Oral Histology
د0سحرغانم Lec.5 CementumThe periodontium is defined as those tissues supporting and investing the tooth and consist of cementum , periodontal ligament, bone the lining alveolus (socket) and that part the of gingival facing the tooth.
CEMENTUM
Cementum is the calcified, avascular mesenchymal tissue that forms the outer layer covering the anatomic root.
Physical properties
1-less hardness than that of dentin & enamel.2-Is light yellow in colour , & somewhat lighter in colour than dentin . And can be distinguished from enamel by its lack of luster & its darker colour.
3-permeable to a variety of materials.
Chemical composition
1-in the fully formed permanent teeth the cementum contains about 45% - 50% inorganic substance & 50% - 55% organic materials & water.
2-the inorganic portion consists mainly of Ca & phosphate in the form of hydroxyapatite.
3-also contain highest fluoride of all the mineralized tissue
4-the organic portion of cementum consists of collagen & protein polysaccharides.
Structures of cementum
Under the light microscope, two kinds of cementum can be differentiated
1. Acellular 2. Cellular
1.Acellular cementum:
that part of the cementum covering 1/3 – 1/2 of the root of a tooth adjacent to the CEJ. It consists of collagen fibers & ground substance.
2.Cellular cementum: that part of the cementum covering the apical 1/2 - 2/3 of the root of a tooth. This cementum is most abundant on the root tip.
cementum is thinnest at the CEJ.(20-50 micron) & thickest toward the apex (150-200 micron).
In some areas, discrete bundles of collagen fibrils can be seen. These bundles known as Sharpey’s fibers, which make up a substantial portion of the cementum.
the cells incorporated into cellular cementum called cementocytes, are similar to osteocytes. They lie in spaces known as lacunae.
A typical cementocyte has numerous cell processes, or canaliculi, radiating from cementocyte cell body. These processes may branch, & they frequently anastomose with those of a neighbouring cell, & most of the processes are directed toward the periodontal surface of the cementum.
Lacunae in the deeper layers of cementum appear to be empty, due to degeneration of cementocytes, because the cementocyte become far away from the origin of nutrition.
Both of acellular & cellular cementum are separated by incremental lines into layers, which indicate periodic formation.
Acellular cementum may occasionally be found on the surface of cellular cementum, but cellular is frequently formed on the surface of acellular cementum.
Cementogenesis
cementum formation is preceded by the deposition of dentin along the inner aspect of Hertwig’s epith. Root sheath.
once dentin formation is under way, breaks occur in epith. Root sheath ,allowing newly formed dentin to come in direct contact with C.T cells of dental follicle & differentiated it to cementoblast.
The cementoblast synthesize collagen & protein polysaccharides, which make up the organic matrix of cementum & known as cementoid (uncalcified cementum matrix).
After some cementum matrix has been laid down, it’s mineralization begins by Ca & phosphate ions which present in tissue fluids & deposited into the matrix & are arranged as unit of hydroxyapatite.
the growth of cementum is a rhythmic process, & as a new layer of cementoid is formed, the old one calcified.
However, a thin layer of cementoid usually observed on the cementum surface, & this cementoid tissue is lined by cementoblasts.
N.B: when the Hertwig’s epith. Root sheath degenerate some sheath cells migrate away from the dentin toward the dental sac & become the epith. Rests of Malassez which remain in PdL.
Cemento-enamel junction
Is the point of the junction between the cementum & enamel at the cervical region of teeth.
The relation between cementum & enamel is variable.
Relationship percentage of Cemento-enamel junction(OMG RULE
1- cementum overlaps enamel for a short distance, 60% this occurs when reduced enamel epith degenerates at it’s cervical termination, permitting connective tissue tocome in direct contact with the enamel surfaces & form a type of cementum termed as Afibrillar cementum.
2- cementum meets enamel in a sharp line 30%
3- small gap exists between cementum & enamel, this 10% occurs when enamel epith in the cervical portion of the root is delayed in it’s separation from dentin.
In such cases there is no junction between cementum & enamel.
Cemento-dentinal junction:
The CDJ in deciduous teeth is somewhat scalloped, while is relatively smooth in permanent teeth.The attachment in either cases is quite firm, although the nature of this attachment is not understood.
The collagen fibrils of cementum & dentin intertwine at their interface in a very complex fashion, & it is not possible to determine which fibrils are of dentinal & which are of cementum origin.
Sometimes dentin is separated from cementum by a zone known as the intermediate cementum layer, which not show characteristic features of either dentin or cementum.
This layer is predominantly seen in the apical 2/3 of roots of molars & premolars. & it is believed that this layer represents areas in which cells of Hertwig’s epith sheath become trapped in a rapidly deposited dentin or cementum matrix.
Cementicles:
Is a calcified ovoid or round nodule found in the periodontal ligment. It may be found single or in groups near the surface of the cementum .Cementicles may be free in the ligament, attached, or embedded in the cementum, & mostly found at a site of trauma.The cementicle develop due to a nidus of epith cells that are composed a calcium phosphate & collagen in the same amount as cementum.
Hypercementosis:
Is an abnormal thickening of cementum, either diffuse or circumscribed.
It may affect all teeth or confined to a single tooth or even affect only parts of one tooth.
It present in 2 types of Hypercementosis:
1- cementum hypertrophy: appear as overgrowth as a spur like extension, & frequently is found in teeth that exposed to great stress, to provide a larger surface area for the attaching fibers (associated with functional teeth)
2- cementum hyperplasia:
is overgrowth occurs in non functional teeth or if it is not correlated with increased function & occasionally associated with chronic periapical inflammation. Mostly is circumscribed & surrounds the root like a cuff.
However, in some cases an irregular overgrowth of cementum occurs with spike like extensions & calcification of Sharpey's fibers & accompanied by numerous cementicles. This type of cemental hyperplasia observed on many teeth of the same dentition as a sequel of injuries to the cementum.
Function of cementum:
1-It seals the surface of the root dentin & cover the ends of the open dental tubules.
2-Serves as an attachment for PdL fibers, these fibers function as an attachment for the Pd ligament fibers to the tooth root & aid in maintaining the tooth in it’s socket.
3- it serves as the major reparative tissue for root surfaces. Damage to roots such as fractures & resorptions can be repaired by the deposition of new cementum.
4- cementum also is the tissue that makes functional adaptation of teeth possible, for e.g., deposition of cementum in an apical area can compensate for loss of tooth substance from occlusal wear.
Clinical considerations:
1- Cementum resist resorption in younger tissue, this is the reason that orthodontic tooth movement results in alveolar bone resorption rather than tooth root loss. The difference in the resistance of bone & cementum to pressure is due to that the bone is richly vascularized, whereas cementum is avascular.
2- Cellular cementum appear similar to bone in structure but does not contain any nerves. Therefore cementum is non sensitive, & scaling when necessary, does not produce pain. However, if cementum is removed, exposure of underlying dentin results in sensitivity.
3- Cementum resorption can occur after trauma or excessive occlusal forces. After resorption has ceased, the damage usually is repaired, either by formation of acellular or cellular cementum or by both of them.
In most cases of repair there is a tendency to reestablish the original outline of the root surface, this is called anatomic repair.
Sometime, the root outline is not reconstructed, in such areas the Periodontal space is restored by formation of a bony projection, & the outline of the alveolar bone in these cases follows that of the root surface, this change is called functional repair.