Aetiology of Malocclusion Flashcards
What are the components of the facial skeleton?
- maxillary base
- mandibular base
- mandibular and maxillary alveolar processes
Malocclusion aetiological factors
- skeletal: size, shape and relatively positions of upper and lower jaws
- muscular: form and function of muscles that surround the teeth e.g. lips, cheeks and tongue
- dentoalveolar: size of the teeth in relation to the size of the jaws
components of the facial skeleton
- mandibular base
- maxillary base
- maxillary and alveolar processes
aetiology of skeletal variation
genetic factors
environmental factors
- masticatory muscles
- mouth breathing
- head posture
Class 1 AP definition
- mandible related normally to maxilla
- jaws are correctly sized
What is a class II occlusion?
- anteroposterior relationship
- mandible placed posteriorly relative to maxilla
- mandible too small (most common) or maxilla too large or both
OR - mandible normally sized but placed too far back due to obtuse cranial base angle
What is the SNA?
-sella-nasion A point angle
-relates the maxilla to anterior cranial base
avg value 81 +/- 3 degrees
What is the SNB?
-Sella-nasion B point angle
-relates the mandible to the anterior cranial base
avg value 78 +/- 3 degrees
What is the ANB
- A-point-nasion B-point angle
- relates the mandible to maxilla
avg value 3 degrees +/- 2
What are the typical cephalometrics of a class II occlusion?
- SNA usually average
- SNB usually decreased
- ANB> 5 degrees
What are the features of a class III occlusion?
- mandible placed anteriorly relative to maxilla
- maxilla too small (most common) or mandible too large or both
- or normally sized jaws but mandible positioned too far forwards as a result of acute cranial base angle
outline the typical cephalometrics of a class III occlusion
- expect SNA to be decreased
- SNB often average
- ANB <1 degrees or negative
In what ways does the facial skeleton vary?
- antero-posterior
- vertical
- transverse
What are the planes looked at when assessing the vertical jaw relationship?
Frankfort plane
- lower orbital rim to superior border of external auditory meatus
Mandibular plane
- lower border of mandible
where do the mandibular and Frankfort planes usually meet at?
the external occipital protuberance
What are the borders for the upper and lower anterior face heights?
Upper anterior face height
- brow ridge to base of nose
Lower anterior face height
- base of nose to inferior aspect of chin
What is the average ratio of the lower anterior face height to the total anterior face height?
55%
What is the average value of the FMPA (Frankfort-mandibular plane angles)?
27 +/- 4 degrees
What are the features of a short facial type?
- LAFH to TAFH <55%
- FMPA < 23 degrees
- deep overbite tendency
- tendency to parallelism of jaws
What is meant by the term ‘arched width discreptancies’?
- disproportional mandibular and maxillary dental arches
- leads to unilateral or bilateral buccal segment cross-bites
- often exaggerated by anteroposterior discreptancies
What is mandibular displacement?
- occurs where inter-arch width discrepancy causes upper and lower posterior teeth to meet cusp to cusp
- mandible forced to deviate from one side to achieve position of inter-cuspatiion or centric occlusion
- may be associated with TMD
What is dento-alveolar disproportion?
Discrepancy between size of teeth and jaws
What causes crowding?
- small jaw, normal sized teeth
OR - large teeth (macrodontia)
what causes spacing in the dentition?
- Large jaws, normal sized teeth
OR - small teeth (microdontia)
