MANDIBULAR CENTRAL INCISORS

These are the first permanent teeth to erupt, replacing deciduous teeth, and are the smallest teeth in either arch

Facial Surfaces:-The facial surface of the mandibular central incisor is widest at the incisal edge. Both the mesial and the distal surfaces join the incisal surface at almost a 90° angle. Although these two surfaces are nearly parallel at the incisal edge, they converge toward the cervical margin. The developmental grooves may or may not be present. When present, they appear as very faint furrows.

Lingual: The lingual surface has no definite marginal ridges. The surface is concave and the cingulum is minimal in size.

Proximal: Both mesial and distal surfaces present a triangular outline.

Incisal: The incisal edge is at right angles to a line passing labiolingually through the tooth reflecting its bilateral symmetry.

Root Surface:-The root is slender and extremely flattened on its mesial and distal surfaces.

Development of occlusion.

A. Occlusion  usually means the contact relationship in function. Concepts of occlusion vary with almost every specialty of dentistry.

Centric occlusion is the maximum contact and/or intercuspation of the teeth.

B. Occlusion is the sum total of many factors.

1. Genetic factors.

-Teeth can vary in size. Examples are microdontia (very small teeth) and macrodontia (very large teeth). Incidentally, Australian aborigines have the largest molar tooth size—some 35% larger than the smallest molar tooth group

-The shape of individual teeth can vary (such as third molars and the upper lateral incisors.)

-They can vary when and where they erupt, or they may not erupt at all (impaction).

-Teeth can be congenitally missing (partial or complete anodontia), or there can be extra (supernumerary) teeth.

-The skeletal support (maxilla/mandible) and how they are related to each other can vary considerably from the norm.

2. Environmental factors.

-Habits can have an affect: wear, thumbsucking, pipestem or cigarette holder usage, orthodontic appliances, orthodontic retainers have an influence on the occlusion.

3.Muscular pressure.

-Once the teeth erupt into the oral cavity, the position of teeth is affected by other teeth, both in the same dental arch and by teeth in the opposing dental arch.

-Teeth are affected by muscular pressure on the facial side (by cheeks/lips) and on the lingual side (by the tongue).

C. Occlusion constantly changes with development, maturity, and aging.

1 . There is change with the eruption and shedding of teeth as the successional changes from deciduous to permanent dentitions take place.

2. Tooth wear is significant over a lifetime. Abrasion, the wearing away of the occlusal surface reduces crown height and alters occlusal anatomy.

Attrition of the proximal surfaces reduces the mesial-distal dimensions of the teeth and significantly reduces arch length over a lifetime.

Abraision is the wear of teeth by agencies other than the friction of one tooth against another.

Attrition is the wear of teeth by one tooth rubbing against another

3. Tooth loss leaves one or more teeth without an antagonist. Also, teeth drift, tip, and rotate when other teeth in the arch are extracted.

Bell stage

The bell stage is known for the histodifferentiation and morphodifferentiation that takes place. The dental organ is bell-shaped during this stage, and the majority of its cells are called stellate reticulum because of their star-shaped appearance. Cells on the periphery of the enamel organ separate into three important layers. Cuboidal cells on the periphery of the dental organ are known as outer enamel epithelium.The cells of the enamel organ adjacent to the dental papilla are known as inner enamel epithelium. The cells between the inner enamel epithelium and the stellate reticulum form a layer known as the stratum intermedium. The rim of the dental organ where the outer and inner enamel epithelium join is called the cervical loop

Other events occur during the bell stage. The dental lamina disintegrates, leaving the developing teeth completely separated from the epithelium of the oral cavity; the two will not join again until the final eruption of the tooth into the mouth

