Occlusion is the way in which the maxillary and mandibular occluding surfaces articulate, both in static or in function. To simplify the description of changes which occur in the dental occlusion during the various stages of the teeth eruption, four stages are assumed: gum pads , primary dentition, mixed dentition and permanent dentition stages.
It extends from birth until the eruption of the first primary tooth, around 6-7 months of age. The newly born baby mouth contains 20 elevations (10 in upper arch and 10 in lower arch) corresponding to the future 20 deciduous teeth. The upper arch is a horse shoe shape, while the lower is a U shape and usually the upper arch overlaps the lower arch in the anteroposterior and transverse direction, in the other words, the upper arch is wider than the lower, and at the same time the lower arch is in a retrognathic position relative to the upper, So it will give an appearance of CL. II pattern.
The maxillary and mandibular anterior gum pads are averted and an anterior open bite relationship is seen while the posterior segments are touching. In this manner the opposing surfaces of the pads provide for a more efficient way of squeezing milk during breastfeeding.
A few babies are born with one or more teeth called Natal teeth. These teeth look like the deciduous teeth but usually with very short or without root. Natal tooth may be a supernumerary one and it's extraction is not necessary unless it causes a painful situation for a breastfeeding mother.
Primary dentition stage
The primary dentition stage extends from the time of eruption of the primary teeth until the eruption of the first permanent tooth around 6 years of age, table (1). By 3 years of age, the eruption of deciduous teeth is usually completed. The usual eruption sequence of deciduous teeth is A-B-D-C-E.
Table (1): Timing of calcification and eruption of primary dentition. There are some characteristics can be seen in the primary dentition stage:
When these teeth erupt in the mouth, they will erupt in a vertical dimension i.e. the upper long axes of the central incisors coincide with the long axes of the lower incisors, and usually there is slightly increase in the Overjet.
The deciduous dentition usually erupt in spaced condition. Spacing is normal throughout the anterior part of the primary dentition but is most noticeable in two locations, called the primate/anthropoid spaces. (Most subhuman primates have these spaces throughout life, thus the name). The primate spaces are located, mesial to the upper canine and distal to the lower canine,
into which the opposing canine tooth interdigitates. These spaces are normally present from the time the teeth erupt and used for early mesial shift. Developmental spaces between the incisors are often present from the beginning, but become somewhat larger as the child grows and the alveolar processes expand. Spacing in the primary incisor region is normally distributed among all the incisors, not just, in the "primate space. This arrangement of the primary incisor teeth with gaps between them may not be very pretty, but it is normal. Adult appearing smile in a primary dentition child is an abnormal, not a normal finding—the spaces are necessary for proper alignment of the permanent incisors.
The anteroposterior molar relationship in the primary dentition stage is described in terms of the relationship between terminal planes. The terminal planes are the distal surfaces of the maxillary and mandibular second primary molars (E). Essentially the two terminal planes can be related to each other in one of three ways:
Flush terminal plane relationship, both terminal planes of maxillary and mandibular 2nd molars are at the same level anteroposteriorly, it is the normal molar relation in primary dentition because the mesiodistal width of the lower E is greater than that of the upper E.
Distal step relationship, the mandibular molar terminal plane is relatively distal to the maxillary molar terminal plane. Lastly,
Mesial step relationship, the mandibular molar terminal plane is relatively mesial to the maxillary molar terminal plane.
Determining the terminal plane relationships, in the primary and mixed dentition stages, is of great importance to the clinician because the erupting first permanent molars are guided by the distal surfaces of the second primary molars as they erupt into occlusion.
In distal step occlusion, the permanent first molar will be in a Class II relation when they erupt. While in mesial step relationship, the permanent first molar will erupt in Class I relation in early mixed dentition.
Changes can occur in deciduous dentition:
By the effect of eruptive force of the permanent teeth, the resorption occurs in the root of the deciduous teeth. Concurrently, at age of 5-6 years the occlusal force will be more, so these occlusal forces together with root resorption will increase the mobility of the deciduous, which facilitate the process of normal shedding.
Spaces try to increase with age due to growth of the jaw and attrition. Since the shape of deciduous teeth is triangular, and these teeth will be subjected to a great amount of wear at incisal edges and attrition of proximal surfaces by mastication, a more spaces are produced between them.
Normal features of gum pads stage:
It's period is 6-7 months.
The mouth contains 20 elevations.
A horse shoe shape upper arch is wider than U- shaped lower arch.
Anterior open bite.
Normal features of primary dentition stage:
Spacing ( primate & developmental).
Vertical inclination of the teeth with +O.J. and O.B.
The timing is variable but the sequence usually constant.