Stability of connected mini-implants and miniplates for skeletal anchorage in orthodontics

The aim of this study was to examine the primary stability of connected mini-implants and miniplates. Three different skeletal anchorage systems were investigated: (1) two 1.5 mm diameter cylindrical mini-implants connected with a 0.021 x 0.025 inch stainless steel (SS) wire, (2) two 1.6 mm diameter tapered mini-implants connected with a 0.021 x 0.025 inch SS wire, and (3) two 2.0 mm diameter cylindrical mini-implants connected by a titanium locking miniplate. Fifteen standardized bovine bone specimens were prepared, five specimens for each experimental group. The connected mini-implants were fixed on the bone specimens. The systems underwent uniaxial pull-out tests at the midpoint of the connecting wire or miniplate using a mechanical testing machine. One-way analysis of variance was used to determine the difference of the pull-out test results between the groups. Both the titanium miniplate and SS wire connection systems showed severe deformation at the screw head, which broke before the mini-implants failed. The 2.0 mm miniplate system showed the highest pull-out force (529 N) compared with the other two wire connection systems (P < 0.001). The 2.0 mm system was also stiffer than the 1.6 and 1.5 mm systems (P < 0.001). The yield force of the 2.0 mm miniplate (153 N) was significantly higher than the 1.5 mm (88 N) and 1.6 mm (76 N) systems (P < 0.001). This in vitro study demonstrated that the connection of two mini-implants with a miniplate resulted in higher pull-out force, stiffness, and yield force to resist pulling force and deformation. Such a set-up could thus provide a stable system for orthodontic skeletal anchorage.