Periodontal ligament (anatomy and structure) l Oral histology and biology MCQs for dental students 

Periodontal ligament (PDL)

• Is a fibrous connective tissue that lies between a tooth and its bony socket (the alveolar bone). Rather than attach directly to the bone, a healthy tooth is suspended in its socket by the fibers of the periodontal ligament. This allows each tooth to have limited individual movement. The PDL space has a narrowest area at the mid-root level. The width of PDL is approximately 0.25 mm. Provides cushioning and acts like a shock absorber. The PDL is noticeably cellular & contains numerous blood vessels.

• The majority of the fibers of the PDL are collagen, & the matrix is composed of proteins & polysaccharides

Development of periodontal ligament:

• The cells of the dental follicle divide & differentiate into the fibroblasts. The formation of PDL occurs after the cells of Hertwig’s epithelial root sheath have separated. The fibroblasts which differentiated from undifferentiated mesenchymal cells in the dental follicle begin to synthesize the fibers & ground substance of the PDL. These fibers embedded in newly developed cementum& alveolar bone.

Cells of periodontal ligament:

• The cells of PDL divided into 4 categories:

1- Synthetic cells:

Fibroblasts:

• found in various stages of differentiation& their progenitor are found in the PDL. In longitudinal sections it appear to be oriented parallel to the oriented bundles of collagen fibers.

Cementoblasts:

• these cells distributed along the surface of the tooth cementum. The progenitor found in PDL

Osteoblasts:

• these cells covering the periodontal surface of the alveolar bone in various stages of differentiation. Collagen fibers of the ligament that penetrate the alveolar bone intervene between these cells. The progenitors are found in the PDL

2- Resorptive cells:

Fibroblasts:

• the fibroblast is capable of both synthesis &resorption of collagen fibers. These cells are found in normal functioning PDL. The progenitor found in PDL.

Cementoclasts:

• these appear as mononuclear or multinucleated giant cells. The functions of these cells are resorption of the cementum. The progenitors for cementoclasts originated from hematopoietic cells

Osteoclasts:

• are cells that resorb bone & tend to be large & multinucleated. These cells are seen regularly in normal functioning PDL. The progenitors are from hematopoietic cells

3-The epithelial rests of malassez:

• The cells are the remnant of the epithelium of Hertwig’s epithelial root sheath & persist in PDL as a network, strands, islands, or tube like structures near & parallel to the surface of the root.

•  When certain pathologic conditions are present, these cells can undergo rapid proliferation & produce a variety of cysts & tumors that are unique to the jaws. 

4- The defense cells

Mast cells:

• Are relatively small, round or oval cell & secret histamine which plays a role in the inflammation. The release of histamine into the extracellular environment causes proliferation of endothelial cells &mesenchymal cells.

Macrophages:

• Are predominantly located adjacent to blood vessels. In PDL play a dual role:

1. Phagocytosing dead cells.
2. Secreting factors that regulate the proliferation of adjacent fibroblasts

 

Extracellular substance: the extracellular substance of the PDL comprises the following:

1- Fibers of periodontal ligament :

A. Collagen fibers: 

• collagen fibers in the PDL are found to be gathered into bundles having clear orientation relative to the PD space, & these are termed principal fibers.

The principal fibers of PDL are arranged in six particular groups:

1. Alveolar crest group: The fiber bundle of this group radiated from the crest of the alveolar process & attached to the cervical part of the cementum
2. Horizontal group: The fiber run at right angles to the long axis of the tooth from the cementum to the bone.
3. Oblique group: The fiber run obliquely. They are attached in the cementum somewhat apically from their attachment to the bone. These fiber bundles are most numerous & constitute the main attachment of the tooth.
4. Apical group: The fibers are irregularly arranged & radiated from the apical region of the root to the surrounding bone
5. Interradicular group: From the crest of the interradicular septum, bundles extend to the furcation of multirooted teeth
6. Trans-septal Fibers: The trans-septal fibers extend from one tooth to an adjacent tooth over the crest of the interdental septum. The fibers blend into the periosteum at the crest of the bony septum

B- Oxytalan fibers:

• Are immature elastic fibers. It runs in an axial direction, one end being embedded in cementum or bone & the other end in the wall of blood vessels. The function of these fibers is supporting the blood vessels of the PDL.

C- Elastic fibers:

• are relatively few and associated with blood vessels

Sharpey’s fibers:

• Are the portions of the principal fibers which are embedded in the root cementum and in the alveolar bone.

