INTRODUCTION

Periodontium

Is defined as the connective tissue organ that attaches the teeth to

bone of jaws and provides a continually adapting apparatus for the support

of teeth during function.

Periodontium comprises of four connective tissue

Two of which are mineralized - Cementum

- A.B.

Two are fibrous - Periodontal ligament

- L.P of gingiva

Periodontal ligament

periodontal ligament is the connective tissue that surrounds the roots

and connects the tooth to the bone.

- It is continuous with the connective tissue of the gingiva and

communicates with the marrow and space this vascular channels in

the bone.

- In a healthy tooth it covers the root up to the cemento-enamel

junction.

- In radiograph periodontal ligament appears as a radiolucent line

paralleling the root surface.

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- Lamina dura in radiograph appear as a radio-dense line which

represent the alveolar bone fibres.

- The average width of periodontal ligament is about 0.15 to 0.38mm.

- periodontal ligament is thinnest in middle portion of root and its

width decrease with age.

At age of 11-16 years 0.21mm.

32-52 years 0.18mm

52-67 years 0.15mm

Periodontal space:

This space lies between two hard tissue i.e. cementum and bone

within this space periodontal ligament is present.

Its rot measures approximately 30mm3 – 100mm3 single root

65-150mm3 multirooted teeth

- The width of periodontal space varies from end to end and from

tooth to tooth and on different location on same tooth.

- With increase in function space widens and with loss of function

thinning and atrophy of fibres takes place.

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Synonyms:

- The other name used for periodontal ligament are Desmodont

Ghomphosis Pericementum, Alveodental ligament; periodontal

membrane, Dental periosteum.

- Most commonly used are periodontal ligament and periodontal

membrane, both neither of them/well describes structure and

function.

- Periodontal ligament is most appropriate term as it provides

continuously between two mineralized tissues bone and

cementum.

Development

The periodontal ligament forms from dental follicle soon

after root deviation begins.

Formation of periodontal ligament starts, after Hertwig’s

epithelial rest sheath cells are separated forming strands

known as epithelial cell rest’s of malassez.

This separation permits the cells of dental follicle to migrate

to the external surface of newly formed root dentine.

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These cells show high degree of mitotic activity, thus

differentiating into different types of cells which give rise to

cells of periodontal ligament namely:

Cementoblast deposit cementum on deviated root.

Osteoblast of the developing alveolar bone.

Fibroblast to synthesize collagen and ground substance of

periodontal ligament.

The fibroblasts cells are oriented obliquely at early stages and

thus fibres secreted by them have the same orientation.

These obliquely oriented fibre bundles get entrapped in bone

and cementum as they are deposited and this give rise to

principal fibres of periodontal ligament.

Before eruption Alveolar bone crest is above cemento-

enamel junction thus fibres are laid down obliquely in a

coronal direction giving rise to oblique fibres.

As eruption begins Alveolar crest coincides with the

cemento-enamel junction and fibres are aligned horizontally.

When the tooth eruption is completed and it is in full

function, alveolar crest below cemento-enamel junction near

apex thus fibres are again aligned obliquely but in an

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opposite direction to first laid fibre bundles, thus forming

alveolar crest fibres.

As the tooth is in function, fibres begin to thicken and are remodeled

constantly over a period of time.

Evolution:

There is fundamental difference between Repitilian and Mammalian

teeth attachment.

In reptiles teeth are ankylosed to bone.

In mammals teeth are suspended in sockets by ligament.

The central point of these changes is “reconstruction of mandible”.

In reptiles mandible consisting of several bones united by sutures.

In mammals the newly acquired cartilage of condyle is the most

important growth site for mandible.

In reptiles the mandibular teeth more with the bones to which they

are fused but in mammals they move as independent units and this

movement is made possible by remodeling of periodontium.

The evolutionary change from reptiles to mammals replaces the

ankylosis of tooth and bone to a ligamentous suspension of the

tooth.

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Structure

- Cellular

- Extracellular

- connective tissue Components

Cellular

Periodontal ligament comprise of three groups of cells. These are:

- Synthetic cells.

- Resorptive cells.

- Progenitor cells.

Apart from these periodontal ligament comprises of :

- Epithelial cells – Epithelial cell rests of malassez.

- Defence cells derived from hemopoietic line.

Synthetic cells:

These include:

- Fibroblast.

