OE L38 Orthodontic Tooth Movement: Cellular Biology

Question 1

What is the side of the tooth with force applied to it known as?

Answer 1

Tension side (bone formation, stretching of PDL fibres and cells)

Question 2

What is the side of the tooth that the force is travelling towards called?

Answer 2

Compression side (bone resorption, PDL compressed)

Question 3

Which cells are involved in orthodontic tooth movement and where are they found?

Answer 3

Most of the cells involved in orthodontic tooth movement are found in the PDL:

• PDL fibroblasts
• MSCs
• Pre-osteoblasts
• Pre-osteoclasts
• Pre-cementoblasts
• Osteocytes
• Endothelial cells
• Neuronal cells
• Macrophages, T cells, neutrophils

Question 4

Where are osteoblasts and osteoclasts found with regards to tooth movement?

Answer 4

• Osteoblasts line the bone surface on the tension side

- Osteoclasts found on compression side

Question 5

What is the function of osteocytes?

Answer 5

• Found embedded in lacunae
• Communicate with other osteocytes, osteoblasts and osteoclasts
• Produce pro-osteoblastic signals on tension side
• Produce pro-osteoclastic signals on compression side

Question 6

What signals are present on the tension side?

Answer 6

• TIMPs
• TGF-β
• IL-10 which causes increased osteoprotegerin (OPG)
• OPG binds to RANKL to reduce osteoclast numbers and reduce resorption
• Increased differentiation of osteoblasts, increases bone deposition

Question 7

What signals are present on the compression side?

Answer 7

• MMPs
• Pro-inflammatory signals e.g. TNF-α and IL-β
• Prostaglandin E release through activity of Cox2 enzyme, leads to RANKL and M-CSF production which activates osteoclasts

Question 8

What are the 3 theories behind orthodontic tooth movement?

Answer 8

• Pressure-tension theory
• Biological piezoelectric theory
• Biomechanical theory

Ultimately, it is most likely a combination of all 3 theories that causes orthodontic tooth movement.

Question 9

Outline the pressure-tension theory.

Answer 9

• On the compression side, the collagen fibres, cells in collagen fibres, BVs and nerve fibre cells are compressed
• This compression causes slow displacement of the tooth into the PDL space
• Compression of PDL creates hypoxic environment, causes necrosis of some cells
• Some BVs rupture, infiltration of immune cells
• Induces a range of chemical signals e.g. cytokines, GFs, prostaglandins

Controlled aseptic inflamation leading to bone resorption

Question 10

Outline the biological piezoelectric theory.

Answer 10

• Pressure placed on bone induces piezoelectric signals
• Receptors on osteocytes sense electric currents
• Tension side: electronegative, favours osteoblastic activity, bone deposition
• Compression side: electric neutrality/positivity, favours osteoclastic activity, bone resorption

Question 11

Outline the biomechanical theory.

Answer 11

• On the tension side the cells are stretched and the cytoskeleton is reorganised
• On the compression side the cytoskeleton is also reorganised
• Geometric remodelling of the cytoskeleton may alter cell metabolism or signal transduction by changing the relative position of different regulatory models, hence altering their chemical interaction

Question 12

Describe the biochemical theory with reference to the changes on the tension side.

Answer 12

• Cells stretched
• Cellular sensing of mechanotransduction via cell surface integrins
• Increased cell proliferation
• Increased angiogenesis, increased immune cells to site
• Increased remodelling of ECM, MMPs play role
• Bone formation

Question 13

Describe the biochemical theory with reference to the changes on the compression side.

Answer 13

• Cells compressed
• PDL cells synthesise IL-1, IL-6 and prostaglandins
• IL1 and 6 upregulate RANKL and MMPs expression by PDL cells and osteoblasts
• Osteoclast activation, bone resorbed
• Increased MMP activity to remodel collagen fibres

Question 14

Why are anti-inflammatory medicines not recommended to relieve orthodontic pain?

Answer 14

• NSAIDs inhibit Cox2 enzyme and prevent production of prostaglandins
• Prostaglandins are important for orthodontic tooth movement
• Therefore, ibuprofen and aspirin will inhibit prostaglandin production and tooth movement will be delayed and inhibited

Question 15

Describe the events that take place in the first 6 hours after braces being placed/tightened.

Answer 15

• First few seconds: fluid fills spaces in PDL and prevents immediate displacement of the tooth
• 5 seconds onwards: PDL fibers and cells distorted, some cell apoptosis and necrosis
• 4-6 hours: minimum of 4-6 hours of continuous compressive force and tooth movement will be initiated

Question 16

What happens to the PDL during orthodontic tooth movement?

Answer 16

• Hyalinastion of PDL tissue
• Afibrillar structure, apoptotic and necrotic cells, no BVs or nerves
• The tissue is degraded by osteoclasts and macrophages
• Extent of hyalinisation is proportional to the amount of force applied
• If heavier forces are applied the hyalinastion period will be longer, weaker force = shorter period = tooth movement achieved sooner

Question 17

What are the 4 phases of orthodontic tooth movement?

Answer 17

• Phase 1: initial movement, displacement of tooth into into PDL
• Phase 2: lag phase, tooth movement stops, hyalanised tissue forms, increase RANKL and osteoclast proliferation
• Phase 3: post-lag phase, movement accelerates, bone resorption and bone apposition
• Phase 4: compression, movement continues

Question 18

Describe the blood vessel response during orthodontic tooth movement.

Answer 18

• Blood vessels are reorganised
• Angiogenesis on tension side
• BVs supply immune cells to both sides

Question 19

Describe the neural tissue response during orthodontic tooth movement.

Answer 19

• Capable of releasing substance P and CGRP

- These modulate signalling pathways leading to an inflammatory response

Question 20

What is the consequence of orthodontic forces being too high?

Answer 20

Root resorption, mediated by odontoclasts on the root surface.