BIOLOGY AND BIOMECHANICS OF TOOTH MOVEMENT
INDIAN DENTAL ACADEMY Leader in continuing dental education www.indiandentalacademy.com www.indiandentalacademy.com
CONTENTS
INTRODUCTION PERIODONTAL LIGAMENT ALVEOLAR BONE TOOTH MOVEMENTS 1. PHYSIOLOGICAL 2. ORTHODONTIC THEORIES OF TOOTH MOVEMENTS EFFECTS OF FORCE MAGNITUDE FACTORS EFFECTING ORTHODONTIC TOOTH MOVEMENT EFFECT OF DRUGS ON RESPONSE TO ORTHODONTIC FORCES. www.indiandentalacademy.com
DELETERIOUS EFFECTS OF ORTHODONTIC FORCES TYPES OF TOOTH MOVEMENTS MECHANICAL PRINCIPLES IN FORCE CONTROL PROPERTIES OF ELASTIC MATERIALS FACTORS AFFECTING ELASTIC PROPERTIES FORCE,MOMENT AND COUPLES IN TOOTH MOVEMENT SYSTEM EQUILIBRIUM SEGMENTED AND CONTINUOUS ARCH MECHANICS CONCLUSION. www.indiandentalacademy.com
INTRODUCTION ď Ž
Orthodontic therapy depends upon the reaction of the teeth, and more generally the facial structures to gentle but persistent force. The main purpose of presenting a discussion on the biophysical principles of tooth movement is to know the facts and histological findings that have a bearing on practical orthodontics.
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ď Ž
In the orthodontic context, biomechanics is commonly used in discussions of the reaction of the dental and facial structures to orthodontic force,
ď Ž
whereas mechanics is reserved for the properties of the strictly mechanical components of the appliance system. www.indiandentalacademy.com
“Tissue consciousness” is a vital prerequisite
to mechanics. There are available today potent tooth-moving appliances that can accomplish almost any desired change, but if their use is not controlled by a profound respect for the biological media in which they work, then tremendous harm can be done.
The forces are applied to the teeth with the objective of getting desired tooth movement, in the desired direction, in the desired amount of time. Thus it is obvious that a sound biological understanding of the orthodontic tooth movement is a must. www.indiandentalacademy.com
PERIODONTAL LIGAMENT
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ď ŽThe
pdl is the soft specialized connective tissue situated between the bone forming the socket wall and the cementum covering the root surfaces. It ranges in width from 0.15 to 0.38mm, with its thinnest portion around the middle third of the root. Like any other connective tissue it consists of cells and an extra cellular compartment of fiber and ground substance. ď Ž
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The cells include Osteoblasts and osteoclasts Fibroblasts Epithelial cells of malasses Macrophages Undifferentiated mesenchymal cells Cementoblasts www.indiandentalacademy.com
The extra cellular compartment: collagen and oxytalan fibers embedded in ground substance consisting mainly of glycosaminoglycans, glycoproteins and glycolipids
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ď Ž
ď Ž
The vast majority of the collagen fibrils in the periodontal ligament are arranged in definite and distinctive fiber bundles. These fiber bundles are arranged in groups and are sometimes called the principal fibers of the ligament. At either end all the principal collagen fiber bundles of the pdl are embedded into cementum or bone. The embedded portion of the fibers is called the Sharpeys fibers.
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The alveolar crest fibers: attached to the cementum just below the CEJ and running downward and outward to insert into the rim of the alveolus. The horizontal group: occurring just apical to the alveolar crest group and running at right angles to the long axis of the tooth from cementum to bone just below the alveolar crest. The oblique group: by far the most numerous in the ligament and running from the cementum in an oblique direction to insert into bone coronally. The apical group: radiating from the cementum around the apex of the root to the bone, forming the base of the socket. The inter-radicular group: found only between the roots of multirooted teeth and running from the cementum to the bone forming the crest of the inter –radicular septum. www.indiandentalacademy.com
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FUNCTIONS OF THE PDL Physical functions: – Transmission of occlusal forces to the bone – Attachment of teeth to the bone – Maintenance of the gingival tissues in proper relationship to the teeth – Resistance to the impact of occlusal forces – Provides a soft tissue housing to protect the vessels and nerves from injury by mechanical forces. www.indiandentalacademy.com
Formative functions: The undifferentiated cells in the pdl serve as precursors for the cementum and bone forming cells. In fact they play a key role in bone remodeling. Nutritional functions: By the way of blood vessels that traverse, the pdl supplies nutrients to the cementum, bone and gingival for their metabolic activities. It also provides lymphatic drainage. www.indiandentalacademy.com
Sensory functions: The innervations of the pdl provide propioceptive and tactile sensation, which detect and localize external forces acting upon individual teeth and serve an important role in neuromuscular mechanism controlling the masticatory musculature. Other functions: – Through the formation, cross linkage and maturational shortening of collagen fibers, it helps in eruption of teeth. – The metabolic activities occurring within the pdl maintain the teeth in position even though the forces acting from extraoral and intraoral muscles are not balanced. www.indiandentalacademy.com
ALVEOLAR BONE
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ď Ž
The alveolar process is that bone of the jaws that contains the sockets (alveoli) for the teeth and consists of outer cortical plates, a central spongiosa and bone lining the alveolus.
ď Ž
The cortical plate and the alveolar plate and the bone lining the alveolus meet at the alveolar crest, usually 1.5 to 2 mm below the level of the CEJ of the tooth it surrounds. www.indiandentalacademy.com
ď Ž
ď Ž
ď Ž
The bone lining the alveolus is specifically called the bundle bone because it is this bone that provides attachment for the pdl fibers. It is perforated by many foramina that transmit nerves and vessels and is therefore sometimes referred to as the CRIBRIFORM PLATE. It is also called as the lamina dura because of its increased radio opacity. The cortical plate consists of surface layers of fine fibered lamellar bone supported by compact Haversian system bone of variable thickness. The trabecular or spongy bone occupying the central part of the alveolar process also consists of fine-fibered membrane bone dispersed in the large trabeculae. The important part of this complex in term of tooth support is the bundle bone. www.indiandentalacademy.com
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TOOTH MOVEMENT
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To a layperson the most rigid thing in the body is his set of teeth. He accepts the fact that they can wear down over the years but if they move he expresses alarm. He knows nothing about the cushioning connective tissue, the periodontal membrane that is as vital as any tissue in the body. He does not know that bone is a vital tissue and also undergoes constant reorganization; that teeth move constantly and imperceptibly through out life
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Physiological tooth movement ď Ž
designates primarily, the slight tipping of the functioning tooth in its socket and, secondarily, the changes in tooth position that occur in young persons during and after tooth eruption.
ď Ž
The minor changes in tooth position observed in growing persons and adults are usually called tooth migration. Tooth migration in both young and older persons is always related to definite tissue changes that can be readily observed in histological sections www.indiandentalacademy.com
ď Ž
With the wearing away process teeth continue to erupt. Contacts are worn and contact points become contact surfaces. Mesial drift compensates for the space created, and as the tooth moves the socket shifts with the tooth. Bone is resorbed ahead of the drifting tooth and deposited behind it.
ď Ž
Resorption is seen as an uneven scalloped margin, with the presence of osteoclasts. Bone deposition appears histologically as concentric lamella of bundle bone laid down in the presence of and with the aid of the bone-building cells the osteoblasts. www.indiandentalacademy.com
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ď Ž
As the alveolus move leaving space for the tooth and the pdl, bony reorganization outside the alveolus occurs. Ahead of the moving tooth, trabaculae show resorption on the side nearest the moving tooth, deposition of bone on the side farther away. Behind the moving tooth bone is deposited on the side away from the tooth to maintain a constant length of the trabecular structure. www.indiandentalacademy.com
ď Ž
The osteoblast first lay down an organic matrix known as the osteoid. This then becomes calcified as calcium salts are deposited in the matrix. The newly calcified tissue is called bundle bone and is basophilic in appearance. The staining properties of bundle bone are related to its high content of cementing substance, consisting essentially of highly polymerized connective tissue polysaccharides.
