Simulation of the clinical procedure by digital intraoral palpation of the greatest prominence of the Infrazygomatic crest for mini-implants insertion

The objective of this cross-sectional retrospective study was to simulate using cone-beam computed tomography (CBCT) in adults, the clinical procedure performed by intraoral digital palpation of the greatest prominence (GP) of the Infrazygomatic crest (IZC) for mini-implants (MIs) insertion. CBCT images of 34 adults (14 men, 20 women), aged 18.0 to 57.7 years (mean, 32.2 years) were selected. On 3D reconstruction, the GP of the IZC region was determined using the anatomical morphology, and its anteroposterior position on the selected axial slice was evaluated relative to the dental reference located between the maxillary first and second molars (U6–U7). On the selected coronal slice, two reference lines were established to evaluate the insertion angle and insertion depth (IZC thickness) for MIs. The same procedure was performed on slices with intervals of 1 mm mesially as well as distally up to reach 4 mm. The right and left sides were measured. In relation to U6-U7, the GP of the IZC was 0.19 mm (±1.79) mesial on the right side and 0.29 mm (±1.65) mesial on the left side. The greatest bone thickness of the IZC was 4.95 mm (±2.39) on the right side, 3.81 mm distal from U6-U7, and 4.79 mm (±2.13) on the left side, 3.71 mm distal from U6-U7. The GP-IZC determined visually on the 3D reconstruction, did not present the greatest bone thickness. The bone tended to gradually become thicker distal to the GP-IZC and the dental reference U6-U7.


Introduction
Patients often reject extractions in orthodontic treatment and demand for alternative treatment to avoid them. (Keles & Sayinsu, 2000) Alternatively, perform distalization of maxillary molars is relatively difficult compared with other types of tooth movements. (Liou et al., 2004) Successful complete maxillary dentition distalization depends on consistent temporary skeletal anchorage with MIs, placed distant from the roots. (Ali et al., 2016;Lima Jr et al., 2022;Wu et al., 2011) The IZC is one of the anatomical sites distant from roots, which allows unobstructed tooth movement during complete distalization of the maxillary dentition, decreasing the chance of root contact and damage. (Uribe et al., 2015;Vargas et al., 2020) The IZC is a pillar of cortical bone located in the lower part of the zygomatic process of the maxilla, intraorally, it is a bony protuberance palpable along the curvature between the alveolar process and the zygomatic process of the maxilla. (Liou et al., 2007) Anatomically, the IZC has two cortical plates: the buccal cortical plate and the cortical plate of the maxillary sinus floor. This anatomical advantage allows bicortical fixation, which contributes in improving the primary stability of the MI. (Jia et al., 2018) According to Liou et al (Liou et al., 2007) and Baumgaertel et al (Baumgaertel & Hans, 2009) the IZC is located on the buccal surface of the zygomatic process of the maxilla, above the first permanent molars. According to De Clerck et al (De Clerck et al., 2002) and Chang et al (Chang et al., 2019) the IZC, located between the first and second molars, is the chosen site for placing MIs at a safe distance from the roots of maxillary molars.
Some studies have evaluated the IZC in terms of bone thickness, MI insertion angle and height using CBCT. (Baumgaertel & Hans, 2009;Lee et al., 2013;Liou et al., 2007;Murugesan & Sivakumar, 2020;Santos et al., 2017) Due to the growing demand of adult patients seeking conservative orthodontic treatment avoiding extractions, and the frequent use of MIs as temporary skeletal anchorage, sometimes the clinician is based solely on his/her knowledge and clinical experience to insert MIs in the IZC region without CBCT of the area. For this reason, the aim of this study was to simulate using CBCT, the clinical maneuver, where the operator decides the insertion site of the MI by intraoral palpation of the greatest Research, Society and Development, v. 11, n. 5, e54211528496, 2022 (CC BY 4.

