Presentation on theme: "The biological basis of the orthodontic Therapy Fengshan Chen Tongji University."— Presentation transcript:
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
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 Never declines to zero. Interrupted forces Declines to zero Intermittent forces Declines 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 4 Orthodontic tooth movement is not the only type 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
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)
Phase 1 Phase 3 Phase 2 Tooth movement (mm) Time (Arbitrary Unit) Phase 1 Phase 3 Phase 2 Tooth movement (mm) Time (Arbitrary Unit) Frontal resorption Undermininging Resorption
Anchorage Newtons 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.
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 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 Cant 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