A. The general arrangement of the arches and the inclinations of the individual teeth are interrelated in such a manner as to allow the most efficient use of the forces of mastication, while at the same time stabilizing and protecting the dental arches.
B. Curve of Spee - The curvature which begins at the tip of the canines and follows the buccal cusp tips of the premolars and molars posteriorly, when viewed from their facial aspect. The Curve of Spee is two dimensional, and curves upward from anterior to posterior. It can readily be seen how the inclination of some of the individual posterior teeth must be offset from the vertical long axis of the body, if their occlusal surfaces are to conform to this curve. Maxillary molar roots are inclined mesially and mandibular molar roots distally.
C. Curve of Wilson - The medio-lateral curvature of the occlusal plane of posterior teeth. This curve is also two dimensional, but in a direction more or less at right angles to that of the Curve of Spee. The purpose of this arc in the occlusal curvature is to complement the paths of the condyles during movements of the mandible. For the occlusal surfaces of the posterior teeth to conform to this curve, the crowns of mandibular posterior teeth must incline to the lingual, while the crowns of maxillary posterior teeth must incline toward the buccal. This curve becomes deeper posteriorly, so that the molars' inclination is greater than that of the premolars. Because of this curve and associated tooth inclinations, the buccal cusps of mandibular molars, and the lingual cusps of maxillary molars usually appear to be longer.
D. Compensating Occlusal Curvature (Sphere of Monson) - The three dimensional curvature of the occlusal plane, which is the combination of the Curve of Spee and the Curve of Wilson. From this definition, it can be seen that this curvature is in the form of a portion of a ball, or sphere. Therefore, this curvature is concave for the mandibular arch and convex for the maxillary arch.
E. Axial Position - The inclination of a tooth from a vertical axis. This inclination is normally described in mesiodistal and faciolingual directions, even though it is usually an inseparable combination of the two. Further, it is normally described in terms of the root's inclination, which means that the crown is normally inclined in the opposite direction. These inclinations are necessary for proper occlusal and in-cisal function of the teeth. There is a wide range of axial positions, as is evident from the following descriptions for individual permanent teeth. As these axial positions are described, it should be of value to relate them to the individual tooth's functions, as well as its inclination relative to the Curves of Spee and Wilson.
1. Maxillary anterior teeth: The maxillary anterior teeth exhibit great inclination of the root toward the lingual, when considering the faciolingual dimension. In the mesiodistal direction, the maxillary incisors' roots incline very slightly toward the mesial, but the canine root inclines toward the distal.
2. Maxillary premolars: Maxillary premolars' root inclinations are slight: toward the lingual in the faciolingual dimension, and toward the distal in the mesiodistal dimension.
3. Maxillary molars: The roots of maxillary molars display great lingual inclination, and moderate mesial inclination.
4. Mandibular anterior teeth: The mandibular incisors and canines exhibit great lingual root inclinations in the faciolingual direction. Mesiodistally, the incisors are nearly straight, or display only minor mesial root inclination, while the canines have slight distal root inclination.
5. Mandibular premolars: Mesiodistally, these teeth show some distal root inclination. An interesting situation occurs in the faciolingual direction, since the first premolar's root inclines lingually, but the second premolar's root is offset buccally, both inclinations being slight.
6. Mandibular molars: The mandibular molars exhibit moderate to great buccal and distal root angulations.
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