Biomechanics of tooth movement

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Biomechanics
of
Orthodontic 
Tooth Movement
Overview
Physiology/Anatomy
Movement/Forces
Orthodontic force
Appliances
What is needed?
What is needed?
• Tooth
• Healthy periodontal ligament
• Bone
• Applied force
Tooth movement is dependant upon physiology of the 
Periodontal ligament and Bone – i.e. Turnover
Tooth
• Means of force application/delivery
• Otherwise ‘inactive’
Periodontal Ligament
• Fibres transmit forces applied to the tooth
• Viscostatic damping of force
• Cells within PDL - Fibroblasts
- Osteoblasts
- Osteoclasts
- Undifferentiated cells
Bone
• Role of Bone in the body
- Structural
- Metabolic
Bone
Structural:
Cortical bone
slow turnover
Metabolic:
• Trabecular bone
constant turnover
Bone Turnover
• Osteclasts
derived from perivascular cells
• Osteblasts
derived from monocytes
Control is by systemic and local factors
Bone – Metabolic Role (systemic control)
Vit D 
(1,25 DHCC)
Gut –
Ca binding
Ca absorption
Bone –
short term:
Ca++ from bone fluid
long term:
Resorption
Deposition
Kidney –
PO4 excretion
Ca++ resorption
PTH
Ca++ 
Serum
Ca++ 
Serum
Local control
• Biologic electricity
• Blood flow
• Microfractures
Local control
• Biologic electricity
• Blood flow
• Microfractures
1. Pietzoelectric effect (V. short duration)
Bending of collagen and bone results in
e-’s moving within crystal lattice 
No signal = bone atrophy
2. Streaming potential 
Movement of ground substance 
results in a potential difference 
+ve on compression
-ve on tension
Affects cell permeability
Local control
• Biologic electricity
• Blood flow
• Microfractures
Sustained pressure 
Alters blood flow in PDL
flow in tension
flow in compression
Affects biochemical environment
Local control
• Biologic electricity
• Blood flow
• Microfractures
Microfractures 
Occur within bond, these accumulate
affecting the microenivironment
Local control
• Biologic electricity
• Blood flow
• Microfractures
Prostaglandins
Cytokines
Cyclic amp
Osteblasts Osteoclasts
Local control (+systemic)
• Biologic electricity
• Blood flow
• Microfractures
Prostaglandins
Cytokines
Cyclic amp
Osteblasts Osteoclasts
Systemic Control
PTH
Vit D
Calcitonin
Tooth movement
Force
Tooth
PDL/Bone
Biological electricity
Blood flow
Microfractures
Osteoblasts (tension)
Osteoclasts (compression)
Resorption and Deposition 
of bone
What happens depends on:
• Level of force
• Duration of force
What happens depends on:
• Level of force
• Duration of force
Heavy force/short duration
1-50Kg / less than 1 sec
Force absorbed by bone bending = Pain
(Pietzoelectric effect)
What happens depends on:
• Level of force
• Duration of force
Heavy force/short duration
1-50Kg / less than 1 sec
Force absorbed by bone bending = Pain
(Pietzoelectric effect)
Heavy force/long duration
1-50Kg / continuous
1-2 secs – PDL fluid displaced 
2-3 secs – PDL tissues compressed = pain
Hours-days – cellular necrosis within bone
= hyalanised (acellular layer)
Removed by osteoclasts, tooth movement in
‘steps’ – Undermining Resorption
What happens depends on:
• Level of force
• Duration of force
Light force/short duration
less than 1Kg / less than 1 sec
Force absorbed by PDL = no effect
(PDL is actively stable – 5-10g)
What happens depends on:
• Level of force
• Duration of force
Light force/short duration
less than 1Kg / less than 1 sec
Force absorbed by PDL = no effect
(PDL is actively stable – 5-10g)
Light force/long duration
less than 1Kg / continuous
Progressive tooth movement occurs
What happens depends on:
• Level of force
• Duration of force
Orthodontic forces
Excessive = pain + undermining resorption
Ideal = socket remodeling
In reality – some undermining 
resorption occurs
Orthodontic force
• Tipping
• Translation
• Rotation
• Extrusion
• Intrusion
Simplest orthodontic movement
Occurs about centre of resistance 
