Periodontal ligament development

Cells from the dental follicle give rise to the periodontal ligaments (PDL).

Formation of the periodontal ligaments begins with ligament fibroblasts from the dental follicle. These fibroblasts secrete collagen, which interacts with fibers on the surfaces of adjacent bone and cementum. This interaction leads to an attachment that develops as the tooth erupts into the mouth. The occlusion, which is the arrangement of teeth and how teeth in opposite arches come in contact with one another, continually affects the formation of periodontal ligaments. This perpetual creation of periodontal ligaments leads to the formation of groups of fibers in different orientations, such as horizontal and oblique fibers.

Tooth eruption Theories

Tooth eruption occurs when the teeth enter the mouth and become visible. Although researchers agree that tooth eruption is a complex process, there is little agreement on the identity of the mechanism that controls eruption. Some commonly held theories that have been disproven over time include: (1) the tooth is pushed upward into the mouth by the growth of the tooth's root, (2) the tooth is pushed upward by the growth of the bone around the tooth, (3) the tooth is pushed upward by vascular pressure, and (4) the tooth is pushed upward by the cushioned hammock. The cushioned hammock theory, first proposed by Harry Sicher, was taught widely from the 1930s to the 1950s. This theory postulated that a ligament below a tooth, which Sicher observed on under a microscope on a histologic slide, was responsible for eruption. Later, the "ligament" Sicher observed was determined to be merely an artifact created in the process of preparing the slide.

The most widely held current theory is that while several forces might be involved in eruption, the periodontal ligaments provide the main impetus for the process. Theorists hypothesize that the periodontal ligaments promote eruption through the shrinking and cross-linking of their collagen fibers and the contraction of their fibroblasts.

Although tooth eruption occurs at different times for different people, a general eruption timeline exists. Typically, humans have 20 primary (baby) teeth and 32 permanent teeth. Tooth eruption has three stages. The first, known as deciduous dentition stage, occurs when only primary teeth are visible. Once the first permanent tooth erupts into the mouth, the teeth are in the mixed (or transitional) dentition. After the last primary tooth falls out of the mouth—a process known as exfoliation—the teeth are in the permanent dentition.

Primary dentition starts on the arrival of the mandibular central incisors, usually at eight months, and lasts until the first permanent molars appear in the mouth, usually at six years. The primary teeth typically erupt in the following order: (1) central incisor, (2) lateral incisor, (3) first molar, (4) canine, and (5) second molar. As a general rule, four teeth erupt for every six months of life, mandibular teeth erupt before maxillary teeth, and teeth erupt sooner in females than males. During primary dentition, the tooth buds of permanent teeth develop below the primary teeth, close to the palate or tongue.

Mixed dentition starts when the first permanent molar appears in the mouth, usually at six years, and lasts until the last primary tooth is lost, usually at eleven or twelve years. Permanent teeth in the maxilla erupt in a different order from permanent teeth on the mandible. Maxillary teeth erupt in the following order: (1) first molar (2) central incisor, (3) lateral incisor, (4) first premolar, (5) second premolar, (6) canine, (7) second molar, and (8) third molar. Mandibular teeth erupt in the following order: (1) first molar (2) central incisor, (3) lateral incisor, (4) canine, (5) first premolar, (6) second premolar, (7) second molar, and (8) third molar. Since there are no premolars in the primary dentition, the primary molars are replaced by permanent premolars. If any primary teeth are lost before permanent teeth are ready to replace them, some posterior teeth may drift forward and cause space to be lost in the mouth. This may cause crowding and/or misplacement once the permanent teeth erupt, which is usually referred to as malocclusion. Orthodontics may be required in such circumstances for an individual to achieve a straight set of teeth.

The permanent dentition begins when the last primary tooth is lost, usually at 11 to 12 years, and lasts for the rest of a person's life or until all of the teeth are lost (edentulism). During this stage, third molars (also called "wisdom teeth") are frequently extracted because of decay, pain or impactions. The main reasons for tooth loss are decay or periodontal disease.

The mixed dentition

I. Transition dentition between 6 and 12 years of age with primary tooth exfoliation and permanent tooth eruption

2. Its characteristic features have led this to be called the ugly duckling stage because of

a. Edentulated areas

b. Disproportionately sized teeth

c. Various clinical crown heights

d. Crowding

e. Enlarged and edematous gingiva

f. Different tooth colors

Age changes in the dentition

I. After the teeth have reached full occlusion, microscopic tooth movements occur to compensate for wear at the contact area (Mesial Drift) and occlusal surfaces (by Deposition of cementum at the root apex)

2. Attrition of incisal ridges and cusp tips may be so severe that dentin may become exposed and intrinsically stained

3. Secondary dentin may be formed in response to dental caries, trauma, and aging and result in decreased pulp size and tooth sensation

ERUPTION OF THE PERMANENT TOOTH

- At the time at which the deciduous tooth erupts the tooth bud for the permanent tooth has already been building up enamel and dentin.

- When the permanent tooth starts to erupt, pressure on the root of the deciduous tooth causes resorption by the osteoclasts.

- Wolff's law states that when two hard tissues exert pressure on one another the softer of the tissues will be resorbed.

- The dentin and cementum of the root of the deciduous tooth is softer than the enamel of the permanent tooth that is why the root of the deciduous tooth is resorbed.

- Most permanent teeth have erupted and have been in use for 2 years before the root is completely formed.

Compensating curvatures of the individual teeth.

- the gentle curvature of the long axes of certain posterior teeth to exhibit a gentle curvature.

-These are probably analogous to the trabecular patterns seen in the femur and therefore reflect lines of stress experienced during function.

Posteruptive tooth movement.

These movements occur after eruption of the teeth into function in the oral cavity. These movements, known collectively as occlusomesial forces.

A. Continuous tooth eruption eruption of teeth after coming into occlusion. This process compensates for occlusal tooth wear.. Cementum deposition and progressive remodelling of the alveolar bone are the growth processes that provide for continuous tooth movement

B. Physiological mesial drift :Tthe tendency of permanent posterior teeth to migrate mesially in the dental arch both before and after they come into occlusion. Clinically, it compensates for proximal tooth wear.

(1) It describes the tendency of posterior teeth to move anteriorly.

(2) It applies to permanent teeth, not deciduous teeth.

(3) The distal tooth have the stronger is the tendency for drift.

(4) It compensates for proximal wear.

(5) In younger persons, teeth drift bodily; in older persons, they tip and rotate.

(6) Forces that cause it include occlusal forces, PDL contraction, and soft tissue pressures. There may be other more subtle factors as well.

Height  of Epithelial Attachment

The height of normal gingival tissue . mesiallv and distallv on approximating teeth, is directly dependent upon the height of the epithelial attachment on these teeth. Normal attachment follows the curvature of the cementoenamel junction if the teeth are jn proper, alignment and contact.