The Influence of loading time on loosening of miniscrews

Abstract

Nowadays orthodontic miniscrews are widely used to provide anchorage and in many cases they have replaced the conventional techniques. Despite many advantages of miniscrews, their success rates are lower than those of dental implants. One of the factors affecting the success rate of miniscrews is the loading time of miniscrew. Aim: The aim of the present study was to determine the appropriate time to begin application of force to increase the success rate of orthodontic miniscrews. In addition, an attempt was made in the present study to evaluate the possible roles of insertion torque, patientEs age and gender and placement side in the success rate of miniscrews. Materials and methods: The samples in the present parallel randomized clinical trial consisted of 186 miniscrews in 93 patients between the second premolars and first molars on the left and right sides of the maxilla. The patients were randomly assigned to three groups (n=31): immediate loading time and 6-week and 14-week loading time. Stratified randomization was carried out based on patient age by dividing patients to two groups: under 16 and over 16 years of age. After application of force, the statuses of the miniscrews were evaluated in relation to mobility for 9 months at 3-, 6-, and 10-week intervals after placement of miniscrews, followed by every 4 weeks. Chi-squared test was used to evaluate the relationship between miniscrew success and the loading time, insertion side (right and left), gender and age. Survival analysis and Kaplan-Meier graphs were used to evaluate the relationships between the success rate and the age and loading time. Results: The success rates in the immediate, 6-week and 14-week loading time groups were 85%, 91.9% and 96.7%, respectively. The success rate of the miniscrews in the 14-week group was significantly higher than that in the immediate group (P=0.027). The overall success rates in the three study groups were significantly higher in subjects over 16 compared to those under 16 years of age (P=0.009). In addition, the success rates in subjects under 16 and in the immediate group were less than those in the other groups. The insertion torque, placement side and gender had no effects on the success rate of miniscrews. Conclusion: The results of the present study suggested that in subjects over 16 years of age the time of initiation of force application had no effect on the failure of miniscrews; however, in subjects under 16, delayed force application resulted in higher success rates.