Dentitions

Tooth Eruption Mechanism

• Dental sac is essential for tooth eruption

  • If not present, there is no eruption

  • If primary tooth is missing, permanent will be missing as well because it is attached lingually to the primary tooth bud

    • Erupt Lingually to primary

• Coronal part regulates osteoclastogenesis needed for eruption

• Basal half regulates osteogenesis needed for eruption

• Dental follicle develops into PDL

Eruption sequence of

• A/1: Central Incisor

• B/2: Lat Incisor

• C/3: Canine

• D: Primary First Molar

  • 4: First premolar

• E: Primary second molar

  • 5: Second Premolar

• 6: First Molar

• 7: Second Molar

Primary: A >B>D>C>E

Permanent Maxillary: 6>1>2>4>5>3>7

Permanent Mandibular: 6>1>2>3>4>5>7

How to determine the delay of tooth eruption

• Based on age: ± 2 years of when it is supposed to erupt

• Contralateral tooth: must erupt within 6 months of contralateral tooth eruption

  • HOW TO KNOW IT’S BEEN 6 MONTHS?: If the tooth has reached the occlusal plane, it has been 6 months

Causes of Tooth Eruption Delay

• Primary: Eruption mechanism defect prevents eruption, not caused by lack of space

  • Cannot be fixed by ortho, will cause ankylosis

• Secondary: Eruption mechanism is normal but there is not enough space to erupt

  • Can be corrected with ortho

• Incisor eruption IMPA angle

• What is the average change of mandibular incisor inclination relative to lower border of mandible

• Mandibular plane mesial to maxillary plane is gonna move how much more than maxillary molars

• 90 degrees

• Constant

• Mandible moves 1.8mm more

Molar Relationships in Primary Dentition

2nd Primary Molar: Compare to distal side

• Flush: both molars align on distal side

• Distal step: Mand molar is more distal

• Mesial step: Mand molar is more mesial

  

Permanent 1st Molar in Mixed Dentition: Compare to mesial side

• Flush: both molars align on mesial side

• Distal Step: Mand molar is more distal

• Mesial Step: Mand Molar is more mesial

  

Flush → Leads to 55% Class I and 45% Class II

Distal step → Leads to 0% Class I and 100% Class II

MEsial step →

• 1mm leads to 75% Class I, 13% Class II and 1% Class 3

• > 2mm leads to 61% Class I, 35% Class II and 4% Class III

Leeway Space

• Primary molars occupy more space than permanent premolars, leaving some space [(C+D+E) -(3+4+5)]

• Mandible: 2.4mm → DOUBLE

• Maxilla: 1.2mm

How do you know if 2nd max molars will fit ?

Measure from distal limit of pterygomaxillary fissure to distal limit of maxillary first molars

• Age +3mm

  

Dental Arch Width changes from 6-45yr

• 6wk-13yo: increase in max and mand inter-canine and inter-molar widths

  • Boys do not have mand intercanine width increase, otherwise their increases are bigger

• after 13yo: slight decrease in inter-canine width; stable inter-molar width

Why may untreated normal occlusion result in crowding from 9-20 yo

• Decrease in arch length (anterior and posterior segment)

• Increase in overjet and overbite

  

• Vertical growth increase

• Not related to 3rd molars

Causes of changes in occlusion 26-45yo

• OJ: did not change

• OB: increased significantly in females

• Crowding is worse in mandibular

• Mesial mgration of dentition