physiology of tooth movement ii /certified fixed orthodontic courses by indian dental academy

Physiology of tooth movement -IIPhysiology of tooth movement -II

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ContentsContents

- - Bone physiologyBone physiology

bone structure, modelling and remodelling,bone structure, modelling and remodelling, osteoblast histogenesis and bone formation,osteoblast histogenesis and bone formation, osteoclast recruitment and bone resorption.osteoclast recruitment and bone resorption.- Wolff`s law of bone transformation- Wolff`s law of bone transformation- Effects of force magnitude, direction, duration- Effects of force magnitude, direction, duration decay.decay.- Drug effects.- Drug effects.


- Anchorage aspects- Anchorage aspects

- Skeletal effects of force - Skeletal effects of force

- Future applications- Future applications

- References - References


Bone physiologyBone physiology

- The initial consideration in defining the physiologic The initial consideration in defining the physiologic basis of tooth movement is the bone morphology basis of tooth movement is the bone morphology (osteology) of the craniofacial complex.(osteology) of the craniofacial complex.

- A systematic study of more than 1000 human skulls A systematic study of more than 1000 human skulls by Atkinson provided the modern basis of by Atkinson provided the modern basis of craniofacial osseous morphology .The vertical craniofacial osseous morphology .The vertical components of the cranium tend to be loaded in components of the cranium tend to be loaded in compression (negative stresses) and the horizontal compression (negative stresses) and the horizontal components are loaded in tension (positive stresses).components are loaded in tension (positive stresses).


- The internal skeletal structure of the midface is similar - The internal skeletal structure of the midface is similar to that of a ladder, vertical rails loaded in to that of a ladder, vertical rails loaded in compression connected by rungs loaded in tension. compression connected by rungs loaded in tension. This is one of the most efficient structures for This is one of the most efficient structures for achieving maximal compressive strength with achieving maximal compressive strength with minimal mass in a composite material.minimal mass in a composite material.

- The maxilla has relatively thin cortices that are - The maxilla has relatively thin cortices that are interconnected by a network of trabeculae, and as it is interconnected by a network of trabeculae, and as it is loaded primarily in compression, the maxilla is loaded primarily in compression, the maxilla is structurally similar to the body of a vertebrae.structurally similar to the body of a vertebrae.


- The mandible on the other hand has thick cortices The mandible on the other hand has thick cortices with radially oriented trabeculae. It is similar to the with radially oriented trabeculae. It is similar to the shaft of a long bone and indicates that the mandible shaft of a long bone and indicates that the mandible is loaded predominantly in bending and torsion. is loaded predominantly in bending and torsion. (Atkinson,1964)(Atkinson,1964)

- Classification of bone tissue-Classification of bone tissue- woven bonewoven bone lamellar bonelamellar bone composite bone composite bone bundle bonebundle bone


- - Woven boneWoven bone is relatively weak, disorganized and is relatively weak, disorganized and poorly mineralized. It serves a crucial role in wound poorly mineralized. It serves a crucial role in wound healing by rapidly filling osseous defects, providing healing by rapidly filling osseous defects, providing initial continuity for fracture segments and initial continuity for fracture segments and osteotomies. The first formed bone in response to osteotomies. The first formed bone in response to orthodontic loading is usually the woven type. It is orthodontic loading is usually the woven type. It is not found in the normal adult skeleton in the steady not found in the normal adult skeleton in the steady states. states. The functional limitations of woven bone are The functional limitations of woven bone are an important aspect both of orthodontic retention and an important aspect both of orthodontic retention and in healing after surgeryin healing after surgery


- - Lamellar boneLamellar bone is strong, highly mineralized and makes is strong, highly mineralized and makes up more than 99pc of the adult skeleton. When new up more than 99pc of the adult skeleton. When new lamellar bone is formed a portion of the lamellar bone is formed a portion of the hydroxyapatite is deposited by the osteoblasts during hydroxyapatite is deposited by the osteoblasts during primary mineralization and the process of crystal primary mineralization and the process of crystal growth completes the remaining mineral portion growth completes the remaining mineral portion (secondary mineralization). Adult human bone is (secondary mineralization). Adult human bone is almost entirely of the remodelled variety. almost entirely of the remodelled variety. The full The full strength of lamellar bone in an orthodontically strength of lamellar bone in an orthodontically moved tooth is not achieved approximately one year moved tooth is not achieved approximately one year after active treatment (retention consideration).after active treatment (retention consideration).


- - Composite boneComposite bone is the deposition of lamellar bone is the deposition of lamellar bone within a woven bone lattice, a process called within a woven bone lattice, a process called cancellous compaction. It is the quickest way of cancellous compaction. It is the quickest way of producing relatively strong bone. It is an important producing relatively strong bone. It is an important intermediary type of bone in the physiologic response intermediary type of bone in the physiologic response to orthodontic loading and is usually the predominant to orthodontic loading and is usually the predominant tissue for stabilization during the early process of tissue for stabilization during the early process of retention. retention. (Roberts, AJO 1984)(Roberts, AJO 1984)


- Bundle boneBundle bone is a functional adaptation of is a functional adaptation of lamellar structures to allow attachment of lamellar structures to allow attachment of tendons and ligaments. Sharpey`s fibres are tendons and ligaments. Sharpey`s fibres are the major distinguishable characteristics of the major distinguishable characteristics of bundle bone.bundle bone.


