Periodontium and periodontal ligament: distinguishing the concepts
Periodontium is the general term for the supporting apparatus of the tooth surrounding it on all sides. The periodontium includes the cementum, periodontal ligament, tooth socket wall, and gingiva. The periodontal ligament is just one component of the periodontium, representing the ligament that holds the tooth root within the bony socket and is located between the root surface and the socket wall.
General functions of the periodontium:
- supportive and cushioning — distribution of masticatory pressure and prevention of direct damage to the alveolar bone;
- barrier — prevention of microorganism penetration into the bone tissue;
- trophic — nourishment of the cementum;
- reflex — ensuring tooth sensitivity through nerve endings.
Functions of the periodontal ligament
The periodontal ligament consists of cells and intercellular substance and performs a number of specialized functions:
- support-retention and cushioning;
- participation in tooth eruption through the activity of myofibroblasts;
- proprioceptive — perception of tooth position and load;
- trophic;
- homeostatic — regulation of proliferative and functional activity of cells, continuous processes of collagen synthesis and resorption, cementum repair, and bone remodeling;
- reparative — participation in bone fracture repair and resorption of tooth tissues;
- protective.
Normally, cementum is little subject to resorption, but under orthodontic forces (e.g., wearing braces), active remodeling occurs primarily in the alveolar bone.
Cellular composition of the periodontal ligament
Fibroblasts are the main and most numerous cells. They are arranged parallel to collagen fibers and have a flattened shape with numerous processes. A well-developed rough endoplasmic reticulum ensures the synthesis of extracellular matrix components, while a well-developed lysosomal apparatus ensures its digestion. They are capable of transforming into myofibroblasts, which ensure tooth eruption. The cells are connected by desmosomes, gap junctions, and tight junctions.
Undifferentiated cells — precursors of various periodontal cell lines; the question of a single cambial source or multiple independent precursors remains open.
Osteoblasts are located along the surface of the alveolar bone wall, active during bone remodeling; they produce and mineralize osteoid. Active ones are cuboidal, inactive ones are flattened. Cementoblasts are localized near the root surface and produce precementum followed by mineralization. Osteoclasts and odontoclasts appear episodically in areas of active resorption.
Immune defense is provided by macrophages, mast cells, and various leukocytes.
Epithelial rests of Malassez
Rests of Malassez are rudimentary formations, remnants of Hertwig's epithelial root sheath. During odontogenesis, the sheath gradually fragments: first turning into a reticular structure, then into cords, and ultimately persisting as separate islands. Three functional states are distinguished:
- resting — small inactive cells;
- degenerating — with pyknotic nuclei and scanty cytoplasm;
- functionally active — large cells with a pale nucleus, developed cytoplasm, and high proliferative activity.
Intercellular substance and fibers
The intercellular substance makes up about 75% of the periodontal ligament volume. The ground substance is 70% water; the rest consists of glycosaminoglycans and glycoproteins.
Fibers are divided into two types: collagen and immature elastic oxytalan fibers. Collagen fibers grow toward each other from the alveolar bone wall and the cementum, forming an intermediate plexus in the middle part. The terminal portions embedded in bone and cementum are called Sharpey's fibers. With age, the fiber diameter decreases.
Types of collagen fibers of the periodontal ligament
- Alveolar crest fibers — connect the tooth neck to the alveolar bone crest, lying in the cervico-lingual plane.
- Horizontal fibers — located deeper, running at right angles to the root surface.
- Oblique fibers — numerically predominant, occupy about two-thirds of the periodontal ligament, running obliquely from the root to the socket wall and taking up the main masticatory load.
- Apical fibers — radiate from the apical foramen area perpendicular to the socket wall.
- Interradicular fibers — present only in multirooted teeth, located in the bifurcation area.
Oxytalan fibers — immature elastic fibers running parallel to the tooth root, most numerous in the cervical region. They interweave into the cementum but do not attach to the bone. Their exact function is not established; they are hypothesized to participate in regulating blood flow depending on the functional load on the tooth.
Key points
- Periodontium — the supporting apparatus of the tooth: cementum, periodontal ligament, socket wall, and gingiva.
- Periodontal ligament — the ligament between the root and the bony socket, performing supportive, trophic, reparative, and proprioceptive functions.
- Main cells of the periodontal ligament are fibroblasts, capable of transforming into myofibroblasts during tooth eruption.
- Osteoblasts, cementoblasts, osteoclasts, odontoclasts, and immune cells are also present in the composition.
- Rests of Malassez — remnants of Hertwig's epithelial root sheath.
- The ends of collagen fibers embedded in bone and cementum are called Sharpey's fibers.
- Five groups of collagen fibers are distinguished: alveolar crest, horizontal, oblique, apical, and interradicular.
- Oxytalan fibers — immature elastic fibers running parallel to the root and presumably regulating blood flow.