muscle strength in orthodontic diagnosis
TRANSCRIPT
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Slideshare
Muscle strength and orthodontic treatment philosophy
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Implications
The same brackets, bands, wires, and mechanics may cause different treatment responses in different patients
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Remember
The worst mistake in orthodontic treatment is …
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cause excessive bite opening
in a patient who already has
an open bite.
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Two general categories of growth rotation
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Descriptive terms summary
• Forward growth direction
• Horizontal grower
• Counter-clockwise grower
• Strong muscled patient
• Downward growth direction
• Vertical grower
• Clockwise grower
• Weak muscled patient
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Every decision you make during ortho-
dontic treatment will be influenced by
the patient’s growth pattern and/or
muscle strength
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Historical Perspective
• Sassouni, McNamara, Tweed, and especially Bjork
• The work of these doctors helps us shape a treatment philosophy
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Sassouni, 1960
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McNamara, 1990
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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
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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
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Bjork’s study
• No treatment performed
• Records taken yearly
• Implants provide a reliable method of superimposition
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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
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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
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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
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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
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Anterior Posterior
• Forward rotator
• Counter-clockwise rotator
• Horizontal growth pattern
• Hypodivergent facial pattern
• STRONG MUSCLED PATIENT
• Backward rotator
• Clockwise rotator
• Vertical growth pattern
• Hyperdivergent facial pattern
• WEAK MUSCLED PATIENT
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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
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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
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Implications of Bjork’s study
• Muscles of mastication exert pressure and tension on different areas of the mandible depending on condylar growth direction
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Implications, continued
• Resorption and apposition of bone, and therefore the morphology of the mandible, differs depending on condylar growth direction
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Conclusion
• Growth direction can be predicted based on mandibular morphology
– This is a very valuable diagnostic tool
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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
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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
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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
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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
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Summary of Growth Mechanics
Vertical grower- note downward
growth direction.
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Summary, Continued
Horizontal grower- note forward growth
direction.
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Strong (l) and weak (r)muscled mandibular shape
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Strong (l) and weak (r) muscled patients
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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
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To increase success rate
• Refer weak muscled patients
• When treating weak muscled patients, use mechanics that limit molar extrusion
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Tweed foundation
• Compared successful and unsuccessful cases
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Successful cases
Note forward mandibular rotation
and lack of molar eruption
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Unsuccessful cases
Note backward mandibular rotation
and molar eruption
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Tweed Results
• Successful cases– Minimal backward
rotation
• Unsuccessful cases– Extreme backward
rotation
• 1mm of molar eruption can lead to 3mm of backward rotation
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So…
• Control of excess molar eruption and the resulting backward mandibular rotation is one of the major goals of orthodontic therapy
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Evaluate this case
Pretreatment- 3mm Class II
note gingival display
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Post treatment
Occlusion is Class I- treatment completed with Class II elastics
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Successful or unsuccessful?
Note molar eruption and man-
dibular rotation.
What caused this?
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Facial photos
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Evaluation
• Poor vertical control
• Vertical component of
Class II elastics was expressed
• What could have been
done to prevent this?
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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
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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.
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Gonial Angle
128º ± 7º
Influences Relative Length Influences Growth Rotation
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Gonial angle
The more acute this angle is, the stronger is the patient’s
musculature
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Shape of lower border of the mandible
Strong muscled-double curve Weak muscled- concave
lower border
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Symphyseal inclination
The more acute the indicated angle, the stronger is the
patient’s musculature
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Symphyseal radiolucency
The more radiopaque the indicated area, the stronger is the
patient’s musculature
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Condylar inclination
Strong muscled- condyle points
forward
Weak muscled- condyle points
backward
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#6 Which has stronger muscles?
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#7 Which is stronger?
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#8 Which is stronger?
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Intramatrix rotation
• Maxillary and mandibular teeth and alveolar processes
• This rotates in conjunction with, but independent of, the maxilla and mandible
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Fulcrum
• The most anterior portion of the dentition where contact occurs
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Type 1 intramatrix rotation
• Strong muscled patients
• Fulcrum at the incisal edges
• Results in normal downward and forward growth
– Best possible development
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Type 1 Intramatrix
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Example of type 1 rotation
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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
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Type 2 Intramatrix
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Why does the fulcrum shift?