The type of bone through which the tooth is displaced The type of bone through which the tooth is displaced must be considered in the treatment plan. Movements must be considered in the treatment plan. Movements in a mesial or distal direction displace the roots in a mesial or distal direction displace the roots through the spongiosa of the alveolar bone. When a through the spongiosa of the alveolar bone. When a tooth is moved into the reorganizing alveolus of a tooth is moved into the reorganizing alveolus of a newly extracted tooth, remodelling is rapid because newly extracted tooth, remodelling is rapid because many differentiating cells present and the limited many differentiating cells present and the limited bone to be resorbed. On the other hand movement in bone to be resorbed. On the other hand movement in a labial or lingual direction should be undertaken a labial or lingual direction should be undertaken with caution due to thin cortical plates which can with caution due to thin cortical plates which can lead to marginal recession.lead to marginal recession.


Modelling and RemodellingModelling and Remodelling

- Bone structure can be changed in 3 principle ways.. - Bone structure can be changed in 3 principle ways.. Osteogenesis, modelling, remodelling.Osteogenesis, modelling, remodelling.

OsteogenesisOsteogenesis – bone is formed on soft tissue and – bone is formed on soft tissue and generally occurs during embryonic development. It generally occurs during embryonic development. It can be intramembranous or endochondral.can be intramembranous or endochondral.

ModellingModelling – Independent sites of resorption and – Independent sites of resorption and formation change the shape or size of a bone. This formation change the shape or size of a bone. This type of bone turnover is a dominant process of facial type of bone turnover is a dominant process of facial growth and development, adaptation to applied loads growth and development, adaptation to applied loads such as headgear, RME, functional appliances.such as headgear, RME, functional appliances.


- It can be seen on cephalometric tracings. From an It can be seen on cephalometric tracings. From an orthodontic perspective, modelling is important in orthodontic perspective, modelling is important in normal growth of the craniofacial structures as well as normal growth of the craniofacial structures as well as the alveolar shape and size during tooth movement.the alveolar shape and size during tooth movement.

- RemodellingRemodelling is a reparative mechanism, coupled is a reparative mechanism, coupled sequence of resorption and formation occur to replace sequence of resorption and formation occur to replace previously existing bone. It helps in maintaining the previously existing bone. It helps in maintaining the structural integrity of bone. The events are apparent at structural integrity of bone. The events are apparent at the microscopic level and not visible on radiographs.the microscopic level and not visible on radiographs.


-The mechanism for internal remodelling of dense -The mechanism for internal remodelling of dense compact bone involves axially oriented cutting and compact bone involves axially oriented cutting and filling cones.filling cones.

-As osteoclasts and osteoblasts are vascularly dependent -As osteoclasts and osteoblasts are vascularly dependent all the bony structural adaptation occurs along all the bony structural adaptation occurs along vascularized surfaces. When a tooth is moved through vascularized surfaces. When a tooth is moved through the cortical bone, there is osseous tissue removal by the cortical bone, there is osseous tissue removal by resorption. In addition the remodelling rate in the resorption. In addition the remodelling rate in the path of tooth movement is elevated. Cutting cones path of tooth movement is elevated. Cutting cones create resorption cavities in the cortical bone.create resorption cavities in the cortical bone.


- If the tooth is moving rapidly, advancement of the If the tooth is moving rapidly, advancement of the resorbing pdl wall can communicate with the resorbing pdl wall can communicate with the resorption cavities in the compacta before they can resorption cavities in the compacta before they can

fill with new bone. fill with new bone. Thus remodelling can contribute Thus remodelling can contribute substantially to resorption efficiencysubstantially to resorption efficiency. . The coordinated The coordinated modelling and remodelling mechanism is highly modelling and remodelling mechanism is highly adaptable to changes in mechanical loading of the adaptable to changes in mechanical loading of the alveolar process. Although the rate of tooth alveolar process. Although the rate of tooth movement is dependent on root resorption the movement is dependent on root resorption the preservation of the alveolar process and the new preservation of the alveolar process and the new


osseous support is directly osseous support is directly

related to orthodontically related to orthodontically

induced osteogenesis.induced osteogenesis.

( RobertsWE, Semin ( RobertsWE, Semin

Orthod 2000)Orthod 2000)


Osteoclast recruitment and bone resorptionOsteoclast recruitment and bone resorption

- - Bone resorption is the rate limiting factor that Bone resorption is the rate limiting factor that determines the rate of tooth movementdetermines the rate of tooth movement. No evidence . No evidence exists to suggest that osteoclasts are produced in the exists to suggest that osteoclasts are produced in the pdl. Preosteoblasts derived from the marrow enter the pdl. Preosteoblasts derived from the marrow enter the pdl and bone through circulation. Porous cortical and pdl and bone through circulation. Porous cortical and trabecular bone allows improved access for trabecular bone allows improved access for osteoclasts. Because osteoclasts originate in the osteoclasts. Because osteoclasts originate in the marrow, production of precursor cells is under both marrow, production of precursor cells is under both systemic (metabolic) and local (hematopoietic) systemic (metabolic) and local (hematopoietic) control. control. ( Roberts DCNA 1981)( Roberts DCNA 1981)


Osteoblast histogenesis and bone formationOsteoblast histogenesis and bone formation

- Osteoblasts are derived from paravascular connective Osteoblasts are derived from paravascular connective tissue cells. The initial mechanically mediated step in tissue cells. The initial mechanically mediated step in osteoblast histogenesis is differentiation of osteoblast histogenesis is differentiation of preosteoblasts from less differentiated precursor cells.preosteoblasts from less differentiated precursor cells.

