Dental Development Stages and Orthodontic Occlusion Criteria

First Stage: Gum Pad (Birth to 2 Years)

This stage lasts from birth until the completion of the deciduous maxillary gum pad.

• Maxillary Gum Pad: Horseshoe shape.
• Mandibular Gum Pad: U shape.
• Transverse Division: Elevated ridges are divided into segments for future deciduous teeth.
• Lateral Sulcus: Located distal to the canine area.

Gum Pad Occlusion

Gum Pad at Rest: The pads are not in contact, and the tongue is projecting.

Occlusal relationships are described in three dimensions:

1. Anteroposteriorly (Overjet): Anterior overjet; the mandibular lateral sulcus is posterior to the maxillary.
2. Vertically: Anterior open bite; posterior segments (Segment D) touch.
3. Transversely: The maxillary gum pad is wider than the mandibular (resulting in overjet).

Second Stage: Deciduous Dentition (2 to 6 Years)

This stage is characterized by generalized spacing in the anterior region.

Primate Space

• Maxillary Arch: Located mesial to the canine (utilized by erupting incisors).
• Mandibular Arch: Located distal to the canine (utilized by early mesial shift, carried by the lower second deciduous molar, E).

Posterior Occlusion: Terminal Planes

The relationship between the distal surfaces of the maxillary and mandibular second deciduous molars determines the terminal plane:

1. Flush Terminal Plane:
   • No early mesial shift: Leads to Class II (half unit).
   • Early mesial shift: Leads to Class I.
2. Mesial Step:
   • No early mesial shift: Leads to Class I.
   • Early mesial shift: Leads to Class III.
3. Distal Step:
   • No early mesial shift: Leads to Class II (full unit).
   • Early mesial shift: Leads to Class II (half unit).

Third Stage: Mixed Dentition (6 to 12 Years)

Leeway Space

Leeway space is the difference between the mesiodistal width of the deciduous canine and molars (C, D, E) compared to their permanent successors (3, 4, 5).

This space is utilized by the Late Mesial Shift, carried by the permanent first molar (lower 6).

• Mandibular Arch: 3.4 mm bilaterally, 1.7 mm unilaterally.
• Maxillary Arch: 1.8 mm bilaterally, 0.9 mm unilaterally.

The Ugly Duckling Stage

This is a normal developmental stage, typically occurring around ages 9 to 10. The erupting permanent canine causes a wedging effect on the central incisors, resulting in a temporary midline diastema. The lateral incisors flare labially, and the central incisors flare distally.

Fourth Stage: Early Permanent Dentition (12+ Years)

Late Teen Crowding

Crowding often occurs due to the cephalocaudal gradient of growth, where the mandible continues to grow later. Mandibular incisors may become locked lingually to the maxillary teeth, causing anterior crowding.

Fifth Stage: Adult Dentition (17 to 21 Years)

Overjet and Overbite Definitions

• Overjet: The horizontal distance, measured from the labial surface of the mandibular incisor to the palatal surface of the maxillary incisor.
• Overbite: The vertical distance, measured from the incisal edge of the maxillary incisor to the incisal edge of the mandibular incisor.

Andrew’s Six Keys to Normal Occlusion

1. Molar Relationship:
   • The mesiobuccal cusp of the maxillary first molar (upper 6) occludes with the buccal groove of the mandibular first molar.
   • The mesiolingual cusp of the maxillary first molar occludes with the central fossa of the mandibular first molar.
   • The distal marginal ridge of the maxillary first molar occludes with the mesial surface of the mandibular second molar (lower 7).
   • Premolars: Buccal cusp to embrasure; lingual cusp to fossa.
   • Maxillary Canine: Cusp tip to embrasure.
2. Crown Angulation (Tipping): Viewed from the facial aspect.
   • Positive: Gingival portion is distal to the incisal portion.
   • Negative: Gingival portion is mesial to the incisal portion.
3. Crown Inclination (Torque): Viewed from the proximal aspect (tangent to the facial surface of the crown).
   • Positive: Gingival portion is lingual to the incisal portion.
   • Negative: Gingival portion is facial to the incisal portion.
4. Absence of Rotation.
5. Tight Contacts.
6. Flat Occlusal Plane (Flat Curve of Spee): A deep curve indicates crowding in the upper arch and spacing in the lower arch.

Roth’s Orthodontic Treatment Goals

1. Centric Occlusion equals Centric Relation.
2. Buccal cusp to embrasure; lingual cusp to fossa.
3. Equal posterior forces along the long axes of the teeth.
4. 0.13 mm space between the upper and lower anterior teeth.

Common Causes of Diastema

• Genetic factors.
• Low attachment of the labial frenum.
• Peg-shaped lateral incisors.
• Unerupted mesiodens (supernumerary teeth).
• Generalized spacing.
• Presence of oral habits (e.g., thumb sucking).
• Proclined maxillary incisors.