Orthodontic Force & Tooth movement

Introduction

• Favourable Orthodontic Force induces an optimal cellular and tissue response, aiding tooth movement.
• Unfavourable force may initiate adverse tissue reactions through an imprecise biological response.

Tooth-Supporting Tissues

• Magnitude, direction, and duration of force, alongside age and health, affect the periodontium response.
• Changes in gingiva, periodontal ligament (PDL), root cementum, and alveolar bone are required.
• These tissues have specific differences in cell population and tissue-adaptation capacity.

Gingiva

• Requires changes for tooth movement
• Free gingiva is in close contact with the enamel surface.
• Margin of free gingiva is located 0.5 to 2 mm coronal to the cementoenamel junction (CEJ).
• Attached gingiva is firmly attached to the underlying alveolar bone/cementum via connective tissue fibers.
• Attached gingiva is comparatively immobile relative to underlying tissue.
• Connective tissue is the predominant component of the attached gingiva.
  • Components include collagen fibres, fibroblasts, vessels, nerves, and extracellular matrix.

Fibroblasts

• Fiber production
• Synthesis of connective tissue matrix

Collagen Fibres

• Bundles of collagen fibrils
• Have distinct orientations
• Provide resilience and tone to the gingiva.
  • Maintain gingival architectural form.
  • Maintain the integrity of dentogingival attachment.
• Collagen turnover rate is lower in the gingiva than in the PDL, due to lower functional stress.
• Transseptal fibers provide firm anchorage for the tooth.
• Remodelling and regeneration of gingival epithelium is a slow process.
• Red patch indicates soft tissue exposure during tooth movement.
• Crease in soft tissue near tooth movement can impede extraction space closure.
• Gingival creasing resolves spontaneously.

Periodontal Ligament

• Layer about 0.25 mm thick
• Functions as soft, vascular, cellular connective tissues.
• Surrounds tooth roots.
• Joins cementum and bundle bone (lamina dura or alveolar bone proper).
• Continuous with gingival lamina propria coronally.
• Separated from the gingiva via collagen fibre bundles connecting alveolar bone/root (alveolar crest fibres).

Development of Periodontal Ligament

• Derived from the dental follicle surrounding the tooth bud.
• Principal fibers develop with eruption.
• Fine fibrils fuse from root cementum and bone surface.
• Fibers increase in quantity and thickness.
• Collagen fibre bundle orientation changes during tooth eruption.

Fully Functional Tooth

• Collagen fiber bundles aggregate into specific groups following occlusal contact
  • Alveolar crest
  • Horizontal
  • Oblique
  • Apical
  • Interradicular fibres
• Individual bundles of the periodontal ligament have a wavy course
  • Allows for (physiologic) mobility

Function of the PDL

• Distributes and resorbs forces during mastication.
• Facilitates tooth movement with orthodontic forces.
• Fibrils are in ground substance filled with mucopolysaccharides (glycosaminoglycans).
• PDL acts as physiologic "cushions" and serves to provide support for compressive forces.
• Exhibits more rapid turnover than collagen fibres.
• Remodelling slows in older individuals.
• Collagen turnover is higher in the PDL than in most alveolar tissues during physiologic conditions.
• Forces are multidirectional, including the vertical and horizontal components.

Root Cementum

• Mineralized tissue covers root surface.
• Similar to bone tissue.
• Contains no blood vessels.
• Lacks innervation.
• Exhibits no physiologic resorption or remodelling.
• Experiences continuous deposition throughout life.
• Primary cementum is acellular.
• PDL principal fibres embed/mineralize during continuous primary cementum formation.
• Sharpey's fibres are a direct continuation of PDL collagen fibres.
• Secondary cementum is cellular and forms after tooth eruption in response to function.
• Attaches PDL fibres to the root.
• Contributes to repair after root surface damage (e.g., cementum resorption during orthodontic treatment).

Alveolar Bone

• Forms/supports the sockets of the teeth
• Composed of dense outer cortical bone plates with cancellous (trabecular) bone in between.
• The thickness of the cortical laminae changes by location.

Functions of Bone

• Cortical bone provides supportive, protective, and mechanical functions.
• Trabecular bone provides hematopoietic functions, energy storage, and calcium/phosphate storage functions.
• Architecture and quantity of bone trabeculae is influenced by genetics, epigenetics, nutrition, hormones, functional, and orthodontic forces.
• Periosteum covers bone tissue
• Bone is differentiated from surrounding tissues
• Its outer fibrous layer is protective.
• Mesenchymal cells in the inner cambium layer produce matrix and orchestrate mineral apposition, becoming osteoblasts.
• Bone constantly changes through modelling and remodelling.

Modelling Processes

• Bone formation/resorption occurs at differing locations.
• This alters the size, shape, and/or location of alveolar bone.

Remodelling Processes

• Results in renewal of bone in the jaw
• The rate of alveolar bone remodelling is subject to regulation.
  • Growth factors, hormones, and/or mechanical loading are regulators
• Osteoblasts/Osteoclasts form basic multicellular units (BMU) to complete bone remodelling.
• Basic multicellular units are located on socket walls, interior surface of the cortical and on the surface of the bone trabeculae
• Osteoblasts secrete collagen fibres and a matrix made of primarily proteoglycans and glycoproteins
• This product is called osteoid
• Osteoid is found on all bone surfaces where bone is deposited.
• Osteoid is not attacked by osteoclasts
• Bone matrix undergoes initial mineralization via calcium and phosphate deposition, turning into woven bone
• Newly created woven bone is poorly organized and poorly mineralized
• Woven bone aids in speedy bone repair/replacement by filling bone defects and providing continuity between segments
• As woven bone matures it becomes reorganized into lamellar bone
• Lamellar bone is highly organized and is well mineralized
• Bone strength increases with ongoing mineralization
• The basic subunit of lamellar bone is a haversian system

Orthodontic Movement

• Involves both modelling and remodelling of the alveolar bone
• Bone resorption occurs on the compression side
• Bone Formation occurs on the tension side of the teeth
• Overall renewal and maturation of alveolar bone remodels.
  • Bone resorption and formation both occur.

