The biological basis of the orthodontic Therapy Fengshan Chen Tongji University

Law 1 In 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 oxytalan Some of them are stretched, torn and ruptured, whereas others are compressed and undergo aseptic necrosis

Ground substance of the PDL Proteoglycans and other proteins Their contents and expression are altered upon tooth movement Water squeezed in and out during tooth movement

Alveolar Bone Thin cortical bone and porous (lamina dura) Fluid pumped in and out of the PDL Trabecular bone underneath Must remodel before teeth can be moved

C:Cell F:Fiber AB: Alveolar bone G:Gingivity CE:

Law 2 Tooth cannot move unless bone apposition and resorption take place.

Susan M. Ott Univ of Washington

Law 3 There 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 forces Interrupted forces Never declines to zero. Interrupted forces Declines to zero Intermittent forces

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

Types of Tooth Movement Eruption Active Passive Lateral drifts Physiological Due to loss of adjacent teeth Orthodontic 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 5 Orthodontic 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 forces Osteoclasts formed Removing lamina dura Tooth movement begins This 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 3 Phase 2 Tooth movement (mm) Time (Arbitrary Unit)

Effects of heavy force on the PDL Heavy, continuous forces Blood supply to PDL occluded Aseptic necrosis PDL 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 6 Frontal 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 3 Phase 2 Tooth movement (mm) Time (Arbitrary Unit)

Frontal resorption Undermininging Resorption Time (Arbitrary Unit) Phase 1 Phase 3 Phase 2 Tooth movement (mm) Frontal resorption Time (Arbitrary Unit) Undermininging Resorption Phase 1 Phase 3 Phase 2 Tooth movement (mm) Time (Arbitrary Unit)

Law 7 Frontal 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 Unit B 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 A Unit B

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 Couple 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 trauma Excessive orthodontic forces Moving 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 8 Orthodontic 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