Cementum: Composition, Structure & Function

• Cementum supports the junction between the tooth root and alveolar bone.
• Cementum is a key part of the periodontium and thus, attachment.

Learning Outcomes

• Describe the composition and structure.
• Distinguish between the 2 main types and their distribution.
• Describe how the structure relates to its function.
• Apply this knowledge to interpret the clinical significance in health and disease.

Cementum Composition

• Inorganic content is 45-50% hydroxyapatite crystal.
• Critical pH is 6.7.
• Organic content is 50-55%.
• Collagen fibers are mainly type I, with minor quantities of types III, V, VI, XII, and XIV.
• Non-collagenous matrix proteins and water are also present.

Overview of Structural Features

• Types of cementum are acellular and cellular.
• Acellular cementum: has extrinsic (Sharpey's) fibers or is afibrillar (no fibers), and is primary cementum.
• Cellular cementum: mixed (intrinsic and extrinsic) or intrinsic (formed by cementoblasts), and is secondary cementum.

Acellular Cementum

• Also known as primary cementum.
• Covers the cervical and middle third of the root (40-70% of root surface).
• The first formed, adjacent to the dentine creating the dento-enamel junction (DEJ).
• Forms slowly as the tooth erupts and continues post-eruption.
• Contains collagen fibers (extrinsic) and non-collagenous matrix proteins.
• No cells are present.

Acellular Cementum Types

• Acellular afibrillar cementum (AAC)
  • Found only in the cervical region covering enamel and dentine
  • Contains no collagen fibers
  • No known functional significance
• Acellular extrinsic fibers cementum (AEFC)
  • The main type of acellular cementum with a key function in support and anchorage

Acellular Cementum Functions

• As part of the periodontium, supports the tooth's position within the alveolar socket
• Through extrinsic fibers forms the attachment with the periodontal ligament fiber bundles, known as Sharpey's fibers, which are formed by fibroblasts and the alveolar bone
• These fibers are oriented perpendicular to the root surface

Cellular Cementum

• Also known as secondary cementum.
• Contains cementocytes.
• Forms quickly compared to primary cementum.
• A layer of cementoid (unmineralized matrix or pre-cementum) is found at the periphery of cellular cementum.
• Contains intrinsic fibers that are parallel to the root surface, and over time, contains extrinsic fibers.

Cellular Cementum Cells

• Cementoblasts form cementum via cementogenesis and originate from the dental sac.
• Cementocytes are found in cellular cementum and originate from cementoblasts trapped in the matrix as a result of the speed of deposition.

Cellular Cementum Distribution

• Cellular intrinsic fiber cementum (CIFC) is found in the apical third of the roots and inter-radicular regions of posterior teeth, and is often absent in single-rooted teeth.
• Cellular mixed stratified cementum (CMSC) is a subcategory of CIFC containing both intrinsic and extrinsic fibers, and over time makes up the bulk of cellular cementum.

Cellular Cementum Functions

• Allows for root reshaping to adjust for movement, such as drifting after extraction.
• Deposition of cementum at the apex to maintain occlusion and compensate for occlusal wear, which is thicker in posterior teeth.
• Can repair itself anatomically, able to repair resorbed or fractured root surfaces to some degree, as well as functionally by plays a role in periodontal disease healing.
• Maintains the width of the PDL.

Cementum Distribution

• Acellular towards the cervical of the roots and cellular towards the apical region.
• Molar teeth undergo heavier occlusal loads, therefore show a difference in the thickness compared to the incisor.

Clinical Significance

• Cementogenesis takes place continuously throughout life.
• Thickness, hardness, and mineral content increase with age.
• Graphs of widths vs. location show a two-step increase in thickness from the CEJ to the root apex regions.

Cemento-Enamel Junction (CEJ)

• The relationship of where cementum and enamel meet varies among teeth and within the same tooth, and may overlap (60-65%), gap (10%), or meet (25-30%).

Structural Abnormalities

• Hypercementosis can occur from excessive deposition of cementum toward the apex, resulting in thicker roots.
  • Causes: local factors such as trauma or inflammation, systemic factors such as Paget's disease, or idiopathic (unknown) cause.

Clinical Implications

• May affect single or multiple teeth, often asymptomatic, and may cause problems for extractions.