A Retrospective Observational Mono-center Study of Miniscrews Placement for Orthodontic Anchorage

The goal of this retrospective observational mono-center study is to evaluate inter- and intra-operator variability and accuracy in miniscrews placement compared to CBCT-guided planning. The main questions it aims to answer are:

• Is there inter- and intra-operator variability in miniscrews placement?
• Is CBCT-guided placement more accurate than direct technique?
• Is there consistency among operators in identifying the third palatal ruga?
• Is there an influence of palatal vault morphology on these results?

The study will be conducted on a sample of 736 cases, that underwent digital planning for palatal mini-implants with the superimposition between the virtual model of the arch and the CBCT or latero-lateral teleradiography. The sample will be provided by a specialist in the field (Professor Giuseppe Perinetti, Adjunct Professor at Vita-Salute San Raffaele University) in anonymized form. From this sample the virtual models, that meet inclusion and exclusion criteria, will be selected and divided in two subgroups according to the palatal vault morphology (deep/flat) using the Korkhaus Index, which relates the distance between the first molars (specifically between the intersection point of the transverse groove and the buccal groove of each first molar) and the height of the palatal vault (i.e., the length of a vertical line perpendicular to the median palatal suture and the previously drawn line) setting the cutoff to determine the depth of the palatal vault at 42%. Below this percentage, the vault is considered flat; above this percentage, it is considered deep.

The operators involved will be orthodontists with varying levels of experience. At two different times, using Meshmixer software, they will digitally place a miniscrew in the left hemi-palate of each model and identify the most medial and lateral point of third palatal ruga. The placement of the miniscrews will be repeated by the same operators on the same models after a period of time.

Before proceeding with the placement, they will be instructed on the clinical references for positioning palatal mini-implants and on the use of Meshmixer software. Two trial placements will be proposed to make operators familiarize with the software and the described positioning parameters in the presence of an expert clinician.

The most adopted references in the literature, which will be therefore illustrated, are as follows:

• Placement at the third palatal ruga or up to 2 mm posteriorly, specifying that some authors discourage positioning directly over the third palatal ruga due to the risk of incorrect mini-implant angulation during insertion
• A distance from the midline ranging from 3 to 5 mm
• Regarding the inclination of the mini-implant, it was specified that Wilmes and co-authors recommend inserting it perpendicularly to the occlusal plane, although this parameter was not considered; what truly matters is the point where the mini-implant penetrates the bone.

The identification of the third ruga and the positioning of the mini-implant will occur at different times and in random order.

Data collected will be analyzed through statistical methods.