Development of dental occlusion

DEVELOPMENT OF OCCLUSION

Tooth development or odontogenesis is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth. For human teeth to have a healthy oral environment, all parts of the tooth must develop during appropriate stages of fetal development. Primary (baby) teeth start to form between the sixth and eighth week of prenatal development, and permanent teeth begin to form in the twentieth week.[1] If teeth do not start to develop at or near these times, they will not develop at all, resulting in hypodontia or anodontia.





The tooth germ is an aggregation of cells that eventually forms a tooth. These cells are derived from the ectoderm of the first pharyngeal arch and the ectomesenchyme of the neural crest. The tooth germ is organized into three parts: the enamel organ, the dental papilla and the dental sac or follicle.

Maxillary (upper) teeth

Primary teeth
Centralincisor
Lateralincisor
Canine
Firstmolar
Secondmolar


Initial calcification
14 wk I.U.
16 wk I.U.
17 wk I.U.
15.5 wk I.U.
19 wk I.U.

Crown completed

1.5 mo
2.5 mo
9 mo
6 mo
11 mo

Root completed

1.5 yr
2 yr
3.25 yr
2.5 yr
3 yr


Mandibular (lower) teeth

Initial calcification

14 wk I.U.
16 wk I.U.
17 wk I.U.
15.5 wk I.U.
18 wk I.U.

Crown completed

2.5 mo
3 mo
9 mo
5.5 mo
10 mo

Root completed

1.5 yr
1.5 yr
3.25 yr
2.5 yr
3 yr


Human Tooth Development Timeline
The following tables present the development timeline of human teeth. Times for the initial calcification of primary teeth are for weeks in utero. Abbreviations: wk = weeks; mo = months; yr = years.

Maxillary (upper) teeth

Permanent teeth
Centralincisor
Lateralincisor
Canine
Firstpremolar
Secondpremolar
Firstmolar
Secondmolar
Thirdmolar
Initial calcification
3–4 mo
10–12 mo
4–5 mo
1.5–1.75 yr
2–2.25 yr
at birth
2.5–3 yr
7–9 yr
Crown completed
4–5 yr
4–5 yr
6–7 yr
5–6 yr
6–7 yr
2.5–3 yr
7–8 yr
12–16 yr
Root completed
10 yr
11 yr
13–15 yr
12–13 yr
12–14 yr
9–10 yr
14–16 yr
18–25 yr


Mandibular (lower) teeth

Initial calcification

3–4 mo
3–4 mo
4–5 mo
1.5–2 yr
2.25–2.5 yr
at birth
2.5–3 yr
8–10 yr
Crown completed
4–5 yr
4–5 yr
6–7 yr
5–6 yr
6–7 yr
2.5–3 yr
7–8 yr
12–16 yr
Root completed
9 yr
10 yr
12–14 yr
12–13 yr
13–14 yr
9–10 yr
14–15 yr
18–25 yr


Stages Of Tooth Development
Tooth development is commonly divided into the following stages: the initiation stage, the bud stage, the cap stage, the bell stage, and finally maturation.

Postnatal development of the dentition

When a child is born, mineralization of all the primary tooth crowns is well happening, with this process also beginning in the first permanent molars. The primary dentition will start to erupt in the first year of life and will be established by the end of the third. The permanent dentition is heralded by eruption of the first molars at around 6 years of age and is completed in most cases, by the appearance of the third molars in the late teenage years.

The jaws at birth

At birth, the maxillary dental arch is characteristically horseshoe-shaped whereas the mandibular arch assumes a wider U-shape. The mucous membrane of both the maxilla and mandible is thickened in the newborn infant to produce gum pads, which cover the alveolar processes containing the developing primary teeth.





Usually the upper jaw overlaps the lower jaw in antero-posterior and in transverse direction, in other ward: the upper jaw is wider than the lower jaw and at the same time,the lower jaw is in a retrognathic position in relation to the upper. On the upper jaw, we can see the lateral sulcus, which express to the distal margin of the upper deciduous canine and the gum pads, which is separated from the masticatory mucosa by a long and continuous groove called gingival groove.





The upper lip at this stage is usually short, and the anterior oral seal of the mouth occurs due to the contact between the lower lip and the tongue. At this age the anterior gum pads is averted anteriorly and when the child closes his mouth there is a space between the anterior gum pads and the only part that is in contact are the future growing E(s) (Es region).
The maxillary (left) and mandibular (middle) gum pads in isolation and occlusion (right). Note the prominent lateral sulci (LS) present in both arches. A, C, external arch width; B, D, internal arch width; E, F, anterior arch length; G, overjet; H, anteroposterior relationship; I, overbite.

