Periodontology Anatomy - Periodontal ligament

The periodontal ligament, which is commonly abbreviated as the PDL, is a group of specialized connective tissue fibers which effectively connect a tooth to the alveolar bone. The PDL inserts into the root cementum on one side and to the alveolar bone on the other.

The tissues of the periodontium are combined to form a dynamic and active group of tissues. The alveolar bone (C) is mostly surrounded by the subepithelial connective tissue of the gingiva. It is covered by the various characteristic gingival epithelia. The cementum which overlays the tooth root 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.

Structure

The PDL is comprised of principal fibers, loose connective tissue, blast and clast cells, oxytalan fibers and Cell Rest of Malassez.

Alveolodental ligament

The primary principal fiber group is the alveolodental ligament. The alveolodental ligament consists of five various fiber subgroups. These fiber subgroups include the following: alveolar crest, horizontal, oblique, apical and interradicular on multirooted teeth. Principal fibers which are not the alveolodental ligament are the transseptal fibers.

These fibers aid the tooth in withstanding the natural compression forces that occur during chewing and remain embedded in the bone. The ends of the principal fibers within the cementum or alveolar bone proper are the Sharpey fibers.

Transseptal fibers

Transseptal fibers (H) extend interproximally over the alveolar bone crest and are embedded in the cementum of adjacent teeth. There they form an interdental ligament. These fibers work together to keep the teeth in alignment. Because these fibers do not have an osseous attachment, they may be considered as belonging to the gingival tissue.

Loose connective tissue

Loose connective tissue is comprised of fibers, extracellular matrix, cells, nerves and blood vessels. The extracellular compartment includes Type 1, 3, and 5 collagen fiber bundles which are embedded in intercellular substance. The PDL collagen fibers are categorized based on how they are oriented and their location along the tooth. The cells include the following: fibroblast, defence cells and undifferentiated mesenchymal cells.

Cell Rest of Malassez

These groups of epithelial cells are located in the mature PDL after the disintegration of Hertwig epithelial root sheath as the root is formed. They form a plexus surrounding the entire tooth. Cell Rests of Malassez can cause a cyst formations later stages of life.

Oxytalan fibers

Oxytalan fibers are natuallry elastic and unique to PDL. It inserts into the cementum and runs in two different directions. The fibers run parallel to the root surface and oblique to the root surface. The function maintains the patency of blood vessels during occlusal loading. Additional research is required in order to determine the function of oxytalan fibres.

Composition

It is estimated that the PDL substance is about 70% water. It is thought to have a significant effect on the tooth's strength and ability to withstand loads. The completeness and vitality of the PDL are crucial in a functioning tooth.

The width of the PDL typically ranges from 0.15 to 0.38mm. The thinnest part is located in the middle third of the root. The width is progressively reduced as a patient ages.