TL;DR

Key Facts

What Is the Periodontium?

The word “periodontium” derives from the Greek peri (around) and odontos (tooth) — literally, “the tissues around the tooth.” It describes not a single organ but a functional complex of four structurally distinct but biologically interdependent tissues that collectively support, suspend, and protect the tooth root within the alveolus. The periodontium is unique in the body: it is the only site where a hard mineralised structure (the tooth root) penetrates a continuous epithelial surface (the gingival epithelium) and anchors within bone via a fibrous ligament (the PDL) rather than direct osseous fusion.

This anatomical arrangement creates a permanent challenge — the epithelial seal at the gingival sulcus must prevent oral micro-organisms from accessing the supporting tissues without limiting the functional mobility of the tooth. The periodontium meets this challenge through constant renewal: the junctional epithelium turns over every 4–6 days, the PDL collagen turns over every 24 hours, and the alveolar bone remodels continuously. This high metabolic activity makes the periodontium both highly responsive to therapeutic intervention and highly vulnerable to the destructive effects of chronic infection.

Why It Matters

The periodontium is the foundation of all restorative, prosthetic, orthodontic, and implant dentistry. Every clinical decision — crown margin placement, implant positioning, orthodontic force application, extraction site management — must respect the anatomy and physiology of the supporting tissues. The most heavily examined periodontal anatomy concepts on board examinations are: the biologic width (STA) and its clinical violation; the PDL fibre groups and their orientations; the distinction between acellular and cellular cementum; the composition and turnover of the junctional epithelium; and the clinical implications of alveolar bone loss for tooth prognosis.

Clinical Relevance

• The JE is the weakest link in the epithelial defence and the site of disease initiation: The junctional epithelium is not keratinised, is only 2–4 cells thick at its apical extent, and has very loose intercellular junctions — properties that allow it to permit the passage of large numbers of neutrophils from the gingival connective tissue into the sulcus as the continuous first-line cellular defence. However, these same properties make it the most permeable and least resistant component of the gingival epithelium. Bacterial products from the subgingival plaque diffuse through the JE into the connective tissue far more readily than they would penetrate the thick keratinised epithelium of the attached gingiva. Periodontal disease begins at the JE — the first histological changes of the initial lesion (vasodilation, increased vascular permeability, PMN emigration) occur in the connective tissue immediately lateral and apical to the JE.
• The width of attached gingiva is clinically significant but not universally so: There is no universally agreed minimum width of attached gingiva required to maintain periodontal health — some sites with minimal or absent attached gingiva remain healthy for decades if plaque control is excellent and there is no frenal pull. However, sites with less than 1 mm of keratinised gingiva are more vulnerable to recession when subjected to frictional trauma, frenal traction, or improperly placed restorations. Attached gingiva width also determines how far apically a crown margin can be placed without impinging on the supracrestal tissue attachment. In sites where restorations are planned to extend subgingivally, or where recession will compromise aesthetics or root coverage, augmentation of keratinised gingiva (free gingival graft) may be required before definitive restoration.
• Sharpey’s fibres are the structural basis for tooth support — their integrity determines tooth mobility: Principal fibre bundles of the PDL insert as Sharpey’s fibres into both the cementum of the root and the bundle bone of the alveolar proper. These fibres are under continuous tension during function, distributing occlusal load across the alveolar bone rather than concentrating it at a single fulcrum point. When these fibres are disrupted by periodontal infection, occlusal trauma, or orthodontic force (in excess of the PDL’s adaptive capacity), the tooth loses its fibrous suspension and mobility increases. Tooth mobility is therefore a clinical marker of PDL integrity — increased mobility signals either irreversible attachment loss, reversible oedema/inflammation of the PDL space, or acute occlusal trauma.
• Cell rests of Malassez have direct pathological significance — they are the origin of periapical and lateral periodontal cysts: The rests of Malassez are small clusters of epithelial cells distributed throughout the PDL, particularly near the cementum surface, as persistent remnants of Hertwig’s root epithelial sheath. In health, they are quiescent. When stimulated by periapical inflammation (necrotic pulp → periapical granuloma) or by periodontal pocket extension, they proliferate and can undergo central liquefaction to form a radicular (periapical) cyst — the most common odontogenic cyst. The distinction between a periapical granuloma and a periapical cyst cannot be made radiographically; it requires histological examination of the curetted tissue. Recognising that the cell rests of Malassez are the origin of these cysts explains why incomplete removal of periapical tissue at extraction can occasionally lead to cyst persistence in the socket.
• Alveolar bone is entirely tooth-dependent — its preservation requires tooth preservation: The alveolar process forms only in response to tooth eruption, and resorbs within months of tooth loss. Unlike the basal bone of the jaw mandible and maxilla (which remodels slowly and incompletely after tooth loss), the alveolar bone is a functional structure whose continued existence is conditional on the presence and function of the teeth it supports. Implant placement in a partially or fully edentulous ridge is limited by the alveolar bone volume that remains after tooth loss and subsequent resorption — explaining why delayed implant placement requires more complex bone augmentation procedures than immediate implant placement at the time of extraction.

