When should a prophy brush not be used during polishing?

A prophy brush should not be used on exposed cementum or dentin because these tissues are softer and more vulnerable compared to enamel. The abrasive nature of a prophy brush can cause excessive abrasion and damage to these surfaces. This could lead to increased sensitivity, potential for recession, and other complications.

Using a prophy brush on healthy enamel surfaces, on the other hand, is generally acceptable as enamel is much harder and can withstand the polishing action without risk of damage. Similarly, while caution is needed with children's primary teeth and unfilled cavities, those surfaces still have some degree of enamel that can be polished if done properly. In contrast, exposed cementum and dentin are not suitable for polishing with a prophy brush due to the likelihood of significant harm.