The curve of Spee: understanding the sagittal organization of mandibular teeth.

The tangent law explained the sagittal organization of the mandibular teeth by geometrical analyses. According to this concept, the long axis of all mandibular teeth must converge at one point in the circle of the "curve of Spee", and this point is diametrically opposed to the condylar point. A cephalometric analysis of 470 patients was the basis for this statistical study of the radius of the curve of Spee. The mean radius was approximately 80 mm. The mandibular incisors followed the tangent law, but the long axis of the posterior mandibular teeth exhibited a progressive differential angle (postero-anterior) with the direction of the tangent (15 degrees, 20 degrees, 25 degrees, 28 degrees). This differential angle generates a horizontal strength component with tendency to forward tooth movement, tightening of the proximal contacts. The use of this cephalometric reference could help the dentist to determine a more suitable organization of the occlusion in the sagittal plane.