Clinical applications of the Mini-Screw-Anchorage-System (M.A.S.) in the maxillary alveolar bone.

AIMS: anchorage control with self-tapping screws has become an important part of the clinical management of the orthodontic patients. Mechanical resistance and sites of insertion of miniscrews as orthodontic anchorage are critical to ensure successful outcomes. Aim of this clinical study was threefold: 1) to measure the mechanical resistance of the M.A.S., 2) to evaluate if the alveolar areas usually selected for mini-screws placement are adequate, 3) to illustrate the most frequent clinical application on the maxillary alveolar bone.

METHODS: two methods were chosen to test these screws mechanically, representing two potential modes of failure during insertion or removal: torsional strenght and bending strenght. Three-dimension images of fifty maxillas have been retrieved from a group of 200 patients, age range between 20 and 40 years with a new type of tomogram called Newtom System. For each area mesio-distal and labio-lingual measurements from four horizontal cuts made at 2-5-8-11 mm below the bone-crest have been evaluated.

RESULTS: the mean value of resistance to breakage in torsion is of 48.7 N.cm (around 5 Kg) for the miniscrew of 1.5 diameter, while the mean value of resistance to breakage in torsion is of 23.4 N.cm (around 2 Kg) for the miniscrew of 1.3 diameter.. The mean value of resistance to breakage in flexion is of 120.4 N (around 12 Kg) for the miniscrew of 1.5 diameter, while the mean value of resistance to the flexion is of 63.7 N (around 6 Kg) for the miniscrew of 1.3 diameter. On the maxillary alveolar bone the highest amount of bone was in mesio-distal dimension between 6 and 5 on the palatal side (minimum 1.9 mm at -11 mm cut; maximum 5.5 mm at -5 mm cut). The smallest amount of bone was in the tuber (minimum 0.2 mm; maximum 1.3 mm). Examination of the labio-palatal dimension demonstrated similar high thickness between 5-6 and 6-7 (minimum 3.7 mm at -11 mm cut; maximum 13.2 mm at -2 mm cut). The smallest amount of bone was recorded on the tuber (minimum 0.6 mm; maximum 4.1 mm). The following clinical applications are described: Closure of the extractions space, Symmetric intrusion of the incisors, Correction of the cant of the plane of occlusion and of the dental midline, Molar intrusion of one or two teeth, Molar distalization with the Distal Jet and miniscrews, Molar mesialization, Intermaxillary anchorage.

CONCLUSIONS: the mechanical resistance of the miniscrews M.A.S. is suitable for their use in orthodontics. The best anatomical zones for their implantation are the interradicular spaces mesial to the first maxillary molars. From our experience to date, the miniscrews are a reliable and convenient system for skeletal anchorage when compared with other more invasive osseo-integrated systems.