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ď Ž
Cells and fiber bundles will be incorporated in bundle bone during its life cycle. When it has reached a certain thickness and maturity, parts of the bundle bone will reorganize into lamellated bone with fine fibrils in its matrix. The lamina dura will subsequently reappear as a somewhat thinner radio opaque line.
ď Ž
This sequence of events is, in principle, the same as that in bone formation after orthodontic tooth movement. www.indiandentalacademy.com
It has been established beyond doubt that bone is biologically plastic and adaptive to developmental and functional forces, responding to pressure with resorption and to tension with bone deposition. It is the property of the teeth to move and reflect various environmental influences by positional modifications throughout life that the orthodontist uses to move teeth to the desired new position. Alveolar bone has been referred to as “the slave of the orthodontist”. The essential processes are there and at work before he attempts guided tooth movement by mechanical appliances. The bony response is primarily mediated by the periodontal ligament, and so the tooth movement is believed to be primarily a periodontal phenomenon. www.indiandentalacademy.com
ORTHODONTIC TOOTH MOVEMENTS ď Ž
ď Ž
Theoretically it should be possible to bring about tooth movement without any tissue damage by using a light force, equivalent to the physiological forces determining tooth position, to capitalize on the plasticity of the supporting tissues. However most current orthodontic techniques do not duplicate the ideal situation; most involve some degree of tissue damage that varies because the forces applied to move the tooth are not equally distributed throughout the pdl www.indiandentalacademy.com
The orthodontic response to light, continuous load is divided into three elements of tooth displacement: ď Ž
Initial strain: occurs in about one week. The displacement produced is about 0.4- 0.9 mm and is due to the pdl displacement, bone strain and extrusion. The fluid mechanics of root displacement in the pdl probably accounts for about 0.3mm of crown movement. www.indiandentalacademy.com
ď Ž
Lag phase: the displacement of the tooth relative to its osseous support stops in about one week. This occurs due to areas of the pdl necrosis (hyalinization). This phase is called the lag phase. It varies from about 2-3 weeks and may be as long as 10 weeks. The duration of the lag phase is directly related to the patient’s age, density of alveolar bone and extent of pdl necrotic zone. www.indiandentalacademy.com
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Progressive tooth movement: after undermining resorption, vitality is restored to the necrotic areas of the pdl, and the tooth movement enters the secondary or progressive tooth movement phase. Frontal resorption in the pdl, and initial remodeling events in the cortical bone ahead of the advancing tooth allow for progressive tooth movement at a relatively rapid rate www.indiandentalacademy.com
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The duration of tooth movement can be divided into two periods:
ď Ž Initial
stage:
when a constant orthodontic force is maintained on the tooth, compression of the pdl occurs. This causes degradation rather than causing proliferation and differentiation. The tissues reveal a glass like appearance when viewed in light microscopy and is termed as Hyalinization. www.indiandentalacademy.com
HYALINISATION ď Ž
it is an unavoidable phenomenon in the initial period of tooth movement. It is partly caused by anatomic and partly by mechanical factors. It is a sterile necrotic area and is limited to 1-2 mm in diameter.
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The process displays three main stages: ď Ž Degeneration:
it starts when the pressure is the highest and narrowing of the membrane is more pronounced. There is retardation of blood flow followed by disintegration of the vessel walls and degradation of blood elements. Cells rupture, the nuclei breakdown leaving unidentifiable cellular elements between the collagen fibrils. In the hyalinised zone, cells cannot differentiate into osteoclasts and so no resorption occurs. Tooth movement stops. www.indiandentalacademy.com
ď Ž Elimination of destroyed
tissue:
Elimination of the hyalinised zone occurs by two mechanisms 1. Resorption of the alveolar bone by osteoclast 2. Invasion of cells and blood vessels from the periphery of the compressed zone by which the necrotic tissue is removed
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ď Ž
Re establishment of tooth attachment: this phase starts by the synthesis of new tissues as soon as the adjacent bone and degenerated membrane tissues have been destroyed. The ligament space is wider than before treatment and the membranous tissue under repair is rich in cells. The pdl is reconstructed in the hyalinised areas. www.indiandentalacademy.com
ď Ž
. Secondary stage of tooth movement: the pdl is considerably widened. Osteoclasts attack the bone over a wider area. Further bone resorption occurs when force is kept constant and within limits. New periodontal fibers are produced and the fibrous attachment apparatus is reorganized. A large number of osteoclasts are seen along the bone surface and tooth movement is rapid. Deposition of bone occurs on the alveolar surface from which the tooth is moving away until the width of the membrane has returned to normal limits. www.indiandentalacademy.com
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THEORIES OF TOOTH MOVEMENT
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Two main theories that have been proposed and are accepted to play a part in the biologic control of tooth movement. They are ď Ž
The Bioelectric theory that relates the tooth movement in part to changes in the bone metabolism controlled by the electric signals that are produced when alveolar bone flexes and bends.
ď Ž
The Pressure Tension theory which relates tooth movement to cellular changes produced by chemical messengers, traditionally thought to be generated by alterations in blood flow through the pdl. www.indiandentalacademy.com
THE BIOELECTRIC THEORY ď Ž
The electric signals that bring about initial tooth movement are piezoelectric. Piezoelectricity is a phenomenon observed in many crystalline materials in which a deformation of the crystal structure produces a flow of electric current as electrons are displaced from one part of the crystal lattice to another. Bone is crystalline in nature and both bone and collagen exhibit peizoeletric effect. www.indiandentalacademy.com
Piezoelectric signals have two unusual characteristics: ď Ž
ď Ž
A quick decay i.e.; when a force is applied, a piezoelectric signal is created in response that quickly dies away to zero even though the force is maintained. The production of an equivalent signal, opposite in direction, when force is released
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When the crystal structure is deformed, electrons migrate from one location to another and an electric charge is observed. As long as the force is maintained, the crystal structure is stable and no further electric events are observed. When the force is released, however, the crystal returns to its original shape, and a reverse flow of electrons is seen. With this arrangement, rhythmic activity would produce a constant interplay of electric signals, whereas occasional application and release of force would produce only occasional electric signals. www.indiandentalacademy.com
The action of any force causes minute distortions in a bone. This leads to regional changes in configuration involving localised surface concavities and convexities.A concavity results in compression and a negative surface charge, and a convexity causes tension and a positive surface charge. This triggers bone deposition and resorption, respectively, by the peizo effect acting on surface cell receptors of osteoblasts and osteoclasts. The bone thereby remodels until biomechanical and bioelectric neutrality is attained. www.indiandentalacademy.com
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If an existing concave surface becomes more concave, the effect is active compression and the action response thereby depository. If an existing concave surface becomes less concave, the action is less compressive and a direction towards tension is seen, the resultant response is resorption. If a convex surface becomes either more or less convex, similarly, the results are believed to be resorption and deposition, respectively.
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ď Ž
A second type of electric signal, which is called the Bioelectric potential, can be observed in bone that is not being stressed. Metabolically active bone or connective tissue cells produce electronegative charges that are generally proportional to how active they are. Inactive cells and areas are nearly electrically neutral. This potential can be altered by applying an external electric field.The effects are felt in the cell membranes. Membrane depolarization triggers nerve impulses and muscle contraction, but changes in membrane potentials accompany other cellular responses as well. The external electric signals probably affect cell membrane receptors, membrane permeability, or both. www.indiandentalacademy.com
Experiments indicate that when low voltage direct current is supplied to the alveolar bone, modifying the bioelectric potential, a tooth moves faster than its control in response to an identical spring. Electromagnetic fields also can affect cell membrane potentials and permeability, and thereby trigger changes in cellular activity.
