The term periodontium arises from greek words “peri”
meaning around and “odont” meaning tooth. Tissues
that invest and support the teeth are collectively termed
Periodontal ligament is an unique multifunctional soft specialized , vascular
and highly cellular connective tissue that surrounds the tooth root and
connects it to the inner wall of the alveolar bone. (1)
It is continuous with the connective tissue of the gingiva and communicates
with marrow spaces through vascular channels in the bone.
1. Periodontal membrane
2. Alveolo-dental ligament
5. Dental periosteum
Enamel organ is surrounded by a condensation of ecto mesenchymal cells called
The part of the dental sac immediately close to the enamel organ is called dental
Once the Hertwig Epithelial root sheath disintegrates leaving behind the Epithelial
rests of Malassez, cells of the dental follicle come close to the surface of newly
The dental follicle cells then differentiate into cementoblasts & lay down
cementum on dentin on the developing root.
The other cells of the dental follicle differentiate into fibroblast & lay down fibers
& ground substance of the periodontal ligament.
As the crown approaches the oral mucosa during tooth eruption, these
fibroblast become active & start producing collagen fibrils. They initially lack
orientation, but they soon acquire an orientation oblique to the tooth.(4)
The first collagen bundles appear in the region immediately apical to the
cemento enamel junction & give rise to gingivodental fiber groups. As tooth
eruption progresses, additional oblique fibers appear & become attached to the
newly formed cementum & bone.
Jan Lindhe, Niklaus P. Lang, Thorkild Karring
Clinical Periodontology and Implant Dentistry 5TH EDITION
The transseptal & alveolar crest fibers develop when the tooth merges into
the oral cavity. Alveolar bone deposition occurs simultaneously with
periodontal ligament organization.
Sharpey‟s fibers are fewer in number & more widely spaced than those
emerging from the cementum.(5)
At the later stage, alveolar fibers extend into the middle zone to join the
lengthening cemental fibers ,attain their classic orientation, thickness &
strength when occlusal function is established.
Jan Lindhe, Niklaus P. Lang, Thorkild Karring
Clinical Periodontology and Implant Dentistry 5TH EDITION
Rearrangement of the fiber ends in the plexus, is supposed to
accommodate tooth eruption without necessitating the
embedding of new fibers into the tooth & the bone.
As the tooth eruption proceeds, the obliquity of the fibers
gradually decreases and the position of the Cememtoenamel
junction , which is originally apical to the crest of the crypt,
comes to the level of the crest and then coronal to the alveolar
SIZE & SHAPE
The Average width -> 0.2mm. It ranges in width from 0.15 to 0.38 ,thinnest portion around
the middle third of the root.
At age of 11 - 16 years - width is 0.21 mm
-> 32 - 52 years - width is 0.18 mm
-> 51 - 67 years - width is 0.18 mm(6)
Progressive decrease of width occurs with increase of age.
The periodontal space is often “ Hour – glass “ shaped , with narrowest
portion in the midroot region near the fulcrum about which the tooth moves.
These are the important elements of the Periodontal
ligament which are collagenous , arranged in bundles and follow
a wavy course when viewed in longitudinal sections.
The principal fibers bundles consist of individual fibers
forming a continuous anastomosing network between tooth and
Transseptal group :
These fibers extend inter proximally over the alveolar crest and are embedded in
the cementum of adjacent teeth.
They are reconstructed even after destruction of the alveolar bone has occurred in
Resist tooth separation, mesial or distal.
Alveolar crest group:
Alveolar crest fibers extend obliquely from the cementum just beneath the
junctional epithelium to the alveolar crest.
Fibres also run from the cementum over alveolar crest and to the fibrous layer of
the periosteum covering alveolar bone
They prevent the extrusion of teeth and resist lateral movements of tooth.(7)
Their incision does not significantly increase tooth mobility.
Jan Lindhe, Niklaus
P. Lang, Thorkild
and Implant Dentistry
These group of fibers extend at right angles to the long axis of the tooth from
-> the cementum to the alveolar bone.
Resist horizontal and tipping forces.
Oblique fibers, the largest group in the periodontal ligament, extend from the cementum in a
coronal direction obliquely to the bone.
They bear the brunt of vertical masticatory stress and transform them into tension on to the
It is the largest prinicpal fibre group
They resist apically directed masticatory forces
The apical group radiate from the cementum to the bone at the fundus of
They do not occur on incompletely formed roots.
