Muscle Strength in Orthodontic Diagnosis
-
Upload
jimprit -
Category
Health & Medicine
-
view
1.016 -
download
3
Transcript of Muscle Strength in Orthodontic Diagnosis
Slideshare
Muscle strength and orthodontic treatment philosophy
Implications
The same brackets, bands, wires, and mechanics may cause different treatment responses in different patients
Remember
The worst mistake in orthodontic treatment is …
cause excessive bite opening
in a patient who already has
an open bite.
Two general categories of growth rotation
Descriptive terms summary
• Forward growth direction
• Horizontal grower
• Counter-clockwise grower
• Strong muscled patient
• Downward growth direction
• Vertical grower
• Clockwise grower
• Weak muscled patient
Every decision you make during ortho-
dontic treatment will be influenced by
the patient’s growth pattern and/or
muscle strength
Historical Perspective
• Sassouni, McNamara, Tweed, and especially Bjork
• The work of these doctors helps us shape a treatment philosophy
Sassouni, 1960
McNamara, 1990
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
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
Bjork’s study
• No treatment performed
• Records taken yearly
• Implants provide a reliable method of superimposition
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
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
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
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
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
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
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
Implications of Bjork’s study
• Muscles of mastication exert pressure and tension on different areas of the mandible depending on condylar growth direction
Implications, continued
• Resorption and apposition of bone, and therefore the morphology of the mandible, differs depending on condylar growth direction
Conclusion
• Growth direction can be predicted based on mandibular morphology
– This is a very valuable diagnostic tool
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
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
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
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
Summary of Growth Mechanics
Vertical grower- note downward
growth direction.
Summary, Continued
Horizontal grower- note forward growth
direction.
Strong (l) and weak (r)muscled mandibular shape
Strong (l) and weak (r) muscled patients
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
To increase success rate
• Refer weak muscled patients
• When treating weak muscled patients, use mechanics that limit molar extrusion
Tweed foundation
• Compared successful and unsuccessful cases
Successful cases
Note forward mandibular rotation
and lack of molar eruption
Unsuccessful cases
Note backward mandibular rotation
and molar eruption
Tweed Results
• Successful cases– Minimal backward
rotation
• Unsuccessful cases– Extreme backward
rotation
• 1mm of molar eruption can lead to 3mm of backward rotation
So…
• Control of excess molar eruption and the resulting backward mandibular rotation is one of the major goals of orthodontic therapy
Evaluate this case
Pretreatment- 3mm Class II
note gingival display
Post treatment
Occlusion is Class I- treatment completed with Class II elastics
Successful or unsuccessful?
Note molar eruption and man-
dibular rotation.
What caused this?
Facial photos
Evaluation
• Poor vertical control
• Vertical component of
Class II elastics was expressed
• What could have been
done to prevent this?
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
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.
Gonial Angle
128º ± 7º
Influences Relative Length Influences Growth Rotation
Gonial angle
The more acute this angle is, the stronger is the patient’s
musculature
Shape of lower border of the mandible
Strong muscled-double curve Weak muscled- concave
lower border
Symphyseal inclination
The more acute the indicated angle, the stronger is the
patient’s musculature
Symphyseal radiolucency
The more radiopaque the indicated area, the stronger is the
patient’s musculature
Condylar inclination
Strong muscled- condyle points
forward
Weak muscled- condyle points
backward
#6 Which has stronger muscles?
#7 Which is stronger?
#8 Which is stronger?
Intramatrix rotation
• Maxillary and mandibular teeth and alveolar processes
• This rotates in conjunction with, but independent of, the maxilla and mandible
Fulcrum
• The most anterior portion of the dentition where contact occurs
Type 1 intramatrix rotation
• Strong muscled patients
• Fulcrum at the incisal edges
• Results in normal downward and forward growth
– Best possible development
Type 1 Intramatrix
Example of type 1 rotation
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
Type 2 Intramatrix
Why does the fulcrum shift?
• Allergies
• Airway problems
• Tongue, lip, and/or finger habits
• Early loss of primary teeth
Example of Type 2 rotation
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?
Type 3 intramatrix rotation
• Weak muscled patients
• Fulcrum on the posterior teeth
• Two possible outcomes
Normal anterior eruption
• Long face
• Good occlusion
Type 3 Intramatrix
Interruption of anterior eruption
• Skeletal open bite
• Dental open bite
Causes of anterior interruption
• Tongue thrust
• Lip habits
• Thumb, finger habits
• Abnormal swallowing pattern
• Mouth breathing
Why is treatment response different?
Determine jaw and intramatrix rotation
Muscle strength?
Intramatrix type?
Muscle strength?
Intramatrix rotation?
Describe the muscle strength
and intramatrix rotation.
Devise a treatment plan. What
additional information do you
need to complete the treatment
plan?
Concepts in facial development
• All faces flatten as they mature
• The mechanics of flattening differ in forward and backward rotators
Strong muscled patients
• Chin grows upward and forward
• Facial musculature “holds teeth back”
Non-extraction treatment, age 9 (l) and age 17 (r)
Weak muscled patients
• Chin grows down and back
• Retrusive pogonion leads to a flat face
Photos were taken 7 years apart