Tooth movement

• Orthodontic tooth movement


  The studies of the different types of removable and fixed appliances and the tissue reactions in response to the forces applied by these appliances in the periodontium, maxillary sutures, and the TMJ region

  
• Conclusions based on animal studies may not always be applicable to humans due to differences in anatomic morphology, growth pattern, turnover rate, and speed of tissue reaction
• Tooth movement


  • Requires changes in the gingiva, periodontal ligament, root cementum, and alveolar bone with their differences in cell population and remodeling capacity

  
• Gingiva


  The free gingiva is in close contact with the enamel surface, and the attached gingiva is firmly attached to the underlying alveolar bone and cementum by connective tissue fibers

  
• Gingival fibers


  • Transseptal fibers
  • Circular fibers
  • Dentogingival fibers
  • Alveologingival fibers
  • Dentoperiosteal fibers

  
• Periodontal ligament (PDL)


  The soft, richly vascular and cellular connective tissue that surrounds the roots of the teeth and joins the root cementum with the alveolar bone

  
• PDL fibers


  • Alveolar crest fibers
  • Horizontal fibers
  • Oblique fibers
  • Apical fibers
  • Interradicular fibers
• Root cementum


  A specialized mineralized tissue covering the root surface that attaches the PDL fibers to the root and contributes to the process of repair after damage
• Alveolar bone


  The bone that forms and supports the sockets of the teeth, consisting of dense outer cortical bone plates with varying amounts of spongy or cancellous bone between them

  
• Cells involved in bone remodeling

  • Osteoblasts
  • Osteocytes
  • Osteoclasts

  
• Bone homeostasis


  Maintaining the net bone mass intact as the activities of osteoblasts and osteoclasts are well-adjusted

  
• Physiologic tooth movement


  1. Changes in tooth position during and after tooth eruption

  2. Slight tipping of the functioning tooth in its socket during chewing process

  
• During masticatory function, the teeth and periodontal structures are subjected to intermittent heavy forces ranging from 1-2kg to as much as 50kg

  
• When a tooth is subjected to heavy loads


  Quick displacement of the tooth within the PDL space is prevented by the incompressible tissue fluid, and the force is transmitted to the alveolar bone, which bends in response

  
• Pain is normally felt after 3 to 5 seconds of heavy force application, indicating the compression of the PDL
• Cusps of upper and lower teeth contact

  • Lasting for 1 second or less
  • Forces are quite heavy, ranging from 1 or 2kg while soft substances are being chewed to as much as 50kg against a more resistant object

  
• Tooth subjected to heavy loads


  1. Quick displacement of the tooth within the PDL space is prevented by the incompressible tissue fluid

  2. Force is transmitted to the alveolar bone, which bends in response

  3. Very little of the fluid within the PDL space is squeezed out during the first second of pressure application

  4. If pressure against a tooth is maintained, fluid is rapidly expressed, and the tooth displaces within the PDL space, compressing the ligament itself against adjacent bone

  
• Pressure against a tooth is maintained

  Fluid is rapidly expressed, and the tooth displaces within the PDL space, compressing the ligament itself against adjacent bone

  
• Prolonged force, even of low magnitude

  Produces remodeling of the adjacent bone

  
• Orthodontic forces


  Act on the PDL and alveolar process
• Orthopedic forces

  More powerful and act on the basal parts of the jaws
• Decisive variables regarding orthodontic forces

  • Application
  • Magnitude
  • Duration
  • Direction
• Continuous forces


  Supplied by some metal springs
• Intermittent forces


  Using smart arch wires, the force is "on" in the body temperature but "off" during cold drinking
• Modern fixed appliance systems are based on light continuous forces from the archwire
• A continuous force may be interrupted after a limited period
• In clinical orthodontics, an interrupted tooth movement may have certain advantages as the tissues are given sufficient time for reorganization, which is favorable for further tissue changes when the force is again activated