This is the rate limiting step in the differentiation of This is the rate limiting step in the differentiation of osteoblasts and the circadian rhythm exerts a strong osteoblasts and the circadian rhythm exerts a strong influence in this sequenceinfluence in this sequence. The process requires 5 . The process requires 5 alternating 12 hr dark/light cycles and this is related alternating 12 hr dark/light cycles and this is related to the varying endocrine levels during the circadian to the varying endocrine levels during the circadian cycle.cycle.


--The important point is that optimal osteoblastThe important point is that optimal osteoblast

histogenesis under physiologic circumstances histogenesis under physiologic circumstances

requires a normal photoperiod.requires a normal photoperiod.

-Disruption of the circadian rhytmn caused by-Disruption of the circadian rhytmn caused by

physiologic stress, travelling or irregular sleep physiologic stress, travelling or irregular sleep

patterns may adversely affect the production of patterns may adversely affect the production of

osteoblasts. (Miyoshi EJO 2001)osteoblasts. (Miyoshi EJO 2001)


Current concepts of bone remodellingCurrent concepts of bone remodelling

- Bone remodelling occurs in small packets of cells - Bone remodelling occurs in small packets of cells called basic multicellular units (BMU) which turn called basic multicellular units (BMU) which turn bone over in multiple bone surfaces. bone over in multiple bone surfaces. At any one time At any one time 20pc of the cancellous bone surface is undergoing 20pc of the cancellous bone surface is undergoing remodellingremodelling. Each BMU is chronologically and . Each BMU is chronologically and geographically separarted from other packets of geographically separarted from other packets of remodelling.( Frost 1991)remodelling.( Frost 1991)


The various steps involved are (The various steps involved are (Hill, 1998Hill, 1998))

Bone resorptionBone resorption

- recruitment and dissemination of osteoclast precursor - recruitment and dissemination of osteoclast precursor cells from blood vessels to bone.cells from blood vessels to bone.

- differentiation and proliferation into osteoclasts- differentiation and proliferation into osteoclasts

- preparation of the bone surface- preparation of the bone surface

- formation of the ruffled border and removal of bone - formation of the ruffled border and removal of bone via proteolytic enzymes and hydrogen ions. via proteolytic enzymes and hydrogen ions.


- Osteoclasts ultimately undergo apoptosis( 12.5 days)- Osteoclasts ultimately undergo apoptosis( 12.5 days)

ReversalReversal

-After the disappearance of osteoclasts there is the-After the disappearance of osteoclasts there is the

release of factors which stimulate osteoblasts.release of factors which stimulate osteoblasts.

Bone formationBone formation

- Chemotactic attraction of osteoblasts to the sites of - Chemotactic attraction of osteoblasts to the sites of resorption defect resorption defect

- Proliferation and differentiation of precursors to - Proliferation and differentiation of precursors to mature osteoblasts.mature osteoblasts.


Wolff’s law of bone transformationWolff’s law of bone transformation

- By 1892 Julius Wolff and others realized that By 1892 Julius Wolff and others realized that mechanical loads can affect bone architecture in mechanical loads can affect bone architecture in living beings, but the mechanisms were unknown and living beings, but the mechanisms were unknown and had no clinical applications.had no clinical applications.

- The methods by which tissue level mechanisms The methods by which tissue level mechanisms support organ bone level functions became apparent support organ bone level functions became apparent by the mid 1990`s.The updated bone physiology and by the mid 1990`s.The updated bone physiology and its clinical relevance have been summarized as its clinical relevance have been summarized as

follows. follows. (Frost AO 2004)(Frost AO 2004)


- We have 2 kinds of bones, the load bearing bones - We have 2 kinds of bones, the load bearing bones (LBB) and few bones which serve different needs.(LBB) and few bones which serve different needs.

- The biologic mechanisms involved include modelling The biologic mechanisms involved include modelling and remodelling.and remodelling.

- Loads on bones cause bone strain that generate Loads on bones cause bone strain that generate signals that some cells can detect and to which they signals that some cells can detect and to which they and other cells respond.and other cells respond.

- Genetically determined threshold ranges of the Genetically determined threshold ranges of the signals help control modelling and remodelling.signals help control modelling and remodelling.


-When bone strain > modelling threshold range -When bone strain > modelling threshold range

(MESm)(MESm)

Modelling can be switched on to strengthen an LBB.Modelling can be switched on to strengthen an LBB.- When bone strain< MSEr, then disuse remodelling When bone strain< MSEr, then disuse remodelling

can be turned on to reduce whole bone strength.can be turned on to reduce whole bone strength.- So if E is the typical peak strain of a LBB, the So if E is the typical peak strain of a LBB, the

following criteria should be satisfiedfollowing criteria should be satisfied- MSEr < E < MSEmMSEr < E < MSEm


Effects of force duration and decay.Effects of force duration and decay.

- The key to producing tooth movement is the The key to producing tooth movement is the application of sustained force, which does not mean application of sustained force, which does not mean the force should be absolutely continuous. But it does the force should be absolutely continuous. But it does mean that the force must be present for a considerable mean that the force must be present for a considerable period of time. Clinical experience shows that in period of time. Clinical experience shows that in humans a threshold of 4-8hrs must be present for humans a threshold of 4-8hrs must be present for effective tooth movement.effective tooth movement.

- Continuous forces produced by fixed appliances Continuous forces produced by fixed appliances produce more amount of tooth movement than produce more amount of tooth movement than removable appliances. removable appliances.


