Book - Oral Histology and Embryology (1944) 6
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Orban B. Oral Histology and Embryology (1944) The C.V. Mosby Company, St. Louis.
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Chapter VI - Cementum
Cementum is the hard dental tissue covering the anatomical roots of the human teeth. It was ﬁrst microscopically demonstrated in 1835 by two pupils of Purkinje.5 It begins at the cervical portion of the tooth at the cemento-enamel junction, and continues to the apex. Cementum furnishes a medium for the attachment of the ﬁbers that bind the tooth to the surrounding structures. It can be deﬁned as a specialized, calciﬁed tissue of mesodermal origin, a modiﬁed type of bone covering the anatomic root of the teeth.
2. Physical Characteristics
The hardness of adult or fully formed cementum is less than that of dentin.“ 2‘ It is light yellowish and is easily distinguished from the enamel by its darker hue; it is somewhat lighter in color than dentin. By means of vital staining and other chemical-physical experiments the cell containing cementum has been proved to be permeable.”
3. Chemical Composition
Adult cementum consists of about 45 to 50 per cent inorganic substances and 50 to 55 per cent organic material and water (see table in chapter on Enamel). The inorganic substances consist mainly of calcium salts. The molecular structure is hydroxyl apatite which, basically, is the same as that of enamel, dentin and bone. The chief constituent of the organic material is collagen.
The development of cementum is known as cementogenesis. During enamel formation the crown of the tooth is covered by the enamel epithelium. The basal part of the epithelium (inner and outer layers) is the
First draft submitted by Emmerich Kotanyi.
Hertwig‘s epithelial root sheath which is of particular importance in root development; it forms the mold into which the root dentin is deposited. Therefore, the newly formed dentin, in this region, is covered at ﬁrst by the epithelium, and is separated by it from the surrounding connective tissue (Fig. 117). Cementum is formed by this connective tissue but it
Fig. 117. Hertwig’s epithelial root sheath at end of forming root. At the side of the root the sheath is broken up and cementum formation begins. (Gottliel).“)
cannot be deposited on the outer surface of the root dentin as long as the epithelial sheath separates it from the dentin. A contact between connective tissue and tooth is accomplished by invasion of connective tissue through the epithelial layers. By this process the epithelial sheath loses its continuity but persists as a network of epithelial strands which lie fairly close to the root surface. The remnants of the epithelial sheath are known as “epithelial rests” of Malassez.“ (See chapter on Periodontal Membrane.) When the separation of the epithelium from the surface of the root dentin has been accomplished, the periodontal connective tissue comes into contact with the root surface and cementum is laid down.
Fig. 118. EpitheliaI sheath is broken and separated from root surface by connective US$116.
In the ﬁrst stage of cementum formation two tissue elements can be observed; First, cells of the connective tissue (undiﬂferentiated mesonchymal cells) are arranged along the outer surface of the dentin (Fig. 118). These change into ﬂat," cuboidal cells and are the cementoblasts. At the same time the second tissue element, pre-collagenous (argyrophil) ﬁbers,
can be seen at right angles to the root surface, and attached to the outer surface of the dentin (Fig. 119). These ﬁbers soon assume a collagenous
Fig. 119. Argy1-ophil ﬁbers of the periodontal membrane. attached to the dentin (silver impregnation).
Fir. 120.—-Cementum ground substance develops from ﬁbers of the periodontal membrane - (silver impregnation).
character and become a. part of the ground substance of the cementum (Fig. 120). This mechanism is similar to that observed in dentinogonesis (see chapter on Dentin).
