What is the difference between an onlay and an overlay?

An onlay is a restorative treatment that covers part of the chewing cusps of a molar, while an overlay covers and protects all cusps. The difference lies in the extent of coverage, and the decision is made by considering ① the amount of remaining tooth structure ② the extent of decay or fracture ③ biting force and occlusal condition together. However, if the tooth has become significantly weakened or the extent of the crack is wide, a crown or another treatment may be more appropriate. The most suitable method will be confirmed and explained after consultation and examination.