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BIOLOGY OF TOOTH MOVEMENT

YASMIN MOIDIN 2008 Batch

Al Azhar Dental CollegeThodupuzha

INTRODUCTION

Orthodontic tooth movement (OTM) is a complex biomechanical process which is initiated by the clinician with the application of a force. The applied force moves the tooth beyond its range of physiologic tooth movement.

Several factors affect and modify the nature and amount of orthodontic tooth movement. The most significant mechanical factors are: magnitude, direction and nature of the force. The inherent biological factors include bone density, age of the person, systemic health, hormones and factors that influence the bone turnover.

PHYSIOLOGIC TOOTH MOVEMENT

Physiologic tooth movement 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.It is of three types:

1. Movement during mastication2. Eruption of tooth3. Tooth migration

MOVEMENT DURING MASTICATION

Tooth movement during masticatory function depends upon the location of neutral axis of the functioning tooth.

Neutral axis is located between the middle and apical regions of the roots in an adult tooth.

For younger persons,the neutral axis is either located in the marginal region or closer to the middle of the root ,if the root is fully developed.

During chewing , the teeth tips slightly around the neutral axis as fulcrum.

Tooth is displaced because of bending of the alveolar process

Movement during mastication is transient. Once the occlusal load is removed, it reverts back to normal position

ERUPTION OF TOOTH

Different teeth move in different directions during eruption

During eruption, upper molar teeth move mainly in mesial direction

Lower molar teeth show variations in direction of movement. Sometimes even a distal direction of movement is observed

Premolars sometimes show lingual movement during eruption

MIGRATION OF TEETH

Migration of teeth is a slow tooth movement

Direction of movement is usually mesial and occlusal

This corresponds to the adult equilibrium stage of tooth eruption

These movements take place to compensate interproximal attrition and occlusal wear

PERIODONTAL LIGAMENT STRUCTURE AND FUNCTION

Each tooth is attached to and separated from the adjacent alveolar bone by a heavy collagenous supporting structure , the periodontal ligament (PDL). The width of the PDL is approximately 0.5mm.

STRUCTURE OF PERIODONTAL LIGAMENT

COMPONENTS OF PERIODONTAL LIGAMENT

1. Collagen fibres 2. Cellular elements 3. Tissue fluids

COLLAGEN FIBRES

The major component of the PDL is a network of parallel collagenous fibers, inserting into cementum of the root surface on one side and into a relatively dense bony plate, the lamina dura,on the other side

These supporting fibers run at an angle, attaching farther apically on the tooth than on the adjacent alveolar bone

This arrangement resists the displacement of the tooth expected during normal function

CELLULAR ELEMENTS

The principle cellular elements in the PDL are undifferentiated mesenchymal cells and their progeny in the form of fibroblasts and osteoblasts

Nerve endings are found within the ligament, both the unmyelinated free endings associated with perception of pain and the more complex receptors associated with pressure and positional information

TISSUE FLIUDS

The PDL space is filled with fluid and is derived from the vascular system

Tissue fluids acts as a shock absorber

RESPONSE TO NORMAL FUNCTION

During masticatory function, the teeth and periodontal structures are subjected to intermittent heavy forces. Tooth contact lasts for one second or less, forces are heavy and tooth is subjected to heavy loads, quick displacement of the tooth within the PDL space is prevented by the incompressible tissue fluid and the force is transmitted to the alveolar bone ,which bends in response and formation of piezoelectric signals.

. Pain is normally felt after 3 to 5 seconds of heavy force application ,indicating that the fluids are expressed and crushing pressure is applied against the PDL in this amount of time. The resistance provided by tissue fluids allows normal mastication with its force applications of 1 second or less, to occur with out pain. Orthodontic tooth movement is made possible by application of prolonged forces.

PHYSIOLOGIC RESPONSE TO HEAVY PRESSURE AGAINST A TOOTH

TIME (SECONDS) EVENT

LESS THAN 1 PDL fluid incompressible, alveolar bone bends, piezoelectric signal generated

1 - 2 PDL fluid expressed, tooth moves with in PDL space

3 – 5 PDL fluid squeezed out, tissues compressed;immediate pain if pressure is heavy

ROLE OF PDL IN ERUPTION AND STABILISATION OF THE TEETH

The phenomenon of tooth eruption makes it plain that forces generated within the PDL itself can produce tooth movement

The eruption mechanism appears to depend on metabolic events with in the PDL including but perhaps not limited to formation, cross-linkage and maturational shortening of collagen fibres.and it continues at a reduced rate into adult life

This mechanism also indicates active stabilization of the teeth against prolonged forces of light magnitude

Active stabilization implies a threshold for orthodontic force

THEORIES OF TOOTH MOVEMENT

Two major theories are:1. The bioelectric theory2. The pressure-tension theory

BIOELECTRIC THEORY

The bioelectric theory relates tooth movement at least in part to changes in bone metabolism controlled by the electric signals that are produced when alveolar bone flexes and bends. This bending and flexing generates electric signals that alter the metabolism of bone.

