Transcript Types of tooth movement

In the name of GOD
Biomechanic and types of tooth movement
Presented by:
Dr Somayeh Heidari
Orthodontist
Reference:
Contemporary Orthodontics
Chapter 8
William R. Proffit, Henry W. Fields, David M.Sarver. 2013. Mosby
Definition of terms
Force
a load applied to an object that will tend to move it to a different position
F = m.a
its units are Newtons or gram. Millimeters/ Seconds2
is usually measured in weight units of Grams or ounces
a force is a vector
Line of Action
Origin /
Point of application
Sense / Direction
Resultant
Center of resistance
a point at which resistance to movement can be concentrated
for an object in free space is the same as the center of mass
for a partially restrained object, determined by the nature of the external
constraints
The center of resistance for a tooth is at the approximate midpoint of the embedded
portion of the root: about halfway between the root apex and alveolar bone crest.
Location of the center of resistance depends on the alveolar bone height,
root length , morphology and number of roots.
Moment
 a force (F) acting at a distance from the center of resistance (d)
 m = F × d (gm-mm)
 moment of a force results in some rotational movement
 it tends to rotate the object around the center of resistance
 it is precisely the situation when a force is applied to the crown of a tooth
F
Distance (d)
M=F×d
Moment of Force
Clinical examples of moments of the forces
Couple
 two parallel forces equal in magnitude and opposite in direction
that
separated by a distance
 the result is a pure moment
 will produce pure rotation around the center of resistance
 couple = magnitude of force(s) × distance between forces
M
F
400 g.mm
d
50 g
F
8 mm
M
50 g
Couple force = F × d
Center of rotation
the point around which rotation actually occurs when an object is moved
If a force and a couple are applied to an object, the center of rotation can
be controlled
Types of tooth movement
 optimum orthodontic force will stimulate cellular activity without completely
occluding blood vessels in the PDL.
 the PDL response is determined not by force magnitude alone, but by force per
unit area, or pressure.
 the distribution of force within the PDL differs with different types of tooth
movement
Pressure = Force / Area
Tipping
 the crown and the root move in opposite directions
 greater movement of the crown than the root
 the center of rotation is apical to the center of resistance
 based on the location of the center of rotation is classified into uncontrolled and
controlled tipping
Uncontrolled Tipping
 the simplest form of orthodontic movement
 produced when a single force is applied against the crown
 the tooth will rotate during movement
 the PDL is compressed near the apex (same side as the force) and at the alveolar
crest (opposite side from the force)
 maximum pressure is at the alveolar crest and the root apex
 progressively less pressure is created as the center of resistance is approached
 only one-half the PDL area is loaded
 high pressure in the two areas is concentrated
 force must be kept quite low:
not exceed approximately 50 gm
 M/F ratio = 0:1 to 5:1
(average root length and 100% alveolar height)
Controlled Tipping
 the center of rotation is at the root apex
 by application a force to move the crown and a moment to control the apex
 concentration of stresses at the alveolar crest
 minimal stress at the root apex
 M/F ratio = 7:1
Translation
 also known as “bodily movement”
 the root apex and crown move in the same direction and the same distance
 PDL area is loaded uniformly
 twice as much force is required for translation
as for tipping
 a horizontal force applied at the center of resistance will result in translation
 two forces are applied simultaneously (a couple and a force are required)
 the center of rotation is at infinity
 M/F ratio = 10:1
Rotation
 pure rotation requires a couple
 no net force acts at the center of resistance
 in theory, rotational force can be more larger than other movements
 in fact, rotational movement without tipping is essentially impossible
 appropriate forces for rotation are similar
to those for tipping
Extrusion
 ideally would produce no area of compression in the PDL, only tension
 practically the tooth tipped at all while being extruded
 even if compressed areas avoided, heavy forces in pure tension would be
undesirable unless for extraction
 about the same magnitude of force as those for tipping
Intrusion
 successful intrusion can be accomplished, only if very light forces are applied
 the force concentration is in a small area at the root apex
 the tooth probably will tip somewhat as it is intruded
Root movement
 keeping the crown stationary and applying a moment and force to move the root
 the center of rotation is at the incisal edge
 root movement requires a large moment
 concentration of stress is in the apex area
 M/F = at or above 12:1
Optimum forces for orthodontic tooth movement
Type of movement
Force* (gm)
Tipping
35-60
Translation
70-120
Root uprighting
50-100
Rotation
35-60
Extrusion
35-60
Intrusion
10-20
*values depend n part on the size of the tooth, smaller values appropriate for incisors,
higher values for multirooted posterior teeth.
Effects of force duration and force decay
 animal experiments
 clinical experiments
 longer force maintenance
after about 4 hours, cyclic nucleotide levels increased
threshold for tooth movement is in the 4-8 hours range
increasingly effective tooth movement
 fixed appliances are more effective unless the removable appliance used almost all
the time
Duration of force has another aspect, related to how force magnitude
changes as the tooth responds by moving.
From this perspective, orthodontic force duration is classified by the rate
of decay as:
Continuous – force
Interrupted – force
Intermittent - force
Continuous force
Interrupted force
Intermittent force
 theoretically light continuous forces produce the must efficient tooth movement.
 the heavier forces are physiologically acceptable only if:
force levels decline so that there is a period of repair and regeneration
before the next activation
or
the force decreases at least to the point that no second and third rounds
of undermining resorption occur
 Experiments has shown that orthodontic appliances should not be reactivated
more frequently than at 3 week intervals.
 A 4 to 6 week appointment cycle is more typical in clinical practice.
 Undermining resorption requires 7 to 14 days : equal or longer period is need
for PDL regeneration and repair before force is applied again.
Thanks for your attention