cementum structure (2) l Oral histology & biology MCQs for dental students

Cementum structure (2)  l Oral histology & biology MCQs for dental students

 

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Cementum

Definition:

• It is the mineralized connective tissue that covers the roots of teeth and serves to attach the periodontal fibers. 

A. Prefunctional stage of Cementogenesis :

• This occurs throughout root formation. This stage consists of two phases (Matrix formation phase and Mineralization phase :

a) Matrix formation:

1. Extrinsic fibers (Sharpey's fibers) if derived from fibroblasts of the periodontal ligament. These are in the same direction of the periodontal ligament principal fibers i.e. perpendicular or oblique to the root surface.
2. Intrinsic fibers if derived from cementoblasts. These fibers run parallel to the root surface and at right angles to the extrinsic fibers (Sharpy's fibers). It is located predominantly at sites undergoing repair, following surface resorption. It plays no role in tooth anchorage.

                         

• It begins after the crown is formed.

• The outer and inner dental epithelium forms the epithelial root sheath of Hertwig.

• The inner dental epithelium will induce the neighboring cells of the dental papilla to differentiate into odontoblasts.

• The predentin and dentin formation occur like that of the crown.

• After dentin formation, the epithelial root sheath of Hertwig will lose its continuity and its cells became the epithelial rests of Malassez.

• Just before the degeneration of the epithelial root sheath of Hertwig's, the inner dental epithelium layer secretes a layer of enamel like protein on the root dentin which was called intermediate cementum. The role of this layer is to seals of the dentinal tubules and is recently found to be named as Hyaline layer of Hopewell-Smith and is considered as an inductive influence for beginning of cementum formation.

• When the cells of the dental sac touch the root dentin it differentiates into cementoblasts and lay down cementum.

• Organic matrix derived from two sources, fibroblast cell, cementoblast cells or both. So, there are three types of cementum collagen fibers:

1. Extrinsic fibers (Sharpey's fibers) if derived from fibroblasts of the periodontal ligament. These are in the same direction of the periodontal ligament principal fibers i.e. perpendicular or oblique to the root surface.
2. Intrinsic fibers if derived from cementoblasts. These fibers run parallel to the root surface and at right angles to the extrinsic fibers (Sharpy's fibers). It is located predominantly at sites undergoing repair, following surface resorption. It plays no role in tooth anchorage.
3. Mixed fiber found at the area where both types of cells produce fibers. It is found in furcation area, apices of roots where the rate of cementum formation is usually rapid.

• Some cementoblasts deposit cementum matrix at various locations around themselves, resulting in their entrapment into the cement matrix converting them into cementocytes.                         

b) Mineralization:

• It begins in the depth of the cementum matrix (cementoid tissue), where, fine hydroxyapatite crystals are deposited between and within the collagen fibrils.

B. Functional stage Cementogenesis:

• This started when the tooth reaches the occlusion and continues throughout life. It is an adaptive role in response to tooth wear and movement and is associated with repair and regeneration of periodontal tissues.

Classification of cementum:

According to the development:

1. Primary cementum: It is that part of cementum that formed during the root formation.
2. Secondary cementum: It is that part of cementum that formed after completion of the root.

According to its cellularity:

1. Acellular cementum: (A) It is clear, structureless and covers the cervical two thirds of the root. It is formed of the calcified ground substance plus the embedded portion of Sharpey's fibers.

Time of formation

• First formed cementum

Formation

• Most of it is formed before the tooth reaches occlusal plane (Prefunctional)

Cells

• Not present

Location

• Coronal ½ - ⅔

Rate of formation

• Slow

Incremental lines

• More and closely arranged

Calcification

• More calcified

Irregularity of fibers

• Regular

 

2. Cellular cementum: (B) It has the same structure of acellular cementum but contain embedded cells (cementocytes).

Time of formation

• Formed after Acellular cementum

Formation  

• Most of it is formed after the tooth reaches occlusal plane Functional

Cells  

• Present

Location

• Apical ⅓ - ½

Rate of formation

• Rapid

Incremental lines

• Less and more apart from each other

Calcification

• Less calcified

Irregularity of fibers

• Irregular

 

 According to the origin of the collagenous matrix:

1. Extrinsic fiber cementum:  Its fibers called Sharpey's fibers and produced by the fibroblast of the periodontal ligament.
2. Intrinsic fiber cementum: Its fibers produced by the developed cementoblasts.
3. Mixed fiber cementum: Its fibers produced from both types of cells, fibroblast and cementoblast.

