3 - Aetiology of malocclusion Flashcards
What are the general aetiology factors of malocclusion?
- skeletal, the shape and relative position of jaws
- muscular, form and function of muscles that surround teeth
- dentoalveolar, size of teeth in relation to jaws
What are the components of the facial skeleton?
- maxillary base
- mandibular base
- maxillary and mandibular alveolar bases
- maxillary complex
What can cause variation in the facial skeleton?
- size and shape of maxilla, mandible and alveolar processes
- size and angle of cranial base
What can cause skeletal variation?
- genetic and environmental factors
- strong hereditary features in class III
What are environmental factors that can affect malocclusion?
- masticatory muscles
- mouth breathing (increased vertical dimension causes teeth to erupt more)
- head posture
Define SNA.
Value between maxilla and anterior cranial base
Define SNB.
Value between mandible and anterior cranial base
Define ANB.
Value between mandible and maxilla
What is the average SNA value for class I?
81° +/- 3°
What is the average SNB value for class I?
78° +/- 3°
What is the average ANB value for class I?
3° +/- 2°
Define S in SNA.
Sella turcica
Define N in SNA.
Nasion
Define A in SNA.
Maximum concavity of upper incisor
Define B in SNB.
Maximum concavity of lower incisor
How does class II present in cephalometrics?
- SNA usually average but can be increased if maxilla is prognathic
- SNB usually decreased
- ANB > 5°
How does class III present in cephalometrics?
- SNA decreased if maxilla deficient
- SNB average or increased if mandible prognathic
- ANB < 1° or negative
What is dentoalveolar compensation?
- dentoalveolar structures can disguise underlying skeletal discrepancies
- muscular forces can incline teeth to appear closer together
- ie. retroclined lowers by muscular strap
What are the average vertical jaw relationship values?
Lower anterior face height = 55% of total anterior face height
Where do you measure UAFH?
Glabella to base of nose
Where do you measure LAFH?
Base of nose to inferior aspect of chin
What is the average FMPA?
27° +/- 4°
What is the FMPA?
- Frankfort plane (inferior of orbit to EAM)
- mandibular plane (menton to gonion)
- angle between these 2 planes
What are the features of long face syndrome?
- convex profile
- incompetent lips (can appear to be straining)
- steeply inclined mandibular plane
- anterior open bite tendency
- backward mandibular growth rotation
What are the cephalometric values for long face syndrome?
- LAFH > 55%
- FMPA > 31°
What are the features of a short face type?
- parallelism of jaws
- forward mandibular growth rotation
- deep overbite tendency
What are the cephalometric values for short facial type?
- LAFH < 55%
- FMPA < 23°
Describe arch width discrepancies.
- disproportion of maxillary and mandibular arches
- unilateral or bilateral segment crossbites
- can be exaggerated by AP discrepancies
What causes mandibular displacement?
- inter-arch width discrepancies causes teeth to meet cusp to cusp
- mandible is forced to deviate to achieve ICP
- can be associated with TMD
What is transverse dentoalveolar compensation?
- the molars in the upper can tilt in order to achieve ICP
- narrow maxilla can lead to flared molars
- broad maxilla can lead to more upright molars
What are the causes of facial asymmetry?
- dental, unilateral crossbite can displace mandible
- true mandibular asymmetry
What are the different types of true mandibular asymmetry?
- hemimandibular hyperplasia
- condylar hyperplasia
What can cause dento-alveolar disproportion?
Crowding
- small jaws and normal sized teeth
- macrodontia
Spacing
- large jaws and normal sized teeth
- microdontia (also linked to missing teeth)