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PRESSURE TENSION THEORY ď Ž
The pressure tension theory, the classic theory of tooth movement relies on chemical rather than electric signals as the stimulus for cellular differentiation and ultimately tooth movement. In this theory, an alteration in blood flow within the pdl is produced by the sustained pressure that causes the tooth to shift position within the pdl space, compressing the ligament in some areas while stretching it in others. Blood flow is decreased where the pdl is compressed, while it usually is maintained or increased where the pdl is under tension. www.indiandentalacademy.com
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Alterations in blood flow quickly create changes in the chemical environment. For instance, oxygen levels certainly would fall in the compressed area, but might increase on the tension side, and the relative proportions of other metabolites would also change in a matter of minutes. These chemical changes, acting either directly or by stimulating the release of other biologically active agents, would stimulate cellular differentiation and activity.
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In essence, this view of tooth movement shows three stages:
Alterations in the blood flow associated with pressure within the pdl,
The formation and\or release of chemical messengers, and
Activation of cells. www.indiandentalacademy.com
EFFECTS OF FORCE MAGNITUDE
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ď ŽWhen
< 1 sec
heavy pressures are applied: Pdl fluid incompressible, alveolar bones bends, piezoelectric signals generated.
1-2 sec
Pdl fluid expressed, tooth move within the pdl space
3-5 sec
Blood vessels within the pdl occludes on the pressure side.
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Mins
Blood flow cut off to the compressed pdl area.
Hours
Cell death in the compressed area
3-5 days
Cell differentiation in adjacent marrow spaces, undermining resorption begins
7-14 days
Undermining resorption removes lamina dura adjacent to compressed pdl, tooth movement occurs.
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ď ŽWhen < 1 sec
light pressure is applied: Pdl fluid incompressible, alveolar bone bends, piezoelectric signal generated.
1-2 sec
Pdl fluid expressed, tooth moves with the pdl space
3-5 sec
Blood vessels in the pdl partially compressed on the pressure side, dilated on the tension side, pdl fibers and cells mechanically www.indiandentalacademy.com
destroyed
Mins
Blood flow altered, oxygen tension begins to change, prostaglandin and cytokines released.
Hours
Metabolic changes occurring, chemical messengers affects cellular activity, enzyme levels change
--4 hrs
Increased cAMP levels, cellular differentiation begins within the pdl
--2 days
Tooth movement beginning as osteoclasts\osteoblasts remodel bony socket. www.indiandentalacademy.com
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FACTORS INFLUENCING ORTHODONIC TOOTH MOVEMENT
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Local tissue reactions are influenced by the anatomic characteristics of the supporting bone into which the tooth is to be moved, the physiologic activity of the tissues that surround the tooth and the force application
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Character of bone Remodeling processes in bone depend on the activity of the cells that act on its surfaces. Thus alveolar bone that is penetrated by numerous canals to transmit blood vessels and contains cancellous bone with marrow spaces at its deeper aspect is favorable for tooth movement. On the other hand, if the bone involved is compact in nature, that is cortical bone, then the surface area where cellular activity can take place is greatly reduced. Here tooth movement is more difficult and slower, and the chances of creating over compression and greater areas of hyalinization are much higher. www.indiandentalacademy.com
Thus it is important that when planning orthodontic treatment, the tooth should remain in spongy bone during movement. Extraction spaces contain tissues undergoing reconstruction, which is rich in cells and vascular supply. Such an area is ideally suitable for tooth movement, and due advantage of this should be taken by commencing treatment as soon as possible following extraction. Thereby one also avoids atrophy and narrowing of the alveolar process, resulting in bone loss and cortical bone formation at the extraction site. www.indiandentalacademy.com
Physiologic activity The strong relapse tendency seen after the orthodontic rotation of teeth is thought to be the result of slow turn over of the gingival fibers mainly the supraalveolar fiber bundles. Turn over varies from person to person and depends on a number of variables such as hormonal balances, age of the patient and health of the patient. Therefore it is necessary to consider these variations during treatment planning, especially if the patient is receiving medications like steroids or anti epileptics, as the threshold for tissue changes or cellular reactions will be influenced. www.indiandentalacademy.com
Force applications\ applied force and time key to orthodontic tooth movement is application of light and sustained force, which does not mean that the force must be continuous, but it must be present for a considerable percentage of time. Experiments have shown that the threshold for orthodontic tooth movement in humans is 4-8 hours.
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Orthodontic force duration is classified by the rate of decay as Continuous Interrupted intermittent
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Continuous forces ď Ž
force maintained at some appreciable fraction of the original from one patient visit to another. Continuous force leads to gradual compression of the pdl on the pressure side of the tooth. If the force is within the limitations where tissue reactions occur, reconstructional changes of the fibrous element as well as direct resorption of the alveolar bone wall take place
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Interrupted forces ď Ž
force levels decline to zero between activations. Here even if the hyalinised zones are established, the pdl has the time to become reconstructed. There is an increase in cell proliferation, which is suitable for further tissue changes following reactivation of the force. Fixed appliances that are constantly present on the tooth can produce both continuous and interrupted forces. www.indiandentalacademy.com
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Intermittent forces ď Ž
force levels decline abruptly to zero intermittently, when the orthodontic appliance is removed by the patient or when a fixed appliance is temporarily deactivated. On the pressure side, the circulation will not be as easily disturbed or hindered unless the force applied is too high. The intermittent force is thought to act as an incitement to cell proliferation. Increase in the cell numbers and direct bone resorptions along the alveolar bone wall are characteristic of this type of tooth movement. The periodontal space increases because the tooth tends to return to its original position following the removal of the force. www.indiandentalacademy.com
In spite of the favorable condition on the side where resorption is seen, tooth movement often will be slower than that seen during application of continuous force, as the time over which the appliance is used is a very important factor. Formation of new tissue and apposition of bone are seen to occur more rapidly under active or constant stretching. Therefore, if the tooth is often allowed to return to its original position, one can expect a limited amount of apposition to occur. www.indiandentalacademy.com
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PRE-ERUPTIVE TOOTH MOVEMENTS These are made by both the deciduous and the permanent tooth germs within the tissues of the jaw before they begin to erupt. The deciduous teeth germs are extremely small and have enough space in the developing jaw. But as they grow rapidly, they become crowded together. This is alleviated by the lengthening of the jaws, permitting the second molar tooth germs to move backwards and anterior tooth germs forward. www.indiandentalacademy.com
At the same time the tooth germs are bodily moving outwards and downwards or upwards. The permanent tooth germs develop lingual to the primary ones in the same bony crypt, except for the molars which develop from the distal extensions of the dental lamina. The canines and incisors gradually shift to occupy a position in their own bony crypts,on the lingual of the roots of their predecessors, while the premolar tooth germs are finally positioned between the divergent roots of the deciduous molars. www.indiandentalacademy.com
Due to lack of space, the upper permanent molar tooth germ develop with their occlusal surfaces facing distally and later swing into position only when the maxilla has grown sufficiently to provide room for such movement. In the mandible, the molars develop inclined towards the mesial direction, which becomes vertical only when sufficient growth has occurred.
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As these preeruptive movements occur in an intraosseous location,such movement is reflected in the patterns of bony remodeling within the crypt wall.eg, during bodily movement in a mesial direction, bony resorption occurs on the mesial surface of the crypt wall, and bony deposition on the distal wall as a filling in process. During eccentric growth only bony resorption occurs, thus altering the shape of the crypt to accommodate the altering shape of the tooth germ.
