Periodontal Ligament Lecture Notes

Periodontal Ligament

Dense, fibrous connective tissue attaching tooth to alveolar bone.
Supports tooth in alveolus; maintains physiologic relation between cementum and bone.

Characteristics

Fibers insert into alveolar bone and cementum.
Cellular, innervated, vascular; contains epithelial cell aggregates.
Occupies narrow space between cementum and alveolus.
Radiolucent line in radiographs paralleling root surface (lamina dura is radiodense line representing alveolar bone proper).
Thickness varies (0.1 – 0.25 mm) depending on age, eruption stage, and function (using LaTeX).
$0.1 - 0.25 \text{ mm}$
Thicker in adolescents.
Maximal dimensions in heavy function.
Thinner ligaments in teeth without antagonists and impacted teeth.
Narrowest near middle of root, wider near apex under normal function.

Types of Periodontal Ligament Fibers

Principal Fiber Groups

Alveolar Crest Group
- Radiates from crest of alveolar process to cervical cementum.
- Attached to cementum just below the cemento-enamel junction and running downward and outward to insert into the rim of the alveolus.
- Resists vertical and intrusive force; anchors tooth to alveolus.
Horizontal Group
- Runs at right angles to long axis of tooth, from cementum to bone near crest.
- Run through the coronal section of the periodontal ligament immediately apical to the alveolar crest at right angles to the axis of the tooth, that is, horizontally from the cementum to bone.
- Resists horizontal or lateral pressures.
Apical Group
- Radiates from apical region of tooth to surrounding bone.
- Radiating from the cementum around the apex of the root of the bone, forming the base of the socket.
- Prevents vestibulo-oral tipping.
Oblique Group
- Courses obliquely; attached to cementum apically from bone attachment.
- By far the most numerous and largest group in the periodontal ligament and running from cementum in an oblique direction to insert into bone coronally.
- Main support of tooth.
- $occlusal \; forces$
- Resists intrusive masticatory forces.
Interradicular Group
- Courses over crest of interradicular septum in furcations of multirooted teeth.
- Found only between roots of multirooted tooth and running from the cementum into the bone forming the crest of the interradicular septum.
- Resists tipping and torque.

Other Fibers

Indifferent Fiber Plexus: Parallel to root surface; loose intersecting network. May be incorporated into mineralized matrices.
Oxytalan Fibers: Interspersed among collagen fibers. Run occluso-apically; associated with blood vessels and nerve fibers. More common on cemental side.
Elastic Fibers: Scarce. Confined to loose connective tissue surrounding neurovascular channels.

Other Features

Principal fiber bundles from bone are more widely separated than those from cementum.
Principal fibers form an interconnecting meshwork.
Alveolar fibers are thick, relatively short, and splay out into smaller fibers that blend with cemental fibers.
Sharpey’s Fibers: Ends of principal fibers embedded in cementum and bone. Mineralized portions of the principal fibers.

Cells

Main function of periodontal ligament cells is to maintain the normal organization of the fiber system by synthesizing new fibers and remove old ones.
Cellular density greatest in young individuals, decreases with age.
Cellular density increases with heavy function, decreases with lack of function.
Fibroblasts: Major cellular population. Synthesize collagen, which aggregates into fibrils and fibers extracellularly. Spindle-shaped or flat disks. Show frequent cell-to-cell contacts of both the adherens (20 to 30 per cell) and the gap junction types.
Fibronex: is the term used to describe the morphologic relationship between intercellular filaments and the sticky attachment glycoproteins fibronectin.
Cementoprogenitor and Osteoprogenitor Cells
- CementoBlasts: Observed during active deposition of cellular cementum; ovoid to cuboidal shape; basophilic cytoplasm.
- OsteoBlasts: Found in periodontal ligament adjacent to bone. Appear only in areas of the periodontal space where alveolar bone is deposited
Osteoclasts and Odontoclasts: Multinucleated cells that are morphologically and functionally indistinguishable. Osteoclasts work with osteoblasts to remodel existing alveolar bone. Odontoclasts resorb mineralized dental tissue (cementum). Located within Howship’s lacunae.
Epithelial Cells: Commonly found close to cementum. Originate from Hertwig’s epithelial root sheath.
Defense Cells: Few in disease-free periodontal ligament.
Undifferentiated Mesenchymal Cells or Progenitor Cells: Perivascular location (within 5 micrometers of blood vessels). Source of new cells for periodontal ligament. Selective deletion of ligament cells occurs by APOPTOSIS or the physiologic cell death, to permit turnover without architectural disruption of tissue.
Macrophages: Important defense cells due to phagocytic activity and mobility. Take up bacteria, dead cells, and foreign bodies.
Leukocytes: Small lymphocytes and plasma cells may appear when periodontium is stressed by disease.

Vascular Supply

Exceptionally well vascularized compared to other connective tissues.
High turnover of cellular constituents.
Main blood supply: superior and inferior alveolar arteries.
Supplied from 3 sources:
1. Branches from the dental artery (longitudinal periodontal artery) just before it enters the apical foramen extend coronally through the periodontal ligament and reach the gingival tissues.
2. Branches of the interalveolar and interradicular arteries extend coronally inside the spongiosa of the interdental and interradicular bone septa, branch within the bone and pass through the Volkmann’s canals (foramina of the cribriform plate) to enter the periodontal ligament from the sides (rami perforantes alveolares).
3. Branches of the periosteal arteries Extend coronally through the facial and lingual membranes of the alveolar processes to reach the gingivae, and communicate with the periodontal blood vessel system via the gingiva.
Functions of blood vessels:
1. Nutritive
2. Waste removal
3. Hydraulic shock absorption

Nerves

Contains both sensory and autonomic nerve endings.
- Sensory: pain and pressure (mechanoreceptors for pressure).
- Autonomic: control of smooth muscles associated with vasculature.
Four types of neural termination described:
1. Free Nerve Endings: Most frequent; treelike configuration. Located at regular intervals along the length of the root and originate largely from unmyelinated fibers Thought to be both nociceptors and mechanoreceptors.
2. Ruffini Corpuscle: Around root apex; dendritic. Mechanoreceptors.
3. Coiled Ending: Midregion of periodontal ligament; function undetermined.
4. Encapsulated Spindle Type Ending: Lowest frequency; associated with root apex; spindle-like endings surrounded by fibrous capsule.

