In the previous studies about prescription of preadjusted appliance, occlusal plane was used as a reference plane for crwon angulation (tip) measurement But this reference plane is not parallel to the he connecting the facial axis points at which the centers of brackets are positioned (Andrews´ plane), due to the curve of Spee. Therefore, we developed a new reference plane unaffected by the curve of Spee and more parallel to the Andrews´ plane. It is an imaginary line connecting mesial and distal marginal ridges of each posterior tooth, and we named it ´marginal ridge plane´ In this study, crown angulations of posterior teeth of 29 normal occlusion samples were measured and measurements from both reference planes were compared. Crown angulation measurements measured from occlusal plane were different from crown angulation measurements from marginal ridge plane in the upper and lower 2nd molars (p < 0.01), and 1st premolars (p < 0.05). These results were analyzed as the crown angulation measurements from occlusal plane were affected by the curve of Spee.
Crown angulations should be vaned according to the amount of curve of Spee to maintain- the continuity of marginal ridges. To solve this problem, determining bracket angulation as the bracket slot is parallel to the marginal ridge plane of each posterior teeth is recommended.
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