1. Structural Organization of the PDL
- Cells of the PDL
- Fibers
- Ground substance
- PDL vascularity
An examination of periodontal and prosthetic interactions from a biomechanical standpoint necessitates an understanding of occlusal force generation and the resultant reactions within the periodontium. A brief review of the nature of masticatory forces that are commonly encountered , as well as the anatomical structures which receive occlusal loads is in order.
Each component of the PDL contributes to the transmission of the forces of mastication from the teeth to supporting bone. The PDL is a dense connective tissue that surrounds the root of the tooth and attaches it to the alveolar bone. In humans, the width of the PDL ranges from 0.15 to 0.38 mm, with it narrowest aspect at the middle third of the root and its widest aspect cervically.1 Occlusal loading in function affects the width of the PDL. If occlusal forces are within physiologic limits, increased function leads to an increase in width through a thickening of the fiber bundles and an increase in diameter and number of Sharpey's fibers.2 Forces that exceed this limit cause lesions that are characteristic of trauma from occlusion. When function is diminished or absent, the width of the PDL decreases. The fibers are reduced in number and density, and they eventually become oriented parallel to the root surface.3
Cells of the PDL
The cellular constituents of the PDL include osteoblasts, osteoclasts, fibroblasts epithelial rests of Malassez, undifferentiated mesenchymal cells, cementoblasts an cementoclasts, as well as neurovascular elements. The osteoblasts and osteoclasts are functionally associated with the alveolar bone, and the cementoblasts and cementoclasts are functionally associated with the cementum. The extracellular constituents of the PDL consist of collagen fibers, oxytalan fibers, ground substance, nerves, and vessels.
Fibers
The dentoalveolar fiber bundles occupy approximately two thirds of the PDL volume.4 It has been found that the volume of the interfascicular spaces increases with age. These spaces contain ground substance, tissue fluid, and neurovascular elements. The volume occupied by vascular elements is 1% to 2% and tends to decrease with age.
| Most of the collagen fibrils are arranged into fiber bundles. Each fiber resembles a spliced rope where individual strands can be continually remodeled while the overall fiber retains its form and function. This remodeling may allow adaptation of the PDL to repeated or continual stresses. The fiber bundles are arranged into principal fiber groups (Fig. 2-1). The portion of the principal fiber that is embedded into either cementum or bone is called a Sharpey's fiber. These fibers occasionally pass through the bone of the alveolar process to continue as principal fibers of an adjacent PDL.5-8 They may also run buccally and lingually to connect with the fibers of the periosteum covering the alveolar process (Figs. 2-2 and 2-3). |  Fig.2.1 Principal fiber groups of the periodontal ligament. (A) Transseptal, (B) Alveolar crest, (C) Horizontal, (D) Oblique, (E) Apical, and (F) Interradicular. |
 Fig 2.2 Frontal view of gingival fiber groups. (A) Circular, (B) Dentogingival, (C) Dentoperiostal, and (D) Alveologingival. |
 Fig. 2.3 Interdental view of gingival fiber groups. (A) Circular, (B) Dentogingival, (C) Dentoperiostal, and (D) Alveologingival. |
The fiber bundles of human PDL are arranged into networks having a complex three-dimensional overlapping arrangement.9,10 The fiber bundles follow a wavy course from the root to the bone with frequent crimping, branching, and anastomosing. The blood vessels take a primarily longitudinal course between the fiber bundles.
 Fig 2.4 Oxytalan fibers. (A) Cementum, (B) Principal oxytalan fiber, (C) Oxytalan tract, and (D) Periodontal vessel. |
In the human PDL oxytalan fibers, which resemble immature elastin fibers, are seen among the collagen fibers. Oxytalan fibers form a network that attaches blood vessels to the cementum (Fig. 2-4).11 Periodontal vessels are linked vertically by fibers or multiple groups of fibers forming tracts. Unique oxytalan-vascular structures consist not only of fibers associated with the walls of individual arteries, veins, and Iymph vessels but a meshwork that surrounds the total vessel complex. Random fiber and vessel associations are also seen where there is no preferential attachment to any particular vascular type. Some researchers hypothesize that these fibers provide support for the blood vessels when the PDL is under function. Others feel that these fibers may influence blood flow and thereby effect tooth support.12 |
Ground substance
The ground substance is a major constituent of the periodontal ligament. Its primary components are hyaluronic acid, glycoproteins, and proteoglycans. Hyaluronic acid and the polysaccharide portion of proteoglycans are glycosaminoglycans (GAGs). Glycosaminoglycans are unbranched polymers composed of repeating disaccharide units.
Sulfated GAGs are found in other tissues exposed to mechanical loading, such as joint cartilage. The compressive stiffness of hyalin articular cartilage has been shown to depend largely on the amount of sulfated GAGs present.13 The binding of water by glycosaminoglycans facilitates the capacity of the articular cartilage to recover from compressive loads. It has been estimated that the ground substance of the PDL consists of 70% water, much of which is in a bound form. This composition is felt to have a significant effect on the viscoelastic character* of the PDL.14
PDL vascularity
The PDL receives its main blood supply through vessels that enter the PDL space through the alveolar bone, but it also receives contributions from the apical and gingival vessels. The main blood vessels within the PDL course parallel to the long axis of the tooth and run between the collagen fiber bundles near the alveolar bone. These main vessels form a flat, basketlike network of capillaries that surround the root. In the cervical region of the PDL are coiled capillaries that have been described as resembling glomeruli (Fig. 2-5).15 Many arteriovenous anastomoses occur in the ligament, particularly in the apical and alveolar crest regions. Axially directed veins drain into a basketlike network at the apical portion of the periodontal space as well as larger veins in the septi of the alveolar bone. These vascular elements may play a role in providing the periodontium with a hydraulic support system that acts in concert with the fiber structure, the viscoelastic ground substance, and the extracellular fluid.
 Fig 2.5 Periodontal blood supply. Left, buccolingual view (frontal section). Right, sagittal view of interdental bone. (A) Subepithelial capillary network of the gingiva, (B) Capillary network of the periodontal ligament, (C) Supraperiosteal arterial, and (D) Arterials penetrating the interdental alveolar bone. |
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