Deﬁnite knowledge of the function of the cementoblasts is incomplete, but it is presumed that they play the same role in cementum formation
Fig. 121. Cementoid tissue on the surfaceﬂgf calciﬁed cementum. Cementoblasts between ers.
as the osteoblasts play in bone formation. In the ﬁrst phase of development the cementoblasts, apparently by enzymatic action, elaborate a homogenous material, the cementoid tissue. In the second phase, calciﬁcation takes place by the deposit of calcium salts in the cementing substance of the intercellular substance. Simultaneously the organic component changes radically, becoming soluble by proteolytic enzymes.“ GEMENTUM 159
During the continuous apposition of cementum a thin layer of non- °¢m°“"°i° calciﬁed matrix, termed cementoid tissue, which is analogous to osteoid tissue and predentin, is seen on the surface of the cementum (Fig. 121). This cementoid tissue is lined by cementoblasts. Connective tissue ﬁbers from the periodontal membrane pass between the cementoblasts into the cementum. These ﬁbers are embedded in the cementum and serve as an attachment for the tooth to the surrounding bone. Their embedded portions are known as Sharpey’s ﬁbers. These were accurately described in 18872 as an essential part of the suspensory apparatus.
Fig. 122. Increment2.l lines in the accellular cementum.
From a morphologic standpoint two kinds of cementum can be differentiated: (a) acellular, and (b) cellular cementum. Functionally, however there is no difference between the two.
Acellular cementum may cover the root dentin from the cemento-enamel junction to the apex, but is often missing on the apical third of the root. Here the cementum may be entirely of the cellular type. The aeellular cementum is thinnest at the cemento-enamel junction (20 to 50 microns), and thickest toward the apex (150 to 200 microns). The apical foramen is surrounded by cementum. Sometimes the cementum extends to the inner wall of the dentin for a short distance, forming a lining of the root canal.
Both acellular and cellular cementum are separated by incremental lines‘ into layers that indicate periodic formation (Fig. 122). Acellular cementum consists of the calciﬁed matrix and the embedded Sharpey’s ﬁbers. The matrix is composed of two elements: the collagenous ﬁbrils and the calciﬁed cementing substance. The ﬁbrils in the matrix are perpendicular to the embedded Sharpey’s ﬁbers and parallel to the cementum surface. The ﬁbrils are less numerous than in lamellated bone and about as numerous as those of bundle bone. Due to identical optical qualities, i.e., the same refractive index, the ﬁbrils and interﬁbrillar cementing substance can be made visible only by special staining methods. Fibrils and Sharpey’s ﬁbers are easily distinguished by means of silver impregnation. In dried ground sections Sha.rpey’s ﬁbers are disintegrated and the spaces and channels which they formerly occupied are ﬁlled with air; the areas thereby become discernible as dark lines.
Fig. 123. The principal ﬁbers ot the periodontal membrane continue into the surface layer of the cementum.
- The term incremental lines was introduced by Salter in 1874 as including the stripes of Remus In the enamel, the contour lines at Owen in the dentin and the stratification In the cementum.
Fig. 124. Cellular cementum on the surface or acellular cementum, and again cov ercd by acellular cementum (incremental lines). The _la.cunae _of the cellular cementum are empty, indicating that this part of the cementum is necrotic.
While the cementum remains relatively thin, Sharpey’s ﬁbers can be observed crossing the entire thickness of the cementum. With further apposition of cementum a larger part of the ﬁbers is incorporated in the cementum. At the same time, the portion of the ﬁbers lying in the deeper layers of the cementum becomes obscure. The attachment proper is probably conﬁned to the most superﬁcial or recently formed layers of cementum (Fig. 123). This would seem to indicate that the thickness of the cementum does not enhance functional efficiency by increasing the strength of attachment of the individual ﬁbers. Continuous apposition of cementum is essential for the continuous eruptive movements of the functioning tooth and the continuous reorganization of the periodontal membrane which is necessitated by these movements. The surface of the cementum is vital Whereas the inner layers become necrotic as recognized by empty lacuni in the cellular cementum (Fig. 124) .
Fig. 125. Cellular cementum forming the entire thickness of cementum. (Orban.W)
Fig. 136 Cementum tyiickest at apex contributing to the length or the root, cnmrmrun
The location of acellular and cellular cementum is not deﬁnite. Layers of acellular and cellular cementum may alternate in almost any arrangement. The acellular cementum, which is normally laid down on the surface of the dentin, may occasionally be found on the surface of cellular cementum (Fig. 124). Cellular cementum is usually formed on the surface of acellular cementum (Fig. 124), but it may comprise the entire thickness of the apical cementum (Fig. 125). It is always thickest around the apex and, in this manner, contributes to the lengthening of the root (Fig. 126).
