Aetiology of Malocclusion

Question 1

What is the skeletal pattern?

Answer 1

A classification of the antero-posterior relationship of the dental bases which support the teeth and alveolar processes.
The relationship of the maxilla to the mandible, it really matters and strongly affects treatment. It is three dimensional:
-Antero-posterior
-Vertical
-Transverse

Question 2

What is the aetiology of malocclusion?

Answer 2

 Skeletal factors
 Soft tissues
 Dento-alveolar factors
– Crowding
-Tooth arch ratio
 Local factors
– Missing teeth, extra teeth, funny teeth, ectopic teeth,
cysts, labial frena, digit habits, trauma.

Question 3

What are molar and incisal classes?

Answer 3

NOT the same as skeletal class!

Question 4

What is skeletal class 1?

Answer 4

The maxillary dental base lies 2-3mm anterior to the mandibular dental base when the teeth are in occlusion.

Question 5

What is skeletal class 2?

Answer 5

The mandibular dental base lies more posteriorly to the maxillary dental base than in the Class I case.

Question 6

What is skeletal class 3?

Answer 6

The mandibular dental base lies more anterior to the maxillary dental base than in the Class I case. (combination of retrognathic maxilla and prognathic mandible is seen in 50% of cases).

Question 7

What does dento-alveolar compensation mean?

Answer 7

When the A/P jaw relationship is Sk 2 or Sk 3 but the soft tissues are favourable, the lips and tongue tend to guide the erupting teeth into better positions than the Skeletal Class would suggest. This compensation means you cannot tell the skeletal class just by looking at the relationship of the teeth. Therefore best to use cephalometric lateral skull radiographs.

Question 8

How do you work out ANB?

Answer 8

SNA - SNB.
Negative number = Sk 3
Positive = Sk 2
around 3.4 = the average aka Sk1

Question 9

Why does skeletal class matter?

Answer 9

Incorrect diagnosis can lead to entirely the wrong treatment:
–Failure to achieve the desired occlusion
–Poor dental aesthetics
–Poor facial aesthetics
–Instability of the result

Question 10

What is the FMPA (Frankfort, Mandibular plane angle)?

Answer 10

27 degrees +- 5 degrees

Question 11

What is the MMA? (Maxillary Mandibular Plane Angle)

Answer 11

27 degrees +- 5 degrees

The upper anterior face height should equal the lower anterior face height.

Question 12

What does Increased Lower Face Height /

High FMPA / MMA mean?

Answer 12

>32 degrees = FMPA/MMA
Occlusal tendencies:
• Reduced overbite or anterior open
bite
Soft tissues
•Wide separation of the lips

Question 13

What does a reduced lower face height mean?

Answer 13

MMA

Question 14

How can you tell the vertical face dimensions clinically?

Answer 14

Average angle: two lines (Frankfort plane and mandibular plane) meet at the back of the head.
Reduced lower face height: Lines meet way behind the head
Increased lower face height: Lines meet anterior to the back of the head

Question 15

Can a person have skeletal discrepancies in more than one plane?

Answer 15

Yes.

Question 16

What comes under the transverse skeletal pattern heading?

Answer 16

– Facial asymmetry: (may require jaw surgery) 
(• Centre-line discrepancies
• Posterior crossbites in the
buccal segments)
– Arch width discrepancy