In the name of GOD

Biomechanics 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

F

F

d

M

M

50 g

50 g

8 mm

400 g.mm

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

Force* (gm) Type of movement

35-60 Tipping

70-120 Translation

50-100 Root uprighting

35-60 Rotation

35-60 Extrusion

10-20 Intrusion

*values depend in 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 after about 4 hours, cyclic nucleotide levels increased

clinical experiments threshold for tooth movement is in the 4-8 hours range

longer force maintenance 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