The biological basis of the orthodontic Therapy

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The biological basis of the orthodontic Therapy

• Fengshan Chen
• Tongji University

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Law 1In orthodontics, tooth moves through bone
and brings the periodontal ligament with it.
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The basis of the Periodontal Ligament (PDL)

• Normal width 0.25 mm or 250 micrometers.
• Cells, fibers, ground substance.

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Cells of PDL

• Fibroblasts
• Osteoblasts, osteoclasts
• Cell rests of Malassez
• Mesenchymal stem cells
• They all proliferate at different stages of tooth
  movement.
• You must know what functions each has in tooth
  movement.

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Fibers of the PDL

• Collagen and oxytalan
• Some of them are stretched, torn and ruptured,
  whereas others are compressed and undergo aseptic
  necrosis

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Ground substance of the PDL

• Proteoglycans and other proteins
• Their contents and expression are altered upon
  tooth movement
• Water squeezed in and out during tooth movement

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Alveolar Bone

• Thin cortical bone and porous (lamina dura)
• Fluid pumped in and out of the PDL
• Trabecular bone underneath
• Must remodel before teeth can be moved

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CCell FFiber AB Alveolar bone GGingivity CE
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Law 2Tooth cannot move unless bone apposition
and resorption take place.
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Susan M. Ott Univ of Washington
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Law 3There will be no tooth movement unless
there is a force.
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The basis of Force

• The force must have the right characteristics
  such as the magnitude and duration ---- it must
  meet certain threshold.

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Force Types

• Light, continuous forces
• Never declines to zero.
• Interrupted forces
• Declines to zero
• Intermittent forces
• Declines to zero

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Force Magnitude (Level)

• In the range of 10 to 200 grams.
• Varies with the type of tooth movement.
• Light, continuous forces are currently considered
  to be most effective in inducing tooth movement.
• Heavy forces cause damages and fail to move the
  teeth.

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Force Duration

• Threshold --- 6 hrs per day.
• No tooth movement if forces are applied less than
  6 hrs/d.
• From 6 to 24 hrs/d, the longer the force is
  applied, the more the teeth will move.

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Law 4Orthodontic tooth movement is not the only
type of tooth movement.
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Types of Tooth Movement

• Eruption
• Active
• Passive
• Lateral drifts
• Physiological
• Due to loss of adjacent teeth
• Orthodontic tooth movement

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Types of Tooth Movement

• Intrusion
• Extrusion
• Tipping
• Bodily movement
• Rotation

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Mechanisms of Tooth Movement

• Piezoelectric theory.

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Mechanisms of Tooth Movement

• Pressure-tension theory.

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The Optimal Force

• High enough to stimulate cellular activity
  without completely occluding blood vessels in the
  PDL (Proffit et al. 2000).
• Actively being investigated in a scientific field
  known as mechanotransduction.

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Law 5Orthodontic tooth movement cannot occur
unless cells are at work.
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• Force --- fluid flow --- cell-level strain
• Deformation of cell membrane leading to
  cytoskeletal changes
• Second messenger pathways
• Gene upregulation in fibroblasts, osteoblasts and
  osteoclasts

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Effect of the light force on the PDL

• Light, continuous forces
• Osteoclasts formed
• Removing lamina dura
• Tooth movement begins
• This process is called FRONTAL RESORPTION

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• Frontal resorption because it occurs between
  the root and the lamina dura.

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Light force leading to frontal resorption

• Phase 1 Mechanical compression and tension of
  the periodontium
• Phase 2 --- Mechanically induced cellular and
  genetic responses no tooth movement
• Phase 3 --- Accelerated tooth movement due to
  frontal bone resorption

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Effects of heavy force on the PDL

• Heavy, continuous forces
• Blood supply to PDL occluded
• Aseptic necrosis
• PDL becomes hyalinized HYALINIZATION
• This process is called UNDERMINING RESORPTION.

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• Undermining resorption because it occurs on the
  underside of lamina dura, not between lamina dura
  and the root.

