The cementum is a specialized calcified avascular mesenchymal tissue that forms the outer covering of the anatomic root. The two main types of cementum are acellular (primary) and cellular (secondary) cementum.
Both consist of a calcifiedInterfibrillar matrix and collagen fibrils.Mineralized tissue covering the root surfaces and, occasionally, small portions of the crown of the teeth. Contains no blood or lymph vessels, has no innervation, does not undergo physiologic resorption or remodeling, but is characterized by continuing deposition throughout life.
It contains collagen fibers embedded in an organic matrix. Its mineral content, which is mainly hydroxyapatite, is about 65% by weight; a little more than that of bone (i.e. 60%).
Functions of cementum:
It attaches the periodontal ligament fibers to the root.
It contributes to the process of repair after damage to the root surface.
Types of cementum:
Acellular afibrillar cementum (AAC) contains neither cells nor extrinsic or intrinsic collagen fibers, apart from a mineralized ground substance. It is a product of cementoblasts and is found as coronal cementum in humans, with a thickness of 1 to 15 Am.
Acellular extrinsic fiber cementum (AEFC) is composed almost entirely of densely packed bundles of Sharpey's fibers and lacks cells. It is a product of fibroblasts and cementoblasts and is found in the cervical third of roots in humans but may extend further apically. Its thickness is between 30 and 230 Am.
Cellular mixed stratified cementum (CMSC) is composed of extrinsic (Sharpey's) and intrinsic fibers and may contain cells. It is a co-product of fibroblasts and cementoblasts, and in humans it appears primarily in the apical third of the roots and apices and in furcation areas. Its thickness ranges from 100 to 1000 gym.
Cellular intrinsic fiber cementum (CIFC) contains cells but no extrinsic collagen fibers. It is formed by cementoblasts, and in humans it fills resorption lacunae.
Intermediate cementum is an ill-defined zone near the cemento-dentinal junction of certain teeth that appears to contain cellular remnants of Hertwig's sheath embedded in calcified ground substance
Permeability of Cementum:
In very young animals, cellular and acellular cementum is very permeable and permits the diffusion of dyes from the pulp and external root surface.The permeability of cementum diminishes with age.
Three types of relationships involving the cementum may exist at the cementoenamel junction.
In about 60% to 65% of cases, cementum overlaps the enamel;
In about 30% an edge-to-edge butt joint exists; and in 5% to 10% the cementum and enamel fail to meet.
In the last instance, gingival recession may result in accentuated sensitivity because the dentin is exposed.
Thickness of Cementum:
Cementum formation is most rapid in the apical regions, where it compensates for tooth eruption, which itself compensates for attrition.
The thickness of cementum on the coronal half of the root varies from 16 to 60 Am, or about the thickness of a hair. It attains its greatest thickness (up to 150 to 200 Am) in the apical third and in the furcation areas.
It is thicker in distal surfaces than in mesial surfaces, probably because of functional stimulation from mesial drift over time.
Hypercementosis (cementum hyperplasia):
It refers to a prominent thickening of the cementum. It may be localized to one tooth or affect the entire dentition.
Because of considerable physiologic variation in the thickness of cementum among different teeth in the same person and also among different persons, distinguishing between hypercementosis and physiologic thickening of cementum is sometimes difficult.
It occurs as a generalized thickening of the cementum, with nodular enlargement of the apical third of the root. It also appears in the form of spike like excrescences (cemental spikes) created by either the coalescence of cementicles that adhere to the root or the calcification of periodontal fibers at the sites of insertion into the cementum.
The etiology of hypercementosis varies:
The spikelike type of hypercementosis generally results from excessive tension from orthodontic appliances or occlusal forces.
The generalized type occurs in a variety of circumstances:
In teeth without antagonists, hypercementosis is interpreted as an effort to keep space with excessive tooth eruption.
In teeth subject to low-grade periapical irritation arising from pulp disease, it is considered compensation for the destroyed fibrous attachment to the tooth. The cementum is deposited adjacent to the inflamed periapical tissue.
Hypercementosis of the entire dentition may occur in patients with Paget's disease.
Cementum Resorption and Repair:
Causes of cementum resorption:
Trauma from occlusion.
Pressure from malaligned erupting teeth, cysts, and tumors;
Teeth without functional antagonists;
Replanted and transplanted teeth;
B- Systemic factors:
hereditary fibrous osteodystrophy,
Cementum resorption appears microscopically as bay like concavities in the root surface multinucleated giant cells and large mononuclear macrophages are generally found adjacent to cementum undergoing active resorption.
Several sites of resorption may coalesce to form a large area of destruction.
The resorption process may extend into the underlying dentin and even into the pulp, but it is usually painless.
reversal lineis the deeply staining line, irregular line that demarcate the newly formed cementum from the root which delineates the border of the previous resorption
Cementum repair requires the presence of viable connective tissue. If epithelium proliferates into an area of resorption, repair will not take place.
Cementum repair can occur in devitalized as well as in vital teeth.
Its fusion of the cementum and alveolar bone with obliteration of the periodontal ligament is termed ankylosis.
Causes of Ankylosis:
Ankylosis occurs in teeth with cemental resorption.
after chronic periapical inflammation,
After tooth replantation, and occlusal trauma and around embedded teeth.
Ankylosis results in resorption of the root and its gradual replacement by bone tissue. For this reason, replanted teeth that ankylose will lose their roots after 4 to 5 years and exfoliate.
When titanium implants are placed in the jaw, healing results in bone formed in direct opposition to the implant without any intervening connective tissue. This may be interpreted as a form of ankylosis. Because resorption of the metallic implant cannot occur, the implant remains indefinitely "ankylosed" to the bone.