ELECTRICAL SIGNALS GENERATED ARE:

1. PIEZOELECTRICITY2. STREAMING POTENTIAL3. BIOELECTRIC POTENTIAL

PIEZOELECTRICITY

It is a phenomenon observed in crystalline materials in which deformation of a crystal structure produces a flow of electric current as electrons from one part of the crystal lattice are displaced to another. Bone and collagen and stress generated potentials in dried bone specimens have piezoelectricity

FEATURES OF PIEZOELECTRICITY

1. Quick decay rate:- When a force is applied a piezoelectric signal is created in response that quickly dies away to zero even though the force is maintained

2. The production of an equivalent signal, opposite in direction when the force is released

ON

OFF

Seconds

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0 1 2 3 4

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STREAMING POTENTIAL

Ions in the fluids that bathe living bone interact with the complex electric field generated when the bone bends, causing temperature changes as well as electric signals. The small voltages that are observed are called streaming potential.

BIOELECTRIC POTENTIAL

Application of orthodontic force by the appliance will cause physical distortion of the alveolar bone which is accompanied by bending of bone . Bone which is deformed by stress becomes electrically charged

Concave surfaces take a negative polarity and convex surfaces a positive polarity

PRESSURE TENSION THEORY

Alterations in blood flow associated with pressure with in the periodontal ligament

Formation and/or release of chemical messengers

Activation of cells

Orthodontic force

Tissue trauma

Release of Ist messengers (PG)

(Extracellular signals are activated)

Conversion into intracellular signals by 2 pathways

IInd messengers

Protein kinase enzymes within the cell

Cellular changes

Remodeling of bone

Synthesis of cAMP

Activation of Ca++

IIIrd messengers

TISSUE REACTIONS TO ORTHODONTIC FORCES

Tissue reactions to orthodontic forces were first described by Sandstedt in 1904,1905 and later by Oppenheim in 1930,1935,1936

Sequence of changes – orthodontic force

Light continuous force

Compression of blood vessels + PDL

Blood flow altered

Prostaglandins (Ist messenger) are released

Synthesis of cyclic AMP activation of Ca++

Metabolic activity

Activation of osteoclasts

TISSUE CHANGES AT PRESSURE ZONE

The bone was deposited on the tension side of the tooth both with heavy and light forces while on the pressure side with light forces alveolar bone was resorbed directly by multinucleated osteoclast cells called frontal resorption or direct resorption

With the application of heavy forces, the periodontal tissues are compressed leading to a cell free zone called the hyalinised tissue, which occurs due to thrombosis of vessels and cell death. on histologic sections, this zone resembles hyaline connective tissue and the process is called hyalinisation

The ideal orthodontic force should not exceed the capillary pulse pressure ,which is about 20-26gm/cm2

In hyalinised areas ,resorption of the alveolus takes place far from the cell free zone in the bone marrow spaces and is called undermining resorption or rear resorption

Tooth movement is delayed because of hyalinization and undermining resorption and the reasons are :-

differentiation and activation of osteoclasts from marrow space take more time

the thickness of bone to be removed from the underside is more

TISSUE CHANGES AT TENSION ZONE

Cellular activity is delayed in areas of tension when compared to pressure zones

It takes 30 hours for increased cellular activity to be seen in tension zone

The stretched periodontal fibers are reconstructed by changes of the original fibrils

Macrophages are found in great numbers in tension zone

There is inflammatory like breakdown and rebuilding of fibrous elements in areas of tension

New unmineralised matrix is laid down around the parts of the fibers that are close to the alveolar wall

After sometime ,osteoid is laid on the whole of the alveolar wall on the tension side

Osteoblasts synthesize the osteoid ,subsequently mineralization of osteoid takes place

Rate of bone deposition is about 30micro meter/day

CONCLUSION

Orthodontic tooth movement consequent to application of force is outcome of complex chains of events ,eventually leading to bone resorption and bone formation

REFERENCES

Contemporary Orthodontics Fourth Edition – WILLIAM R PROFFIT

Orthodontics Diagnosis and Management of Malocclusion and Dentofacial Deformities - OM PRAKASH KHARBANDA

Orthodontics – Exam Preparatory Manual for Undergraduates Second Edition – SRIDHAR PREM KUMAR

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