Normal Periodontium

Periodontium literally means “around the tooth”.

Definition:

It is the functional unit of tissue, investing and supporting the tooth.

Functions:

  • Attachment of teeth to the bone and to one another.
  • Adaptation to the changes due to ageing and wear.
  • Defence against noxious substances in oral cavity.
  • Resistance towards masticatory forces.

Components:

  • Gingiva        
  • Periodontal Ligament 
  • Cementum
  • Alveolar Bone

Gingiva

 

It is a fibrous connective tissue and is that part of oral mucosa which covers the coronal (tooth bearing) portion of the alveolar bone and the cervical region of the teeth. It normally covers tooth root to a level just coronal to CEJ. Anatomically gingiva is divided into:

  • Marginal/free/unattached gingiva.
  • Attached gingiva.
  • Interdental gingiva.

Normal gingival

Gingiva

A) Macroscopic features of gingiva:

Marginal gingival:

  • The unattached sleeve like portion of the gingiva surrounding the neck of tooth.
  • It covers about 1.0 to 1.5 mm of the tooth surface.
  • Demarcated from attached gingival by free gingival groove (present in 30-40% of adults and often corresponds to the location of the bottom of the gingival sulcus). 

Attached gingiva:

  • Firm, resilient and tightly bound to underlying periosteum and cementum.
  • Lies between free gingival groove and mucogingival junction.
  • Attached gingiva is normally covered by keratinized or para-keratinized epithelium.
  • Width of attached gingiva = width of keratinized gingiva – marginal gingiva.
  • Width of the attached gingiva is greatest over labial surface of maxillary incisors (3.5-4.5 mm).
  • Narrowest over buccal surface of mandibular premolars (1.8 mm).
  • It increases with age due to supra-eruption of teeth of teeth with age.
  • A width of attached gingiva below 1.0 mm, increases the risk of developing gingival and periodontal diseases, unless strict oral hygiene is practiced. 

Interdental gingiva:

  • Occupies the interdental space beneath the areas of contact.
  • It can be pyramidal (in case of anterior teeth) or col shaped (in case of premolars and molars). Col is non-keratinized and is highly susceptible to infections).
  • In case of diastema, interdental papilla is absent. 

B) Microscopic features of gingiva:

Gingiva is composed of overlying keratinized stratified squamous epithelium, epithelium-connective tissue interface and underlying connective tissue.

Gingival Epithelium:

  • It includes oral epithelium, sulcular epithelium and junctional epithelium.
  • Principal cell type: keratinocyte.
  • Other cells (non-keratinocytes or clear cells: langerhans cells, merkels cells, melenocytes).
  • Epithelium is attached to connective tissue by basal lamina (300-400 A⁰ thick).
  • Basal lamina consists of lamina lucida and lamina densa.

Gingival connective tissue:

  • It is less cellular and is primarily composed of collagen fibers and ground substance.
  • Its main components are: collagen fibers, proteoglycans, vessels and fibroblasts. 

Gingival sulcus:

It is a V-shaped crevice or space between tooth and free gingiva.

Boundaries of gingival sulcus:

    • Surface of tooth on one side
    • Epithelial lining of free margin of gingival on other side.
    • Junctional epithelium at base.

Depth of gingival sulcus:

    • Histological depth: 1.8 mm
    • Probing depth: 2-3 mm

C) Junctional epithelium (JE):

  • The term JE represents the tissue that is attached to the tooth on one side and to the sulcular epithelium and connective tissue on the other side.
  • Non-keratinized and poorly stratified epithelium.
  • It is highly permeable but not completely permeable.
  • Lies apical to gingival sulcus.
  • Length of JE is 0.25-1.35 mm.
  • JE tapers from its coronal end (15 to 30 cells wide) to its apical end (1 to 3 cells wide).
  • Attached to enamel by basement lamina and hemidesmosomes.
  • A small number of leukocytes, lymphoid cells (especially, small lymphocytes), are also seen within the JE of clinically normal gingiva.
  • JE has a high turnover rate. It is found to be approximately 5 to 10 days.
  • After mechanical separation of JE from the tooth surface, regeneration of epithelial attachment takes place in about 7 days. 
  • JE acts as the main passageway for the entry of neutrophils into the gingival sulcus. 

