In the upper and lower posterior segments, the goal in orthodontic treatment is to level the marginal ridges, align the central fossae, and level the curve of Wilson. One of the most difficult areas to achieve this key three dimensional relationship is in the lower second bicuspid-lower first molar area.

The average occlusal – gingival height of the lower second bicuspid is 7 mm, and that of the first molar is 5 mm. The center of the clinical crown, where bracket slots are normally placed, is then 3.5 mm below the buccal cusp tip for the lower second bicuspid and 2.5 mm for the lower first molar.  When placing bracket slots in this position on the lower first molar, particularly convertible brackets with extended tie wings, there are frequently interferences between the brackets and the upper first molars. This is accentuated by the frequent lingual inclination of the entire lower posterior segments.

To eliminate this interference, the lower first molar brackets are frequently seated more gingivally, which misaligns the marginal ridges. Also, because of the more gingival placement, the bracket is located on a wider buccal – lingual portion of the tooth. This rolls the tooth lingually, misaligning the central fossae and accentuating the lower curve of Wilson. So, this is not the best solution.

Solutions-Placement of tubes on first molars (similar to second molar tubes) is most advantageous in minimizing this problem, as convertible brackets extend approximately 1.5 mm more occlusally than thinner tubes. Also, acrylic “turbos” can be placed on upper incisors or the mesial-lingual cusp of upper first molars. This will separate the occlusion until the curve of Wilson begins to level in the lower arch. As a last resort (which is infrequently needed) the first molar bracket can be placed more gingival, and step bends can be added to correct the inevitable three dimensional discrepancy.