Periodontal Ligament (PDL)

Periodontal Ligament (PDL)

• A dense, regular fibrous connective tissue that occupies the periodontal space between the root cementum and the alveolar bone, attaching them together.
• It's considered a type of joint called gomphosis comprised of fibrous tissue
• It is continuous with both gingival connective tissue above the alveolar crest and the pulp at the apical foramen.
• The continuity explains why inflammation from the pulp spreads to involve the periodontal ligament and other apical supporting tissues.
• The width is about 0.25mm, allowing the physiological movement of the tooth.
• Variation in width occurs regarding location, function, and age between teeth and within an individual tooth.
• The periodontal space (containing the PDL) is narrower in non-functional, un-erupted teeth and wider in teeth with high occlusal loads.
• Dentists assess gum health and periodontal disease by inserting a periodontal probe into the gingival sulcus, the natural space between the tooth and gum
• In healthy gums, the probe stops within 1-2mm inside the tooth's gingival socket.
• In periodontal disease, the probe goes deeper than 2mm.

Periodontal Ligament in Research

• Current research focuses on its role in tooth eruption mechanisms, tooth support, involvement in inflammatory periodontal diseases, and how it can be reattached if lost due to pathology or aging.
• Research addresses why PDL remains a soft connective tissue and does not calcify.
• "Signalling System" measures and maintains the periodontal space, with failure leading to ankylosis (cementum fused with bone, PDL calcified).

Functions of PDL

• Provides tissue attachment between the tooth and alveolar bone, responsible for tooth support, protection, resisting displacing forces, and protecting dental tissues from excessive occlusal forces.
• Responsible for the mechanism by which the tooth attains and maintains its functional position, i.e., tooth eruption and support.
• Maintains and repairs cementum and alveolar bone through cellular content like osteo/cemento blasts.
• Neurological control of mastication occurs via mechanoreceptors in the PDL; loss of PDL results in receptor loss.

Components of PDL

• PDL comprises of stroma of fibers (collagen, oxytalan, and reticulin), ground substance (GAGs, proteoglycans, and glycoproteins), cells (fibroblasts, cementoblasts, osteoblasts, osteoclasts, immune cells, and epithelial cells), blood vessels, and nerves.

Fibers

• 90% of PDL is collagenous, along with small amounts of oxytalan and reticulin fibers.

Collagen

• 80% type I, 15% type III
• Small amounts of types V and VI exist
• Fibrils show the classical banding of collagen
• Typically 50nm in diameter (small and uniform)
• Small amounts found with types IV and VII (in basement membrane) with epithelial cell rests of Malassez and blood vessels.
• Type XII collagen which is non fibrous, is linked to other collagens
• Much collagen gathers into bundles (the principal fiber); 5µm in diameter.
• Collagen fibrils are subunits within each principal fiber
• Fibroblasts are closely involved with PDLs and responsible for collagen synthesis and degradation, surrounding and enveloping fiber bundles.

Collagen Origination in Different Regions of PDL

• Alveolar crest fibers are at the top
• Horizontal fibers exist
• Oblique fibers exist
• Apical fibers are around the apex
• Inter-radicular fibers are in multi-root teeth only
• Transseptal fibers are between two teeth to attach them together

The Extent of Individual Fibres Across the Width of the Periodontal Ligament

• Tooth and bone-related fibers intercalate in an intermediate plexus.
• Fibers cross the ligament's entire width, branching to join neighboring fibers and form a 3D network

The Role of PDL in Tooth Eruption

• During eruption, fibroblasts of the dental follicle become active when the crown approaches the mucosa, producing fibrils (collagen).
• Fibrils initially lack orientation, then become oblique.
• As tooth eruption progresses, detaching and reattaching of PDL occurs within the zone of shear, which is a site of remodeling during eruption.
• Additional oblique fibers appear and attach to the newly formed cementum and bone.
• Trans-septal and alveolar crest fibers develop when the tooth merges in the oral cavity.
• Alveolar bone is disposed simultaneously with PDL organization

Appearance of Fibers

• Principal fibers wavy course better accommodate occlusal forces.
• Sharpey's Fibres are collagen fibers inserted into cementum and bone
• Sharpey's fibres from the cementum side are more numerous but smaller, in comparison, than those from the alveolar bone.
• Cemental Sharpey's fibres appear first, then Sharpey's fibres emerging from bone
• There is fast turnover (remodelling) of collagen in the periodontal ligament, fastest towards the root apex due to the amount of stress

Oxytalan

• Immature elastin fibres (pre-elastin.)
• Attaches to cementum and leaves to the ligament in different directions.
• Rarely incorporated in bone (doesn't attach to the bone).
• Courses vary by region and terminate around blood vessels and nerves.
• Fibres are 0.5µm-2.5µm in diameter (larger than collagen).
• Oxytalan's thickness indicates a role in tooth support undergoing heavy occlusal loading or playing a role in aiding fibroblast migration in the ligament and giving elasticity.

Ground Substance

• Its percentage differs between tissues, mainly secreted by fibroblasts, comprised of hyaluronate glycosaminoglycans, proteoglycans (proteodermatan sulphate, PG1), glycoproteins (fibronectin, tenascin)
• Functions include ion and water binding/exchange, control of collagen synthesis, fibre orientation, tooth support/eruption mechanisms (providing pressure), potential involvement of fibronectin in cell migration and orientation

Role of PDL Tissue

• Scientists discovered, in vitro, that hyaluronidase and chondroitinase(enzymes that dissolve GAGs/proteoglycans) prevent calcification of the PDL tissue by inhibiting tissue mineralization.
• Calcium-binding proteins (ex: S100A4) have a specific role in inhibiting tissue mineralization.

