This document discusses the periodontal ligament (PDL) and its role in orthodontic tooth movement. It contains the following key points: 1. The PDL is a collagenous structure that attaches the tooth to the alveolar bone. When orthodontic forces are applied, the PDL mediates bone resorption and deposition, allowing tooth movement. 2. There are different theories that explain how orthodontic forces stimulate the PDL to remodel bone, including the pressure-tension theory. This theory proposes that light forces cause bone resorption on the pressure side and deposition on the tension side through changes to blood flow and chemical mediator release. 3. Heavy forces can

2. Maj Dr. M.Golam Soroar
FCPS Part-II Trainee
Dept of Orthodontics &
Dentofacial Orthopedics
DDC&H

3. Orthodontic treatment is based on the
principle that if prolonged pressure is applied
to a tooth, the tooth will move as a result of
selective bone resorption and deposition
around the root. Because the bony response
is mediated by the periodontal ligament,
tooth movement is primarily a periodontal
ligament phenomenon.

4.  Each toot is attached to and separated from the
adjacent alveolar bone by a heavy collagenous
structure, the periodontal ligament(PDL).
Normally it occupies approximately 0.5mm in
width around all parts of the root.
 Components of PDL:
i) Collagen fibres
ii) Cellular elements; including osteoblast and
osteoclast, fibroblast, ECRM, macrophages,
cementoclast and undifferentiated
mesenchymal cells.
iii) Vascular elements.
iv) Neural elements.
V) The tissue fluids(ECM).

8. Functions of PDL
1.Periodontal ligament suspends and secure
the tooth, it’s bony socket and isolate the
tooth from the socket wall, so the root
does not conflict with the bone during
chewing.
2.Periodontal ligament comes with nerve fibers
that transmit pressure and pain sensation.
3.Periodontal ligament is supplied with
blood vessels that feed the cement, and
bones.

9.  4. Periodontal ligament contains
cementoblasts, which produce cementum
throughout the life of the tooth, while
osteoblasts maintain bone tissue of the tooth
socket.
 5. In the complex function in response to
strong pressure(ie; orthodontic pressure),
the cells osteoclasts can cause resorption of
bone tissue and, sometimes, resorption of
cement.

10. 1. Physiologic tooth movement:
i) Tooth eruption
ii) Migration or drifting
iii) Changes of tooth position
during mastcation
2. Pathologic tooth movement.
3. Orthodontic tooth movement.

11. Whenever a force of sufficient magnitude(optimum
orthodontic force) is applied to a healthy tooth
for a sufficient duration of time, its surrounding
bony socket is remodeled and tooth migrates to
a new position. This type of migration is termed
as orthodontic tooth movement.
Types of orthodontic tooth movement:
a) Tipping
b) Bodily movement/ translation
c) Rotation
d) Extrution
e) Intrution

12. Force: Force may be defined as an act upon a
body that changes or tends to change the rest
state or of motion of that body.
Optimum orthodontic force: The force which
moves teeth most rapidly in the desired
direction, with the least possible damage to the
tissue and minimum discomfort to the patient.
The optimum orthodontic force should be
equivalent to the capillary pulse pressure, which
is 20-25gm/ sq cm of root surface area.

13. Types of orthodontic force:
1. According to the force magnitude:
a) Light force
b) Heavy force
2. According to the duration:
a) Continuous force
b) Interrupted force
c) Intermittent force

14.  Orthodontic force: Those forces which range
from 50gm to 300gm and are capable of
inducing movement of a tooth or a group of
teeth.
 Orthopedic force: Those forces which are
normally higher than 300gm and are capable
of producing orthopedic movement.

15. Continuous force: Force is maintained at some
appreciable fraction of the original from one
visit to the next. eg; NiTi coil spring
Interrupted: Force level declines to zero
between activations. eg; expansion screw.
Intermittent: Force level declines abruptly to
zero when the appliance is removed by the
patient. eg; removable plates, headgear.

16. Optimum force for orthodontic tooth
movement:
Tipping, Extrution, Rotation - 35-60gm
Intrution- 10-20gm
Bodily movement- 70-120gm
Root uprighting- 50-100gm

17. During Tipping movement

18. During bodily movement

19. Intrusion

20. There are three distinct yet overlaping stages
of tooth movement are:
a) Initial phase
b) Lag phase
c) Post lag phase
Initial phase: Characterized by sudden
displacement of tooth into the socket,
pressure is transmitted to the alveolar bone
as the PDL fluid is incompressible so there is
bone bending.

21. Lag phase: Immediately after initial phase
there is a little or no tooth movement
because of hyalinization.
Post lag phase: The third phase of tooth
movement, characterized by increased tooth
movement, as because the hyalinized tissue
is removed by the osteoclasts.

22. Depending on the magnitude of the force
applied during orthodontic tooth movement
there are two types of resorption occurs on
the socket wall:
a) Direct or frontal resorption
b) Undermined or rearward resorption.
Direct/frontal resorption: When light force is
applied to a tooth, the bone resorption starts
at the lamina dura of the socket wall. This
type of resorption is known as direct/frontal
resorption.

