Impact of bone quality, implant type, and implantation site preparation on insertion torques of mini-implants used for orthodontic anchorage

Mini-implants are widely used as skeletal anchorage in orthodontics. To reduce implant loss rate, sufficient primary stability is required. This study quantitatively analysed the impact of bone quality and pre-drilling diameter on the insertion torque of five different mini-implants. Twenty pig bone segments were dissected and embedded in resin. The insertion torques of two different mini-implant types (Tomas Pin, Dentaurum, Germany, 8 and 10 mm; and Dual Top, Jeil, Korea, 1.6 mm × 8 and 10 mm plus 2 mm × 10 mm) were measured. After preparation of the implant sites using pilot drill diameters 1.0, 1.1, 1.2 and 1.3 mm, 30 implants were inserted into each bone segment. Five reference implants were inserted into each segment for comparison. Micro CT evaluated bone compacta thickness. Insertion moments of orthodontic mini-implants, and hence primary stability, varied strongly depending on compacta thickness, implant design, and pre-drilling at the implant site. The Dual Top consistently showed higher primary stability than the Tomas Pin. Insertion moments higher than 230 Nmm resulted in fractures in some cases. Compacta thickness, implant design and preparation of implant site affect the insertion torque of mini-implants for orthodontic anchorage. To avoid fractures and high bone stresses, optimum pre-drilling diameters should be chosen.