The placement and removal torques of mini-implants were evaluated as an index of implant stability. We examined factors affecting the initial and long-term stability of mini-implants.
We measured the placement and removal torques of 134 mini-implants placed in buccal posterior alveolar bone and assessed the relationships among placement and removal torques, placement period, age, sex, and cortical bone thickness. The mini-implants were machine-surfaced, 1.6 mm in diameter and 6 mm long. A torque screwdriver was used to measure the peak torque values.
Results and Conclusions
The placement and removal torques averaged approximately 8 and 4 N cm respectively. A torque of 4 N cm suggests sufficient anchorage capability for mini-implants. No significant correlation between placement and removal torques was found. Placement torque was significantly related to age and cortical bone thickness in the maxilla, whereas removal torque was not significantly related to placement period, age, sex, or cortical bone thickness.
In an attempt to investigate risk factors for the failure of mini-implants, these authors examined the influence of placement period, age, sex, and cortical bone thickness to identify factors affecting the initial and long-term stability of mini-implants. To do this, they determined the placement and removal torque values of machine-surfaced mini-implants in 52 orthodontic patients. The sample consisted of 10 male (25 implants) and 42 female (109 implants) subjects, with ages from 13.9 to 63.5 years. Titanium mini-implants were placed in the buccal posterior alveolar bone in all subjects as anchors for orthodontic treatment.
Some studies have suggested that there might be a relationship between placement torque and the forces required to remove a mini-implant after its use. But, in this study, removal torque was not significantly related to placement torque. Therefore, removal torque is not affected by the degree of firmness when tightening the mini-implant. To identify other factors affecting torque, the researchers investigated the placement and removal torques of the mini-implants according to placement period, age, sex, and cortical bone thickness. No sex differences were found in either placement or removal torque. No significant correlations were found between removal torque and age or placement period, but placement torque was significantly related to both age and placement period. There was a significant correlation between cortical bone thickness and placement torque in the maxilla, but not in the mandible. The main messages of this study were that torque of 4 N cm is sufficient for orthodontic anchorage with machine-surfaced mini-implants, and removal torque was not significantly related to placement torque.