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Muscle Strength in Orthodontic Diagnosis


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Understanding mandibular growth rotation is an integral component of orthodontic diagnosis

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Muscle Strength in Orthodontic Diagnosis

  1. 1. SlideshareMuscle strength and orthodontic treatmentphilosophy
  2. 2. ImplicationsThe same brackets, bands, wires, and mechanicsmay cause different treatment responses indifferent patients
  3. 3. RememberThe worst mistake in orthodontic treatment is …
  4. 4. cause excessive bite opening in a patient who already has an open bite.
  5. 5. Two general categories of growth rotation
  6. 6. Descriptive terms summary• Forward growth • Downward growth direction direction• Horizontal grower • Vertical grower• Counter-clockwise • Clockwise grower grower• Strong muscled patient • Weak muscled patient
  7. 7. Every decision you make during ortho-dontic treatment will be influenced bythe patient’s growth pattern and/ormuscle strength
  8. 8. Historical Perspective• Sassouni, McNamara, Tweed, and especially Bjork• The work of these doctors helps us shape a treatment philosophy
  9. 9. Sassouni, 1960
  10. 10. McNamara, 1990
  11. 11. 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
  12. 12. 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
  13. 13. Bjork’s study• No treatment performed• Records taken yearly• Implants provide a reliable method of superimposition
  14. 14. 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
  15. 15. 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
  16. 16. 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
  17. 17. 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
  18. 18. 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
  19. 19. 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
  20. 20. 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
  21. 21. Implications of Bjork’s study• Muscles of mastication exert pressure and tension on different areas of the mandible depending on condylar growth direction
  22. 22. Implications, continued• Resorption and apposition of bone, and therefore the morphology of the mandible, differs depending on condylar growth direction
  23. 23. Conclusion• Growth direction can be predicted based on mandibular morphology – This is a very valuable diagnostic tool
  24. 24. 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
  25. 25. 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
  26. 26. 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
  27. 27. 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
  28. 28. Summary of Growth Mechanics Vertical grower- note downward growth direction.
  29. 29. Summary, Continued Horizontal grower- note forward growth direction.
  30. 30. Strong (l) and weak (r)muscled mandibular shape
  31. 31. Strong (l) and weak (r) muscled patients
  32. 32. 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
  33. 33. To increase success rate• Refer weak muscled patients• When treating weak muscled patients, use mechanics that limit molar extrusion
  34. 34. Tweed foundation• Compared successful and unsuccessful cases
  35. 35. Successful cases Note forward mandibular rotation and lack of molar eruption
  36. 36. Unsuccessful cases Note backward mandibular rotation and molar eruption
  37. 37. Tweed Results• Successful cases – Minimal backward rotation• Unsuccessful cases – Extreme backward rotation• 1mm of molar eruption can lead to 3mm of backward rotation
  38. 38. So…• Control of excess molar eruption and the resulting backward mandibular rotation is one of the major goals of orthodontic therapy
  39. 39. Evaluate this case Pretreatment- 3mm Class II note gingival display
  40. 40. Post treatmentOcclusion is Class I- treatment completed with Class II elastics
  41. 41. Successful or unsuccessful? Note molar eruption and man- dibular rotation.What caused this?
  42. 42. Facial photos
  43. 43. Evaluation• Poor vertical control• Vertical component of Class II elastics was expressed• What could have been done to prevent this?
  44. 44. 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
  45. 45. 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.
  46. 46. Gonial AngleInfluences Relative LengthInfluences Growth Rotation 128º ± 7º
  47. 47. Gonial angleThe more acute this angle is, the stronger is the patient’smusculature
  48. 48. Shape of lower border of the mandibleStrong muscled-double curve Weak muscled- concave lower border
  49. 49. Symphyseal inclinationThe more acute the indicated angle, the stronger is thepatient’s musculature
  50. 50. Symphyseal radiolucencyThe more radiopaque the indicated area, the stronger is thepatient’s musculature
  51. 51. Condylar inclinationStrong muscled- condyle points Weak muscled- condyle pointsforward backward
  52. 52. #6 Which has stronger muscles?
  53. 53. #7 Which is stronger?
  54. 54. #8 Which is stronger?
  55. 55. Intramatrix rotation• Maxillary and mandibular teeth and alveolar processes• This rotates in conjunction with, but independent of, the maxilla and mandible
  56. 56. Fulcrum• The most anterior portion of the dentition where contact occurs
  57. 57. Type 1 intramatrix rotation• Strong muscled patients• Fulcrum at the incisal edges• Results in normal downward and forward growth – Best possible development
  58. 58. Type 1 Intramatrix
  59. 59. Example of type 1 rotation
  60. 60. 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
  61. 61. Type 2 Intramatrix
  62. 62. Why does the fulcrum shift?• Allergies• Airway problems• Tongue, lip, and/or finger habits• Early loss of primary teeth
  63. 63. Example of Type 2 rotation
  64. 64. 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?
  65. 65. Type 3 intramatrix rotation• Weak muscled patients• Fulcrum on the posterior teeth• Two possible outcomes
  66. 66. Normal anterior eruption• Long face• Good occlusion
  67. 67. Type 3 Intramatrix
  68. 68. Interruption of anterior eruption• Skeletal open bite• Dental open bite
  69. 69. Causes of anterior interruption• Tongue thrust• Lip habits• Thumb, finger habits• Abnormal swallowing pattern• Mouth breathing
  70. 70. Why is treatment response different?
  71. 71. Determine jaw and intramatrix rotation
  72. 72. Muscle strength?Intramatrix type?
  73. 73. Muscle strength?Intramatrix rotation?
  74. 74. Describe the muscle strengthand intramatrix rotation.Devise a treatment plan. Whatadditional information do youneed to complete the treatmentplan?
  75. 75. Concepts in facial development• All faces flatten as they mature• The mechanics of flattening differ in forward and backward rotators
  76. 76. Strong muscled patients• Chin grows upward and forward• Facial musculature “holds teeth back”
  77. 77. Non-extraction treatment, age 9 (l) and age 17 (r)
  78. 78. Weak muscled patients• Chin grows down and back• Retrusive pogonion leads to a flat face
  79. 79. Photos were taken 7 years apart