Functions- They prevent tooth tipping,resist forces of luxation .
Inter radicular fibers
These fibers fan out, from the cementum to the teeth on the furcation
areas of multirooted teeth.
They resist forces of luxation and roatation.
Study conducted by EINAR KVAM
To study TOPOGRAPHY OF PRINICPAL FIBRES(1973)
Untreated human premolars were fixed in 4 % buffered Formaldehyde,
The fibrous tissue of root surfaces were studied in SEM.
1.They occur in smaller bundles
2.fibre bundles appear to increase by additional layer of peripheral bundles
3.On root surface->outer portion of fibre bundles interlaced with fibrous
In demineralized sections
Principal fibres bundles transversed the cementum.
Principal fibres connecting the periodontal ligament were
arranged in ladder like fashion.
At Cemento Dentinal junction, fibres ended as finger like
projections,rendering an irregular borders between dentin
The collagen bundles of the periodontal ligament are embedded into
cementum and alveolar bone – similar to that of tendon inserting into bone.
These fibers are called as Sharpey‟s fibers.
These fibers level to concentrate on the crestal region. Orientation is
similar to that of adjacent periodontal ligament bundles. The Bone adjacent to
the periodontal ligament that contains a great number of sharpeys‟ fibers are
called as Bundle bone.
Some sharpey‟s fibers are completely calcified but most contain an uncalcified
central core within a calcified outer layer.(9)
INTERMEDIARY PLEXUS OF
The principal fibers frequently run a wavy course from cementum to
bone. It may appear as fibers arising from cementum & bone are
joined in the midregion of the periodontal ligament space, giving rise
to a zone of distinct appearance, called Intermediate plexus.
INDIFFERENT FIBER PLEXUS
In addition to these fiber types, shakle ford has described small
collagen fibers in association with larger collagen fibers. These fibers
run in all directions forming a plexus that has been termed the
“indifferent fiber plexus”.(10)
CELLS OF THE PERIODONTAL
1) CONNECTIVE TISSUE CELLS
A. SYNTHETIC CELLS
B. RESORPTIVE CELLS
2). EPITHELIAL REST CELLS -
Epithelial rests of malassez
3). DEFENSE CELLS
2. Mast cells
CONNECTIVE TISSUE CELLS
Most predominant cell type of periodontal ligament.
-> found between the fibers of the periodontal ligament, where they are
surrounded by fibers & ground substance.
Ovoid or elongated cells oriented along the principal fibres.
Large cells with an extensive cytoplasm
Associated with protein synthesis & secretion (e.g. RER, several Golgi
complex, & many secretary vesicles) (11)
Periodontal ligament fibroblasts are large cells with an extensive cytoplasm
They have a well-developed cytoskeleton .
Adherens and gap junctions, reflecting the functional demands placed on the cells.
Aligned along the general direction of the fiber
Extend cytoplasmic processes that wrap around them.
The collagen fibrils of the bundles are continuously being remodeled by the
The remodeling of collagen in the ligament is not confined to any intermediate zone,
but occurs across the entire width of the ligament.
Because of exceptionally high rate of turnover of collagen in the ligament, any
interference in the fibroblast function by disease rapidly produces a loss of the tooth‟s
Found on the surface of the alveolar bone .
Ultrastrucutre is same as that osteoblasts anywhere in the body.
Bone forming cells derived from osteoprogenitor cells.
Seen on surfaces of bone giving an epithelium like appearance. Shape varies
(seen as oral, triangular, cuboidal) etc.
Nucleus is ovoid and euchromatic
Cytoplasm is basophilic ( because of abundant rough endoplasmic reticulum)
Responsible for laying down the organic matrix of bone including the
collagen fibers. Alkaline phosphatase present in osteoblasts is responsible for
ELECTRON MICROGRAPH – OSTEOBLAST –CROSS
SECTIONOrban's Oral Histology and Embryology
Maintain the integrity of the lacunae and canaliculi.
Open the channels for diffusion of nutrition through the bone.
Play a role in removal or deposition of matrix and of calcium when required.
Osteoblasts first migrate away from the bone surface into the body of
periodontal ligament before eventually taking up their functional position.
Osteoblasts form a cell layer over forming bone surface and act as barrier
that control ion flux into out bone(14)
These are the cells responsible for secreting the organic matrix (mainly
collagen) of cementum.
