Effect of cortical shell and bone quality on the pullout comparison of mini-implants

Anchorage, which is defined as resistance against undesired tooth movements is one of the most important factors in success of orthodontic treatments. In recent years, mini-implants have been started to use instead of uncomfortable headgears and nance appliance etc. Mini-implants, which were firstly mentioned by Konami, are the small sized screws with 1 to 2.5 mm in diameter and 4 to 21 mm in length. Pullout test is the most common method for measuring the anchorage capacity of the mini-implants. In recent studies, organic bone tissue obtained from animal and human cadavers and synthetic bone blocks produced from polyurethane were used as a standard testing material. Bone mineral density (BMD), thickness of the cortical bone and implant design are the main factors that affecting pullout performance of the implants. There are several studies in literature that are investigating these factors. The main advantage of using synthetic foam is omitting the bone quality bias via using a uniform and standard material. In addition to this, because of ethical issues and difficulties about providing and conserving organic specimens, synthetic are more preferable. In this study, 9 pieces of PU foams cortical shells including were produced with several cortical thicknesses and foam densities, in order to determine the effect of cortical shell's thickness and density on the pullout performance of the mini-implants. Experimental results showed that there is positive correlation between pullout performance and these two variables. Groups with 2 mm and 3mm cortical shell have significantly higher results then the groups with 1 mm cortical shell. Groups which have grade 50 cortical shell have higher results then the groups with lower grade cortical shell. Results showed that cortical shell thickness is directly correlated with the pullout strength and although the density of the cortical shell has an effect on pullout the most dominant factor is the cortical shell thickness.