Primary stability of orthodontic miniscrews inserted into femurs of osteopenic rats

Aim: To test the hypothesis that bone quality may affect the stability of anchorage devices implanted in a rat model.
Methods: Twenty male Wistar rats were divided into 2 groups: Group 1 (rats treated with FK506) and Group 2(rats treated with saline solution vehicle). The immunosuppressant tacrolimus was used with the intention of causing a condition of osteopenia in Group 1. A total of 20 orthodontic mini-implants were used. One mini-implant was inserted in the right femur of each rat. After sacrificing the animals, blocks of bone tissue containing the mini-implants were removed for mechanical pull-out tests to be performed in a universal test machine at a crosshead speed of 0.5 mm/s. The maximum insertion force values and pull-out test were recorded and submitted the nonparametric analyses including the Kruskal-Wallis test and Mann-Whitney tests (p<0.05). The test for Pearson's correlation was used to verify the correlation between bone mineral density and the pull-out test. The dose of FK506 was 2 mg/kg/day. Densitometric analysis was performed to observe the effect of FK506 on the bone mineral density.
Results: The mean insertion force values were similar in both groups, with no statistically significant difference between them (P > 0.05). The mean pull-out force values were higher in Group 2 than in Group 1, with statistically significant difference between the groups (P < 0.05).
Conclusions: Bone quality may affect the stability of mini-implants. Orthodontic mini-implants did not present good primary stability in a rat model with osteopenia.