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BONE’S STRUCTURAL ADAPTATIONS TO MECHANICAL USAGE
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In health and disease the architecture of a whole bone, such as the mandible, depends on both cartilage and bone. Some general cartilage roles a) In children, cartilage growth determines a bone’s length and a joint’s shape, size and alignment. b) During growth a cartilage layer at the bony attachments of fascia, ligaments and tendons controls the local growth, and migration during growth, of those attachments. This includes the mandibular insertions of the masseter, pterygoids and temporalis www.indiandentalacademy.com
Some general bone roles: a) Bone provides rigid levers for muscles to act on, and support for joints and teeth.
b) Lamellar and woven bones serve somewhat different purposes and can respond differently to mechanical and nonmechanical influences.
c) Modeling drifts and remodeling BMUs (basic multicellular units) can each result in bone turn over, alter the shape and size of bone. Each can also respond in its own way to aging, hormones, disease, drugs and mechanical influences. www.indiandentalacademy.com
A load (force) on a bone deforms or strains it. This stretches intermolecular bonds in the bone that resist with an elastic force called a stress. Living bone may depend more on strain than stress to generate the signals that control its biological reactions to mechanical loads.
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Modeling and Remodeling: Two bone-biologic activities can affect a boneâ&#x20AC;&#x2122;s architecture. Modeling by resorption and formation can move a boneâ&#x20AC;&#x2122;s surfaces in tissue space to shape and size it. Remodeling by BMUs (Basic Multicellular Units) can turn bone over in small packets. Each activity can respond in its own way to mechanical and other influences.
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Bone modeling: Osteoblasts in formation drifts can form new bone on large regions of periosteal, cortical-endosteal and trabecular surfaces. Osteoclasts in resorption drifts can resorb bone from other similar surfaces. Various stimuli can trigger bone modeling or drift. These drifts usually maintain a boneâ&#x20AC;&#x2122;s shape while it increases in size. Such drifts also move tooth sockets around in the mandible and maxilla in response to orthodontic forces. www.indiandentalacademy.com
A) Lamellar or woven bone can each provide formation drifts on periosteal, cortical-endosteal and trabecular surfaces, but larger stimuli are needed to make woven bone form than lamellar bone. B) Woven bone can appear in fracture healing, some neoplasm, infections, and in reaction to large mechanical loads. It can arise in the marrow cavity ahead of a tooth socket containing a tooth subjected to excessive orthodontic forcesâ&#x20AC;&#x201C; undermining resorption. Woven bone drifts can add bone much faster than lamellar drifts. C) Lamellar drifts can thicken or thin a cortex or trabecula no more than about 2 mm/year, a limit that may decrease with age. www.indiandentalacademy.com
Macromodeling, minimodeling and micromodeling: Drifts control if, when, where and how much bone formation and resorption happen. The naked eye can see their effects so they provide macromodeling. On trabeculae these drifts provide minimodeling, since it takes magnification to see them. During any bone formation a different, cell-level activity determines the microscopic organization and â&#x20AC;&#x153;grainâ&#x20AC;? of the new tissue.
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It organizes lamellar and woven bone differently. It always aligns lamellar boneâ&#x20AC;&#x2122;s grain parallel to the major compression or tension loads on it while it was forming. Therefore lamellar boneâ&#x20AC;&#x2122;s grain in a mandible, tooth socket, maxilla or femur can show the orientation of the major mechanical loads on it during its formation. Boneâ&#x20AC;&#x2122;s structural adaptations to mechanical usage (SATMU) respond to some average of many strains, not to single ones, and large strains influence modeling much more than small ones no matter how frequent. www.indiandentalacademy.com
In sum: By moving bone surfaces in tissue space, global (refers to bone as a whole) modeling can increase but not decrease bone mass and strength. Decreased modeling simply slows down such increases. Obviously single resorption drift must remove bone locally. Remodeling: Small â&#x20AC;&#x153;packetsâ&#x20AC;? called BMUs (Basic Multicellular Units) provide bone remodeling, as distinguished from the modeling described above. In an Activation-ResorptionFormation (ARF) sequence a BMU replaces some old bone with new bone, to create a new bone packet or Basic Structural Unit www.indiandentalacademy.com (BSU).
The BMU “rho fractions”: A completed BMU can resorb more, or less, or equal to the bone than it makes. Let rho equal any such deficit or excess of resorption over formation. When more bone is formed than removed then it is said to be having a positive rho and the vice versa. Remodeling happens on periosteal, haversian, corticalendosteal and trabecular surfaces or “envelopes” Normally rho may be positive only on the periosteal envelope, where completed BMUs may resorb a bit less bone than they make. www.indiandentalacademy.com
Rho approaches zero on the haversian envelope, where net resorption and formation tend to equalize. Rho is usually negative on cortical-endosteal and trabecular surfaces where BMUs usually resorb a bit more bone than they make throughout life. Global remodeling can remove or conserve bone but apparently cannot add to it. Increased remodeling tends to remove bone next to marrow and make a bone weaker. Decreased remodeling tends to conserve bone and its strength. www.indiandentalacademy.com
Microdamage and its thresholds:
Mechanical fatigue damage (microdamage) normally occurs in bone in life. Remodeling BMUs usually repair the damage and keep it from accumulating. This is done by removing and replacing the damaged bone with new bone. Overloading the bone can increase microdamage and remodeling that repair it.
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The microdamage threshold: When loaded below about 2000 mE (microstrain), BMUs can easily repair what little microdamage occurs. At and above 4000 mE enough microdamage can occur to overwhelm the repair mechanisms, resulting in accumulations of damage that can cause fatigue failures of trabeculae or whole bones. In this 2000â&#x20AC;&#x201C;4000 mE range, merely doubling the size of the strains can increase microdamage hundreds of times. www.indiandentalacademy.com
Strains lesser than 1500mE( MESm ) trigger lamellar drifts. The largest normally allowed peak bone strain lies below the 1500mE range. Strains in the range of 3000-4000mE(MESp) and above usually trigger woven bone formation. Strains above MESm approaching 3000mE(MESr) increase bone microdamage, which then increases BMU creations to repair it. Lamellar drifts add,reshape and strengthen bone, thus reducing future strains under the same mechanical load towards that strain region. Woven bone drifts suppress lamellar drifts ,but strengthen bone faster. However strains at this range also increases microdamage alarmingly. www.indiandentalacademy.com
Orthodontic forces above the peak bone strain can cause damage to the teeth and the sockets. The nonmechanical factors that can influence modeling and remodeling include hormones, vitamins, drugs, disease, inflammation (including infection), genetics (including race and species), nutrition, climate and occupation. Errors in the bones structural adaptation to mechanical usage by modeling and remodeling can and do cause skeletal disease and problems encountered in orthopedic and maxillofacial surgeries, orthodontics and dentistry. www.indiandentalacademy.com
LATEST CONCEPTS OF FORCE MAGNITUDE
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Martina Von Bohl et al., did a study on beagle dogs to evaluate histological changes in the periodontal structures after using high and low continuous forces during experimental tooth movement. The aim of this study was to evaluate the rate of tooth movement and tissue reaction after standardized application of low and high orthodontic force that lead to low and high pressures in the pdl of different teeth within one experimental animal.