Fig. 127. Cementum lacuna and canaliculi ﬁlled with air (ground section).
The cells in cellular cementum (cementocytes) are similar to osteocytes. They lie in spaces designated as lacunae. Frequently, the cell body has the shape of a plum stone, with numerous long processes radiating from the cell body. These processes may branch, and frequently anastomose with those of a neighboring cell. Most of the processes are directed toward the periodontal suﬂface of the cementum, differing in this respect from the evenly distributed processes of the bone cells.
Some canaliculi, containing processes of the cementocytes, have been said to anastomose with peripheral branches of the dentinal tubuliﬁ» 9" The cells are irregularly distributed throughout the thickness of the cellular cementum. The cavities can be best observed, however, in ground sections of dried teeth, where they appear as dark spider-like ﬁgures (Fig. 127). The dark appearance is due to the fact that the spaces are ﬁlled with air; these spaces also can easily be ﬁlled with dyes.
Fig. 128. Cemento—enamel junction. A. cementum and enamel meet in a. sharp line. B. cementum overlaps enamel.
6. Cemento-Enamel Junction
The relation between cementum and enamel at the cervical region of the teeth is variable.“ In about 30 per cent of the examined teeth the cementum meets the cervical end of the enamel in a sharp line (Fig. 128, A). Here, the cementum, as Well as the enamel, tapers into a knife-edge. In other teeth, about 60 per cent, the cementum overlaps the cervical end of the enamel for a short distance (Fig. 128, B). Developmentally, this may occur only when the enamel epithelium, which normally covers the entire enamel, degenerates in its cervical end, permitting the connective tissue, which is responsible for the deposition of cementum, to come in contact With the enamel surface.
Fig. 129. Variations at the cemento-enamel junction. 4. Enamel epithelium attached to dentin surface preventing cementum formation. 1 BhEnamel epithelium breaking continuity of cementum near the cemento-enamel unc on.
In about 10 per cent of all teeth various aberrations of the cementeenamel junction may be observed. Occasionally, the enamel epithelium which covers the cervical part of the root does not separate from the dentin surface at the proper time.‘ In other words, it remains attached to the dentin of the root for variable distances (Fig. 129, A), and prevents the formation of cementum. In such cases there is no cemento-enamel junction, but a zone of root dentin is devoid of cementum and covered by enamel epithelium. In other instances cementum is formed at the cemento-enamel junction for a short distance only and, following it apically, Her-twig’s epithelial root sheath remains in contact with the dentin in a limited area (Fig. 129, B). Enamel spurs, pearls or drops 1nay be formed by such epithelium.“
7. Cemento—Dentinal Junction
The surface of the dentin upon which the cementum is deposited is normally smooth in permanent teeth. The eemento-dentinal junction,
Fig. 130. Intermediate layer of cementum.
in deciduous teeth, however, is sometimes scalloped. The attachment of the cementum to the dentin, in either case, is quite ﬁrm although the nature of this attachment has not been fully investigated.
Sometimes the dentin is separated from the cementum by an intermediate layer, known as the intermediate cementum layer, which does not exhibit the characteristic features of either dentin or cementum (Fig. 130). This layer contains large and irregular cells which can be regarded as embedded connective tissue cells. The development of this layer may be due to localized, premature disintegration of 1'-lertwig ’s epithelial sheath after its cells have induced the diﬂerentiation of odontoblasts, but before the production of dentinal intercellular substance. It is found mostly in the apical two-thirds of the root. Sometimes it is a continuous layer; sometimes it is found only in isolated areas.“
The functions of cementum are, ﬁrst, to anchor the tooth to the bony socket by attachment of ﬁbers; second, to compensate by its growth for loss of tooth substance due to occlusal wear; third, to enable, by its continuous growth, the continuous vertical eruption and mesial drift of the teeth; and fourth, to make possible the continuous rearrangement of the principal ﬁbers of the periodontal membrane.
