You should understand how the periodontal ligament (PDL) responds to orthodontic forces (light or heavy). Physiological changes and responses of the bone or the connective tissue should be understood. The Pressure -Tension appearance of the PDL is very important.

To refresh your memory, read the following paragraph with the attached figure as quoted from “Handbook of Orthodontics” 2nd Ed, 2015: “When a force is placed on a tooth, bone is deposited on the tension side of the periodontal ligament and resorbed on the pressure side (Fig. 5.1). On the pressure side, when the force is light, multinucleate cells resorb bone directly. However, if the forces are greater and exceed capillary blood pressure, cell death can occur and a cell-free area forms. This is described as hyalinization, due to the glass-like appearance of these regions when viewed with light microscopy resembling hyaline cartilage.”