D) Gingival fibers (on the basis of their location, origin and insertion): 

a)      Dentogingival group:

Extend from cervical cementum to CT of free and attached gingiva. They provide gingival support.

b)      Circular group:

Fibers within the gingiva that encircles the tooth in a ring like fashion. This group maintains the position of free marginal gingiva.

c)       Semicircular group:

Arise just beneath the circular fibers, from the cementum of proximal root surface and then traverse the facial or lingual free marginal gingiva to get inserted into the opposite side of the same tooth.

d)      Transseptal group:

They run between the cementum of two adjacent teeth. These fibers rapidly reform after excision. They are important in maintaining the integrity of dental arch.

e)      Alveologingival group:

They serve to join attached gingiva and alveolar bone.

f)       Dento-periosteal group:

They anchors tooth to the bone.

g)      Interpapillary group:

They give support to interdental gingiva, as they extend from interdental gingiva in faciolingual direction.

E) Clinical features of normal gingival:

  1. Colour: Ideally, it is coral pink.

            Colour depends upon:

      • Degree of vascularity.
      • Keratinization.
      • Pigmentation.
      • Thickness of Epithelium.
  1. Contours: Thin, knife edge margins.
  2. Consistency: Firm and resilient, except in case of movable free gingiva.
  3. Texture: Attached gingiva shows stippling in 30-40% cases. Stippling gives attached gingiva an “orange peel” appearance.
  4. Position: It denotes the level at which gingival margin is attached to the tooth surface.

F) Blood supply of gingiva:

a)      Supraperiosteal arterioles (They mainly supply free gingival and gingival sulcus).

b)      Arterioles from crest of interdental septa (supply attached gingival).

c)       Vessels of PDL (supply col region). 

Periodontal ligament (PDL)

 

Periodontal ligament is composed of white collagenous connective tissue fibers present around the roots of the teeth and connecting them to adjacent inner wall of alveolar process.
It is highly vascular.
Width of PDL: 0.10-0.25 mm
Shape of PDL is like an hourglass apicocoronally. 

Periodontal structures

Periodontium

A) Composition:

PDL Fibers:

  • Main component of PDL is type I collage. Type III collagen also accounts for 15-20% of the total.
  • PDL also contains two immature forms of elastin: Oxytalin and Eluanin. They are thought to regulate vascular flow.

Cellular components of PDL:

Connective tissue cells:

Which includes synthetic cells [Fibroblasts (most numerous), osteoblasts, cementoblasts)] and resorptive cells (fibroblasts, osteoclasts, cementoclasts).

Epithelial rests of Malassez:

Considered as remnants of Hertwig’s root sheath. They proliferate when stimulated and participate in the formation of periapical cysts and lateral root cysts.

Defense cells:

These include neutrophils, macrophages, lymphocytes, mast cells, and eosinophils.

Cells related to neurovascular elements.
Progenitor cells.

Ground substances:

  • Glycosaminoglycans (such as hyaluronic acid and proteoglycans)
  • Glycoproteins (like fibronectin and laminin)
  • Around 70% water content.
  • Cementicles may also be present in PDL. Cementicles are calcified masses, which may lies at some distance from the root or may be attached to it. 

B) Structure:

  • The collagen fibers of mature PDL are arranged into bundles that traverse the PDL space obliquely, and are known as principal fibers and the portions of these fibers which got inserted into cementum and bone are known as ‘Sharpey’s fibers’.
  • The bundles present between the principal fiber bundles and investing nerves and blood vessels are known as secondary fibers.

C) Functions of PDL:

  1. Attachment of tooth to bone
  2. Transmits occlusal force to the bone
  3. Sensory
  4. Nutritive
  5. Formative
  6. Regenerative

D) Fiber groups of PDL: (According to location):

  1. Alveolar crest group:

This fiber group runs from cementum (just beneath CEJ) to the alveolar crest. It mainly resists lateral tooth movement. It also prevents extrusion of teeth.

  1. Horizontal group:

Run perpendicular to the long axis of the tooth from cementum to bone.

  1. Oblique group:

It is the most numerous group of fibers. They run obliquely from cementum in coronal direction and get inserted into the bone.

