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