Introduction
The Periodontium refers to the tissues investing and supporting the teeth, including the cementum, periodontal ligament (PDL), alveolar bone and gingiva.The PDL is a soft, specialized connective tissue that connects the tooth to the alveolar bone.
The embryologic origin of the PDL is from the dental follicle.
The average thickness of periodontal ligament in a young adult is 0.21 mm, in a mature adult is 0.18 mm and in older adult is 0.15 mm.
Other terms which were previously used for periodontal ligament were
(a) Desmodont
(b) Gomphosis
(c) Pericementum
(d) Dental periosteum
(e) Alveolodental ligament
(f) Periodontal membrane.
On radiograph, periodontal ligament appears as a radiolucent area of 0.4 to 1.5 mm, thickness between the radiopaque cementum and radiopaque lamina dura of the alveolar bone. This is also known as Periodontal Ligament Space
Development of Periodontal Ligament
Periodontal ligament is developed from Dental Follicle.Dental follicle is composed of mesenchymal cells of the dental follicle proper in the centre and the perifollicular mesenchymal cells in the periphery.
You can read development of tooth in detail from this link.
Components of Periodontal Ligament
The periodontal ligament is a highly cellular and complex vascular structure composed of fibres which are principally collagenous. The ground substance contains cells, vessels and neural elements.
Cells present in PDL
Various types of cell are found in a functioning periodontal ligament. This helps in the synthesis and resorption of the alveolar bone and fibrous connective tissue of the ligament and the cementum.1) Synthetic Cells
A) Osteoblasts
Osteoblasts are derived from the multipotent mesenchymal cells. They cover the periodontal surface of the alveolar bone.
Osteoblasts are basophilic cuboidal cells, with abundant rough endoplasmic reticulum,
mitochondria and vesicles. These cells contact one another through tight junctions and desmosomes.
B) Fibroblasts
Fibroblasts are found in the periodontal ligament surrounded by fibers and ground substance. Fibroblasts show adherens and gap junction.
The functions of fibroblasts are as follows:
A. Formation and remodeling of periodontal ligament fibers,
B. Maintain the width of the periodontal ligament,
C. Production of collagen, elastin, structural connective tissue proteins, glycoproteins and glycosaminoglycans.
D. Secretion of active collagenase and enzyme matrix metalloproteinases.
C) Cementoblasts
Cementoblasts are found on the surface of the cementum. They are distributed in the same manner as the osteoblasts are on bone surface.
Prominent cytoplasmic processes are present in cementoblast depositing cellular cementum and absent in cementoblast depositing acellular cementum. Cells contact one another through desmosomes and gap junctions.
2) Resorptive Cells
A) Osteoclasts
Multinucleated osteoclasts are the cells which resorb the bone.
The surface of an osteoclast which is in contact with the bone has a ruffled border. The presence of osteoclasts indicates resorption of the alveolar bone. Histochemical test shows the presence of acid phosphatase in lysosomes.
B) Fibroblasts
Fibroblast cells are capable of both synthesis and resorption. The collagen fibers of periodontal ligament can be resorbed by the mononuclear fibroblasts. These fibroblasts are present in the periodontal ligament for the remodeling process. They exhibit lysosomes, which contain collagen fragments undergoing digestion.
C) Cementoclasts
Cementoclasts are found in the periodontal ligament but the cementum is not remodeled like alveolar bone and periodontal ligament. Under certain conditions, the resorption of cementum can occur. The mononuclear cementoclasts or multinucleated giant cells are located in Howship’s lacunae. These are found on surface of the cementum.
3) Progenitor Cells
These are the undifferentiated mesenchymal cells. They constitute an important cellular component of the periodontal ligament. They have the capacity to undergo mitotic division and replace the differentiated cells dying at the end of their life span. The progenitor cells are located in perivascular region.
In orthodontic therapy, after application of force to a tooth or after wounding, burst of mitosis occurs in the progenitor cells. This stimulates the proliferation and differentiation of cells of periodontal ligament. From the perivascular area these cells enter the periodontal ligament.
4) Defence cells
Monocytes/macrophages, mast cells and eosinophils are normal inhabitants of the PDL. Neutrophils and lymphocytes can be seen in response to inflammation.
A. Monocytes
They are located mainly around blood vessels and nerves and not in the dense connective tissue of the PDL.
B. Macrophages
These are fully matured monocytes in the tissue and are responsible for phagocytosing particulate matter and invading organisms.
C. Mast Cells
These are frequently associated with blood vessels. The function of these cells is production of histamine, heparin and factors associated with anaphylaxis.
D. Eosinophils
These cells are seldom seen in the PDL; they display the property of phagocytosis.