A true periodontal pocket will not form because apical proliferation of the epithelium along the root, a key element of pocket formation, is not possible owing to the ankylosis.
Exposure of cementum to oral cavity:
Cementum becomes exposed to the oral environment in cases of gingival recession and as a consequence of loss of attachment in pocket formation.
The cementum is sufficiently permeable to be penetrated in these cases by organic substances, inorganic ions, and bacteria. Bacterial invasion of the cementum occurs commonly in periodontal disease caries of the cementum also can develop.
It is the portion of the maxilla and mandible that forms and supports the tooth sockets (alveoli).
It forms when the tooth erupts to provide the osseous attachment to the forming periodontal ligament; it disappears gradually after the tooth is lost.
The alveolar process consists of the following:
An external plate of cortical bone formed by haversian bone and compacted bone lamellae
The inner socket wall of thin, compact bone called the alveolar bone proper, which is seen as the lamina dura in radiographs. Histologically, it contains a series of openings (cribriform plate) through which neurovascular bundles link the periodontal ligament with the central component of the alveolar bone, the cancellous bone.
Cancellous trabeculae, between these two compact layers, which act as supporting alveolar bone. The interdental septum consists of cancellous supporting bone enclosed within a compact border.
basal bone: is the portion of the jaw located apically but unrelated to the teeth
Osteoblasts, the cells that produce the organic matrix of bone, are differentiated from pluripotent follicle cells.
Alveolar bone is formed during fetal growth and consists of a calcified matrix with osteocytes enclosed within spaces called lacunae.
The osteocytes extend processes into canaliculi that radiate from the lacunae which form an anastomosing system through the intercellular matrix of the bone, which brings oxygen and nutrients to the osteocytes through the blood and removes metabolic waste products
Haversian systems (osteons) are the internal mechanisms that bring a vascular supply to bones too thick to be supplied only by surface vessels; it's in the outercortical plates and the alveolar bone proper.
Bone consists of 2/3 inorganic matter and 1/3 organic matrix.
Inorganic include minerals as calcium and phosphate, along with hydroxyl, carbonate, citrate, and trace amounts of other ions such as sodium, magnesium,and fluorine.
Organic matrix consists mainly (90%) of collagen type 1 with small amounts of non collagenous proteins such as osteocalcin, osteonectin, bone morphogeneticprotein, phosphoproteins, and proteoglycans.
Bonedeposition by osteoblasts is balanced by resorption byosteoclasts during tissue remodeling and renewal.
Remodeling is the major pathway of bony changes in shape, resistance to forces, repair of wounds, and calcium and phosphate homeostasis in the body.
Bone resorption process:
- Ten Cate described the sequence of events in the resorption process as follows:
1. Attachment of osteoclasts to the mineralized surface of bone
2. Creation of a sealed acidic environment through action of the proton pump, which demineralizes bone and exposes the organic matrix
3. Degradation of the exposed organic matrix to its constituent amino acids by the action of released enzymes, such as acid phosphatase and cathepsine
4. Sequestering of mineral ions and amino acids within the osteoclast.
The socket wall consists of dense, lamellated bone, some of which is arranged in haversian systems, and bundle bone.
Bundle bone is the term given to bone adjacent tothe periodontal ligament that contains a great numberof Sharpey's fibers; it is characterized bythin lamellae arranged in layers parallel to the root, with intervening appositional lines.
Cancellous bone is found predominantly in the interradicularand interdental spaces and in limited amountsfacially or lingually, except in the palate.
In the embryo and newborn, the cavities of all bonesare occupied by red hematopoietic marrow. It found only in the ribs, sternum, vertebrae and skull.
The red marrow gradually undergoes a physiologic change to the fatty or yellow inactive type of marrow. In the adult, the marrow of the jaw is normally of the lattertype, and red marrow is
Periosteum and Endosteum
They are layers of differentiated osteogenic connective tissue that covers all bone surfaces
The tissue covering the outer surface of bone is termed periosteum, whereas the tissue lining the internal bone cavities is called endosteum.
The periosteum consists of an inner layer composed of osteoblasts surrounded by osteoprogenitor cell and an outer layer rich in blood vessels and nerves and composed of collagen fibers and fibroblasts.
Bundles of periosteal collagen fibers penetrate the bone, binding the periosteum to the bone.
The endosteum is composed of a single layer of osteoblasts and sometimes a small amount of connective tissue.
It consists of cancellous bone bordered by the socket wall cribriform plates (lamina dura or alveolar bone proper) of approximating teeth and thefacial and lingual cortical plates.
If roots are too close together, an irregular "window" can appear in the bone between adjacent roots.
The mesiodistal angulation of the crest of the interdentalseptum usually parallels a line drawn between the cemento-enamel junctions of the approximating teeth.
The mesiodistal and faciolingual dimensions and shape of the interdental septum are governed by the size and convexity of the crowns of the two approximating teeth, as well as by the position of the teeth in the jaw and their degree of eruption.
The bone contour normally conforms to the prominence of the roots, with intervening vertical depressions that taper toward the margin.
The height and thickness of the facial and lingual bony plates are affected by the alignment of the teeth and angulation of the root to the bone.
Fenestration and Dehiscence
Fenestration: Isolated areas in which the root is denuded of bone and the root surface is covered only by periosteum and overlying gingiva and the marginal bone is intact.
Dehiscence: : Isolated areas in which the root is denuded of bone and the root surface is covered only by periosteum and overlying gingiva, the denuded areas extendto marginal bone.
Such defects occur more often on the facial bone than on the lingual, are more common on anterior teeth than on posterior teeth, and are frequently bilateral.
The cause of these defects is not clear, but it may due to: