Chapter 14. Dental tissues. III
Cementum: structure and composition
Physical properties 194
Chemical composition 194
Cementum is the mineralized connective tissue that covers the root of the tooth, helping to attach it, via the periodontal ligament, to alveolar bone. Although a thin layer, it is formed continuously throughout life, allowing for readjustment of attaching periodontal ligament fibres. Different systems of classification of cementum exist depending on the presence or absence of cells and/or the nature of origin of its collagen fibres. Unlike bone, it is devoid of blood vessels and nerves and has only a very limited capacity for remodelling.
• know the composition and main structural features of cementum
• be familiar with the various types of cementum and be able to compare and contrast it with bone, with which it has a number of similarities
• be aware of its clinical importance during orthodontic tooth movement and its significance in periodontal disease and regeneration.
Cementum is the thin layer of calcified tissue covering the dentine of the root. It is one of the four tissues that support the tooth in the jaw (the periodontium), the others being the alveolar bone, the periodontal ligament and the gingiva. It is thickest at the root apex (up to 200 μm, although it may exceed 600 μm) and thinnest cervically (10–15 μm). Its prime function is to give attachment to collagen fibres of the periodontal ligament. Cementum is formed slowly throughout life and this allows for continual reattachment of the periodontal ligament fibres. Developmentally, cementum is said to be derived from the investing layer of the dental follicle. Similar in chemical composition and physical properties to bone, cementum is, however, avascular and has no innervation. It is also less readily resorbed than bone, a feature that is important for permitting orthodontic tooth movement. The development of cementum is described in Chapter 10.
In any single section of a tooth, three arrangements of the junction between cementum and enamel may be seen:
• In the most common (60%), the cementum overlaps the enamel for a short distance.
• In 30% of cases, the cementum and enamel meet at a butt joint.
• In the remaining 10% of specimens, the cementum and enamel fail to meet and the dentine between them is exposed.
All three patterns may be present in a single tooth
Cementum is pale yellow with a dull surface. It is softer than dentine. Permeability varies with age and type of cementum, the cellular variety being more permeable than the acellular.
The chemical composition of cementum is somewhat similar to that of bone and comprises on a wet weight basis 65% inorganic material, 23% organic material and 12% water.
The principal inorganic component is hydroxyapatite, the crystals being thin and plate-like and similar to those in bone. Other trace elements can be found within the hydroxyapatite and such substitutions are particularly found towards the external surface of the cementum. Fluoride is the most common ionic substitution, and is found in higher levels in acellular cementum than in cellular cementum. Calcification of cementum is possibly initiated by the root dentine and continues on and around the collagen fibres found in cementum.
The organic matrix is collagen, with non-collagenous proteins being thought to be the same as those found in alveoar bone (see page 221). Collagen accounts for 90% of the organic matrix, the majority being type I collagen with some trace amounts of type III, mainly at the insertion points of the extrinsic Sharpey fibres. Trace amounts of other types of collagen have also been reported. The non-collagenous matrix accounts for 10% of the organic matrix. Important non-collagenous molecules found in cementum are the proteoglycans decorin and biglycan, bone sialoprotein (BSP), osteonectin, osteopontin, tenascin and fibronectin. It has been claimed that there may be present a cementum-specific glycoprotein called cementum attachment protein (CAP) that promotes the attachment of mesenchymal cells to the extracellular matrix.
The various types of cementum encountered may be classified according to the presence or absence of cells and/or the nature and origin of the organic matrix.
Acellular and cellular cementum
Acellular cementum is the first-formed cementum and usually covers the cervical two-thirds of the tooth. Acellular cementum is formed more slowly than cellular cementum. It is more highly mineralized than cellular cementum and usually lacks a layer of precementum (cementoid). Into acellular cementum the periodontal ligament fibres pass roughly perpendicularly to insert as Sharpey fibres.
Cellular cementum is found mainly in the apical and inter-radicular areas, overlying the acellular cementum. However, deviations from the normal arrangement are common and sometimes several layers of each variant alternate. Cellular cementum contains cells (cementocytes), representing entrapped cementoblasts. The spaces that the cementocytes occupy in cellular cementum are called lacunae, and the channels that their processes extend along are the canaliculi. Adjacent canaliculi are often connected, and the processes within them contact at gap junctions. Cementocytes are relatively inactive. Their cytoplasmic/nuclear ratio is low and they have sparse, if any, representation of the organelles responsible for energy production and for protein synthesis and secretion. In ground sections, the cellular contents are lost, air and debris filling the voids to give the dark appearance seen in transmitted light. Unlike osteocytes in bone, cementocytes:
• are not arranged circumferentially around blood vessels in the form of osteons
• are more widely dispersed
• are more randomly arranged
• have canaliculi that are preferentially oriented towards the periodontal ligament, their chief source of nutrition.
