Cone Beam Computed Tomography References for Placement Site and Insertion Angle of a Palatal Orthodontic Miniscrew in the Posterior Maxillary Area

Background: To strengthen the safety and stability of palatal orthodontic miniscrew, the interradicular width, and the thickness of palatal mucosal and bone should be considered, providing a reference for placement site and insertion angle of palatal orthodontic miniscrew. Methods: The imaging data of 90 adult patients were selected for this study, and the jaw bone was reconstructed by scanning. On the maxillary palatal aspect, distances of 12, 14, 16 and 18 mm from the palatal apex of the rst maxillary molar between the maxillary second premolar and rst molar, and between the maxillary rst and second molars were selected as measurement points. Linear measurements included the interradicular width of the palatal root between adjacent teeth, as well as the palatal mucosal thickness and bone thickness if the miniscrew was inserted at 30°, 45°, 60° and 90° relative to the long axis of the tooth. Data comparisons were performed via one-way analyses of variance and least signicant difference tests. Results: The greater the insertion angle, the shorter the interradicular width and mucosal thickness, the difference among all these angles was statistically signicant (P < 0.05)(cid:0)an angle between 60–90° was safer for insertion. The higher the placement site, the greater the interradicular width and mucosal thickness, the difference among all placement sites was statistically signicant (P < 0.05); placement sites between 14–16mm had greater interradicular widths. Conclusions: It is safer to insert a palatal orthodontic miniscrew at a 60–90° angle 14–16 mm away from the palatal apex of the rst maxillary molar between the maxillary second premolar and rst molar. Further, we found it is safer to insert a palatal orthodontic miniscrew at a 60° angle 14–16 mm away from the palatal apex of the rst maxillary molar between the maxillary rst and second molars.


Introduction
In recent years, miniscrews have achieved remarkable therapeutic effects and attracted increasing attention from clinicians [1]. Miniscrews have been used to provide stable anchorage by insertion on the palatal aspect of the posterior maxillary region of the dental arch. Palatal miniscrews can accomplish various orthodontic tooth movements, such as depressing elongated maxillary molars and correcting the buccal tilt of maxillary molars (Fig. 1). Moreover, miniscrews have the advantages of size diversity, exibility of insertion placement, and convenient insertion; therefore, they can be used as conventional orthodontic auxiliary materials [2]. However, the process of insertion may damage the root or the periodontal membrane of the adjacent teeth and penetrate the maxillary sinus, causing peri-implant in ammation, ultimately leading to the miniscrew detachment. The failure rate of miniscrew ranges between 7-40% [3]. Many elements in uence its success rate [4]; one determining factor is the placement site [5]. Herein, cone beam computed tomography (CBCT) was used to measure the interradicular width of the palatal root between adjacent teeth in the posterior maxillary area, as well as the mucosal and bone tissue thicknesses of the insertion path after a miniscrew was inserted at different angles on the palatal aspect. This study aimed to provide data that could serve as a reference for the clinical selection of palatal orthodontic miniscrew placement sites and insertion angles. Measuring points CBCT scan data were imported into Invivo Dental 5.0 software (Anatomage, USA) in DICOM 3.0 standard le formats. The orbito-ear plane was used as the reference plane to adjust the image in the coronal plane. On the maxillary palatal mucosa, distances of 12, 14, 16 and 18 mm from the palatal apex of the rst maxillary molar were selected as reference points (Fig. 2). The interradicular width of the palatal root between adjacent teeth as well as the mucosal and bone tissue thicknesses of the insertion path were measured when the miniscrews were inserted at 30°, 45°, 60° and 90° angles relative to the long axes of the teeth (Fig. 3). All linear measurements were performed by the same researcher. Measurements were separated by one week, and each measurement was repeated three times.

Statistical analysis
The intra-class correlation (ICC) coe cient was used to evaluate the consistency of the three measurements. SPSS 26.0 software was used to conduct one-way analyses of variance and least signi cant difference paired comparisons for the mean values obtained via the three measurements. P < 0.05 indicated a statistically signi cant difference.

Results
The ICC value obtained from comparing the measured values of the same measurement item was 0.981-0.996; since the ICC > 0.75, the three measurements exhibited good consistency.

Comparison of interradicular width at various miniscrew insertion angles and placement sites
The larger the insertion angle, the shorter the interradicular width; the difference among all these angles was statistically signi cant (P < 0.05). The interradicular width was shorter and the probability of contacting the adjacent root was higher when the miniscrew was inserted at an angle of 90°, as shown in Tables 1 and 2. The lower the insertion placement site, the shorter the interradicular width; the difference was statistically signi cant (P < 0.05). Compared with that at 12 mm site, miniscrew was inserted at 14-16 mm sites exhibited larger interradicular widths, as shown in Tables 1 and 2. At this point, the probability of damaging the roots of adjacent teeth can be reduced.

