Learners always found these crisscrossing fibers difficult to understand. In this informative blog, we will explain these periodontal ligament fibers in an easy to understand manner, without excluding important details.
Periodontal ligaments (PDL) are one of the important components of the periodontium (the investing and supporting tissues of the human tooth). Besides periodontal ligaments, other components that invest and support the human tooth are cementum, alveolar bone, and part of the gingiva that is facing towards the teeth.
The periodontal ligaments are the dense soft connective tissue. Most of the periodontal ligament fibers are present between cementum and the bone lining the tooth socket. The periodontal ligaments (PDL) are derived from the dental follicular cells. The periodontal ligaments with cementum and alveolar bone form a specialized fibrous joint that is known as gomphosis.
Functions Performed by the Periodontal ligaments
The periodontal ligament fibers perform the following main functions.
- The PDL fibers support the tooth in the socket and keep the gingival tissues in their proper relationship with teeth and the alveolar bone.
- The Periodontal ligament fibers act as a shock absorber and therefore, they withstand and absorb the impact of masticatory forces.
- There are cells in the periodontal ligament space and those cells form bone, cementum, and periodontal ligaments. Besides these cells, there are stem cells that can differentiate into different cell families according to the requirement.
Cells in the Periodontal ligament space
The following cells are present within the PDL space.
Fibroblasts
These are the principal cells of the periodontal ligament. The fibroblasts they achieve remodeling (synthesis and degradation) of the collagen fibers. Increase expression of matrix metalloproteinases by these cells aggressively destroy collagen fibers during periodontal disease.
Epithelial Cells
These cells in the periodontal ligament space are the remnants of Hertwig epithelial root sheath cells (HERS) known as the rest cells of malassez. These cells are present close to cementum in the form of clusters or strands. The epithelial rest cells of malassez have a role in the repair and regeneration of the periodontium.
Undifferentiated cells
These cells are located around the blood vessels (perivascular cells). These cells are the source of new cells for the PDL space as after the deletion of old cells by programmed cell death (apoptosis) there is a requirement of new cells. These undifferentiated cells can differentiate into various cell types present in the PDL space.
Bone and Cementum Cells
Besides these cells, other cells that are present in the periodontal ligament space are bone and cementum cells. The cementum cells are the cementoblasts and the cementoclasts and bone cells that are present are osteoblasts and osteoclasts. Besides these cells, there are also defense cells that are present and those include mast cells, macrophages, and eosinophils.
Periodontal Ligament fiber bundles
The periodontal ligament fiber bundles mainly comprise of type 1 and Type 3 collagen fibers. These collagen fibers are arranged in fiber bundles. Besides these other fibers that are present in small amounts are Elastin, Oxytalan, and Elaunin fibers.
The PDL ligament fiber bundles are arranged into two groups.
- Principal fiber bundles (originate from cementum and insert into alveolar bone).
- Gingival Ligaments (originate either from cementum or alveolar bone and inserts into gingiva).
Principal Fiber Bundles
The principal fiber bundles are arranged into five groups.
Alveolar crest fibers
The alveolar crest fibers originate from the cementum just below the cementoenamel junction (CEJ) and insert at the crest of the alveolar bone socket.
Horizontal fibers
The horizontal group of fibers presents below the alveolar crest group and they originate from the cementum and at a right angle to the long axis of the tooth inserts into the alveolar bone.
Oblique group
The oblique group of fibers originates from the cementum and in an oblique direction inserts into the alveolar bone lining the socket coronally. The oblique group of fibers is more in number as compared to other principal fiber groups.
Apical Group
These fibers are present at the base of the socket. The apical group of PDL fibers originates from the cementum around the apex of the root and inserts into the adjacent alveolar bone.
Interradicular group
These group of fibers they originate from the interradicular cementum present at the furcation areas of the root and inserts into the interradicular septum of the alveolar bone. The interradicular fibers are only present in the multi-rooted teeth.
These all principal fiber groups are embedded in each end into cementum and another end into the alveolar bone. The embedded portion of these PDL fiber bundles is referred to as Sharpey’s fibers. These Sharpey’s fibers in the acellular cementum are fully mineralized while in the cellular cementum and the alveolar bone these fiber bundles are only mineralized at their periphery.
Figure 1 The orientation and names of the Principal fibers (follow the color and numbers) of the PDL.
- Alveolar crest fibers
- Horizontal fibers
- Oblique group
- Apical Group
- Interradicular group
- Transseptal group (part of Gingival fiber group)
Gingival ligaments
The gingival ligaments are arranged into five groups.
Dentogingival group
The dentogingival group of fibers they originate from the cervical cementum (near cementoenamel junction) and inserts into the connective tissue of the free gingiva. Among the gingival group of fibers, these are the most frequent fiber bundles.
Alveologingival group
These fibers originate from the crest of the alveolar bone and inserts into lamina propria of the free gingiva.
Circular group
This fiber group forms a band around the neck of the tooth. Together with the gingival group of fibers, these fibers bind the gingiva to the neck of the tooth.
Dentoperiosteal Group
These group of fibers they originate from the cementum slightly below the dentogingival fibers and inserts into the outer cortical plate of the alveolar bone.
Transseptal Fibers
These fibers originate from cementum just below the cementoenamel junction (CEJ) passes over the alveolar crest and inserts into the cementum of an adjacent tooth. These fibers are also known as interdental ligaments as they connect all the teeth in the dental arch.
The turnover time (formation of new fibers and replacement of old fibers) of transseptal fibers is low as compared to other fiber groups. Therefore, the prolonged retention period is required after orthodontic treatment. Post retention relapse is associated with these fibers.
Figure 2 The orientation and names of the Gingival fibers of the PDL (follow the color and numbers). Transseptal group of fibers is in figure 1
- Dentogingival group
- Alveologingival group
- Circular group
- Dentoperiosteal Group
Blood Supply of the Periodontal ligaments
The PDL has a rich blood supply and is supplied by the superior and inferior alveolar arteries. The branches of these arteries enter the periodontal ligament space as perforating arteries. The blood supply is more in the PDL space of the posterior teeth as compared to the anterior teeth.
Nerve Supply of the Periodontal ligaments
Nerve fibers enter into the PDL space from the apical region towards the gingival margin. Few nerve fibers enter form the later walls of the tooth socket. Most of the nerve ending in the PDL space are the free nerve endings.