- Osteoblast.

- Cementoblast.

Fibroblast

These cells are the most numerous and densely packed in

periodontal ligament.

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They appear as ovoid or flattened with numerous cytoplasmic

processes.

Shape of these cells vary according to the orientation of fibres,

where active fibres and fibroblasts are densely packed cells

appear elongated along the fibre length.

In the loose connective tissue cells are ovoid, or spindle shaped.

These cells are surrounded by fibres and ground substance.

periodontal ligament fibroblasts are attached to one another by

a. gap junction.

b. Simplified desmosomes.

They are involved in both synthesis and resorption of collagen

fibres.

Osteoblasts:

These cells are found on surface of alveolar bone in periodontal

ligament.

They constitute modified endosteum instead of periosteum as an

outer layer i.e. outer layer is cellular instead of fibrous as seen in

osteoblasts found elsewhere in body.

A prominent nucleus lies towards the base of these cells.

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Collagen fibres of periodontal ligament penetrating the bone

intervene between these cells.

Cementoblasts

These cells resemble osteoblasts.

These cells are found in portion of dental follicle adjacent to root

and at the boundary between the periodontal ligament and tooth

surface.

These cells have a diameter of about 8-12µm and display several

cytoplasmic processes.

Resorptive cells

- Osteoclasts.

- Fibroblasts / fibroclasts.

- Cementoclasts.

Osteoclasts

These are the bone resorbing cells and tend to be large and

multinucleating but may be small and mononuclear.

Derived from circulating monocytes.

Under light microscope These are present in Howship’s lacunae.

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Under electron microscope exhibit numerous mitochondria,

lysosomes, Golgi sacules, few ribosomes and few rough

endoplasmic reticulum.

The part of plasma membrane of these cells adjacent to bone is

continually resorbed and has characteristic folds giving rise to

ruffled or striated border.

This border is separated from the rest of plasma membrane by a

zone of specialized membrane devoid of organelles known as clear

zone.

Resorption of bone margin occurs in two stages:

1. Remineralization of bone margin.

2. Disintegration of orgenic matrix.

Osteoclasts are rich in acid phosphatase (present in

lysosomes).

The presence of osteoclasts in periodontal ligament indicates

active resorption or previous resorption in that area.

Fibroclasts

Collagen fibres of the mammalian periodontal ligament are resorbed

under normal physiologic conditions by mononuclear fibroblasts.

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These cells contain lysosomes – which bring about

resorption.

There presence indicate active resorption occurring during

physiologic turnover or remodeling of periodontal ligament.

Cementoblasts:

Resembles osteoclasts and are occasionally found in

periodontal ligament.

Origin is unknown.

Resorption of cementum may take place under certain

circumstances and in such instances these cells are present in

Howships lacunae on the surface of cementum.

Progenitor cells:

Like all connective tissues periodontal ligament also contain

progenitor cells or undifferentiated mesenchymal cells that undergo

mitotic division.

But very little is known about progenitor cells of periodontal

ligament.

Such as it is not known whether a single population of

progenitor cells give rise to all the synthetic cells of periodontal

ligament or there are a number of population.

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There presence is evident from mitosis occurring after the

application of a pressure such as in orthodontic treatment and after

wounding.

These cells lie close to blood vessels.

These cells have a close-faced nucleus and very less

cytoplasm.

Epithelial Cell Rests of Malassez

Periodontal ligament contains epithelial cells that lie close to

cementum.

Described by malassez in 1884.

They are remnants of Hertwig’s epithelial rest sheath that are

embedded in periodontal ligament during deviation.

Under electron microscope these cells exhibit tonofilaments

and are attached to each other by desmosomes.

Role of these cells under normal physiologic conditions is

unknown, but under pathologic condition. These cells may

proliferate to give rise to cysts such as periapical and lateral root

cysts and tumours of jaws.

Seen in form of islands / clumps / tubules.

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Defence cells

- Mast cells.

- Macrophages.

- Eosinophils.

Mast cells

These cells are sometimes seen in periodontal ligament.

These are small round / oval in shape about 12-15 µm in dia.

Contain numerous cytoplasmic granules such as heparin,

serotonin, histamine

Granules have a dia-0.5-1µm and are membrane bound.

These granules play a role in inflammation reaction

especially histamine.

Cells degenerate in response to Ag-Ab reaction.

