Introduction Histology of supporting structure Types of tooth movements Phases of orthodontic tooth movements Biological changes by tooth movements Theories of tissue reactions Replacement resorption

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Dr. Mohammed Alruby
Biomechanical consideration to
orthodontic force
Prepared by:
D. Mohammed Alruby
‫فحم‬ ‫وجدوه‬ ‫االماس‬ ‫حللوا‬ ‫فالذين‬ ‫شئ‬ ‫كل‬ ‫في‬ ‫تدقق‬ ‫وال‬ ‫شئ‬ ‫كل‬ ‫تحلل‬ ‫فال‬ ‫سعيدا‬ ‫تعيش‬ ‫ان‬ ‫اردت‬ ‫اذا‬

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Dr. Mohammed Alruby
Introduction
Histology of supporting structure
Types of tooth movements
Phases of orthodontic tooth movements
Biological changes by tooth movements
Theories of tissue reactions
Replacement resorption

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Dr. Mohammed Alruby
Introduction;
When an orthodontic force applied to the crown of the tooth, it transmits through the roots to
PDL and alveolar bone, according to the direction of force, there is an area of tension and
another compression.
Bone resorption occurs at pressure area and bone deposition at tension area.
PDL: periodontal ligament: soft Richey vascular and cellular connective tissue, approximately
0.20---0.25mm wide surrounds the roots of the teeth and join the root cementum with lamina
dura or alveolar bone proper.
Histology of supporting structure
= periodontal ligament:
- Cellular elements
- Fibrous elements
- Interstitial or ground substances
- Neurovascular elements
= alveolar bone.
A- Periodontal ligament: the main component of PDL are:
1- Cellular elements of PDL:
a- Forming cells:
- Osteoblast: bone forming cells, normally, a layer of osteoblast line the alveolar socket wall
and the intervene between the PDL fibers that are inserted into bundle bone layer
- Fibroblast: PDL fibers forming cells
- Cementoblast: found in a layer adjacent to the root surface in different stages of maturity
with their progenitor cells
b- Resorptive cells:
- Osteoclast: usually are multinucleated large cells, rich in acid phosphatase enzyme, they
are usually present during physiologic tooth movement and remodeling of alveolar bone:
they: demineralize the bone and disintegrate the organic matrix, their progenitor cells are
theought to be blood monocytes
- Fibroclast: disintegrate fibers
- Cementoblast: resorb cementum
c- Progenitor cells:
small cells with closed face nucleus and very little cytoplasm
- UMC: undifferentiated mesenchymal cells
- Monocyte
d- Epithelial rest of malaseze:
arise as a result of breakdown of epithelial root sheath at the time of cementogesis
e- Defensive cells:
- Macrophages: share in clearance of bone resorption and fiber elements
- Mast cells: seen in the inflammatory reactions during orthodontic tooth movement
2- Fibrous component:
a- Collagen fibers: constitute the main bulk of PDL fibers, and found in 5 groups:
1- Alveolar crest group: from cervix to alveolar crest
2- Horizontal group: from cementum to bone horizontally
3- Oblique group: the main attachment, run obliquely from cementum to bone in an
apical direction
4- Apical group: circumscribe the apex, responsible for resistance to rotation

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Dr. Mohammed Alruby
5- Inter-radicular group: an intermediate plexus midway between bone and roots, this
is area of fiber remodeling in response to minor movement
Dentogingival, transeptal, circular, dentoperiosteal, alveologingival
b- Oxytalan fibers: immature elastic fibers that resist dissolution by acids unlike collagen
Runs from cementum or bone to blood vessels
Play a role in supporting the blood vessels against dissolution and compressive strain
Restricted to blood vessels and characterized by their increased elasticity
3- Interstitial or ground substances:
= organic matrix surrounding the PDL elements
= it is chemically composed of CHO linked with varying amount of protein
= CHO- protein divided into: proteoglycan that has increased CHO and decreased protein
: glycol protein that has decreased CHO and increased protein
Proteoglycan; one protein contains hyaluronic acid and other contain chondrotein B
= ground substances of PDL is in continuous state of remodeling process, it involves de-
polymerization of old ground substances and removal of broken down product
4- Neurovascular elements;
Myelinated; pain sensation
Non-myelinated: blood vessels walls
B- Alveolar bone:
The alveolar bone surrounds the tooth at level approximately 1mm apical to the cemento-enamel
junction. This part of alveolar bone that covers the alveolus is referred to as lamina dura, cortical
bone
The alveolar bone is constantly renewed in response to functional demands, bone forming cells
and bone resorptive cells are responsible for this remodeling
Types of tooth movements
1- Physiologic tooth movements:
It is the movement of the tooth in the alveolus during function or it is the change in tooth
position by occlusal and inter-proximal wear of crown during adult life.
As the tooth and alveolar bone drift together, PDL itself undergoes a corresponding process of
drift the permit differential movements between the roots of the teeth and their supporting
structure.
2- Orthodontic tooth movements:
Basically, no great differences exist between the tissue reactions observed in physiologic tooth
migration and those observed in orthodontic tooth movement, that the tooth is moved rapidly
during treatment so the
There are two main differences between orthodontic and physiologic movements:
a- Orthodontic movements are more rapidly and thus produce extensive and marked tissue
changes
b- In some cases, orthodontic movements, may often against physiologic tooth movement and
periodontal shift
Factors affecting tissue reaction in the periodontium
1- Orthodontic force duration: There are 3 types of force duration:
a- Continuous force: can be obtained by using wires with low deflection load and high
working range as in leveling phase, this force decrease slowly but never stop or reach
zero also using niti open coil spring