Comparison of mucosal thickness at various miniscrew insertion angles and placement sites
The lower the insertion angle, the greater the mucosal thickness and this difference was statistically signi cant (P < 0.001), as shown in Tables 3 and 4. Compared with that at 30° and 45°, the mucosal thickness was reduced at 60°-90°, and the length of the miniscrew penetrating the bone tissue was greater. The higher the insertion placement site, the greater the mucosal thickness, signi cantly decreasing the length of miniscrew penetrating the bone tissue (P < 0.001), as shown in Tables 3 and 4.
Compared with that at 18 mm site, miniscrews inserted at 14-16 mm sites exhibited shorter mucosal thicknesses, and the length of the miniscrew penetrating the bone tissue was greater.

Comparison of bone tissue thickness at various miniscrew insertion angles and placement sites
When the miniscrew was inserted at 30°, 45°, and 60°, the higher the insertion placement site, the shorter the bone tissue thickness (P < 0.001), as shown in Tables 5 and 6. Since the maxillary sinus was avoided when the miniscrew was inserted at a 90° angle, there was no signi cant difference in the bone tissue thickness at different insertion placement sites (P > 0.05), as shown in Tables 5 and 6. Compared with that at 18 mm site, miniscrews inserted at 14-16 mm sites exhibited greater bone thicknesses.

Measuring Marks
A previous study [6] used CBCT to measure and analyse the alveolar bone thickness in the buccal, proximal, and distal directions in horizontal sections at 2, 5, 8 and 11 mm at the top of the maxillary alveolar crest. Similarly, the present study also used CBCT to measure the interradicular width of the palatal root in the posterior maxillary area as well as the mucosal and bone tissue thickness along the insertion path. Most studies on the safety of miniscrew insertion used a distance 2-3 mm away from the alveolar crest as the initial measurement point [7], and measured every 2-3 mm. Although this method permits easy determination of the marker points, the conclusions drawn from those studies are di cult to apply in clinical practice. In clinical practice, doctors cannot directly observe the alveolar crest, although palatal apex attrition of the maxillary molars is rare [8]. Therefore, the palatal apex of the rst maxillary molar is generally selected as the reference point in clinical practice, and a periodontal probe was used to determine the distance, selecting placement sites on the palatal mucosa. The innovation of the present study was to use the palatal apex of the rst maxillary molar as a reference point, allowing the conclusions drawn from this research can be directly applied to clinical practice. Our method has the advantages of high operability and allows the determination of measurement points. On the palatal aspect, we chose 12 mm from the palatal apex of the rst maxillary molar as the starting measurement point. Using a 2 mm interval, distances of 12, 14, 16 and 18 mm from the palatal apex of the rst maxillary molar were selected as measurement points. Linear measurements were obtained from these sites, including the interradicular width as well as the mucosal and bone tissue thicknesses along the insertion path.