  1. Apical group:

Extend in an irregular fashion from cementum at apex of the tooth to the bone forming the base of the alveolus. They are absent in incompletely formed roots.

  1. Interradicular group:

They are found between the roots of multi-rooted teeth, running from cementum into the crestal bone of the interradicular septum.

  1. Transseptal group:

These fibers run from one tooth to another above the alveolar crest. These fibers may be considered as belonging to the gingiva because they do not have any osseous attachment.

E) Blood supply of PDL:

It is mainly derived from inferior alveolar artery to the mandible and superior alveolar artery to the maxilla, via three sources:

  • From vessels in apical area,
  • Vessels penetrating interdental alveolar process,
  • Anastomosing vessels from gingiva.

CEMENTUM

A) Definition:

It is a bone like substance, covering the roots of the teeth and providing attachment for the periodontal fibers.

B) Features: 

  • Mesenchymal tissue forming the outer covering of the anatomic root.
  • It is calcified.
  • Avascular.
  • Light yellow in color.
  • It has a continuous deposition, at least intermittently, throughout life.
  • Relatively brittle structure and can get fractured by trauma.
  • It is quite permeable. Dyes and radioactive substances can diffuse from surrounding connective tissue to the pulp via cementum and vice versa.
  • It has same mineral and crystal pattern as dentin and bone, but it is not as hard as them.

C) Thickness of cementum:

  • At coronal half: 16-60 µm.
  • At apical third: 150-200 µm.
  • Thickness is greatest in apical third and in furcation areas.
  • Thickness is more on distal surface of the tooth than on mesial.
  • On normal human teeth thickness of cementum increases with increasing age.
  • No direct relationship is seen between thickness of cementum and functional stress. Thick layers of cementum have been seen on un-erupted and impacted teeth.

D) Composition:

a) Organic matrix (50%):

  • Collagen fibers (90% -Type I collagen, less than 5% -Type III collagen, also Type V, XII, XIV)
  • Non-collagenous substances (fibronectin, osteonectin, osteocalcin, bone sialoprotein, alkaline phosphatase).
  • Cells (cementoblasts, cementoclasts).
  • Adhesion molecule [cementum attachment protein (CAP)].

b) Inorganic (about 46%):

  • Hydroxyapatite crystals.

E) Classifications:

a) On basis of location:

  • Radicular cementum: Present on root surface.
  • Coronal cementum: Present on enamel covering some part of the crown.

b) On the basis of cells present:

Acellular cementum (primary cementum):

  • It does not contain cells.
  • It is the first formed cementum.
  • Forms with the root formation and eruption.
  • Thickness: 30-230 µm.
  • Present at cervical 3rd or half of the root.
  • Mainly composed of sharpey’s fibers.

Cellular cementum (secondary cementum):

  • Contains cells called cementocytes.
  • Formed after tooth eruption is completed.
  • It is less calcified than acellular cementum.
  • Present at apical 3rd of root surface.

c) On basis of fibers present (Schroeder):

Acellular afibrillar cementum:

  • 1-15 µm thick.
  • Found in coronal cementum.
  • It lacks cells and collagen fibers (both intrinsic and extrinsic).
  • Consists of mineralized ground substance, produced by cementoblasts.

Acellular extrinsic fiber cementum:

  • 30-230 µm thick.
  • Present at the cervical 3rd of the tooth root but it may extend towards the apex.
  • Consists of densely packed bundles of sharpey’s fibers.
  • It lacks cells.
  • It is a product of fibroblasts and cementoblasts.

Cellular mixed stratified cementum:

  • 100-1000 µm thick.
  • Present at the apical 3rd of roots and at the furcation area of multi-rooted teeth.
  • Consists of fibers (both extrinsic and intrinsic) and may contain cells.
  • It is a product of fibroblasts and cementoblasts.

Cellular intrinsic fiber cementum:

  • It serves to fill resorption lacunae.
  • Consists of intrinsic fibers and cells.
  • It is a product of cementoblasts.

Intermediate cementum:

  • Also known as Layer of Hopewell Smith.
  • Present near cementodentinal junction of certain teeth as an ill-defined zone.
  • Consists of cellular remnants of Hertwig’s epithelial root sheath embedded in a calcified ground substance.

