Orthodontic Biomechanics Flashcards
What are the 3 laws of motion?
- Law of inertia
- Law of acceleration
- Law of action and reaction
What is the 1st law of motion: the law of inertia?
- An object at rest will remain at rest…
- Unless acted on by an unbalanced force
and
- An object in motion will continue with constant speed and direction
- Unless acted on by an unbalanced force
What is the 2nd law of motion: law of acceleration?
- An object acted upon by an unbalanced force will accelerate in the direction of the force
- The ball accelerates only while your foot is in contact with the ball
What is the third law of motion: law of action and reaction?
For every action there is an equal and opposite reaction
What are mechanics?
The science that deals with the action of forces on the form and motion of bodies
mechanics in ortho: forces are applied to teeth via brackets and wires
What are the characteristics of a force?
- magnitude (how much grams are applying to force (ounces, pounds, tons, a unit of force)
- direction (line of action)
- sense (where its going N,S,E,W, occlusal/gingival/mesial/lingual/buccal)
What is the center of resistance?
- Also known as center of gravity
- Point in which an object or body is in equilibrium - at this point the tooth or object is going to remain stable and not move
What is the movement of a free body?
Relationship of the line of action of the force with the center of resistance
What is a constrained body?
- A tooth is a constrained body (not floating in space)
- The center of resistance will vary with alveolar height (bone loss or no bone loss the resistance changes)
What is moment?
Measure of the tendency of a body to rotate around a point or axis perpendicular to any given plane
What are examples of simple forces?
- Pull (tension)
- Push (pressure)
- push and pull are the same
- Extrude
- Intrude
What are the different force systems in ortho?
- Point of application of the force: almost always at the crown level
- Direction of the force (3 planes of space)
- Amplitude of the force (varies)
Point of application of the force: almost always at the _______ level
crown
In orthodontics, a simple force applied to the crown of a tooth will result in ________ of the crown
tipping
The crown will tip in the direction of the force, the apex will tip…
in the opposite direction
What is done when a tooth needs movement more than a simple force?
- Translation
- Rotation
- need to have these to avoid tipping
What is a couple?
Two forces equal in magnitude and opposite in direction act on an object
- need to have a couple to avoid tipping
What is the result of a couple?
- pure moment, since translatory effect of the forces cancel
- The object will spin around its center of resistance (rotation)
Couples result in…
all rotation
no translation (because one force going to right and one force going to left = same amount of force on both sides
If one side is greater than the other then itll move that way, whereas if equal, they will cancel so NO translation)
How do you obtain translation?
bodily movement
a combination of a simple force and a moment of force
- If we have an incisor on the right and want to translate it to the left
if we place the force on the bracket, the tooth is going to translate AND rotate so the crown goes to the left but the root is going to go to the right, to counteract this, we will need to place another force in opposite direction to minimize rotation by adding a COUPLE in the OPPOSITE direction of the rotation
farther away the line of the force to center of resistance = more rotation and inc in translation
In orthodontics, _________ is often the movement of choice
translation
Describe the 3D forces applied to this pt
if we want to bring this tooth down, it will NOT be in a good position because of the sagittal discrepancys (if its brought down it’ll be in crossbite because it’s intruded), SO it needs to be brought DOWN AND have a labial force to bring it forward to avoid having it in a crossbite
(have to think in 3D - difficult to translate what you see on a screen (2D) to thinking in 3D)
Where is the center of resistance commonly located?
More or less in center of tooth and
cervical 1/3rd of root
important to know because depending on
where the force is applied and the action
of the forec, that tooth is going to move
that way
remember: we must move the __________
Roots
- when you say youre moving teeth also need to move the roots, NOT just the crowns, if the crowns move and the roots dont, then the ‘crown follows apex’ so the tooth will essentially go back to where it initially was placed aka where the root remains