Factors Affecting Insertion
When placing a screw on the maxillary palatal aspect, care should be taken to avoid damaging the adjacent root and penetrating the maxillary sinus. In clinical practice, the placement site of the palatal miniscrew is carefully selected along the extension line of the contact point of the two adjacent teeth. Since the interradicular width as well as the mucosal and bone tissue thicknesses along the insertion path vary with height, the placement site and insertion angle of the miniscrew should be further scrutinized. Previous studies have con rmed the insertion angle of the miniscrew is a key factor affecting insertion stability [9]; an angulated placement is strongly recommended in the maxillary posterior area.
Park et.al. [3] reported that when a miniscrew was inserted at an angle between 30-40°, the tip of the miniscrew could be inserted in the apical site, allowing more width to prevent damaging the root. Mai et.
al. [10] found that when the miniscrew was inserted at a 70°-80° angle, the thickness of the penetrating cortex increased, which enhanced the stability of the miniscrew.
The present study's ndings showed that, as the placement height was increased, the interradicular width also increased. Furthermore, even if the placement site remains the same, different insertion angles will lead to differing interradicular widths when the screw passes through the palatal mucosa and bone plate to reach the palatal root plane. If the miniscrew was inserted at 30°, the position of the miniscrew was higher when approaching the palatal root plane, leading to a greater interradicular width. If inserted at 90°, the miniscrew was lower in the palatal root plane and had a shorter interradicular width between the palatal root of adjacent teeth. For instance, the interradicular widths at the 14 mm and 16 mm sites were only 3.6 mm and 3.8 mm, respectively, if inserted at 90° between the maxillary rst and second molars. Previous studies showed that ≥ 1 mm of bone around the miniscrew increases the stability [11]. At this point, the miniscrew diameter should not exceed 1.6 mm. The palatal aspect of the maxillary molars differs from the buccal aspect. The maxillary rst and second molars have two buccal roots and one palatal root each. Due to the scattered buccal roots, the interradicular widths are greater palatally than they are buccally, and this difference is even greater between the maxillary second premolar and rst molar. Even if inserted at 14 mm and 16 mm sites with an angulation of 90° between the maxillary second premolar and rst molar, the interradicular widths were 4.8 mm and 5.2 mm, respectively. The space was even su cient to accommodate a 2 mm diameter screw. Because of the maxillary palatal miniscrew, most applications are used to depress the maxillary molars and correct the buccal tilt of the molars. Therefore, if the insertion angle is too oblique, such as 30°, a reaction force opposite to the insertion direction will be generated, making the screw susceptible to detachment.
When considering the stability of palatal orthodontic miniscrew, the mucosal thickness should be considered in addition to the bone thickness [12]. The palatal mucosa is denser and thicker than the buccal mucosa, and its thickness varies with the placement height. Since the main objective of orthodontic miniscrews is to gain maximum retention, they are placed in areas with thinner soft tissue and thick bone tissue. Since the depth of bone penetration needs to be ≥ 5 mm for most screws, an extremely thick mucosa will inevitably reduce the penetration into the bone, affecting the miniscrew's stability [11]. Hendriks et. al. [13] proposed that the maxillary palatal mucosa was very thick, reaching up to 6 mm. The maxillary palatal mucosa differs from the buccal mucosa; the further from the neck to the palatal midline, the thicker the mucosa. The results of this study also demonstrated that the bone tissue thickness decreased gradually from the gingival margin to the palatal dome, while the mucosal thickness gradually increased.
Due to the greater distance from the greater palatine neurovascular bundle [14], the palatal mucosa is dense and thick, and the trauma during insertion is also minimal. Therefore, there is a low risk of clinical injury to the greater palatine neurovascular bundle.

Selection Of Orthodontic Miniscrews
Although parameters such as diameter, length, and shape determine the quantity and quality of osteointegration, it is hard to make a conclusive statement regarding the optimal miniscrew dimensions under orthodontic loading. Previous studies have proposed that miniscrews can maintain certain stability only when the penetration depth is ≥ 5 mm [15]. Therefore, to guarantee an adequate length of miniscrew penetrates the bone, a minimum length of 10 mm is recommended. The results of this experiment showed that the mucosal thickness was excessive if inserted 18 mm from the palatal apex of the rst maxillary molar. At this point, the length of the miniscrew penetrating the palatal bone tissue is less than the recommended value, and even if a miniscrew with a length of 10 mm is used, the probability of detachment is increased.
Miniscrew dimensions are another key parameter affecting the insertion stability. The diameter and length of miniscrews commonly used in clinical practice range between 1.2-2 mm and 6-11 mm, respectively [11]. Previous studies have shown that for a miniscrew length < 8 mm, stability increases with increasing length. However, for lengths > 8 mm, there is no signi cant relationship between stability and length. At that point, the diameter becomes more signi cant to miniscrew stability [16].

Limitation and future research direction
The limitation of this experiment is that it did not consider the in uence of gender and bone density on the stability of miniscrew insertion, which will be explored in future studies. Currently, the palatal miniscrew is widely used in clinical practice. This study is clinically useful because the conclusions drawn from our research can be directly, conveniently, and safely applied to clinical practice.

Conclusions
Based on the present study's results, we propose a more comprehensive consideration of different miniscrew placement sites and insertion angles. It is safer to insert a palatal orthodontic miniscrew at a 60-90° angle 14-16 mm away from the palatal apex of the rst maxillary molar between the maxillary second premolar and rst molar; further, we found it is safer to insert a palatal orthodontic miniscrew at a 60° angle 14-16 mm away from the palatal apex of the rst maxillary molar between the maxillary rst and second molars.  Distances marked along the maxillary palatal aspect.
Distances were marked 12, 14, 16, and 18 mm from the palatal apex of the rst maxillary molar in the posterior maxillary region of the dental arch.

Figure 3
The miniscrews inserted along the long axis of the tooth. The miniscrews were inserted at different insertion angles relative to the long axis of the tooth on the palatal aspect: a: 30°, b: 45°, c: 60°, d: 90°.