- If the idea of providing brief intervals of no pressure If the idea of providing brief intervals of no pressure and resumed blood flow (for vitality of pdl) is ever to and resumed blood flow (for vitality of pdl) is ever to be used clinically, it will be necessary to modify fixed be used clinically, it will be necessary to modify fixed appliances to do this.appliances to do this.

- Duration of force has another aspect, related to how Duration of force has another aspect, related to how force magnitude changes as the tooth responds by force magnitude changes as the tooth responds by moving. With many orthodontic devices the force moving. With many orthodontic devices the force may drop all the way to zero.may drop all the way to zero.


- Force duration can be classified as the following types.- Force duration can be classified as the following types. continuouscontinuous- force levels maintained at some - force levels maintained at some

appreciable fraction of the original form till the next appreciable fraction of the original form till the next visit.visit.

interruptedinterrupted – force levels decline to zero between – force levels decline to zero between activations.activations.

intermittentintermittent – force levels decline abruptly to zero – force levels decline abruptly to zero intermittently and then return to the original intermittently and then return to the original sometime later. These forces are produced by patient sometime later. These forces are produced by patient activated appliances- removable plates, headgear, activated appliances- removable plates, headgear, elastics.elastics.


- Intermittent forces can also become interrupted Intermittent forces can also become interrupted between adjustments of the appliance.between adjustments of the appliance.

- There is an important interaction between force There is an important interaction between force magnitude and how rapidly the force declines as the magnitude and how rapidly the force declines as the tooth moves. Continuous forces ( light versus heavy).tooth moves. Continuous forces ( light versus heavy).

- Also the forces can decay rapidly so that the force Also the forces can decay rapidly so that the force declines to zero after the tooth moves only a short declines to zero after the tooth moves only a short distance. (light versus heavy).distance. (light versus heavy).

- There is no doubt that light continuous forces There is no doubt that light continuous forces produce the most efficient tooth movement. produce the most efficient tooth movement.


- Despite the clinician`s best efforts some areas of Despite the clinician`s best efforts some areas of undermining resorption do occur in every patient. undermining resorption do occur in every patient. The heavy forces are acceptable, only if the force The heavy forces are acceptable, only if the force levels decline so that there is a period of repair and levels decline so that there is a period of repair and regeneration before the next activation or if the force regeneration before the next activation or if the force decreases at least to the point that no second and third decreases at least to the point that no second and third rounds of undermining resorption occur.rounds of undermining resorption occur.

- Experience has shown that orthodontic appliances Experience has shown that orthodontic appliances should not be reactivated more frequently than a 3 should not be reactivated more frequently than a 3 week interval.week interval.


- Undermining resorption requires 7 to 14 days. When Undermining resorption requires 7 to 14 days. When this is the mode of tooth movement and when force this is the mode of tooth movement and when force levels decline rapidly, tooth movement is essentially levels decline rapidly, tooth movement is essentially complete in this length of time.complete in this length of time.

- With the use of light, continuous forces producing With the use of light, continuous forces producing frontal resorption there is no need of activation of the frontal resorption there is no need of activation of the appliance. But with the use of stiffer appliances appliance. But with the use of stiffer appliances where forces drop to zero tooth moves for the first 10 where forces drop to zero tooth moves for the first 10 days followed by an equal or longer period of pdl days followed by an equal or longer period of pdl repair. Activating an appliance too frequently, short repair. Activating an appliance too frequently, short


cuts the repair process and damages the tooth and bonecuts the repair process and damages the tooth and bone

that a longer appointment cycle would have prevented that a longer appointment cycle would have prevented

or atleast minimized.or atleast minimized.


Clinical consideration – Use of intermittent Clinical consideration – Use of intermittent mechanicsmechanics

- The kinetics of continuous mechanics are well known The kinetics of continuous mechanics are well known but the response to intermittent loads such as but the response to intermittent loads such as headgear and removable appliances is unclear. Here headgear and removable appliances is unclear. Here tooth movement is related directly to the number of tooth movement is related directly to the number of hours each day the force is applied. Inspite of hours each day the force is applied. Inspite of sufficient motivation and co operation the sufficient motivation and co operation the effectiveness of similar therapy between patients is effectiveness of similar therapy between patients is inconsistent.inconsistent.

- Thus additional variables are involved.Thus additional variables are involved.


- It is said that a 3 hr period of continuous loading is - It is said that a 3 hr period of continuous loading is necessary to achieve maximum displacement of a necessary to achieve maximum displacement of a tooth root in the pdl. If it is assumed that 3 hrs are tooth root in the pdl. If it is assumed that 3 hrs are needed for pdl activation or recovery , continuous needed for pdl activation or recovery , continuous wearing of the headgear is expected to be more wearing of the headgear is expected to be more effective than longer periods of wear with frequent effective than longer periods of wear with frequent release of force. So it is recommended that the release of force. So it is recommended that the patient not remove the headgear at all during their patient not remove the headgear at all during their daily interval of wear. Removing the device for meals daily interval of wear. Removing the device for meals or sports compromises the biomechanical activation or sports compromises the biomechanical activation


of the periodontium. of the periodontium. The complex biologic responseThe complex biologic response associated with irregular force application probably associated with irregular force application probably is the main factor in the unpredictable response to is the main factor in the unpredictable response to headgear or removable appliances. headgear or removable appliances. - Another possible variable in the response to Another possible variable in the response to intermittent forces is the circadian rhythm of pdlintermittent forces is the circadian rhythm of pdl proliferation and differentiation. Maximal cell proliferation and differentiation. Maximal cell proliferation in the pdl occurs during the resting proliferation in the pdl occurs during the resting hours, night time for humans.hours, night time for humans.


