Cementum and the Periodontium

Cementum Overview

• Cementum is a specialized tissue that surrounds and supports teeth.
• Cementum anchors teeth in the maxillary and mandibular bones.
• The periodontium includes cementum, alveolar bone, periodontal ligament (PDL), and gingiva.

Periodontium

• The word "periodontium" is derived from the Greek words "peri" (around) and "odont" (tooth).
• The periodontium's primary functions are attaching and supporting teeth in the jaws.
• It functions during relaxed states and activities like sleeping, speaking, and eating, and provides a barrier against oral microflora.
• Preservation of the periodontium's structure depends on stimulation from tooth function and mastication.
• Periodontal structures exist in a constant state of homeostasis to adapt to external forces.
• Cementoblasts and osteoblasts have receptors for parathormone (PRT) and parathormone receptor protein (PTHrP).
• Vertical forces from mastication increase its ability to withstand pressure through remodeling.
• Instead of concentrating in one point, vertical force gets distributed through the PDL and surrounding bone.

Cementum Definition and Characteristics

• A cement is a binder that sets, hardens, and adheres to other materials.
• Cementum is a calcified substance covering the root of a tooth.
• It attaches teeth to the alveolar bone by anchoring the periodontal ligaments.
• Cementum is a hard, avascular connective tissue.
• Physiologic resorption or remodeling should not be seen in cementum, and deposition continues throughout life.
• Cementum is a covering of calcified tissue that adheres firmly to radicular dentine and connects to the periodontal ligaments.
• Cervically, the thickness is 10-15µm, increasing apically to 50-200µm and potentially exceeding 600µm at the root apex.
• Cementum closer to the apex is thicker to compensate for greater mechanical stress.
• The composition of cementum varies depending on its location in the tooth.
• A highly responsive mineralized tissue that maintains root integrity and preserves tooth position.
• The mineralized tissue, functions in tooth repair and regeneration.
• There is always a layer of uncalcified pre-cementum.
• Similar in composition to bone, cementum is not innervated or vascularized, lacking a lamellar appearance or marrow spaces.

Physical and Chemical Properties of Cementum

• Cementum has a pale yellow and dull surface.
• It is softer and more permeable than dentine, providing a cushioning effect.
• Variable permeability is dependent on age and type, (cellular type is more permeable and it reduces with age).
• Cementum is easily abraded cervically, leading to dentine exposure.
• The inorganic component is mainly hydroxyapatite, containing other thin, plate-like calcium crystals.
• The organic component consists of collagen type I, non-collagenous elements (sialoprotein and osteopontin).

Classification of Cementum

• Classified by the presence of cells (cellular or acellular) and the origin of fibers (intrinsic or extrinsic).
• Types of combination include: Acellular extrinsic fibre cementum (AEFC), Cellular intrinsic fibre cementum (CIFC), Mixed fibre cementum (MFC), Cellular mixed stratified cementum (CMSC), and Afibrillar cementum.
• Cellular cementum contains cementocytes, while acellular cementum covers the dentine.
• Cellular cementum is mainly in the apical and inter-radicular areas.
• Acellular cementum appears structureless and lacks cells.
• A dark line exists between the hyaline layer and acellular cementum, marking afibrillar cementum.
• Differences in both cellular and acellular cementum come from differences in the formation rate of both tissues.
• Lacunae, which contain cementocytes, are more widely spaced in incremental lines.

Cementocytes

• Cementocytes exist in different relations between cellular and acellular cementum, determined by their existence.
• Cementocytes are trapped in lacunae that have canaliculi for cylindrical processes.
• Canaliculi are oriented towards the periodontal ligament, to maintain origin contact.
• Cementocytes are usually inactive, with low cytoplasmic/nuclear ratio, and have minute amounts of protein synthesizing organelles.
• Cementum is deposited irregularly, resulting in incremental lines of Salter, occurring at weekly intervals.
• Incremental lines of Salter mark differences in mineralization and organic matrix composition, and are more calcified.

Origin of Fibers in Cementum

• Cementum's organic matrix is derived from cementoblasts (intrinsic fibers parallel to the root surface) and fibroblasts of the periodontal ligament (Sharpey fibers as extrinsic fibers).
• Fibroblasts become cementoblasts and help in making and attaching Sharpey fibers.
• Cellular cementum contains intrinsic fiber and acellular cementum contains extrinsic fibers.
• Mixed fiber cementum contains both types.

