Andrew’s six keys of normal occlusion

Andrew’s six keys of normal occlusion

1. Andrew’s six keys of normal occlusion Prof Dr Maher Fouda Mansours Egypt

2. Andrew’s Six Keys to Normal Occlusion Lawrence Andrew, in 1972,12 outlined six keys to normal occlusion after studying 120 non-orthodontic models and comparing them with the best 1150 finished orthodontic cases . Normal occlusion

3. Andrew’s Six Keys to Normal Occlusion The established six keys where not only purposeful due to its presence in all 120 orthodontic normals , but also due to the fact that in treated models, the absence of one of the six was able to predict defective incomplete end result. Normal occlusion

4. Key I: Molar Relationship The first of the six keys is molar relationship. 1. The distal surface of the distobuccal cusp of the upper first permanent molar occluded with the mesial surface of the mesiobuccal cusp of the lower second molar . Key I molar relation. (A) Improper molar relationship. (B) Improved molar relationship. (C) More improved molar relationship. (D) Proper molar relationship

5. Key I: Molar Relationship The first of the six keys is molar relationship . 1. I t is possible for the mesiobuccal cusp of the upper first year molar to occlude in the groove between the mesial and middle cusps of the lower first permanent molar, while leaving a situation unreceptive to normal occlusion. Key I molar relation. (A) Improper molar relationship. (B) Improved molar relationship. (C) More improved molar relationship. (D) Proper molar relationship

6. Key I: Molar Relationship The first of the six keys is molar relationship. 1. The closer the distal surface of the distobuccal cusp of the upper first permanent molar approaches the mesial surfaces of the mesiobuccal cusp of the lower second molar, the better the opportunity for normal occlusion. Key I molar relation. (A) Improper molar relationship. (B) Improved molar relationship. (C) More improved molar relationship. (D) Proper molar relationship

7. 2. The mesiobuccal cusp of the upper first permanent molar fell within the groove between the mesial and middle cusps of the lower first permanent molar. 3. The canines and premolars enjoyed a cusp–embrasure relationship buccally , and a cusp–fossa relationship lingually . Key I: Molar Relationship The first of the six keys is molar relationship . Key I molar relation. (A) Improper molar relationship. (B) Improved molar relationship. (C) More improved molar relationship. (D) Proper molar relationship

8. Key II crown angulation or tip: Long axis of crown measured from line 90° to occlusal plane Key II: Crown Angulation, The Mesiodistal Tip • The term crown angulation refers to angulation (or tip) of the long axis of the crown, not to angulation of the long axis of the entire tooth. .

9. Key II crown angulation or tip: Long axis of crown measured from line 90° to occlusal plane Key II: Crown Angulation, The Mesiodistal Tip • • The gingival portion of the long axis of each crown was distal to the incisal portion, varying with the individual tooth type.

10. Key II: Crown Angulation, The Mesiodistal Tip • The long axis of the crown for all teeth, except molars, is judged to be the mid-developmental ridge, which is the most prominent and innermost vertical portion of the labial or buccal surface of the crown.

11. Key II: Crown Angulation, The Mesiodistal Tip The long axis of the molar crown is identified by the dominant vertical groove on the buccal surface of the crown.

12. Key II crown angulation or tip: Long axis of crown measured from line 90° to occlusal plane Key II: Crown Angulation, The Mesiodistal Tip Crown tip is expressed in degrees, plus or minus . The degree of crown tip is the angle between the long axis of the crown (as viewed from the labial or buccal surface) and a line bearing 90° from the occlusal plane.

13. Key II: Crown Angulation, The Mesiodistal Tip A ‘plus reading’ is assigned when the gingival portion of the long axis of the crown is distal to the incisal portion and a ‘minus reading’ when the gingival portion of the long axis of the crown is mesial to the incisal portion.

14. Normal occlusion is dependent upon proper distal crown tip, especially of the upper anterior teeth since they have the longest crowns. Key II: Crown Angulation, The Mesiodistal Tip

15. The degree of the tip of incisors determines the amount of mesiodistal space they consume and, therefore, has a considerable effect on posterior occlusion as well as anterior esthetics . Key II: Crown Angulation, The Mesiodistal Tip

16. Key II: Crown Angulation, The Mesiodistal Tip In normal occlusion, the crown angulation was positive for all teeth

17. Key II: Crown Angulation, The Mesiodistal Tip according to Andrew

18. Key III: Crown Inclination ( Labiolingual or Buccolingual Inclination) Crown inclination refers to the labiolingual or buccolingual inclination of the long axis of the crown, not to the inclination of the long axis of the entire tooth . The inclination of all the crowns had a consistent scheme . Key III crown inclination is determined by the resulting angle between a line 90° to the occlusal plane and a line tangent to the middle of the labial or buccal clinical crown. (A) shows tooth with positive crown torque and (B) shows tooth with negative torque .

19. Key III: Crown Inclination ( Labiolingual or Buccolingual Inclination) Anterior teeth (central and lateral incisors): Upper and lower anterior crown inclination was sufficient to resist overeruption of anterior teeth and also to allow proper distal positioning of the contact points of the upper teeth in their relationship to the lower teeth, permitting proper occlusion of the posterior crowns.