Functions of the Periodontal Ligament

Formative: Develops from mesenchymal tissue of dental follicle; differentiates into fibroblasts, cementoblasts, osteoblasts.
Supportive: Anchors teeth to alveoli; multistage shock absorption. Acts together as a hydraulic damper or shock absorber with all the components of the periodontal ligament with the ground substance of the ligament, the tissue fluid, and blood in blood vessels all acting as a viscoelastic system.
Nutritive: Provided by blood vessels.
Homeostatic: Continuous remodeling of fiber system. Remodeling subjected to functional, metabolic, and aging effects. Gentle, atraumatic stimulation in the form of a brief orthodontic separation of contacting teeth leads to an acceleration in the rate of cell division. It must be emphasized that the continuous remodeling of the periodontal ligament fiber system is subjected to functional, metabolic and aging effect.
Sensory: Pain and tactile sensation. Necessary for proper positioning of jaws during function; estimating pressure in mastication; identifying tapped tooth. The periodontal ligament can register 2 types of sensation: pain and pressure. The FREE NERVE ENDINGS OF SENSORY FIBERS, which are especially plentiful in the periodontal ligament nearest the cementum, are probably responsible for pain perception and the RUFFINI-LIKE ENDINGS seem to be mechanoreceptors for the reception of proprioceptive stimuli or pressure. In addition, afferent nerve fibers, the periodontal ligament contains autonomous, probably sympathetic fibers, which for the most part are unmyelinated and form basket-like structure around the blood vessels. These fibers are responsible for regulation of the smooth muscle cells of all the vessels of the periodontal ligament, that is, they regulate the local blood supply.

Development of the Periodontal Ligament

Forms from the dental follicle as well as those of the cementum and alveolar bone.
Dental follicle is defined in the bell stage of the tooth germ as a layer rich in fibers and cells covering the entire tooth germ.
Transformation begins at root formation and disintegration of Hertwig’s epithelial root sheath.
Cemental fibers attach to cementum; alveolar fibers attach to alveolar bone; intermediate fibers are between these two fibers.
Continuous with the intermediate fibers, apical to the epithelial diaphragm is the cushion hammock ligament, which is believed to prevent the apical movement of the epithelial diaphragm, and helps the tooth to erupt towards the oral cavity.
Intermediate Plexus: Zone of loose collagen fibers that radiate out from bone and cementum and intertwine into a latticework. Sicher has called this the INTERMEDIATE PLEXUS.
Sicher and Orban believed that this zone persisted after eruption and that within it occurred the adaptive separating and resplicing of collagen fiber bundles necessary for tooth movement.
It has been conclusively shown that within the completely formed periodontal ligament of erupted functioning teeth, an intermediate plexus no longer exist.
The orientation of the periodontal fiber bundles therefore depends on the stage of tooth eruption.

Physiologic Changes

Tooth position changes mesially and occlusally during migration.
Mesial Migration/Physiologic Mesial Drifting: Teeth drift mesially with age.
Basis is loss of enamel from proximal contact areas through attrition.
Remodeling during mesial migration is seen in alveolar bone structure.
Bone surfaces mesial to tooth show resorption; distal surfaces show apposition.
On the mesial, the periodontal ligament is relaxed and the interstitial space becomes round; while on the distal, the periodontal ligament is stretched and the interstitial space becomes elliptical
1. In newly erupted teeth, the periodontal ligament is still wide, because roots may not be complete yet, but as soon as the teeth receives pressure, such is relayed to the bone as pulling tension and stimulate bone formation and periodontal ligament becomes narrower but teeth has much stronger attachment.
2. Increased functional stresses will lead to development of thicker principal fiber bundles and less interstitial tissue. In loss of function, the principal fibers become less and poorly developed and interstitial spaces become wider.
3. The periodontal ligament in the region of a permanent mandibular canine tends to become thinner with age, probably as the result of deposition of cementum and bone.

Clinical Consideration

Excessive forces produce necrosis in periodontal ligament where compression is maximal. Necrotic areas in the periodontal ligament occur by continuous forces no greater than 25 g. Necrosis is initiated during the first two hours of force application and therefore is irreversible.
Periodontal ligament health is important to dental health and function.
Renewal capability is important for repair, such as orthodontic tooth movements.If the movement of teeth is within physiologic limits, the initial compression of the periodontal ligament on the pressure side is compensated for by bone resorption, whereas in the tension side bone apposition is seen.
Adjustment period needed after restoration of bridge abutment, teeth opposing bridges/dentures, and teeth used as anchorage for removable bridges.
Acute trauma may produce fracture/resorption of cementum, tears of fiber bundles, hemorrhage, and necrosis.
Periapical area of tooth is often site of pathologic lesion (dental granuloma, cyst).
Integrity of periodontium must be maintained. Gingivitis can lead to periodontitis (destruction of periodontal ligament and bone).