Orthodontic tooth movement

Question 1

What is the pressure-tension theory?

Answer 1

When a force is placed on a tooth, there is bone depostion on the tension side, and bone resorption on the pressure side

Question 2

What is hyalinisation and when does it occur?

Answer 2

This occurs when the force is high and exceeds capillary blood pressure, therefore causing cell death and formation of a cell-free area.
In light microscopy, hyalinisation appears as a glass-like region, resembling hyaline cartilage.

Question 3

Force

Answer 3

A load applied to an object that has both magnitude and direction. Forces can be represented visually by vectors.

Question 4

Centre of resistance

Answer 4

The point at which bodily movement or translation of an object will result when a force is applied. This is difficult to determine on the tooth since it is embedded in bone, however it is estimated to be around one third to halfway down the root of a healthy single-rooted tooth.

Question 5

Moment

Answer 5

When a force is applied to a body at a distance from the centre of resistance, a rotational effect or moment is created.
It is the product of the force and the distance from the centre of resistance - therefore, the larger the distance, the greater the rotation.

Question 6

Couple

Answer 6

Represents 2 equal and opposite forces. It exerts no net bodily movement of teeth, as the forces are opposite in direction and cancel each other out.

Question 7

What are the two types of movement which can be generated on a tooth to cause movement?

Answer 7

Tipping movement
Bodily movement

Question 8

What is a tipping movement and which appliance brings about this?

Answer 8

These movements are easy to generate by point contact on the crown of the tooth. The active components on a removable appliance cause this type of movement, whereby only the position of the crown is changed, rather than the entire tooth as a unit.

Question 9

What is a bodily movement and which appliance brings about this?

Answer 9

This movement is more difficult to produce, and requires a couple as well as a force, to prevent rotational effects.
This is essentially how the edgewise slot of a fixed orthodontic appliance works.

Edgewise slot - a rectangular wire is passed through the slot of the bracket, therefore creating a couple within the bracket slot. This will control root position and allow bodily movement of the tooth in the direction of the force.

Question 10

What is the bone bending theory?

Answer 10

This theory suggests that when a force is applied to a tooth, it is distributed throughout the PDL fibres, as well as the bone, causing it to bend slightly. Bone is more elastic than other tissues, and so, it allows this to occur.

Question 11

Which bioelectric signals are produced during tooth movement?

Answer 11

Tension side - this is the side on which the force is applied; it is an electronegative area which favours osteoblastic activity

Pressure side - this is the side on which the tooth moves; it is electropositive and favours osteoclastic activity.

Question 12

What happens to alveolar bone when it bends or flexes?

Answer 12

It releases electric signals, which are similar to piezoelectric signals.

Question 13

What are the features of piezoelectric signals?

Answer 13

1. Quick decay rate - starts when force is applied and finishes quickly even if the force is maintained
2. Equal signals produced on opposite side when force is released

Question 14

Which signalling molecules are involved in orthodontic tooth movement?

Answer 14

1. Arachidonic acid metabolites
2. Prostaglandins - cytokines and growth factors