Abstract
The application of force to bone has been shown to produce a change in its shape or morphology. This paper reviews the literature relating to the principles of such force application by extra-oral appliances to the maxilla and mandible both in animals and humans. The significance of direction of force, amount of force, and timing of force application is stressed. Studies of craniofacial changes associated with the use of face-bows and cervical straps in orthodontic treatment are closely examined. The post-treatment changes, after the extra-oral forces have been discontinued, are also discussed.