Aim of the study is to assess the effectiveness of Invisalign® First clear aligners compared to tooth-borne Hyrax-type maxillary expander on palatal morphology in growing subjects using 3D technology.

Aim of the study is to assess the effectiveness of Invisalign® First clear aligners compared to tooth-borne Hyrax-type maxillary expander on palatal morphology in growing subjects using 3D technology. This two-arm parallel-group randomized prospective clinical trial will include patients with: indications for maxillary expansion treatment, age between 6 and 10 years old, mixed dentition, good standard of oral hygiene, good compliance during treatment as assessed by the practitioner. Patients will be assigned to one of two expansion methods (arm A: Invisalign® First clear aligners, arm B: tooth-borne Hyrax-type maxillary expander) according to a computer-generated randomization list shortly before the start of expansion Primary outcome measure will be the change in palatal morphology (palatal volume and surface area variations). Secondary outcomes will be: arch dimensions (upper inter-canine width, upper inter-molar width, anterior segment length, posterior segment length), complications, patients' satisfaction and periodontal health. Due to the nature of the intervention blinding will not be possible. Potential statistical differences between the two intervention groups will be evaluated.

Inclusion Criteria:

• patients who have been diagnosed with transverse maxillary deficiency;
• mixed dentition phase with cervical vertebral maturation stage (CVMS) less than 4;
• fully erupted upper and lower first molars;
• transversal discrepancy ≤5mm
• demonstration of a good standard of oral hygiene (determined through questioning and clinical examination);
• subjects willing to consent to the trial and comply with the trial regime.

Exclusion Criteria:

• any general medical health problems which may influence gingival health, such as those necessitating antibiotic cover as bacteremia prophylaxis, diabetes mellitus, epilepsy, or physical or mental disability;
• permanent teeth extraction-based treatment (third molars excluded);
• morphologic crown anomalies;
• auxiliary treatment during arch expansion stage (such as crossbite elastics);
• posterior interproximal reduction;
• orthognathic surgery treatment planned;
• cleft palate or severe facial deformities.