Cortical bone microdamage affects primary stability of orthodontic miniscrew

Background

The aim of this study was to investigate the effects of orthodontic miniscrew pitch and thread shape on microdamage in cortical bone. The relationship between the microdamage and primary stability was also examined.

Methods

Ti6Al4V orthodontic miniscrews and 1.0-mm-thick cortical bone pieces from fresh porcine tibia were prepared. The orthodontic miniscrews had custom-made thread height (H) and pitch (P) size geometries, and were classified into three groups: control geometry; H_CP_C (H_C; thread height = 0.12 mm, P_C; pitch size = 0.60 mm), geometry with a narrower pitch; H_CP_N (H_C; thread height = 0.12 mm, P_N; pitch size = 0.30 mm), and geometry with a taller thread height; H_TP_C (H_T; thread height = 0.36 mm, P_C; pitch size = 0.60 mm). The orthodontic miniscrews were inserted into a pilot hole in the cortical bone, and maximum insertion torque and Periotest value were measured. After insertion, the samples were stained with basic fuchsin. Histological thin sections were obtained and the bone microdamage parameters, i.e., total crack length and total damage area, and insertion state parameters, i.e., orthodontic miniscrew surface length and bone compression area were calculated.

Results

The orthodontic miniscrews with the taller thread height resulted in lower primary stability with minimal bone compression and microdamage; however, the narrower thread pitch led to maximum bone compression and extensive bone microdamage.

Conclusions

A wider thread pitch reduced microdamage, and decreased thread height resulted in increased bone compression, ultimately resulting in increased primary stability.