objectives: nitric oxide (no) is a signaling molecule that mediates mechanical bone loading. cyclic guanosine 3', 5' monophosphate (cgmp) is a no-induced effector molecule. the aim of this study was to assess the effect of no-cgmp pathway on orthodontic tooth movement (otm) in rats by use of two phosphodiesterase 5 (pde5) inhibitors namely sildenafil and tadalafil as chemical tools. materials a...