Aetiology of malocclusion - skeletal Flashcards
malocclusion
prevalence
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
general aetiological factors (3 groups)
skelatal
muscular
dentoalveolar
skeletal aetiological factos
size
shape
relative positions of the upper and lower jaws
muscular aetiological factors
form and function of the muscles that surround the teeth
i.e. lips, cheek, tongue
dentoalveolar malocclusion aetiological factors
size of teeth in relation to size of the jaws
components of the facial skeleton
- 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
malocclusion results from
disharmony between the components of the facial skeleton
variation in facial skeleton in
- 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.
aetiology of skeletal variation
- 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.
3 planes of aetiological skeletal variation
antero-posterior
vertical
transverse
class I
antero-posterior
- 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.
lateral cephalometry
- 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
how are lateral cephalometry reproducible
- patient positioned in a cephalostat a set distance from the cone and the film
radiographic technique
NHP
ALARA
- Aluminium soft tissue filter
- Thyroid collar
- Triangular collimation
- Rare earth screen
- LANEX screen
- Fastest film possible (60-70kV)
2 ways to analyses lateral cephalometrograms
hand traced on paper - need light box
digitised on a compute
3 cephalometrics to know
- SNA - relates maxilla to anterior cranial base
- SNB - relates mandibule to anterior cranial base
- ANB - relates mandible to maxilla
SNA
relates maxiall to anterior cranial base
SNB
relates mandible to anterior cranial base
ANB
relates mandible to maxilla
SNA for class I
Ave value 81o +/- 3o
SNB class I
ave value 78o +/- 3o
ANB class I
Ave value 3o +/- 2o
class I cephalometrics
- 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
antero-posterior class II
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
S
sella
N
nasion
A and B
curvature above maxillla and below mandible
class II cephalometrics
- SNA usually average but may be increased if maxilla prognathic.
- SNB usually decreased.
- ANB > 5°
antero-posterior class III
- 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
class III Cephalometrics
- Expect SNA to be decreased if maxilla deficient.
- SNB often average but may be increased if mandible prognathic.
- ANB < 1° or negative
dento-alveolar compensation for malocclsuion
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
2 main average clinical values to assess for vertical jaw relationships
frankfurt plane and mandibular plane
upper anterior face height and lower anterior face height
frankfort plane
lower orbital rim to superior border of external auditory meatus
manibular plane
lower border of mandible
Frankfort-Manibular Planes Angle
planes normally meet at external occipital protuberance
upper anterior face height
brown ridge (glabellla) to base of nose
lower anterior face height
base of nose (sub nasale) to inferior aspect of chin (soft tissue menton)
LAFH to TAFH average ratio
50%
common dento-alveolar compensation for class III
proclined upper incisors
retroclined lower incisors
2 main average cephalometric values to assess
frankfurt and mandibular plane angle FMPA
upper anterior face height to lower anterior face height LAFH to TAFH
cephalometric frankfurt plane
orbitale to porion
cephalometric value mandibular plane
menton to gonion
cephalometric upper anterior face height
nasion to anterior nasal spine
cephalometric lower anterior face height
anterior nasal spine to menton
average cephalometric value for FMPA
27o +/- 4o
average cephalometric value of LAFH to TAFH
55%
long facial type vertical jaw relationship
- LAFH to TAFH proportion > 55%
- FMPA > 31°
- Steeply inclined mandibular plane.
- Backward mandibular growth rotation.
- Anterior open bite tendency.
porion
opening of external acoustic meatus
short facial type - vertical jaw relationship
- LAFH to TAFH proportion < 55%
- FMPA < 23°
- Tendency to parallelism of jaws.
- Forward mandibular growth rotation.
- Deep overbite tendency
2 types of transverse skeletal malocclusions
arch width discrepancies
mandibular displacement
arch width discrepancies
Disproportion of maxillary and mandibular dental arches.
- Causes unilateral or bilateral buccal segment cross-bites.
- Often exaggerated by antero-posterior discrepancies
mandibular displacement
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.
transverse dentoalveolar compensation
moulded so teeth meet together reasonably well
transverse arch width discrepanacies causes
mandibular displacement
- Mandible forced to deviate to one side to achieve position of inter-cuspation.
Possible association with TMD.
dental cause of facial asymmetries
displacement of normal mandible due to unilateral cross-bite
check facial asymmetries
- dental
- skeletal
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
2 classes of facial asymmetries
dental cause
true mandibular asymmetry
true mandibular asymmetry
Hemi-mandibular hyperplasia/elongation.
Condylar hyperlasia.
- Whole face may be affected by mild expressions of hemi-facial microsomia.
arch size discrepancies due to
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)
crowding caused by
- Small jaws, normally sized teeth
- Large teeth (macrodontia) unusual
dento alveolar disproportion
- Discrepancy between size of teeth and jaws to accomodate them
spacing causes
- Large jaws, normally sized teeth
- Small teeth (microdontia)
- Hypodontia (absent teeth)
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