4.4 Cementum, the Periodontal Ligament and Oral Mucosa
Key Takeaways
- Cementum is roughly 50% inorganic hydroxyapatite and 50% organic matrix and water, making it softer than dentine and easily removed by over-instrumentation.
- Cementum overlaps enamel in about 60% of teeth, meets it edge to edge in about 30%, and leaves a gap of exposed dentine in about 10%.
- The periodontal ligament is 0.15 to 0.38 mm wide with an hourglass profile that is narrowest at the mid-root fulcrum.
- Oblique fibres are the largest and most numerous principal group and resist axial masticatory loading.
- Masticatory mucosa covers the attached gingiva and hard palate and is keratinised, whereas lining mucosa is non-keratinised with a loose submucosa.
Cementogenesis & The Cementoenamel Junction
Cementum is a specialized, avascular mineralised tissue covering the anatomical root. Composed of 50% inorganic hydroxyapatite and 50% organic matrix (Type I collagen) and water, its principal role is anchoring the principal collagen fibres of the periodontal ligament to the root.
Primary vs Secondary Cementum
- Acellular Extrinsic Fibre Cementum (AEFC / Primary Cementum): Covers the coronal two-thirds of the root. Formed slowly by cementoblasts before tooth eruption; contains no embedded cells (acellular). It is composed almost entirely of densely packed Sharpey's fibres (extrinsic collagen fibres derived from the PDL) entering at 90-degree angles, providing the principal attachment of the tooth to alveolar bone.
- Cellular Intrinsic Fibre Cementum (CIFC / Secondary Cementum): Confined to the apical third of the root and the furcations of multi-rooted teeth. Formed rapidly after eruption; contains embedded cementocytes housed within lacunae, whose canaliculi communicate oriented toward the periodontal ligament blood supply. It repairs root resorption defects and undergoes continuous deposition to compensate for occlusal tooth wear.
Cementoenamel Junction (CEJ) Patterns (Choquet's Relationships)
Three distinct relationships occur at the junction between enamel and cementum:
- Cementum Overlaps Enamel (~60% of teeth): Cementum extends over the cervical edge of enamel.
- Edge-to-Edge Meeting (~30% of teeth): Cementum and enamel meet in a clean line.
- Gap Junction (~10% of teeth): A distinct gap exists between enamel and cementum, leaving underlying dentine exposed. This 10% subset of teeth is acutely susceptible to cervical dentine hypersensitivity and cervical root caries upon gingival recession.
Periodontal Ligament (PDL) Architecture
The periodontal ligament is a dense, highly cellular, vascular fibrous connective tissue situated between the radicular cementum and the alveolar bone proper (cribriform plate / lamina dura). Its width ranges from 0.15 to 0.38 mm, exhibiting an hourglass shape that is narrowest at the mid-root fulcrum (~0.15 mm).
Principal Collagen Fibre Groups
Composed of Type I (~80%) and Type III (~20%) collagen bundles, the principal dentoalveolar fibres are divided into five functional groups based on their trajectory:
| Fibre Group | Origin & Insertion | Primary Mechanical Function |
|---|---|---|
| Alveolar Crest Fibres | Extends from the cervical cementum just below the junctional epithelium obliquely downward to the alveolar crest. | Resists extrusive (pulling) forces and lateral tipping movements. |
| Horizontal Fibres | Extends at right angles from the cervical third of the cementum to the alveolar bone. | Resists horizontal and lateral displacing forces. |
| Oblique Fibres | Largest and most numerous group (two-thirds of all fibres); extends obliquely from cementum coronally to the alveolar bone. | Converts vertical compressive masticatory loads into tensile stress on alveolar bone, preventing apical vascular strangulation. |
| Apical Fibres | Radiates from the apical cementum to the fundus of the socket. | Resists extrusive and tipping forces; cushions the neurovascular bundle. |
| Interradicular Fibres | Runs from the furcation cementum to the interradicular alveolar crest in multi-rooted teeth. | Resists rotational, tilting, and vertical torque. |
Cervical Margin / Gingiva
┌────────────────────────┐
│ Alveolar Crest Fibres │ <── Resists Extrusion & Tilting
├────────────────────────┤
│ Horizontal Fibres │ <── Resists Lateral Displacement
├────────────────────────┤
│ Oblique Fibres │ <── Largest Group (Translates
│ (Slanted Coronally │ Compression into Tension)
│ to Bone) │
├────────────────────────┤
│ Apical Fibres │ <── Protects Apical Bundle
└────────────────────────┘
Apex / Alveolus
- Gingival Fibre Groups: Distinct from the dentoalveolar group, these include the dentogingival, alveologingival, circular, dentoperiosteal, and transseptal fibres. The transseptal fibres span interproximally between the cementum of adjacent teeth coronal to the alveolar crest. Because they possess low metabolic turnover, they maintain interdental contact but are heavily implicated in post-orthodontic rotational relapse.
