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BDS3 Orthodontics > Aetiology of malocclusion - skeletal > Flashcards

Aetiology of malocclusion - skeletal Flashcards

1
Q

malocclusion

prevalence

A

Most prevalent in western developed countries

  • Has increased in incidence and severity over the past 200 years
    • Mixed gene pool?
    • High survival rate of young population?
  • Decreased jaw function due to dietary refinement
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2
Q

general aetiological factors (3 groups)

A

skelatal

muscular

dentoalveolar

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3
Q

skeletal aetiological factos

A

size

shape

relative positions of the upper and lower jaws

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4
Q

muscular aetiological factors

A

form and function of the muscles that surround the teeth

i.e. lips, cheek, tongue

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5
Q

dentoalveolar malocclusion aetiological factors

A

size of teeth in relation to size of the jaws

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6
Q

components of the facial skeleton

A
  • Maxillary base
  • Mandibular base
  • Maxillary and mandibular alveolar processes
    • The maxillary complex is attached to the anterior cranial base while the mandible articulates with the posterior cranial base
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7
Q

malocclusion results from

A

disharmony between the components of the facial skeleton

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8
Q

variation in facial skeleton in

A
  • The size and shape of the maxilla, mandibular and alveolar processes and their relationship to each other in all three planes of space.
  • The size and angle of the cranial base.
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9
Q

aetiology of skeletal variation

A
  • Genetic and environmental factors.
  • Familial studies show strong hereditary component to shape of face and jaws.
  • Certain features of malocclusion are hereditary e.g. class III
  • Possible environmental factors:
    • Masticatory muscles
    • Mouth breathing
    • Head posture
  • Long standing debate in literature over relative importance of genetics vs environment.
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10
Q

3 planes of aetiological skeletal variation

A

antero-posterior

vertical

transverse

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11
Q

class I

antero-posterior

A
  • 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.
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12
Q

lateral cephalometry

A
  • Standardised lateral radiographs of the face and base of skull
  • Reproducible - patient positioned in a cephalostat a set distance from the cone and the film
  • Cephalometry is the analysis and interpretation of these radiographs
  • Many different methods of analysis have been published
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13
Q

how are lateral cephalometry reproducible

A
  • patient positioned in a cephalostat a set distance from the cone and the film
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14
Q

radiographic technique

A

NHP

ALARA

  • Aluminium soft tissue filter
  • Thyroid collar
  • Triangular collimation
  • Rare earth screen
  • LANEX screen
  • Fastest film possible (60-70kV)
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15
Q

2 ways to analyses lateral cephalometrograms

A

hand traced on paper - need light box

digitised on a compute

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16
Q

3 cephalometrics to know

A
  • SNA - relates maxilla to anterior cranial base
  • SNB - relates mandibule to anterior cranial base
  • ANB - relates mandible to maxilla
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17
Q

SNA

A

relates maxiall to anterior cranial base

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18
Q

SNB

A

relates mandible to anterior cranial base

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19
Q

ANB

A

relates mandible to maxilla

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20
Q

SNA for class I

A

Ave value 81o +/- 3o

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21
Q

SNB class I

A

ave value 78o +/- 3o

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22
Q

ANB class I

A

Ave value 3o +/- 2o

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23
Q

class I cephalometrics

A
  • SNA relates maxilla to anterior cranial base – Ave value 81° +/- 3°
  • SNB relates mandible to anterior cranial base – Ave value: 78° +/- 3°
  • ANB relates mandible to maxilla – Ave value: 3° +/- 2
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24
Q

antero-posterior class II

A

Mandible placed posteriorly relative to maxilla.

  • Mandible too small (most commonly), maxilla too large, or combination of both.
  • Mandible normally sized but placed too far back due to obtuse cranial base angle.
  • Teeth erupt into post-normal (class II) occlusion
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25
Q

S

A

sella

26
Q

N

A

nasion

27
Q

A and B

A

curvature above maxillla and below mandible

28
Q

class II cephalometrics

A
  • SNA usually average but may be increased if maxilla prognathic.
  • SNB usually decreased.
  • ANB > 5°
29
Q

antero-posterior class III

A
  • Mandible placed anteriorly relative to maxilla.
  • Maxilla too small (most commonly), mandible too large, or combination 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

30
Q

class III Cephalometrics

A
  • Expect SNA to be decreased if maxilla deficient.
  • SNB often average but may be increased if mandible prognathic.
  • ANB < 1° or negative
31
Q

dento-alveolar compensation for malocclsuion

A

Dento-alveolar structures may disguise underlying skeletal discrepancy esp Class III

