Tissue Reactions to Orthodontics

Question 1

What is the primary principle on which orthodontic treatment is based?

Answer 1

If prolonged pressure is applied to a tooth, tooth movement will occur as the bone around the tooth remodels

Question 2

Describe the important tooth anatomy to orthodontics

Answer 2

• Each tooth is attached to and separated from alveolar bone by PDL
• PDL is heavy collagenous supporting structure
• PDL fibres resist displacement of tooth during normal function

Question 3

Describe 2 theories of biological control of tooth movement

Answer 3

1. Bioelectric (piezoelectricity and bioelectric potentials)

2. Pressure tension theory (cellular changed and chemical messengers)

Question 4

What is the bioelectric theory of tooth movement?

Answer 4

Relates tooth movement to changes in bone metabolism controlled by the electric signals that are produced when alveolar bone and collagen flex and bend

Question 5

What is piezoelectricity?

Answer 5

A phenomenon observed in crystal structures in which deformation produces flow of current as electrons are displaced (bone mineral is crystal structure)

Question 6

What are bioelectric potentials?

Answer 6

Observed in bone which is not being stressed

Question 7

What is the pressure tension theory of tooth movement?

Answer 7

Relates tooth movement to changes in bone metabolism controlled by chemical rather than electric signals

Question 8

Describe 3 ways the pressure tension theory is said to work

Answer 8

1. Alteration in blood flow associated with pressure within PDL
2. Formation and/or release of chemical messengers
3. Activation of cells

Question 9

Name 2 sites of remodelling following orthodontic loading

Answer 9

1. Compression (force going towards)

2. Tension (force going away from)

Question 10

Describe 4 changes on the compression side during orthodontic loading

Answer 10

1. Compression of blood vessels
2. Attraction of osteoclasts
3. Resorption of bone
4. Production of fibrous tissue in Howship’s lacunae

Question 11

Describe 3 changes on the tension side during orthodontic loading

Answer 11

1. Stretching of PDL fibres
2. Stimulation of osteoblasts
3. Deposition of bone

Question 12

Name 2 major cells in the bone responsible for remodelling its structure

Answer 12

1. Osteoblasts

2. Osteoclasts

Question 13

Describe osteoblasts and their function

Answer 13

• Derived from mesenchymal cells
• Function to construct ECM of bone
• Control osteoclast function

Question 14

Describe osteoclasts and their function

Answer 14

• Large multinucleated cells of the monocyte lineage
• Adhere to bone surface and secrete hydrolytic enzymes
• Found in Howship’s Lacunae

Question 15

Describe how bone remodelling occurs

Answer 15

• Old bone is rapidly destroyed by short lived osteoclasts
• Pit left by resorption are filled by long lived osteoblasts which fill pits with new bone
• Ratio of osteoclast to osteoblast activity can result in net loss or gain of bone

Question 16

Name 2 ways bone remodelling is controlled

Answer 16

1. Systemic hormone

2. Local factors

Question 17

Give an example of a hormone which systemically affects bone remodelling

Answer 17

1. Parathyroid hormone

Question 18

Name a local factor which influences bone remodelling

Answer 18

Prostaglandins

Question 19

Describe how osteoblasts control osteoclasts

Answer 19

• Many factors e.g PTH, PGE2 influence the expression of RANK ligand expression on osteoblasts
• RANK ligand in combination with other factors e.g VEGF, induce the differentiation of osteoclasts from their precursors
• Osteoblasts secrete OPG to oppose RANK ligand effects

Question 20

Describe the typical tooth movement response after application of moderate orthodontic load

Answer 20

Split into 3 phases

 1. Initial phase in PDL and support bone
 2. Lag phase bone adjacent to crushed areas in PDL is removed
 3. Progressive tooth movement

Question 21

Name 2 types of bone resorption

Answer 21

1. Frontal resorption

2. Undermining resorption

Question 22

What is frontal resorption?

Answer 22

If orthodontic force does not occlude blood vessels in PDL steady remodelling of the tooth socket will result in smooth continuous tooth movement

Question 23

What is undermining resorption?

