Location and Borders of Cementum
Cementum covers the outer surface of the tooth root (radix dentis) and the neck of the tooth (cervix dentis). It is a component of the periodontium, the complex of tissues supporting the tooth.
- Cervical junction (Cementoenamel junction - CEJ): At the neck of the tooth, cementum meets the enamel. There are four possible variants of this junction: edge-to-edge butt joint; enamel overlapping cementum; cementum overlapping enamel; or a gap where enamel and cementum fail to meet, leaving an exposed area of dentin.
- Root canal boundary: Cementum may extend into the root canal up to the physiological apex (apical constriction), where the cementodentinal junction is formed.
- Relation to the periodontium: The periodontium (periodontium), a dense connective tissue situated between the root and the alveolar bone, lies immediately external to the cementum.
Classification: Types of Cementum
Based on its structure and anatomical location, cementum is divided into two main types:
- - Acellular (primary) cementum: Lies directly on the dentin and covers the coronal and middle portions of the root.
- - Cellular (secondary) cementum: Covers the apical third of the root and the furcation areas of multirooted teeth. As a rule, it is deposited over the acellular cementum, but occasionally it may lie directly on the dentin.
Histological Structure
Cementum is formed continuously throughout life by cells on its outer surface. Cellular cementum consists of cellular elements and an extracellular matrix:
- - Cementoblasts: Active bone-like cells ("builders") that secrete successive layers of cementum. During the formation of acellular cementum, they recede outward; during the formation of cellular cementum, some become trapped within the matrix.
- - Cementocytes: Cells located in specialized spaces called lacunae. They derive from entrapped cementoblasts and closely resemble osteocytes in structure.
- - Extracellular matrix: Composed of ground substance and collagen fibers running in various directions. Some fibers run longitudinally, parallel to the surface. The majority are radial fibers known as Sharpey's fibers; these fibers are embedded into the dentin on one end and into the periodontal ligament on the other.
Main Functions
- - Supportive: Participates in forming the supporting apparatus of the tooth. Cementum anchors the periodontal ligament fibers to the root and neck of the tooth.
- - Protective: Protects the root dentin from damaging external influences.
- - Reparative: Participates in tissue repair processes (e.g., during root fractures or periodontal therapy).
Clinical Significance and Pathologies
When the root surface near the crown becomes exposed (gingival recession), the clinical root size decreases relative to the anatomical root. With age and ongoing exposure of areas not covered by enamel, the clinical crown becomes larger than the anatomical crown.
A benign tumor known as a cementoma can arise from odontogenic connective tissue capable of differentiating into cementum. It is typically closely associated with the tooth roots and may consist of fibroblastic tissue containing dense calcified cementum-like masses that can coalesce into large conglomerates as they grow. Several forms of cementomas are recognized: benign cementoblastoma (true cementoma, appearing macroscopically as a yellowish-white tissue of varying density), cementifying fibroma, periapical cemental dysplasia, and gigantiform cementoma.