Understanding mandibular growth rotation is an integral component of orthodontic diagnosis

1. Slideshare
Muscle strength and orthodontic treatment
philosophy

2. Implications
The same brackets, bands, wires, and mechanics
may cause different treatment responses in
different patients

3. Remember
The worst mistake in orthodontic treatment is …

4. cause excessive bite opening
in a patient who already has
an open bite.

7. Two general categories of growth
rotation

16. Descriptive terms summary
• Forward growth • Downward growth
direction direction
• Horizontal grower • Vertical grower
• Counter-clockwise • Clockwise grower
grower
• Strong muscled patient • Weak muscled patient

17. Every decision you make during ortho-
dontic treatment will be influenced by
the patient’s growth pattern and/or
muscle strength

18. Historical Perspective
• Sassouni, McNamara, Tweed, and especially
Bjork
• The work of these doctors helps us shape a
treatment philosophy

19. Sassouni, 1960

20. McNamara, 1990

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

28. Anterior Posterior
• Forward rotator • Backward rotator
• Counter-clockwise • Clockwise rotator
rotator
• Vertical growth pattern
• Horizontal growth
pattern • Hyperdivergent facial
• Hypodivergent facial pattern
pattern • WEAK MUSCLED
• STRONG MUSCLED PATIENT
PATIENT

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

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

39. Summary of Growth Mechanics
Vertical grower- note downward
growth direction.

40. Summary, Continued
Horizontal grower- note forward growth
direction.

41. Strong (l) and weak (r)
muscled mandibular shape

42. Strong (l) and weak (r) muscled patients

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

45. Tweed foundation
• Compared successful and unsuccessful cases

46. Successful cases
Note forward mandibular rotation
and lack of molar eruption

47. Unsuccessful cases
Note backward mandibular rotation
and molar eruption

49. Tweed Results
• Successful cases
– Minimal backward
rotation
• Unsuccessful cases
– Extreme backward
rotation
• 1mm of molar eruption
can lead to 3mm of
backward rotation

50. So…
• Control of excess molar
eruption and the
resulting backward
mandibular rotation is
one of the major goals
of orthodontic therapy

51. Evaluate this case
Pretreatment- 3mm Class II
note gingival display

52. Post treatment
Occlusion is Class I- treatment completed with Class II elastics

53. Successful or unsuccessful?
Note molar eruption and man-
dibular rotation.
What caused this?

54. Facial photos

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

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.

61. Gonial Angle
Influences Relative Length
Influences Growth Rotation
128º ± 7º

63. Gonial angle
The more acute this angle is, the stronger is the patient’s
musculature

65. Shape of lower border of the mandible
Strong muscled-double curve Weak muscled- concave
lower border

67. Symphyseal inclination
The more acute the indicated angle, the stronger is the
patient’s musculature

69. Symphyseal radiolucency
The more radiopaque the indicated area, the stronger is the
patient’s musculature

71. Condylar inclination
Strong muscled- condyle points Weak muscled- condyle points
forward backward

78. #6 Which has stronger muscles?

79. #7 Which is stronger?

80. #8 Which is stronger?

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

85. Type 1 Intramatrix

86. Example of type 1 rotation

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

88. Type 2 Intramatrix

89. Why does the fulcrum shift?
• Allergies
• Airway problems
• Tongue, lip, and/or
finger habits
• Early loss of primary
teeth

90. Example of Type 2 rotation

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

93. Normal anterior eruption
• Long face
• Good occlusion

94. Type 3 Intramatrix

95. Interruption of anterior eruption
• Skeletal open bite
• Dental open bite

97. Causes of anterior interruption
• Tongue thrust
• Lip habits
• Thumb, finger habits
• Abnormal swallowing pattern
• Mouth breathing

98. Why is treatment response different?

99. Determine jaw and intramatrix
rotation

100. Muscle strength?
Intramatrix type?

102. Muscle strength?
Intramatrix rotation?

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

106. Strong muscled patients
• Chin grows upward and forward
• Facial musculature “holds teeth back”

107. Non-extraction treatment, age 9 (l) and age 17 (r)

108. Weak muscled patients
• Chin grows down and back
• Retrusive pogonion leads to a flat face

109. Photos were taken 7 years apart