Application and effectiveness of a mini-implant- and tooth-borne rapid palatal expansion device: the hybrid hyrax

Rapid palatal expansion (RPE) is used for treatment of skeletal crossbites. It may be combined with a face mask if the maxilla is to be protracted. Conventional tooth-borne appliances rely on an almost complete dentition to transmit the relatively high forces to the bony structures of the maxilla and midface. In most situations, tooth-borne appliances produce adverse effects such as buccal tipping of the lateral teeth, imposing the risk of recessions and vestibular bone fenestrations. To overcome these drawbacks, an RPE appliance was developed that utilizes mini-implants anteriorly in the palate for skeletal anchorage. Because this device is also attached to the first molars, it can be denominated as a bone- and tooth-borne appliance (hybrid hyrax). The objective of this clinical pilot study was to investigate its dental and skeletal effects.

RPE was performed in 13 patients (seven females, six males; mean age 11.2 years). In 10 patients with a skeletal Class III occlusion, a face mask was used simultaneously for maxillary protraction. Three-dimensional scans of the individual study models were digitally superimposed for the assessment of the dental effects. Skeletal effects were evaluated by lateral cephalograms taken before and after RPE and protraction.

The time needed to achieve the intended expansion ranged from 4 to 14 days (mean 8.7 ± 3.6 days). The mean expansion in the first premolar/first primary molar region was 6.3 ± 2.9 mm and 5.0 ± 1.5 mm in the first molar region. The Wits appraisal changed from -5.2 ± 1.3 mm to -2.5 ± 1.5 mm (mean improvement 2.7 ± 1.3 mm). The right first molar migrated 0.4 ± 0.6 mm mesially and the left one 0.3 ± 0.2 mm.

The hybrid hyrax is effective for RPE and can be employed especially in patients with reduced anterior dental anchorage. Since most teeth are not in the appliance, regular orthodontic treatment can start early. The combination of the hybrid hyrax with a face mask for maxillary protraction appears to be effective in minimizing mesial migration of the dentition.