B Melsen C Verna Authors’ affiliations: Birte Melsen, Carlalberta Verna, Department of Orthodontics, Royal Dental College, Aarhus University, Denmark Correspondence to: Prof. Birte Melsen, DDS Royal Dental College Department of Orthodontics Aarhus University Vennelyst Boulevard DK-8000 Aarhus C Denmark Tel: +45 8 942 4037 Fax: +45 8 619 6029 E-mail: orthodpt@odont.aau.dk To cite this article: Prog. Orthod. 1, 1999; 10–22 Melsen B, Verna C: A rational approach to orthodontic anchorage Copyright © Munksgaard 1999 ISSN 1399-7513 A rational approach to orthodontic anchorage Abstract: The outcome of orthodontic treatment is often compromised by the loss of anchorage. Yet surprisingly, forces acting on the anchorage unit that bring about the loss have received very little interest. This paper discusses the biologic background for anchorage control; specifically, the impact of the orthodontic force systems and of occlusal forces on the cellular responses of periodontal tissues supporting the anchorage teeth. Also discussed is the influence of various anchorage devices on the interaction between vertical and sagittal directions of dentofacial development. The importance of taking the vertical forces into consideration is stressed. A rational approach to orthodontic anchorage is suggested. We advocate the use of intra-oral and extra-dental devices when possible, as well as rigid appliances that stimulate the patient’s sensation of occlusion. Clinical cases are presented to demonstrate the clinical application of this new approach. Key words: active unit; anchorage; biomechanics; biological anchorage; extrusion; posterior stability; posterior rotation; reactive unit Introduction All orthodontic appliances are in a state of equilibrium. They function by delivering equal and opposite forces to the two units between which they are placed. Force sys- tems configured as such are always far from being desir- able. In cases where one of the units is to be kept as anchorage, various solutions have been suggested — all in an effort to deliver differential force systems to these two units. Traditionally anchorage has been classified as either intra-oral or extra-oral. The intra-oral anchorage systems are subclassified further as intra-arch, inter-arch, and ex- tra-dental. The intra-arch anchorage is based on the as- sumption that more teeth offer more anchorage than fewer teeth and the tipping is easier than translation. Whereas, the inter-arch anchorage rests upon the desire of moving teeth in one direction in the upper and in the opposite direction in the lower jaw. In the case of the extra-oral anchorage, the reactive forces are transferred to