Lecture 13: Mixed Dentition

What is the mixed dentition period?

Ages between 6-13 years where primary dentition is replaced with permanent dentition

What is incisor liability?

The size difference between small primary incisors and larger permanent incisors.

Why does incisor liability matter clinically?

It predicts crowding unless spacing or arch adaptation compensates.

How do lower permanent incisors erupt relative to primary incisors?

Lingually

How do upper permanent incisors erupt relative to primary incisors?

Labially/buccally

What is the terminal plane?

The distal surfaces of the primary second molars, which determine early molar occlusion

What are the three types of terminal plane relationships?

1. Flush terminal plane

2. Mesial step

3. Distal step

What are the four major dental arch measurements?

1. Arch length

2. Arch width

3. Arch perimeter

4. Intercanine distance

How is arch length measured?

From the midline to a tangent touching the distal surfaces of second primary molars

True or false: arch length is stable

True— unless they are altered by eruption of crowding

What causes arch length loss in the lower arch?

Lingual eruption of lower incisors

What causes arch length gain in the upper arch?

Labial eruption of upper incisors

How is arch width measured?

• Canine width: cusp tip to cusp tip

• Molar width: occlusal groove to occlusal groove

True or false: There is significant arch width change in the lower arch between the change from primary to permanent dentition

False: There’s no significant change from primary to mixed dentition

True or false: The maxillary arch width increases more than mandibular arch width

True

What is intercanine distance?

The distance between canine cusp tips

When does intercanine distance increase?

During incisor eruption, then stabilizes

What is arch perimeter?

The total length of the arch measured along contact points

How is arch perimeter measured?

From distal of second primary molar → around arch → to distal of opposite molar

Why is arch perimeter important?

It determines space availability for permanent teeth

______ plane determines early molar occlusion.

Terminal

Arch perimeter decreases as _____ is lost.

leeway space

Mixed dentition is the most _____ stage of dental development.

variable

Why is arch length so stable?

Arch length does not naturally increase, only changes due to eruption or crowding

Why does the lower arch lose arch length when the lower incisors erupt lingually?

It reduces the anterior arch depth

Why does the upper arch gain arch length when the upper incisors erupt labially?

It increases anterior arch depth

What is the typical increase in intercanine distance?

3 mm

Why is leeway space clinically important?

It allows late mesial shift of molars and helps achieve Class I molar occlusion

Arch perimeter _____ as leeway space is lost and permanent teeth erupt.

decreases

True or false: Root growth does not cause eruption

True

True or false: Eruption is driven by follicular pressure and bone remodeling.

True

List the steps for permanent tooth eruption

1. Permanent crown forms

2. Root develops

3. Primary root resorbs

4. Primary tooth exfoliates

5. Permanent tooth erupts

What stops a permanent tooth from erupting further?

Contact with the opposing tooth at the occlusal plane

Mixed dentition radiographs show _____ variability.

high

Which permanent tooth eruption sequence is considered normal?

Canine → first premolar → second premolar

Which permanent tooth eruption sequence is associated with serial extraction cases?

First premolar → canine → second premolar

Which permanent tooth eruption sequence is associated with crowding?

First premolar → second premolar → canine

True or false: Depending on growth and dental age, some children can get permanent teeth early or late

True

True or false: dental age always equals chronological age

False: tooth development does not always match the child’s actual age.

Teeth may erupt _____, _____, ______, or ______ depending on space and eruption path.

Buccally, lingually, mesially, distally

Congenitally missing teeth occur in ______ of the population.

3–5%

Which permanent tooth is most commonly missing?

Maxillary lateral incisor

Which mandibular tooth is most commonly missing?

Mandibular second premolar

True or false: If a primary tooth is missing, the permanent successor is also missing.

True

What is ankylosis?

Fusion of a primary tooth’s root to bone, preventing eruption.

Why does ankylosis become more obvious in mixed dentition?

Adjacent teeth continue vertical eruption, while the ankylosed tooth stays submerged

What is the recommended management for ankylosis?

1. Monitor (50% self‑correct)

2. Extract if exfoliation fails or if it disrupts the occlusal plane

Why do canines or premolars failing to erupt?

Due to lack of space or ectopic path

90% of impacted canines are associated with missing _____ or ______

lateral incisors; peg laterals

What does a peg lateral look like?

A small, conical lateral incisor that disrupts canine guidance and eruption

That ectopic eruption can affect permanent tooth _____ and must be monitored early.

alignment

What is the most common ankylosed primary tooth?

Primary second molar

What are the clinical consequences of ankylosis?

• Loss of arch length

• Tipping of adjacent teeth

• Impaction of permanent successor

• Occlusal plane distortion

When should ankylosed teeth be extracted?

• If exfoliation fails

• If they block permanent eruption

• If they distort the occlusal plane

What is impaction?

Failure of a tooth to erupt into the arch due to lack of space, ectopic path, or obstruction

What is a stationary tooth?

A tooth that fails to erupt despite root development, often due to ankylosis or ectopic eruption

What is ectopic eruption?

A tooth erupting in an abnormal path, often causing root resorption of adjacent teeth

What is the clinical significance of ectopic eruption?

It can cause irreversible damage and must be detected early

True or False: Missing primary teeth are common.

False: they are rare

True or False: Ankylosis becomes more obvious as adjacent teeth erupt.

True

True or False: Ectopic eruption self‑corrects.

False: it often requires intervention