[Headgears in orthodontics: mechanical principles and clinical application]

Understanding of the mechanics of displacement of molars during headgear treatment makes an efficient clinical application possible, while undesirable molar displacements can be prevented or compensated. Of paramount importance is the relationship of the line of force to the center of resistance of the tooth to be moved. This relationship is examined in three dimensions. Subsequently the applicability of mechanical principles in clinical use of headgears is discussed.