Miniscrews can be used to provide absolute anchorage during orthodontic treatment. If we could obtain the optimum design or shape of the miniscrew, we might be able to reduce its size and lessen the chance of root contact. In addition, miniscrews are placed at several angles, and orthodontic forces are applied in various directions for clinical requirements. In this study, we used finite element analysis to investigate changes in stress distribution at the supporting bone and miniscrew by changing the angle and the shape of the miniscrew and the direction of force.
Three types of miniscrews (cylindrical pin, helical thread, and nonhelical thread) were designed and placed in 2 types of supporting bone (cancellous and cortical). The miniscrews were inclined at 30°, 40°, 45°, 50°, 60°, 70°, 80°, and 90° to the surface of the supporting bone. A force of 2N was applied in 3 directions.
Significantly lower maximum stress was observed in the cancellous bone compared with the cortical bone. By changing the implantation angle, the ranges of the maximum stress distribution at the supporting bone were 9.46 to 14.8 MPa in the pin type, and 17.8 to 75.2 MPa in the helical thread type. On the other hand, the ranges of the maximum stress distribution at the titanium element were 26.8 to 92.8 MPa in the pin type, and 121 to 382 MPa in the helical thread type. According to the migration length of the threads in the nonhelical type, the maximum stresses were 19.9 to 113 MPa at the bone, and 151 to 313 MPa at the titanium element. By changing the angle of rotation in the helical thread type, the maximum stress distributions were 25.4 to 125 MPa at the bone, and 149 to 426 MPa at the titanium element. Furthermore, the maximum stress varied at each angle according to the direction of the applied load.
From our results, the maximum stresses observed in all analyzed types and shapes of miniscrews were under the yield stress of pure titanium and cortical bone. This indicates that the miniscrews in this study have enough strength to resist most orthodontic loads.
Anchorage control in orthodontic treatment is an important factor influencing treatment outcome. In traditional orthodontic treatment, extraoral appliances such as headgear and various intraoral appliances are used to prevent the anchorage loss. However, patient cooperation is required for headgear, and anchorage loss is often observed despite the use of these appliances. Recently, miniscrews were introduced as absolute anchorage devices in orthodontic treatment, requiring no patient compliance. Further-more, excellent treatment results have been reported by using miniscrews for orthodontic anchorage in various malocclusions.
A major problem with miniscrews is their high failure rate. Several factors relating to the causes of failure have been reported: gingival inflammation, diameter of the miniscrew, implanted site, and root proximity of the miniscrew. Among them, we recently reported that root proximity is recognized as a crucial factor for failure. To avoid root contact, the miniscrew should be as small as possible. However, if the miniscrew is too small, it cannot resist the orthodontic force, resulting in miniscrew failure. Therefore, the relationship between the orthodontic load and the stress distribution of the miniscrew should be investigated to establish the optimal design for the minimum size of a miniscrew that can resist the orthodontic load.
Finite element analysis is useful for simulating stress distribution in the biologic and medical fields. A previous study evaluated the stress distribution of the surrounding bone. However, no study has investigated the stress distribution at the miniscrew itself. Unlike dental implants, mechanical interdigitation at the cortical bone rather than ossointegration (at the cortical or can-cellous bone) is required for the stability of miniscrews. Therefore, in this study, we analyzed the stress distribution mainly at the cortical bone surrounding the miniscrew, and at the miniscrew with various angles, shapes, and directions of force using finite element analysis.
Material and methods
Three types of miniscrews were designed ( Fig 1 ). The first type, called a pin, is cylindrical (1.3 × 9.1 mm diameter) ( Fig 1 , A ). The second type is called a helical thread miniscrew (1.3 × 9.1 mm diameter, 0.25 mm thread height, and 0.8 mm thread pitch) ( Fig 1 , B ). The third type, called a nonhelical thread miniscrew, has noncontiguous ring-shaped threads with the same size and shape of thread as the second type ( Fig 1 , C ). These miniscrews were assumed to be made of pure titanium. The material properties of the elements in the finite element model are shown in Table I .