Aetiology of Malocclusion 1 - Skeletal Causes

Question 1

Give examples of skeletal aetiological factors

Answer 1

Size, shape and relative positions of the upper and lower jaws

Question 2

Give examples of muscular aetiological factors

Answer 2

Form and function of the muscles that surround the teeth ie - lips, cheeks and tongue

Question 3

Give examples of dentoalveolar aetiological factors

Answer 3

Size of the teeth in relation to the size of the jaws

Question 4

What are the components of the facial skeleton?

Answer 4

Maxillary base
Mandibular base
Maxillary and mandibular alveolar processes

Question 5

How does malocclusion arise?

Answer 5

From disharmony between the components of the facial skeleton

Question 6

What factors can affect skeletal variation?

Answer 6

Genetic factors - hereditary shape of face and jaws
Environmental factors - masticatory muscles, head posture
Combination of both

Question 7

Which malocclusion is commonly seen as hereditery?

Answer 7

Class III malocclusion

Question 8

Describe an antero-posterior relationship (Class I)

Answer 8

Mandible related normally to maxilla, such that teeth erupt into class I occlusion
Jaws usually correctly sized but may have bi-maxillary protrusion or retrusion

Question 9

What is lateral cephalometry?

Answer 9

Standardised lateral radiographs of the face and base of skull

Question 10

Why are lateral cephalograms used?

Answer 10

They are reproducible and can be used to monitor treatment and growth
Measurements can be used to analyse the size and shape of the jaws

Question 11

What are the cephalometric angles?

Answer 11

SNA angle relates maxilla to anterior cranial base
SNB angle relates mandible to anterior cranial base
ANB angle is the difference between SNA and SNB, it relates the mandible to maxilla

Question 12

Describe an antero-posterior relationship (Class II)

Answer 12

Mandible placed posteriorly relative to maxilla
Mandible too small (most common), maxilla too large or combo of both
Mandible normally sized but placed too far back due to obtuse cranial base angle
Teeth erupt into post-normal (class II) occlusion

Question 13

What are the average class I cephalometrics?

Answer 13

SNA 81º
SNB 78º
ANB 3º

Question 14

What are the average class II cephalometrics?

Answer 14

SNA usually average
SNB usually decreased
ANB >5º

Question 15

Describe an antero-posterior relationship (class III)

Answer 15

Mandible placed anteriorly relative to maxilla
Maxilla too small (most common), mandible too large, or combo of both
Normally sized jaws but mandible positioned too far forwards due to acute cranial base angle
Teeth erupt into pre-normal (class III) occlusion

Question 16

Describe average class III cephalometrics

Answer 16

Expect SNA to be decreased if maxilla is deficient
SNB often average but may be increased if mandible is prognathic
ANB <1º or negative

Question 17

What is dento-alveolar compensation?

Answer 17

Dento-alveolar structures may disguise underlying skeletal discrepancy
Forces from lips, cheeks and tongue tend to incline teeth towards a position of soft tissue balance

Question 18

Give examples of dento-alveolar compensation seen in a class III malocclusion

Answer 18

Proclined upper incisors
Retroclined lower incisors