A COMPARATIVE HISTOMORPHOLOGICAL AND MICRO CT STUDY OF THE PRIMARY STABILITY AND THE OSSEOINTEGRATION OF THE SYDNEY MINI-SCREW: AN ANIMAL STUDY USING NEW ZEALAND RABBITS

Introduction: Failure rate of orthodontic miniscrews (MSs) is 7-50%. To address this problem and to promote primary stability of the miniscrew (MS), we recently designed and developed The Sydney Mini Screw (SMS, Patent number: PCT2009014) which can be used with injectable bone graft substitutes (iBGS). The aim of this study was to assess in vivo dispersion of bone graft substitutes (BGS) and the integration of the SMS to the cortical and trabecular bone using New Zealand femur and tibia rabbit model. Method: Twenty-four MSs were randomly placed in each proximal tibia and femur of 6 New Zealand rabbits with an open surgery process. Aarhus MS was used as a control and the effect of injection of BGS was studied by implanting SMS with and without BGS injection. The dispersion and integration of the MS were studied by using micro CT (μCT) and histochemical analysis at two time points, 0 day and 8 weeks post-implantation. Results: BGS were successfully injected to the SMS and thereafter hardened in situ to fill the bone void. After 8 weeks, μCT results revealed that the iBGS were resorbed and bone tissue was formed around the MS and within its lateral exit holes. The osteointegration of the SMS samples showed similar histologic characteristics to that of Arhus controls, and initial drilling for injection of bone cements into SMS did not seem to affect adjacent bone quality. Conclusion: Results of this pilot animal study showed the high potential of SMS and the developed technique to promote the primary stability of MS. Keywords: Primary stability; orthodontic miniscrew; injectable bone graft substitute