars on the availability of the interradicular space.

A limitation of our study was the use of periapical radiographs to assess the interradicular space since they provide limited, two-dimensional representations of three-dimensional anatomic structures.21 

Although all periapical radiographs were made using the long-cone paralleling technique, thus providing images with minimal distortion, the use of cone-beam computed tomography with three-dimensional images would provide more accurate and reliable results.22 Moreover, the use of periapical radiographs is the practical approach to assess the safe space for miniscrew placement.23 Therefore, it might be preferable to use cone-beam computed tomography to assess the amount of interradicular space in different dentoskeletal patterns in a future study.

The authors acknowledge the study performed by Poggio et al.,10 which described the safe zones for miniscrew placement in subjects with minor or no malocclusions. In the present study, similar methodology was applied to assess the availability of interradicular spaces in subjects with skeletal discrepancies.

• The availability of interradicular space was mainly influenced by the axial inclination of teeth due to dentoalveolar compensatory changes for variations in sagittal skeletal discrepancies.

• For all skeletal patterns, the safest zone in the interradicular space of the posterior maxilla was the space between the second premolar and the first molar. In the posterior mandible, the safer zones were located between the first and second premolars and between the first and second molars.

• Understanding the relationship between the skeletal pattern and the availability of interradicular space may aid the clinician in planning appropriate surgical sites for miniscrew implant placement.

This study was supported by a grant from the Faculty of Dentistry, Chiang Mai University, and grant MRG5080344 and MRG5080347 from Thailand Research Funding. We would like to acknowledge the assistance of M. Kevin O'Carroll, Professor Emeritus of the University of Mississippi School of Dentistry, Jackson, Miss, and Faculty Consultant, Chiang Mai University Faculty of Dentistry, Thailand, in the preparation of the manuscript.