Periodontal Anatomy – Cementum
The cementum is a special type of calcified substance which covers the tooth’s roots. The cementum is included as part of the periodontium. It anchors the periodontal ligament and attaches the teeth to the alveolar bone.
The cells which are present within the cementum are the entrapped cementoblasts, or the cementocytes. Each cementocyte lies in its lacuna, which is similar to the pattern found in the bone. In addition, these lacunae have canals. However, unlike those which are found in bone, the canals which are in the cementum do not contain nerves, nor do they radiate outward. These canals orient toward the periodontal ligament. They contain cementocytic processes which diffuse nutrients from the ligament.
Following the apposition of cementum in layers, the cementoblasts which are not entrapped in the cementum, collect along the cemental surface. This occurs on the outer covering of the periodontal ligament. These cementoblasts are able to form additional layers of cementum in the event the tooth suffers an injury.
Sharpey fibers are part of the principal collagenous fibers of the periodontal ligament. These fibers are embedded in the cementum and the alveolar bone which attach the tooth to the alveolus.
When the cementum is visible on the teeth, this suggests that there is root exposure. This often occurs as a result of gingival recession. It may also be sign of periodontal disease.
The cementum then connects to the enamel and forms the cementoenamel junction (CEJ). This is called the cervical line.
Once the cementoid reaches the necessary thickness, the cementoid which surrounds the cementocytes is mineralized and is considered to be cementum. As a result of the apposition of cementum on the dentin, the dentinocemental junction (DCJ) forms. This interface is not as defined compared to the dentinoenamel junction (DEJ). The cementum and dentin are from a similar embryological background, which differs from that of the enamel and dentin.
The dentinocemental junction (DCJ) is a relatively smooth area which exists in permanent teeth. While the attachment of cementum to the dentin is firm, it is not fully understood.
The various categories or types of cementum are classified based on the presence or absence of cementocytes. This is in addition to whether the collagen fibers are extrinsic or intrinsic. Fibroblasts and some cementoblasts are thought to secrete extrinsic fibers. However, only cementoblasts will secrete intrinsic fibers. The extrinsic fibers within the acellular extrinsic fiber cementum, travel in a perpendicular pattern to the surface of the root. This allows the tooth to attach to the alveolar bone through the periodontal ligament (PDL), which is consistent with the cementodentinal junction (CDJ). While acellular cementum contains extrinsic collagen fibers, cellular cementum is thick. It contains both extrinsic and intrinsic collagen fibers. The first cementum which forms during the development of the tooth is acellular extrinsic fiber cementum. The acellular layer of cementum is a living tissue. It does not incorporate cells into its structure and typically predominates on the coronal half of the root. The main types of cementum include the following:
- Acellular Extrinsic Fibers Cementum (AEFC)
- Cellular Intrinsic Fibers Cementum (CIFC)
- Mixed Stratified Cementum (MSC)