20. Key III: Crown Inclination ( Labiolingual or Buccolingual Inclination) A, Improperly inclined anterior crowns result in all upper contact points being mesial, leading to improper occlusion. B, Demonstration, on an overlay, that when the anterior crowns are properly inclined the contact points move distally, allowing for normal occlusion.

21. Key III: Crown Inclination ( Labiolingual or Buccolingual Inclination) Spaces resulting from normally occluded posterior teeth and insufficiently inclined anterior teeth are often falsely blamed on tooth size descrepancy .

22. Key III: Crown Inclination ( Labiolingual or Buccolingual Inclination) In normal occlusion, the crown inclination for all teeth was negative except maxillary central and lateral incisors

23. Key III: Crown Inclination ( Labiolingual or Buccolingual Inclination) Upper posterior teeth (canines through molars): A lingual crown inclination existed in the upper posterior crowns. It was constant and similar to the canines through the second premolars and was slightly more pronounced in the molars.

24. Key III: Crown Inclination ( Labiolingual or Buccolingual Inclination) Lower posterior (canines through molars): The lingual crown inclination in the lower posterior teeth progressively increased from the canines through the second molars.

25. Tip and Torque The clinical implication of the tip and torque is that they collectively affect the upper anterior crowns and total occlusion. Andrew’s wagon wheel concept. (A, B) Unbent rectangular archwire with vertical wires soldered at 90°, spaced to represent the upper central and lateral incisors. (C–E) As the anterior portion of the archwire is torqued lingually , the vertical wires begin to converge until they become the spokes of a wheel when the archwire is torqued 90° progressively .

26. Tip and Torque In lingual crown torque, for every 4˚, there is 1˚ mesial convergence of central and lateral incisor crowns, at the gingival portion. The ratio is approximately 4:1. Andrew described this phenomenon as the ‘wagon wheel concept’ Andrew’s wagon wheel concept. (A, B) Unbent rectangular archwire with vertical wires soldered at 90°, spaced to represent the upper central and lateral incisors. (C–E) As the anterior portion of the archwire is torqued lingually , the vertical wires begin to converge until they become the spokes of a wheel when the archwire is torqued 90° progressively. Andrew’s wagon wheel concept. (A, B) Unbent rectangular archwire with vertical wires soldered at 90°, spaced to represent the upper central and lateral incisors. (C–E) As the anterior portion of the archwire is torqued lingually , the vertical wires begin to converge until they become the spokes of a wheel when the archwire is torqued 90° progressively .

27. Key IV: Rotations • The fourth key to normal occlusion is that the teeth should be free from undesirable rotations.

28. Key IV: Rotations • The molar , as for example, if rotated, would occupy more space than normal, creating a situation unreceptive to normal occlusion. A rotated incisor on the other hand occupies less space.

29. CLINICAL SIGNIFICANCE Rotated tooth • By correcting a rotated tooth, space can be gained in posterior segment as a rotated posterior tooth occupies more space. Key IV: Rotations

30. CLINICAL SIGNIFICANCE Rotated tooth • For correction of an anterior tooth, space is required as rotated anterior tooth occupies less space. Key IV: Rotations

31. Key V: Tight Contacts • The fifth key is that the contact points should be tight (no spaces). • Persons who have genuine tooth-size discrepancies pose special problems, but in the absence of such abnormalities tight contact should exist.

32. Key V: Tight Contacts Without exception, the contact points on the nonorthodontic normals were tight.

33. Key VI: Occlusal Plane • The planes of occlusion found on the normal models ranged from flat to slight curves of Spee . • A flat plane should be a treatment goal as a form of overtreatment. A, A deep curve of Spee results in a more confined area for the upper teeth, creating spillage of the upper teeth progressively mesially and distally. B, A flat plane of occlusion is most receptive to normal occlusion. C, A reverse curve of Spee results in excessive room for the upper teeth

34. Key VI: Occlusal Plane • Intercuspation of teeth is best when the plane of occlusion is relatively flat . • A deep curve of Spee results in a more contained area for the upper teeth, making normal occlusion impossible. A deep curve of Spee results in a more confined area for the upper teeth, creating spillage of the upper teeth progressively mesially and distally. A flat plane of occlusion is most receptive to normal occlusion .

35. Key VI: Occlusal Plane The remaining upper teeth , anterior and posterior to the first premolar, are progressively in error. • A reverse curve of Spee is an extreme form of overtreatment, allowing excessive space for each tooth to be intercuspally placed . A reverse curve of Spee results in excessive room for the upper teeth

36. • There is a natural tendency for the curve of Spee to deepen with time, for the lower jaw’s growth downward and forward sometimes is faster and continues longer Key VI: Occlusal Plane than that of the upper jaw, and this causes the lower anterior teeth, which are confined by the upper anterior teeth and lips, to be forced back and up, resulting in crowded lower anterior teeth and/or a deeper overbite and deeper curve of Spee . A, A deep curve of Spee results in a more confined area for the upper teeth, creating spillage of the upper teeth progressively mesially and distally. B, A flat plane of occlusion is most receptive to normal occlusion. C, A reverse curve of Spee results in excessive room for the upper teeth

37. References