• Allergies
• Airway problems
• Tongue, lip, and/or finger habits
• Early loss of primary teeth
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Example of Type 2 rotation
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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?
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Type 3 intramatrix rotation
• Weak muscled patients
• Fulcrum on the posterior teeth
• Two possible outcomes
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Normal anterior eruption
• Long face
• Good occlusion
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Type 3 Intramatrix
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Interruption of anterior eruption
• Skeletal open bite
• Dental open bite
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![Page 97: Muscle Strength in Orthodontic Diagnosis](https://reader034.vdocuments.us/reader034/viewer/2022042701/55a7545d1a28abb85d8b4647/html5/thumbnails/97.jpg)
Causes of anterior interruption
• Tongue thrust
• Lip habits
• Thumb, finger habits
• Abnormal swallowing pattern
• Mouth breathing
![Page 98: Muscle Strength in Orthodontic Diagnosis](https://reader034.vdocuments.us/reader034/viewer/2022042701/55a7545d1a28abb85d8b4647/html5/thumbnails/98.jpg)
Why is treatment response different?
![Page 99: Muscle Strength in Orthodontic Diagnosis](https://reader034.vdocuments.us/reader034/viewer/2022042701/55a7545d1a28abb85d8b4647/html5/thumbnails/99.jpg)
Determine jaw and intramatrix rotation
![Page 100: Muscle Strength in Orthodontic Diagnosis](https://reader034.vdocuments.us/reader034/viewer/2022042701/55a7545d1a28abb85d8b4647/html5/thumbnails/100.jpg)
Muscle strength?
Intramatrix type?
![Page 101: Muscle Strength in Orthodontic Diagnosis](https://reader034.vdocuments.us/reader034/viewer/2022042701/55a7545d1a28abb85d8b4647/html5/thumbnails/101.jpg)
![Page 102: Muscle Strength in Orthodontic Diagnosis](https://reader034.vdocuments.us/reader034/viewer/2022042701/55a7545d1a28abb85d8b4647/html5/thumbnails/102.jpg)
Muscle strength?
Intramatrix rotation?
![Page 103: Muscle Strength in Orthodontic Diagnosis](https://reader034.vdocuments.us/reader034/viewer/2022042701/55a7545d1a28abb85d8b4647/html5/thumbnails/103.jpg)
![Page 104: Muscle Strength in Orthodontic Diagnosis](https://reader034.vdocuments.us/reader034/viewer/2022042701/55a7545d1a28abb85d8b4647/html5/thumbnails/104.jpg)
Describe the muscle strength
and intramatrix rotation.
Devise a treatment plan. What
additional information do you
need to complete the treatment
plan?
![Page 105: Muscle Strength in Orthodontic Diagnosis](https://reader034.vdocuments.us/reader034/viewer/2022042701/55a7545d1a28abb85d8b4647/html5/thumbnails/105.jpg)
Concepts in facial development
• All faces flatten as they mature
• The mechanics of flattening differ in forward and backward rotators
![Page 106: Muscle Strength in Orthodontic Diagnosis](https://reader034.vdocuments.us/reader034/viewer/2022042701/55a7545d1a28abb85d8b4647/html5/thumbnails/106.jpg)
Strong muscled patients
• Chin grows upward and forward
• Facial musculature “holds teeth back”
![Page 107: Muscle Strength in Orthodontic Diagnosis](https://reader034.vdocuments.us/reader034/viewer/2022042701/55a7545d1a28abb85d8b4647/html5/thumbnails/107.jpg)
Non-extraction treatment, age 9 (l) and age 17 (r)
![Page 108: Muscle Strength in Orthodontic Diagnosis](https://reader034.vdocuments.us/reader034/viewer/2022042701/55a7545d1a28abb85d8b4647/html5/thumbnails/108.jpg)
Weak muscled patients
• Chin grows down and back
• Retrusive pogonion leads to a flat face
![Page 109: Muscle Strength in Orthodontic Diagnosis](https://reader034.vdocuments.us/reader034/viewer/2022042701/55a7545d1a28abb85d8b4647/html5/thumbnails/109.jpg)
Photos were taken 7 years apart