Biology of Tooth Movement - I

1. THEORIES OF TOOTH MOVEMENT

2. CONCEPT OF CENTER OF RESISTANCE

3. TYPES OF TOOTH MOVEMENT

TOOTH MOVEMENT

• Force– Application

– Elimination

• Effects of Force

– On bone and teeth

– Tension Bone Formation Osteoblasts

– Compression Bone Resorption Osteoclasts

• Magnitude of Force

– > 7-8 grams

• Duration of Force

– > 4-6 hours

Periodontal Ligament

• PDL occupies a space approximately

0.5 mm in width around all parts of the root

• Components of PDL:

– Periodontal ligament fibers

Response to Normal Function

• Tooth contacts last for 1 second or less

• Forces of mastication

• Incompressible PDL fluid:– displacement of the tooth within the PDL space is

prevented by the incompressible tissue fluid

• Bone Bending

• Is bone bendable?

• Bone bending and piezoelectric currents

• Maintenance of normal bone structure and volume

Prolonged Compression

Role of PDL in Stabilization of

Teeth• The light prolonged pressures against the teeth

are not in perfect balance, as would seem to be required if tooth movement were not to occur.

• Active Stabilization:

• The ability of the PDL to generate a force and maintain teeth in their position despite unbalanced soft tissue balance.

• The current concept is that active stabilization can overcome prolonged forces of a few grams at most, perhaps up to the 5 to 10 gm/cm2 often observed as the magnitude of unbalanced soft tissue resting pressures.

Periodontal Ligament and Bone

Response to Sustained Force

Resorption of Compression Side:

• Frontal Resorption

• Undermining Resorption

• Depends on:

– Duration of force

– Magnitude of force

– Vascularity of tissue

Biologic Electricity

• Piezoelectricity is a phenomenon observed in many crystalline materials in which a deformation of the crystal structure produces a flow of electric current as electrons are displaced from one part of the crystal lattice to another.

• The piezoelectricity of many inorganic crystals like those in bone has been recognized for many years and has been used in everyday technology (e.g., the crystal pickup in inexpensive phonographs). Organic crystals also can be piezoelectric, and collagen in the PDL is an excellent example.

Piezoelectricity

• Piezoelectric signals have two unusualcharacteristics:

• (1) a quick decay rate (i.e., when a force isapplied, a piezoelectric signal is created inresponse that quickly dies away to zero eventhough the force is maintained) and

• (2) the production of an equivalent signal,opposite in direction, when the force isreleased

Mechanism of Piezoelectricity

• The migration of electrons within the

crystal lattice as it is distorted by pressure.

• When the force is released, however, the

crystal returns to its original shape, and a

reverse flow of electrons is seen

• Rhythmic Activity?

Streaming Potential

• Ions in the fluids that bathe living bone

interact with the complex electric field

generated when the bone bends, causing

electric signals in the form of volts as well

as temperature changes.

• The small voltages that are observed in

body fluids are called the “streaming

potential.”

Reverse Piezoelectricity

• There is also a reverse piezoelectric

effect. Not only will the application of force

cause distortion of crystalline structure and

with it an electric signal, but also

application of an electric field can cause a

crystal to deform and produce force in

doing so.

Pressure–Tension in Periodontal

Ligament

• Stage of PDL response to pressure:

• (1) initial compression of tissues and

alterations in blood flow associated with

pressure within the PDL

• (2) the formation and/or release of

chemical messengers

• (3) activation of cells

What is MUST if a tooth has to

move through bone?

BONE RESOPRTION AT

COMPRESSION SITE

Which Cells Resorb Bone?

OSTEOCLASTS

From where do these

osteoclasts come?

Osteoclast stem cells – OsteoclastPogenirator cells

Local Macrophages

Monocytes from Bone Marrow

Formation and Activation of

Osteoclasts

INFLAMMATION

Light

pressure

Heavy

pressure

<1 second

1-2 seconds

PDL fluid incompressible, alveolar bone bends, piezoelectric signal

generated PDL fluid expressed, tooth moves within PDL space

3-5

seconds

Blood vessels within PDL partially compressed on pressure side,

dilated on tension side; PDL fibers and cells mechanically distorted

MinutesBlood flow altered, oxygen tension begins to change; prostaglandins

and cytokines released

HoursMetabolic changes occurring: chemical messengers affect cellular

activity, enzyme levels change

~4 hoursIncreased cAMP levels detectable, cellular differentiation begins

within PDL

~2 daysTooth movement beginning as osteoclasts/osteoblasts remodel bony

socket

3-5 seconds Blood vessels within PDL occluded on pressure side

Minutes Blood flow cut off to compressed PDL area

Hours Cell death in compressed area

3-5 daysCell differentiation in adjacent narrow spaces, undermining

resorption begins

7-14 daysUndermining resorption removes lamina dura adjacent to

compressed PDL, tooth movement occurs

Hyalinization

• Hyalinized Tissue

– Excessive forces

– Occlusion of blood vessels

– Sterile tissue necrosis

– Invasion of macrophages to remove hyalinized

tissues – Associated with Undermined Resorption

– Tooth does not move unless the hyalinized area is

removed

– Formation of repaired tissue structures

TYPES OF TOOTH

MOVEMENT

Center of Mass / Gravity

Center of Mass

Center of Resistance

Center of Resistance

• In case of alveolar bone resorption

• In case of root resorption

Center of Rotation

TYPES OF TOOTH

MOVEMENT

Effect of Force

• On a Tooth

Effects of Force Distribution and

Types of Tooth Movement

• The PDL response is determined not by

force alone, but by force per unit area, or

pressure.

Optimum Forces for Orthodontic Tooth Movement

Type of movement Force (gm)

Tipping 35-60

Bodily movement

(translation)70-120

Root uprighting 50-100

Rotation 35-60

Extrusion 35-60

Intrusion 10-20

Tipping

Bodily Tooth Movement

Intrusion

Continuous force, 24 hours per day, produces the most

efficient tooth movement, but successful tooth

movement can be produced by shorter durations, with

a threshold at about 6 hours.

Questions?