biomechanics- both lectures

lectures given 10/23/2026

tldr

application of force → tissue response → orthodontic movement

what are the principles of biomechanics?

prolonged pressure applied to a tooth → movement will occur

bone is removed in areas of pressure and added in areas of tension

tooth moves through the bone carrying its attachment apparatus with it

tooth movement is primarily a phenomenon of the PDL

biological basis of orthodontics

tooth movement is a function of the periodontal ligament

no PDL = no movement

why do implants and ankylosed teeth not move?

no PDL

are endo treated teeth or teeth wtih open apices are contraindicated to ortho movement?

no! they have a PDL, so they can be moved

how long does the reorganization of PDL fibers take post-ortho tx?

3-4 mo

how long does the reorganization of collagenous fibers (gingival fiber network) take post-ortho tx?

4-6 mo

how long does the reorganization of elastic/supracrestal fibers take post-ortho tx?

1 year

what are the principal fibers of PDL?

transseptal (cause rotational replapse)

alevolar crest group

horizontal

oblique (majority)

apical

interradicular

what factors affect tooth movement?

manner of force application- continuous (NiTi wires), interrupted (invisalign), and intermittent

amount of force- light/heavy

duration force

direction of force

occusal function

age

threshold for tooth movement

prolonged application of force is necessary for tooth movement- minimum for duration of force is 4-6 hrs (important consideration for removable appliances)

if less than 4 hrs you won’t get movement

greatest efficiency with up to 24hrs/day- fixed appliances more efficient than removable

initial strain/primary displacement

1-3 days

occurs at tissue level

compression of PDL

bone bending

creates area of pressure/tension

compression/tension of blood vessels

compression → resorption of bone

tension → deposition of bone

lag phase

2-10 weeks

may be longer with heavy forces

cellular recruitment

neovascularization

cell differentiation

PDL reorganization

lag phase- body treats it like a traumatic event (except with hyalinization)

secondary tooth movement

after lag phase + on

cellular recruitment

pressure areas = resorption of bone by osteoclasts

tension areas = deposition of bone by osteoblasts

frontal resorption

undermining resorption

undermining resorption

width of PDL maintained

rate of tooth movement gradually increases

pressure

PDL fibers and bone are compressed

tension

PDL is stretched

force magnitude

blood vessels in PDL are compressed

partial occlusion force magnitude

lower relative forces

blood vessels adapt to new environment

localized angiogenesis to bypass occluded areas

absolute occlusion force magnitude

heavier forces

complete occlusion of blood flow

cells die/hypoxic areas/necrosis of immediate area

hyalinization

loss of organization

frontal resorption

light force applied → cells from PDL go onto surface of alveolar bone

osteoclasts from within the PDL resorb front of bone

happens between root and lamina dura

light force- smooth continuous tooth movement

movement occurs within 3-4 days

undermining resorption

heavy forces- even more pressure and even more compression of ligament

occurs on the outer surface of lamina dura

pressure is too much for cells to be recruited

ligament loses cellular organiztion = hylinization, hypoxic appearance, necrosis

body resorbs by cells generated from the marrow spaces away from it in underlying bone

aka resorbing from the inside

delay in tooth movement (7-14 days)

clinical characteristics of optimal force

produces rapid tooth movement

minimal pt discomfort

lag phase of tooth movement

no marked mobility of tooth

histological characteristics of optimal force

maintained vitality of teeth

initiated maximal cellular response

produces frontal resorption

initial phase of bone remodeling in relation to ortho

rapid tooth movement

a few days

displacement of tooth in periodontal space

lag phase of bone remodeling in relation to ortho

no/low displacement rate

possibly due to hyalinization

possibly due to time required for absorption of thicker compact bone of lamina dura

heavy forces → considerable lag phase

post-lag phase of bone remodeling in relation to ortho

rate of tooth movement gradually/suddenly increases

static

forces on bodies at rest (teeth)

kinetics

forces on bodies in motion (accelerating at/changing certain velocity)

force = mass x acceleration (in gravity)

requires two bodies

defined by Newton’s

vectors

have magnitude and direction; all orthodontic forces are vectors

priniciple of transmissibility

external forces acting on a tooth is independent of where along the line it is applied

ie the tooth will move the same with a force pushing from the buccal as pulling from the lingual

newton’s first law

inertia and equilibrium

object at rest stays at rest/object in motion stays in motion unless acted upon by outside force

all ortho systems are at equilibrium / the body is said to be at equilibrium

newton’s second law

F = MA; A=F/M

force depends on magnitude of mass of body and direction of which it acts

acceleration is change in speed over time

newton’s third law

every action has an equal and opposite reaction

center of resistance (Cres)

point at which a force will produce movement so that all points on the tooth move in parallel, straight lines (translation)

depends on bone level and root length

moves apically with perio disease

for healthy single rooted teeth → 1/3 from apex

for molars → at the furcation

center of rotation (Crot)

the pivot point, the point about which a tooth will rotate

is NOT an anatomic point

the orthodontist controls the center of rotation

how do you find the center of rotation?

superimpose the two positions of the tooth, draw a line through the long axis of each, where the lines intersect

can be within tooth structure or outside of it

how is rotational movement created?

ortho forces are applied at the crown, not through the Cres of the tooth

how can you move teeth around Crot?

crown lingual/root buccal

crown lingual/apex still

translation along the line of force

movement of the force

tendency of a force to produce rotation

because brackets/attachments are on crowns and not at the Cres orthodontic forces are acting at some distance from Cres

the further the force is from Cres, the bigger the moment

moment (Mf)

force x distance from Cres

units = gram x mm

direction of the moment

take the line of force and draw it around Cres → that is the direction of the moment

t/f forces in opposite directions can cause moments in the same direction

true

t/f you can put 2 forces on a tooth so the moments cancel to get translation

true

what is the optimum force for intrusion?

