Mandibular Second Deciduous Molar.

-This tooth resembles the lower first permanent molar that is d

istal to it in the dental arch.

-There are two roots and five cusps. The three buccal cusps are all about the same size. This is in contrast to the lower first molar where the 'distal' cusp is smaller that the mesiobuccal and distobuccal cusps.

-The distal of the three buccal cusps may be shifted of onto the distal marginal ridge.  

NOTE

-Upper molars have three roots, lowers have two roots.

-Upper and lower second deciduous molars resemble first permanent molars in the same quadrant.

-Upper first deciduous molars vaguely resemble upper premolars. -Lower first deciduous molars are odd and unique unto themselves.

-First deciduous molars (upper and lower) have a prominent bulge of enamel on the buccal at the mesial. These help in determining right and left.

THE DECIDUOUS DENTITION

I. The Deciduous Dentition

-It is also known as the primary, baby, milk or lacteal dentition.

diphyodont, that is, with two sets of teeth. The term deciduous means literally 'to fall off.'

  There are twenty deciduous teeth that are classified into three classes. There are ten maxillary teeth and ten mandibular teeth. The dentition consists of incisors, canines and molars.

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.

FORMATION OF THE PERMANENT DENTITION

Twenty deciduous tooth buds are formed initially.
Proliferative activity of the dental lamina during the bell stage that leads to formation of permanent tooth buds (cap stage) lingual of each deciduous tooth germ.
Molars have no predecessors; they are formed by posterior proliferation of the dental lamina.

HARD TISSUE FORMATION

Hard tissue formation starts at the late stages of the bell stage.
Differentiatioin of cells into odontoblasts and ameloblasts.
The cells of the inner dental epithelium will become ameloblasts.
The cells of the dental papilla opposite to the inner dental epithelium will become odontoblasts.
Dentin is formed before enamel.
Dentin initiates the formation of enamel.

ROOT FORMATION

The root of the tooth is composed by dentin and cementum.
Dentinogenesis is initiated by the odontoblasts.
Odontoblasts are formed as epithelial cells continue to proliferate from the cervical loop as a double layer of cells known as Hertwig's root sheath.

TOOTH SHAPE

The shape of the crowns results from the interaction of inner dental epithelium and the dental papilla.
The cells of the inner dental epithelium have a programmed proliferation.
This internal program determines the tooth form.

The fate of the dental lamina

Rests of Serres
The rest of Serres are rests of the dental lamina identified in the gingival soft tissues.
They are round to ovoid aggregates of epithelial cells that have clear cytoplasm (glucogen rich).
They result from early breakup of the dental lamina during bell stage.

Rests of Malassez
The rests of Malassez result from breakup of the Hertwig's root sheath during root formation.
They can be identified in the periodontal ligament and are responsible for the development of radicular cysts.