- Differentiation to preosteoblasts, the key rate limiting Differentiation to preosteoblasts, the key rate limiting step in osteoblast differentiation, occurs during the step in osteoblast differentiation, occurs during the late resting and early arousal periods. late resting and early arousal periods.

- These data suggest that wearing headgear or a These data suggest that wearing headgear or a removable appliance at night is more effective than removable appliance at night is more effective than wearing the same appliance for an equal period wearing the same appliance for an equal period during the day. (Roberts AJO1979)during the day. (Roberts AJO1979)


Effects of force magnitudeEffects of force magnitude

--It is generally considered that a light force over a It is generally considered that a light force over a certain distance moves a tooth more rapidly and with certain distance moves a tooth more rapidly and with less injuries than a heavy one. What is considered a less injuries than a heavy one. What is considered a light force or a heavy force depends upon the mode of light force or a heavy force depends upon the mode of application and the mechanical arrangement of the application and the mechanical arrangement of the recipient units. The purpose of applying a light force recipient units. The purpose of applying a light force is to prevent undue compression and to cause less is to prevent undue compression and to cause less discomfort and pain to the patient. discomfort and pain to the patient.


- When light prolonged forces are applied to a tooth, When light prolonged forces are applied to a tooth, the partially compressed pdl causes movement of the partially compressed pdl causes movement of tooth within the socket. Changes in chemical tooth within the socket. Changes in chemical environment lead to an increase in the level of cyclic environment lead to an increase in the level of cyclic AMPs which appear AMPs which appear about 4 hrsabout 4 hrs after sustained after sustained pressure.pressure.

- Removable appliances worn for less than 4-6 hrs per Removable appliances worn for less than 4-6 hrs per day will produce no orthodontic effects.day will produce no orthodontic effects.

- For a tooth to move osteoclasts must be formed For a tooth to move osteoclasts must be formed adjacent to the compressed area and it takes almost adjacent to the compressed area and it takes almost


- 48 hours48 hours before the first osteoclast can appear in the before the first osteoclast can appear in the compressed area. Injected parathyroid hormone can compressed area. Injected parathyroid hormone can reduce this time period and can induce osteoclasts reduce this time period and can induce osteoclasts within a few hours as compared to mechanical within a few hours as compared to mechanical deformation.deformation.

- At the same time but lagging behind so that the pdl At the same time but lagging behind so that the pdl becomes enlarged, osteoblasts form bone on the becomes enlarged, osteoblasts form bone on the tension side and begin remodelling on the pressure tension side and begin remodelling on the pressure side.side.

- The course of events is different when the forces are The course of events is different when the forces are heavy. heavy.


- There is a complete occlusion of blood vessels in the There is a complete occlusion of blood vessels in the pdl and instead of osteoclast stimulation a sterile pdl and instead of osteoclast stimulation a sterile necrosis ensues within that area.necrosis ensues within that area.

- In clinical orthodontics it is difficult to avoid In clinical orthodontics it is difficult to avoid pressure that produces at least some avascular areas pressure that produces at least some avascular areas in the pdl. It has been suggested that releasing in the pdl. It has been suggested that releasing pressure against a tooth at intervals while pressure against a tooth at intervals while maintaining the pressure for enough hours to maintaining the pressure for enough hours to produce a biologic response could help in produce a biologic response could help in maintaining tissue vitality.maintaining tissue vitality.


- It is possible in the future that interrupted force of It is possible in the future that interrupted force of this type will become clinically useful ,if methods for this type will become clinically useful ,if methods for activating and de activating springs can be worked activating and de activating springs can be worked out. out.

- The smooth progression of tooth movement with light The smooth progression of tooth movement with light force may be an unattainable ideal when continuous force may be an unattainable ideal when continuous forces are used. In clinical practice, tooth movement forces are used. In clinical practice, tooth movement proceeds in a more stepwise fashion because of the proceeds in a more stepwise fashion because of the inevitable areas of undermining resorption. inevitable areas of undermining resorption.


Force distribution and types of tooth Force distribution and types of tooth movementmovement

- TippingTipping is the simplest form of tooth movement. It is the simplest form of tooth movement. It leads to concentration of pressure at certain areas of leads to concentration of pressure at certain areas of the pdl. Only one half of the pdl area that could be the pdl. Only one half of the pdl area that could be loaded is actually loaded.loaded is actually loaded.

- Also the hyalinized zone is located a short distance Also the hyalinized zone is located a short distance from the alveolar crest. It is chiefly the coronal from the alveolar crest. It is chiefly the coronal portion of the tooth that is moved because relatively portion of the tooth that is moved because relatively few fibre bundles that resist tooth movement exist on few fibre bundles that resist tooth movement exist on the tension side. the tension side.


- In tipping only one half of the pdl area is actually - In tipping only one half of the pdl area is actually loaded. The loading diagram consists of two loaded. The loading diagram consists of two triangles covering half the total pdl area.triangles covering half the total pdl area.

- Forces used to tip the teeth must be kept quite low. - Forces used to tip the teeth must be kept quite low. Clinical experience with humans suggest that the Clinical experience with humans suggest that the force must not exceed 50gms.force must not exceed 50gms.