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In the past many authors have described the formation of hyalinization zone in the pdl as a result of localized ischaemia. They reported that, after excessive compression of the pdl, blood supply is cut off which leads to necrosed areas and arrest of tooth movement. Removal of the necrotic tissue and bone resorption from adjacent marrow space allow the resumption of tooth movement. In the absence of necrotic areas, cells start remodeling process at the tension site, and rate of tooth movement increases. The outcome of the new study are contradictory to this commonly accepted theory. www.indiandentalacademy.com
An orthodontic appliance was placed on second premolar and first molar by exerting a continuous and constant force of 25 gm on one side and 300 gm on the other side of the mandible. Tooth movement was recorded weekly. Dogs were sacrificed after one, four, twenty, forty and eighty days for histological evaluation. The results showed large individual differences in the rate of tooth movement after using high or low forces and that the force level had no influence on the amount of tooth movement. www.indiandentalacademy.com
Hyalinization was not only found in the phase of arrest, i,e between 4 and 20 days, but also after 40 and 80 days of tooth movement. This suggest that the development and removal of necrotic tissue is a continuous process during tooth displacement instead of a single event. It was also found that the location of the hyalinization zone was different from those of earlier reports. They were not found in the area of the central plane but lingually and bucally from it. This is probably the consequence of local stress and shear concentrations caused by local irregularities in the bone morphology. www.indiandentalacademy.com
The other significant finding of this study is that the teeth on which high forces were applied did not move faster than the ones displaced by low forces. Areas of hyalinization were found to be more in the tooth displaced with higher forces but these areas were present in both situations throughout the tooth movement. The appearance of the necrotic tissue might be related to force magnitude, but seems to have no significance for the rate of tooth movement. This means that once tooth movement has started, bone remodeling takes place at a certain rate, independent of force magnitude. www.indiandentalacademy.com
Furthermore, the data show a large individual variation which could be due to differences in bone metabolic capacity. Bone density, morphologic differences and genetic factors could also influence the remodeling process and subsequent tooth movement.
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EFFECTS OF DRUGS ON THE RESPONSE TO ORTHODONTIC FORCE
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Agents that stimulate tooth movement are rare but under some circumstances vitamin D administration can enhance response to orthodontic forces. Direct injection of prostaglandin into the pdl has shown to increase the tooth movement, but this is very painful and not practical. www.indiandentalacademy.com
Two types of drugs are known to depress the response to orthodontic forces: -Biophosphates- used in treatment of osteoporosis. Osteoporosis is a problem seen mostly in postmenopausal females but may be seen in both the sexes with aging. Thus, medication for this purpose is seen with adult orthodontic patients. Estrogen therapy used for the same condition has little or no impact on ortho treatment. Therefore if a patient taking biophosphates for treatment of osteoporosis comes for orthodontic taerapy, it would be worthwhile to discuss with her physician the possibility of switching over to www.indiandentalacademy.com estrogen temporarily.
-Prostaglandin inhibitors: Since prostaglandins play an important role in chemical mediation of tooth movement, inhibitors of its activity would affect movement. Drugs that affect the PG activity fall into two main categories: 1.Corticosteroids
and NSAIDs
2.Agents
that have mixed agonist and antagonist effects on various PGs.
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Steroids reduce the synthesis of PG by inhibiting formation of arachidonic acid,the precursors for PG, whereas the NSAIDs act by inhibiting the conversion of arachidonic into PGs. Fortunately only potent PG inhibitors like indomethasin used for treatment of arthritis interfere with tooth movement, while the common analgesics seem to have little or no inhibiting effect on tooth movement at the dose levels used with orthodontic patients. www.indiandentalacademy.com
DELETERIOUS EFFECT OF ORTHODONTIC FORCE
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Pain If heavy pressure is applied to a tooth, pain develops immediately as the pdl is literally crushed. If appropriate orthodontic force is applied, the patient feels little or nothing immediately. Several hours later, patient feels a mild aching sensation which lasts for 2 to 4 days, then disappears until the orthodontic appliance is reactivated. The tooth is quite sensitive to pressure. This suggests inflammation at the apex, and the mild pulpities that usually appears soon after orthodontic force is applied probably contributes to the pain www.indiandentalacademy.com
If the source of pain is ischemic areas, strategies to temporarily relieve pressure and allow blood flow through the areas should help. If light forces are used the amount of pain to the patient can be decreased by having them engage in repetitive chewing of gum or plastic wafer placed between teeth during the first 8 hours after the orthodontic appliance is activated. Presumably this works by temporarily displacing the teeth enough to allow some blood flow through the compressed areas, thereby preventing build-up of metabolic products that stimulate pain receptors. www.indiandentalacademy.com
Mobility
Orthodontic tooth movement requires both remodeling of bone and reorganization of the pdl itself. Fibers become detached from the bone and cementum, then reattach later. Radiograpically it can be observed that the pdl space widens during ortho tooth movement leading to some mobility. A moderate increase in mobility is an expected response to orthodontic tooth movement. The heavier the force, greater the amount of undermining resorption expected, greater the mobility that will develop. If a tooth becomes extremely mobile during treatment, it should be taken out of occlusion and all forces should be discontinued until the mobility decreases to moderate levels. www.indiandentalacademy.com
Effects on pulp Although pulpal reactions to orthodontic treatment are minimal, there is probably a moderate and transient inflammatory response within the pulp, which contributes to the discomfort that the patients feel for the first few days after appliance activation. There are occasional reports of loss of tooth vitality during ortho treatment. If a tooth is subjected to heavy continuous force, there is a sequence of abrupt movements, which could sever the blood vessels as they enter. www.indiandentalacademy.com
Effects on root structure When ortho forces are applied, there is usually an attack on the cementum of the root, just as there is an attack on the adjacent bone, but repair of the cementum also occurs. Rygh and co-workers have shown that the cementum adjacent to the hyalinsed areas of the pdl are attacked by the clast cells and can lead to severe root resorption. It is seen that if cementum is removed from the root surface, then it is restored in the same way that the alveolar bone is removed and then www.indiandentalacademy.com replaced.
Repair of the damaged root restores its original contours; unless the attack on the root surface produces large defects at the apex that eventually become separated from the root surface. Once an island of cementum or dentin has been cut totally free from the root surface, it will be resorbed and will not be replaced. Permanent loss of root structure after ortho treatment appears primarily at the apex. Sometimes there is a reduction in the lateral aspect of the root in the apical region. www.indiandentalacademy.com
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Types of tooth movements that may lead to apical root resorptions include: Prolonged tipping,notably of the anterior teeth • Distal tipping of the molars, causing resorption of the distal roots of the molars • Prolonged continuous bodily movement of small teeth such as upper lateral incisors. • Intrusion • Extensive edgewise torque of anterior teeth in the more mature young and adult patients. •
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Effects on height of alveolar bone Another effect of orthodontic treatment might be loss of alveolar bone height. Since the presence of orthodontic appliances increases the amount of gingival inflammation, even with good hygiene, this side effect might seem even more likely. Fortunately, excessive loss of crestal bone height is almost never seen as a complication of ortho treatment. The reason is that the position of the tooth determines the position of the alveolar bone. When teeth erupt or are moved, they bring alveolar bone with them. www.indiandentalacademy.com
TYPES OF TOOTH MOVEMENTS
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Theoretically tooth movement is divided into three types, viz, Pure translation Pure rotation Combination of translation and rotation
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Before we go into details about the various types of tooth movement possible, a few concepts and definitions have to be understood ď ŽFORCE: a load applied to an object that will tend to
move it to a different position in space. Force has both direction and magnitude. ď ŽCENTER
OF ROTATION: it is the point around
which the body seems to have rotated. The center of rotation is not a fixed point and can be changed by the manner of force application. www.indiandentalacademy.com
ď Ž
CENTER OF RESISTANCE: a point at which
resistance to movement can be concentrated for mathematical analysis. For an object in free space, the center of resistance is the same as the center of mass. For an object, which is partially restrained, the center of resistance will be determined by the nature of the external restraints. The center of resistance for a tooth is approximately the midpoint of the embedded portion of the root for a single rooted tooth and at a point just below the furcation for a multi-rooted tooth. www.indiandentalacademy.com
MOMENT: is force acting at a distance. If the line of action of an applied force does not pass through the center of resistance a moment is created. Not only will the force tend to translate the object to a different position, it will also tend to rotate the object around the center of resistance. It is defined as the product of the force times the perpendicular distance from the point of force application to the center of resistance.
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COUPLE: two forces equal in magnitude and opposite in direction. A couple will produce pure rotation, spinning the object around its center of resistance. The combination of force and couple can change the way an object will rotate while it is being moved.
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PURE TRANSLATION It occurs when all points on the tooth move an equal distance in the same direction. This is brought about when the line of action of an applied force passes through the center of resistance of the tooth.
