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It is the fibrous structure of the connective tissue, with neural and vascular components, that joins the cementum that covers the root with the alveolar bone.
Despite its fibrous nature, the periodontal ligament is a highly cellular structure that can perform a number of important functions that are essential for the long-term health of the chewing apparatus.
Origin and development
The periodontal ligament is obtained embryologically from the ectomesenchymal tissue of the dental follicle that surrounds the developing tooth in its bony crypt.
At the time of tooth eruption, the collagen cells and fibers in the dental follicle, that is, the future periodontal ligament, are oriented primarily with their longitudinal axis parallel to the root surface.
Follicle remodeling in a periodontal ligament begins at the cementum-enamel junction and continues in an apical direction.
Functions of the periodontal ligament
The periodontal ligament serves primarily as a supporting function by attaching the tooth to the surrounding alveolar bone itself . This function is mainly mediated by the main fibers of the periodontal ligament that form a strong fibrous junction between the root cementum and the bone.
The periodontal ligament also serves as a shock absorber by mechanisms that provide resistance to light as well as heavy forces. The light forces are dampened by intravascular fluid that is forced out of the blood vessels.
Moderate forces are also absorbed by extravascular tissue fluid that is forced out of the periodontal ligament space into the adjacent medullary spaces. The main forces are absorbed by the main fibers.
The periodontal ligament also fulfills an important remodeling function by providing cells capable of forming and reabsorbing all the tissues that make up the insertion apparatus, that is, bone, cementum, and the periodontal ligament.
Undifferentiated ectomesenchymal cells, located around blood vessels, can differentiate into specialized cells that form bone (osteoblasts), cementum ( cementoblasts ), and connective tissue fibers (fibroblasts).
The cells that resorb bones and teeth (osteoclasts and odontoclasts) are generally multinucleated cells derived from blood-borne macrophages.
The periodontal ligament also has a sensory function. The myelinated dental nerves that pierce the bottom of the alveoli rapidly lose their myelin sheath as they branch out to supply both the pulp and the periodontal ligament.
The periodontal ligament is rich in nerve endings that are primarily receptors for pain and pressure. Finally, the periodontal ligament provides a nourishing function that maintains the vitality of its various cells.
The ligament is well vascularized, and the main blood supply originates from the dental arteries that enter the ligament through the bottom of the alveoli. There are significant anastomoses between blood vessels in the adjacent medullary spaces and the gingiva.
Major fibers of the periodontal ligament
The periodontal fibers of the periodontal ligament are composed mainly of bundles of type I collagen fibrils. They have been classified into various groups based on their anatomical location.
The following constitute the major fiber groups of the periodontal ligament:
- Fibers of the alveolar ridge.
- Horizontal fibers.
- Oblique fibers.
- Periapical fibers.
- Interradicular fibers.
In addition to collagen fibers, the periodontal ligament also contains oxytalan fibers that are related to the microfibrillar component of elastic fibers. They generally run parallel to the root surface, although occasionally they can be inserted into the cementum.