The Gingiva

The gingiva is the most coronally positioned component of the periodontium and the only part directly visible to the clinician without instrumentation. It forms a continuous soft tissue collar around each tooth and across the interdental spaces, providing the mucosal seal against the oral environment and serving as the first clinical indicator of periodontal health or disease. Clinically, the gingiva is divided into three zones based on location, mobility, and histological characteristics.

Anatomy of the Gingiva

Gingival Epithelium

The gingiva is covered by three histologically distinct epithelial types that differ in keratinisation, thickness, and permeability — each adapted to its specific microenvironment and functional demands.

Oral epithelium covers the outer surface of the attached and free gingiva. It is orthokeratinised or parakeratinised stratified squamous epithelium — thick (200–300 µm), with a well-developed stratum corneum and extensive rete peg projections into the underlying connective tissue. The thick keratinisation provides excellent resistance to mechanical trauma and forms an effective barrier against bacterial product penetration. The rete peg interlocking with connective tissue papillae provides the mechanical anchoring that makes attached gingiva immobile and resistant to shear forces.

Sulcular epithelium lines the lateral wall of the gingival sulcus (free gingiva inner surface). It is non-keratinised thin squamous epithelium that faces the tooth surface across the sulcular fluid-filled space. It lacks the rete pegs and stratum corneum of the oral epithelium and is therefore more permeable to bacterial products — but it is still thicker and more resistant than the JE. Bacterial endotoxins and metabolic products diffusing through the sulcular epithelium trigger the connective tissue inflammatory response of gingivitis.

Junctional epithelium (JE) forms the epithelial seal at the base of the sulcus, attaching the gingiva to the tooth surface. It is a unique, non-keratinised, non-stratifying epithelium derived embryologically from the reduced enamel epithelium that remains after crown formation. Key characteristics: (1) only 15–20 cell layers thick at the gingival crest, tapering to 2–3 cells at the apical terminus; (2) attached to the tooth surface by hemidesmosomes and an internal basal lamina on its dental surface — the only epithelium in the body that forms an attachment directly to a mineralised surface; (3) has very large intercellular spaces and loose desmosomes that allow continuous neutrophil emigration from the connective tissue into the sulcus; (4) turns over every 4–6 days — the fastest epithelial turnover in the body; (5) approximately 2 mm long on a healthy tooth, situated on the enamel surface of a fully erupted tooth.

Gingival Connective Tissue

The gingival connective tissue (lamina propria) is predominantly composed of Type I collagen (60–65% of dry weight), with smaller amounts of Type III collagen (young/healing tissue), elastin (in vessel walls but not gingival fibres), and ground substance (proteoglycans, glycoproteins). Gingival fibroblasts are the dominant cell type, responsible for collagen synthesis and degradation. The gingival connective tissue contains a characteristic set of fibre groups — the gingival fibres — that are distinct from the PDL principal fibres and serve specific functions in maintaining the free gingival position and in anchoring the free and attached gingiva:

• Dentogingival fibres: From cementum just below the CEJ, fanning outward into the free gingiva — the most numerous; they maintain the free gingival position coronal to the CEJ.
• Alveologingival fibres: From the alveolar crest, running coronally into the free gingiva.
• Circular fibres: Encircling the tooth within the free gingiva without cementum insertion; they form the “cuff” that maintains the gingival collar shape.
• Dentoperiosteal fibres: From cementum, passing over the alveolar crest into the periosteum of the alveolar process.
• Transseptal fibres: Passing from the cementum of one tooth, over the alveolar crest, to the cementum of the adjacent tooth — they maintain the interdental relationship of adjacent teeth and are critical for orthodontic retention. They are entirely within the interdental gingiva above the alveolar bone.