Macrophages

These cells may also be present in the ligament.

These are delivered from blood monocytes.

These cells need to be differentiation from fibroblasts.

Fibroblasts contains phagocyted mat in their cytoplasm.

Macrophages have a nucleus with regular contour.

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Cell surface is raised because of microvilli.

Cytoplasm has numerous free ribosomes, lysosomes.

Less of RER, and Golgi apparatus not well

developed.

Relationship between cells

Cells of periodontal ligament form a three dimensional

network and there processes surround collagen fibres and these are

attached to one another and to cells of bone, cementum by gap

junction.

Extracellular substances:

Fibres Ground substance

- Collagen - Glycosaminoglycans

- Oxytalan - Glycoproteins

- Elavanin

Fibres

Collagen

These comprise the majority of fibres of periodontal

ligament.

Collagen is a specific high molecular weight protein.

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Protein macromolecules are clustered to form fibrils which

are packed to form fibres.

These fibres are gathered into bundles having clear orientation

relative to periodontal space and are known as principal fibres of

periodontal ligament.

Collagen are of five types periodontal ligament consists of

Type I collagen predominantly and a little of type III collagen.

Principal fibres

These comprise of five groups of fibres.

1. Alveolar crest fibres:

Alveolar crest fibres extend obliquely from cementum below the

functional epithelium to the alveolar crest.

Function – these fibres prevent extrusion and lateral tooth movement of

tooth. There incision does not significantly increase tooth mobility.

2. Horizontal group

Restricted to coronal ½ of periodontal ligament.

These fibres run horizontally at right angles to long axis of

tooth from cementum to alveolar bone.

Function: prevent lateral tooth movement

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3. Oblique group

Occupy middle 2/3rd of periodontal ligament.

These fibre bundles account for the main attachment of tooth.

These fibres extend in a coronal direction obliquely from cementum

to bone.

Function:

These fibres bear the forces of vertical masticatory stresses and

transform them into tension on the alveolar bone.

4. Apical group

These fibres are irregularly arranged and radiate from cementum to

bone at the fundus of socket.

Function: Resist forces of luxation, prevent tooth tipping.

Provide for protection of blood vessels and nerves of periodontal

ligament.

5. Inter-radicular group:

From the crest of inter-radicular septum, bundles extend to furcation

of multi-rooted teeth.

Function:

Resist tooth tipping, torque and luxation of tooth.

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Transeptal group – These fibres extend interproximally over the alveolar

crest to be embedded into the cementum of adjacent teeth.

Collagen fibres are embedded on one side into cementum and on the

other into bone. These embedded fibres are known as “Sharpey’s

fibres”.

The principal fibres run a wavy course from cementum to bone.

These fibres appear to join in mid-region of ligament giving rise to a

zone of distinct appearance known as “Intermediate plexus”.

Previously considered to be zone of remodeling this zone is an

artifact as seen under electron microscope which arises due to plane

of sectioning.

Oxytalan:

This is an immature elastic fibre restricted to walls of blood vessels.

Their orientation differ from that of principal fibres.

These fibres run axially i.e. one end is in either bone/cementum and

other in blood vessels.

They from a complex network close to apex.

Function is unknown but may play a role in supporting blood

vessels of periodontal ligament.

Eluanin:

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This is the other form of immature elastic fibres and along with

collagen fibres it forms a meshwork extending from cementum to

bone and sheathing the collagen fibre bundles.

Ground substance

The space between cells, fibres, blood vessels, nerves in periodontal

ligament is occupied by ground substance.

Function Exchange of metabolites between microcirculation and cells.

Enables tooth to withstand stress along with fibres.

Comprises of 70% water

Two major components are:

Glycosaminoglycans Proteoglycans and Hyaluronic acid.

Glycoproteins Fibronectin and laminin.

Both components comprise of proteins and polysaccharides

but of different nature and arrangement.

They can be demonstrated by histochemical method under

electron microscope and light microscope.

Chemicals used for Electron microscope Ruthenium Red

Light microscope Alcian blue, 8GX and toulidiene blue.

Glycoproteins contain a group 1,2 glycol which can be

demonstrated in light microscope by use of periodic Acid Schiff method.

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In electron microscope by Periodic acid silver methanamine

technique.

periodontal ligament contains a glycoprotein “fibronectin” which is

present in filamentous form. It contains chemical groups that attach to

surface of fibroblasts, collagen proteoglycans and fibrin.

