Microdamage in cortical bone due to the overtightening of orthodontic microscrews

The possibility of skeletal anchorage achieved with mini-implants has greatly broadened the spectrum of treatment modalities in orthodontics. Aim of this study was to investigate the microstructural alterations in cortical bone due to overtightening of orthodontic microscrews during the insertion procedure.

After having prepared three fresh pelvic porcine bone segments and removed the periosteum, we drilled 25 holes 1 mm in diameter into the segments. Using a screwdriver, we then manually inserted a total of 20 orthodontic microscrews from the Aarhus Anchorage System (1.5 mm x 7.7 mm) into the holes: ten screws were inserted about 6.7 mm deep, leaving the last 1 mm of the thread visible outside the bone. The other ten screws were initially inserted 7.7 mm (until the screw thread was no longer visible and the screw neck was slightly touching the cortex). Those last ten screws were then tightened by another quarter-turn. Five drill-holes were left empty, serving as a reference. The three pelvic bone segments were sawed into 25 smaller bone-specimens, 20 segments containing one screw and five with only the drill-hole. They were cortically cross-sectioned and processed for scanning electron microscope analysis. All the microcracks in the peri-implant bone tissue at least 50 microm long were documented for each sample. Statistical analysis was carried out according to the "number of cracks", "accumulated length of all cracks", "maximum radius of crack alteration", and "longest crack". The specimens were then divided into two subgroups according to cortical thickness.

The group of overtightened implants showed higher values in the following categories: number of cracks, accumulated length of all cracks, maximum radius of crack alteration, and longest crack in comparison to the reference samples without screws and the group of less deeply-inserted implants. However, we identified no correlation between cortical thickness and the degree of microdamage.

We demonstrate that there was more microstructural damage in cortical bone due to overtightening through deep insertion of orthodontic microscrews than occurred at the lower insertion depth. Extensive osseous microdamage may detract from the stability of immediately-loaded microscrews and implants due to the bone-remodeling processes initiated by microdamage. The subject of just how serious this potential risk is should be addressed in future studies.