Micro-CT evaluation
The experimental animals were sacrificed and micro-CT was performed on all specimens by harvesting samples that included the implanted part of the tibia. Micro-CT (SkyScan 1173, Zwijnaarde, Belgium) was performed using an aluminum filter in a mid-resolution of 20 μm at 30 kV, 60 μA. The newly formed bone area ratio (the ratio of the area of new bone formation at the space between two threads in the pitch) and the bone-to-implant contact ratio (the ratio of bone directly attached to the thread of the screw) were evaluated. The quality of new bone was evaluated based on a total of 42 pictures analyzed by CTAn, a micro-CT analysis program. The bony tissue of the new bone was analyzed with an ROI (region of interest) at a width of 0.99 mm and a height of 2.48 mm. The bone fraction directly attached to the screw thread was analyzed with an ROI at a width of 0.97 mm and a height of 2.48 mm. For quantification of the quantity and quality of the new bone, the following items were evaluated: percent bone volume (bone volume/trabecular volume), specific surface (bone surface/volume ratio), trabecular bone pattern factor (the parameter of the connectedness of these bone patterns), trabecular thickness (the thickness of the trabeculae), trabecular number (the number of trabeculae), and trabecular separation (the distance between the trabeculae).