The removal of unwanted material from the walls of a hard surface. Removal of hard deposits on the root surface (root curettage) can lead to improved gingival health. Periapical curettage removes granulation tissue surrounding the root apex and any associated intra-bony cyst lining. Subgingival curettage is the removal of inflamed tissue and the epithelial lining of a periodontal pocket. Curettage may be performed with hand instruments (curettes) or ultrasonically.