The Periodontal Ligament

The periodontal ligament is the fibrous connective tissue occupying the periodontal space between the root cementum and the alveolar bone proper (cribriform plate). It is one of the most specialised connective tissues in the body — simultaneously a suspension apparatus (distributing occlusal load), a mechanoreceptor system (proprioceptive feedback for jaw closure), a nutrient conduit (vascular supply to the avascular cementum and bundle bone), and a pool of progenitor cells (for cementum, bone, and PDL regeneration). The PDL space measures approximately 0.15–0.38 mm in health; it is narrowest at the fulcrum point (middle third of the root) and widest at the alveolar crest and apex.

Principal Fibre Groups

PDL Cell Types

• Fibroblasts (most numerous): Responsible for continuous PDL collagen synthesis and degradation — the mechanism underlying the 24-hour collagen turnover rate. PDL fibroblasts are unique in that they simultaneously produce new collagen and activate its phagocytic degradation; this combined synthesis-degradation activity allows rapid PDL remodelling in response to orthodontic forces. They also secrete the ground substance (hyaluronan, versican, tenascin) of the PDL matrix.
• Osteoblasts and osteoclasts: Line the alveolar bone surface within the PDL space, responsible for the continuous remodelling of bundle bone in response to physiological loading and pathological stimuli. Orthodontic tooth movement depends on osteoclast activity on the pressure side and osteoblast activity on the tension side of the PDL space.
• Cementoblasts: Line the cementum surface within the PDL, responsible for the continuous deposition of cellular cementum throughout life (explaining why the apical cementum thickens with age). They also participate in cementum repair after injury.
• Epithelial cell rests of Malassez (ERM): Small clusters of quiescent epithelial cells distributed throughout the PDL, representing persistent remnants of Hertwig’s epithelial root sheath (HERS) — the bilaminar epithelial structure that induces root dentine formation. ERMs are found in a continuous network near the cementum surface in any section of the PDL. They are quiescent in health but proliferate when stimulated by periapical inflammation, mechanical trauma, or periodontal pocket extension — forming periapical granulomas and subsequently radicular cysts.
• Undifferentiated mesenchymal cells (progenitor cells): A population of perivascular progenitor cells capable of differentiating into fibroblasts, osteoblasts, and cementoblasts. They are critical for PDL regeneration after periodontal surgery and are the cellular basis for guided tissue regeneration (GTR) procedures.
• Macrophages, mast cells, and lymphocytes: Immune surveillance cells normally present at low numbers; massively increased during periodontal inflammation.

Functions of the PDL

The PDL performs five principal functions simultaneously: (1) Physical/supportive: anchors the tooth to bone via Sharpey’s fibres and distributes occlusal loads across the alveolar bone through the tensile properties of the oblique fibres; (2) Sensory/proprioceptive: the PDL contains Ruffini corpuscles and free nerve endings that provide precise proprioceptive information about tooth position and occlusal load magnitude — allowing reflexive jaw muscle control during mastication (the “periodontal bite reflex”); (3) Nutritive: the PDL vasculature is the sole nutrient supply for the avascular cementum and for the bundle bone; (4) Resorptive: the PDL contains cementoclasts and osteoclasts that can resorb cementum and bone under pathological or therapeutic (orthodontic) conditions; and (5) Reparative/regenerative: PDL progenitor cells can regenerate cementum, PDL, and alveolar bone — the target of regenerative periodontal surgery.

Cementum

Cementum is a mineralised connective tissue covering the root dentine surface from the CEJ to the root apex. It is produced by cementoblasts derived from the dental follicle. Cementum is unique among mineralised tissues in that it is avascular (no blood vessels), alymphatic (no lymphatics), and lacks innervation — it receives its nutrient supply entirely from the PDL vasculature and via diffusion. Unlike alveolar bone, cementum does not resorb physiologically — it accumulates throughout life. The thickness of cementum increases continuously with age, particularly in the apical third and at furcations.