F) Extrinsic and intrinsic fibers of cementum:

 Extrinsic fibers:

They are Sharpey’s fibers that are continuous with the principal fibers of the periodontal ligament. Since these fibers were originally produced by periodontal ligament fibroblasts, they are considered as “extrinsic” to the cementum.  These fibers are orientated more or less perpendicularly to the surface of cementum and play a major role in tooth anchorage.

 Intrinsic fibers:

These fibers are produced by cementoblasts and they are orientated more or less parallel to the surface of cementum. They play no role in tooth anchorage but are present in cementum predominantly at sites undergoing repair.

G) Cementoenamel junction (CEJ):

 Usually 3 types of relationships are found

a)      In 60-65% cases- cementum is overlapping the enamel.

b)      In about 30% cases- cementum and enamel lies edge to edge, this is known as butt joint.

c)       In about 5-10% case- cementum fails to meet the enamel.

H) Cementodentinal junction (CDJ):

It is the interface between cementum and internal root canal dentine at the terminal apical area of the root. The width of CDJ remains almost constant throughout life . During root canal treatment, obturation material should be filled till CDJ.

I) Functions of cementum:

  • Acts as an attachment aperture for the PDL fibers to the tooth root.
  • Covers the root, thus seals the dentinal tubules and prevents dentinal sensitivity.
  • Continuous deposition of cementum at the apex, contribute to the compensation of wear during ageing.
  • It helps to maintain the width of the PDL space.
  • It allows PDL fiber rearrangement.
  • As cementum is resorbed less than bone, it allows orthodontic tooth movement.
  • It serves as a medium by which damage to the root surface is repaired.

J) Resorption of cementum:

  • It has been seen that cementum is resorbed less than bone. But some remodeling does occur in cementum.
  • According to various radiographic studies, the renewal rate of cementoblasts and the turnover rate of PDL fibers near cementum, are found to be much slower than the turnover close to the alveolar bone.
  • The collagen fibers entering the cementum are found to be the last fiber structures in the periodontium to lose their collagen properties after trauma from occlusion, or by scurvy or disuse atrophy.
  • With the increasing age, the number and size of the resorbed areas also increases.
  • Other evidences for internal cemental resorption includes: Cementolysis which is further enhanced by administration of parathyroid hormone.

Alveolar Bone

 

It is that portion of the jaws (maxilla and mandible), which forms and supports the tooth sockets (alveoli).

It rests on basal bone.

The main aim of preventive periodontics and of periodontal therapy is the preservation of alveolar bone.

The alveolar processes are tooth-dependent structures, they develop as the teeth erupt and they resorb to a great extent, after the tooth loss.

Composition:

Inorganic content (67%):

  • Forms the two third content of the bone.
  • Mainly consists of minerals calcium and phosphate, also contains hydroxyl, carbonate, citrate and trace amounts of other ions (like sodium, magnesium and fluorine).

Organic content (33%):

  • Forms one third of the bone.
  • Mainly collagen Type I (90%), Type III, V, XII, XIV.
  • Non-collagenous proteins (in small amounts): osteocalcin, osteonectin, bone morphogenic protein, phosphoproteins and proteoglycans.

Cellular components:

Osteoblasts:

They are uninucleated, cuboidal cells. They are differentiated from pluripotent follicle cells. They synthesize both collagenous and non-collagenous bone proteins. The main function of osteoblasts is bone formation by synthesizing organic matrix of bone.

Osteocytes:

Some osteoblasts that got entrapped in lacunae are known as osteocytes. They are smaller than osteoblasts and have lesser amount of synthetic and secretory organelles. They got oxygen and nutrients via canaliculi( they are th processes of osteocytes that radiate from the lacunae). These canaliculi also help them to remove metabolic waste products.

Bone lining cells:

They cover most of the quiescent bone surfaces in adult skeleton. These cells along with other cells like osteocytes, bone forming cells and their connecting cell processes, are capable of regulating plasma calcium concentration.