  
• Intermittent force

  Acts during a short period and is induced primarily by removable appliances, especially functional appliances where these appliances are worn for 14-16 hours a day and removed for the rest of day
• Reactions in PDL to heavy, continuous loads in experimental tipping of first molars in rats indicated that up to a certain level of stress or duration, the reactions occur mainly in the periodontal membrane with increasing vascularization, cell proliferation, fiber formation, and osteoid application on the bone surfaces
• Beyond a certain level of stress or duration, decreased vascular supply in the PDL and destruction of cells between stretched fibers occur
• A light force over a certain distance moves a tooth more rapidly and with fewer injuries to the supporting tissues than a heavy one
• Purpose of applying a light force

  To increase cellular activity without causing undue tissue compression and to prepare the tissues for further changes
• Another reason for applying light forces

  It results in less discomfort and pain to the patient
• Initial phase of tooth movement

  1. Application of a force to the crown of a tooth leads to tooth movement within the alveolus and initially a narrowing of the PDL, causing compression in limited areas and by that impedes vascular circulation and cell differentiation

  2. Advanced cellular and vascular changes may occur within a few hours of the application of the orthodontic force

  3. The cells undergo a series of changes, starting with a swelling of the mitochondria and the endoplasmatic reticulum, and continuing with rupture and dissolution of the cell membrane

  4. Undifferentiated precursor cells along the alveolar bone wall differentiate into osteoclasts and fibroblasts in young humans after 30 to 40 hours

  
• Hyalinization phase/Lag Phase


  1. A limited hyalinized zone may persist from 3 to 4 weeks in a young patient, while its duration is longer in adults

  2. At the pressure side, the degradation of the cells and vascular structures gives the tissue a glasslike appearance under the light microscope, termed hyalinization

  3. Hyalinization represents a sterile necrotic area, characterized by three main stages: degeneration, elimination of destroyed tissue, and establishment of a new tooth attachment

  4. Degeneration starts where the pressure is highest and the narrowing of the PDL is most obvious, that is, around the bone spicules

  5. In the hyalinized zones, there are no cells that can differentiate into osteoclasts, since osteoclast precursor cells cannot enter from the compressed and degenerated blood vessels

  6. Osteoclasts are formed and start the resorption from the alveolar bone marrow spaces (undermining bone resorption)

  7. The necrotic structures are removed, and the hyalinised area repopulated by cells

  8. The peripheral areas of the hyalinized compressed tissue are eliminated by an invasion of cells and blood vessels from the undamaged PDL

  9. The hyalinized tissue is ingested by the phagocytic activity of macrophages and is removed completely and the re-establishment will start

  
• Secondary phase / Post-lag phase

  1. The PDL space is now wider than before the start of the treatment and the tissue under repair is rich in cells

  2. On the pressure side, the osteoclastic bone resorption continues with a predominantly direct bone resorption, as long as the force is kept within certain limits or gentle reactivation

  3. Cell proliferation usually occurs after 30 to 40 hours in young humans

  4. A 'pre-bone' protein matrix, or osteoid, is produced by osteoblasts on the tension side

  5. The originally periodontal fibers become embedded in the osteoid, which continuously mineralizes to bone tissue in its deeper layer

  6. Extensive remodeling takes place in the deeper cell-rich layers of the periosteum, a reaction that tends to restore the thickness of the supporting bone

  
• Pressure-Tension Theory


  The area of the periodontium in the direction of force is under pressure while the area of periodontium opposite to the direction of force is under tension

  
• Blood Flow Theory/Fluid Dynamic Theory

  When a light continuous force applied to the tooth, the PDL vascular fluid, cellular fluid and interstitial fluid escape through tiny vascular channels creating a favorable local environment for bone resorption

  
• Bone Bending or Piezoelectric Theory


  The effects of physical distortion of the alveolar bone by the forces from orthodontic appliance may electrically charge the bone and exhibits a phenomenon called piezoelectricity and this may be responsible for the cellular reactions observed during tooth movement

  
• Mechanochemical Theory


  Application of physical stress to the bones changes the solubility of the hydroxyapatite crystals and results in remodeling of bone