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Pure translation can be of three types: INTRUSION:
translation of the teeth along its long axis in an apical direction EXTRUSION:
translation of teeth along its long axis in an occlusal direction They are axial type of translation and the center of rotation lies at infinity. www.indiandentalacademy.com
Intrusion Intrusion is primarily done for anterior teeth. More rapid intrusion is obtained by light continuous forces than other types of tooth movement. Forces applied must not act for excessively long period if root shortening is to be avoided. A carefully measured intruding force may cause root resorptions, but there may be no visible shortening of the roots. Each anterior tooth may be intruded by forces as light as 20 to 30 gms. This light force produces very short hyalinization period and the teeth will intrude rapidly. www.indiandentalacademy.com
Small resorbed lacunae of the root surface may be observed even with this light forces. This resorption is located between the middle and apical thirds. It occurs as a result of tipping of individual tooth during intrusion. Intruded teeth vary in their reactions according to the magnitude of the force exerted. Generally teeth in young patients are intruded more rapidly and with less tendency to shortening of the apical portion of the root. www.indiandentalacademy.com
Extrusion Extrusion of the tooth involves the more prolonged stretch and displacement of supra alveolar fiber bundles than of the principle fibers of the middle and apical thirds. Some of the principle fibers groups may be subjected to stretch for a certain time as the tooth is moved, but these will rearrange after a short retention period(4 to 5 months). Only the supra alveolar fibers remain stretched for longer, leading to a certain degree of relapse. www.indiandentalacademy.com
ď ŽBODILY MOVEMENT:
translation of teeth in mesiodistal or labio-lingual direction. Bodily movement of a tooth is usually produced from two-point contact of the applied force. It involves moving the tooth parallel to its long axis. Therefore the force is distributed over relatively large areas of the alveolar bone wall.
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When small forces are used, the hyalinised zones that occur will generally be of shorter duration than those seen during tipping movements. The reason for this is that the local forces in these hyalinised zones are smaller, thus allowing resorption of the alveolar bone wall to occur. The tooth movement following such applied forces is quite favorable since there is steady bone resorption as well as steady pdl fibers pull on the tension side.
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Shortly after the movement is initiated, there is no bodily movement in the strict mechanical sense but rather a slight tipping movement. The tooth will be subjected to a couple. The degree of tipping varies according to the size of the arch and the width of the brackets. The result is compression on the pressure side with formation of hyalinised zone between the marginal and middle thirds of the root. Gradually increased stretching on the tension side tends to prevent further tipping. New bone layers are formed along these stretched fiber bundles. www.indiandentalacademy.com
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A light initial force is preferable in initial bodily movements, especially during first 5 to 6 weeks.The optimal magnitude of the force to be applied depends on the resistance exerted by the stretched fiber bundles. During the secondary period, a force within the range of 150- 200gms have proved favourable for bodily movement of premolars and canines..it may,however,become necessary to apply a force of around 300gms during the final closure of spaces to bring the tooth being moved in contact with the anchor tooth. www.indiandentalacademy.com
PURE ROTATION ď ŽA
displacement of the body produced by a couple, characterized by the center of rotation coinciding with the center of resistance, i.e; the movement of points of the tooth along the area of a circle, with the center of resistance being the center of the circle. www.indiandentalacademy.com
Pure rotation can be divided into two types: ď ŽTRANSVERSE ROTATIONS:
tooth displacements during which the long axis orientation changes: a) TIPPING: the simplest type of tooth movement in which the crown moves in one direction and the root in the opposite direction. If a force is applied against the crown of the tooth, and if this force has a one-point contact, then a tipping effect is produced. Tipping tends to concentrate compression on a small periodontal area. Its greatest effects are seen usually at the marginal root area. Local pressure zones and areas of hyalinization are a common occurrence in the marginal regions of the pdl during tipping movements. www.indiandentalacademy.com
The compressive forces generated at the root apex can cause extensive hyalinization and therefore increase the risk for apical root resorption. In clinical situation, tipping movements are often used when moving teeth in a labiolingual direction. The labial and lingual bone plates consist of dense cortical bone, and compensatory apposition of bone at these sites following initial tipping movements is comparatively slow.
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During the secondary period of movement, compensatory bone remodeling is seen in the periosteal surfaces of both the pressure side and the tension side. With an increase in the thickness of the new bone layers formed on the tension side adjacent to the tooth moved, resorption will occur of the old bone on the corresponding periosteal side. This illustrates that there is a tendency for the alveolar plate to maintain its original thickness.
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Tipping movements can be further divided into controlled and uncontrolled tipping: 1.
2.
Uncontrolled tipping: this describes a movement that occurs about a center of rotation that lies close to or apical to the center of resistance. Here the crown moves in one direction and the root in the opposite direction. Controlled tipping:this type of movement occurs when a tooth tips about a center of rotation at its apex. Here the crown moves in one direction but the root is prevented from moving in the opposite direction. www.indiandentalacademy.com
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b) TORQUE : This can be considered as a reverse tipping characterized by lingual movement of the root. The tooth moves about a center of rotation at or close to the incisal edge. Much bone undergoes resorption during this type of tooth movement and so root movements require lots of time. www.indiandentalacademy.com
Torque During the initial movement of torque, the pressure area is close to the middle region of the root. This occurs because the pdl is normally wider in the apical third than in the middle third. After resorption of the bone areas corresponding to the middle third, the apical surface of the root will gradually begin to compress adjacent pdl fibres and a wider pressure area will be exerted. www.indiandentalacademy.com
ď ŽLONG
AXIS ROTATION: Here the orientation of the long axis is not altered. The tooth rotates about its center of resistance. Here the center of rotation is the long axis of the tooth.
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The tissue transformation that occurs during rotation is influenced by the anatomic arrangement of the supporting structures. Various factors are involved in the movement of rotation. The anatomic factor is primarily related to the position of the tooth, its form,and its size. Except for the upper centrals and the lower premolars, most teeth have an oval root form. This implies that during rotation, a parallel movement between the root and bone surface takes place mainly on the buccal and lingual sides of the root. In practice most teeth to be rotated will create two pressure sides and two tension sides. www.indiandentalacademy.com
Rotation might cause variations in the type of tissue response observed on the pressure sides. Hyalinization and undermining bone resorption take place in one pressure zone while direct bone resorption occurs in other. It is favourable to apply a light force during the initial period. After rotation for 3-4weeks, undermining resorption is complete and direct bone resorption prevails on the pressure side. Root resorptions may occur on one side of the pressure sides and frequently on both pressure sides, but the resorbed lacunae of the root will be repaired over the retention period of 6-8 weeks. www.indiandentalacademy.com
On the tension side, new bone spicules will be formed along stretched fiber bundles arranged more or less obliquely. This stretch of the pdl fibers coincide with the formation of cellular cementum along the root surface. Very little cementum will be seen on pressure side. In the apical region less new bone will be formed during rotation, but some fiber groups are frequently elongated and arranged obliquely. www.indiandentalacademy.com
The method of treatment influences the final result of the rotating tooth. If the tooth is moved interruptedly with a light force that acts over a certain distance, and then held in position by the appliance until reactivation, more fiber bundles will be rearranged during the treatment period. The relapse tendency will be markedly reduced in such conditions. The degree of relapse is especially pronounced when the tooth is rotated rapidly with a typical continuous force. www.indiandentalacademy.com
COMBINATION OF BOTH Any movement that is not pure rotation or translation can be termed a combination of both translation and rotation. This type of movement is often seen in routine clinical practice.