5) Epithelial Rests of Malassez
These are remnants of Hertwig’s epithelial root sheath scattered throughout the PDL. They are arranged as clumps of cells which are closely packed and have a prominent nucleus and a scanty cytoplasm.
Extracellular Substance present in PDL
Extracellular substance comprises the following.1. Fibers
a. Collagen
b. Elastic
2. Ground substance
a. Proteoglycans
b. Glycoproteins
1) Fibers
A. Collagen Fibers
Collagen fibers are the principal fibers of the periodontal ligament. The periodontal ligament is basically made up of a type I and type III collagen.
There are approximately twelve types of collagen, which are similar in chemical structure. Each collagen is produced by a different gene. Vitamin C helps in the formation and repair of collagen.
Type I collagen forms the bulk (80%) of fibres in the PDL.
Small amounts of types V and VI and traces of types IV and VII have also been found in the
PDL.
The rate of turnover of the collagen is faster in PDL compared with all other connective
tissues.
The presence of type III perhaps explains this rapid turnover.
They are arranged in bundles running from the tooth to the bone. These fibres are known
as the principal fibre bundles of the PDL. The terminal portion of the principal fibres of the PDL inserted in the cementum and alveolar bone is known as Sharpey’s fibres.
Principal Fibers of Periodontal Ligament (PDL)
The principal fibres of the PDL can be grouped under five divisions according to the manner in which they are arranged.1) Alveolar Crest Group Fibers
2) Horizontal Fibers
3) Oblique Fibers
4) Apical Fibers
5) Inter-Radicular Fibers
Intermediate Plexus
The principal fibers course from cementum to bone. Under light microscope, it is seen that some fibers arising from cementum and bone are joined in the mid-region of the periodontal space. This gives rise to a distinct zone called as intermediate plexus. The remodeling of fibers takes place in intermediate plexus.
B. Elastic Fibers
Elastic fibers are of the following types.
1. Mature elastic fibers
2. Immature elastic fibers
2) Ground Substance
The ground substance is present in between the cells and fibers of the periodontal ligament. All the anabolic and catabolic substances pass through ground substance. Hence, for proper functioning of the periodontal ligament its integrity should be maintained.
Ground substance is a gel-like matrix and consists of hyaluronate, glycoprotein, proteoglycans glycosaminoglycans and substrate adhesion molecules. Two major groups of substances are, proteoglycans and glycoproteins.
Blood Supply
Nerve Supply
Lymphatic Supply
Lymph channels draining the area beneath the junctional epithelium pass into the PDL and
accompany the blood vessels into the periapical region. They continue through the alveolar bone to the inferior dental canal in the mandible and the infraorbital canal in the maxilla and then finally drain into submandibular nodes.
Cementicles
Cementicles are the small calcified bodies present in periodontal ligament. These are not the cells of cementum. They may form into the large calcified bodies and fuse with cementum, or they remain free. When they join with cementum, cementicles form the excementoses.Functions of Periodontal Ligament
The Periodontal Ligament Functions are Physical, Formative and Remodeling, Nutritive and Sensory.
1) Physical Function
The primary function of the PDL is to protect the tooth from masticatory forces by transmitting these forces to the supporting alveolar bone and also protect nerves and vessels in the PDL.
The viscoelastic system of PDL acts as a shock absorber protecting the tooth from undue forces.
The PDL is responsible for maintaining the tooth in functional position during tooth eruption.
In addition, it serves as the tissue of attachment between the tooth and alveolar bone.
2) Formative and Remodeling Function
The cells of the PDL take part in the formation, resorption, maintenance and repair of
alveolar bone and cementum during physiologic tooth movement.
There is continual turnover of periodontal fibers. The half-life of collagen in the PDL
is about one-fifth of that in the gingiva and about 1⁄15th of that in skin. Turnover slows down with age.
The PDL constantly undergoes remodelling, i.e. degenerated old cells and fibres are
replaced by new ones, thus maintaining the homeostasis of the tissue.
It enables the PDL to maintain a width (Periodontal Ligament Space) of 0.2–0.3 mm despite being surrounded by hard tissues such as cementum and bone.
3) Nutritive
The periodontal ligament has a rich vascular supply which anastomoses with that in the gingiva and the adjacent marrow spaces of the alveolar bone essential in the maintenance of an adequate nutritional supply during functional demands.
4) Sensory
The PDL contains sensory receptors which perform the sensory functions of pressure and pain.
Periodontal nerve fibres have both myelinated and unmyelinated structures. The myelinated fibres are 5–15 μm in diameter and mainly convey pressure sensation. Proprioceptors and unmyelinated fibres are about 0.5 μm in diameter and convey the sensation of pain to the higher neurological centres.