As distinct layer of cells on the root surface, similar to osteoblastic layer but
not regular in arrangement.
Differs from periodontal ligament fibroblasts in->
Appear near the cementum.
Less rough Endoplasmic Recticulum
Orban’s Oral Histology and Embryology,Surinder Nath
Bone resorbing cells.
Present on the surface of bone where it is removed. At such
locations the cells occupy pits called Resorption bays or lacunae of
20 – 100 µm diameters.
upto 20 or more
Numerous mitochondria lysosomes
Osteoclasts are formed by fusion of mononuclear cells arising from
They do not cover the whole of resorbing surface at any one time,
rather they service a much larger area by demonstrating considerable
Located in howships lacunae.
Origin unknown but believed to be same as that of
Since constant deposition of cementum occurs, these cells
are not seen during normal functioning of cementum.
1.Cytoplasma exhibit numerous lysozomes and mitochondria
2.Ruffled border->part of plasma memberane lying adjacent
4.Less rough endoplasmic retiuculum.
EPITHELIAL CELL RESTS OF
Form a latticework in the periodontal ligament and appear as either isolated
clusters of cells or interlacing strands.
Represent the remnants of developmental Hertwig epithelial root sheath which
is involved in differentiation of root odontoblasts and also secrete enamel like
proteins on the root surface.
Arranged in close packing of cuboidal cells,completely surrounded by connective
Located closer to cementum than to alveolar bone.
Situated in the periodontal ligament at a distance of 15–75 μm from the
cementum on the root surface.(17)
As age increases the epithelial cell rest move vertically and are located mostly in
the vertical region in the gingiva.
EPITHELIAL RESTSOrban's Oral Histology and Embryology PAGE -186
Loe and waerhaug (1960)(18)
After reimplantation studies observed that ->
Ankylosis and subsequent root resorption never occurred when a periodontal
ligament that contained epithelial cell rest was retained.
These authors suggest that epithelial cell rest may be the factor in limiting the
resorption and play a role in maintenance of periodontal space.(18)
Have the ability to phagocytose unwanted material.
Macrophages are usually described as „fixed‟ when they are attached to fibers
or as motile (free).(19)
Fixed macrophages resembles fibroblasts but free macrophages are rounded.
Nuclei smaller and stain intensely than fibroblasts. (kidney shaped).
Cytoplasm contain numerous lysosomes which help in digesting material
phagocytosed by macrophages.
Macrophages may fuse together to form multinucleated giant cells.
MULTIVESICULATED BODY OF MACROPHAGE
Orban's Oral Histology and Embryology,Page 129
These are round or oval cells.
Nucleus small centrally placed Microvilli (filopodia) present on cell surface.
Presence of numerous granules in the cytoplasm. Granules are known to
contain acid mono polysacharrides.
The cell membrane of mast cells contain antibodies which react with the
This leads to rupture of the cells which discharge histamine and serotonin.
The discharge in turn lead to local reactions like utricaria or severe general
reactions like Anaphylactic shock.
PERIODONTAL LIGAMENT IN HEALTH
DR JOHNN KAZIMM
DEPARTMENT OF PERIODONTICS
I.T.S DENTAL COLLEGE, HOSPITAL & RESEARCH
CENTRE, GREATER NOIDA
8 th November 2013
MODERATOR: Dr Annapurna Ahuja
Formed by fibroblasts.
The predominant collagens of the periodontal ligament are type I, III, and
XII, within individual fibrils having a relatively smaller average diameter than
tendon collagen fibrils.
Collagen type IV found in basement membrane of Junctional epithelium
and epithelial rests of PDL.
These fibers of the periodontal ligament have a structural requirement to
withstand intensive forces from mastication and to accommodate tooth
The collagen fibril diameters of the periodontal ligament are relatively small,
with mean diameter of order 45 – 55 m which is much less compared to
other connective tissues e.g. Tendon fibril – >250 m.
SURFACE OF CEMENTUM
COLLAGEN FIBRES EMBEDDED IN
Orban's Oral Histology and Embryology,Page , Page 156
FUNCTIONAL ADAPTATIONS OF THE
COLLAGEN FIBRES OF THE
The internal orientation of collagen fibres in connective tissues suggested that
collagenous bundles could resist axially directed forces.