Tooth Movement

• Includes mesial or distal direction, involving changes in the less dense trabeculae or interdental septum.
• Tooth movement towards a newly extracted socket accelerates bone-resorbing activities.
• Labial or lingual movement towards thin cortical plates requires caution to avoid iatrogenic responses.
• Animal studies have shown bone apposition accommodates bone movement; the apposition pace may be slow.
• Rapid or large directional movements of the teeth buccally can impact the health of the bone.
  • Thinning, dehiscence and fenestration can occur

Orthodontic Tooth Movement Biology Through Bone

• Related to the physiologic process of mesial tooth migration
• More substantial tissue changes are observed in orthodontic tooth movement
• Modeling of the tooth socket results after force is applied to the crown
• Cell signalling and force type allows teeth to undergo movement
• Orthodontic tooth movement uses mechanical force and transforms to biological signals
  • Cells sense mechano- and oxygen changes
• This transduction promotes intracellular communication.
• The modelling of hard/soft tissue allows movement in response to orthodontic force
• Mechanical forces are perceived via changes in:
  • Tissue and or cell strain
  • Fluid flow induced shear stress changes
  • Oxygen changes
• Progenitor cells in the PDL and periosteum and in alveolar bone initiate signalling through mediators.
  • These mediators are Wnt, BMP, TNFα, IL1p, NO, CSF1, VEGF, PGE2
• These mediators cause osteoclast/blast recruitment via formative and resorptive activities and relocation

Fundamental Studies

• Kingsley and Walkhoff suggested tooth movement depends on extensibility, compressibility and elasticity of bone
• Schwalbe and Flourens theorized tension is associated with bone deposition and pressure is associated with bone resorption
• Sandstedt's early 1900s experiments confirmed Schwalbe and Flourens' theory.
• Bone deposition occurs at tension areas, and bone resorption occurs at areas of pressure
• Sandstedt demonstrated that lighter forces lead to bone resorption along the alveolar wall.
• Also demonstrated that heavier compressive foces lead to necrosis within PDL and alveolar wall (hyalinized tissue).

Schwarz

• Tissue response is correlated with capillary blood pressure
• Below capillary pressure, rapid alveolar bone resorption and tooth movement occurs.
• Above capillary BP, tissue ischemia results, leading to necrosis, delayed tooth movement and "suffocation of the peridental membrane".

Bone Cell Modelling

• Differential osteoclastic and osteoblastic activity is induced via orthodontic force.
• Alveolar bone that lines the compressed PDL exhibits osteoclastic activity upon appliance activation
• Osteoblastic activity occurs along tensed regions.
• Osteoclast precursor cells are recruited from the circulatory system and bone marrow where hematopoietic stem cells reside
• Mononuclear Osteoclast precursor cells fuse, differentiate and mature into fully functional multinucleated cells
• Osteoprogenitor cells must differentiate into osteoblasts to obtain formative activity along the tooth socket
• Wise/king: mechanical orthodontic forces cause recruitment and activation of bone forming/resorbing cells
• Tooth movement results from relocation of the tooth socket
• Next, shift of socket occurs and bone cell signalling through environmental changes and recruitment is activated for movement
• If forces are high, the necrotic tissue will need to be removed which prolongs tooth translation

Orthodontic Forcing

• Shift will occur within socket
• Cells sense change and trigger signalling
• Osteoblasts and osteoclasts need to be recruited
• Light force leads to linear tooth translation after a shift
  • Osteoclasts resorb bone
  • Osteoblasts form bone
• Heavier force will requires necrotic tissue to be eliminated by macrophages
  • Increases time to movement

Biomechanical Cues

• Mechanical signals initiate biological cellular responses through sensors.
• Mechanotransduction is the translation of changes into cellular signals.
• Tissue load occurs and is converted to a cellular, biochemical signal communicated to other cells.
• Very likely involves alveolar bone osteocytes, osteoblasts, plus PDL and periosteal mesenchymal precursor cells
• Movement occurs initially within the PDL -Mechanical and directional force cause an adaptive change -Compression will occur within the capillaries, causing low oxygen
• These activations lead to resorption and formation.
• The primary bone cell to respond to forces is the osteocyte
• Osteocytes are terminally differentiated blasts
• 90-95% are in bone lacunae
• Gap junctions cause direct contact to osteoclasts and lining cells
• Mechanical forces apply stress/strain and are detected in the fluid or as compression
• Integrin stimulation occurs and soluble factors are released through parocrine or autocrine systems -These factors help with regulation of osteogenesis
• The described mechanotransduction also directly stimulates blasts and progenitor cells -Stimulates shear stress signalling to drive protein and PGE2 proliferation

Mechanical Orthodontic Movement

• Mechanical load is applied in the periodontal space
• Leads to changes in localized regions of the PDL and bone from stretch and force -Osteocytes are the first cells to detect
• Intracellular mediators help with cell-cell signalling to illicit a cellular response

Physical Methods for Orthodontic Tooth Movement

• Local bone damage to accelerate tooth movement
• Local bone injuries were an idea in the 1890s, improved further in 1959 with corticotomy using horizontal cuts and sub apical osteotomy
• The procedure involves grafting to assist
• These localized cuts are now known as periodontally accelerated osteogenic orthodontics
• New cortical techniques without elevation or with less incision or used with piezocision have begun These listed techniques have been found effective and safe The effectiveness is limited around 3-4 months