24. Undermined/rearward resorption: When heavy
sustained force is applied to a tooth, there is
formation of a hyalinized area at the compressed side
and osteoclasts appear within the adjacent marrow
spaces and start resorption from the underside of the
lamina dura. This type of resorption is known as
undermined/rearward resorption.
Hyalinization: Due to application of a sustained
heavy force to a tooth, there is total occlusion of
blood vessel and cut off blood supply to the area of
pressure side, leading to a sterile necrosis and
formation of a clear cell free zone in the PDL and
adjacent socket wall. This is called hyalinization.
The ‘socalled’ hyalinized area is actually an area of
focal necrosis that resembles hyaline connective
tissue in histological section, but there is nothing
formation of hyaline tissue in this process.

27. Orthodontic pressure
Mechanical trauma and alteration of blood
flow(increase/decrease) to cellular elements
Cell injury and imbalance in intra cellular chemical
composition
Release of membrane phospholipid and production of
arachidonic acid
Release of chemical mediators and cytokynes,eg; PG, IL, NO
Bradikynin etc.
Initiation of inflammatory procedure, eg; activation of cells,
pain production and other signs of inflammation

28. >1sec- PDL incompressible, alveolar bone
bends, piezoelectric signal generated.
1-2 sec- PDL fluid expressed, tooth moves
within the PDL space.
3-5 sec- Blood vessels within the PDL partially
compressed on the pressure side,
dilated on the tension side, PDL
fibres and cells are mechanically
distorted.
Minutes- Blood flow altered, oxygen tension
changed, prostaglandin and cytokines released.
Houres- Metabolic changes: chemical messengers
affect cellular activity, enzyme level change.
4 hours- Increased cAMP level, cellular differentiation
begins within PDL.
2 days- Tooth movement begins as osteoclasts and
osteoblasts remodel the bony socket.

29. >1sec- PDL incompressible, alveolar bone
bends, piezoelectric signal generated.
1-2 sec- PDL fluid expressed, tooth moves
within the PDL space
3-5 sec- Blood vessels within PDL occluded on the
pressure side.
Minutes- Blood flow cut off to the compressed PDL
area.
Hours- Cell death in the compressed area.
3-5 days- Cell differentiation in the adjacent marrow
spaces, undermined bone resorption begins.
7-14 days- Undermined bone resorption removes
lamina dura adjacent to the compressed PDL, tooth
movement occurs.

30.  Numerous theories have been put forward to
explain the orthodontic tooth movement.
Accepted theories are as follows:
 1. Pressure tension theory
 2. Blood flow theory
 3. Bone bending piezoelectric theory

31.  The pressure-tension theory is the classic theory
of tooth movement that relies on chemical as
the stimulus of cellular differentiation and
ultimate tooth movement. This theory has
illustrated three distinct stages of tooth
movement following application of a sustained
pressure by developing areas of pressure and
tension; eg
a) Alteration of blood flow.
b) Formation and/or release of chemical
mediators.
c) Activation or differentiation of
boneremodeling cells.

33. All the above mentioned changes are different
following light and heavy pressure.
Changes following light pressure
On the pressure side:
1. Compression of the PDL, becomes 1/3 of the
original length.
2. Blood flow is decreased.
3. Oxygen level falls down.
4. Changes in the relative proportion of other
metabolites.
5. Stimulation of the release of biologically active
substances including Prostaglandin-e,
Interleukin-1, Nitric oxide etc.

34. 6. Stimulation of cell differentiation; eg:
Osteoclast( and also osteoblast)
7. The first wave of osteoclasts is derived from
the local cell population but the second one
is from distant areas via blood flow.
8. These osteoclasts attack the adjacent
lamina dura and remove bone in the process
of ‘frontal resorption.

37. ACTIVATION OF OSTEOCLAST

39. On the tension side:
1. Stretching of PDL.
2. Maintained or increased blood flow. May be
decreased if PDL is over stretched.
3. Mechanical diformation of cells within the PDL.
4. Release of chemical mediators like PGE,IL-1
etc.
5. Activation or stimulation of cell
differentiation, eg: osteoblast(and also
osteoclast).
6. The osteoblasts are derived from local cell
population.
7. Osteoblasts form new bone on the adjacent
lamina due.

40. Changes following heavy pressure
On the pressure side:
1. Total occlusion of blood vessel and cut off
blood supply.
2. Formation of an area of sterile necrosis,
which is termed as “hyalinization”.
3. Appearance of osteoclasts within the
adjacent marrow space.
4. Osteoclasts attack on the underside of the
lamina dura and start resorption in the
process of “undermined” or ‘rearward”
resorption.

42. On the tension side: The PDL become
overstretched leading to tearing of blood
vessels and ischemia and less activity of
osteoblast. Thus with extreme pressure a net
increase in osteoclastic activity as compared
to bone formation with the result that the
tooth becomes loosened in its socket.
In addition pain and hyperamia of the gum
may occur.