The elevations of the Es cannot be seen clearly, until the age of 5 months, the anterior opening of the mouth will facilitate the feeding process without discomfort to the mother, at this age usually the labial frenum is attached to the incisive papillary region and after the eruption of the deciduous it will migrate in upward direction and gives the incisive papillary attachment this is due to alveolar bone formation in association with the development of the deciduous teeth.




Occasionally a child is born with teeth already present or that undergo precocious eruption within the oral cavity
• Natal teeth are present at birth.
• Neonatal teeth erupt within the first month of life; and
• Pre-erupted teeth appear within the second and third months of life.
Natal and neonatal teeth occur in around 1: 3000 children and are usually mandibular primary incisors, although rarely they can be supernumerary teeth. They are often poorly developed, mobile and can cause ulceration of the mouth and nipple during suckling. If these teeth give rise to problems, they should be removed.

Deciduous Dentition

Usually the deciduous teeth begin to erupt at the sixth months of age until 2.5 – 3 years of age. (All the deciduous teeth will be erupted, completely, at the age of third years), and can be a source of some malaise for the child. Teething is associated with tender swollen gums, irritability, finger sucking, drooling and nocturnal crying. It is generally selflimiting but can be relieved by allowing the child to chew on clean, hard, cool objects and the provision of appropriate analgesia, either in the form of an elixir or lignocaine based teething gel. Occasionally, primary tooth eruption is preceded by the appearance of an
eruption cyst, a small bluish-type swelling, which overlies the erupting tooth. These generally disappear, but can be excised if they persist.

Deciduous Dentition

Eruption sequence of deciduous dentition:
This includes:
Lower (As) at 6th month,
Lower (Bs) at 7th month,
Upper (As) at 8th month,
Upper (Bs) at 9th moth,
Lower (Ds) at 12th month,
Upper (Ds) at 14th month,
Lower (Cs) at 16th month, upper (Cs) at 18th month,
Lower (Es) at 20th month,
Then finally, upper (Es) at 24th month.


The first sign of the formation of the deciduous teeth is at the age of 4-6 months IUL, and their roots will be completely formed after 12 – 18 months after their eruption. When these teeth erupt in the mouth, they will erupt in a vertical direction i.e. the upper long axes of the centrals coincide with the long axis of the lower incisors, and usually there is slightly increase in the over-jet.




These teeth usually present in their crept in a rotated manner, this is to occupy a less space in the child’s mouth; so that, when they erupt, they will erupted in a spaced conditions, and these spaces usually present at the mesial aspect of the upper canine and the distal aspect of the lower canine.
These spaces are termed as primary spaces or anthropoid spaces, since it looks like the spaces that are present between the teeth of the higher Apes.

Such spaces also present mesial to the Ds, these spaces that are anterior to the Ds, try to close in a way that: the distal surface of the D (upper and lower) will be located on the same terminal plane and usually the lower D is larger in mesiodistal dimension than the upper D. The Es when erupt, they erupt in the same terminal plane (flush terminal).












In summery the complete primary dentition is classically associated with a number of characteristic features:

• The arches are semi-circular in shape;

• The incisors are spaced, upright and associated with a positive overjet and overbite;



• Primate or anthropoid spaces are present, mesial to the maxillary primary canines and distal to the mandibular canines;
• The molar and canine relationship is class I; and
• The distal edges of the second primary molars are flush in the vertical plane.

Changes in spaces:

The spaces of the deciduous teeth try to increase with age due to the growth of the jaws in: antroposterior, transverse and vertical direction, and due to: attrition; since the shape of the deciduous teeth is triangular and these teeth will be subjected to a great amount of attrition due to wear at the incisal edges; so, the spaces will be increased, especially anteriorly due to the attrition. This attrition will occur at the incisal edges and the proximal surfaces since the deciduous teeth mostly converted into edge to edge relationship at a later stage.




At the age of 5.5 - 6 years the permanent teeth begun to erupt, and these teeth contains an eruptive cyst, and this is filled with fluid, this fluid will exert a pressure on the roots of the deciduous teeth causing their resorption with the aid of special enzyme which is produced at this stage of age.

The roots resorption of the deciduous teeth means decrease in the root length and since the occlusal forces at the age of 5-6 years are more than those of 3 years; so, these occlusal forces together with the root resorption will increase the mobility of the deciduous teeth and if the deciduous teeth in closed case (without spacing), this will produce attrition of the proximal surfaces due to friction produced by movement during mastication, as the mobility progress the spaces will be increased and this will facilitate the process of normal shedding of the incisors.




Usually, the permanent teeth when erupt ,they are located at the palatal or lingual aspect of the deciduous incisors, causing their resorption during eruption, but sometimes, the permanent teeth could be deflected from the roots of incisors; therefore, this process will not happen in the normal way, and the permanent erupt ,while the deciduous is stay in its space.