Types of Cementum

The Cemento-Enamel Junction (CEJ)

The CEJ is the anatomical landmark at which cementum and enamel meet at the cervical region of the tooth. Three patterns of CEJ relationship are possible: (1) Cementum overlapping enamel (~60–65% of teeth) — the most common; cementum extends coronally onto enamel for a small distance; (2) Edge-to-edge meeting (~30% of teeth) — cementum and enamel abut with no overlap; (3) Gap between cementum and enamel (~5–10% of teeth) — dentine is exposed at the cervical region between the two tissues; this variant is associated with dentine hypersensitivity when the root is exposed by recession. The CEJ is also the anatomical reference point for measuring clinical attachment level (CAL) — the distance from the CEJ to the base of the probing pocket defines attachment loss and is the primary measure of periodontitis severity.

Alveolar Bone

The alveolar bone is the portion of the maxilla and mandible that forms the tooth sockets (alveoli) and supports the teeth. It is a highly specialised bone that is entirely tooth-dependent — it forms in response to tooth eruption and undergoes progressive resorption following tooth loss, a process that continues throughout life and accounts for the dramatic ridge resorption seen in edentulous patients. Alveolar bone is the most metabolically active bone in the body, capable of rapid remodelling in response to changing functional demands — making it the structural basis for orthodontic tooth movement and for the bone loss of periodontitis.

Bundle Bone, Alveolar Bone Proper, and Supporting Bone

Alveolar bone is anatomically divided into the alveolar bone proper (inner socket wall) and the supporting alveolar bone (the bulk of the alveolus). The alveolar bone proper — also called the cribriform plate or lamina dura (the radiographic term) — is the thin layer of specialised bone lining the tooth socket, into which the Sharpey’s fibres of the PDL insert. Its inner surface (facing the PDL) is described as bundle bone — a zone of bone containing the embedded Sharpey’s fibre insertions, incompletely mineralised, and histologically distinct from the lamellar bone that forms the bulk of the alveolus. The cribriform plate is perforated by numerous small canals (Volkmann canals carrying blood vessels from the PDL; the anatomical basis for the term “cribriform” — from Latin for sieve) that allow the vascular communication between the PDL and the supporting bone vasculature.

The supporting alveolar bone comprises the cortical plates (buccal and lingual compact bone) and the cancellous (trabecular) bone between them. The thickness and density of the cortical plates varies markedly by location: the buccal cortical plate over the maxillary anterior teeth may be as thin as 0.5 mm and is highly susceptible to resorption after extraction; the lingual plate of the mandibular molar region may be several millimetres thick. This variation has direct implications for implant placement (thin cortical plates = risk of fenestration), bone grafting (thin plates limit graft containment), and flap design in periodontal surgery.

Bone Remodelling in the Periodontium

Alveolar bone remodels through the coordinated activity of osteoblasts (bone formation) and osteoclasts (bone resorption). Osteoclasts are multi-nucleated cells of haematopoietic origin that adhere to bone surfaces via integrins, form the ruffled border, and secrete hydrochloric acid (to dissolve mineral) and lysosomal enzymes (to degrade organic matrix) into the Howship’s lacunae they create. The RANKL/OPG axis regulates osteoclast formation and activity — RANKL (receptor activator of NF-κB ligand), produced by osteoblasts and periodontal stromal cells, activates osteoclast precursors; osteoprotegerin (OPG), also produced by osteoblasts, acts as a decoy receptor that inhibits RANKL-RANK interaction and limits osteoclast activity. In periodontitis, the bacterial LPS-stimulated IL-1β and TNF-α from activated macrophages and fibroblasts dramatically upregulate RANKL expression while downregulating OPG, shifting the balance toward osteoclast activation — the molecular mechanism of the alveolar bone loss that characterises periodontitis.

