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: Metabolic:
Cortical bone • Trabecular bone
slow turnover constant turnover
Bone Turnover
Control is by systemic and local factors
• Osteclasts • Osteblasts
derived from perivascular cells derived from monocytes
Bone – Metabolic Role (systemic control)
Kidney –
PO4 excretion
Ca++ resorption
PTH
Ca++ Gut – Ca++
Serum Ca binding Serum
Ca absorption
Vit D
(1,25 DHCC) Bone –
short term:
Ca++ from bone fluid
long term:
Resorption
Deposition
Local control
• Biologic electricity
• Blood flow
• Microfractures
Local control
• Biologic electricity
1. Pietzoelectric effect (V. short duration)
• Blood flow Bending of collagen and bone results in
e-’s moving within crystal lattice
• Microfractures 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
Sustained pressure
• Microfractures 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
PTH
Systemic Control Vit D
Calcitonin
Force
Tooth movement
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
Heavy force/short duration
• Duration of force 1-50Kg / less than 1 sec
Force absorbed by bone bending = Pain
(Pietzoelectric effect)
What happens depends on:
• Level of force
Heavy force/short duration
• Duration of force 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
Light force/short duration
• Duration of force 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
Light force/short duration
• Duration of force 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
Orthodontic forces
• Duration of force Excessive = pain + undermining resorption
Ideal = socket remodeling
In reality – some undermining
resorption occurs
Orthodontic force
• Tipping Simplest orthodontic movement
• Translation Occurs about centre of resistance
(1/3 from root apex)
• Rotation Forces are high at apex and alveolar crest,
reduce to zero at centre of resistance
• Extrusion
• Intrusion
Orthodontic force
• Tipping Simplest orthodontic movement
• Translation Occurs about centre of resistance
(1/3 from root apex)
• Rotation Forces are high at apex and alveolar crest,
reduce to zero at centre of resistance
• Extrusion
• Intrusion
Force – 50-75g
Orthodontic force
• Tipping Bodily movement
• Translation All of PDL is uniformly loaded
• Rotation
• Extrusion
• Intrusion
Orthodontic force
• Tipping Bodily movement
• Translation All of PDL is uniformly loaded
• Rotation
• Extrusion
• Intrusion
Force – 100-150g
Orthodontic force
• Tipping Rotary movement
• Translation Theoretically need high force
• Rotation
• Extrusion
• Intrusion
Orthodontic force
• Tipping Rotary movement
• Translation Theoretically need high force
BUT
• Rotation Tipping occurs
= excessive compression of PDL
• Extrusion
• Intrusion
Force – 50-100g
Orthodontic force
• Tipping Vertical movement
• Translation Need to produced tension in fibres
of PDL
• Rotation
• Extrusion
• Intrusion
Orthodontic force
• Tipping Vertical movement
• Translation Need to produced tension in fibres
of PDL
• Rotation
• Extrusion
• Intrusion
Force – 50g
Orthodontic force
• Tipping Vertical movement
• Translation Forces concentrated at root apex
• Rotation
• Extrusion
• Intrusion
Orthodontic force
• Tipping Vertical movement
• Translation Forces concentrated at root apex
• Rotation
• Extrusion
• Intrusion
Force – 15-25g
Orthodontic force duration
• Ideal
• Intermittent
• Interrupted
Orthodontic force duration
• Ideal Light continuous force
• Intermittent Achievable with fixed appliances
• Interrupted
Orthodontic force duration
• Ideal
• Intermittent Force decays between adjustments
• Interrupted 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 Minimal effect
• Root transient inflammatory response
can cause loss of vitality:
• PDL compromised teeth
excessive force
• Bone inappropriate movement
Orthodontic adverse affects
• Pulp
• Root Some resorption of root occurs
• PDL usually repaired by cementum
Repairs occur during ‘rest’ periods
• Bone 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 Minimal transient damage
Unless:
• Bone 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 Rotation
Intrusion Extrusion
When to use what appliance….
Springs / Screws
Tipping (Individual or groups of teeth)
Bodily movement Rotation
Removable Accidental!!
Intrusion Extrusion
FABP
(Groups of teeth)
When to use what appliance….
Tipping
Bodily movement Rotation
Fixed
Intrusion Extrusion
Adv / Disadv
Removable: Fixed:
Adv: Adv:
• Cheap • All tooth movements possible
• Oral hygiene
• Anchorage Disadv:
• ‘Simple to use’ ? • Patient co-operation
• Patient co-operation ? • Oral hygiene
• Better tolerated ? • Anchorage
Disadv: • Require skilled operator
• Limited tooth movements (tipping) • Cost ?
• NOT ‘simple to use’
Summary
• Physiology of tooth movement
• Biomechanics of achieving tooth movement
• ‘Review’ of available appliances