Cementum is deposited rhythmically, resulting in unevenly spaced incremental lines (of Salter). Some believe the lines represent an annual cycle:
• In acellular cementum, incremental lines tend to be close together, thin and even.
• In the more rapidly formed cellular cementum, the lines are further apart, thicker and more irregular.
Extrinsic, intrinsic and mixed fibre cementum
Cementum has also been classified according to the nature and origin of its fibrous matrix:
• When the collagen is derived from the periodontal ligament as Sharpey fibres, the cementum is referred to as extrinsic fibre cementum. These Sharpey fibres continue into the cementum in the same direction as the principal fibres of the ligament (i.e. perpendicular or oblique to the root surface).
• When collagen fibres are derived from cementoblasts, the cementum is referred to as intrinsic fibre cementum. The fibres run parallel to the root surface and approximately at right angles to the extrinsic fibres.
• When both extrinsic and intrinsic fibres are present, the tissue may be termed mixed fibre cementum.
Acellular extrinsic fibre cementum
Incorporating the source of origin of the collagen as well as the presence or absence of cells, two main varieties of cementum can be classified. Acellular extrinsic fibre cementum (AEFC) is located mainly over the cervical two-thirds of the root and constitutes nearly all of the cementum in some teeth (e.g. incisors and premolars). AEFC is the first-formed cementum and layers attain a thickness of approximately 15 μm. For this type of cementum all the collagen is derived as Sharpey fibres from the periodontal ligament (the ground substance itself may be produced by the cementoblasts). This type of cementum corresponds with primary acellular cementum. It is therefore formed slowly and the root surface is smooth. The fibres are generally well mineralized.
Cellular intrinsic fibre cementum
Cellular intrinsic fibre cementum (CIFC) contains cells and is composed only of intrinsic fibres running parallel to the root surface. The absence of Sharpey fibres means that intrinsic fibre cementum has no role in tooth attachment. It is found in the apical and inter-radicular region of molars. It may be a temporary phase, with extrinsic fibres subsequently gaining a reattachment, or may represent a permanent region without attaching fibres. It generally corresponds to secondary cellular cementum and, as it is formed more rapidly, has a cementoid seam on its outer surface.
Cellular mixed stratified cementum
Towards the root apex, and in the furcation areas of multirooted teeth, the acellular extrinsic fibre cementum and the cellular intrinsic fibre cementum commonly may be present in alternating layers known as cellular mixed stratified cementum.
Chemical differences exist between the different forms of cementum. For example, dentine sialoprotein, fibronectin and tenascin are not present in acellular extrinsic fibre cementum, but are found in cellular intrinsic fibre cementum.
A further type of cementum can be identified that contains no collagen fibres. This afibrillar cementum is sparsely distributed as a thin, acellular layer (difficult to identify at the light microscope level) that covers cervical enamel or intervenes between fibrillar cementum and dentine. It consists of a well-mineralized ground substance that may be of epithelial origin.
It is often reported that an ‘intermediate layer’ can sometimes be visualized between cementum and the hyaline layer of dentine. This is referred to as intermediate cementum; it is said to be characterized by wide, irregular branching spaces and is most commonly found in the apical region of cheek teeth. The spaces may interconnect with dentinal tubules. The nature, origin and function of this layer are controversial. Some differences in glycoprotein species can be found in this intermediate layer. In rats, the intermediate layer is rich in the bone-related glycoproteins bone sialoprotein and osteopontin. In many human teeth, the collagen fibres from the AEFC intermingle with the dentine matrix with no increase in glycoprotein levels.
The clinical significance of the interface between cementum and dentine relates to regeneration of the periodontium following periodontal surgery. Although a layer of cementum may regenerate, subsequent histological examination may show a ‘space’ between regenerated cementum and surface dentine, perhaps indicating an absence of a true union.
Although cementum is less susceptible to resorption than bone under the same pressures, most roots of permanent teeth still show small, localized areas of resorption. The resorption is carried out by multinucleated odontoclasts and may continue into the root dentine. Resorption deficiencies may be filled by deposition of mineralized tissue from a layer of cementoblast-like cells. A reversal line may be seen separating the repair tissue from the normal underlying dental tissues. Where repair is slow, the repair tissue may be acellular. However, where the repair tissue is formed rapidly it may be cellular.