The biological basis of the orthodontic

Therapy

Fengshan ChenTongji University

Law 1In orthodontics, tooth moves through bone and brings the periodontal ligament with it.

The basis of the Periodontal Ligament (PDL)

Normal width 0.25 mm or 250 micrometers. Cells, fibers, ground substance.

Cells of PDL

Fibroblasts Osteoblasts, osteoclasts Cell rests of Malassez Mesenchymal stem cells They all proliferate at different stages of tooth movement. You must know what functions each has in tooth

movement.

Fibers of the PDL

Collagen and oxytalanSome of them are stretched, torn and

ruptured, whereas others are compressed and undergo aseptic necrosis

Ground substance of the PDL

Proteoglycans and other proteinsTheir contents and expression are

altered upon tooth movementWater squeezed in and out during

tooth movement

Alveolar Bone

Thin cortical bone and porous (lamina dura)Fluid pumped in and out of the PDLTrabecular bone underneathMust remodel before teeth can be moved

C:CellF:Fiber

AB: Alveolar bone

G:GingivityCE:

Law 2Tooth cannot move unless

bone apposition and resorption take place.

Susan M. OttUniv of Washington

Law 3There will be no tooth movement

unless there is a force.

The basis of Force

The force must have the right characteristics such as the magnitude and duration ---- it must meet certain threshold.

Force Types

Light, continuous forcesNever declines to zero.

Interrupted forcesDeclines to zero

Intermittent forcesDeclines to zero

Force Magnitude (Level)

In the range of 10 to 200 grams. Varies with the type of tooth movement. Light, continuous forces are currently considered to be

most effective in inducing tooth movement. Heavy forces cause damages and fail to move the teeth.

Force Duration

Threshold --- 6 hrs per day. No tooth movement if forces are applied less than 6 hrs/d. From 6 to 24 hrs/d, the longer the force is applied, the more

the teeth will move.

Law 4Orthodontic tooth movement is not the only type of tooth

movement.

EruptionActivePassive

Lateral driftsPhysiologicalDue to loss of adjacent teeth

Orthodontic tooth movement

Types of Tooth Movement

Types of Tooth Movement

Intrusion Extrusion Tipping Bodily movement Rotation

Mechanisms of Tooth Movement

Piezoelectric theory.

Mechanisms of Tooth Movement

Pressure-tension theory.

The Optimal Force

“High enough to stimulate cellular activity without completely occluding blood vessels in the PDL” (Proffit et al. 2000).

Actively being investigated in a scientific field known as mechanotransduction.

Law 5Orthodontic tooth movement cannot occur unless cells are

at work.

Force --- fluid flow --- cell-level strain Deformation of cell membrane leading to cytoskeletal changes Second messenger pathways Gene upregulation in fibroblasts, osteoblasts and osteoclasts

Effect of the light force on the PDL

Light, continuous forcesOsteoclasts formedRemoving lamina duraTooth movement beginsThis process is called “FRONTAL

RESORPTION

“Frontal resorption” because it occurs between the root and the lamina dura.

Light force leading to frontal resorption

Phase 1 – Mechanical compression and tension of the periodontium Phase 2 --- Mechanically induced cellular and genetic responses; no tooth

movement Phase 3 --- Accelerated tooth movement due to frontal bone resorption

Phase 1

Phase 3Phase 2

Tooth

movem

en

t (m

m)

Time (Arbitrary Unit)

Effects of heavy force on the PDL

Heavy, continuous forcesBlood supply to PDL occludedAseptic necrosisPDL becomes “hyalinized” – “HYALINIZATION”This process is called “UNDERMINING

RESORPTION”.

“Undermining resorption” because it occurs on the underside of lamina dura, not between lamina dura and the root.

Law 6Frontal resorption occurs in

the PDL, whereas undermining resorption

occurs underneath the lamina dura.

Heavy force leading to undermining resorption

Phase 1 – Mechanical compression and tension of the periodontium Phase 2 --- Continuing mechanical compression; little cellular and genetic

responses; no tooth movement Phase 3 --- Cells recruited from the undermining side of lamina dura, not within

the PDL, to induce undermining bone resorption

Phase 1

Phase 3Phase 2

Tooth

movem

en

t (m

m)

Time (Arbitrary Unit)

Phase 1

Phase 3Phase 2

Tooth

movem

en

t (m

m)

Time (Arbitrary Unit)

Phase 1

Phase 3

Phase 2

Tooth

movem

en

t (m

m)

Time (Arbitrary Unit)

Frontal resorption

Undermininging Resorption

Law 7Frontal resorption facilitates orthodontic tooth movement,

whereas undermining resorption impedes

orthodontic tooth movement.

Anchorage

Newton’s law: for every action, there is reaction. Defined as “resistance to unwanted tooth

movement.” The “anchorage value” of any tooth is roughly

equivalent to its root surface area. Thus, molars and canines generally have higher anchorage values than incisors and bicuspids.

Anchorage types

Reciprocal anchorage. Reinforced anchorage. Stationary anchorage. Cortical anchorage.

Reciprocal anchorage

Both units move roughly equal distance. Exemplified by closing a diastema

between two central incisors.

Reinforced anchorage Unit A has substantially more anchorage value than Unit

B. Thus, Unit A moves little but Unit B moves a lot. Exemplified by retracting anterior teeth to close an

extraction space by using posterior teeth as a reinforced anchorage unit.

Unit BUnit A

Biomechanics of Tooth Movement

Center of Resistance --- A point on the tooth around which the tooth shall move. For most teeth, COR is 2/5 way between the apex and the crest of the alveolar bone.

Center of Rotation --- The point around which rotation occurs when an object is being moved.

Force and Couple

Force Is applied by orthodontic appliances. Induces tipping, translation, intrusion, extrusion and/or

rotation. Couple

Two forces of opposite directions and with non-overlapping points of application.

Translation of teeth occurs in response to appropriate force couples.

Potential Complications of Orthodontic Tooth movement

The pulp Root resorption Alveolar bone height

Orthodontic effects on the pulp

Rare if light, continuous forces are applied. Occasional loss of tooth vitality.

History of previous traumaExcessive orthodontic forcesMoving roots against cortical bone

Endodontically treated teeth can be moved like natural teeth, with proper management.

Root resorption

More accurately, resorption of root cementum and dentin. Normal ageing process in many individuals Likely occurring in many cases but not to the degree of

clinical significance. Root resorption induced by light orthodontic forces is

reversible (by regeneration and repair of cementum and/or dentin).

Can lead to tooth mobility in severe cases.

Generalized Root Resorption

Affects most, if not all, teeth; maxillary incisors more susceptible than other teeth.

Could be moderate or severe but commonly in the range of up to 2.5 mm.

Etiology largely unknown but predisposing factors include conical roots with pointed apices, distorted tooth form, or a history of trauma.

Localized Root Resorption

Can’t always be distinguished from generalized root resorption.

Maxillary incisors more susceptible than other teeth. Only in rare cases can the causes, such as heavy

orthodontic forces, be pinpointed. Etiology largely unknown.

Law 8Orthodontic tooth movement

remains one of the most successful procedures with

predictable outcome in medicine and dentistry.

Orthodontics and dentofacial orthopedics requires thorough knowledge in biology (of bone, cartilage, teeth, muscles, nerves and other soft tissues), biomechanics, biometrics,

material science, clinical skills and practice management in addition to interpersonal skills.

Why study tooth movement?

Up to 70% of the Chinese population have malocclusion that warrants orthodontic correction.

Currently, less than 20% of the Chinese patients seeks orthodontic treatment. However, I believe more and more people will seek orthodontic with the development of society