Ankylosis

• Bisphosphonate, a drug used to treat osteoporosis, can have a negative side effect: destruction of ground substance leading to increased osteogenic factors, resulting in ankylosis (loss of PDL and direct tooth/bone contact).

Cells

• Fibroblasts are closely associated with the PDLs and responsible for the synthesis and degradation of collagen, surrounding/enveloping the fiber bundles.
• They aid in the regeneration of tooth support apparatus and an essential role in adaptive response.
• They are very active cells, metabolically, with large nuclei containing one/more prominent nucleoli, abundant ER, and a massive Golgi apparatus.
• Periodontal ligament fibroblasts differ in origin from gingival fibroblasts:
  • PDL fibroblasts come from ectomesenchymal cells (neural crest)
  • Gingival fibroblasts come from mesoderm.
• They exhibit a variety of shapes, are spindle-shaped, and can be larger/smaller in size.
• Periodontal ligament fibroblast activity is modulated by growth factors and cytokines, which are bioactive molecules that may be endogenous (internal) or exogenous (external).
• Motile or contractile cells capable of generating the force for tooth eruption
• Receives chemotactic stimuli to induce the localized movement of fibroblasts guided by collagen fibers.
• Fibroblasts have cilia & a basal body underneath
• Fibroblasts, also fibroclasts, secrete enzymes that have proteolytic activity, e.g., collagenases (Acid phosphatase & cathepsins)
• Fibroblasts contain intracellular collagen profiles

Epithelial Cells

• Aggregations of epithelial cell rests: rests of Malassez are remnants of the developmental epithelial root sheath of Hertwig and are a normal feature
• Network of strands parallel to the long axis of the root.
• Closely packed cuboidal cells stain more purplish deeply and decrease in size as the tooth is out in the oral cavity (with age).
• Variations in location according to age, clusters in cross sections.
• Not much-detected activity, but can give rise to cysts/tumors if stimulated inappropriately.

Immune Cells

Hematopoietic stem cells Hematopoietic stem cells give rise to lymphoid cells (adaptive immune response) and myeloid cells (innate immune response).

• Polymorphonuclear leukocytes (PMNs) include neutrophils, eosinophils, basophils, and mast cells.
• Macrophages: 4% of the cell population; derivatives of monocytes, phagocytosis, attacking organisms and production of interferon, prostaglandin, and growth factors.
• Mast cells have a large number of granules and release histamine, heparin, and anaphylactic factors when stimulated.

Blood Supply

• The periodontal ligament has a rich blood supply unlike the dental pulp which is less vascularized.
• This allows the PDL to remodel easily, making it more adaptable compared to the pulp and also reflects their defensive capabilities.
• PDL relies on the gingiva and underlying bone for protection unlike the pulp which is well-protected by multiple layers of hard tissue.
• Rich blood supply is derived from superior or inferior alveolar arteries (a.a.i.) with the dental artery (a.d.), Intraseptal artery (a.i.), and rami perforantes (terminal branches of a.i.).
• Blood supply to periodontium fed through three sources: branches entering the periodontal ligaments apically, branches entering the alveolar spongy bone (coming out of its perforations), and branches feeding from gingiva.
• Vessels leaving the bony fenestrations form a capillary plexus around teeth.
• Major vessels between principal fibres have an average diameter of 20µm.
• Veins pass through the alveolar walls into intra-alveolar venous networks, prominent around the apex of the alveolus instead of accompanying arteries.

Nervous Supply

• Nerves enter via the apical region and the alveolar wall.
• Nerve fibres are either myelinated (5µm in diameter, sensory, faster) or non-myelinated (0.5µm in diameter, sensory and autonomic, slower).

Mechanoreception

• Why is it important? Because there is a receptive field confined to each tooth
• 75% of mechanoreceptors have cell bodies in the trigeminal ganglion (TG) with the other 25% in the mesencephalic nucleus.
• TG is associated with touch, pressure, and movement during mastication, swallowing, and speech, enabling unconscious detection of tooth contact for reflex control of mandibular movement.
• The Mesencephalic role (carry impulses to other muscles such as neck muscles) is lost after tooth extraction
• These modulate the hypoglossal cranial nerve to modulate tongue position and neck musculature.
• Patients with occlusal and temporomandibular disorders often report pain and dysfunction of neck musculature or not controlling the tongue position.

Clinical Considerations

• Periodontal disease is a chronic inflammatory periodontal disease.
• This condition affects the periodontal ligament leading to the destruction and loss of periodontal ligament tissue and adjacent alveolar bone.
• Treatment objectives include stopping disease progression
• Regeneration of the lost tissue occurs through restoring bone defects and reattaching the PDL fibres.
• Orthodontic treatment repositioning relies on tooth (detachment/reattachment) to the preferred area. This force activates osteoblasts on the tension side (where movement is going away from) and osteoclasts in the compression side (pulling site). At the tension side, the periodontal space widens, and the osteoblasts start to build new bone, while at the compression side, the periodontal space becomes narrower, and the crest of the alveolar bone is slightly deformed.