- TranslationTranslation is obtained by establishing a couple of is obtained by establishing a couple of forces acting along parallel lines and distributing the forces acting along parallel lines and distributing the force over the whole pdl area. force over the whole pdl area. Hence twice as much Hence twice as much force would be required for translation as for tipping.force would be required for translation as for tipping.

- Hyalinization during initial bodily movement occurs Hyalinization during initial bodily movement occurs largely due to mechanical factors. Shortly after largely due to mechanical factors. Shortly after initiation of movement , no bodily movement occurs initiation of movement , no bodily movement occurs but instead a slight tipping is noticed. A tooth moved but instead a slight tipping is noticed. A tooth moved bodily may also become slightly extruded unless the bodily may also become slightly extruded unless the archwire has been adjusted.archwire has been adjusted.


- In - In RotationRotation of a tooth around its long axis the of a tooth around its long axis the force is distributed over the entire pdl rather force is distributed over the entire pdl rather than over a narrow vertical strip, allowing than over a narrow vertical strip, allowing larger forces to be applied than in other larger forces to be applied than in other movements. Correction of a rotated tooth is movements. Correction of a rotated tooth is largely influenced by the anatomic largely influenced by the anatomic arrangement of the supporting structures. Most arrangement of the supporting structures. Most teeth to be rotated create two pressure and two teeth to be rotated create two pressure and two tension sides. tension sides.


- Hyalinization and undermining resorption take place Hyalinization and undermining resorption take place in one pressure zone while direct bone resorption in one pressure zone while direct bone resorption occurs in the other. After rotation for 3 -4 weeks occurs in the other. After rotation for 3 -4 weeks direct bone resorption prevails on the pressure side.direct bone resorption prevails on the pressure side.

- The presence of elastic and oxytalan fibres in the The presence of elastic and oxytalan fibres in the marginal region increase the contraction of marginal region increase the contraction of supraalveolar structures, leading to a relapse supraalveolar structures, leading to a relapse tendency.tendency. Free gingival fibres remain stretched and Free gingival fibres remain stretched and displaced for as long as 232 days and possibly longer. displaced for as long as 232 days and possibly longer. Hence overrotation is recommended.Hence overrotation is recommended.


- - ExtrusiveExtrusive movements produce no areas of movements produce no areas of compression within the pdl, only tension. Use of compression within the pdl, only tension. Use of light forces results in movement of alveolar bone light forces results in movement of alveolar bone with tooth.with tooth.

- In young individuals- there is a prolonged stretch and - In young individuals- there is a prolonged stretch and displacement of the supraalveolar fibres than the displacement of the supraalveolar fibres than the fibres in the middle and apical thirds. fibres in the middle and apical thirds. Reaarangement occurs after a fairly short retention Reaarangement occurs after a fairly short retention period.period.


- In adults – although the fibres are stretched during In adults – although the fibres are stretched during extrusion they are less readily elongated and extrusion they are less readily elongated and rearranged after treatment.rearranged after treatment.

- Periapical radiographs reveal calcified bone in the Periapical radiographs reveal calcified bone in the apical area after 4 -5 weeks.apical area after 4 -5 weeks.


- IntrusionIntrusion requires careful control of force requires careful control of force magnitude . Light force is required as force is magnitude . Light force is required as force is concentrated at a small area at the tooth apex.concentrated at a small area at the tooth apex.

- Unlike extruded teeth the intruded teeth in young Unlike extruded teeth the intruded teeth in young patients undergo only minor positional change after patients undergo only minor positional change after treatment.treatment.

- Intrusion may result in the formation of new bony Intrusion may result in the formation of new bony spicules in the marginal region. In young patients the spicules in the marginal region. In young patients the intruded tooth remains fairly stable.intruded tooth remains fairly stable.


- Optimum forces for orthodontic tooth movementOptimum forces for orthodontic tooth movement- Tipping 35-60 gmsTipping 35-60 gms- Translation 70-120 gmsTranslation 70-120 gms- Rotation 35-60 gmsRotation 35-60 gms- Extrusion 35-60 gmsExtrusion 35-60 gms- Intrusion 10-20 gmsIntrusion 10-20 gms- Root uprighting 50-100 gmsRoot uprighting 50-100 gms


Drug effectsDrug effects

- Agents that stimulate tooth movement include vitamin Agents that stimulate tooth movement include vitamin D as well as prostaglandins. Also a number of drugs D as well as prostaglandins. Also a number of drugs that inhibit tooth movement are frequently that inhibit tooth movement are frequently encountered in the clinical set up.encountered in the clinical set up.

- BiphosphonatesBiphosphonates – These are drugs used in the – These are drugs used in the treatment of osteoporosis .ex Alendronate. Medication treatment of osteoporosis .ex Alendronate. Medication of these drugs is usually encountered in the older of these drugs is usually encountered in the older adult orthodontic patients. They act as specific adult orthodontic patients. They act as specific inhibitors of osteoclast mediated bone resorption. inhibitors of osteoclast mediated bone resorption.


- In patients on this drug the bone remodelling In patients on this drug the bone remodelling necessary for tooth movement occurs at a slower rate.necessary for tooth movement occurs at a slower rate.

- Prostaglandin inhibitors –Prostaglandin inhibitors –

These include corticosteroids and NSAID`S. These These include corticosteroids and NSAID`S. These drugs inhibit the formation of prostaglandins via the drugs inhibit the formation of prostaglandins via the arachidonic pathway. The possibility of difficulty in arachidonic pathway. The possibility of difficulty in tooth movement must be kept in mind while treating tooth movement must be kept in mind while treating these patients. these patients.


