Aetiology of Malocclusion 1 - Skeletal Causes Flashcards
What are the skeletal aetiological factors?
- Size
- Shape
- Relative positions of upper and lower jaws
What are the muscular aetiological factors?
- Form and function of the muscles that surround the teeth i.e. lips, cheeks and tongue
What are the dentoalveolar aetiological factors?
- Size of teeth in relation to size of jaws
Malocclusion prevalence possible explanation?
- 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
What are the components of 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.
What is antero-posterior skeletal relationship?
- 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
How to assess antero-posterior jaw relationship?
- Palpate maxilla and mandible on lips
What are the 3 planes of space?
- Antero-posterior
- Vertical
- Transverse
What is lateral Cephalometry?
- 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
What is the radiographic technique for Cephologram?
- 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)
What is computer digitisation?
- Use digital software to trace and identify landmarks on lateral cephalogram
- Value in red highlight Eastman analysis (most widely used in UK)
What are Class I Cephalometrics?
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°
What is class II antero-posterior relationship
- 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
What are the Class II Cephalometrics?
- SNA usually average but may be increased if maxilla prognathic.
- SNB usually decreased
- ANB > 5°
What is Class III antero-posterior relationship?
- 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
What are 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
What is dento-alveolar compensation?
- 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.
What is dento-alveolar compensation in sever class III malocclusion?
- Proclined upper incisors
- Retroclined lower incisors
How do the planes exhibit in Vertical jaw relationship? Clinical values
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%.
What are the average Cepholometric values for Vertical jaw reltionship?
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%.
What are features of Long facial type?
- LAFH to TAFH proportion > 55%
- FMPA > 31°
- Steeply inclined mandibular plane
- Backward mandibular growth rotation
- Anterior open bite tendency
What are features of short facial type?
- LAFH to TAFH proportion < 55%
- FMPA < 23°
- Tendency to parallelism of jaws.
- Forward mandibular growth rotation.
- Deep overbite tendency
What are arch width discrepancies?
- Disproportion of maxillary and mandibular dental arches.
- Causes unilateral or bilateral buccal segment cross-bites.
- Often exaggerated by antero-posterior discrepancies
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 to one side to
achieve position of inter-cuspation - Possible association with TMD
What is true mandibular asymmetry?
- Hemi-mandibular hyperplasia/elongation
- Condylar hyperplasia
What is a dental cause of facial asymmetry?
- Displacement of normal mandible due to unilateral cross-bite
- Whole face may be affected by mild expressions of hemi-facial microsomia
What is dento-alveolar disproprotion
- 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
What is transverse dentoalveolar compensation?
- Narrow maxilla with flared molars
- Broad maxilla with upright molars
What is transverse dentoalveolar compensation?
- Narrow maxilla with flared molars
- Broad maxilla with upright molars
