Effect of cortical bone thickness and density on pullout strength of mini-implants: An experimental study

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Anchorage, which is defined as resistance against undesired tooth movements, is one of the most important factors in success of orthodontic treatment. In recent years, mini-implants have been used instead of uncomfortable headgears and Nance appliances. The pullout test is the most common method for measuring the anchorage capacity of mini-implants. The aim of this study was to investigate the effects of cortical bone thickness and cortical layer bone density on pullout strength of mini-implants. Mini-implants were placed in polyurethane foam blocks representing 3 different cortical thicknesses (1mm, 2mm, and 3mm) and 3 different cortical bone densities (grade 40, grade 45, and grade 50). Pullout tests were performed with 5mm/min loading rate. Load vs displacement values were recorded during the tests. Comparing cortical thicknesses on the same bone densities, statistically significant differences were found between 1mm, 2mm, and 3mm thicknesses. Likewise, comparing bone densities on the same cortical thicknesses, statistically significant differences were found among all groups. Spearman rank tests showed that both cortical thickness and cortical bone density are correlated with pullout strength (rs, 0.548; P<0.001 and rs, 0.691; P<0.001, respectively). Although both factors are positively correlated with pullout strength, the effect of cortical bone density was the dominant factor affecting primary stability.