Intermediate plexus:

• In the mid- section of PDL the fibers bundles arising from both sides of cementum and alveolar bone joined together by small, thick strands of collagen fibers.

2- Ground substance:

• This occupies the space between cells, fibers, blood vessels, & nerves in the PDL. In addition all anabolites reaching the cells from the microcirculation in the ligament & all catabolites passing in the opposite direction must pass through the ground substance.
• The ground substance is made up of 2 major groups of substances

1. Proteoglycans
2. Glycoproteins

• Both groups are composed of protein & polysaccharides.

Structures present in Periodontal ligament connective tissue

1- Blood vessels:

• The arterial vessels of the PDL are derived from 3 sources:

1. Branches from apical vessels that supply the dental pulp.
2. Branches from intra-alveolar vessels. These run horizontally penetrating the alveolar bone to enter the PDL.
3. Branches from gingival vessels. These enter the PDL from the coronal direction. 

2- Lymphatics

• A network of lymphatic vessels, following the path of the blood vessels, provides the lymph drainage of the PDL. The flow is from the ligament toward & into the adjacent alveolar bone.

3- Nerves:

• Usually are associated with blood vessels, pass through foramina in the alveolar bone, including the apical foramen, to enter the PDL.
• The nerve fibers are either of large diameter &myelinated or of small diameter in which may or may not be myelinated. The large diameter fibers concerned with pressure & the small ones with pain.

4- Cementicles :      see part 1 of pulp

Functions of periodontal ligament:

• The PDL has the following functions:

1- Supportive:

• When a tooth is moved as a result of force during mastication or of an orthodontic force, one side compressed, while other side widened.
• The fibers of PDL in compressed side act as a cushion for the displaced tooth to prevent pressure on the blood vessels & other structures. While in the widened parts the collagen fibers of PDL prevent the tooth from being moved too far.
• In addition the fibers of PDL attaching the cementum of the root to alveolar bone & keep the tooth in its socket

2- Sensory:                                                                        

• The PDL by its nerve supply, provides a most efficient proprioceptive mechanism (is the sense of the relative position of neighboring parts of the body. It is the sense that indicates whether the body is moving with required effort, as well as where the various parts of the body are located in relation to each other, found chiefly in muscles, tendons, joints, and the inner ear, that detects the motion or position of the body or a limb by responding to stimuli arising within the organism ) , allowing to detect any foreign bodies such as stone during bitten soft food, in order to protect both the supporting structures of the tooth & the substance of the crown from the effects of excessively masticatory force.

3- Nutritive:

• Through the blood vessels of the PDL, anabolites& other substances required by the cells of the ligament, cementum, & superficial osteocytes of the alveolar bone are provided.
• The blood vessels also concerned with removal of catabolites (unnecessary cell products)
• Occlusion of blood vessels leads to necrosis of cells in the affected part of the ligament.

4- Homeostatic: 

The ability or tendency of an organism or cell to maintain internal equilibrium by adjusting its physiological processes.

 

Clinical considerations

• The primary role of the PDL is to support the tooth in the bony socket. Its thickness varies in different individuals, in different teeth in the same person, & in different locations on the same tooth. The thickness of the PDL maintained by the functional movements of the tooth. It is thin in functionless & embedded teeth & wide in teeth that are under excessive occlusal stresses.

• Acute trauma to the PDL may produce pathologic changes such as fractures or resorption of the cementum, tears of fiber bundles, hemorrhage, & necrosis. The adjacent alveolar bone is resorbed, the PDL is widened, & the tooth becomes loose. When trauma is eliminated, repairs usually take place.

• During orthodontic tooth movement. If the movement of teeth is within physiologic limits, the initial compression of the PDL on the pressure side is compensated by bone resorption, whereas on the tension side bone depositions occur.

• Application of large forces result in necrosis of PDL & alveolar bone on the pressure side, & movement of the tooth will occur only after the necrotic bone resorbed by osteoclasts located on its surface.

• The PDL in the periapical area of the tooth is often the site of a pathologic lesion that progress from the pulp to apical PDL & replace its fiber bundles with granulation tissue. This lesion, called a dental granuloma, may contain epithelial cells that undergo proliferation & produce a cyst.

• Gingivitis, or inflammation of the gingiva, is the most common dental disease. If not controlled or treated, periodontitis may develop, & destruction may extend into the PDL & bone. Once destroyed by periodontitis, the PDL & the alveolar bone are very difficult to regenerate.