Skeletal Causes of Malocclusion Flashcards
what is the aetiology of skeletal variation?
- genetic and environmental factors
- hereditary component to shape of face & jaws
what environmental factors could cause differing skeletal variations?
- masticatory muscles
- mouth breathing
- head posture
What is an antero-posterior relationship in relevance to skeletal variation?
- mandible related normally to maxilla
- jaws usually correctly sized but may have bi-maxillary protrusion or retrusion
what is the SNA on a lateral cephalogram?
relates maxilla to anterior cranial base
what is the SNB on a lateral cephalogram?
relates mandible to anterior cranial base
what is the ANB on a lateral cephalogram?
relates mandible to maxilla
what is the average SNA value on a lateral cephalogram for a class 1 occlusion?
81º +/- 3º
what is the average SNB value on a lateral cephalogram for a class 1 occlusion?
78º +/- 3º
what is the average ANB value on a lateral cephalogram for a class 1 occlusion?
3º +/- 2º
What is a class 2 antero-posterior relationship
mandible placed posteriorly relative to maxilla
why does a class II occlusion occur?
- mandible too small/maxilla too large or combination of bone
- mandible normally sized but placed too far back due to obtuse cranial base angle
how do the SNA/SNB/ANB angles differ in class II occlusion from the norm?
SNA = usually average but may be increased if maxilla protrudes
SNB = usually decreased
ANB = >5º
what is a class III antero-posterior relationship?
mandible placed anteriorly relative to maxilla
why does a class III antero-posterior relationship occur?
- maxilla too small/mandible too large or combination of both
- mandible positioned too far forwards due to acute cranial base angle
how do the SNA/SNB/ANB angles differ in class III occlusion from the norm?
SNA = decreased if maxilla deficient
SNB = often average but may be increased if mandible prognathic
ANB = <1º or negative
What is the Frankfurt plane?
lower orbital rim to superior border of external auditory meatus
what is the mandibular plane?
lower border of mandible
where do the Frankfurt plane and Mandibular plane normally meet?
occipital protuberance
What is the average ratio of LAFH to TAFH?
50%
How is the upper anterior face height measured?
brow ridge (glabella) to base of nose
How is the lower anterior face height measured?
base of nose (sub nasale) to inferior aspect of chin (soft tissue menton)
what is the average value of FMPA?
27º +/- 4º
If a patient has a ‘long facial type’ what would the LAFH to TAFH be?
> 55%
If a patient has a ‘long facial type’ what would the FMPA be?
> 31º
What are the clinical signs of a long face?
- steeply inclined mandibular plane
- backward mandibular growth rotation
- anterior open bite
If a patient has a ‘short facial type’ what would the LAFH to TAFH be?
< 55%
If a patient has a ‘short facial type’ what would the FMPA be?
< 23º
What are the clinical signs of a short facial type?
- parallelism of jaws
- forward mandibular growth rotation
- deep overbite
What do arch width discrepancies cause?
Unilateral or bilateral segment cross-bites
what is a transverse jaw relationship?
Disproportion of maxillary and mandibular dental arches
(e.g maxilla sits out from mandible vice versa)
What is the dental cause of facial asymmetry in relation to jaw?
displacement of normal mandible due to unilateral cross-bite
