Presentation on theme: "The biological basis of the orthodontic Therapy"— Presentation transcript:
1The biological basis of the orthodontic Therapy Fengshan ChenTongji University
2Law 1 In orthodontics, tooth moves through bone and brings the periodontal ligament with it.
3The basis of the Periodontal Ligament (PDL) Normal width 0.25 mm or 250 micrometers.Cells, fibers, ground substance.
4Cells of PDL Fibroblasts Osteoblasts, osteoclasts Cell rests of MalassezMesenchymal stem cellsThey all proliferate at different stages of tooth movement.You must know what functions each has in tooth movement.
5Fibers of the PDL Collagen and oxytalan Some of them are stretched, torn and ruptured, whereas others are compressed and undergo aseptic necrosis
12Law 3 There will be no tooth movement unless there is a force.
13The basis of ForceThe force must have the right characteristics such as the magnitude and duration ---- it must meet certain threshold.
14Force Types Light, continuous forces Interrupted forces Never declines to zero.Interrupted forcesDeclines to zeroIntermittent forces
15Force 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.
16Force 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.
17Law 4 Orthodontic tooth movement is not the only type of tooth movement.
18Types of Tooth Movement EruptionActivePassiveLateral driftsPhysiologicalDue to loss of adjacent teethOrthodontic tooth movement
19Types of Tooth Movement IntrusionExtrusionTippingBodily movementRotation
30The 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.
32Law 5 Orthodontic tooth movement cannot occur unless cells are at work.
33Force --- fluid flow --- cell-level strain Deformation of cell membrane leading to cytoskeletal changesSecond messenger pathwaysGene upregulation in fibroblasts, osteoblasts and osteoclasts
34Effect of the light force on the PDL Light, continuous forcesOsteoclasts formedRemoving lamina duraTooth movement beginsThis process is called “FRONTAL RESORPTION
35“Frontal resorption” because it occurs between the root and the lamina dura.
36Light force leading to frontal resorption Phase 1 – Mechanical compression and tension of the periodontiumPhase Mechanically induced cellular and genetic responses; no tooth movementPhase Accelerated tooth movement due to frontal bone resorptionPhase 1Phase 3Phase 2Tooth movement (mm)Time (Arbitrary Unit)
37Effects of heavy force on the PDL Heavy, continuous forcesBlood supply to PDL occludedAseptic necrosisPDL becomes “hyalinized” – “HYALINIZATION”This process is called “UNDERMINING RESORPTION”.
38“Undermining resorption” because it occurs on the underside of lamina dura, not between lamina dura and the root.
39Law 6 Frontal resorption occurs in the PDL, whereas undermining resorption occurs underneath the lamina dura.
40Heavy force leading to undermining resorption Phase 1 – Mechanical compression and tension of the periodontiumPhase Continuing mechanical compression; little cellular and genetic responses; no tooth movementPhase Cells recruited from the undermining side of lamina dura, not within the PDL, to induce undermining bone resorptionPhase 1Phase 3Phase 2Tooth movement (mm)Time (Arbitrary Unit)
41Frontal resorption Undermininging Resorption Time (Arbitrary Unit) Phase 1Phase 3Phase 2Tooth movement (mm)Frontal resorptionTime (Arbitrary Unit)UnderminingingResorptionPhase 1Phase 3Phase 2Tooth movement (mm)Time (Arbitrary Unit)
43Anchorage 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.
45Reciprocal anchorage Both units move roughly equal distance. Exemplified by closing a diastema between two central incisors.
46Reinforced 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 AUnit B
47Biomechanics 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.
49Force and Couple Force Couple Is applied by orthodontic appliances. Induces tipping, translation, intrusion, extrusion and/or rotation.CoupleTwo forces of opposite directions and with non-overlapping points of application.Translation of teeth occurs in response to appropriate force couples.
51Potential Complications of Orthodontic Tooth movement The pulpRoot resorptionAlveolar bone height
52Orthodontic 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 boneEndodontically treated teeth can be moved like natural teeth, with proper management.
53Root resorptionMore accurately, resorption of root cementum and dentin.Normal ageing process in many individualsLikely 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.
55Generalized 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.
56Localized 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.
58Law 8 Orthodontic tooth movement remains one of the most successful procedures with predictable outcome in medicine and dentistry.
59Orthodontics 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.
60Why 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