The Healthy Periodontium
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The gingiva, periodontal ligament (PDL), alveolar bone, and cementum collectively form the periodontium, with the latter three collectively known as the attachment apparatus.
Oral mucosa can be divided into two layers; the stratified squamous epithelium (oral epithelium) and the underlying connective tissue (lamina propria). Keratinized epithelium contains the following layers: Stratum basale (basal layer), Stratum spinosum (prickle layer), Stratum granulosum (granular layer), and Stratum corneum (keratinized layer). There are three functional types of oral mucosa:
- Lining mucosa – covers all oral soft tissue except for the gingiva, hard palate and the dorsal surface of the tongue, and consists of:
- Epithelium – generally non-keratinized stratified squamous oral epithelium, but if exposed to unusual friction it can become keratinized. May contain Langerhans cells, melanocytes, and Merkel cells.
- Lamina propria – loose connective tissue that carries blood vessels, lymphatic vessels, and nerves.
- Masticatory mucosa – covers the gingiva and hard palate, and consists of:
- Epithelium – keratinized or parakeratinized stratified squamous epithelium.
- Lamina propria – consists of a thick papillary layer of loose connective tissue and a deep reticular layer of dense connective tissue.
- Specialized mucosa – keratinized epithelium that covers the dorsal surface of the tongue, embedded with taste buds and special surface projections called papillae.
GINGIVA
The gingiva refers to the oral mucosa that covers the alveolar processes and the cervical portion of teeth. The gingival epithelium can be classified according to its histological appearance or position.
- Oral epithelium – composed of keratinized stratified squamous epithelium with rete pegs.
- Sulcular epithelium – composed of thin, non-keratinized stratified squamous epithelium without rete pegs, the sulcular epithelium lines the gingival sulcus and extends from the coronal limit of the junctional epithelium to the crest of the gingival margin.
- Junctional epithelium – composed of non-keratinized stratified squamous epithelium, directly connected to the tooth structure via hemidesmosomes.
Junctional epithelium refers to a ~1mm band of epithelium that surrounds the tooth like a collar. It is formed when the reduced enamel epithelium and the oral epithelium coalesce during tooth eruption, but can be formed or reformed after injury, surgery, or even around newly placed implants. The junctional epithelium is usually 10–29 cells thick near the base of the gingival sulcus, and only 2–3 cells thick at the apical end. The cell layer adjacent to the external basal lamina is called the proliferative cell layer and is responsible for most of the cellular division. The desquamative layer can be found at the coronal end of the band, forming the base of the gingival sulcus. The specialized part of the junctional epithelium that actually provides the attachment to tooth structure is called the epithelial attachment. Junctional epithelial cells are often larger than sulcular epithelial cells and have two basal laminas:
- internal basal lamina – faces the tooth and facilitates attachment to the tooth. The internal basal lamina consists of a lamina densa (adjacent to the enamel) and a lamina lucida (attached to hemidesmosomes).
- external basal lamina – faces the connective tissue.
The junctional epithelium acts as a more permeable gateway for the passage of bacterial products from the sulcus into the connective tissue and for fluid and cells from the connective tissue into the sulcus. In periodontal disease the following changes are evident:
- Degeneration of underlying connective tissue under the attachment.
- Migration of junctional epithelium – the junctional epithelium proliferates along the root surface (becomes elongated) and the coronal portion detaches.
The free gingiva is the collar of tissue surrounding the tooth which is not directly attached to the tooth or alveolar bone. The free gingiva includes:
- gingival margin – the most coronal portion of gingiva.
- free gingival groove – demarcation of free and attached gingiva (not always be present).
- gingival sulcus – located between the gingival margin and the tooth surface.
- interproximal (interdental) gingiva – fills the interdental spaces.
The attached gingiva is directly attached to the underlying periosteum of the alveolar bone and indirectly attached to the cementum by the connective tissue fibers and epithelial attachment. The major proportion of the organic matrix of cementum is composed of Type I (90%) and type III collagen. The attached gingiva sits between the free gingiva and the more moveable alveolar mucosa. The mucogingival junction is the transition point from attached gingiva to alveolar mucosa.
Gingival fibers (supracrestal connective tissue fibers) are continuous with the periodontal ligament and function to support the gingiva. The high proportion of type I collagen provides tensile strength, while type IV collagen branches in between the type I collagen bundles and integrates with the basement membrane and blood vessel walls. Recall that ascorbic acid (Vit C) is essential for the hydroxylation of proline and lysine, which is required in collagen formation. Gingival fibers can be categorized according to their orientation:
- Dentogingival fibers – extend out laterally and coronally into the lamina propria of the gingiva, inserting into the cementum. These fibers sit apical to the epithelial attachment.
- Dentoperiosteal fibers – extend over the alveolar crest to the periosteum of the cortical plates of bone, inserting into the cervical cementum.
- Circular fibers – surrounds teeth, inserting into the cementum and lamina propria of the alveolar crest and free gingiva. Function to resist rotation.
- Alveologingival fibers – extends out through the lamina propria into the free gingiva, inserts in crest of alveolar process.
- Transseptal fibers – from tooth to tooth above the alveolar crest, inserting into the cementum (not alveolar crest). Function to maintain arch integrity.
Stippling refers to the normal “orange peel” like surface texture of the attached gingiva, a result of the microscopic elevations and depressions of connective tissue projections underneath the epithelium. An absence of stippling could be a result of:
- underlying edema.
- the destruction of gingival collagen fibers.
- normal variation (not always present).