  • Forces from lips, cheeks and tongue tend to incline teeth towards a position of soft tissue balance.

most noticiable a-p, transversely but can also affect vertical

32
Q

2 main average clinical values to assess for vertical jaw relationships

A

frankfurt plane and mandibular plane

upper anterior face height and lower anterior face height

33
Q

frankfort plane

A

lower orbital rim to superior border of external auditory meatus

34
Q

manibular plane

A

lower border of mandible

35
Q

Frankfort-Manibular Planes Angle

A

planes normally meet at external occipital protuberance

36
Q

upper anterior face height

A

brown ridge (glabellla) to base of nose

37
Q

lower anterior face height

A

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

38
Q

LAFH to TAFH average ratio

A

50%

39
Q

common dento-alveolar compensation for class III

A

proclined upper incisors

retroclined lower incisors

40
Q

2 main average cephalometric values to assess

A

frankfurt and mandibular plane angle FMPA

upper anterior face height to lower anterior face height LAFH to TAFH

41
Q

cephalometric frankfurt plane

A

orbitale to porion

42
Q

cephalometric value mandibular plane

A

menton to gonion

43
Q

cephalometric upper anterior face height

A

nasion to anterior nasal spine

44
Q

cephalometric lower anterior face height

A

anterior nasal spine to menton

45
Q

average cephalometric value for FMPA

A

27o +/- 4o

46
Q

average cephalometric value of LAFH to TAFH

A

55%

47
Q

long facial type vertical jaw relationship

A
  • LAFH to TAFH proportion > 55%
  • FMPA > 31°
  • Steeply inclined mandibular plane.
  • Backward mandibular growth rotation.
  • Anterior open bite tendency.
48
Q

porion

A

opening of external acoustic meatus

49
Q

short facial type - vertical jaw relationship

A
  • LAFH to TAFH proportion < 55%
  • FMPA < 23°
  • Tendency to parallelism of jaws.
  • Forward mandibular growth rotation.
  • Deep overbite tendency
50
Q

2 types of transverse skeletal malocclusions

A

arch width discrepancies

mandibular displacement

51
Q

arch width discrepancies

A

Disproportion of maxillary and mandibular dental arches.

  • Causes unilateral or bilateral buccal segment cross-bites.
  • Often exaggerated by antero-posterior discrepancies
52
Q

mandibular displacement

A

Occurs where inter-arch width discrepancy causes upper and lower posterior teeth to meet cusp to cusp.

  • Mandible forced to deviate to one side to achieve position of inter-cuspation.

Possible association with TMD.

53
Q

transverse dentoalveolar compensation

A

moulded so teeth meet together reasonably well

54
Q

transverse arch width discrepanacies causes

A

mandibular displacement

  • Mandible forced to deviate to one side to achieve position of inter-cuspation.

Possible association with TMD.

55
Q

dental cause of facial asymmetries

A

displacement of normal mandible due to unilateral cross-bite

56
Q

check facial asymmetries

  • dental
  • skeletal
A

relax mandible Pt

get into RCP

touch teeth to first contact

  • slide to cross bite –> displacement
  • close directly into cross bite - have true mandibular asymmetry - skeletal - surgical Tx
57
Q

2 classes of facial asymmetries

A

dental cause

true mandibular asymmetry

58
Q

true mandibular asymmetry

A

Hemi-mandibular hyperplasia/elongation.

Condylar hyperlasia.

  • Whole face may be affected by mild expressions of hemi-facial microsomia.
59
Q

arch size discrepancies due to

A

dento alveolar disproportion

Discrepancy between size of teeth and jaws.

  • Crowding caused by:
    • Small jaws, normally sized teeth
    • Large teeth (macrodontia).
  • Spacing caused by:
    • Large jaws, normally sized teeth
    • Small teeth (microdontia)
    • hypodontia (absent teeth)
60
Q

crowding caused by

A
  • Small jaws, normally sized teeth
  • Large teeth (macrodontia) unusual
61
Q

dento alveolar disproportion

A
  • Discrepancy between size of teeth and jaws to accomodate them
62
Q

spacing causes

A
  • Large jaws, normally sized teeth
  • Small teeth (microdontia)
  • Hypodontia (absent teeth)
    *

BDS3 Orthodontics (13 decks)

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