21. What do these studies tell us?
• The most unattractive facial profiles are long
face profiles
• Most Class II malocclusions are vertically
normal or excessive
• Therefore, control in the vertical dimension is
vitally important in orthodontic treatment
22. Importance of Vertical Control
• Recognized by Professor Arne Bjork
– 1951-1965 – Chairman of the orthodontic
department at the Royal College of Dentistry in
Copenhagen, Denmark
– Authored a study in which he superimposed
cephalometric x-rays on upper and lower metallic
implants placed in 248 untreated, growing
children
23. Bjork’s study
• No treatment performed
• Records taken yearly
• Implants provide a reliable method of
superimposition
24. Importance of this study
• Can never be duplicated due to ethical
concerns
– Not treating malocclusions in a timely manner is
now unethical
– Placing implants in children for observation only is
now unethical
25. Results
• Condyle seems to be the driving force behind
craniofacial development
• Condylar growth direction depends on the
location of the growth cells on the head of the
condyle
– This is an inherited trait
26. Cellular proliferation
• If it occurs on the
anterior surface of the
head of the condyle:
– Mandible will rotate in a
forward (counter-
clockwise) direction
Chin moves forward
with growth
27. Cellular proliferation, continued
• If it occurs on the
posterior surface of the
head of the condyle:
– Mandible will rotate in a
backward (clockwise)
direction
Chin moves down with
growth
29. Facts about muscle strength
• 85% of the population are predominately
strong muscled
• Occlusal force can be 6 times more powerful
in strong muscled patients than in weak
muscled patients
– Bite opening is more easily induced in weak
muscled patients
30. Location of growth cells
• Can be anywhere on the
condylar head
• Most patients have both
forward and backward
rotation characteristics
– The most difficult ortho cases
are extreme forward and
especially extreme backward
rotators
31.
32. Implications of Bjork’s study
• Muscles of mastication
exert pressure and
tension on different
areas of the mandible
depending on condylar
growth direction
33. Implications, continued
• Resorption and
apposition of bone, and
therefore the
morphology of the
mandible, differs
depending on condylar
growth direction
34. Conclusion
• Growth direction can be
predicted based on
mandibular morphology
– This is a very valuable
diagnostic tool
35. How does this affect treatment?
• Most orthodontic mechanics are extrusive
• Molar extrusion exceeding the amount
associated with normal growth can lead to
excessive backward mandibular rotation
– This is to be avoided because long faces are very
undesirable from an esthetic standpoint
36. Treatment, continued
• Strong muscled patients usually easily resist
the extrusive components of mechanics
• Weak muscled patients are often susceptible
to the extrusive mechanics
– Since weak muscled patients are already long
faced patients, this extrusion can be very harmful
37. Rules to ALWAYS Remember
• The same brackets, bands, wires, and
mechanics system will produce different
treatment responses in different patients
– Muscle strength often determines these
responses
• The worst mistake in orthodontic treatment is
to cause over-eruption of molars in a weak
muscled patient
38. Review: facts about molar extrusion
• Mechanics produce extrusive forces
• Eruption is expressed more in weak muscled
patients because masticatory muscles do not
prevent it
• Excessive molar extrusion leads to backward
mandibular rotation
43. Important Points
• Not all patients exhibit pure horizontal or
vertical growth.
• The direction of eruption differs in the growth
patterns.
– Horizontal pattern- deep bite plus mesial eruption
can lead to lower arch crowding
– Vertical pattern- vertical eruption leads to no arch
length increase with growth
44. To increase success rate
• Refer weak muscled
patients
• When treating weak
muscled patients, use
mechanics that limit
molar extrusion
55. Evaluation
• Poor vertical control
• Vertical component of
Class II elastics was expressed
• What could have been
done to prevent this?
56. Mandibular morphological differences
between strong and weak muscled
patients
Qualitative evaluation
Many patients have both strong and weak muscled
characteristics
The main goal is to identify the extremes
57.
58.
59.
60. Gonial angle (Angle of the mandible)
• The angle formed by
the intersection of a
line tangent to the
posterior border of the
ramus and the
mandibular plane. It
determines inclination
of the ramus to the
mandibular plane. It
indicates mandibular
growth direction.
82. Intramatrix rotation
• Maxillary and
mandibular teeth and
alveolar processes
• This rotates in
conjunction with, but
independent of, the
maxilla and mandible
83. Fulcrum
• The most anterior portion of the dentition
where contact occurs
84. Type 1 intramatrix rotation
• Strong muscled patients
• Fulcrum at the incisal
edges
• Results in normal
downward and forward
growth
– Best possible
development
87. Type 2 intramatrix rotation
• Strong muscled patients
• Fulcrum in the middle
of the arch
• Super-eruption of
anteriors leads to
dental deep bite
– Class II, div. II
characteristics
91. Question
• A 10 year old patient comes into your office.
She presents with a Class II malocclusion with
a Type 2 intramatrix rotation. She has
mandibular retrognathism and a deep bite.
From an orthodontic perspective,
– What does she need?
– What appliance will help her meet her needs?
92. Type 3 intramatrix rotation
• Weak muscled patients
• Fulcrum on the posterior teeth
• Two possible outcomes
104. Describe the muscle strength
and intramatrix rotation.
Devise a treatment plan. What
additional information do you
need to complete the treatment
plan?
105. Concepts in facial development
• All faces flatten as they mature
• The mechanics of flattening differ in forward
and backward rotators