Mixed Dentition Period Notes

The Mixed Dentition Period

The period during which both the primary and the permanent teeth are present in the oral cavity is known as mixed dentition.

The mixed dentition period can be divided into three stages:

• First transitional stage
• Intertransitional stage
• Second transitional stage

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1. First transitional stage

The first transitional stage is characterized by the eruption of first Permanent molars and exchange of primary incisors with permanent Incisors.

Eruption of first molars:

• In both the jaws, the first permanent molars erupt more or Less in a perpendicular orientation to the occlusal plane.
• The anteroposterior relation between the two opposing first Molars after eruption depends on:
  • Their positions previously occupied within them jaws
  • The sagittal relation between the maxilla and mandible
  • Ratios of the mesiodistal crown dimensions of the upper and lower deciduous molars
• The occlusal relationship is established by the ‘cone and funnel’ mechanism with the upper palatal cusp (cone) sliding into the lower occlusal fossa (funnel).
• Ideally, the eruption of the permanent molars into a class i molar relationship is desired.
• Since the flush terminal relationship is more common in deciduous dentition, it is more common for the permanent molars to erupt into an end-to-end relationship.
• The desired class i relationship is established in the following ways:
• Early mesial shift:
  
  • If the deciduous dentition is a spaced dentition with the flush terminal relationship of second deciduous molars, the eruptive force of the permanent molars causes closing of any existin spaces between the primary molars or primate spaces effectively causing a decrease in arch length

• Late mesial shift:
  • When no spaces exist, the erupting first permanent molar is not able to close the space. In these cases when the primary canine and molar exfoliates the permanent molar migrates mesially to use up the ‘leeway space’ (the difference between the mesiodistal width of the primary canine, first and second molars and their permanent successors.
  • This averages 1.8 mm in the maxilla and 3.4 mm in the mandible).
• Primary molar guidance of permanent dentition:
  • The primary second molar relationship can give clues to the eventual permanent molar relationship. If the deciduous arches terminate in a mesial step, the permanent molars may erupt directly into a normal, angle class i occlusion (meaning that the mandibular first molar is half a cusp mesial to its antagonist).
  • In few cases, it may develop into a class iii relationship. A definite distal step guides the molars into a Distal relationship which generally does not improve with age, instead, deteriorates.

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Incisal transition:

The period of transition is critical for the development of the dentition. During the transition, there are some factors that determine the successful alignment of the permanent teeth towards an ideal arch form.

• The interdental spacing of primary incisors:
  • Interdental spacing averages 4 mm in the maxillary arch and 3 mm in the mandibular arch. The amount of anterior spacing in the primary dentition is one of the first clinical observations  made in examining a young child. Lack of interdental spacing must be considered a serious handicap towards normal incisor alignment.
• Intercanine arch width growth:
  • Only negligible amount of growth occurs from three years (completion of primary dentition) to 6–7 years when the permanent mandibular incisors begin to erupt. A notable amount of growth occurs with the eruption of the incisors and canines.
• Intercanine arch length increase:
  • When the permanent incisors erupt, they assume a somewhat more anteriorly inclined position (labial inclination) than the\ deciduous incisors.
  • This position of the incisors averages 2.3 mm and results in an increase in intercanine arch length of approximately 3 mm without any change in intercanine width.
  • This makes another characteristic difference in tooth axis.
  • The interincisal angle between the maxillary and mandibular Incisors is about 150° in primary dentition, whereas it is about 123° in permanent dentition which makes permanent dental arch circumferences wider.
• Favourable variations in the size ratio between the primary and permanent teeth:
  • The permanent incisors are usually larger in total width than the primary incisors they replace (the difference is called the incisal liability).
  • Permanent canine, premolars are usually smaller in total width than primary canine and molar they replace (the difference is leeway space).
  • The sum of the mesiodistal width of the permanent lateral incisors is generally smaller than of the primary lateral incisors by about 1 Mm in the maxilla and 3 mm in the mandible.
  • This carries out the smooth exchange of the lateral incisors.
  • The leeway space, in addition to providing space for the permanent canine, also allows the permanent molars to move mesially when the deciduous molars are replaced with the mesiodistal smaller premolars.
  • Moreover, the leeway space is greater in the mandible than in the maxilla and facilitates a greater mesial movement of the lower permanent molars in relation to the upper ones, which consequently results in a change from a possible cusp-to cusp molar Relationship to a normal molar interrelation.

