aetiology og malocclusion: skeletal factors Flashcards
3 dimensions of skeletal pattern and describe each
antero-posterior (skeletal class)
vertical: facial proportions
transverse: facial asymmetry, arch width discrepancy
define skeletal CLASS
Classification of the antero-postero relationship of the dental bases which support the teeth and alveolar processes
define skeletal class 1
maxillary dental base lies 2-3mm anterior to mandibular dental base when teeth in occlusion
define skeletal class 2
maxillary dental base more than 2-3mm anterior to mandibular dental base when teeth in occlusion
3 causes of skeletal class 2
- retrognathic mandible
- prognathic maxilla
- both
likely skeletal class if cranial base angle is
a. obtuse
b. acute
a. obtuse: class 2
b. acute: class 3
define skeletal class 3
mandibular dental bases more anterior than the maxillary dental bases
how should the pt be sitting to measure skeletal class
upright with the head in the free postural position
3 causes and % of causes of skeletal class 3
retrognathic maxilla (25%) prognathic mandible (25%) combination of both (50%)
what is dentoalveolar compensation
when the antero-posterior jaw relationship is class 2 or 3 but the soft tissues are favourable –> lips and tongue guide erupting teeth in to better positions than the skeletal class would suggest
describe dentoalveolar compensation in class 2 (3)
retroclined upper incisors
proclination of lower incisors –> class I incisors, class 2 base
both
dentoalveolar compensation in class 3 (3)
proclined upper incisors
retroclination of lower incisors
both
what radiograph is needed to see skeletal class
lat cephalometric
what 2 points on the faced are used in cephalometric analysis
sella (depression in sphenoid bone containing pituitary gland)
nasion (intersection of nasal bone and frontal bone)
what do SNA and SNB measure
SNA: how far forward/back upper incisors are (A= dental bases of maxillary incisors
SNB: how far forward/ back lower incisors are (B= dental bases of mandibular incisors)
describe the distribution of cephalometric analysis
normal distribution (most people skeletal class I, very sk3 and sk2 to each side
median ANB (SNA-SNB) (degrees)
3.4 degrees
4 outcomes of diagnosing wrong skeletal class
- failure to achieve desired occlusion
- poor dental aesthetics
- poor facial aesthetics
- instability of result
describe average facial proportions (vertical)
1/3: top of head to top of ear
1/3: top of ear to bottom of nose
1/3: bottom of nose to bottom of chin
describe the frankfort plane
top of eam- inferior margin of orbit. HORIZONTAL
what does the mandibular plane measure
top of eam to bottom of chin
normal value of
a. frankfort-mandibular angle
b. maxillary plane- mandubular angle
a. frankfort-mandibular angle: 27+- 5 degrees
b. maxillary plane- mandubular angle: 27+- 5 degrees
what points mark
a. upper anterior face height B
b. lower anterior face height A
a. upper anterior face height: superior margin of orbit/ eyebrow - maxillary plane/ bottom of nose
b. lower anterior face height: maxillary plane/ bottom of nose - bottom of chin
which of these changes with face height
lower face height A
usual value of A/B
50-55
2 probs of inc face height
aesthetics: wide separation of lips
malocclusion: reduced overbite/ anterior open bite (negative overbite)
reduced lower face height
a. facial features 2
b. occlusal tendencies 2
c. soft tissues 1
a. facial features 2: reduced lower face height, prominent chin (look overclosed, edentulous)
b. occlusal tendencies 2: increased overbite, hard to correct
c. soft tissues 1: lips completely cover teeth
where frankfort plane and mandibular angle meet
a. on average
b. with reduced face height
c. with increased face height
a. on average: at back of head
b. with reduced face height: way behind back of head
c. with increased face height: anterior to back of head
2 intra oral features associated with asymmetry
centre line discrepancies posterior crossbites (in buccal segments)