Biologic Width and Supracrestal Tissue Attachment

The biologic width — now termed supracrestal tissue attachment (STA) in the 2017 World Workshop classification — describes the combined vertical height of the junctional epithelium and the supracrestal connective tissue attachment (the zone of collagen fibres attached to the cementum above the alveolar bone crest). Gargiulo, Wentz, and Orban (1961) measured these dimensions in autopsy specimens and established the widely cited figures: JE = 0.97 mm ± 0.29 mm; supracrestal CT attachment = 1.07 mm ± 0.25 mm; sulcus depth = 0.69 mm — giving a total of approximately 2.73 mm from the gingival crest to the alveolar bone crest.

The clinical relevance of STA is that it is physiologically constant and will re-establish itself if violated. When a crown margin, preparation finish line, or restorative material is placed apical to the most coronal attachment of the JE — encroaching on the STA — the body responds by resorbing the alveolar bone apically until the STA has re-established its required 2 mm of attachment above the new bone crest. The result is chronic inflammation, gingival recession, pocket formation, and bone loss at the restoration site. Prevention requires: (1) measuring the distance from the gingival crest to the alveolar bone (by probing to bone under anaesthesia — “bone sounding”); (2) ensuring that the restoration margin is at least 3 mm from the alveolar bone crest (2 mm STA + 1 mm sulcus depth); and (3) performing crown lengthening surgery (apical flap repositioning + osseous surgery) before final preparation if the existing anatomy does not provide adequate space for the planned margin placement.

Blood Supply and Nerve Supply

The periodontium is supplied by three arterial sources that form an anastomotic plexus providing redundant supply — a clinically important feature for surgical flap design. The supraperiosteal vessels (terminal branches of labial, buccal, mental, and palatal arteries) supply the attached and free gingiva from the oral surface; the PDL vessels (branches of dental arteries that also supply the pulp, entering through the apical foramen and multiple lateral canals) supply the PDL and give off branches that penetrate the cribriform plate to supply the bundle bone; and the alveolar vessels (from the superior and inferior alveolar arteries via nutrient canals through the alveolar bone) supply the cancellous and cortical bone. The anastomotic connections between these three systems mean that periodontal surgical flaps can be elevated over considerable areas without compromising tissue viability — provided that the base of the flap preserves the supraperiosteal vessels.

The periodontium is densely innervated with both mechanoreceptors and nociceptors. The PDL contains Ruffini corpuscles (slowly adapting mechanoreceptors that detect tooth position, loading duration, and direction of force — the basis of periodontal proprioception), Meissner’s corpuscles (rapidly adapting, detecting velocity of loading), and free nerve endings (pain, thermal). The density and precision of PDL mechanoreception is dramatically higher than for implants (which have no PDL and rely on bone-anchored osseoperception and mucosal mechanoreceptors) — explaining why implant-supported prostheses show reduced tactile discrimination and increased risk of overloading compared with tooth-supported restorations.

