5 - Physiology of tooth movement and appliances overview

Question 1

What are the different types of tooth movement?

Answer 1

• physiological (mesial drift and tooth eruption)
• orthodontic (externally generated forces)

Question 2

Described the physiological basis of orthodontic tooth movement.

Answer 2

• external force applied to tooth
• bony remodelling occurs around the tooth to facilitate movement
• movement is dictated by PDLs

Question 3

Describe the differential pressure theory.

Answer 3

In areas of compression the bone is resorbed and in areas of tension, bone is deposited.

Question 4

What are the different theories of orthodontic tooth movement?

Answer 4

• piezo-electric theory (historic)
• differential pressure theory
• mechano-chemical theory

Question 5

What are the types of orthodontic appliance?

Answer 5

• removables
• functionals
• fixed

Question 6

What are the different types of orthodontic tooth movement?

Answer 6

• tipping
• bodily
• intrusion
• extrusion
• rotation
• torque (move root instead of crown)

Question 7

How much force is required to produce a tipping movement?

Answer 7

35-60 grams

Question 8

How do functional appliances work?

Answer 8

• mandible is postured away from normal position
• facial muscles are stretched which generates forces that are transmitted to teeth and alveolus
• restrict maxillary growth but promote mandibular growth and remodel the glenoid fossa
• uppers become more retroclined and lowers become more proclined

Question 9

How much force is required to produce bodily movement?

Answer 9

150-200 grams

Question 10

How much force is required to produce an intrusion movement?

Answer 10

10-20 grams

Question 11

How much force is required to produce an extrusion movement?

Answer 11

35-60 grams

Question 12

How much force is required to produce a rotational movement?

Answer 12

35-60 grams

Question 13

How much force is required to produce a torque movement?

Answer 13

50-100 grams

Question 14

Describe the histological changes brought about by light orthodontic force.

Answer 14

• hyperaemia within the PDL
• appearance of osteoclasts and osteoblasts
• resorption of lamina dura from pressure side (clasts)
• deposition of osteoid on tension side (blasts)
• frontal remodelling of socket
• PDL reorganises
• gingival fibres remain distorted

Question 15

Why is relapse common after light orthodontic forces?

Answer 15

The gingival fibres do not reorganise and remain distorted which can move tooth back to original position

Question 16

Describe the histological changes brought about by moderate orthodontic forces.

Answer 16

• occlusion of vessels of PDL on pressure side
• hyperaemia of vessels of PDL on tension side
• hylinisation on pressure side
• period of stasis 10-14 days
• increased endosteal vascularity (ie undermining resorption)
• sudden movement of tooth (can feel loose)
• PDL heals and remodels

Question 17

Describe the histological changes brought about by excessive orthodontic forces.

Answer 17

• extensive lateral root resorption and undermining resorption
• PDL necrosis

Question 18

What are the side effects of excessive orthodontic force?

Answer 18

• pain
• necrosis and undermining resorption
• 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

What is the threshold of duration of wear required to see some tooth movement?

Answer 20

6 hours/day

Question 21

How does age affect tooth movement?

Answer 21

Movement occurs more quickly in younger patients

Question 22

What is alveolar necking?

Answer 22

• thinner bone where a tooth has been lost
• thick cortical plates
• difficult to move tooth through

Question 23

Does RCT prevent orthodontic treatment?

Answer 23

Can be orthodontically treated if there is no associated pathology and the PDL is intact