- youth, since stippling is not generally visible until after the emergence of adult teeth.
Gingival crevicular fluid (GCF) is a clear transudate that slowly discharges from the gingival sulcus. Also known as sulcular fluid, GCF helps to maintain the structure of junctional epithelium and plays an integral part in the antimicrobial defense of periodontium. The GCF rate of flow is related to the degree of gingival inflammation, an increase in gingival crevicular flow is often one of the first detectable signs of periodontal inflammation (inflammatory exudate). GCF contains:
- enzymes, serum proteins and inflammatory mediators.
- antibodies directed against dental plaque bacteria.
- cellular tissue, including epithelial cells and neutrophils.
PERIODONTAL LIGAMENT
The periodontal ligament (PDL) refers to a specialized connective tissue that attaches the tooth to bone, inserting into root cementum on one side and into the cortical surface of the alveolar bone on the other. The periodontal ligament fibers are primarily composed of collagen, and the ground substance is composed of a variety of proteins and polysaccharides. The PDL is a dynamic connective tissue that can change in thickness according to age, function or pathology. Tooth eruption and trauma can also alter the PDL. The periodontal ligament consists of:
- principal fibers.
- connective tissue.
- fibroblast cells.
- oxytalan fibers.
- cell rests of Malassez.
- ground substance.
Fibroblasts are the most common cell in the periodontal ligament and appear as ovoid or elongated cells oriented along the principle fibers. Cementoblasts and osteoblasts are found in close proximity to the cementum and bone respectively.
Collagen is a protein composed of different amino acids including glycine, proline, hydroxyproline and hydroxylysine. The amount of collagen in a tissue can be determined by the amount of hydroxyproline. Oxytalan and eluanin are two forms of immature elastin found in the PDL. The average width of the periodontal ligament space is about 0.2mm, but this can vary considerably. It is also not static, shrinking in hypofunction, and increasing in hyperfunction.
Principal fibers of the periodontal ligament are bundles of collagen fibers (mainly type I) grouped according to the direction they extend.
- Transseptal fibers – extend over the crest of the alveolar bone from the cementum of one tooth to the tooth adjacent. These fibers maintain the integrity of the dental arches by keeping the teeth aligned.
- Alveolar crest fibers – extend from the cervical cementum of the tooth to the alveolar crest. These fibers function to resist occlusal and lateral forces.
- Horizontal fibers – extend from the cementum to the alveolar bone perpendicular to the tooth surface, positioned apical to the alveolar crest fibers.
- Oblique fibers – extend from the cementum to the alveolar bone in the middle third of the tooth. The majority of periodontal ligament fibers are oblique fibers that function to resist forces along the long axis of a tooth.
- Apical fibers – radiate out from the apical cementum of the tooth to bone. These fibers function to resist forces along the long axis of a tooth away from bone.
- Interradicular fibers – present in multirooted teeth only, extending from the cementum in the furcation area to surrounding bone.
Sharpey’s fibers are the terminal ends of principal fibers of the periodontal ligament that insert into the cementum and into the periosteum of the alveolar bone. Sharpey’s fibers can also be found where muscle attaches to the periosteum of bone, or in the sutures between cranial bones. They are predominantly composed of strong type 1 collagen.
The indifferent fiber plexus refers to smaller collagen fibers that are associated with the larger, principal collagen fibers, extending in all directions to form a plexus. Reticular fibers are primarily composed of type III collagen. Type IV collagen is found in the basal lamina.
The periodontal ligament serves many functions:
- Physical attachment of the tooth to the bone via principal fibers.
- Remodeling of the cementum (cementoblasts and cementoclasts), connective tissue (fibroblasts) and alveolar bone (osteoblasts, osteoclasts). Remodeling makes the orthodontic movement of teeth possible.
- Provide nutrition through blood vessels and fluid movement within the PDL.
- Carries sensory information via the trigeminal nerve which provides tactile feedback and proprioception.
Nerve fibers located in the PDL space transmit tactile, pressure and pain sensations via the trigeminal nerve. These nerve bundles divide into single myelinated fibers but lose their myelin sheath towards the terminus, and end in one of four receptors.
- Free endings – tree-like architectural configuration that transmit pain sensations.
- Ruffini-like mechanoreceptors – mainly found in the apical region, these receptors are the primary mechanoreceptors in the periodontal ligament.
- Coiled Meissner corpuscles – mechanoreceptors found mainly in the midroot region of the PDL, providing tactile perception.
- Spindle-like pressure/vibration receptors.
ALVEOLAR BONE
The periodontal ligament fibers imbed into the alveolar bone proper, a perforated cribriform plate that allows for the passage of nerve fibers and blood vessels. This alveolar bone consists of two layers, compact lamellar bone, and bundle bone. Supporting alveolar bone surrounds the alveolar bone proper and consists of an inner and outer plate of compact lamellar bone which surrounds spongy (cancellous) bone. In areas where the bone is thinner (maxillary or mandibular anterior region) the cortical plate of the alveolar bone may fuse with the cribriform plate of the alveolar bone proper (ie. no cancellous bone). The outer and inner compact lamellar bone of the supporting alveolar bone is thicker in the mandible when compared to the maxilla, which can inhibit local anesthetic penetration. Bone is roughly two thirds inorganic matter, composed primarily of calcium and phosphate in the form of hydroxyapatite crystals. About one third of the bone is organic matrix, mainly consisting of Type I collagen.
CEMENTUM
The cementum is considered to be part of the periodontium because it serves as support for the fibers of the periodontal ligament.
