To the best of our knowledge, this is the first study that specifically evaluates the anatomical characteristics of the Palatal Posterior Supra-Alveolar Insertion Site (PPSAIS).
PPSAIS is located cranially compared to the alveolar bone of the maxillary posterior dentition and laterally to the palatal process of the maxillary bone. It is demarcated latero-cranially by maxillary sinus cortical bone, medio-cranially by the cortical bone of the nasal cavity, medially by the palatal cortical bone, and caudally by the alveolar process (Fig. 3).
Consequently, PPSAIS overall presents three cortical plates (sinus, nasal, and palatal) and well-represented trabecular bone within them. The presence of several cortical plates potentially offers the possibility to have a better miniscrew stabilization.
It can be considered a strategic insertion site for palatal posterior miniscrew application. Some authors used this insertion site to obtain several orthopedic and orthodontic treatment effects such as skeletal palatal expansion [13, 15, 20], intrusion of maxillary posterior dental elements [18, 19, 21, 22], and upper molar distalization [23].
PPSAIS offers numerous potential benefits. It is accessible on the palatal side of maxillary arch, and consequently, it presents attached gingiva. This characteristic seems to offer an advantage in terms of miniscrew survival [24].
Moreover, it is located in the posterior part of the palate, and this aspect makes it suitable for the application of more effective posterior expansion forces [13, 15, 20], distalization forces [23], and intrusion forces for posterior maxillary dentition [18, 19, 21, 22].
Literature showed that different authors published case reports using PPSAIS at different antero-posterior levels. Specifically, some authors placed TADs between the second premolar and the first molar [13, 15]; others opted for placing TADs between the second molar and the first molar [18, 19].
To analyze the ideal characteristics of PPSAIS, it is essential to evaluate what are the insertion sites that offer an adequate amount of total bone thickness for optimal miniscrew primary stability.
In this study, the evaluation of bone availability at different antero-posterior levels showed that the maximum amount of total bone thickness was found between the second premolar and the first molar (P2-M1). At this level, total bone thickness is statistically significantly greater compared to the other sagittal sites (Table 1 and Additional file 1: Table S1) and on average around 2 mm ticker (Tables 1 and 2). This datum indicated a general bone reduction in the posterior part of the palate.
No significant differences were noticed comparing the total bone thickness at the first molar furcation site (M1F) and to the M1-M2 insertion site (Additional file 1: Table S1). This finding could be related to the presence of the upper first molar palatal root that could reduce the overall quantity of bone at furcation (M1F).
To support clinicians during miniscrew insertion, this study was conducted describing characteristics of PPSAIS using anatomical references visible during miniscrew insertion procedures.
For this purpose, in each considered cross-sectional scan, it was used as starting insertion point the 90° projection on the palatal mucosa of the middle point of the segment connecting the free gingival margin with mucosa overlying mid-palatal suture (Fig. 3). This point, named “zeroP,” can be approximately visualized by the clinician during miniscrew insertion, it is located in the transition zone of the palate and dento-alveolar process, and it was used as reference point to perform the outcomes evaluation at different corono-apical levels.
In fact, outcomes were also evaluated 2 mm cranial to this starting point along the profile of palatal mucosa and 2 mm caudal to it. Statistically, the maximum average amount of bone was found 2 mm caudal (+ 2P) to zeroP (8.3 mm) at P2M1 and with 75° of insertion axes (Table 2). Two millimeters cranial to zeroP (− 2P), the amount of bone seems to be significantly reduced (Table 1 and Additional file 1: Table S1).
The amount of total bone could be affected by the insertion inclination axis. However, comparing the 3 axes inclination (45°, 60°, and 75°), no significant differences were found. All the evaluated sites showed on average 8 mm of total bone thickness. Moreover, considering descriptive statistics data evaluation, it is possible to conclude that the amount of total bone varies according to individual patient characteristics. Consequently, the ideal miniscrew inclination could be obtained only with a preliminary CBCT evaluation.
The ideal miniscrew placement in the PPSAIS could be obtained when TADs placement is performed perforating cortical plate of the palatal vault (Fig. 4) and placing miniscrew tip in contact with nasal and maxillary sinus cortical plates [15, 16].
This approach could offer tricorticalism stabilization and the application of higher apical expansion force, thus improving biomechanics force application and potentially achieving better skeletal treatment effects (Fig. 4). On this regard, some authors proposed a miniscrew CBCT planning performed with the aim to reach the above-mentioned tricorticalism TADs placement approach [16].
Cortical bone characteristics are fundamentals for optimal miniscrew primary stability.
Results showed that the cortical bone of the palatine vault in different considered insertion sites may vary from 1.2 to 1.9 mm. These findings are coherent with previously published studies [5, 25, 26] and confirm an adequate amount of cortical bone for primary miniscrew stability.
Descriptive statistics showed cortical bone values tend to increase in the most caudal (+ 2P) and anterior (P2-M1) insertion sites (Tables 1 and 3). Moreover, data showed that insertion angle does not affect the amount of cortical bone thickness. Overall, quantitative differences of the cortical bone thickness of different evaluated insertion sites showed significant statistical differences (Table 1 and Additional file 1: Table S2). However, these differences could be clinically not significant.
The appraisal of palatal mucosa thickness usually presents difficulties in retrospective CBCT studies.
When lingual dorsum and palatal mucosa are in mutual contact, during CBCT examination, it is not possible to distinguish them from each other during subsequent imaging evaluation. Different methods have been proposed to overcome these limitations [27, 28]. In this study, we used a validated approach [29] matching the CBCT volume data and the digital models by means of suitable software. This methodology allowed to identify distinctly the profile of the palatal mucosa in all the analyzed cross-sectional scans.
The characteristics of the palatal mucosa are crucial for proper miniscrew selection, as the extent of mucosal thickness directly affects miniscrew length selection.
The findings of this study showed that at the level PPSAIS palatal mucosa is particularly thick with average values ranging from 4 to 7 mm (Tables 1 and 4). Palatal mucosa thickness showed no clinically significant differences (Table 1 and Additional file 1: Table S3) comparing different sagittal (P2-M1, M1F, M1-M2) and vertical insertion sites (− 2P, zeroP, + 2P). Data also showed that palatal mucosal thickness increases slightly with the inclination of the insertion axis relative to the occlusal plane (Tables 1 and 4).
This finding could be related to the geometric relationship of the insertion axes with the palatal mucosa layer rather than with a real anatomic increment of the mucosa thickness. However, this relative increment of mucosa thickness in high angle insertion axes has clinical relevance. Miniscrew inserted at a high angle to the palatal plane passing through thicker palatal mucosa should be longer and with the longest neck extension (Fig. 4). So, the proper selection of these miniscrew features is important to ensure adequate bone penetration and warrant optimal neck adaptation.
Finally, results showed that vertical skeletal growth pattern significantly affects considered outcomes. Hypodivergent subjects showed on average 1 mm of additional total bone depth, 0.2 mm of supplementary cortical bone, and 0.5 mm of reduced mucosa thickness compared to mesodivergent and hyperdivergent high angle patients. This data provides important information that can be relevant for optimal patient selection. Further studies could be necessary to better estimate the impact of skeletal characteristics on specific insertion sites of palatal posterior supra-alveolar bone.