Ethical considerations
This cross-sectional study was approved by the Institutional Committee of Ethics with the register number 3.005.777.
The CBCT images of 34 adults (14 men, 20 women), aged 18.0 to 57.7 years (mean age, 32.2 years) of mestizo ethnicity were selected from the database of a private tomographic laboratory in Santo Domingo, Dominican Republic, to assess the IZC crest region, according to the objectives mentioned. The CBCT images were acquired using a Promax 3D Max (Plan-meca Inc, Roselle, IL) device with an acquisition protocol of 90 kVp, 8-14 mA, 0.2mm-thick slices, 0.2-mm voxel size, and 20 × 20 cm field of view. The inclusion criteria were CBCT images of adult patients over 18.0 years old, no craniofacial anomalies, trauma or asymmetries, presence of complete maxillary and mandibular dentition with the exception of the third molars, no radiographic signs of bone resorption or periapical problems, and good image quality. All CBCT images that do not comply with the aforementioned inclusion criteria were excluded from this study.

Measurement protocol
Initially, each CBCT image was adjusted in order to set the head position, considering the sagittal slice perpendicular to the Frankfurt plane, the axial slice perpendicular to the midpalatal suture and the coronal slice perpendicular to the maxillary occlusal plane (plane formed between the distobuccal cusps of both maxillary first molars). All adjustments and measurements were performed using On Demand 3D software (On Demand 3D software version 1, Cyber Med, Seoul, South Korea) ( Figure   1). Source: Authors. Research, Society and Development, v. 11, n. 5, e54211528496, 2022 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v11i5.28496 4 First, in the 3D reconstruction, the greatest prominence of the IZC region (GP-IZC) was determined in the frontal view and corroborated with the lateral view to determine the insertion site of the MI (Figure 2).

Source: Authors.
The GP-IZC is comparatively the site chosen by digital palpation for MI insertion, when the clinician seeks for a site of greater prominence in the IZC region. Immediately, on the selected axial slice was evaluated its anteroposterior position relative to the dental reference located at the contact point of the crowns of the maxillary first and second molars (U6-U7) on each side. Second, on the selected coronal slice, two reference lines were established to evaluate the insertion angulation of the MI. The first reference line was the maxillary occlusal plane, a plane formed between the distobuccal cusps of both maxillary  Third, on the selected coronal slice, the height of the MI insertion point (greatest prominence of the IZC) was measured from the distobuccal cusp of the maxillary first molar to the GP-IZC.
All measurements were performed on the right and left sides.

Statistical analysis
A previously trained and calibrated investigator (O.M.A.R) performed all measurements on CBCTs under identical conditions. To test the reliability of the measurements, 30 CBCTs were randomly selected for re-measurement by simple random sampling 2 weeks after the initial measurement. The Dahlberg's error test was used to verify the reliability of the two-time measurements. All data collected were tabulated in an electronic database and later subjected to specific statistical analyses using the SPSS statistical program (version 25.0; IBM, Chicago, IL, USA).
Descriptive statistics were performed to identify the general and specific characteristics of the study sample. The normality assumptions were verified using the Kolmogorov-Smirnov test.
The paired Student's t-test was used to compare the measurements between the two sides and on the same side.
Comparisons of measurements between sexes were performed using the Mann-Whitney U test. The level of significance used was P ˂ 0.05.

Results
The Dahlberg's error test showed satisfactory reliability of the two-time measurements (< 20% error). The mean mesiodistal position of the GP-IZC was located 0.19 mm (±1.79) mesial to U6-U7 on the right side and 0.29 mm (±1.65) mesial to U6-U7 on the left side (Table 1).
The mean value of the greatest bone thickness of the IZC was 4.95 mm (±2.39) on the right side and 4.79 mm (±2.13) on the left side (Tables 2 and 3). There were no statistically significant differences when comparing the mean bone thickness values of the IZC region between genders.
The mean MI height above the maxillary occlusal plane was located 17.92 mm (± 2.23) on the right side and 17.58 mm (± 2.37) on the left side.  Statistically significant differences found on comparing the mean bone thickness values between each slice studied on the same side are shown in Table 4 for the right side and in Table 5 for the left side. The bone tended to gradually become thicker distally in relation to the dental reference U6-U7 and the GP-IZC.  Comparing the right and left sides according to MI insertion angulation at each slice studied, statistically significant differences were found in most slices, except at point 0 (P = 0.123) and point 4 mm distally (P = 0.069) ( Table 6). The mean value for MI insertion angulation on the right side was 63.28° and 65.14° on the left side.