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OPTIMAL FORCES FOR ORTHODONTIC TOOTH MOVEMENTS Type of movement
Tipping Translation Root uprighting Rotation Extrusion Intrusion
force( gms) 35-60 70-120 50-100 35-60 35-60 10-20
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MECHANICAL PRINCIPLES IN FORCE CONTROL
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FORCE Force is the load applied to an object that will tend to move it to a different position in space. It is the application of a force that will bring about orthodontic tooth movement. A force is a vector, and is defined by the characteristics of a vector. Vectors have both magnitude and direction. Magnitude represents its size and the direction its line of action, sense and point of origin. Forces in orthodontics exhibit what is known as the principle of transmissibility. This principle says that the external effect of a force acting on tooth is independent of where along its linewww.indiandentalacademy.com of action the force is applied.
When two forces are acting at the same point, the total effect of the two can be represented as the resultant force and can be determined by the parallelogram of forces.
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When there are more than one force systems acting on a body,then they can be divided into 1. Co-planar 2. Non co-planar, depending on the plane of action. They can be further broken down into Concurrent and non-concurrent force systems , depending on whether all the forces of the system intersect at a common point. If they do then it is a concurrent system, if not then it is non-concurrent. www.indiandentalacademy.com
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SIGN CONVENTION
A positive sign is given to all crown movement in • • • • •
Mesial Labial or buccal Anterior Lateral Extrusive direction
A negative sign to
• • • • •
Distal Lingual or palatal Posterior Medial Intrusive movements
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FORCES, MOMENTS, AND COUPLES IN TOOTH MOVEMENTS
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A moment is a measure of the tendency to rotate. A moment is produced in one of two ways. If a single force is applied to a body that does not act through the center of resistance, the force causes the tendency for the body to rotate. This moment, the moment of force (Mf), is quantitatively equal to the magnitude of the applied force times the perpendicular distance between the line of the applied force and center of resistance. Mf is increased equally by either applying a larger force to the tooth or applying the force further away from the center of resistance. www.indiandentalacademy.com
A moment can also be applied to a tooth with a couple,called moment of couple (Mc). The magnitude of Mc is equal to the value of one of the forces of the couple times the perpendicular distances between the two parallel forces. The magnitudes of Mc is increased by either increasing both of the forces of the couple or increasing the distance between the two forces
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To produce different types of tooth movements, it is necessary that the ratio between the applied moment and force on the crown be altered. As the moment force ratio is altered so the center of rotation will be changed. There are few instances in which desirable types of tooth movement can be produced by single forces applied to the crown alone. If this is done, the root will move in the opposite direction.
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The m/f determines the control that an orthodontic appliance will have on both active and reactive units.
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EQUIVALENT FORCE SYSTEMS The application of forces and couples in orthodontics is at the brackets and not at the center of resistance. It is impractical to place forces and moments at the centers of resistance, instead an equivalent force system can be placed on the tooth at the brackets or tube. If two forces are to be equivalent then the sum of all the forces and moments of each system should be equal to that of the second system. www.indiandentalacademy.com
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The control of root position during movement requires both a force to move the tooth in the desired direction, and a couple to produce the necessary counter-balancing moment for control of root position. The simplest way to determine how a tooth will move is to consider the ratio between the moment created when a force is applied to the crown of a tooth(Mf) and the counterbalancing moment generated by a couple within the bracket (Mc). www.indiandentalacademy.com
Mc/Mf = 0
Pure tipping
0<Mc/Mf < 1
Controlled tipping
Mc/Mf = 1
Bodily movement
Mc/Mf > 1
Torque
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The distance from the point of force application to the center of resistance can and does vary, so the moment to force ratios have to be adjusted if root length, amount of alveolar bone support or point of force application differs from the usual condition. It is because of this that the Mc/Mf ratio is believed to give a more precise description of how a tooth will respond. www.indiandentalacademy.com
SYSTEM EQUALIBRIUM
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Newtonâ&#x20AC;&#x2122;s third law of motion states that for every action there is equal and opposite reaction. The single forces and couples of orthodontic appliances are no exceptions. Static equilibrium requires that the sum of both the forces and moment acting on an appliance in any plane must be equal to zero to maintain the system in equilibrium. Each moment must be opposed by an equal and opposite tendency to rotate in the opposite direction. www.indiandentalacademy.com
Force system can be defined as statically determinate, meaning that the moments and forces can be readily discerned, measured and evaluated, or as indeterminate. Statistically indeterminate systems are too complex for precisely measuring all forces and moments involved in the equilibrium www.indiandentalacademy.com
Determinate systems in orthodontics are those in which a couple is created at one end of an attachment, with only a force and no couple at the other. When the wire is tied into a bracket on both ends, a statically indeterminate two couple system is created. The determinate force systems are advantageous in orthodontics because they provide much better control of the magnitude of forces and couples. www.indiandentalacademy.com
One couple systems In orthodontics one couple systems are found when two conditions are met. 1)Â Â A cantilever or auxillary arch wire is placed into a bracket or tube. 2) The other end of the spring or auxillary arch wire is tied to a tooth or a group of teeth that are to be moved, with a single point of force application.
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Two couple system When a wire is placed into two brackets the forces of equilibrium always act at both brackets. There are three possibilities for placing a bend in the wire to activate it. 1.Symmetric V bends, which creates equal and opposite couples at the brackets. The forces at each bracket are equal and opposite, and therefore cancel each other out. A symmetrical V bend is not necessarily half way between two teeth or two groups of teeth. www.indiandentalacademy.com
If two teeth are involved but one is bigger than the other, equal and opposite moments would require placing the bend closer to the large tooth, to compensate for the longer distance from the bracket to its center of resistance. The same would be true if two groups of teeth had been created by tying them into the equivalent of a single large multi-rooted tooth, as when posterior teeth are grouped into a stabilizing segment and used for anchorage to move a group of for incisors. www.indiandentalacademy.com
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2. Asymmetric V bend, which creates unequal and opposite couples, and net equilibrium forces that would intrude one unit and extrude the other. Although the absolute magnitude of the forces involved cannot be known with certainty, the relative magnitude of the moments of the associated equilibrium forces can be determined. The bracket with the larger moment will have a greater tendency to rotate than the bracket with the smaller moment, and this will indicate the direction of equilibrium forces. www.indiandentalacademy.com
As the bend moves closer to one of the two equal units, the moment increases on the closer unit and decreases on the distant one, while the equilibrium forces increase. When the bend is located 1\3rd of the distance along the wire between two equal units no moment is felt at the distant bracket, only a single force. When the bend moves closer than that to one bracket, moments at both brackets are in same direction and equilibrium forces increases further. www.indiandentalacademy.com
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3. Step bend which creates two couples in the same direction regardless of its location between the two brackets. The location of a V bend is a critical variable in determining its effect, but the location of a step bend has little or no effect on either the magnitude of the moments or the equilibrium forces. www.indiandentalacademy.com
LOAD DEFLECTION RATE
A characteristic of an ortho appliance, the load deflection or torque â&#x20AC;&#x201C; twist rate, is involved in the delivery of a constant force. By definition the load deflection rate gives the force produced per unit activation. For a tooth moving under a continuous force, as the load-deflection rate becomes lower the change in force value is reduced. With regard to active members a low load-deflection rate is desirable for two important reasons. www.indiandentalacademy.com
A mechanism with low L-D rate will maintain a more desirable stress level in the pdl. Also a low L-D rate offers greater accuracy in control over force magnitude. If a low L-D rate is desirable for an active member then the opposite is true for the reactive member. The reactive member should be relatively rigid; that is it should have a high L-D rate. The anchorage potential of a group of teeth can be enhanced if the teeth displace as a unit. www.indiandentalacademy.com
ELASTIC MATERIALS
properties
and
The elastic behavior of any material is defined in terms of its stress-strain response to an external load. Both stress and strain refer to the internal state of the material being studied.
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STRESS: is the internal distribution of the load, defined as force per unit area, STRAIN: is the internal distortion produce by the load, defined as the deflection per unit length. When a force is applied to an appliance, its response can be measured as deflection produced by the force, which is bending or twisting.
