DEFINITION The period duringwhich both primary & permanent teeth are in themouth together is known as the mixed dentition period.
Successional teeth : Those permanent teeth that follow into a place in the arch once held by primary tooth E.g.-Incisors, Canines, Premolars Accessional teeth : Those permanentteeth that erupt posteriorly to the primary teeth E.g.-Molars
Phases of Mixed Dentition # First transitional period # Inter transitional period # Second transitional period
First Transitional PeriodIt is characterized by : The emergence of first permanent molars The exchange of primary incisors with permanent incisors
Emergence of first permanent molars The location & relationship of firstpermanent molar depends much upon the distal surface relationship between the upper & lower second primary molars.
“Changes in Occlusion”• Flush terminal plane may lead to: - Class I [ with late mesial shift ] - Class II - End-to-end• Mesial step may lead to: - Class I [ normally ] - Class III• Distal step leads to: - Class II
FLUSH TERMINAL PLANE (FTP) Class II Class I (desirable) FTP End-to-end
MESIAL STEP Class III Class I (desirable) MESIAL STEP
Early Mesial shiftIf the primary dentition is spaced dentition with flush terminal relationship of second primary molars, theeruptive forces of the permanent molar causes closing of any existing spaces between the primary molars (primate spaces) effectively causing a decrease in arch length.
Late Mesial shift When no spaces exist, the erupting first permanentmolar is not able to close spaces. In these cases when the primary molars exfoliate the permanent molars migrate mesially to use up the Leeway space.
Late Mesial shift Class IFTP Molar moves into this space
Exchange of incisors The permanent incisors are largerthan the primary teeth they replace.This difference between the amount of space needed for theaccommodation of the incisors & theamount of space available for this is called INCISAL LIABILITY. 7 mm 6 mm (maxillary arch) (mandibular arch)
“Overcoming Incisal Liability”1) Interdental spacing of primary incisors2) Intercanine arch width growth3) Labial positioning of permanent incisors4) Favorable size ratio between primary & permanent incisors
1) Interdental spacing of primary incisorsGood interdental spacing of primary incisorsallows for better alignment of the largerpermanent incisors. Proper alignment Crowding
3) Labial positioning of the permanent incisorsPermanent incisors erupt to a more labialposition & are angled more labially, therebyincreasing the arch length. 130* 160* PRIMARY PERMANENT
4) Favorable size ratio between the primary & permanent incisors Size ratio may be favorable or unfavorable.• Favorable : large primary, small permanent• Unfavorable : small primary, large permanent
UGLY DUCKLING STAGE (Broadbent phenomenon) It is a transient/self-correcting malocclusion seen in the maxillary incisor region between 8-9 yrs of age. Erupting permanent canines displace the roots of lateral incisors mesially, resulting in transmission of force on to the roots of central incisors which also get displaced mesially.
A resultant distal divergence of crowns of two central incisors causes a midline spacing. This situation has been described by Broadbent as the UGLY DUCKLING STAGE as children tend to look ugly during this phase of development.
Inter-transitional PeriodIn this period the maxillary & mandibulararches consists of sets of primary &permanent teeth i.e.- permanent incisors- primary canines & molars- first permanent molars
Features of Inter-transitional period• Under the influence of tongue, themandibular lateral incisors assume theirproper position in the arch & its initial linguallocation is eliminated.• Small discrepancies are corrected bypressure exerted by tongue & lips.• No fixed intercusp relation is present, thishelps in mesial drift of mandibular teeth.
Second Transitional PeriodIt is characterized by the replacement of theprimary molars & canines by the premolars &permanent canines respectively.The combined mesiodistal width of thepermanent canines & premolars is usually lessthan that of the primary canines & molars.This surplus space is called LEEWAY SPACE .It is - 1.8 mm in Maxilla (0.9 mm each side) - 3.4 mm in Mandible (1.7 mm each side)
Features of Second Transitional Period • Early Duckling stage is corrected. • Arch loses its roundness. • Most common sequence of eruption is : 4-3-5 in maxilla 3-4-5 in mandible • This sequence is essential to prevent crowding & utilization of Leeway space. • The dental arch length & arch circumference decreases with mesial force.