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BDS3 ORTHODONTICS > Skeletal Causes of Malocclusion > Flashcards

Skeletal Causes of Malocclusion Flashcards

1
Q

what is the aetiology of skeletal variation?

A
  • genetic and environmental factors
  • hereditary component to shape of face & jaws
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2
Q

what environmental factors could cause differing skeletal variations?

A
  • masticatory muscles
  • mouth breathing
  • head posture
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3
Q

What is an antero-posterior relationship in relevance to skeletal variation?

A
  • mandible related normally to maxilla
  • jaws usually correctly sized but may have bi-maxillary protrusion or retrusion
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4
Q

what is the SNA on a lateral cephalogram?

A

relates maxilla to anterior cranial base

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5
Q

what is the SNB on a lateral cephalogram?

A

relates mandible to anterior cranial base

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6
Q

what is the ANB on a lateral cephalogram?

A

relates mandible to maxilla

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7
Q

what is the average SNA value on a lateral cephalogram for a class 1 occlusion?

A

81º +/- 3º

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8
Q

what is the average SNB value on a lateral cephalogram for a class 1 occlusion?

A

78º +/- 3º

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9
Q

what is the average ANB value on a lateral cephalogram for a class 1 occlusion?

A

3º +/- 2º

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10
Q

What is a class 2 antero-posterior relationship

A

mandible placed posteriorly relative to maxilla

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11
Q

why does a class II occlusion occur?

A
  • mandible too small/maxilla too large or combination of bone
  • mandible normally sized but placed too far back due to obtuse cranial base angle
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12
Q

how do the SNA/SNB/ANB angles differ in class II occlusion from the norm?

A

SNA = usually average but may be increased if maxilla protrudes
SNB = usually decreased
ANB = >5º

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13
Q

what is a class III antero-posterior relationship?

A

mandible placed anteriorly relative to maxilla

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14
Q

why does a class III antero-posterior relationship occur?

A
  • maxilla too small/mandible too large or combination of both
  • mandible positioned too far forwards due to acute cranial base angle
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15
Q

how do the SNA/SNB/ANB angles differ in class III occlusion from the norm?

A

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

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16
Q

What is the Frankfurt plane?

A

lower orbital rim to superior border of external auditory meatus

17
Q

what is the mandibular plane?

A

lower border of mandible

18
Q

where do the Frankfurt plane and Mandibular plane normally meet?

A

occipital protuberance

19
Q

What is the average ratio of LAFH to TAFH?

A

50%

20
Q

How is the upper anterior face height measured?

A

brow ridge (glabella) to base of nose

21
Q

How is the lower anterior face height measured?

A

base of nose (sub nasale) to inferior aspect of chin (soft tissue menton)

22
Q

what is the average value of FMPA?

A

27º +/- 4º

23
Q

If a patient has a ‘long facial type’ what would the LAFH to TAFH be?

A

> 55%

24
Q

If a patient has a ‘long facial type’ what would the FMPA be?

A

> 31º

25
Q

What are the clinical signs of a long face?

A
  • steeply inclined mandibular plane
  • backward mandibular growth rotation
  • anterior open bite
26
Q

If a patient has a ‘short facial type’ what would the LAFH to TAFH be?

A

< 55%

27
Q

If a patient has a ‘short facial type’ what would the FMPA be?

A

< 23º

28
Q

What are the clinical signs of a short facial type?

A
  • parallelism of jaws
  • forward mandibular growth rotation
  • deep overbite
29
Q

What do arch width discrepancies cause?

A

Unilateral or bilateral segment cross-bites

30
Q

what is a transverse jaw relationship?

A

Disproportion of maxillary and mandibular dental arches
(e.g maxilla sits out from mandible vice versa)

31
Q

What is the dental cause of facial asymmetry in relation to jaw?

A

displacement of normal mandible due to unilateral cross-bite

BDS3 ORTHODONTICS (8 decks)

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