Aetiology of Malocclusion - Skeletal & Soft Tissue Factors

Question 1

3 main factors affecting occlusal development?

Answer 1

1. Skeletal factors (size, shape and relative positions of jaws)
2. Soft tissue factors (form and function of the muscles which surround teeth)
3. Dental factors (size of dentition in relation to jaw size)

Question 2

What can the variation in the AP relationship between the upper and lower jaw be due to?

Answer 2

• Variation in size of jaws

- Variation in the position of jaws in relation to the cranial base

Question 3

How can transverse skeletal relationship affect occlusion?

Answer 3

• If the lower jaw is wider, buccal crossbite arises

- If the lower jaw is too narrow, lingual occlusion of the lower teeth arises

Question 4

How can transverse vertical relationship affect occlusion? What scenarios lead to AOB or deep overbite?

Answer 4

• Mandible with a high gonial angle and posterior growth rotation tends to produce a longer vertical dimension of the face (and AOB is severe cases)
• Mandible with low gonial angle and anterior growth rotation tends to produce a shorter vertical dimension (and deep overbite)

Question 5

How can lip form affect occlusion?

Answer 5

• If the lips are full and everted, the underlying teeth may be proclined
• If the lips are thin and vertical, the underlying teeth may be retroclined

Question 6

How can lip line affect occlusion?

Answer 6

• The level at which lips meet is called the lip line
• Ideally lower lip covers incisor third of upper 1s
• In skeletal class II, high lower lip may function behind the upper incisors proclining them (causing Class II div 1)
• In other skeletal class II cases, high lower lip functions in front of the upper 1 causing retroclination (Class II div 2)

Question 7

How can tongue position and size affect occlusion?

Answer 7

• If the tongue is large and forward in position it can procline the upper and lower incisors (bimaxillary proclination) causing AOB or incomplete overbite
• Where the lips are incompetent the tongue can be used to produce an anterior oral seal during swallowing

Question 8

What is the neutral zone?

Answer 8

The lips and cheeks function outside and the tongue within the dental arch has led to the concept of neutral zone between the inner and outer perimeters of the arches.
This is where the forces of the lips, cheeks and tongue are in balance which keep the teeth in position

Question 9

When does digit sucking start to affect the dentition?

Answer 9

If it persists for over 6 hours per 24 hours

Defects will disappear if the habit ceases by 7-8 yrs

Question 10

What are the affects of persistent digit sucking on the dentition?

Answer 10

• Asymmetrical AOB or incomplete overbite
• Increase in OJ due to proclination of upper 1s (lower 1s may be retroclined)
• Upper arch narrowed by pressure from cheeks and depression of tongue - this causes a cusp to cusp relationship and then mandibular displacement laterally forming a unilateral buccal crossbite
• Defects will disappear if the habit ceases by 7-8 yrs