biomechanics and appliances Flashcards
optimum tooth movement is produced by
1) light continuous force
key terms to remember
1) elastic behavior is dictated by its stress strain response
2) stress is internal distribution of the load (F/A)
3) strain is internal distortion produced by the load (deflection/length)
elastic limit
1) point on stress/stain curve where any deformation occurs
yield point
1) 0.1% deformation occurs
ultimate tensile strength
1) maximum stress a material can withstand while being stretched or pulled before necking (cross section starts to contract)
properties of a good wire
1) high strength
2) low stiffness
3) high range
4) high formability
5) weldable and solderable
stainless steel high chromium contnet
1) stiff, strength
2) bendable and deformable
cobalt chromium alloy
1) bendable, more expensive
NiTi wires
1) nickel, titanium, and naval ordinance lab
2) two major properties
- shape memory
- superelastic
3) originally, it was a springy and strong form, but now it has super elasticity
stress stain curve of NiTi
1) more stress deforms the wire until the strain plateaus
2) then as the teeth straighten, it comes down at a lower force
two more properties
1) resilience
- area under the stress strain curve
out to the proportional limit
- the storage capacity of the wire
- combination of strength and springiness
2) formability
- amount of permanent deformation a wire can withstand before failing
archwire sequence
1) round flexible wire for alignment
2) rectangular flexible wire to fill the slot
3) rectangular formable wire to detail finish
mechanical concepts in ortho
1) force
2) center of resistance
3) center of rotation
4) moment
5) couple
force
1) load applied to object that will tend to move it into a different position in space
- point of application
- magnitude
- line of action
- direction (sense)
center of mass
1) a point in mass that behaves as if the whole mass is concentrated in it
center of resistance
1) point of concentrated resistance to movement
2) applied force must pass through for a free object to move linearly without any rotation
—
1) if the object is embedded in bone, the CR changes to the center of the object that is EMBEDDED
- depends on the alveolar bone height and root length
center of resistance for a tooth
1) midpoint of the embedded portion of the tooth
2) usually furcation area for multirooted
moment
1) if force doesn’t pass through the CR, there will be rotation around the center of rotation, which can be at the CR or apical to it or at the apex
2) rotational + linear movement
3) moment (M) = force (F) x perpendicular distance (D) from CR
clinical examples of moments of a force
1) mesial at the molar bracket tends to rotate the tooth, mesial in
2) expansion force on a molar tips the crown buccally
3) intrusive force at the molar creates a moment tipping the crown buccally
couple
1) created by two parallel forces equal in magnitude and opposite in direction
2) moment
- magnitude of one of the forces multiplied by the perpendicular distance between two forces
- pure rotation around CR regardless of point of application of the couple
couple of second order
1) engaging a wire in an angulated bracket
- causes spinning motion of the tooth
2) can upright a molar and close diastema
couple in the third order
1) engaging a rectangular wire in a bracket slot
- inclination of root (torque)
round arch wire vs rectangular wire
1) moment of the force = moment of couple
- only works when wire contacts the bracket in 2 points!
ribbon arch
1) taller
edgewise
1) wider
types of toot hmovment
1) tipping
2) translation
3) root movement
4) rotation
5) intrusion and extrusion
uncontrolled tipping
1) cannot control amount of rotation
2) a retainer
- contacts at one point, not counteracting force
3) Mc/Mf = 0
moment to force ratio
Mc/Mf = 0 (uncontrolled)
- center of rotation at CR
0<Mc/Mf<1 (controlled)
- center of rotation moves further apically from the CR
Mc/Mf = 1 (translation)
-center of rotation approaches infinity
Mc/Mf >1 (torqueing)
- center of rotation approaches incisal edge
intrusion movement
1 )force is concentrated over a small area at the apex
2) light force is required 10-20 gm
- or tooth would get shorter
extrusion
1) ideally no areas of compression within PDL
2) only tension
3) same force as tipping
forces for different movements
1) tipping 25-60 gm
2) bodily movement 70-120
3) root uprighting 50-100
4) rotation 35-60
5) extrusion 35-60
6) intrusion 10-20
edward angle
1) four appliance designs
- e-arch
- pin and tube
- ribbon arch
- edgewise appliance
e-arch appliance
1) ligatures form a heavy labial arch were used to bring malposed teeth to the line of occlusion
2) ONLY capable of tipping
3) very painful
ribbon arch
1) vertical slot appliance
2) well adapted to bring teeth into alignment, butt too flexible to allow precise positioning of roots
edgewise appliance
1) horizontal slop appliance
2) arch wire inserted at 90 degree angles
bracket slot dimension
1) 22 slot
2) 18 slot
3) how tall the slot for the wire is
bracket placement is important
1) where the tooth move
2) how long the treatment last
3) final look
indirect bonding
1) you place it on the study models where you want them
2) make a suckdown, then use the template and light cure it
bands
1) restrict to second molars
2) things that cause a lot of force
3) more secure for surgeries
4) the separator is painful and more plaque
clear or ceramic brackets
1) more expensive
2) more friction, can slow movement a little
lingual braces
1) you cannot see them
2) restricts tongue space
1) self ligating brackets
1) self-closing and holds wire in position
2) offer better hygiene
invisalign
1) clear trays that help move teeth
