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BDS3 Orthodontics > Aetiology of Malocclusion 1 - Skeletal Causes > Flashcards

Aetiology of Malocclusion 1 - Skeletal Causes Flashcards

1
Q

What are the skeletal aetiological factors?

A
  • Size
  • Shape
  • Relative positions of upper and lower jaws
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2
Q

What are the muscular aetiological factors?

A
  • Form and function of the muscles that surround the teeth i.e. lips, cheeks and tongue
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3
Q

What are the dentoalveolar aetiological factors?

A
  • Size of teeth in relation to size of jaws
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4
Q

Malocclusion prevalence possible explanation?

A
  • Most prevalent in western developed countries
  • Increased in incidence and severity over past 200 years
  • Mixed gene pool
  • High survival rate of young population
  • Decreased jaw function due to dietary refinement
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5
Q

What are the components of 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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6
Q

What is antero-posterior skeletal relationship?

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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7
Q

How to assess antero-posterior jaw relationship?

A
  • Palpate maxilla and mandible on lips
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8
Q

What are the 3 planes of space?

A
  • Antero-posterior
  • Vertical
  • Transverse
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9
Q

What is lateral Cephalometry?

A
  • Standardised lateral radiographs of face and base of skull
  • Reproducible as patient positioned in a cephalostat at a set distance form cone and film
  • Cephalometry is analysis and interpretation of these radiographs
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10
Q

What is the radiographic technique for Cephologram?

A
  • NHP (natural head position) or Frankfurt plane horizontal

ALARA (minimise radiation dose)
- Aluminium soft tissue filter
- Thyroid collar
- Triangular collimation
- Rare earth screen
- LANEX screen
- Fastest film possible (60-70kV)

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11
Q

What is computer digitisation?

A
  • Use digital software to trace and identify landmarks on lateral cephalogram
  • Value in red highlight Eastman analysis (most widely used in UK)
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12
Q

What are Class I Cephalometrics?

A

SNA (relates maxilla to anterior cranial base)
- Average value 81° +/-3°

SNB (relates mandible to anterior cranial base)
- Average 78° +/-3°

ANB (relates mandible to maxilla)
- Average 3° +/-2°

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13
Q

What is class II antero-posterior relationship

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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14
Q

What are the Class II Cephalometrics?

A
  • SNA usually average but may be increased if maxilla prognathic.
  • SNB usually decreased
  • ANB > 5°
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15
Q

What is Class III antero-posterior relationship?

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
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16
Q

What are 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
17
Q

What is dento-alveolar compensation?

A
  • 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.
18
Q

What is dento-alveolar compensation in sever class III malocclusion?

A
  • Proclined upper incisors
  • Retroclined lower incisors
19
Q

How do the planes exhibit in Vertical jaw relationship? Clinical values

A

Frankfurt plane
– Lower orbital rim to superior border of external auditory meatus
Mandibular plane
– Lower border of mandible
Planes normally meet at external occipital protuberance
Upper anterior face height
– Brow ridge (glabella) to base of nose.
Lower anterior face height
– Base of nose (sub nasale) to inferior aspect of chin (soft tissue menton).
Average ratio of LAFH to TAFH = 50%.

20
Q

What are the average Cepholometric values for Vertical jaw reltionship?

A

Frankfurt plane:
– Orbitale to Porion.
Mandibular plane:
– Menton to Gonion.
- Average value of FMPA = 27° +/- 4°
Upper anterior face height:
– Nasion to Anterior Nasal Spine.
Lower anterior face height:
– Anterior Nasal Spine to Menton.
* Average value of LAFH to TAFH = 55%.

21
Q

What are features of Long facial type?

A
  • LAFH to TAFH proportion > 55%
  • FMPA > 31°
  • Steeply inclined mandibular plane
  • Backward mandibular growth rotation
  • Anterior open bite tendency
22
Q

What are features of short facial type?

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

What are 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
24
Q

What is 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
25
Q

What is true mandibular asymmetry?

A
  • Hemi-mandibular hyperplasia/elongation
  • Condylar hyperplasia
26
Q

What is a dental cause of facial asymmetry?

A
  • Displacement of normal mandible due to unilateral cross-bite
  • Whole face may be affected by mild expressions of hemi-facial microsomia
27
Q

What is dento-alveolar disproprotion

A
  • 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

28
Q

What is transverse dentoalveolar compensation?

A
  • Narrow maxilla with flared molars
  • Broad maxilla with upright molars
28
Q

What is transverse dentoalveolar compensation?

A
  • Narrow maxilla with flared molars
  • Broad maxilla with upright molars

BDS3 Orthodontics (10 decks)

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