10-20g

what is the optimum force for tipping, rotation, extrusion?

35-60g

what is the optimum force for root uprighting?

50-100g

what is the optimum force for translation?

70-120g

couple

two equal and opposite non-colinear forces

the only thing that can cause PURE rotation

can never move Cres

moment of a couple

couples result in pure rotation about the center of resistance regardless of where the couple is applied on the object

applied moment

couples

edgewise appliances

allow us to create couples in 3 planes of space

types of tooth movement

tip, translation, torque (root movement), rotation

results from the application of moments and forces

uncontrolled tipping

apply forces at bracket, crown moves lingual, apex moves labial (tip and apex move in opposite direction)

Crot = Cres

simplest movement to produce

greatest stress at crown and apex

controlled tipping

forces moved closer to Cres, only the crown moves

Crot is at the apex

system of forces

force AND couple- force to move crown, couple to control apex position

minimal stress at apex

greatest movement occurs further away

translation

line of force right through Cres (Crot at infinity)

Crot = at infinity

bodily movement

single force at Cres

stresses uniformly distributed along the root

root and crown move the same distance in the same direction Crot at infinity

torque

line of force is above

Cres/the whole thing reverses

Crot a the crown and the root moves

root movement

change in tooth’s axial inclination

greater stress at apical portion

rotation

pure rotation requires a couple

if single force is used → Crot is outside of tooth

no net force acts at Cres

viewed from the occlusal

anchorage

goal- maximize desired tooth movement while minimizing undesirable side effects

resistance to unwanted tooth movement

teeth themselves have the ability to resist movement although vary greatly in their ability to do so

these differences are due to variations in their root surface area- more surface area → more resistance to movement

intraoral- intra-arch or inter-arch

extraoral

intraoral anchorage

TPA- transpalatal arch

lingual arch

nance appliance

lip bumper

TAD- temporary anchorage device

inter-arch elastics

TPA- transpalatal arch

connects maxillary 1st molars

creates an anchor unit of the collective 6 roots

lingual arch

resists mesial movement of mandibular 1st molar

contoured to lingual surface of incisors

nance appliance

resists mesial movement of maxillary 1st molars

acrylic button on anterior portion of palate

lip bumper

resists mesial movement of mandibular 1st molars, can get distal movement

acrylic bar anterior to incisors

lingual pressure from lip musculature gives posterior pressure

inter-arch elastics

have horizontal and vertical vectors

extraoral anchorage

headgear

usually paired with TPA or expander

provides a distal force

can also provide superior or inferior force depending on design

orthodontic forces

move teeth

lighter forces

think grams

orthopedic forces

move bones

heavier forces- 500g (1lb or more)

think pounds

treat skeletal discrepancies

application to maxilla or mandible

maxilla and midface orthopedic forces

maxillary expansion

restrain growth

augment growth

mandible orthopedic forces

restrain growth

augment growth

maxillary dental expansion

correct dental crossbite

tipping teeth buccally

no orthopedic effect (single light forces)

can be achieved with archwires alone

maxillary palatal expansion

rapid palatal expansion (RPE)

corrects skeletal crossbites

expansion occurs mostly at midpalatal suture

heavy orthopedic forces

palatal suture during growth- infancy to early childhood

primary dentition

straight line

contraindication for expansion

palatal suture during growth- childhood

early mixed/mixed dentition

interdigitation

easy to open

most appliances will work

palatal suture during growth- early adolescence

complex interdigitation

most common sequelae for RPE in adult pts = gingival clefting (recession)

methods of maxillary palatal expansion

haas appliance- soft and hard tissue borne

hyrax appliance- fully tooth borne, will get some dental expansion, 30% skeletal opening

rapid palatal expansion

suture opening

non-parallel

more pronounces anterior and inferior- triangular

occusal- base is anterior

frontal- base is inferior

age limit- suture fusion, mid/late teens

must be stabilized for 4-6mo to re-establish suture (25%) relapse

MARPE

mini-screw assisted rapid palatal expansion

use of TADs (temporary anchorage devices)

best for adult pts after sutures have fused

70% skeletal opening

SARPE

surgically assisted rapid palatal expansion

incisions to release the maxilla

incisions to release the midpalatal suture

best for adult pts after sutures have fused who require 10+mm of expansion

highly stable (10-15% relapse)

restrain maxillary growth

excessive maxillary growth → class II

prognathic maxilla

forces designed to oppose natural growth

as maxilla is restricted, allows for mandibular growth to catch up

corrects max/mand discrepancies

headgear

cervical pull headgear

restrain maxillary growth

high pull headgear

restrain maxillary growth

straight pull headgear

restrain maxillary growth

dental effects of cervical pull

down and back

extrude and translate

dental effects of high pull

up and back

intrude and translate

dental effects of straight pull

translate

augment maxillary growth

insufficient maxillary growth → class III

retrognathic maxilla

forces follow natural growth direction → down and forward

reverse pull headgear- facemask

reverse pull headgear

500-1000g (1-2lbs)

at least 14 hrs a day

RPE recommended- mobilizes maxillary complex

side effects- proclination U incisors, retroclination L incisors, soft tissue irritation

restriction of mandibular growth

excessive mandibular growth → class III

prognathic mandible

forces oppose natural growth direction

hold mandibular growth

very controversial

chin cups

augment mandibular growth

insufficient mandibular growth → class II

retrognathic mandible

forces follow natural direction of growth

advance mandible

functional appliance- herbst, MARA, twin block

herbst, MARA, twin block

class II correctors

designed to posture mandible forward when pt occludes

part skeletal/part dental correction- varies between appliance