Osteoclasts:

These are derived from monocytes. They are multinucleated cells. They are irregular oval or club shaped and about 50-100um in size. These cells are present in Howship’s lacunae( baylike depressions in the bone).

Osteoprogenitor cells:

They generate osteoblasts. They are long, thin stem cells.

Functions:

  • It anchors the teeth and their soft investing tissues.
  • It resolves the forces generated by various functions like intermittent tooth contact , mastication etc.

Morphology:

  • Alveolar bone shows a lot of variations in its morphology, depending upon:
  1. The state of eruption, position, size and shape of the teeth.
  2. Forces to which it is subjected.
  3. Finite thickness below which the bone cannot survive, but gets resorbed.
  • The margin of the alveolar bone usually follows the contour of CEJ. Thus shows scalloping of bony margin. Scalloping is more prominent on facial aspect of anterior teeth than on molars.
  • In case of two adjacent teeth which are erupted to different occlusal planes, the interproximal bone will be inclined towards the root of less erupted tooth.
  • In case of rotated teeth, bone margin is located more coronally and is less scalloped than that of the adjacent normally positioned tooth.
  • In case of teeth with abnormal buccolingual position, on prominent side: there will be a thin layer of cortical bone with little or no spongiosa, and apically positioned bone margin, dehiscence or fenestration. On contralateral side: there will be thick bone and more coronally placed marginal ledge.

Periosteum and endosteum:

Periosteum:

The tissue covering the outer surface of bone is known as periosteum. It is also know as “mother of the bone”.

The periosteum consists of:

  1. Inner layer (composed of osteoblasts surrounded by osteoprogenitor cells): It has potential to differentiate into osteoblasts,
  2. Outer layer: It is rich in blood vessels and nerves and composed of collagen fibers and fibroblasts. Bundles of periosteal collagen fibers penetrate the bone, binding the periosteum to the bone.

Endosteum:

The tissue lining the internal bone cavities. The endosteum is composed of a single layer of osteoblasts and sometimes a small amount of connective tissue.

Parts of alveolar bone (divided anatomically, but it functions as a unit):

Alveolar bone proper:

  • It is identified anatomically as the lamina cribriformis and radiographically as the lamina dura.
  • It is the thin lamella of bone surrounding the tooth root and giving attachment to the principal fibers of PDL.
  • It is seen as lamina dura in radiographs, and appears denser than adjacent supporting structures, due to the mineral orientation around the fiber bundles. But the mineral content of lamina dura and the supporting bone is the same.
  • Also known as cribriform plate as it is perforated by many openings by which neurovascular bundles connect the PDL with the central component of alveolar bone.

Supporting alveolar bone:

  • It surrounds the alveolar bone proper and gives support to the socket.
  • It has two parts:
    1. Cortical plates: They form the outer and inner plates of alveolar bone. They consist of compact bone.
    2. Cancellous bone: It is also known as trabecular bone or spongy bone. It fills the area between the cortical plates and the alveolar bone proper.

Bone remodeling:

  • It is the process which allows replacement of old bone by new bone, in response to functional demands. It involves two processes: bone resorption and bone apposition. Bone resorption is seen on the pressure side and bone apposition on the tension side of the moving tooth.
  • Usually the teeth erupts continuously and migrates mesially to compensate for attritional reduction of teeth in occlusal and mesio-distal dimensions.
  •  Areas of deposition: They exhibit layer of recently deposited bundle bone containing no marrow spaces or osteons. Bundle bone contains collagen fibers (running perpendicular to the bone). Apposition areas are mainly seen on apical one third and on distal aspect of the alveolus. 
  • Areas of resorption: They have rough uneven surfaces with numerous cavities and spicules. Histologically, they are covered with multinucleate osteoclasts and gives moth-eaten appearance. Resorption areas are mostly seen on mesial aspect of alveolus.
  • Various bone resorbing factors are:

Systemic factors: Parathyroid hormone, vitamin D3, thyroid hormone.

Local factors: Prostanoids, lipo-oxygenase metabolites,IL-1, TNF-α, TNF-B, IL-6.

Growth factors: EGF, TGF-A,TGF-B, PDGF.

Bacterial factors: Lipopolysaccherides, capsular material, peptidoglycans, lipoteichoic acids.

 

References:

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