The arrangement of majority of periodontal ligament collagen fibres into
horizontal and obliquely directed groups may therefore be adapted to resist axial
Such an arrangement has mechanical advantages over a simple radial arrangement.
The overlapping arrangement of the bundles may also be advantageous in
resisting intrusive forces, as axial displacement of the tooth would tend to
straighten out the fibre bundles and compress the blood vessels.
IMMATURE FORMS OF ELASTIN
Run parallel to the root surface via vertical direction and bend to attach to
cementum in the cervical third of the root.
Restricted to the walls of the blood vessels.
Regulate vascular flow and play a part in supporting the blood vessels of the
Form a three dimensional meshwork that extends from cementum to
peripheral blood vessels.
Important for fibroblast adhesion and migration.
Elastic meshwork in periodontal ligament is composed of elaunin fibres and
Tranmission Electron Microgrpah of
Oxytalan Fibre in Cross Section
Orban's Oral Histology and Embryology,Page Page-192
GROUND SUBSTANCE OF THE
The ground substance is the gel like matrix which is synthesized by the cells
of the fibroblast family.
Two types of glyco conjugates to form the major constituents of the ground
Composed of specific core of proteins to which one or more glycosamino
glycans (GAG) chains are covalently attached through a sequence, usually the
trisaccharide galactose – xylose.
Linear polymers of dissacharide which contain a hexosamine (usually D- Nacetyl glucosamine or D-N- acetyl galactosomine) and a hexuronic and either
D- glucuronic acid.
Currently termed as hyaluronan.
Is a non- sulphated glycosaminoglycans.
Not seen in periodontal ligament but unique among glycosoaminoglycans, as it
does not contain hexuronic acid.
Three distinctly related glycoprotiens of the extra cellular matrix have been
localized in the decalcified sections of human periodontal ligament, namely:
Fibronectin-> Present in loose connective tissues and in embroyonic
basement membrane (not in adults)
Laminin-> Major glycoprotein component of basement membranes.
Tenascin-> Present in the glycoproteins of periodontal ligament with a
smaller role in cell attachment and organization of basement membrane.
The blood supply to the supporting structures of the tooth is derived from
the inferior & superior alveolar arteries to the mandible & maxilla, & it
reaches the periodontal ligament from three sources:
Penetrating vessels from the alveolar bone
Anastomosing vessels from the gingiva.(23)
The branches of the apical vessels supply the apical region of the
periodontal ligament before the vessels enter the dental pulp.
The trans-alveolar vessels are branches of the intraseptal vessels that perforate
the lamina dura & enter the ligament.
The intraseptal vessels continue to vascularize the gingiva.
Vessels within the periodontal ligament are contained in interstitial spaces of
loose connective tissue between the principal fibers & are connected in a
netlike plexus that runs longitudinally & closer to the bone than the
Louis F rose,Brian,Cohen.Periodontics,Surgery Page 23
Numerous branches arise from the intraalveolar vessels to run horizontally,
penetrate the alveolar bone & enter the ligament, they are called „perforating
The blood supply increases from the –>
1.Incisors to the molars,
2.Greatest in the gingival third of single rooted teeth, in the apical third
3.Least in the middle third
4.Equal in the apical & middle third of the multi rooted teeth,
5.Greater on the mesial & distal surfaces than on the facial & lingual surfaces
6.Greater on the mesial surfaces of mandibular molars than on the distal
This pattern of distribution has a clinical importance.
In the healing of the extraction wounds, new tissues invader from
perforations & formation of blood clot occupying the socket is more rapid in
its gingival & apical areas.
Within the ligament, the arteries occupy the areas of loose connective tissue
called as „interstitial areas‟ between the principal fiber bundles. Vessels course
in a apico-occlusal direction with numerous transverse connections.
Small fibers in the periodontal ligament, particularly the A and C fibers.
The trigeminal nerve innervates the periodontal ligament through maxillary
nerve or inferior alveolar branches.
The majority of the nerves supplying the periodontal ligament pass through
foramina on the alveolar bone to enter the periodontal ligament space close to
the tooth apex, while others enter via the lateral aspect of the alveolar wall.
INNERVATION OF PERIODONTAL
These nerve bundles terminates as mentioned into four types :
FREE NERVE ENDING
Located at regular intervals along the length of the root.
Appear dendrite and end in terminal expansions among the periodontal ligament fiber bundles.
