Biological and mechanical principles of orthodontic tooth movement

Biological and mechanical principles of orthodontic tooth movement B. Mechanical principles Sara El-kilani Specialist orthodontist

Learning outcomes �To define centre of resistance, force, couple and moment of force. �To illustrate various types of tooth movements. �To describe how tooth movement is influenced by age factor.

Centre of resistance �Is the point in a body at which resistance to movement can be considered concentrated. �Is the point at which bodily movement or translation of an object will result when a force is applied.

�centre of resistance of a single rooted tooth is about half way between the root apex and the crest of the alveolar bone, For a multi-rooted tooth, the centre of resistance is estimated to be 1 -2 mm apical to the furcation.

Force �a load applied to an object that has both magnitude and direction. Type of tooth movement Force magnitude Tipping 30 -60 g Bodily movement 100 -150 g Rotational movement 50 -75 g Extrusion 50 -75 g Intrusion 15 -25 g

Moment �When a force is applied to a body at a distance from the centre of resistance, moment is created which result in tooth tipping. �Moment is the product of the force and the distance from the centre of resistance, so the greater the distance the greater the tipping.

Couple �It represents two equal and opposite forces. A couple exerts no net force to bodily move a tooth, as the forces are opposite in direction and cancel each other out. �A couple acting alone on a tooth will produce a purely rotational movement.

Tipping and bodily tooth movement �Tipping movements are relatively easy to generate by point contact on the crown of a tooth and this is how the active components on a removable appliance work.

�Bodily movement is more difficult to produce and requires the combination of a force and a couple to control the rotational effect.

�This is essentially how the edgewise slot of a fixed appliance works. By placing a rectangular wire into the slot, a couple is created within the bracket slot, which will control root position and allow the tooth to move bodily in the direction of applied force. teeth will tip and then upright as they move along the archwire, giving the impression of bodily movement.

Effect of age factor on tooth movement �The rate of tooth movement is more in children compared with adults for the following reasons: 1. Physiologic tooth movement is greater. 2. Growth can be utilized.

3. The cellular response is quicker. 4. The alveolar bone has a greater proportion of osteoclasts. 5. The periodontal ligament is more cellular. 6. The width of the periodontal ligament is increased in newly erupted teeth, and so a greater force can be applied before constriction of the blood vessels occurs.

References �Cobourne M. T. , Di. Biase A. T. Handbook of orthodontics.