A previous study  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 , and measured every 2–3 mm. Although this method permits easy determination of the marker points, the conclusions drawn from those studies are difficult 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 . Therefore, the palatal apex of the first 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 first 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 first 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 first 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 confirmed the insertion angle of the miniscrew is a key factor affecting insertion stability ; an angulated placement is strongly recommended in the maxillary posterior area. Park et.al.  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.  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 findings 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 first and second molars. Previous studies showed that ≥ 1 mm of bone around the miniscrew increases the stability . 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 first 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 first molar. Even if inserted at 14 mm and 16 mm sites with an angulation of 90° between the maxillary second premolar and first molar, the interradicular widths were 4.8 mm and 5.2 mm, respectively. The space was even sufficient 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 . 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 . Hendriks et. al.  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 , 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 . 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 first 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 . Previous studies have shown that for a miniscrew length < 8 mm, stability increases with increasing length. However, for lengths > 8 mm, there is no significant relationship between stability and length. At that point, the diameter becomes more significant to miniscrew stability .
Limitation and future research direction
The limitation of this experiment is that it did not consider the influence 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.