Clinical Considerations

• Probing pocket depth vs. clinical attachment level — the critical distinction for periodontitis staging: Probing pocket depth (PPD) measures from the gingival margin to the base of the probing pocket — it reflects both the degree of gingival recession/swelling and the level of the JE attachment. Clinical attachment level (CAL) measures from the CEJ to the base of the probing pocket — it is CEJ-referenced and is therefore a true measure of tissue destruction regardless of gingival position. In a patient with 2 mm of recession and a 4 mm PPD: PPD = 4 mm but CAL = 2 + 4 = 6 mm — Stage III periodontitis. In a patient with 2 mm of gingival hyperplasia (pseudopocket) and 4 mm PPD: PPD = 4 mm but CAL = 4 − 2 = 2 mm (attachment not lost). The 2017 staging system is based on CAL, not PPD — failure to use CAL will systematically underestimate disease severity in patients with recession.
• The furcation entrance is narrower than most instruments — it defines the limits of both NSPT and prognosis: The average furcation entrance (the distance from the outer buccal root surface to the start of the furcation) is 0.75–0.78 mm in maxillary molars and approximately 0.75 mm in mandibular molars. Standard Gracey curettes are 0.75–0.9 mm wide at the blade — at or beyond the furcation entrance width. Even thin ultrasonic tips (0.5 mm) cannot access the full depth of a Class II furcation. The anatomical inaccessibility of furcation areas is the primary reason furcation involvement reduces periodontal prognosis and the primary indication for surgical access (furcation plasty, root resection, hemisection, or extraction) when NSPT fails to control disease at furcated sites.
• Cementum loss during root planing is irreversible and clinically significant: Aggressive root planing removes not only contaminated cementum but also sound acellular cementum and exposes dentinal tubules — producing dentine hypersensitivity, weakening the root surface, and removing the substrate for PDL re-attachment. Current periodontal debridement guidelines emphasise biofilm and deposit removal, not the aggressive root surface reduction historically associated with “root planing.” The endpoint of subgingival instrumentation is a clean, calculus-free root surface — not an ultra-smooth, glassy root surface achieved by removing all cementum. Excessive root planing is an iatrogenic procedure that should be avoided.
• Alveolar bone pattern (horizontal vs. angular) has direct implications for prognosis and surgical planning: Horizontal bone loss (bone level at the same height across the alveolus, parallel to the CEJ) is the predominant pattern in chronic generalised periodontitis. Angular (vertical, infrabony) bone loss describes a defect in which the bone loss is greater adjacent to one tooth than an adjacent tooth, creating an angular defect. Infrabony defects (one-, two-, or three-walled, depending on the number of bone walls remaining) are the target of regenerative periodontal procedures — they have a better regenerative potential than horizontal defects because the residual bone walls provide scaffolding for new bone, PDL, and cementum formation. Understanding the defect morphology (identified by probing and CBCT) determines the surgical approach: osseous surgery (resective) for horizontal defects; regenerative surgery (GTR, bone grafts, EMD) for infrabony defects.
• Mucogingival junction position is fixed — gingival margin position is not: The mucogingival junction (MGJ) — the boundary between the keratinised attached gingiva and the non-keratinised alveolar mucosa — is a fixed anatomical landmark whose position does not change significantly over a lifetime (except with significant alveolar bone loss exposing the mucosa above the bone crest). The gingival margin, by contrast, can move coronally (hyperplasia, oedema) or apically (recession) in response to disease, trauma, or treatment. When recession has occurred to the point where the attached gingiva is absent or minimal, the gingival margin may be at or below the MGJ — the clinical significance of this is that there is no keratinised tissue at the gingival margin, which increases susceptibility to further recession, impairs maintenance of hygiene at restoration margins, and is a contraindication to subgingival crown margin placement.

Common Mistakes & Misconceptions

• Misconception: “The junctional epithelium is keratinised like the oral epithelium.”
  Correction: The JE is non-keratinised and non-stratifying. It lacks both a stratum granulosum and a stratum corneum. Its cells are large, with prominent intracellular spaces and reduced desmosomes that permit neutrophil transmigration. These characteristics make it the most permeable component of the gingival epithelial system and the primary site of bacterial product entry into the periodontium — the structural basis for gingivitis initiation at the JE-sulcus junction.
• Misconception: “Cementum resorbs physiologically, like alveolar bone.”
  Correction: Cementum does not resorb physiologically. Unlike alveolar bone (which undergoes continuous resorption-deposition remodelling), cementum is deposited throughout life and does not resorb under normal conditions — it accumulates progressively, particularly in the apical third. Cementum resorption is a pathological event occurring in root resorption (orthodontic overloading, trauma, replantation after avulsion, idiopathic root resorption), in ankylosis, and in certain cystic and neoplastic conditions. The absence of physiological cementum resorption is clinically exploited in endodontic treatment — the apical constriction (cementodentinal junction) is the target for canal preparation and filling because it represents the narrowest point of the root canal at the boundary of the vital PDL-containing cementum.
• Misconception: “The PDL space is the same width throughout its length.”
  Correction: The PDL space is hourglass-shaped in cross-section — widest at the alveolar crest (0.38 mm) and at the apex (0.35 mm), and narrowest at the fulcrum point (approximately the middle third of the root, 0.15 mm). This variation reflects the biomechanics of tooth loading: the PDL is compressed most at the fulcrum point during lateral forces, requiring less space; the crestal and apical regions experience more tension and require more space for fibre arrangement. The PDL space is radiographically visible as the lamina dura–PDL space–root outline pattern on periapical radiographs. Loss of lamina dura definition and PDL space widening are radiographic signs of periapical pathology and occlusal trauma, respectively.
• Misconception: “Biologic width violation only occurs if a crown margin is placed on bone.”
  Correction: Biologic width violation occurs any time a restoration margin, preparation finish line, or restorative material encroaches on the supracrestal tissue attachment — which extends approximately 2 mm above the alveolar bone crest. A margin placed 1 mm below the gingival crest in a site where the gingival crest is only 2 mm above the bone crest has already violated the biologic width. The clinical test is bone sounding under anaesthesia — the distance from the gingival crest to the alveolar bone is measured, and margins must not be placed closer than 3 mm from the bone (leaving 2 mm for STA and 1 mm of sulcus).
• Misconception: “All five PDL principal fibre groups are present at every location around every tooth.”
  Correction: The interradicular fibres are present only at multi-rooted teeth (the furcation area between roots). All other groups (alveolar crest, horizontal, oblique, apical) are present on all teeth. Additionally, the relative proportions of fibre groups vary by root location and functional demands — the oblique fibres predominate in the middle and apical thirds because axial loading is the primary force direction on most teeth, while alveolar crest and horizontal fibres are more prominent coronally where lateral forces are resisted. Knowing which fibre group is lost first in periodontitis (alveolar crest fibres — the most coronal group, destroyed by the advancing lesion as the pocket deepens) is a board exam favourite.

