Biomechanics of Orthodontic Tooth Movement

Question 1

What are the 3 phases of physiologic tooth movement? Which of these does orthodontic tooth movement fall under?

Answer 1

• pre-eruptive
• eruptive
• post-eruptive (orthodontic tooth movement - manipulates this)

Question 2

How does the relationship between force and velocity affect tooth movement?

Answer 2

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

Question 3

What is biomechanics in context of ortho tooth movement?

Answer 3

interactions between mechanical forces -ortho appliances (physics) and biological systems (biology- PDL, bone)

Question 4

How heavy can the forces of occlusion be and what does that tell us?

Answer 4

over 50kg

teeth must withstand repeated heavy forces and remain stable in position

Question 5

What kind of force is an orthodontic force?

Answer 5

sustained force

Question 6

Explain what happens in the images below (with no force, sustained force)

Answer 6

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)

Question 7

Label the image

Question 8

Why is the amount of orthodontic force put on a tooth critical? (explain light, moderate, heavy) How much is capillary blood pressure?

Answer 8

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)}

Question 9

What is the difference between light force and heavy force on the PDL?

Answer 9

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)

Question 10

Explain frontal vs undermining resorption using graph.

Answer 10

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

Question 11

Why do the optimal forces for each type of movement differ?

Answer 11

because the area where the force is applied to PDL changes depending on the type of tooth movement

Question 12

Why is a lighter force required for tipping movement vs bodily movement?

Answer 12

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

Question 13

Why is the lowest optimal force required for intrusion movements?

Answer 13

all the force is concentrated into small area at apex of tooth → force must be low to not overload the PDL

Question 14

What is the centre of resistance vs centre of rotation?

Answer 14

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

Question 15

When does rotation happen?

Answer 15

when the force applied to an object is not perpendicular to the centre of resistance

Question 16

What is a moment (M_f)?

Answer 16

tendency for a body to rotate around a specific point as a response to a force delivered away from the centre of resistance

moment = force x distance

Question 17

Where is the centre of resistance on a tooth? Why is orthodontic force always going to produce a moment?

Answer 17

approximately halfway between crestal bone of PDL space and the apex

orthodontic forces must be placed on the crown therefore will always be away from the centre of resistance and will produce a moment (Mf)

Question 18

Where is the centre of rotation of a tooth?

Answer 18

below the centre of resistance

Question 19

What is a couple (M_c)? (3)

Answer 19

a pair of equal and opposite noncollinear forces

therefore requires 2 points of contact

force couple creates pure rotation

Question 20

Why do fixed appliances allow bodily control/3d control but removable appliances allow only simple tipping movement?

Answer 20

because multiple points of application can be put on a tooth with fixed appliances but cannot with removable

Question 21

How are couples used to achieve bodily movements in fixed appliances?

Answer 21

1. orthodontic force applied to the tooth
2. as the tooth moves, a moment will also be created causing the tooth to tip
3. as the tooth tips, the bracket engages with the wire at these 2 points
4. the wire hitting the bracket creates a force which is a couple
5. the rotational force from the couple cancels out the rotation from initial force of the tooth movement
6. the net result is the tooth only moves in the direction of the force with no rotation (tipping)