Anchorage aspectsAnchorage aspects

- Anchorage is defined as the resistance to tooth - Anchorage is defined as the resistance to tooth movement. In planning orthodontic tooth movement movement. In planning orthodontic tooth movement it is simply not possible to consider only the teeth it is simply not possible to consider only the teeth whose movement is desired. Reciprocal affects must whose movement is desired. Reciprocal affects must be carefully analyzed, evaluated and controlled.be carefully analyzed, evaluated and controlled.

-The threshold for tooth movement appears to be quite -The threshold for tooth movement appears to be quite low and the tooth movement is proportional to low and the tooth movement is proportional to pressure up to a certain point. pressure up to a certain point.


- When that point is reached the amount of movement - When that point is reached the amount of movement becomes more or less independent of the pressure becomes more or less independent of the pressure magnitude. The optimum force level for movement is magnitude. The optimum force level for movement is the lightest force and resulting pressure that produces the lightest force and resulting pressure that produces a near maximum response. Forces greater than that a near maximum response. Forces greater than that though effective would be unnecessarily traumatic though effective would be unnecessarily traumatic and stressful to anchorage.( Yoshikawa,AJO 1985)and stressful to anchorage.( Yoshikawa,AJO 1985)


-- In the first case A1-M1- pressure on M1 is optimal and In the first case A1-M1- pressure on M1 is optimal and on A1 is suboptimal. (on A1 is suboptimal. (anchorage preservedanchorage preserved))

- In the second case A2-M2- though pressure is less on - In the second case A2-M2- though pressure is less on A2 as compared to M2-anchor teeth move as much as A2 as compared to M2-anchor teeth move as much as the teeth whose movement is desired (the teeth whose movement is desired (anchorage anchorage lostlost).).

- In the third case A3-M3-with extreme force the anchor - In the third case A3-M3-with extreme force the anchor teeth move more than the teeth whose movement is teeth move more than the teeth whose movement is desired. This case is theoretic and is not encountered desired. This case is theoretic and is not encountered clinically. Hence it proves that heavy forces destroy clinically. Hence it proves that heavy forces destroy anchorageanchorage. .


Skeletal effects of orthodontic force.Skeletal effects of orthodontic force.

- Orthodontic force applied to the teeth has the potential - Orthodontic force applied to the teeth has the potential to radiate outwards and affect distant skeletal to radiate outwards and affect distant skeletal locations. If the distant effects could change the locations. If the distant effects could change the pattern of jaw growth, there could be a possibility of pattern of jaw growth, there could be a possibility of correcting skeletal malocclusions ( correcting skeletal malocclusions ( principle ofprinciple of growth modificationgrowth modification).).

- - Effects of force on the maxilla-Effects of force on the maxilla- the important growth the important growth sites of the maxilla are the sutures that attach the sites of the maxilla are the sutures that attach the maxilla to the different bones and also the maxilla to the different bones and also the midpalatine suture.midpalatine suture.


- It is difficult to know theoretically the amount of It is difficult to know theoretically the amount of force at the sutures required to alter growth. When a force at the sutures required to alter growth. When a force is applied to the teeth only a fraction of the force is applied to the teeth only a fraction of the pressure in the pdl is experienced at the sutures, pressure in the pdl is experienced at the sutures, because the area of the sutures is much larger.because the area of the sutures is much larger.

- The effect of this much force can result in undesirable The effect of this much force can result in undesirable tooth movements. Hence full time application of tooth movements. Hence full time application of heavy forces is not recommended. Heavy intermittent heavy forces is not recommended. Heavy intermittent forces are less likely to produce damage. It would beforces are less likely to produce damage. It would be


convenient if part time application of heavy force convenient if part time application of heavy force produced more skeletal than dental effect. It was said produced more skeletal than dental effect. It was said that 24 hr headgear wear produced more undesirable that 24 hr headgear wear produced more undesirable tooth movement than 12-16 hrs wear. This could be tooth movement than 12-16 hrs wear. This could be another argument for part time rather than full time another argument for part time rather than full time headgear wear.headgear wear.

- For tooth movement there is a definite threshold – - For tooth movement there is a definite threshold – unless force is applied for 6 hrs per day, no unless force is applied for 6 hrs per day, no remodelling can take place. If any similar threshold remodelling can take place. If any similar threshold applies to the sutures is unknown.applies to the sutures is unknown.


- It is also known that - It is also known that growth hormone is primarily growth hormone is primarily released during eveningreleased during evening and the addition of new and the addition of new bone at the epiphyseal plates of the long bones occurs bone at the epiphyseal plates of the long bones occurs mostly at night.mostly at night.

- Facial growth also follows this pattern and so it likely - Facial growth also follows this pattern and so it likely that the effect of headgear wear is greatest at the night that the effect of headgear wear is greatest at the night time. So it is time. So it is important to stress that a patient should important to stress that a patient should begin wearing headgear immediately after dinner begin wearing headgear immediately after dinner rather than waiting for bedtimerather than waiting for bedtime. .


- Effects of force on the mandible-Effects of force on the mandible- If the mandible If the mandible like the maxilla grows largely in response to the like the maxilla grows largely in response to the growth of the surrounding soft tissues it should be growth of the surrounding soft tissues it should be possible to alter its growth by either pushing it back possible to alter its growth by either pushing it back or pulling it forward.or pulling it forward.

- But the attachment of the mandible to the rest of the But the attachment of the mandible to the rest of the facial skeleton via the TMJ is very different from the facial skeleton via the TMJ is very different from the sutural attachments of the maxilla.sutural attachments of the maxilla.