Clinical and physiologic aspect of ground substance

Proteoglycans play a role in transmitting pressure and thus

dispersing mechanical forces to protect periodontal ligament from

damage.

Changes in ground substance are seen during tooth eruption.

Ground substance are also found to play a role in tooth

mobility as studied by Picton in 1984.

Interestitial tissue

Some of blood vessels, lymphatics, and nerves of periodontal

ligament are surrounded by loose connective tissue known as interstitial

tissue.

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Structures in connective tissue

Contains:

- Blood vessels

- Lymphatics

- Nerves

- Cementicles

Blood vessels

The arterial vessels of periodontal ligament are delivered from three

sources:

1. Branches from apical vessels that supply dental

pulp.

2. Branches from intra-alveolar vessels that

penetrate the alveolar bone horizontally to enter periodontal

ligament.

3. Branches of gingival vessels. They enter

periodontal ligament from coronal direction.

The arterioles and cappaliries form a rich network adjacent to bone.

There is a rich vascular plexus also at the apex and cervical part of

ligament. Venous plexus run axially to drain to the apex.

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Lymphatics

A network of lymphatic vessels along the blood vessels provides the

lymph drainage of periodontal ligament. The flow is from periodontal

ligament toward and into adjacent alveolar bone.

Nerves

Associated with blood vessels and nerves pass through the foramina

in the alveolar bone including apical foramen to enter the ligament. In

region of apex, they run toward the cervix, whereas along the length of root

run both coronally and apically.

Nerves are of both large (myelinated) diameter as well as small

(may or may not myelinated) diameter. Large fibres are concerned with

sense of touch and end in different type of endings such as knob like,

spindle-like etc. small are responsible for pain.

Cementicles

Calcified bodies cementicles are sometimes found in periodontal

ligament seen commonly in old people. These may join to form large

calcified masses. As cementum thickens, it envelops these masses. Origin

is not known, but it is thought that degenerated epithelial cells form nidus

for their calcification.

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Functions of periodontal ligament

The functions of periodontal ligament are:

1. Physical

2. Nutritive

3. Sensory

4. Homeostatic (formative and remodeling).

1) Physical functions of periodontal ligament include the

following:

a. Transmission of occlusal forces to bone.

b. Attachment of teeth to bone.

c. Maintenance of gingival tissue in the proper relationship to the

teeth.

d. Resistance to impact of occlusal forces.

e. Provision of a soft tissue cassing to protect the vessels and nerves

from injury by mechanical forces.

As a result of forces, (on tooth) during mastication or due to

orthodontic forces, part of PD space is narrowed and periodontal ligament

gets compressed. Other parts are widened. Compressed part provides

support for the loaded tooth. Collagen fibres acts as a cushion for the

displaced tooth.

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2) Sensory

The periodontal ligament is abundantly supplied with sensory nerve

fibres capable of transmitting tactile pressure and pain sensation by the

trigeminal pathways.

This proprioceptive mechanism, allows the organism to detect the

application of most delicate forces to the teeth and very slight displacement

of the teeth. E.g., Bitings on a small hard object such as stone while eating.

3) Nutritive :

The periodontal ligament supplies nutrients to the cementum

(Cementocytes) bone (Superficial osteocyctes) gingiva and metabolites to

other cells of ligament via blood vessels.

4) Homeostatic

Cells of periodontal ligament participate in the formation and

resorption of cementum and bone and occur in physiologic tooth

movement in the accommodation of periodontium to the occlusal forces

and in the repair of injuries.

Periodontal ligament is constantly undergoning remodeling. Old

cells are replaced by new ones.

The rate of formation and the differentiation of fibroblasts affect the

rate of formation of collagen, cementum and bone.

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Rate of collagen synthesis is faster than other any connective tissue

of the body.

When homeostatic mechanism, is upset derangement of

periodontium occurs.

If periodontal ligament is irrepairably damged ankylosis may occur

due to bone deposition in periodontal space.

Theories in periodontal ligament

Three theories relative to the mechanism of tooth support have been

considered:

1) Tensional theory

It states that principal fibres of periodontal ligament have major

responsibility in supporting the tooth and transmitting forces to the

bone. When a force is applied to the crown, principal fibres first

unfold and straighten and then transmit forces to the alveolar bone,

causing an elastic deformation of the bony socket finally when

alveolar bone has reached its limit, load is transmitted to basal bone.