Subdivided into simple & compound forms.(25)
Is coiled form found in midregion of periodontal ligament whose function & structure have not been
Consists of spindle like endings surrounded by fibrous capsule.
These are calcified masses found in the periodontal ligament.
These bodies found in the older individuals and they remain free, they may
fuse into large calcified masses or they may be joined with the cementum.
As the cementum thickens with age , it may envelope the bodies and will be
adherent to the cementum , they form excementosis. Their origin is
It was thought that degenerated epithelial cells form the nidus for their
FUNCTIONS OF PERIODONTAL
Provision of a soft tissue "casing" to protect the vessels and nerves from injury
by mechanical force
Transmission of occlusal forces to the bone.
Attachment of the teeth to the bone.
Maintenance of the gingival tissues in their proper relationship to the teeth.
Resistance to the impact of occlusal forces (shock absorption) .
SHOCK ABSORPTION – THREE THEORIES
Force is applied to the crown,fibres first unfold,straighten and then transmits forces to the alveolar
bone causing an elastic deformation of socket, when the alveolar bone has reached its limit, the
load is transmitted to the basal bone.
Displacement of tooth to be primarily controlled by the fluid movements.When forces are
transmitted to the tooth, the extracellular fluid passes from the periodontal ligament into the
marrow spaces of the bone through the foramina in the cortical layer. Perforations in the lamina
dura link the periodontal ligament with the cancellous portion of the alveolar bone.After depletion
of the tissue fluids , the fibre bundles absorb and tighten, this leads to blood vessel stenosis ,
arterial back pressure and passage of blood ultra filtrates into the tissues there by replenishing the
THIXOTROPIC GEL THEORY:
Periodontal ligament has behaviour of thixotropic gel(property of becoming fluid when shaken or
stirred and then becoming semisolid again). The physiologic response of the periodontal ligament
may be explained by changes in the viscosity of the biologic system.(26)
FORMATIVE AND REMODELING
The periodontal ligament is constantly undergoing remodeling. Old cells and
fibres are broken down and replaced by new ones. The rate of formation and
differentiation of the fibroblasts affects the rate of formation of collagen,
cementum and bone.
The periodontal ligament supplies nutrients to the cementum, bone, and gingiva
by way of the blood vessels and provides lymphatic drainage.
Sensory: The periodontal ligament is abundantly supplied with sensory nerve
fibers capable of transmitting tactile, pressure, and pain sensations by the
Nerve bundles pass into the periodontal ligament from the periapical area
and through channels from the alveolar bone that follow the course of the
They divide into single myelinated fibers, which ultimately lose their myelin
EXTERNAL FORCES &
The periodontium exists for the purpose of supporting teeth during function
& depends on the stimulation it receives from function for the presentation
of its structure. Therefore, a constant & sensitive balance is present between
external forces & the periodontal structures.
The periodontal ligament also depends on stimulation provided by function
to preserve its structure. Within physiologic limits, the periodontal ligament
can accommodate & function with an increase in diameter & number of
Forces that exceed the adaptive capacity of the periodontium produce injury
called Trauma from occlusion.
The periodontal ligament also atrophies, appearing thinned, & the fibers are
reduced in no. & density, disoriented, & ultimately arranged parallel to root
surface. This phenomenon is called disuse atrophy or a functional
Trauma to the ligament due to mechanical separation can produce
pathological changes such as fracture or resorption of the cementum, tears of
fiber bundles, hemorrhage and necrosis.
This result in resorption of bone and periodontal ligament is widened, so the
teeth become loose.
Orthodontic tooth movement depends on resorption and formation of
both bone and periodontal ligament ->stimulated by both tension and pressure
Application of large forces->necrosis of periodontal
In SCLERODERMA, the periodontal ligament is uniformly widened at the
expense of surrounding alveolar bone.
Malignancy, both primary and metastatic, can affect the alveolar ridge and often
presents as periodontal disease.
A uniform widening of the periodontal ligament can be an early sign of
osteosarcoma. Irregular destruction of the periodontal bone without tooth
displacement is frequently the result of squamous cell carcinoma or metastatic
EFFECTS OF AGING ON THE
Reduction in vascularity, elasticity & reparative capacity are some of the
common manifestations of ageing, generally noticed in all tissues.
Gottlieb & Orban believed that with age , gingival recession & alveolar bone
resorption occur & termed it as senile atrophy.(29)
Decreased number of fibroblasts with more irregular structure is seen.
