Periodontium: The Periodontal Ligament (PDL)

• what is the PDL?

  The Periodontal Ligament (PDL) is the fibrous connective tissue that holds the tooth in its alveolus (socket).

  
• what is the PDL like in healthy teeth?

  the PDL is a soft tissue between the cementum of the tooth root and the wall of the socket (the alveolar bone).
• what is the PDL like in radiographs?

  the PDL is radiolucent, appearing as a dark region between the tooth and socket

  
• show how the average width of the PDL space changes with age

  • 0.21mm at 11-16yrs.

  • 0.18mm at 32-52yrs.

  • 0.15mm at 51-67yrs

  
  

  (younger= wider, -> gets thinner as we are older)
• what is the PDL composed of?

  a network of thick collagen fiber bundles (types I, III, and XII) that anchor on two sides: the cementum and the alveolar bone.

  
• What are the functions of the PDL?

  (1) A cushion during occlusion

  (2) A pressure sensor (mechanoreception)

  (3) A key role in tooth drift

  (4) A source of new periodontal cells

  (5) A key role in tooth eruption
• (1) A cushion during occlusion

  
  

  describe how the PDL is a cushion during occlusion

  • The thick collagen fiber bundles have considerable tensile strength.

  • Their orientations around the tooth differ depending on the region (major fiber groups).

  
• (1) A cushion during occlusion

  
  

  what cell produces and maintains the Collagen fiber bundles in PDL?

  fibroblasts

  (the most abundant cell type found in the PDL)
• (1) A cushion during occlusion

  
  

  what can visualize PDL fiber orientations?

  special histological stains and crosspolarized light
• (2) A pressure sensor (mechanoreception)

  
  

  describe how the PDL is a pressure sensor (mechanoreception)

  • The PDL is well-innervated by mandibular and maxillary branches of the trigeminal nerve

  • These nerves are intimately connected with the ligament fiber bundles, sending sensory information back to the brain to let you know how hard you are biting and if you are occluding your teeth properly

  
• (3) A key role in tooth drift

  
  

  describe how the PDL has a key role in tooth drift


  tooth drift: when teeth gradually move through the jaw (tooth drift) in response to physical stimuli or during growth of the jawbones

  
  

  This is mediated by the PDL:

  • Pressure on the leading side of the tooth initiates a bone-resorption response.

  • Tension on the trailing side initiates a bone deposition response
• (3) A key role in tooth drift

  
  

  what does pressure on the leading side of the tooth initiate?

  a bone-resorption response.

  (which makes room within the bone for the tooth to room in that direction)
• (3) A key role in tooth drift

  
  

  what does tension on the trailing side initiate?

  a bone deposition response

  (which increases bone to maintain the consist width of the PDL space)
• (3) A key role in tooth drift

  
  

  why is a healthy periodontal ligament is required for orthodontic tooth movement?

  because without it, teeth will not move!
• (4) A source of new periodontal cells

  
  

  describe how the PDL is a source of new periodontal cells

  There is a lot of tissue turnover within the PDL:

  • Collagen fiber bundles are broken down and re-assembled.

  • Alveolar bone is deposited or resorbed.

  • Cementum layers are added and removed.

  • This requires a reservoir of cells to replenish these tissues
• (4) A source of new periodontal cells

  
  

  what are the cells in the reservoir of cells in the PDL?

  • Fibroblasts

  (Collagen production and turnover)

  
  

  • Osteoblasts

  (Alveolar bone production)

  
  

  • Cementoblasts

  (Cementum production)

  
  

  • Osteoclasts

  (Hard tissue resorption)

  
  

  • Epithelial cells

  (Function not known. Acellular cementum production maybe?)

  
  

  • Macrophages

  (Immune responses)

  
  

  • Neurovascular cells

  (Nerves and blood vessels)

  
  

  • Stem cells

  (Produce new periodontal cells for repair and regeneration)
• (4) A source of new periodontal cells

  
  

  what are fibroblasts for?

  Collagen production and turnover
• (4) A source of new periodontal cells

  
  

  what are Osteoblasts for?

  Alveolar bone production
• (4) A source of new periodontal cells

  
  

  what are Cementoblasts for?

  Cementum production
• (4) A source of new periodontal cells

  
  

  what are osteoclasts for?

  Hard tissue resorption
• (4) A source of new periodontal cells

  
  

  what are Epithelial cells for?

  - form a net around each tooth root

  (epithelial rests of malassez)

  
  

  Function not known. Acellular cementum production maybe?
• (4) A source of new periodontal cells

  
  

  what are macrophages for?

  Immune responses
• (4) A source of new periodontal cells

  
  

  what are Neurovascular cells for?

  nerves and blood vessels

  - nourishes and maintains the cell types and tissues in PDL
• (4) A source of new periodontal cells

  
  

  what are stem cells for?

  Produce new periodontal cells for repair and regeneration
• (5) A key role in tooth eruption?

  
  

  describe how the PDL has a key role in tooth eruption

  • The mechanism(s) of tooth eruption are poorly understood.

  • Some research has demonstrated that PDL development is needed to maintain tooth eruption.

  • But other studies have shown that this isn't the case.

  • Does the PDL have a role in tooth eruption?

  Research is ongoing!
• what does the PDL normally maintain a balance between?

  hard tissue removal (resorption) and hard tissue deposition
• what can cause imbalance between hard tissue removal (resorption) and hard tissue deposition?

  Trauma, infection/disease, and other factors can contribute to an imbalance in these processes
• what does Excessive resorption lead to?

  root and alveolar bone resorption of varying severity

  
  

  -> If severe enough, this can lead to tooth loss
• what does Excessive alveolar bone or cementum deposition lead to?

  partial or complete dental ankylosis (fusion of teeth to the jawbone).

  
  

  -> This has consequences for tooth extraction and orthodontic tooth movement