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Dr. Mohammed Alruby
b- Dissipating (Reitan interrupted type): it is a continuous force that shows decrease
amount of force within short time as elastic ring and rapid maxillary expansion, this
type has a period of recovery, reorganization and cell proliferation prior to force re-
application
c- Intermittent force: force associated with removable appliances, the force is active when
the appliance is in the mouth and stop when removed, also extra-oral forces considered
as intermittent force.
It appears that this kind of force that start in a continuous mode and then becomes
interrupted is biologically favorable, particularly when its initial magnitude is low
2- Magnitude of force:
Light force produce direct bone produce hyalinization that causing undermining resorption
and so delayed movement.
The purpose of applying a light force is it increase cellular activity without causing undue
tissue compression and prepare tissue for further changes
Various studies demonstrate that teeth subjected to high force magnitude revealed
hyalinization more than teeth exposed to light force
Light force gives less discomfort and less pain to patients
3- Types of movement direction:
a- Tipping: controlled and uncontrolled tipping: easiest type of tooth movement, when the
single force is applied to a bracket on a round wire, the tooth tips about its center of
rotation, located in the middle of the root, close to its center of resistance.
b- Translation (bodily movement): movement of the body in straight line without changing
the angle, during translation all the points have less hyalinization with true bodily
movement because more uniform pressure
c- Rotation: movement of body along its long axis or along fixed reference frame
d- Torque: movement of root with little or no movement of crown, the center of rotation is
at incisal edge or the bracket
e- Vertical movements: extrusion and intrusion
4- Other factors:
- Age: biologic response to orthodontic force in adult is slower than children because denser
and PDL is less cellular
- Occlusal function: movement of interdigitated teeth is rather more difficult than that of teeth
with inter-occlusal clearance
- Drugs: aspirin like drugs reduce the rate of tooth movements which inhibit prostaglandin
synthesis (Chemby and Tunkay 1986)
Administration of prostaglandin E2 enhance tooth movement (more safe analgesic ------
paramol)
- Sex
- Nutrition.
N: B: guide of force level for tooth movement:
Tipping: ------ 30 to 60gm
Bodily: ---------100 to 150gm
Torque: ---------50 to 100gm
Rotation: --------50 to 75gm
Extrusion: -------50 to 75gm
Intrusion: --------15 to 25gm

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N: B: pain and force magnitude:
There are 3 types of pain degrees:
1- Pain produced only by heavy pressure placed on the tooth with an instrument like band
pusher, the patient is not a ware by this type
2- Pain or discomfort during clinching of the teeth, the patient maintain the ability to
masticate a normal diet without difficulty
3- Spontaneous pain is present or patient unable to masticate food with normal consistency
N: B: threshold stress level: the level of stress at which the bone resorption can begin, if
this stress increased beyond this level, there is amount of frontal resorption on the alveolar
process
N: B: differential orthodontic force is that force which causes minimum discomfort,
minimum loosening of teeth, minimum damaged of tooth supporting structure and at the
same time tooth moving rapidly e.g 150—200mg for canines and 300gm for 1st
molar
N: B: ideal orthodontic force:
According to Schwartz: that force which induce pressure in PDL not exceeding the capillary
blood pressure not more than 30mm Hg
Or: that force which result in tooth movements of about 1mm/month. He suggested that
force to be 15—20gm/cm of the root surface.
Or: that force which moves teeth efficiently into their desired position, without causing
discomfort or tissue damaged to the patient
N: B: light force more likely to move teeth gradually whereas heavy force demonstrate a
marked lag phase followed by a period of rapid movement
Phases of orthodontic tooth movement
1- Displacement phase; initial phase:
Initial reaction of the tooth following application of force with fraction of seconds
The fluid compartment within the PDL plays an important role in the transmission and damping
of force acting on tooth
Factors affect displacement magnitude:
a- Root length, alveolar bone height, which determine the location of center of resistance and
rotation
b- Age, it suggested that there is reduce in biological response of the PDL and thus delay tooth
movement in adult.
This phase lasts 24 hours to 2 days.
2- Delay phase: lag phase:
= Characterized by very little or no movement and formation of hyalinization in PDL
= Duration of this phase depend on the amount of force applied, as if force is high so the delay is
longer because the hyalinization created is large and there is undermining resorption. But in light
force this phase is short because there is no hyalinization and frontal resorption is noticed.
= Extended for 2 to 3 weeks but may be longer than that.
3- Acceleration and linear phase: post lag phase:
= Characterized by rapid tooth movement as the hyalinized zone is removed and bone undergoes
resorption
= Magnitude of force directly affect the rate of tooth movement as high force excess 100gm used
to retract canine have been shown to produce lag phase up to 21 days before tooth movement. But