Decreased Collagen synthesis with increasing age.
Decrease in no. of periodontal fibers. The fiber bundles were thicker,
broader and more highly organized.
Areas of hyalinization were present.
Decreased organic matrix production & epithelial cell rests.
Increased amount of elastic fibers.
The surfaces of the periodontal alveolar bone were jagged & uneven & an
irregular insertion of fibers were seen.
EFFECTS OF AGING
In children, periodontal ligament space is wider in children partly because of
thinner cementum & alveolar cortical plates.(30)
It contains less fibers & more blood vessels. Alveolar bone has larger
marrow spaces & fewer trabeculae than in adults. Marrow is also more
Radiographically, distance from the Cementoenamel junction to the
alveolar bone crest of primary canines & molars in the range of 0 to 2 mm.
The susceptibility to periodontal disease is more significant for the rate
of periodontal destruction than the length of time plaque is present
(the age effect)(31)
1. Michael G. Newman, DDS, Henry Takei, DDS, Perry
R. Klokkevold Carranza’s Clinical Periodontology:
2. Shantipriya Reddy, Reddy e,ssentials of Clinical
Periodontology and Periodontics (492)
• 3. Antonio Nanci Ten Cate's Oral Histology:
Development, Structure, and Function Elsevier Health
4. Periodontics Revisited,(pg 16) Shalu Bath
5. Michael G. Newman, DDS, Henry Takei, DDS, Perry R.
Klokkevold Carranza’s Clinical Periodontology:Elsevier
6. Md. Abdul Kadir Intentional Replantation of Molar
Tooth City Dental College J. Volume-9, Number-1, January2012
7. Essentials of Oral Histology By K. Chatterje 01-Dec2006 - Medical 106 pg
8. EINAR KVAM,Topography of principal fibers European
Journal of Oral Sciences;81:(7)pages 553–557,1973
9.Krishna kumar Review of Clinical,Periodontology 2007
10. Shackleford JM: The indifferent fiber plexus and its
relationship to principal fibers of the periodontium.
Am J Anat 1971; 131:427.
11.Surindar Nath Bhaskar Orban's Oral Histology and
Embryology pg 180
12.Antonio Nanci Structure of periodontal tissues in
health and disease Periodontology 2000, Vol. 40, 2006,
13V Dhakray, M Mittal, P Khanna, M Jain, B Yadav Evolution And
History Of The Periodontal Ligament - A Review Internet
Journal of Medical Technology:6,1
14. Antonio Nanci Ten Cate's Oral Histology: Development,
Structure, and Function Elsevier Health Sciences 2008
15. Ultrastructural and morphometric analyses of human
cementoblasts and periodontal fibroblasts.A Yamasaki,
G G Rose, G J Pinero, C J Mahan,Journal of Periodontology,
16. Textbook of Human Histology,By Inderbir Singh pg 104
17. Jan Lindhe, Niklaus P. Lang, Thorkild Karring
Clinical Periodontology and Implant Dentistry Page 30, 5 th
18.Loe Waerhuaug j Experimental Reimplantaion of
teeth.Archives of oral biology 3; 176- 183
19.E Robert burns A human cell biology.Mosby 2007
20.Stanley Leonard Robbins Pathology Volume 2
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Newman,The periodontal ligament in health and
disease pg 446
22. Ajit Varki and Nathan Sharon Essentials of
Glycobiolgy 2nd edition
23. Cohen L.Vascular Literature of mandible. J dent res
24. Michael G. Newman, DDS, Henry Takei, DDS, Perry R.
Klokkevold Carranza’s Clinical Periodontology:Elsevier
25. Antonio Nanci Ten Cate's Oral Histology:
Development, Structure, and Function Elsevier Health
26. Periodontics Revisited,(pg 19) Shalu Bathia
27. Michael G. Newman, DDS, Henry Takei, DDS,
Perry R. Klokkevold Carranza’s Clinical
Periodontology:Elsevier pg 44
28.Surnder nath bhaskar,Orbans oral histology &
emryology pg 200
29.Gottlieb b,Orba b,Active and passive continous
eruption of teeth,J dent res 13:214,1933
30. Richard Welbury, Monty S. Duggal, Marie Thérèse
Hosey,Paediatric Dentistry ,Oxford pg 416
31. Van der Velden U,Effect of age on the periodontium
J Clin Periodontol 1984 May;11(5):281-94.