Tissue Changes During Orthodontic Tooth Movements

The histological or tissue changes incidental to orthodontic treatment are explained under the following headings:

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Orthodontic-Tooth-Movement-Timeline

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Tissue Changes at Pressure Zone

If light forces are applied frontal resorption will occur.
If heavy forces are applied hyalinization and undermining resorption will occur.

Orthodontics Biology Of Tooth Movement Frontal resorption Side shows resorption of bone Side shows deposition of bone

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Frontal Resorption

As orthodontic tooth movement starts osteoclasts get activated.
Osteoclasts are derived from local population or blood supply.
Activated osteoclasts start resorption by resorbing adjacent lamina dura. This process of bone resorption is also known as frontal resorption.
Resorption starts from the PDL side of the alveolar bone.
Frontal resorption takes place after two days of orthodontic force application.

Frontal Resorption In Orthodontics

If the orthodontic force increases more than the capillary pulse pressure, i.e. 20–26 gm/cm2 blood vessels get compressed or occluded.
As PDL gets compressed the blood supply of the compressed area gets cut off
Now the cells that become activated osteoclasts remain inactive.
The sterile necrotic area is visible in compressed PDL.
This is when seen in a light microscope appears as an area devoid of cells and this is known as a hyalinized area and the process is known as hyalinization.
Hyalinization is a reversible process.
The effect of hyalinization is that it does not allow the tooth to move.
In hyalinization, the tooth can be moved in the condition in which bone beneath the hyalinized area undergoes resorption.
Hyalinization lasts for one to two weeks after which resorption occurs by undermining resorption.

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Resorption is also known as indirect resorption.
As there is the occurrence of hyalinization, chances of frontal resorption are diminished.
After some days hyalinized zone is invaded by the cells from adjacent normal areas of PDL.
With this osteoclasts also appear in adjacent marrow spaces.
These osteoclasts resorb bone adjacent to the hyalinized PDL zone from the underside. This is known as undermining resorption as bone resorption occurs from the underside of lamina dura.
Resorption of bone occurs from the endosteal part.
Hyalinization and undermining resorption lead to delayed tooth movement while tooth movement is efficient with frontal resorption.
Bone deposition occurs at the rate of 15µm/day

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In areas of tension cellular activity becomes delayed as compared to the pressure zone.
In the tension zone, 30 hours are required for increased cellular activity.
Macrophages are abundant in the tension zone.
Inflammation and remodeling of fibrous elements over evident in the tension zone.
A new unmineralized matrix is laid down in the fibers which are close to the alveolar wall.
Later on osteoid which is synthesized by osteoblasts is laid down on the complete alveolar wall over the tension zone.
Bone deposition occurs at the rate of 30µm/day.

Tissue Changes in Other Areas

Orthodontics Biology Of Tooth Movement Tissue Changes in Other Areas