- In order to restrict the mandibular growth appropriate In order to restrict the mandibular growth appropriate pressure should be created within the joint .pressure should be created within the joint .


- The presence of articulation disc complicates the - The presence of articulation disc complicates the situation, making it difficult to determine exactly situation, making it difficult to determine exactly what areas in and around are being loaded. In what areas in and around are being loaded. In addition the rounded joint surface makes it difficult to addition the rounded joint surface makes it difficult to load the entire area.load the entire area.

- It is fair to say that controlling excessive mandibular - It is fair to say that controlling excessive mandibular growth is an important unsolved problem and we growth is an important unsolved problem and we cannot restrain mandibular growth with anything like cannot restrain mandibular growth with anything like the effectiveness of similar treatment for the maxilla.the effectiveness of similar treatment for the maxilla.


Future applicationsFuture applications

- In future the principles gleaned from molecular In future the principles gleaned from molecular biology and tissue engineering are expected to biology and tissue engineering are expected to become part of the therapeutic regimen utilized become part of the therapeutic regimen utilized during treatment. Orthodontic treatment may now during treatment. Orthodontic treatment may now involve involve delivery of bioactive agentsdelivery of bioactive agents in combination in combination with conventional biomechanics. with conventional biomechanics.

- Numerous agents have activities that can accelerate or Numerous agents have activities that can accelerate or prevent tooth movement and methods of local prevent tooth movement and methods of local pharmaceutical delivery are soon being developed. pharmaceutical delivery are soon being developed.


- Ex – localized application of calciotropic agents has Ex – localized application of calciotropic agents has significant effects on bone remodelling dynamics and significant effects on bone remodelling dynamics and may be used in canine retraction into extraction may be used in canine retraction into extraction spaces or impair tooth movement in high anchorage spaces or impair tooth movement in high anchorage cases.cases.

- Advances in technologic bioassays may also Advances in technologic bioassays may also facilitate the biologic diagnostics and monitoring of facilitate the biologic diagnostics and monitoring of bone turnover and metabolism.bone turnover and metabolism.


ReferencesReferences

- Hill P A, Bone Remodelling BJO1998;25;101-107.- Hill P A, Bone Remodelling BJO1998;25;101-107.- W E Roberts, Bone physiology of tooth movement, W E Roberts, Bone physiology of tooth movement,

ankylosis and osseointergration. Seminars in Orthod ankylosis and osseointergration. Seminars in Orthod 2000;6(3)173-184.2000;6(3)173-184.

- Frost H M, Wolff`s law and bone`s structural Frost H M, Wolff`s law and bone`s structural adaptations to mechanical usage. AO 1994;64,175-adaptations to mechanical usage. AO 1994;64,175-188188

- Frost H M, A 2003 update of bone physiology and Frost H M, A 2003 update of bone physiology and Wolff`s law .AO 2004 74(1) 3-15.Wolff`s law .AO 2004 74(1) 3-15.


- Roberts WE, Ferguson: Cell kinetics of the Roberts WE, Ferguson: Cell kinetics of the periodontal ligament.periodontal ligament.

- Roberts WE, Osseous adaptation to continuous Roberts WE, Osseous adaptation to continuous loading of rigid endosseous implants.AJO1984,86;95loading of rigid endosseous implants.AJO1984,86;95

- Atkinson, Balance the magic word:AJO1964:50,189 Atkinson, Balance the magic word:AJO1964:50,189 - RobertsonWE, GoodwinWC:Cellular response to RobertsonWE, GoodwinWC:Cellular response to

orthodontic force .Dent Clin Nort Amer1981,25:3orthodontic force .Dent Clin Nort Amer1981,25:3- Miyoshi K, Igarashi K, Mitani H. Tooth movement Miyoshi K, Igarashi K, Mitani H. Tooth movement

and changes in pdl in response to orthodontic forceand changes in pdl in response to orthodontic force


in rats vary depending on the time of the day the force in rats vary depending on the time of the day the force is applied. EJO 2001,23;329-338.is applied. EJO 2001,23;329-338.

- Roberts et al, Circadian periodicity of the cell kinetics Roberts et al, Circadian periodicity of the cell kinetics of rat molar pdl. AJO1979,76:316of rat molar pdl. AJO1979,76:316

- Quinn RS, Yoshikawa. A reassessment of force Quinn RS, Yoshikawa. A reassessment of force magnitude in orthodontics.AJO1985:88,252-260magnitude in orthodontics.AJO1985:88,252-260

- Proffit W, Contemporary orthodontics.3Proffit W, Contemporary orthodontics.3 rdrd edition. edition.


- Graber TM, Orthodontics, Current principles and Graber TM, Orthodontics, Current principles and techniques.3techniques.3rdrd edition. edition.

- Ravindra Nanda. Biomechanics and Esthetic Ravindra Nanda. Biomechanics and Esthetic strategies in clinical orthodontics.strategies in clinical orthodontics.

- Roberts WE, Hartsfield JK. Bone development and Roberts WE, Hartsfield JK. Bone development and function. Seminar Orthod 2004,10;100-112function. Seminar Orthod 2004,10;100-112

- Roberts WE, Huja S, Roberts JA. Bone modelling Roberts WE, Huja S, Roberts JA. Bone modelling biomechanics, molecular mechanisms and clinical biomechanics, molecular mechanisms and clinical perspective. Seminar Orthod2004,10;123-161perspective. Seminar Orthod2004,10;123-161


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