Biomechanics of Orthodontic Tooth Movement Flashcards
What are the 3 phases of physiologic tooth movement? Which of these does orthodontic tooth movement fall under?
- pre-eruptive
- eruptive
- post-eruptive (orthodontic tooth movement - manipulates this)
How does the relationship between force and velocity affect tooth movement?
not the same as usual physics
rate of tooth movement increases quickly for a small amt of force then plateaus, as force further increases tooth movement decreases
→ peculiar, due to biomechanics
What is biomechanics in context of ortho tooth movement?
interactions between mechanical forces -ortho appliances (physics) and biological systems (biology- PDL, bone)
How heavy can the forces of occlusion be and what does that tell us?
over 50kg
teeth must withstand repeated heavy forces and remain stable in position
What kind of force is an orthodontic force?
sustained force
Explain what happens in the images below (with no force, sustained force)
A. normal tooth at rest
B. with sustained force, liquid is expelled a bit from PDL space, PDL stretches/tenses on side away from force and compresses on side that force is pushing
C. osteoclasts are recruited on the side with compression (resorption), osteoblasts recruited on side with tension (deposition)
Label the image
Why is the amount of orthodontic force put on a tooth critical? (explain light, moderate, heavy) How much is capillary blood pressure?
light forces squash the PDL fibres but blood vessels remain intact
as we put more force on, PDL fibres we start to occlude the blood vessels
with heavy forces, we completely obstruct the blood vessels which leads to death of PDL
capillary bp ~ 26g/cm2
(exceeding this force will lead to death of PDL)
What is the difference between light force and heavy force on the PDL?
light forces: blood vessels distort, blood flow altered, osteoclasts recruited and works at interface between PDL and alveolar bone, tooth moves as bone is resorbed (3-5days)
heavy forces: blood vessels occluded, blood flow completely obstructed, sterile necrosis results in hyalinised PDL (within hours), undermining resorption (osteoclasts cannot be recruited from PDL, instead recruited from alveolar bone and cause resorption of alveolar bone from outside-in) → takes 7-14 days instead (undermining resorption vs frontal resorption - the heaveier the force, the slower the movement)
Explain frontal vs undermining resorption using graph.
heavy force: big pause happens, then osteoblasts that move close enough to the PDL space cause a big suddent movement (in the area undermined)
light force:
→ undermining resorption can cause root resorption compared to frontal
Why do the optimal forces for each type of movement differ?
because the area where the force is applied to PDL changes depending on the type of tooth movement
Why is a lighter force required for tipping movement vs bodily movement?
force on PDL stays same but the area over which the force works is different
when tipping: movement is within the socket, force works over a lesser area
bodily movement: larger area cos whole of root surface being pushed onto alveolar bone
Why is the lowest optimal force required for intrusion movements?
all the force is concentrated into small area at apex of tooth → force must be low to not overload the PDL
What is the centre of resistance vs centre of rotation?
centre of resistance: fixed point that force must pass through to move object in linear manner (translation), consistent with centre of mass (free floating object)
centre of rotation: unfixed point around which an object rotates, determined from original and final position
When does rotation happen?
when the force applied to an object is not perpendicular to the centre of resistance