(1/3 from root apex)
Forces are high at apex and alveolar crest, 
reduce to zero at centre of resistance
Orthodontic force
• Tipping
• Translation
• Rotation
• Extrusion
• Intrusion
Simplest orthodontic movement
Occurs about centre of resistance 
(1/3 from root apex)
Forces are high at apex and alveolar crest, 
reduce to zero at centre of resistance
Force – 50-75g
Orthodontic force
• Tipping
• Translation
• Rotation
• Extrusion
• Intrusion
Bodily movement
All of PDL is uniformly loaded
Orthodontic force
• Tipping
• Translation
• Rotation
• Extrusion
• Intrusion
Bodily movement
All of PDL is uniformly loaded
Force – 100-150g
Orthodontic force
• Tipping
• Translation
• Rotation
• Extrusion
• Intrusion
Rotary movement
Theoretically need high force
Orthodontic force
• Tipping
• Translation
• Rotation
• Extrusion
• Intrusion
Rotary movement
Theoretically need high force
BUT
Tipping occurs 
= excessive compression of PDL
Force – 50-100g
Orthodontic force
• Tipping
• Translation
• Rotation
• Extrusion
• Intrusion
Vertical movement
Need to produced tension in fibres
of PDL
Orthodontic force
• Tipping
• Translation
• Rotation
• Extrusion
• Intrusion
Vertical movement
Need to produced tension in fibres
of PDL
Force – 50g
Orthodontic force
• Tipping
• Translation
• Rotation
• Extrusion
• Intrusion
Vertical movement
Forces concentrated at root apex
Orthodontic force 
• Tipping
• Translation
• Rotation
• Extrusion
• Intrusion
Vertical movement
Forces concentrated at root apex
Force – 15-25g
Orthodontic force duration
• Ideal
• Intermittent
• Interrupted
Orthodontic force duration
• Ideal
• Intermittent
• Interrupted
Light continuous force
Achievable with fixed appliances
Orthodontic force duration
• Ideal
• Intermittent
• Interrupted
Force decays between adjustments 
e.g. Removable appliance springs
Initially force is too high, decays to ideal, 
then to zero
Results in undermining resorption, which 
repairs between visits
Orthodontic force duration
• Ideal
• Intermittent
• Interrupted Force only present when appliance 
worn 
e.g. Headgear
Heavy force used, needs at least 12hours/day for
tooth movement to occur.
Optimal 14-16 hours/day
250g/side for anchorage
450g/side for distal movement
Orthodontic adverse affects
• Pulp
• Root
• PDL
• Bone
Orthodontic adverse affects
• Pulp
• Root
• PDL
• Bone
Minimal effect 
transient inflammatory response
can cause loss of vitality:
compromised teeth
excessive force
inappropriate movement
Orthodontic adverse affects
• Pulp
• Root
• PDL
• Bone
Some resorption of root occurs 
usually repaired by cementum
Repairs occur during ‘rest’ periods
BUT permanent damage occurs to root apex
commonly lose 1-2mm root length
At risk: distorted apices
thin roots
compromised teeth
excess force
history of previous idiopathic resorption
Orthodontic adverse affects
• Pulp
• Root
• PDL
• Bone
Minimal transient damage 
Unless:
excess force maintained
existing periodontal disease
Orthodontic adverse affects
• Pulp
• Root
• PDL
• Bone Minimal transient damage
BUT : loose ½ -1mm of alveolar crest
When to use what appliance….
Tipping
Bodily movement
Intrusion Extrusion
Rotation
When to use what appliance….
Tipping
Bodily movement
Intrusion Extrusion
Rotation
Removable
FABP
(Groups of teeth)
Springs / Screws 
(Individual or groups of teeth)
Accidental!!
When to use what appliance….
TippingBodily movement
Intrusion Extrusion
Rotation
Fixed
Adv / Disadv
Removable:
Adv:
• Cheap
• Oral hygiene
• Anchorage
• ‘Simple to use’ ?
• Patient co-operation ?
• Better tolerated ?
Disadv:
• Limited tooth movements (tipping)
• NOT ‘simple to use’
Fixed:
Adv:
• All tooth movements possible
Disadv:
• Patient co-operation
• Oral hygiene
• Anchorage
• Require skilled operator
• Cost ?
Summary
• Physiology of tooth movement
• Biomechanics of achieving tooth movement
• ‘Review’ of available appliances