Microcomputed Tomographic and Biomechanical Analysis of Orthodontic Microscrew Stability Under Continuous or Intermittent Force

Purpose: To evaluate and compare the influence of continuous and intermittent forces on stability of titanium microscrews. Materials and Methods: One hundred forty-four microscrews were inserted bilaterally in the intraradicular zones of the maxillary first molar and second premolar in 36 beagles. Loads were delivered consecutively in the continuous group (n = 12), in cycles of 12 hours on/paused for 12 hours in intermittent group A (n = 12), and in cycles of 24 hours on/paused for 24 hours in intermittent group B (n = 12). The on/ off cycles were repeated for 1, 3, 5, or 7 weeks, after which the animals were sacrificed, and microcomputed tomography (micro-CT) and pull-out testing were performed. Results: The micro-CT parameters of the microscrews in all three groups increased gradually with loading time. The value of peak load at extraction (Fmax) increased and reached a peak at week 5 but dropped slightly at week 7. In the continuous group, all measurements were lower than those in the intermittent groups at all times examined. All values in intermittent group A were higher than those in intermittent group B. Conclusion: An intermittent loading regimen appears to be more favorable for obtaining stability than continuous loading, and a 12-hour/12-hour on/off loading cycle is superior to a 24-hour/24-hour on/off protocol in promoting bone-implant contact. Keywords: bone-implant contact, continuous force, intermittent force, microscrews, stability