(which is labeled
B). Rather than being a passive entity like paint on a wall, cementum is a dynamic entity within the
periodontium. It is attached to the
alveolar bone (C) by the fibers of the
periodontal ligament and to the soft tissue of the
gingiva by the
gingival fibers (H). The cells of cementum are the entrapped cementoblasts, the cementocytes. Each cementocyte lies in its
lacuna, similar to the pattern noted in bone. These lacunae also have canaliculi or canals. Unlike those in bone, however, these canals in cementum do not contain nerves, nor do they radiate outward. Instead, the canals are oriented toward the periodontal ligament and contain cementocytic processes that exist to diffuse nutrients from the ligament because it is vascularized. After the apposition of cementum in layers, the cementoblasts that do not become entrapped in cementum line up along the cemental surface along the length of the outer covering of the periodontal ligament. These cementoblasts can form subsequent layers of cementum if the tooth is injured.
Sharpey fibers are part of the principal collagenous fibers of the
periodontal ligament embedded in the cementum and
alveolar bone to attach the tooth to the alveolus. If cementum can be observed on teeth, it can imply that the roots are exposed, showing that the clinical crown (the exposed part of the tooth) is bigger than the anatomical crown (the surface of the tooth covered by enamel). This is often due to
gingival recession and may be an indication of
periodontal disease.
Cementoenamel junction The cementum joins the enamel to form the
cementoenamel junction (CEJ), which is referred to as the
cervical line. Three possible types of transitional interfaces may be present at the CEJ. The traditional view was that certain interfaces dominated in certain oral cavities. The CEJ may exhibit all of these interfaces in an individual's oral cavity, and there is even considerable variation when one tooth is traced circumferentially.
Dentinocemental junction When the cementoid reaches the full thickness needed, the cementoid surrounding the cementocytes becomes mineralized, or matured, and is then considered cementum. The dentinocemental junction (DCJ) is formed because of the apposition of cementum over the dentin. This interface is not as defined, either clinically or histologically, as that of the dentinoenamel junction (DEJ), given that cementum and dentin are of common embryological background, unlike that of enamel and dentin. The dentinocemental junction (DCJ) is a relatively smooth area in the permanent tooth, and attachment of cementum to the dentin is firm but not understood completely.
Types The different categories of cementum are based on the presence or absence of cementocytes, as well as whether the
collagen fibres are
extrinsic or
intrinsic. It is thought that
fibroblasts, and some
cementoblasts, secrete extrinsic fibres, but only cementoblasts secrete intrinsic fibres. The extrinsic fibres within acellular extrinsic fibre cementum, travel perpendicular to the surface of the root and allow the tooth to attach to the alveolar bone by the periodontal ligament (PDL), continuous with the cementodentinal junction (CDJ). Acellular cementum only contains extrinsic collagen fibres. Whereas, cellular cementum is quite thick and contains both extrinsic and intrinsic collagen fibres. The acellular layer of cementum is living tissue that does not incorporate cells into its structure and usually predominates on the coronal half of the root; cellular cementum occurs more frequently on the apical half. Cellular cementum contains cells and is the medium of attachment of collagen fibres to the alveolar bone. It is also responsible for minor repair of any resorption by continued deposition to keep the attachment apparatus intact. Acellular cementum does not contain cells and has a main purpose of adaptive function.
Composition Cementum is slightly softer than
dentin and consists of about 45% to 50% inorganic material (
hydroxylapatite) by weight and 50% to 55%
organic matter and water by weight. The organic portion is composed primarily of
collagen and
proteoglycans. Cementum is avascular, receiving its nutrition through its own imbedded cells from the surrounding vascular
periodontal ligament. The cementum is light yellow and slightly lighter in color than
dentin. It has the highest fluoride content of all mineralized tissue. Cementum also is permeable to a variety of materials. It is formed continuously throughout life because a new layer of cementum is deposited to keep the attachment intact as the superficial layer of cementum ages. Cementum on the root ends surrounds the apical foramen and may extend slightly onto the inner wall of the pulp canal.
Development Cementum is secreted by cells called
cementoblasts within the root of the tooth and is thickest at the
root apex (the end of the root where the nerves and blood vessels enter the tooth). These cementoblasts develop from undifferentiated
mesenchymal cells in the connective tissue of the
dental follicle or sac. Cementoblasts produces cementum in a rhythmic manner on intervals indicating periods of activity and periods of rest, producing so-called incremental lines of Salter. Incremental lines of Salter are the only incremental lines in the tooth that are hypercalcified, because there is a much smaller organic portion (collagen fibers) than inorganic portion (hydroxyapatite crystals) of cementum, so when the cementoblasts rest they leave a space for the inorganic portion. Conversely, in enamel ameloblasts (incremental lines of Retzius) and dentin odontoblasts (incremental lines of von Ebner), the inorganic portion is much greater than the organic portion, so when ameloblasts and odontoblasts rest, they leave a space for the organic portion and become hypocalcified. Cementum is capable of repairing itself to a limited degree, but cannot regenerate. It is not resorbed under normal conditions. ==Clinical significance==