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Dr. Mohammed Alruby
in light force can induce tooth translation without lag phase at rate that are still clinically
significant
= Light continuous force is much more conductive to orthodontic tooth movement because the cell
biology system remains in a constantly responsive state
Physiologic response to light force
1- Displacement phase:
= PDL fluid incompressible, alveolar bone beads, piezoelectric signal generated
= tooth moves within PDL space
= blood vessels within PDL partially compressed on pressure side and dilated on tension side
= PDL fibers and cells mechanically distorted
2- Delay phase:
= blood flow altered, oxygen tension begins to change
= prostaglandin and cytokines released
= increase AMP level, cellular differentiation begins within PDL
3- Acceleration phase:
= tooth movement begins as osteoclastic and osteoblastic remodeling starts.
Physiologic response to high force
1- Displacement phase:
= blood vessels of PDL is occluded on pressure side and dilated on tension side
2- Delay phase:
= blood flow cut and cell death at compressed area
= undermining resorption
3- Acceleration phase:
= undermining resorption remove lamina dura adjacent to compressed PDL and tooth
movement.
Biological changes by tooth movement
Tissue response in periodontium:
= application of continuous force on the crown lead to tooth movement within the alveolus with
narrowing of periodontal membrane, particularly at marginal area
= osteoclast differentiate along bone wall after 30 to 40 hours
= bone resorption occurs during hyalinization has undermining nature because there are no living
cells within compressed periodontal membrane
Hyalinization:
= most frequent complication preventing rapid tooth movement occurs when the force presses the
tooth hard against the alveolar bone wall
= the periodontal membrane responds with degeneration and sterile necrosis(hyalinization)
instead of proliferation and differentiation of cells
= generally limited to 1 -2mm diameter and by light microscope this tissue appears glass like and
take 1 –2 days to develop
= if hyalinization is occurring, the tooth not capable for further movement until pass three phases:
a- Tissue degeneration
b- Elimination of damaged tissue
c- Establishment of new attachment

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Dr. Mohammed Alruby
= elimination of damaged tissue starts when cellular elements such as macrophages or osteoclast
from adjacent undamaged area invade the necrotic tissue
= undermining resorption: the eating cells resorb the underside of bone immediately adjacent to
necrotic PDL area and remove it together with the necrotic tissue
= reestablishment of tooth attachment in the hyalinized area starts by synthesis of new tissue
elements as soon as the adjacent bone and degenerated membrane tissue have been removed
Histology of area of compression:
= changes depend on whether exceed blood capillary or not exceed, if excessive forces are used
hyalinization may be extensive, area of hyalinization is less common with true bodily tooth
movements because of more uniform pressure distribution along the root
a- Compression below capillary blood pressure:
= Proliferation of fibroblast and other cell of PDL
= osteoclast come to lie within shallow depression known as Howship’s lacuna
= resorption does not take place over along continuous manner so some PDL become detached
and others remain intact
= experimental work has shown that the application of direct current to the alveolar process close
to a tooth accelerates its movements in response to orthodontic force
= injection of prostaglandin into the alveolar mucosa has been shown to accelerate orthodontic
tooth movement
b- Compression exceeding capillary blood pressure
= where the capillary vessels crushed, the cells die and the compressed appears structure less
under light microscope (hyalinization)
= if pressure is reduced this hyalinization area rapidly become re-vascularized and colonized by
new cells
= when force is reduced by movement so that the areas become only gently compressed, however,
the hyalinized area will be vascularized and direct surface resorption can proceed
Histology of area of tension
= the periodontal fibers are stretched, and with excessive forces some may torn and blood vessels
may be ruptured
= the area of tension in contrast to compression, there is no difference between the heavy and light
forces
= within few days, cellular proliferation occurs among the fibroblast of PDL and osteoblast lining
the socket
= osteoid laid down along the socket wall and this becomes calcified and reorganized as woven
bone
N: B: the irregular and vascular woven bone is very susceptible to resorption, but is progressively
remodeled into mature bone
Histology at supra-alveolar:
= the transeptal fibers that pass between adjacent teeth also adjust to tooth movement, if two teeth
are being moved a part the fibers stretch but the residual tension is sufficient to promote tooth
movement
= it is good practice to avoid elongating the transeptal fibers because when teeth are approximated
these fibers not shorten, so the interdental contact may be less