2. Intertransitional stage:

• In the intertransitional period which lasts about 1.5 years, asymmetry in the emergence and associated differences in height levels and lengths Of clinical crowns of the corresponding left and right teeth are made Up.
• Under the influence of the tongue, the mandibular lateral incisors attain the proper sites within the dental arch and their initial lingual \ location is eliminated.
• Small rotations are corrected by the pressure exerted by the tongue and lips if the spatial conditions in the dental arches permit these movements.
• There is wearing of deciduous teeth with the attrition of the cusp tips and occlusal morphology approaching that of a plane.
• The anteroposterior relation between two jaws is not fixed in occlusion. A fixed intercuspal relationship is absent. No interferences from the occlusal contacts are present so the mandibular teeth attain a slightly More mesial position.

3. Second transitional stage:

This stage is characterized by the exchange of primary canines and primary molars with permanent canines and premolars.

• After the eruption of incisors, there follows a pause of about one to two years, and the next tooth to erupt is the lower canine and the first premolar at 9–10 years. The maxillary canine and second premolar then erupt at 11–12 years and the period is terminated by the appearance of the second molars at 12 years.
• School children tend to look unusual during the exchange of their incisors, especially in the upper arch when the permanent incisors erupt. Because of the pressure of erupting permanent canines on the developing roots of lateral incisors, the crowns of erupting incisors flare more Laterally producing diastema.
• Many parents worry about these conditions. The transitional malalignment during the exchange period of the upper anterior teeth is called ‘the ugly duckling stage’ or ‘broadbent phenomenon’. This phenomenon is self-correcting, and normally, the incisors gradually straighten with the eruption of the lateral incisors and canines.
• The transition from the ugly duckling to a mature stage of dentition is also called the prepubertal period. This is correlative with the maturation of the child as a whole.

The most common sequence of eruption of permanent lateral teeth in the maxilla is 4-3-5 and in the mandible 3-4-5.

• As mentioned previously, because the permanent canine is larger than the primary canine, crowding is very common immediately after the exchange of the canines.
• This phenomenon is more prevalent in the mandible, but this crowding gets alleviated after the exfoliation of the second primary molar. If this sequence of eruption is changed to 4- 3-5 or 4-5-3, the leeway space will not be utilized as efficiently, and in such cases, the dentition will become crowded without the leeway space helping to improve.
• After the exchange of the lateral teeth has been completed and dental arch up to the first molar is established, the second permanent molars begin to erupt.
• The dental arch length is reduced just prior to the eruption of the second molar by the mesial force.
• The arch circumference may also become shortened than that of the primary dental arch by the utilization of the leeway space with the exchange of the second primary molar to the second premolar.
• Therefore, it is quite possible that eruption of the second molar may accentuate the crowding if it was already present in the dentition.
• In some cases, the permanent molar erupts prior to the second premolars. If the space after the extraction of the second primary molar is not maintained, that space will be lost rapidly.
• A careful monitoring is required.

Self Correcting Malocclusion In Mixed Dentition Stage

Self-correcting anomalies are the anomalies which arise in a child’s developing dentition during the period of transition from that of gum pads stage to the onset of permanent teeth and get corrected on their own without any dental treatment. The only treatment needed is the reassurance to the parents who are worried about these conditions.

The Mixed Dentition Period Conclusion

• The period during which both the primary and permanent teeth are present in the oral cavity is known as mixed dentition. The mixed dentition period can be Divided into three stages.
• First transitional stage
  • Emergence of the first permanent molar
  • Incisors transition
  • Establishment of occlusion
• Intertransitional stage
  • Containing both sets of dentition
  • Four permanent incisors, left and right first permanent molars
• Second transitional stage
  • Emergence of canines, premolars and the second permanent molars
  • Establishment of occlusio