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Law 6Frontal resorption occurs in the PDL,
whereas undermining resorption occurs underneath
the lamina dura.
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Heavy force leading to undermining resorption

• Phase 1 Mechanical compression and tension of
  the periodontium
• Phase 2 --- Continuing mechanical compression
  little cellular and genetic responses no tooth
  movement
• Phase 3 --- Cells recruited from the undermining
  side of lamina dura, not within the PDL, to
  induce undermining bone resorption

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Tooth movement (mm)
Frontal resorption
Time (Arbitrary Unit)
Undermininging Resorption
Tooth movement (mm)
Time (Arbitrary Unit)
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Law 7Frontal resorption facilitates
orthodontic tooth movement, whereas undermining
resorption impedes orthodontic tooth movement.
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Anchorage

• Newtons law for every action, there is
  reaction.
• Defined as resistance to unwanted tooth
  movement.
• The anchorage value of any tooth is roughly
  equivalent to its root surface area. Thus,
  molars and canines generally have higher
  anchorage values than incisors and bicuspids.

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Anchorage types

• Reciprocal anchorage.
• Reinforced anchorage.
• Stationary anchorage.
• Cortical anchorage.

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Reciprocal anchorage

• Both units move roughly equal distance.
• Exemplified by closing a diastema between two
  central incisors.

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Reinforced anchorage

• Unit A has substantially more anchorage value
  than Unit B. Thus, Unit A moves little but Unit
  B moves a lot.
• Exemplified by retracting anterior teeth to close
  an extraction space by using posterior teeth as a
  reinforced anchorage unit.

Unit B
Unit A
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Biomechanics of Tooth Movement

• Center of Resistance --- A point on the tooth
  around which the tooth shall move. For most
  teeth, COR is 2/5 way between the apex and the
  crest of the alveolar bone.
• Center of Rotation --- The point around which
  rotation occurs when an object is being moved.

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Force and Couple

• Force
• Is applied by orthodontic appliances.
• Induces tipping, translation, intrusion,
  extrusion and/or rotation.
• Couple
• Two forces of opposite directions and with
  non-overlapping points of application.
• Translation of teeth occurs in response to
  appropriate force couples.

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Potential Complications of Orthodontic Tooth
movement

• The pulp
• Root resorption
• Alveolar bone height

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Orthodontic effects on the pulp

• Rare if light, continuous forces are applied.
• Occasional loss of tooth vitality.
• History of previous trauma
• Excessive orthodontic forces
• Moving roots against cortical bone
• Endodontically treated teeth can be moved like
  natural teeth, with proper management.

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Root resorption

• More accurately, resorption of root cementum and
  dentin.
• Normal ageing process in many individuals
• Likely occurring in many cases but not to the
  degree of clinical significance.
• Root resorption induced by light orthodontic
  forces is reversible (by regeneration and repair
  of cementum and/or dentin).
• Can lead to tooth mobility in severe cases.

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Generalized Root Resorption

• Affects most, if not all, teeth maxillary
  incisors more susceptible than other teeth.
• Could be moderate or severe but commonly in the
  range of up to 2.5 mm.
• Etiology largely unknown but predisposing factors
  include conical roots with pointed apices,
  distorted tooth form, or a history of trauma.

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Localized Root Resorption

• Cant always be distinguished from generalized
  root resorption.
• Maxillary incisors more susceptible than other
  teeth.
• Only in rare cases can the causes, such as heavy
  orthodontic forces, be pinpointed.
• Etiology largely unknown.

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Law 8Orthodontic tooth movement remains one of
the most successful procedures with predictable
outcome in medicine and dentistry.
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Orthodontics and dentofacial orthopedics requires
thorough knowledge in biology (of bone,
cartilage, teeth, muscles, nerves and other soft
tissues), biomechanics, biometrics, material
science, clinical skills and practice management
in addition to interpersonal skills.
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Why study tooth movement?

• Up to 70 of the Chinese population have
  malocclusion that warrants orthodontic
  correction.
• Currently, less than 20 of the Chinese patients
  seeks orthodontic treatment. However, I believe
  more and more people will seek orthodontic with
  the development of society