An archwire doesn't move a tooth directly — it moves the periodontal ligament, and the ligament tells the bone what to do. This chapter follows that chain: from the fibrous tissue holding a tooth in its socket, to the pressure and tension it feels under load, to the cells and molecular signal that turn "squeezed" into "resorbed" and "stretched" into "built."
Every mechanism in the last two chapters can fail in a specific, recognizable way. A blocked path stalls a tooth in impaction. The follicle that clears bone can instead trap fluid and become a cyst. A ligament that never regenerates after injury lets bone fuse directly onto root, freezing a tooth in place. And a primary tooth's own exfoliation depends on the same resorption machinery being aimed at its root instead of bone. This chapter follows each failure back to the normal process it broke.