The periodontal ligament, commonly abbreviated as the PDL, is a group of specialized connective tissue fibers that essentially attach a tooth to the alveolar bone within which it sits. It inserts into root cementum one side and onto alveolar bone on the other.
The PDL consists of principal fibres, loose connective tissue, blast and clast cells, oxytalan fibres and Cell Rest of Malassez.
The main principal fiber group is the alveolodental ligament, which consists of five fiber subgroups: alveolar crest, horizontal, oblique, apical, and interradicular on multirooted teeth. Principal fibers other than the alveolodental ligament are the transseptal fibers.
All these fibers help the tooth withstand the naturally substantial compressive forces that occur during chewing and remain embedded in the bone. The ends of the principal fibers that are within either cementum or alveolar bone proper are considered Sharpey fibers.
- Alveolar crest fibers (I) run from the cervical part of the root to the alveolar bone crest
- Horizontal fibers (J) attach to the cementum apical to the alveolar crest fibers and run perpendicularly from the root of the tooth to the alveolar bone..
- Oblique fibers (K) are the most numerous fibers in the periodontal ligament, running from cementum in an oblique direction to insert into bone coronally. These fibres resist vertical & intrusive forces
- Apical fibers are found radiating from cementum around the apex of the root to the bone, forming base of the socket or alveolus.
- Interradicular fibers are only found between the roots of multirooted teeth, such as premolars and molars. They extend from radicular cementum to interradicular alveolar bone.
Transseptal fibers (H) extend interproximally over the alveolar bone crest and are embedded in the cementum of adjacent teeth; they form an interdental ligament. These fibers keep all the teeth aligned. These fibers may be considered as belonging to the gingival tissue because they do not have an osseous attachment.
【Loose connective tissue】
Loose connective tissue contains fibres, extracellular matrix, cells, nerves and blood vessels. The extracellular compartment consists of Type 1, 3, and 5 collagen fibers bundles embedded in intercellular substance. The PDL collagen fibers are categorized according to their orientation and location along the tooth. The cells include fibroblast, defence cells and undifferentiated mesenchymal cells.
【Cell Rest of Malassez】
These groups of epithelial cells become located in the mature PDL after the disintegration of Hertwig epithelial root sheath during the formation of the root.They form a plexus that surrounds the tooth. Cell Rests of Malassez might proliferate during inflammation which may lead to radicular cyst formation in later life.
Oxytalan fibres are unique to the PDL and are elastic in nature. It inserts into cementum and runs in 2 directions; parallel to root surface and oblique to root surface. The function is thought to maintain the patency of blood vessels during occlusal loading. Further research is needed to determine the function of oxytalan fibres.
The PDL substance has been estimated to be 70% water, which is thought to have a significant effect on the tooth's ability to withstand stress loads. The completeness and vitality of the PDL are essential for the functioning of the tooth.
The PDL ranges in width from 0.15 to 0.38mm with its thinnest part located in the middle third of the root. The width progressively decreases with age.
The PDL is a part of the periodontium that provides for the attachment of the teeth to the surrounding alveolar bone by way of the cementum.
The PDL appears as the periodontal space of 0.4 to 1.5 mm on radiographs, a radiolucent area between the radiopaque lamina dura of the alveolar bone proper and the radiopaque cementum.
PDL cells are one of the many cells derived from the dental follicle and this occurs after crown formation is completed and when the roots start developing. These cells will remodel the dental follicle to form the PDL. Formation of PDL will start at the cementoenamel junction and proceeds in an apical direction.
Effects of Mechanical Forces on the Development of PDL
Movement of teeth is determined by two factors; deposition of bone on the tension side and resorption of the bone on the compression side of the periodontal ligament (PDL). During this movement, bone remodelling is initiated by the PDL in which forces are transmitted from the tooth to the alveolar bone. Fibroblasts of the PDL will react to mechanical stress, therefore affecting osteoblastogenesis and osteoclastogensis of the cells. When mechanical stimuli are introduced to the cells osteocytes in the PDL will differentiate into osteoclasts which will then reform and remodel the bone structure in the affected area. For example, orthodontic treatment involves application of a mechanical force on to the teeth to align them and this is done through this complex combination of physical and cellular processes.
Functions of PDL are supportive, sensory, nutritive, and remodelling.
The PDL is a part of the periodontium that provides for the attachment of the teeth to the surrounding alveolar bone by way of the cementum. PDL fibres also provide a role in load transfer between the teeth and alveolar bone. (PDL fibres absorb and transmit forces between teeth and alveolar bone. It acts as an effective support during the masticatory function.)
PDL is heavily innervated; it involves mechanoreception, nociception and reflexes. Periodontal mechanoreceptors are present in pdl. They will transmit information about the stimulated tooth, direction and amplitude of forces.
It maintains the vitality of the surrounding cells. (PDL is heavily anastomosed). There are 3 principal sources of blood vessels which are apical vessels, perforating vessels and gingival vessels. Apical vessels originate from vessels that supply the pulp. Perforating vessels originate from lamina dura and the vessels perforate the socket wall (cribriform plate). Gingival vessels are derived from the gingival tissue. Outer layers of blood supply in PDL may help in mechanical suspension and support of the tooth while inner layers of blood vessels supply surrounding PDL tissues.
There are progenitor cells in the periodontal ligament that can differentiate into osteoblasts for the physiological maintenance of alveolar bone and, most likely, for its repair as well.
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