Answer 23

• When orthodontic forces occlude blood vessels and the area is hyalinised
• Resorption in this area will not take place until osteoclasts removed bone adjacent to hyalinised areas of PDL
• When osteoclasts reach PDL the tooth will move

Question 24

Associate frontal and undermining resorption with the phases of tooth movement

Answer 24

• Undermining resorption occurs in the lag phase until osteoclasts reach PDL
• Frontal resorption occurs in progressive tooth movement stage which occurs once osteoclasts have reached PDL

Question 25

Describe the changes which occur in 1-2 seconds on light or heavy sustained pressure against a tooth

Answer 25

- PDL fluid incompressible and alveolar bone bends | - Tooth moves within PDL space

Question 26

Describe how tooth movement occurs with regards to time if light pressure is applied

Answer 26

3.5 secs - Blood vessels in PDL partially compressed on one side and dilated on other, cells and fibres are distorted Minutes - Blood flow altered and prostaglandins and cytokines released Hours - Metabolic changes occur and affect cellular activity Days - Tooth movement begins as osteoclasts and blasts remodel socket

Question 27

Describe how tooth movement occurs with regards to time if heavy pressure is applied

Answer 27

3.5 secs - Blood vessels within PDL completely occluded on one side Minutes - Blood flow cut off to compressed PDL area Hours - Cell death in area Days - Cell differentiation in adjacent marrow spaces and resorption begins

Question 28

What is the rate limiting factor in tooth movement?

Answer 28

Efficiency of bone resorption

Question 29

Describe 4 cellular reactions in pressure areas when optimal orthodontic forces are applied

Answer 29

1. Cellular proliferation 2. Osteoclasts migrate to PDL 3. Resorption of bone 4. Tooth movement occurs

Question 30

Describe 5 cellular reactions in tension areas when optimal or excessive orthodontic forces are applied

Answer 30

1. Stretching of PDL fibres 2. Cellular proliferation of fibro and osteoblasts 3. Increase length of PDL fibres 4. Deposition of osteoid 5. Remodelling and reattachment of PDL fibres

Question 31

Describe 4 cellular reactions in pressure areas when excessive orthodontic forces are applied

Answer 31

1. Capillary blood vessels are occluded resulting in death of PDL cells 2. Cellular proliferation occurs adjacent to hyalinised sections of PDL 3. Resorption occurs in hyalinised area from cancellous bone towards lamina dura 4. Tooth movement occurs

Question 32

Describe the ideal relationship between the level of force and tooth movement

Answer 32

The force should be just high enough to stimulate cellular activity without completely occluding blood vessels in PDL

Question 33

Describe the ideal relationship between the duration of force and tooth movement

Answer 33

Clinical experience suggests threshold for force duration for approx. 6 hours that increasingly effective tooth movement is produced if force is maintained longer than this duration (6 hours in any 24 hours)

Question 34

What is the optimum orthodontic force for tooth movement?

Answer 34

20-25 g/cm2 of the root surface

Question 35

Describe 3 types of durations of forces

Answer 35

Continuous - Force maintained at some fraction of the original from one visit to the next Interrupted - Force levels decline to 0 between activations Intermittent - Force levels decline abruptly to 0 intermittently when orthodontic appliance is taken out or fixed appliance is deactivated

Question 36

How is light continuous force achieved?

Answer 36

Fixed appliances

Question 37

Describe how interrupted force is achieved

Answer 37

Removable appliance springs

Question 38

Describe how intermittent forces are achieved

Answer 38

- Removable appliances or headgear being worn | - Must be worn for minimum 6 hours a day due to threshold for force duration

Question 39

Describe 3 factors which affect pain during orthodontic treatment

Answer 39

Attitude - Attitude and discomfort have correlation on compliance Age - Adult subjects tend to perceive more pain than younger patients Appliance type - Fixed appliances have higher values for pain

Question 40

Name 5 adverse effects of orthodontics

Answer 40

1. Crestal bone loss 2. Root resorption 3. Pulpal damage 4. PDL damage 5. Decalcification

Question 41

Describe the loss of crestal bone during orthodontics

Answer 41

- Slight reduction can occur but is usually of little clinical relevance - In active periodontitis crestal bone should not be put at risk as bone resorption is already occuring - Orthodontic tooth movement avoided in this case

Question 42

What are the most common teeth for root resorption to occur during orthodontic treatment?

Answer 42

Upper incisors or first permanent molars

Question 43

Describe how pulpal damage can occur during orthodontics

Answer 43

If a pulp is fibrotic with poor blood supply, orthodontic movement can strangulate vessels resulting in necrosis

Question 44

Name 2 occasions when damage to the PDL by orthodontic forces is not minimal and transient

Answer 44

1. Excessive force used | 2. Pre-existing periodontal disease

Question 45

Describe decalcification as an adverse effect of orthodontics

Answer 45

- Fixed appliances predispose to plaque accumulation as tooth cleaning is difficult - Lesions can regress following removal of appliance but some patients may be left with enamel scarring