The influence of miniscrew insertion torque.

Objective: The aim of this in vitro study was to evaluate the progressive development of surface microdamage produced following the insertion of orthodontic miniscrews (OMs) into 1.5 mm thick porcine tibia bone using maximum insertion torque values of 12 Ncm, 18 Ncm, and 24 Ncm. Methods: Aarhus OMs (diameter 1.5 mm; length 6 mm) were inserted into 1.5 mm porcine bone using a torque limiting hand screwdriver set at 12 Ncm, 18 Ncm, and 24 Ncm. A custom rig equipped with a compression load cell was used to record the compression force exerted during manual insertion. A sequential staining technique was used to identify microdamage viewed under laser confocal microscopy. Virtual slices were created and stitched together to form a compressed two-dimensional composition of the microdamage. Histomorphometric parameters, including total damage area, diffuse damage area, maximum crack length, maximum damage radius, and maximum diffuse damage radius, were measured. Kruskal-Wallis Tests and Wilcoxon Rank-Sum Tests were used to analyse the generated data. Results: All OMs inserted using 12 Ncm failed to insert completely, while partial insertion was observed for two OMs inserted at 18 Ncm. Complete insertion was achieved for all OMs inserted at 24 Ncm. Histomorphometrically, OMs inserted using 24 Ncm produced a significantly larger diffuse damage area (P < 0.05; P < 0.05) and maximum diffuse damage radius (P < 0.05; P < 0.05), for both the entry and exit surfaces, respectively, compared with the 12 Ncm and 18 Ncm groups. Conclusions: Insertion torque can influence the degree of OM insertion and, subsequently, the amount of microdamage formed following insertion into 1.5 mm thick porcine tibia bone. An increase in insertion torque corresponds with greater insertion depth and larger amounts of microdamage.

Influence of cortical bone thickness on miniscrew microcrack formation.

INTRODUCTION: The aim of this in-vitro study was to investigate the influence of cortical bone thickness on the amount of surface microdamage produced after insertion of orthodontic miniscrews (OM) in porcine tibia bone. METHODS: Aarhus OMs (Medicon, Tuttlingen, Germany; diameter, 1.5 mm; length, 6 mm) were inserted into 1.0 mm (group A; n = 10), 1.5 mm (group B; n = 10), and 2.0 mm (group C; n = 10) of porcine cortical bone using a torque-limiting hand screwdriver set at 18 Ncm. A sequential staining technique was used to identify microdamage under laser confocal microscopy. Virtual slices were stitched together using ImageJ software (National Institutes of Health, Bethesda, Md) to form a compressed 2-dimensional composition of the microdamage. The ImageJ software was used to quantify the total damage area, diffuse damage area, maximum crack length, maximum damage radius, and maximum diffuse damage radius. Kruskal-Wallis tests and Wilcoxon rank sum tests were used to analyze the data. RESULTS: All OMs in group A (1.0 mm) were inserted completely; however, 2 OMs from group B (1.5 mm) and all OMs in group C (2.0 mm) failed to insert completely. The entry surface of group C (2.0 mm) exhibited significantly higher amounts of total damage, diffuse damage area, maximum crack length, and maximum crack damage radius compared with groups A (1.0 mm) and B (1.5 mm). The maximum crack length observed on the entry and exit surfaces ranged from 1.03 to 3.06 mm. CONCLUSIONS: In this study, we demonstrated a higher level of microdamage after the insertion of OMs into 2.0-mm thick cortical bone compared with 1.0-mm thick cortical bone. Therefore, clinicians need to consider the thickness of the cortical bone at the insertion site, because mechanisms to reduce cortical bone thickness would likely reduce the amount of microdamage formed. A safety zone of 3.5 mm from the OM is also recommended for OMs inserted into 1.0- and 1.5-mm cortical bone thicknesses to minimize any detrimental effects after targeted remodeling.