Many investigators do not agree to this theory.

2) Viscoelastic system theory

This theory states that fluid movement is mainly responsible for

displacement of tooth; fibres have the secondary role.

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When forces are applied on tooth extracellular fluid from

periodontal ligament passes into marrow spaces of bone through

foramina in the cortical layer. After the depletion of tissue fluids, the

fibre bundles absorb the slack and lighten, causing blood vessel

stenosis, arterial back pressure cause ballooning of the vessels and

passage of blood ultrafiltrates into the tissues, thereby replenishing

the tissue fluids.

3) Thixotropic theory

It states that periodontal ligament behaves like a thixotrophic gel

i.e. property of becoming fluid when shaken or stirred and then

becoming semi-solid again.

Physiologic response of periodontal ligament is explained by

changes in the viscosity of biologic system. According to Schroeder,

presence of organized collagen fibres makes the theory untenable.

Age Changes in periodontal ligament

The principal fibres of the periodontal ligament are thicker in aging

human than in younger individuals. Interfibrillar areas are reduced in size.

There is a decrease in ratio of ground substance to collagen. Fewer cells are

seen.

Periodontal ligament show hyalinization and chondroid

degeneration related to reduced vascular supply.

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- Calcified bodies are common in periodontal ligament of elderly

humans.

- Occassionally they increase in number and appear to calcify a

complete fibre bundle producing ankylosis.

- Epithelial cell rests may undergo calcification with age.

Clinical Consideration:

- Primary role of periodontal ligament is to support tooth in socket.

- Thickness of periodontal ligament varies and decrease with age.

- Periodontal ligament is wide in tooth under function and is thin in

functionless teeth.

Thus the tooth long out of function is poorly adapted to carry the

load suddenly placed on tooth by a restoration and thus may account for

inability of a patient to use the restoration immediately after its placement.

Acute trauma to periodontal ligament due to accidental blow

condensing of a foil, rapid tooth separation may produce pathologic

changes such as:

- Fracture and resorption of cementum and bone.

- Tear of fibre bundles.

- Hemorrhage and necrosis.

- periodontal ligament widening.

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- Tooth becomes loose.

- Repair occurs on elimination of trauma.

Orthodontic movement depends on resorption and formation of both

periodontal ligament and bone.

Pressure causes bone resorption.

Tension causes bone formation.

The periodontal ligament in the periapical area of tooth is often the

site of pathologic lesion.

Infectious disease pulp progress to apical periodontal ligament and

replace its fibres by granulation tissue giving rise to dental

granuloma which may develop into cyst later on.

Acute alveolar abscess.

Acute PD abscess.

Diseases of periodontal ligament are often irreversible

From gingiva it progresses to periodontal ligament, it is a slow

process. Once destroyed PDL is difficult to regenerate.

Summary and Conclusion

Periodontal ligament is a connective tissue structure that surrounds

the roots of teeth and attaches them to bony alveolus.

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It is composed of a group of dento-alveolar fibres called principal

fibres which help to support the tooth.

Periodontal ligament has rich vascular supply (nourishes) and is

well innervated these nerve help prevent trauma to teeth.

Periodontal ligament thus forms an important structure of dento-

alveolar unit.

The periodontal disease has rapidly taken over dental caries which

was considered as the primary disease of oral cavity.

Damage of periodontal ligament results in loss of tooth. Thus a

thorough understanding of periodontal ligament is necessary for all

health care personnel.

References

1. Periodontal Ligament – Oral Histology and Embryology by

Orban’s.

2. Tencate A.R.

3. Tooth supporting structures Page 30, Feemin A.Caranza Jr. and

Angela MUBIOS, Clinical Periodontology, 8th Edition.

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PERIODONTAL LIGAMENT

CONTENTS

INTRODUCTION

DEFINITION

EVOLUTION AND DEVELOPMENT

STRUCTURE OF PDL-EPITHELIUM

- CELLULAR COMPONENT

- EXTRACELLULAR COMPONENT

STRUCTURE IN CONNECTIVE TISSUE

FUNCTIONS

THEORIES

AGE CHANGES

CLINICAL CONSIDERATION

SUMMARY & CONCLUSION

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