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Tapered orthodontic miniscrews induce bone-screw cohesion following immediate loading.

The aim of this study was to investigate the initial stability of tapered orthodontic miniscrews (T-type screws) after placement, the necessity of a healing period, and the propriety of immediate loading. Twenty male Wistar rats with a mean age of 20 weeks were divided into two groups. In the immediate-loading groups, straight orthodontic miniscrews (S-type screws) and T-type screws (five rats each) underwent experimental traction force for 2 weeks (W) immediately after placement. In the healing groups (S- and T-type, five rats each), force was applied for 2 W after a 6-W healing period. The right tibia in each rat was identified as the test limb, while the left tibia in each rat was used as the control group, and underwent no experimental force during the experimental period. The screw-to-bone contact was observed histologically and the bone-screw contact ratio was calculated. Scheffe's test was performed to compare the bone-screw contact ratio in each group using statistical software package (SPSS 8.0 for Windows). In the control group, the bone-screw contact ratio improved from 34.8 +/- 16.0 to 74.8 +/- 12.0 per cent with S-type screws in proportion to the experimental period (2 to 8 W, respectively). With the T-type screws in the test group, there was no significant difference between the immediate-loading and healing groups. In the immediate-loading group, the bone-screw contact ratio with T-type screws was significantly greater (82.3 +/- 15.0 per cent) than with the S-type screws (33.3 +/- 11.8 per cent; P < 0.05), suggesting that T-type screws can be used for orthodontic anchorage immediately after placement.