Longer, conical miniscrews most successful for orthodontic anchorage

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By Sarah Guy, MedWire Reporter

The length and shape of miniscrew implants used for orthodontic anchorage has an effect on their success, with longer, and conical rather than cylindrical screws significantly associated with success, report Italian researchers.

Furthermore, the team's study findings indicate that miniscrews applied to medium quality bone have a significantly lower failure rate than those applied to high- and low-quality bone.

"Titanium miniscrews are highly adaptable and well suited to the demands and timeframes of current orthodontic treatment," say B Giuliano Maino (University of Ferrara and Insurbia, Vicenza) and colleagues in Progress in Orthodontics.

They found that the method of insertion of miniscrews (pre-drilling vs drill-free) and patient skeletal type (defined according to the angle of inclination to their mandibular plane in relation to the anterior cranial base) had no effect on miniscrew success rates.

Using data from 144 patients, aged a mean 24.6 years, with 324 miniscrews applied, the team assessed factors linked to the success of their treatment, evaluated clinically, and based on the endurance and clinical usefulness of the screws over the course of patients' orthodontic treatment.

In all, 296 applications were successful after a mean implantation duration of 13.7 months and the authors report that a miniscrew length of 10 mm had the best success rate, with a significantly higher rate of treatment failure for screws of 8 mm versus 10 mm.

Miniscrew diameter had no effect on the success of treatment; however conical-shaped screws had fewer failures than cylindrical ones, although this was not a significant finding in comparison with the total mean failure rate.

Significantly fewer miniscrew failures occurred in patients with medium bone quality (torque 10-15 Ncm), at 5.5%, compared with those with soft (torque 5-10 Ncm) or hard (torque >15 Ncm), at 25.0% and 19.2%, respectively, and this did not differ by miniscrew shape.

"A possible explanation for this apparent discrepancy might be that there is increased local damage to the osseous structures upon insertion of a mini-implant into more compact bone, where critical temperatures of over 47C will be reached in cases where pre-drilling is carried out," write Maino and co-investigators.

They conclude that the causes of orthodontic miniscrew failure appear to be varied and "might also be attributable to factors that were not included as objectives of this study, such as metabolic disturbance, smoking, and specific local parafunctions."

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