Biomechinal

The biological basis
of the orthodontic
Therapy
www.indiandentalacademy.com

Law 1
In orthodontics, tooth moves through
bone and brings the periodontal
ligament with it.

The basis of the Periodontal
Ligament (PDL)
 Normal width 0.25 mm or 250 micrometers.
 Cells, fibers, ground substance.

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.

Fibers of the PDL
 Collagen and oxytalan
 Some of them are stretched, torn and
ruptured, whereas others are compressed
and undergo aseptic necrosis

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

AlveolarBone
 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

C:Cell
F:Fiber
AB: Alveolar
bone
G:Gingivity
CE:

Law 2
Tooth cannot move unless bone
apposition and resorption take place.

Susan M. Ott
Univ of Washington

Law 3
There will be no tooth movement unless
there is a force.

The basis of Force
 The force must have the right characteristics
such as the magnitude and duration ---- it must
meet certain threshold.

Force Types
 Light, continuous forces
 Never declines to zero.
 Interrupted forces
 Declines to zero
 Intermittent forces
 Declines to zero

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.

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.

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

 Eruption
 Active
 Passive
 Lateral drifts
 Physiological
 Due to loss of adjacent teeth
 Orthodontic tooth movement
Types of Tooth Movement

Types of Tooth Movement
 Intrusion
 Extrusion
 Tipping
 Bodily movement
 Rotation

Mechanisms of Tooth Movement
 Piezoelectric theory.

Mechanisms of Tooth Movement
 Pressure-tension theory.

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.

Law 5
Orthodontic tooth movement cannot
occurunless cells are at work.

 Force --- fluid flow --- cell-level strain
 Deformation of cell membrane leading to cytoskeletal changes
 Second messenger pathways
 Gene upregulation in fibroblasts, osteoblasts and osteoclasts

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

 “Frontal resorption” because it occurs
between the root and the lamina dura.

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
Phase 1
Phase 3
Phase 2
Toothmovement(mm)
Time (Arbitrary Unit)

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”.

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

Law 6
Frontal resorption occurs in the PDL,
whereas undermining resorption
occurs underneath the lamina dura.

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
Phase 1
Phase 3
Phase 2
Toothmovement(mm)
Time (Arbitrary Unit)www.indiandentalacademy.com

Phase 1
Phase 3
Phase 2
Toothmovement(mm)
Phase 1
Phase 3
Phase 2
Toothmovement(mm)
Frontal resorption
Undermininging
Resorption

Law 7
Frontal resorption facilitates
orthodontic tooth movement, whereas
undermining resorption impedes
orthodontic tooth movement.

Anchorage
 Newton’s 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.

Anchorage types
 Reciprocal anchorage.
 Reinforced anchorage.
 Stationary anchorage.
 Cortical anchorage.

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

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 BUnit A

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.

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.

Potential Complications of Orthodontic
Tooth movement
 The pulp
 Root resorption
 Alveolar bone height

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.

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.

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.

Localized Root Resorption
 Can’t 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.

Law 8
Orthodontic tooth movement remains one
of the most successful procedures with
predictable outcome in medicine and
dentistry.

Orthodontics and dentofacial orthopedics requires thorough knowledge in
biology (of bone, cartilage, teeth, muscles, nerves and othersoft tissues),
biomechanics, biometrics, material science, clinical skills and practice
management in addition to interpersonal skills.

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

Biomechinal

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