The attachment of the ﬁbers of the periodontal connective tissue to the surface of the tooth is the medium by which functional connection between tooth and surrounding tissues is established. Due to physiological movements of the functioning tooth, ﬁbers have to be replaced continually. In order to maintain a functional relationship new cementum has to be deposited continuously on the surface of the old cementum.‘ By this continued formation of cementum new ﬁbers of the periodontal membrane are attached to the surface of the root, and loosened or degenerated Sharpey’s ﬁbers are thus continuously replaced. By this mechanism an adequate attachment of the tooth to the supporting tissues is maintained. The morphologic evidence of the continuous formation of cementum is shown by the presence of cementoblasts and a layer of cementoid tissue on the surface of the cementum. Cementoid tissue may be found on acellular (Figs. 121, 122, 124), as well as on cellular (Fig. 125) cementum.
The continuous deposition of cementum is of great biologic importance. In contrast to the ever alternating resorption and new formation of bone, cementum is not resorbed under normal conditions. If a layer ages or, functionally speaking, loses its vitality, the periodontal connective tissue and cementoblasts must produce a new layer of cementum on the surface to keep the attachment apparatus intact. In bone the loss of vitality can be recognized by the fact that the bone cells degenerate and the bone lacunae are empty. Lowered vitality in acellular cementum cannot be so readily ascertained but, in cellular cementum, the cells in the deepest layers may degenerate and the lacunae may be empty (Fig. 124). This indicates necrosis of the cells. On the surface, the lacunae contain normal cementocytes. The nuclei of degenerating cells in the deeper layers are pyknotic and the cells are shrunken: near the surface the cells ﬁll the entire space of the cementum lacunae (Fig. 125) and the nuclei stain dark.
Hypercementosis designates an abnormal thickening of the cementum. It may be diﬁuse or circumscribed, i.e., it may affect all teeth of the dentition, or it may be conﬁned to a single tooth. It may even affect only certain parts of one tooth. If the overgrowth improves the functional qualities of the cementum, it is termed a cementum hypertrophy; if overgrowth occurs in nonfunctional teeth or if it is not correlated with increased function, it is termed hyperplasia.
Fig. 131. Pronglike excementoses.
In localized hypertrophy a spur or pronglike extension of cementum may be observed (Fig. 131). This condition is frequently found in teeth which are exposed to great stress. The pronglike extensions of cementum provide a larger surface area for the attaching ﬁbers, thus securing a ﬁrmer anchorage of the tooth to the surrounding alveolar bone.
Localized hyperplasia of cementum may sometimes be observed in areas where enamel drops have developed on the dentin. The hyperplastic cementum, covering the enamel drops (Fig. 132), is occasionally irregular and sometimes contains round bodies which may be calciﬁed epithelial rests. The same type of embedded calciﬁed round bodies are frequently found in localized areas of hyperplastic cementum (Fig. 133). Such knoblike projections are designated as excementosis. They, too, develop around disintegrated, degenerated epithelial rests.
Fig. 132. Irregular hyperplasia or cementum on the surface or an enamel drop.
Extensive hyperplasia of the cementum of a tooth is found, occasionally, in connection with chronic periapical inﬂammation. Here the hyperplasia is circumscribed and surrounds the root like a cuff.
A thickening of the cementum is often observed on teeth which are not in function. The hyperplasia may extend around the entire root of the nonfunctioning teeth or may be localized in small areas (see chapter on Periodontal Membrane). Hyperplasia of cementum in nonfunctioning teeth is characterized by the absence of Sharpey’s ﬁbers.
Fig. 133. Excementoses in bifurcation of a. molar. (Gottliebﬂ)
Fig. 134.. Extensive spikelike hyperplasia of cementum
The cementum is thicker around the apex of all teeth and in the bifurcation of multirooted teeth than on other areas of the root. This thickening can be observed in embedded, as well as newly erupted, teeth.”
