The Blood flow theory explains orthodontic tooth movement by emphasizing the role of vascular changes in the periodontal ligament (PDL). When force is applied, it alters the blood flow in the PDL, leading to a cascade of biological responses including bone remodeling. Reduced blood flow in the compressed area induces bone resorption, while increased flow in the tension area promotes bone formation. This dynamic vascular response is critical for the remodeling necessary for tooth movement. Hence, blood flow alterations directly regulate the cellular activity responsible for orthodontic tooth movement.

Reference: Graber's Orthodontics, 7th Edition.