Periodontal Anatomy – Periodontal ligament

The periodontal ligament, which is often referred to as the PDL, is a group of specialized connective tissue fibers which attach the tooth to the alveolar bone. On one side, it inserts into the root cementum and it inserts into the alveolar bone on the other.

Structure

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

Alveolodental Ligament

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 which 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.

Transseptal fibers

The transseptal fibers extend interproximally just over the alveolar bone crest. They are embedded into the cementum of the adjacent teeth where they form an interdental ligament. These fibers work to maintain alignment of the teeth. Some consider these fibers to belong to the gingival tissue as they do not contain an osseous attachment.

Loose Connective Tissue

The loose connective tissue contains the following: 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 collagen fibers of the PDL are classified based on how they are oriented and their specific location on the tooth. The cells contain defence cells, fibroblasts, and undifferentiated mesenchymal cells.

Cell Rest of Malassez

This group of epithelial cells is located in the mature PDL following the disintegration of Hertwig epithelial root sheath, which occurs during the formation of the tooth’s root. These cells then form a plexus surrounding the tooth. The Cell Rests of Malassez may especially prolific when there is inflammation. This can lead to lead to the formation of radicular cysts later in life.

Oxytalan Fibers

Oxytalan Fibers are specific to the PDL and they are naturally elastic. The fibers insert into the cementum and runs in two different directions including parallel to root’s surface and oblique to root’s surface. These fibers are thought to maintain the patency of blood vessels in occlusal loading, however, additional research is needed to determine their primary function.

Composition

The composition of the PDL is estimated to be about 70% water. It is believed that this has a significant impact on the ability of the tooth withstand heavy stress loads. The vitality and completeness of the PDL is critical in a healthy, functional tooth.

The PDL can range in its width from about 0.15 to 0.38mm. This measurement is taken at its thinnest portion which is located in the middle third section of the root. As patients age, the width slowly and continually decreases. The PDL is included in the periodontium which offers attachment of the teeth to the surrounding alveolar bone through the cementum. In radiographs, the PDL appears as the periodontal space of about 0.4 to 1.5 mm. It appears as a radiolucent area which is located between the radiopaque lamina dura of the alveolar bone proper and the radiopaque cementum.