In some cases an irregular overgrowth of cementum can be found with spikelike extensions and calciﬁcation of Sharpey’s ﬁbers, accompanied by numerous cementicles. This type of cementum hyperplasia can, occasionally, be observed on many teeth of the same dentition and is, at least in some cases, the sequelae of injuries to the cementum (Fig. 134).
10. Clinical Considerations
The fact that cementum appears to be more resistant to resorption than bone renders orthodontic treatment possible. When a tooth is moved by means of an orthodontic appliance, bone is resorbed on the side of pressure new bone is formed on the side of tension. On the side toward which the tooth is moved pressure is equal on the surfaces of bone and cementum. Resorption of bone, as well as of cementum, may be anticipated. However, in careful orthodontic treatment cementum resorption, if it occurs, is usually localized and shallow. Moreover, it is readily repaired if the intensity of pressure is reduced and the surrounding connective tissue remains intact. If resorption is extensive it may indicate a. systemic disorder, possibly of the endocrine system.
Excessive lateral stress may compress the periodontal connective tissue between bone and cementum and cause bleeding, thrombosis and necrosis. After resorption of the damaged tissues, accompanied by bone resorption, repair may take place.
It has been the aim of some investigators to determine why resorption takes place in some cases and not in others when external conditions seem to be identical. The reasons are as yet unknown. The potentiality to form new cementum does not seem to be equal in all individuals; in some, cementum forms readily; in others it does not. The latter are those cases which react unfavorably to trauma or any kind of irritation. These are the cases that develop periodontal diseases easily. The dilference in cementum formation may be explained by constitutional factors. Cementum resorption without obvious cause is called idiopathic.
Severe resorption of cementum may be followed by resorption of the dentin. After resorption has ceased the damage is usually repaired, either by formation of acellular (Fig. 135, A) or cellular (Fig. 135, B) cementum, or by alternate formation of both (Fig. 135, 0). In most cases of repair there is a tendency to re-establish the former outline of the root surface. However, if only a thin layer of cementum is deposited on the surface of a deep resorption, the root outline is not reconstructed and a baylike recess remains. In such areas sometimes the periodontal space is restored to its normal width by formation of new bone, so that a proper functional relationship will result. The outline of the alveolar bone, in these cases, follows that of the root surface (Fig. 136). In contrast to anatomical repair, this change is called functional repair.“
Fig. 135. Repair of resorbed cementum. 4. Repair by acellular cementum (a:). B. Repair by cellular cementum (2). C. Repair ﬂrst by cellular (ac) and later by acellular (wz) cementum. D = Dentin; I1’. = line or resorption; P = periodontal membrane.
If teeth are subjected to acute trauma, such as a blow, smaller or larger fragments of cementum may be severed from the dentin. The tear occurs frequently at the cemento-dentinal junction, but may also be in the cementum or dentin. Transverse fractures of the root may heal by formation of new cementum uniting the fragments.
Frequently, hyperplasia of cementum is secondary to periapical inﬂammation or extensive occlusal stress. The fact is’ of practical significance in so far as the extraction of such teeth necessitates the removal of bone. This also applies to extensive excementoses, as shown in Fig. 133. These can anchor the tooth so tightly to the socket that the jaw or parts of it may be fractured in an attempt to extract the tooth. These facts indicate the necessity of taking roentgenograms before an extraction. Small fragments of roots, left in the jaw after extraction of vital teeth, may be surrounded by cementum, and remain in the jaw without causing any disturbance.
Fig 136. Functional repair of cementum resorption by bone apposition. Normal width of periodontal membrane re-established.
If the cementum does not cover the cervical part of the root, recession of the gingiva will expose the highly sensitive dentin in the cervical area. When calculus is removed it is frequently impossible to avoid the removal of the thin cementum covering the cervical region of the exposed root. As the individual gets older, more cementum is gradually exposed and subject to the abrasive action of some dentifrices. Since the cementuin is the softest of the hard dental tissues, a considerable amount of cementum may be removed by these mechanical means.” The denuded dentin is then highly sensitive to thermal, chemical or mechani cal stimuli. The hypersensitivity can often be relieved with astringent chemicals which coagulate the protoplasmic odontoblastic processes.
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