This prospective parallel cohort study aimed to compare the short-term stability of maxillary expansion achieved with tooth-borne (Hyrax) and bone-borne expanders.

A total of 36 patients (mean age: 12.3 ± 0.6 years) with transverse maxillary deficiency (≥8 mm) were randomly assigned to two groups: Group A (tooth-borne Hyrax) and Group B (bone-borne expander with four mini-screws). Both groups followed the same activation protocol and underwent an 8-month retention period. After this period, the expansion devices were removed, and relapse was assessed at 12 months post-expansion (T2), corresponding to 4 months after device removal. Digital dental models were analyzed using linear measurements, and statistical analysis was conducted using t-tests (p ≤ 0.05).

The present prospective study aimed to compare, through a parallel cohort study, the stability of the results obtained at the dental level of tooth-borne (hyrax) and bone-borne maxillary expansion.

It has been hypothesized that some degree of post-treatment relapse is always present and that there are no significant differences between the two types of RME.

The clinical investigation was conducted following the ethical principles of the World Medical Association Declaration of Helsinki. The parents or guardians were informed of the content, risks, and benefits of the study, and written consent was obtained. The local ethics committee approved the investigation (Rif. N. 5951- Prot. N. 250/2020). Of the 200 eligible subjects, 26 were included. The patients were randomly divided into two groups, Group A and Group B, using the statistical software Clinstat (Martin Bland, York, UK). Group A was treated with a tooth-borne hyrax expander, with two bands placed on the first permanent molars. Group B was treated with a bone-borne expander supported by four mini-screws, two inserted in the paramedian position and two in the parapalatal position. The mini-screws were placed using a computer-guided technique, assisted by a surgical guide (Easy Driver; Uniontech Orthodontic Lab, Parma, Italy), created with a 3D printer (Stratasys OrhoDesktop; Stratasys, Rehovot, Israel). The insertion site, angle, length, and diameter of the miniscrews were planned using specialized software that matched CBCT images with STL files of the digital dental models. Both the mini-screws (Benefit system; PSM Medical Solutions, Tuttlingen, Germany) and the expansion devices were inserted in the same session. In both groups, the same expansion screw (10 mm; hyrax click; Dentaurum, Ispringen, Germany) and the same activation protocol were used: 4 turns on the first day, followed by 3 turns per day for the next twelve days. An 8 mm screw opening (0.2 mm per turn) was achieved in both groups. At the end of the planned expansion, an 8-month retention period was expected. At this time, the expansion device was removed, and impressions were taken in both groups (T1).

At 12-month follow-up (T2), new impressions were taken in both groups to evaluate, at dental level, the relapse in the short term. Fig 1 The plaster study models deriving from the impressions taken at T1 and T2 were scanned using a 3D scanner (Raptor3DX platform VYLO) and saved as stereolithographic files (STL). The measurements on the three-dimensional models were carried out by two operators (M.M. and R.G.) using dedicated software (Exocad DentalDB 3.1 Rijeka 8349 Version 3.1) and repeated one month later to evaluate the intra-operator and inter-operator variability. Statistical analysis Data were collected on an Excel database (Microsoft, Redmond, WA, USA) and analysed using statistical analysis software (Python 3.6). Descriptive statistical analysis (mean values and standard deviation) was performed to evaluate the relapse at twelve months (T2), after four months since the device was permanently removed.

The measurements on the eight-month and twelve-month models (ΔT2-T1) were performed, and the T-test was used to analyse the differences between the two groups.

Cohen's kappa coefficient (k) was used to evaluate the interobserver variability. Intraclass correlation coefficient was used to assess the intra-observer variability.

In all analyses, a P ≤0.05 was considered as an indicator of statistical significance.

Inclusion Criteria:

• Caucasian children in late mixed dentition or permanent dentition,
• unilateral or bilateral transverse maxillary deficiency of at least 8 mm,
• no patients with skeletal Class III and/or open bite,
• no previous orthodontic treatment,
• good oral hygiene,
• good quality of the plaster models to allow a suitable scanning.

Exclusion Criteria:

• systemic syndrome involved