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For orthodontic purposes three major properties of materials are critical in defining their clinical usefulness: 1. 2. 3.
Strength Stiffness/springiness Range.
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Strength Three different points on a stress-strain diagram can be taken as representatives of the strength of a material. 1. Proportional limit: the point at which any permanent deformation is first observed.
2. Yield strength: the point at which a deformation of 0.1% is measured.
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3. Ultimate tensile strength: the maximum load the wire can sustainâ&#x20AC;Śthis point is reached after the permanent deformation and is greater than the yield strength. 2
Strength is measured in stress units (gms/cm )
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Stiffness and springiness are reciprocal properties. Each is proportional to the slope of the elastic portion of the force-deflection curve. The more horizontal the slope, the springier the wire, and the steeper the slope, the stiffer the wire.
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Range is defined, as the distance the wire will bend elastically before permanent deformation occurs. It is measured in millimeters or any length units. If the wire is deflected beyond its yield strength, it will not return to its original shape, but clinically useful spring back will occur unless the failure point has been reached. In many cases orthodontic wires are deformed beyond their elastic limit. Their spring back properties in the portion of the load- deflection curve between the elastic limit and the ultimate strength are important in determining the clinical performance. www.indiandentalacademy.com
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These three major characteristics are related by the formula Strength = Stiffness x Range. Two other characteristics of clinical importance can also be described on the stress- strain: Resiliency: is the area under the stress- strain diagram upto the proportional limit. It represents the energy storage capacity of the wire, which is a combination of strength and springiness. Formability: is the amount of permanent deformation that a wire can withstand before failing. It represents the amount of permanent bending the wire will tolerate www.indiandentalacademy.com before it breaks.
FACTORS AFFECTING ELASTIC PROPERTIES
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Material   Precious metal alloys : are the first used materials for orthodontic purposes, primarily because nothing else could tolerate the intra-oral conditions. The introduction of stainless steel in the 1970s made the use of precious alloys obsolete.
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Stainless steel and cobalt 窶田hromium alloys : both these metals have considerable higher strength and springiness along with equivalent corrosion resistance compared to the precious metal alloys and so replaced them in orthodontic practice. The properties of these steel wires can be controlled over a reasonably wide range by varying the amount of cold working and annealing during manufacture.
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Stainless Steel is softened by annealing and hardened by cold working. Elgiloy, the cobalt-chromium alloy, has the advantage that it can be supplied in a softer and therefore more formable state, and then can be hardened by heat treatment after being shaped.
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Nickel-titanium (NiTi) alloys. Has proved very useful in clinical orthodontics because of its exceptional springiness. Niti alloys have two remarkable properties that are unique in dentistry---shape memory and super elasticity. Shape memory refers to the ability of the material to “remember” its original shape after being plastically deformed while in the martensitic form.
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Nitinol was marketed in the late 1970â&#x20AC;&#x2122;s for orthodontic use in a stabilized martensitic form, with no application of phase transition effects. Nitinol is exceptionally springy and quite strong but has poor formability. In the late 1980â&#x20AC;&#x2122;s new nickel-titanium wires with an active austenitic grain structure appeared. These wires exhibit the other remarkable property of niti alloys--super elasticity. This group subsequently is referred to as A-NiTi. www.indiandentalacademy.com
Over considerable range of deflection, the force produced by A-Niti hardly varies. This means that an initial arch wire would exert about the same force whether it was deflected a relatively small or a large distance, which is a unique and extremely desirable characteristic. The unique force-deflection curve for A-NiTi wire occurs because of a phase transition in grain structure from austensite to martensite, in response not a temperature change but to applied force. The transition is a mechanical analogue to the thermally induced shape memory effect www.indiandentalacademy.com
Beta-Titanium: In the early 1980’s, after nitinol but before A-NiTi, Beta-Ti material (TMA) was developed primarily for orthodontic use. It offers a highly desirable combination of strength and springiness as well as reasonably good formability. This makes it an excellent choice for arch wires, especially rectangular wires, for the late stages of edgewise treatment.
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Effects of size and shape Each of the major elastic properties â&#x20AC;&#x201C;strength, stiffness and range-is substantially affected by the change in the geometry of a beam. Both the cross section and the length are of great significance in determining its properties. Changes related to size and shapes are independent of the material.
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Diameter: doubling the diameter of the wire increases its strength by 8 times, i.e; the large wire can resist 8 times as much force before permanently deformed,or can deliver 8 times as much force. Doubling the diameter, however, decreases springiness by a factor of 16 and range by a factor of 2.
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Length and attachment: If the length of a cantilever spring is doubled, its bending strength is cut in half, but its springiness increases 8 times and its range 4 times. Length changes affect torsion quite differently from bending: springiness and range in torsion increase proportionally with length, while torsional strength is not affected by length.
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The way in which a beam is attached also affects its properties. An arch wire can be tied tightly or loosely, and the point of loading can be any point along the span. A supported beam like an arch wire is 4 times as springy if it can slide over the abutments rather than if the beam is firmly is attached. With multiple attachments, as with an arch wire tied to several teeth, the gain in springy from loose ties of an initial arch wire is less dramatic but still significant.
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CONCLUSION
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The design of efficient orthodontic appliance does not occur by trial and error. Instead, an approach based on sound biologic and physical principles leads to development of appliances with predictable actions. We should be able to define and quantify forces, moments, couples and equilibriums associated with appliances. If the force systems acting on a tooth cannot be defined, their effect on cells and tissues will be difficult to understand. Biomechanics thus analyses the reaction of dental and facial structures to orthodontic forces. www.indiandentalacademy.com
Many variables affect the outcome of orthodontic treatment. Some are partially or totally out of the clinicians control such as growth, bone-pdl-gingival responses, and neuromuscular adaptation to changes in jaw and tooth positions. Factors that are in the control of the clinician are the magnitude and direction of the forces, couples,moments and moment to force ratio exerted by the appliance. A thorough understanding of the physical principles operating in orthodontic appliances eliminates appliances as an uncontrolled variable affecting the final result. www.indiandentalacademy.com
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SEGMENTED ARCH MECHANICS This is considered an organized approach to using one couple and two couple systems for most tooth movement so as to obtain both more favorable force levels and better control. The essence of the segmented arch system is the establishment of welldefined units of teeth, so that anchorage and movement segments are clearly defined. www.indiandentalacademy.com
The desired tooth movement is accomplished with cantilever springs where possible, so that the precision of one couple approach is available, or with the use of two couple systems through which at least net movements and the directions of equilibrium forces can be known.
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Typical segmented arch treatment would call for initial alignment within posterior and anterior segments, the creation of appropriate anchorage and tooth movement segments, vertical leveling, space closure with differential movement of anterior and posterior segments, and perhaps the use of auxillary torquing arches.
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The advantages of the segmented arch approach are the control that is available, and the possibility of tooth movements that cannot be achieved with continues arch wires. The disadvantage is the greater complexity of the appliance, and the greater amount of time needed to install, adjust and maintain it.
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CONTINUOUS ARCH MECHANICS Continuous is one that is tied into the brackets on all the teeth. An extremely complex multicouple force system is established when the wire is tied into place. In general, the mechanical efficiency of a continuous arch wire system is less than that of a segmented system, but its fail â&#x20AC;&#x201C; safe properties are better.
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The advantages and disadvantages are just the reverse of those with segmented arch approach. Continuous arch treatment is not as well defined in terms of forces and moments that will be generated at any one time. But continuous arch wires often take less chair time because they are simpler to make and install, and because they have excellent fail-safe property in most applications.
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Required moment to force ratios for different types of tooth movement: •
Uncontrolled tipping
0:1 to 5:1
•
Controlled tipping
7:1
•
Translation
10:1
•
Root movement/ torque
12:1
•
Rotations
(net forces at the center of resistance is nil, only a couple is seen) www.indiandentalacademy.com
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