Effect of mini-implant length and diameter on primary stability under loading with two force levels

Mini-implants are widely utilized as anchorage units in orthodontic treatment. Nevertheless, there are factors that interfere with their clinical performance. The aim of this study was to examine the impact of length and diameter on the primary stability of two different types of orthodontic mini-implants loaded with two force levels. A total of 90 self-drilling mini-implants were inserted in bovine ribs in vitro, 62 of which were used in data analysis. The mini-implants were of two types, Aarhus (n = 29) and Lomas (n = 33), of two lengths (7 and 9 mm, n = 26 and n = 28, respectively), and of two diameters (1.5 and 2 mm, Lomas only, n = 6 and n = 8, respectively). A closed nickel-titanium (NiTi) coil spring was attached to each mini-implant. Half of the preparations were loaded with a low force of 0.5 N and the other half with a force of 2.5 N. Mini-implant deflections during force application were non-invasively registered using a three-dimensional (3D) laser-optical system. The results were analysed with analysis of variance for the effects of implant type, implant length, and force level, and with a t-test for the study of the effect of diameter in two different diameter variants of the same (Lomas) implant. In the low-force group, implant displacements were not statistically significant difference according to the investigated parameters. In the high-force group, the 9 mm long mini-implants displaced significantly less (10.5 +/- 7.5 mu m) than the 7 mm long (22.3 +/- 11.3 mu m, P < 0.01) and the 2 mm wide significantly less (8.8 +/- 2.2 mu m) than the 1.5 mm implants (21.9 +/- 1.5 mu m, P < 0.001). The force level at which significance occurred was 1 N. The rotation of the Lomas mini-implants in the form of tipping was significantly higher than that of the Aarhus mini-implants at all force levels. Implant length and diameter become statistically significant influencing parameters on implant stability only when a high force level is applied.