Mean
There were no statistically significant differences when comparing the mean bone thickness values of the IZC region between the right and left sides (P >0.05).
Between sexes, there were no statistically significant differences when comparing the mean bone thickness values in the 9 coronal slices studied in the IZC region (P >0.05).

Discussion
Sometimes the clinician is based solely on his/her ability and clinical experience to insert MIs in the IZC region without CBCT of the area. For this reason, the clinical procedure performed by intraoral digital palpation of the GP-IZC for MI insertion in adults was simulated using CBCT.
In the present study, the mean position of the greatest prominence of the IZC was found to be 0.19 mm (± 1.79) mesial on the right side and 0.29 mm (± 1.65) mesial on the left side, relative to the dental reference U6-U7. This was determined visually in the 3D reconstruction of each patient, representing the clinical procedure by digital palpation performed by the clinician.
By the way, Liu et al (Liu et al., 2017) observed that the U6-U7 region is the most ideal safe zone for placing MIs in the buccal alveolar bone in the IZC region for maxillary dentition distalization.
The mean greatest bone thickness of the IZC was 4.95 mm (± 2.39) on the right side and 4.79 mm (± 2.13) on the left side, and it was located 3.81 mm distal to the dental reference U6-U7 on the right side and 3.71 mm distal on the left side. It was 17.92 mm (± 2.23) height above the maxillary occlusal plane on the right side and 17.58 mm (± 2.37) on the left side. The overall mean bone thickness was 4.35 mm. In relation to the mean IZC greatest prominence found, the IZC greatest thickness was located 4 mm distally on both sides. The bone tended to gradually become thicker distally in relation to the dental reference U6-U7 and the GP-IZC.
Some studies have evaluated the IZC in terms of bone thickness, MI insertion angle, and height using CBCT. In 2007, Liou et al (Liou et al., 2007) measured the thickness of the IZC above the mesiobuccal root of the maxillary first molars at different angles and positions and recommended 6 mm as the minimal IZC thickness for sustaining an MI, inserting it 14 to 16 mm height at an angle of 55° to 70° to the maxillary occlusal plane. Baumgaertel et al (Baumgaertel & Hans, 2009) investigated the thickness of the IZC at three sites above the maxillary first molar and found an average bone depth of 6.17 to 7.05 mm at the Research, Society and Development, v. 11, n. 5, e54211528496, 2022 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v11i5.28496 lowest measurement level selected; thereafter, the values gradually decreased until reaching the most apical measurement of 2.97 to 3.6 mm. Placement at the first level would violate the minimal safety recommended distance for MIs insertion. In addition, Santos et al (Santos et al., 2017) evaluated the IZC at two selected points above the distobuccal root of the maxillary first molars and found an overall mean IZC thickness of 2.49 mm and 2.29 mm, respectively.
In the present study, the average bone thickness of IZC was lower than the values obtained by Liou and Baumgaertel. (Baumgaertel & Hans, 2009;Liou et al., 2007) This was probably owing to the following circumstances. First, a certain safety distance is required. Maino et al (Maino et al., 2005) recommended 0.5 mm as the minimal safety distance to any adjacent anatomical structure, and Baumgaertel (Baumgaertel & Hans, 2009) also used 0.5 mm; nevertheless, Liou et al (Liou et al., 2004) recommended 2 mm. In our study, 1.25 mm was the minimal safety distance used. Therefore, it was necessary to place the MIs in a more apical position, which resulted in a concomitant reduction of the IZC bone thickness. Second, the amount of alveolar process height and depth; the buccal bone available surrounding the posterior maxillary roots in several patients was very thin to support an MI; therefore, it was necessary to insert the MIs in a more apical position, resulting in a reduction of the IZC thickness. Third, maxillary sinus pneumatization size; roots of the maxillary posterior teeth frequently cause convolutions in the floor of the sinus, resulting in a reduction of the IZC thickness. Fourth, root length and buccolingual inclination of the maxillary first and second molars; the greater the root length and buccal root inclination, the lesser the thickness of the IZC.
