Physiology of Tooth Movement

Question 1

What is the physiological basis of orthodontics?

Answer 1

If an external force is applied to a tooth, the tooth will move as the bone around it remodels
If a tooth has no PDL or is ankylosed, it will not move

Question 2

What are the different theories for orthodontic tooth movement?

Answer 2

Piezo-electric theory
Differential pressure theory
Mechano-chemical theory

Question 3

What is differential pressure theory?

Answer 3

In areas of compression bone is resorbed and in areas of tension bone is deposited

Question 4

What happens in mechano-chemical theory at a cellular level?

Answer 4

Cell shape changes occur within the PDL and adjacent alveolar bone
This causes the production and release of cytokines
The cytokines regulate the action of target cells

Question 5

Describe mechano-chemical theory

Answer 5

Mechanical loading causes fluid movement, stretching and compression of the PDL fibres
This is detected by osteocytes which product cytokines which will activate osteoblasts to recruit osteoclasts
This causes bone resorption

Question 6

Describe a cell interaction within the PDL

Answer 6

Cytokines activate osteoblasts to produce prostaglandins and leukotrienes
These cause the osteoblasts to produce secondary messengers
These initiate the production of RANKL, CSF and IL-1
IL-1 increases production of RANKL
RANKL and CSF cause monocytes to fuse and form osteoclasts
RANKL stimulates osteoclasts to become active and resorb the bone

Question 7

What do osteoblasts do in areas of compression?

Answer 7

Expose the osteoid layer giving osteoclasts access to resorb the bone
Osteoblasts send signals to osteoclasts (RANKL) to recruit and activate osteoclasts to resorb bone

Question 8

What to osteoblasts do in areas of tension?

Answer 8

They are flattened, covering the osteoid layer and preventing osteoclasts from gaining access to the bone
Osteoblasts secrete collagen and other proteins forming the organic matrix into which they then secrete hydroxyapatite which forms new bone

Question 9

What regulates bone modelling?

Answer 9

Osteoblasts release OPG which prevents osteoclasts differentiation and suppress their activity
The balance between RANKL and OPG therefore regulates bone modelling

Question 10

What are the types of orthodontic appliances?

Answer 10

Removable
Functional
Fixed

Question 11

What are the types of tooth movement?

Answer 11

Tipping
Bodily movement
Intrusion
Extrusion
Rotation
Torque

Question 12

How do functional appliances work?

Answer 12

The mandible is postured away from its normal rest position
The facial musculature is stretched which generates forces transmitted to the teeth and alveolus

Question 13

What are the clinical effects of a twin block?

Answer 13

Retrocline of upper incisors
A lateral open bite - may spontaneous close

Question 14

What is the mode of action of functional appliances?

Answer 14

Skeletal change of 30%, growth of mandible and restraint of maxilla
Dentoalveolar change of 70%, retroclincation of upper teeth, proclination of lower teeth
Mesial migration of the lower teeth
Distal migration of the upper teeth
This achieves class I

Question 15

What are the optimum forces required for moving teeth?

Answer 15

Tipping - 35-60g
Bodily movement - 150-200g
Intrusion - 10-20g
Extrusion - 35-60g
Rotation - 35-60g
Torque - 50-100g

Question 16

What happens when light force is applied to a tooth?

Answer 16

Hyperaemia within the PDL
Appearance of osteoclasts and osteoblasts
Resorption of lamina dura from pressure side (osteoclasts)
Apposition of osteoid on tension side (osteoblasts)
Remodelling of socket - frontal resorption
PDL fibres reorganise
Gingival fibres appear not to become reorganised but remain distorted

Question 17

What happens when moderate force is applied to a tooth?

Answer 17

Occlusion of vessels of PDL on pressure side
Hyperaemia of vessels in PDL on tension side
Cell free areas on pressure side (hylinisation)?
Period of stasis
Increased endosteal vascularity - undermining resorption
Relatively rapid movement of tooth with bone deposition on the tension side - tooth may become slightly loose
Healing of PDL - reorganisation and remodelling

Question 18

What are the unwanted side effects of excessive force?

Answer 18

Pain
Root resorption - significant if greater than 1/3 root length lost
Anchorage loss
Possible loss of tooth vitality

Question 19

What factors affect the response to orthodontic force?

Answer 19

Magnitude
Duration
Age
Anatomy

Question 20

How does magnitude affect tooth movement?

Answer 20

Light forces allow slow continuous tooth movement
Moderate/heavy forces give rapid movement initially then 10-14 days with little movement while undermining resorption occurs

Question 21

How does age affect tooth movement?

Answer 21

It takes longer to move teeth in older patients
Teeth will still move

Question 22

How does anatomy affect tooth movement?

Answer 22

Can’t move teeth if there is no bone
Soft tissues - anterior open bite
Teeth which have been root treated - can only treat if intact PDL and no periapical pathology

Question 23

What are the side effects of orthodontic force?

Answer 23

Pain and mobility
Pulpal changes
Root resorption
Loss of alveolar bone support
Relapse