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
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
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 treatmentOcclusion is Class I- treatment completed with Class II elastics
Successful or unsuccessful? Note molar eruption and man- dibular rotation.What caused this?
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.
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