Oral Mucosa Histology
The oral mucosa is categorized into three functionally and histologically distinct types:
1. Masticatory Mucosa (Keratinised / Parakeratinised)
- Anatomical Distribution: Attached gingiva and the hard palate.
- Histological Profile: Stratified squamous epithelium that is either orthokeratinised (surface squames are completely anucleate and packed with cytokeratin) or parakeratinised (surface cells retain pyknotic, condensed nuclei). It exhibits a thick, dense collagenous lamina propria firmly anchored to underlying periosteum (mucoperiosteum) without an intervening submucosa (except laterally on the palate where adipose and minor salivary glands reside). Deep, interlocking epithelial rete pegs interdigitate with connective tissue papillae, resisting heavy shear friction during bolus mastication.
2. Lining Mucosa (Non-Keratinised)
- Anatomical Distribution: Buccal and labial mucosa, floor of the mouth, ventral tongue, soft palate, and alveolar mucosa.
- Histological Profile: Non-keratinised stratified squamous epithelium. Consists of stratum basale, stratum spinosum, and stratum superficiale (no stratum granulosum or corneum). The underlying lamina propria is loose with abundant elastic fibres, resting upon a distinct submucosa containing minor salivary glands and adipose tissue. Rete pegs are short, blunt, or absent, conferring high mobility and elastic compliance.
3. Specialised Mucosa (Lingual Papillae)
Confined to the dorsal and lateral surfaces of the tongue, featuring four varieties of papillae:
- Filiform Papillae: Most numerous; small, cone-shaped projections covered by heavily keratinised epithelium. Provide abrasive mechanical traction during mastication; contain no taste buds.
- Fungiform Papillae: Mushroom-shaped, scattered red elevations predominantly at the tip and lateral margins; contain taste buds on their apical surface.
- Circumvallate Papillae: 8 to 12 large, dome-shaped papillae arranged in an inverted V-shape along the sulcus terminalis. Each papilla is encircled by a deep circular moat or crypt into which open the serous glands of von Ebner (whose lipase and watery secretions flush food debris). The lateral crypt walls contain numerous taste buds.
- Foliate Papillae: Parallel vertical mucosal folds on the posterolateral borders of the tongue containing lateral taste buds.
| Non-Keratinocyte Cell | Layer & Morphology | Specific Biological Role |
|---|---|---|
| Langerhans Cells | Stratum spinosum; dendritic "clear cells" containing Birbeck (tennis-racket) granules. | Antigen-presenting cells; uptake foreign antigens, express MHC-II and CD1a, and migrate to regional lymph nodes to activate T-lymphocytes. |
| Melanocytes | Stratum basale; dendritic cells of neural crest origin. | Synthesize melanin within melanosomes and transfer pigment to adjacent keratinocytes, producing physiological pigmentation. |
| Merkel Cells | Stratum basale; associated with an unmyelinated nerve terminal. | Mechanoreceptors for light touch and tactile discrimination (slow-adapting type I). |
| Inflammatory Cells | Transient throughout basal and spinous layers. | Intraepithelial T-lymphocytes and neutrophils guarding against pathogen invasion. |
The periodontal ligament also carries a proprioceptive role that has direct clinical consequences. Its mechanoreceptors allow fine discrimination of occlusal load and are the reason a patient can detect a high restoration measured in tens of micrometres. That input is lost when a tooth is extracted and replaced by an implant, which is osseointegrated rather than suspended in a ligament; implant-supported restorations therefore lack proprioceptive feedback and shock absorption, which is why occlusal schemes for implants are designed with lighter contacts and why implants do not move physiologically or respond to orthodontic force.
An 8-year-old child presents with a deep carious lesion on permanent tooth 36 that extends through the inner third of dentine. The pulp is vital and asymptomatic. Histologically, if the original primary odontoblasts are destroyed by bacterial toxins and newly differentiated odontoblast-like cells derived from pulpal mesenchymal stem cells secrete a tertiary dentine matrix characterized by atubular, irregular architecture, how is this tertiary dentine classified?