Related Topics

References & Sources

1. Gargiulo AW, Wentz FM, Orban B (1961). Dimensions and relations of the dentogingival junction in humans. Journal of Periodontology, 32(3):261–267. [Original biologic width measurements — 0.97 mm JE + 1.07 mm CT attachment]
2. Schroeder HE, Listgarten MA (1977). Fine Structure of the Developing Epithelial Attachment of Human Teeth. 2nd ed. Basel: Karger. [Definitive ultrastructural description of junctional epithelium and hemidesmosome-basal lamina attachment]
3. Nanci A, Bosshardt DD (2006). Structure of periodontal tissues in health and disease. Periodontology 2000, 40(1):11–28. [Comprehensive histological review of all four periodontal tissues]
4. Bosshardt DD, Selvig KA (1997). Dental cementum: the dynamic tissue covering of the root. Periodontology 2000, 13(1):41–75. [Cementum types, formation, and clinical relevance]
5. Pihlstrom BL (2001). Periodontal risk assessment, diagnosis and treatment planning. Periodontology 2000, 25(1):37–58. [Clinical application of periodontium anatomy to diagnosis and planning]
6. Tonetti MS, Greenwell H, Kornman KS (2018). Staging and grading of periodontitis. Journal of Periodontology, 89(Suppl 1):S159–S172. [2017 classification — supracrestal tissue attachment terminology]
7. Ten Cate AR (2012). Ten Cate’s Oral Histology: Development, Structure, and Function. 8th ed. St. Louis: Elsevier Mosby. [Standard oral histology reference — all four periodontium components]

Summary

The periodontium comprises four interdependent tissues: gingiva (free gingiva forming the sulcus with sulcular and junctional epithelium; attached gingiva keratinised and immobile; interdental col non-keratinised and most susceptible to disease); periodontal ligament (fibrous connective tissue 0.15–0.38 mm wide containing five principal fibre groups as Sharpey’s fibres, fibroblasts with 24-hour collagen turnover, Malassez epithelial rests, cementoblasts, and progenitor cells); cementum (acellular primary on the coronal two-thirds with extrinsic Sharpey’s fibres; cellular secondary on the apical third with cementocytes; avascular and inalymphatic; accumulates throughout life without physiological resorption); and alveolar bone (cribriform plate/bundle bone with Sharpey’s fibre insertions; supporting cortical and cancellous bone; most metabolically active bone in the body; entirely tooth-dependent). The supracrestal tissue attachment (biologic width) of ~2 mm must be respected in all restorative planning to prevent chronic inflammation and bone loss at restoration margins.

About the Author

Dr. Andries Smith

Founder, Dental Panda

Dr. Andries Smith founded Dental Panda in 2020. As an immigrant to the United States, he had to take the INBDE exam, even though he was practicing dentistry for over 10 years. This revealed an opportunity. Andries noticed that INBDE prep course companies were putting profit over students. With his expertise and experience in dentistry, he created free dental wiki resources for students and the general public to have access to.