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|The tissues of the periodontium combine to form an active, dynamic group of tissues. The alveolar bone (C) is surrounded for the most part by the subepithelial connective tissue of the gingiva, which in turn is covered by the various characteristic gingival epithelia. The cementum overlaying the tooth root (B) is attached to the adjacent cortical surface of the alveolar bone by the alveolar crest (I), horizontal (J) and oblique (K) fibers of the periodontal ligament.|
The periodontal fiber or 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. These fibers help the tooth withstand the naturally substantial compressive forces which occur during chewing and remain embedded in the bone.
Functions of PDL are supportive, sensory, nutritive, homeostatic and eruptive.
Structure of the PDL 
It consist of cells, and extracellular compartment of fibers. The cells are fibroblast, epithelial, undifferentiated mesenchymal cells, bone and cementum cells. The extracellular compartment consists of collagen fibers bundles embedded in ground substance. The PDL substance has been estimated to be 70% water and is thought to have a significant effect on the tooth's ability to withstand stress loads. The PDL is a part of the periodontium that provide 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 radio opaque cementum.
Types of fibers 
- The PDL collagen fibers are categorized according to their orientation and location along the tooth. They are:
Transseptal fibers 
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 gingiva because they DO NOT have osseous attachment.
Alveolar crest fibers 
Alveolar crest fibers (I): extend obliquely from the cementum just beneath the junctional epithelium to the alveolar crest. These fibers prevent the extrusion of the tooth and resist lateral tooth movements.
Horizontal fibers 
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 
Oblique fibers (K): are the most numerous fibers in the periodontal ligament, running from cementum in an oblique direction to insert into bone coronally.
Apical fibers 
radiating from cementum around the apex of the root to the bone, forming base of the socket
Interradicular fibers 
Interradicular fibers are only found between the roots of multi-rooted teeth, such as molars. They also attach from the cementum and insert to the nearby alveolar bone.
- Elastic fibrils:
The completeness and vitality of the PDL are essential for the functioning of the tooth. Damage to the PDL may result in ankylosis of the tooth to the jawbone, making the tooth lose its continuous eruption ability. Dental trauma, such as subluxation, may cause tearing of the PDL and pain during function (eating).
- Herbert F. Wolf; Klaus H. Rateitschak (2005). Periodontology. Thieme. pp. 12–. ISBN 978-0-86577-902-0. Retrieved 21 June 2011.
- Zadik Y (December 2008). "Algorithm of first-aid management of dental trauma for medics and corpsmen". Dent Traumatol 24 (6): 698–701. doi:10.1111/j.1600-9657.2008.00649.x. PMID 19021668.