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= the free gingival fibers which pass from the neck of the tooth into CT system, when the tooth is
rotated so the fibers is stretched, the residual tension within the supra-alveolar CT plays important
role for relapse
Remodeling of alveolar process:
Any labiolingual tooth movement is associated with remodeling of the alveolar process. Sub-
periosteal apposition occurs on the bone surface from which tooth moves and bone resorption on
the bone surface toward the root move, so the alveolar process drifts with the tooth
Soft tissue response:
Lack of proper exercise, stasis of circulation and irritation lead to hyperemic gingival tissue,
edematous and puffy.
Most of orthodontic therapy occurs at a time when the endocrine system is undergoing great
changes, so the patient prone to abnormal tissue response during treatment with pocket formation
Theories of tissue reaction
1- Classical theory: pressure tension theory: (Oppenhim 1911, Schwarz 1932, Sandsdt 1905)
= during tooth movement, there is pressure side and other tension side on the PDL
= cell proliferation decreased at pressure side and increase at tension side
= dis-organization of fibers at pressure side and production increase at tension side
= bone resorption at pressure side and deposition at tension side
= oxygen level increase at tension side and decrease at pressure side.
2- Blood flow theory: fluid dynamic theory: Bein 1966
= according to this theory tooth movement occurs as a result of alteration in fluid dynamic in the
periodontal ligament, periodontal membrane space is confined space and passage of fluid in and
out this space limited due to presence of hard tissue in both sides.
= the periodontal membrane space contains the following interacting fluid system:
a- Vascular element system
b- Interstitial fluid
c- Cellular element
d- Viscous and ground substances
= when force of short duration as in mastication is applied to the tooth, the fluid of PDL escapes
through tiny vascular channels. The fluid gets replenished by diffusion from capillary walls and
re-circulation of the interstitial fluid as soon as force is removed.
= however force of longer duration as in orthodontic tooth movement, the interstitial fluid in the
periodontal ligament spaces gets squeezed out and moves toward apex and cervix margin
= by orthodontic force, the blood vessels of PDL compressed between the principles fibers of
ligament which result in stenosis, the blood vessels in area beyond stenosis (balloon up forming
aneurysms)
= formation of aneurysms causes blood gases to escape inti the interstitial fluid, thus create
favorable environment for resorption.
= Bein suggested that due to decreased oxygen, the chemical environment at the side of the
vascular stenosis is decreased which is favorable to bone resorption.
3- Mechanical theory: Justus and Luft 1970
Application of physical stress to the bone lead to changes the solubility of the hydroxyl
apatite crystals which affect remodeling of bone.

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This theory is not widely used and accepted.
4- Bone bending: Piezoelectrical theory: bioelectrical theory:
= proposed by Baumaind 1969 based on observation of Farrar 1876, when the force is applied to
the teeth it cause deformation or beading of alveolar bone, which forms area of concavity has –ve
charge and near to the root of moved tooth --------- bone deposition, and other part of bone which
is convex and has +ve charges ---------- bone resorption.
= deformation of crystalline structure produce flow of current as a result of displacement of
electron from one part to another crystal lattice
= source of Piezoelectricity in alveolar bone:
1- Hydroxyapatite crystals
2- Collagen fibers
3- Collagen hydroxyapatite interface
4- Muco-polysaccharide of ground substances
= Piezoelectric effect first described by Fukada and Yasuda 1957, it is characterized by two unique
properties:
1- Quick delay rate: quick decay rate:
when force is applied on teeth, crystalline substances of bone get deformed and electric charges
are produced as a result of migration of electrons from one location to another
when a force is applied piezoelectricity is generated which immediately goes to zero level, even if
the force is continuously applied.
2- Reversed piezoelectricity:
Production of equivalent current but in opposite direction when force is released
When force is released, a reverse flow of electrons occurs as the crystals returns to their original
shape
Chemical mediators of orthodontic tooth movement
They are substances that released from the cells, plasma, or damaged tissue itself as:
Arachidonic acid
Cytokines: interleukins, tumor necrosis, interferons
Second messages: CAMP, tyrosine kinase, inositol phosphatase
Ankylosis:
= in rare cases a tooth may not move at all, regardless to the amount of external force applied on
it------------ ankylosis
= as the PDL fibers is absent and therefore cannot serve as an inter-mediatory between the root
structure and alveolar bone
= the contact point is direct fusion of cementum layer to the cortical bone of tooth socket
= in case of severe dental trauma, such as avulsion or intrusion, there is injury to the periodontal
membrane resulting in direct fusion of the alveolar bone with tooth