[Orthodontics: interface between biology and biomechanics]

As far as craniofacial development is concerned, the orthodontist is mainly interested in growth of the jaws, craniofacial growth patterns and physiologic backgrounds of bone remodelling after tooth displacement. In biomechanics, the principle "action is equal to reaction" is one of the most important issues to deal with. The art in orthodontics is to distribute the reaction forces or to use them on purpose in an attempt to achieve one of the treatment goals. Reaction forces can be neutralized intra-orally. In this respect the location of the center of resistance of the tooth or a group of teeth needs to be defined carefully in order to have a good understanding of the biomechanical principles of tooth displacement. The action forces can be neutralized extra-orally. When no reaction forces on other teeth are applied, there will be no side effects of the reaction forces on other teeth. Research on the biomechanical principles of the headgear does offer some clinical information. Recently in orthodontics, the use of implants has been introduced. It can be used to neutralize reaction forces intra-orally. Moreover, the implants can be used primarily as anchorage during orthodontic tooth movement and later on to replace missing teeth. Recent animal studies at our department have shown that non-axial loading of implants with forces higher than 5 Newton can be used.