Acellular Extrinsic Fiber Cementum (AEFC)

• Collagen is derived from Sharpey's fibers in the periodontal ligament (PDL).
• The ground substance may be produced by cementoblasts.
• AEFC is mainly found in the cervical two-thirds of the root and forms the bulk of the cementum in premolars.
• It is the first formed cementum, well-mineralized, and can reach 15µm in thickness.

Cellular Intrinsic Fiber Cementum (CIFC)

• CIFC contains intrinsic fibers parallel to the root surface and does not play a role in tooth attachment to the periodontal ligament.
• It is found in the apical third of the root and inter-radicular area.
• CIFC accumulates with age and can cover the mid-root portion.

Mixed Fiber Cementum (MFC)

• Contains both extrinsic and intrinsic fibers with different orientations.
• Extrinsic fibers are perpendicular, while intrinsic fibers run parallel to the root surface.
• Extrinsic fibers are ovoid or round, measuring 5-6µm in diameter.
• Intrinsic fibers are very thin, ranging from 1-2µm in diameter.
• Cellular mixed fiber cementum forms quickly and is less mineralized.
• Acellular mixed fiber cementum forms slowly and is well mineralized.

Afibrillar Cementum

• Afibrillar cementum is the first layer and lacks collagen fibers, however, it is filled with sparsely distributed, well-mineralized ground substance.
• It likely originates from epithelial cells, with the hyaline layer appearing as part of uncalcified afibrillar cementum.
• It is thin, acellular, may overlap with enamel, and is located between fibrillar cementum and dentine.

Cementum Relation with Surrounding Tissues

• Includes the cemento-enamel junction (CEJ), cemento-dentinal junction (CDJ), and attachment to the periodontal ligament.

Cemento-Enamel Junction:

• Involves three possible patterns: cementum overlapping enamel and meeting edge to edge, or cementum failing to meet, which exposes dentine.
• All three patterns may occur in the same tooth, but one pattern may be more dominant.
• Exposed dentine can lead to cold sensitivity.

Cemento-Dentinal Junction:

• It acts as an interface between tissues with different levels of mineralization that develop simultaneously.
• It is clinically significant for maintaining tooth function and root surface repair.
• The junction has lower mineral content, including the hyaline layer and afibrillar cementum, and consists of organic and inorganic components.
• It serves as the anchoring site for periodontal fibers into dentine.
• Can be referred to as the innermost cementum layer, superficial layer of root dentine, intermediate cementum, and hyaline layer.
• Contains wide, irregular spaces that may connect with dentinal tubules.

Intermediate Cementum (Hyaline Layer of Hopewell-Smith)

• It is the first layer of cementum, formed by inner Hertwig's epithelial root sheath (HERS) cells and deposited on the root's surface.
• Deposition occurs before HERS disintegrates, sealing dentinal tubules.
• Lies between Tomes' granular layer of dentin and secondary cementum.
• It is about 10 µm thick, more mineralized than adjacent dentin and cementum, with wide irregular spaces for entrapped cells.
• In humans, is considered a product of Hertwig's epithelial root sheath (HERS).
• There may be no distinct layer to avoid direct contact between dentinal tubules and cementum fibers.

Cementum-Periodontal Ligament Junction

• Periodontal ligament (PDL) fibers embed into pre-cementum's organic matrix, secreted by cementoblasts.
• Pre-cementum mineralizes and incorporates extrinsic (Sharpey's) fibers.
• Approximately 30,000 principal PDL fibers attach to cementum per square millimeter.

Resorption and Repair of Cementum

• Cementum is less susceptible to normal physiological resorption than bone.
• Localized resorption is caused by multinucleated odontoclasts (cementoclasts) and active tissue degradation, and may extend into dentin.
• When resorption occurs, the body deposits new cementum to repair the damaged area.
• Repaired tissue may have a reversal line between it and the original cementum.
• The repair tissue resembles cellular cementum and has a thicker precementum layer (15 µm) and lower mineralization/crystal size.

Clinical Considerations

Exposed Root Dentine:

• Thin cementum is easily removed by abrasion, especially cervically.
• Exposed dentine is rapidly dissolved and leads to cervical caries and hypersensitivity.

Root Fracture Repaired by Cementum Callus:

• Cemental callus formation repairs root fractures and is similar to the fibroid that forms when wounded.

Cementicles:

• Are abnormal secretions of cementum in the periodontal ligaments, and found in 35% of human roots.
• Not always attached and are located in the apical and middle root areas.

Hypercementosis:

• An increased cementum thickness throughout life due to chronic periapical inflammation or Paget's disease of bone.
• Attachment of periodontal ligaments is replaced by cementum.