The crown of the tooth, which is influenced by the shape of the internal enamel epithelium, also takes shape during this stage. Throughout the mouth, all teeth undergo this same process; it is still uncertain why teeth form various crown shapes—for instance, incisors versus canines. There are two dominant hypotheses. The "field model" proposes there are components for each type of tooth shape found in the ectomesenchyme during tooth development. The components for particular types of teeth, such as incisors, are localized in one area and dissipate rapidly in different parts of the mouth. Thus, for example, the "incisor field" has factors that develop teeth into incisor shape, and this field is concentrated in the central incisor area, but decreases rapidly in the canine area. The other dominant hypothesis, the "clone model", proposes that the epithelium programs a group of ectomesenchymal cells to generate teeth of particular shapes. This group of cells, called a clone, coaxes the dental lamina into tooth development, causing a tooth bud to form. Growth of the dental lamina continues in an area called the "progress zone". Once the progress zone travels a certain distance from the first tooth bud, a second tooth bud will start to develop. These two models are not necessarily mutually exclusive, nor does widely accepted dental science consider them to be so: it is postulated that both models influence tooth development at different times.Other structures that may appear in a developing tooth in this stage are enamel knots, enamel cords, and enamel niche.

HISTOLOGY OF SALIVARY GLANDS

Parotid: so-called watery serous saliva rich in amylase
Submandibular gland: more mucinous
Sublingual: viscous saliva

Parotid Gland:  The parotid is a serous secreting gland.

There are also fat cells in the parotid.

Submandibular Gland

This gland is serous and mucous secreting.

There are serous demilunes

This gland is more serous than mucous

Also fat cells

Sublingual Gland

Serous and mucous secreting

Serous cells in the form of demilunes on the mucous acini.

more mucous than serous cells

Minor Salivary Glands

Minor salivary glands are not found within gingiva and anterior part of the hard palate
Serous minor glands=von Ebner below the sulci of the circumvallate and folliate papillae of the tongue; palatine, glossopalatine glands are pure mucus; some lingual glands are also pure mucus

Functions

Protection: lubricant (glycoprotein); barrier against noxious stimuli; microbial toxins and minor traumas; washing non-adherent and acellular debris; calcium-binding proteins: formation of salivary pellicle
Buffering: bacteria require specific pH conditions; plaque microorganisms produce acids from sugars; phosphate ions and bicarbonate
Digestion: neutralizes esophageal contents, dilutes gastric chyme; forms food bolus; brakes starch
Taste: permits recognition of noxious substances; protein gustin necessary for growth and maturation of taste buds
Antimicrobial: lysozyme hydrolyzes cell walls of some bacteria; lactoferrin binds free iron and deprives bacteria of this essential element; IgA agglutinates microorganisms
Maintenance of tooth integrity: calcium and phosphate ions; ionic exchange with tooth surface
Tissue repair: bleeding time of oral tissues shorter than other tissues; resulting clot less solid than normal; remineralization

Differences Between the Deciduous and Permanent Teeth

1. Deciduous teeth are fewer in number and smaller in size but the deciduous molars are wider mesiodistally than the premolars. The deciduous anteriors are narrower mesiodistally than their permanent successors. Remember the leeway space that we discussed in the unit on occlusion?

2. Their enamel is thinner and whiter in appearance. Side by side, this is obvious in most young patients.

3. The crowns are rounded. The deciduous teeth are constricted at the neck (cervix).

4. The roots of deciduous anterior teeth are longer and narrower than the roots of their permanent successors.

5. The roots of deciduous molars are longer and more slender than the roots of the permanent molars. Also, they flare greatly.

6. The cervical ridges of enamel seen on deciduous teeth are more prominent than on the permanent teeth. This 'bulge' is very pronounced at the mesiobuccal of deciduous first molars.

G. Deciduous cervical enamel rods incline incisally/occlusally.

The Transition from the Deciduous to the Permanent Dentition.

1. The transition begins with the eruption of the four first permanent molars, and replacement of the lower deciduous central incisors by the permanent lower central incisors.

2. Complete resorption of the deciduous tooth roots permits exfoliation of that tooth and replacement by the permanent (successional) teeth

3. The mixed dentition exists from approximately age 6 years to approximately age 12 years. In contrast, the intact deciduous dentition is functional from age 2 - 2 /2 years of age to 6 years of age.

4. The enamel organ of each permanent anterior tooth is connected to the oral epithelium via a fibrous cord, the gubernaculum. The foramina through which it passes can be seen in youthful skulls

The deciduous second molars are particularly important. It is imperative that the deciduous second molars be preserved until their normal time of exfoliation. This prevent mesial migration of the first permanent molars.

Use a space maintainer in the event that a second deciduous molar is lost prematurely