In contrast, the values obtained by Santos et al (Santos et al., 2017) were significantly lower than those obtained in the present study because the MI insertion site selected by them was even more apical. In summary, the more apical the selected MI insertion site, the lesser the bone thickness.
As mentioned above, Liou (Liou et al., 2007) recommended 6 mm as the minimal IZC thickness for sustaining a MI, nevertheless, Costa et al (Costa et al., 1998) and Miyawaki et al (Miyawaki et al., 2003) mentioned that the cortical bone quality and quantity are the major factors associated with primary stability. Farnsworth et al (Farnsworth et al., 2011) reported that the average cortical thickness of the IZC is only 1.44 to 1.58 mm. As generally accepted, cortical bone thickness of more than 1 mm is required for good stability of orthodontic MI. The thickness of the buccal cortical bone was not directly evaluated in the present study, but the total thickness of the IZC according to the MI insertion angle was measured from the buccal cortical bone to the cortical bone of the MSF. In this way, the MI insertion reaches bicortical fixation, achieving adequate primary stability by mechanical retention.
Among the risks mentioned during the insertion of MIs in the IZC is also the possibility of penetrating and damaging the maxillary sinus. Jia et al (Jia et al., 2018) determined the incidence of IZC MIs penetration into the maxillary sinus in clinical practice using CBCT. The overall success rate of MIs inserted in the IZC was 96.7%, and 78.3% penetrated into the maxillary sinus. They concluded that the incidence of penetration of IZC MIs into the sinus may be high, and penetration through double cortical bone plates with limitation of penetration depth within 1 mm is recommended. On the other hand, Chang et al (Chang et al., 2022) evaluated the success rate of MIs placed in the Infrazygomatic crest in relation to maxillary sinus penetration. They observed that 48% of MI penetrated the maxillary sinus with a mean penetration length of 3.23 mm, however they concluded that maxillary sinus perforation has no significant effect on MI survival, and none patient reported any adverse signs or symptoms of sinus perforation. In the present study, the importance of bicortical penetration of MIs in the IZC to obtain adequate primary stability was considered.
Lee et al (Lee et al., 2013) found that the IZC was clinically thicker in males than in females. In contrast, Santos et al (Santos et al., 2017) mentioned that no statistically significant differences were detected between gender. In the present study, there were no statistically significant differences when comparing the mean bone thickness values of the IZC region between sexes.
In concordance with Lee et al (Lee et al., 2013), we found no statistically significant differences when comparing the mean bone thickness values of the IZC region between the right and left sides. Comparing the right and left sides according to MI insertion angulation at each slice studied, differences were found in most slices, except at point 0 and point 4 mm distally.
The average insertion angle of MI on the left side was higher compared to the right side.
A limitation of this study was that individuals in the sample were from a specific country with possible ethnic skeletal features; therefore, caution is recommended when extrapolating these results to other populations. Nevertheless, the results presented can be used as clinical knowledge to guide procedures of MI insertion in the IZC region.
As a suggestion to conduct a future study, it is recommended to have a larger sample size to clarify the findings presented in this research.

Conclusion
Sometimes the operator is based solely on his/her ability and clinical experience to insert MIs in the IZC without aid of CBCT. For this reason, the clinical procedure performed by intraoral digital palpation of the GP-IZC for MI insertion in adults was simulated using CBCT, to have predictable clinical reference in the insertion of MIs.
The GP-IZC determined visually on the 3D reconstruction, did not present the greatest bone thickness. The bone tended to gradually become thicker distal to the GP-IZC (MI insertion site selected by the operator) and the dental reference U6-U7.