Aim: Mini-implants are a relatively new device for absolute anchorage control in orthodontics. Their failure due to loosening is a multifactorial problem. Improper positioning of mini-implants with different diameters is evaluated in this study via the finite element method. Methods: Twelve 3D finite element models of a mandibular posterior segment were designed and included the first molar, a mini-implant, the periodontal ligament, and spongy and cortical bone. They were similar except for the mini-implant position and diameter (1.3, 1.6, and 2.2 mm). A vertical force of 300 N was applied to the occlusal molar surface. The von Mises stress and energy produced by the applied occlusal forces were assessed in the mini-implant and bone. Results: The stress findings were between 15.284 and 359.77 MPa for the mini-implants based on their diameters and locations. The mini-implant energy findings were between 0.00084 and 0.258 mJ. These ranges for bone stress and energy changes were 17.611 and 129.45 MPa and 0.296 and 1.575 mJ, respectively. Conclusion: A decrease in the distance between the mini-implant and tooth root has a direct effect on different parameters to explain mini-implant loosening. ORTHODONTICS (CHIC) 2012;13:e51–e57.

Key words: absolute anchorage, finite element method, mini-implant loosening