 

Physical properties:

1. Thickness: the thickness of cementum at the cervical area is about 20-50 μ and gradually increases in thickness till it reaches its maximum about 150-200 μ at the apex and at the bifurcation of the root.
2. Color: pale yellow in color. It can be distinguished from enamel by its lack of luster and darker hue.
3. Permeability: it is permeable from both the dentin and the periodontal ligament side.

 

Chemical properties:

1. Inorganic substances (45-50%): Consist mainly of calcium phosphate in the form of hydroxy apatite crystals.
2. Organic matrix (55-50%): Consist primarily of collagen type I (90%).

 

Cementum structure:

Cementocytes:

• These cells are like the osteocytes, has a granular cytoplasm and faintly stained nucleus.
• The cells have cytoplasmic processes which can anastomose with neighboring cells and are mostly directed toward the periodontal ligament to provide nutrition.
• The cell body lies in lacunae and the cell processes are present in canaliculi.
• Cementocytes in the deepest layer of cementum show signs of degeneration.

Incremental lines of Salter:

• Both cellular and acellular cementum is separated by incremental lines into layers which indicate periodic formation.
• Histo-chemical studies indicate that incremental lines are highly mineralized areas with less collagen and more ground substance than other portions of cementum.
• The incremental lines are roughly parallel to the long axis of the root.
• The cellular cementum is formed at a faster rate than the acellular cementum so the incremental lines are therefore located further apart than in acellular cementum.

 

Cemento-enamel junction:

• It is the relation between the enamel and cementum edges at the cervix of the tooth. This relation is found to be either one of three forms.

Edge to edge

1. Incidence : 30%
2. Cause : Proper root development
3. Description : The cementum edge of the tooth meets the enamel edge in a sharp line.

Denuded area of dentin (bare dentin)

1. Incidence : 10%
2. Cause : Epithelial root sheath of Hertwig delayed in its separation from the dentin, a zone of the root dentin became devoid of cementum.
3. Description : A bare dentin area found to be so sensitive at the cervix of the tooth especially if there is a gingival recession.

Cement overlap enamel

1. Incidence : 60%
2. Cause : Removal of the cervical part of the reduced dental epithelium permits dental sac cells to contact with the enamel surface and differentiate into cementoblasts.
3. Description : It is called afibrillar cementum because its matrix doesn't contain collagen fibrils.

 

Function of cementum:

1. Cementum is the medium for the attachment of the fibers of the periodontal ligament to the alveolar bone.
2. Compensation of the lost tooth structure with wearing or fracture by deposition of new cementum at the apical part of the root.
3. Repair any damaged area of the root resulted from resorption.

 

Age changes:

1. Smooth surface becomes irregular.
2. Continues deposition of cementum occurs with age in the apical area. This could be a two edges weapon as it maintains tooth length, while obstructs the apical foramen.
3. Resorption of root dentin occurs with aging which is covered by cemental repair.
4. The permeability of cementum decreases gradually.
5. Cementicles: calcified nodules found in the periodontal ligament, single or multiple near the cemental surface. It could be free, attached or embedded in cementum. It appears by aging or at site of trauma.

                        Hypercementosis:

• It is an abnormal thickening of cementum.
• It may affect one tooth or may be generalized in all teeth.  
• It is either limited to a small area of the root or through the whole root length.
• It has two types:

a. Cementum hypertrophy:

• It is physiologic thickening of the cementum found in the good functioning teeth as a response to increase the function of the tooth to increase the root surface area and thus permitting more periodontal fibers (Sharpey's fibers) to be attached to the tooth.

b. Cementum hyperplasia:

• It is overgrowth of the cementum in a non-functioning or embedded tooth. It is characterized by the absence of Sharpey's fibers.  

 

Clinical considerations:

1. Cellular cementum is like bone but has no nerves, therefore it is non-sensitive to pain so in scaling procedures it gives no pain, but if cementum is removed, dentin is exposed and causes sensitivity on scaling.
2. Cementum is resistant to resorption especially in younger patients. Thus, orthodontic tooth movement causes alveolar bone resorption and not tooth root loss.