Inclination of mandibular canine in relation to the condylo-occlusal line

Abstract

Statement of problem: Geometrically, to best resist occlusal forces, the mandibular incisor presents generally an orthogonal inclination (approximately 90 ) to the radius of rotation during the closing movement of the mandible, the centre of rotation being the condyles.Purpose: The objective of this study was to confirm the application of this geometrical principle, named "tangent law", to the inclination of the mandibular canine. If the tangent law is applicable to the mandibular canine, the mean angle between the long axis inclination of the mandibular canine is orthogonal to the condylo-occlusal line.Material and methods: Measurements were taken from sixty lateral cephalometric radiographs in a strict sagittal plane to analyze the incisors, and in the canine plane to analyze the canines.Results: The measurements, close to 90 , confirmed the tangent law for the mandibular incisor (mean: 89.38 AE1.77), and also confirmed our hypothesis about the inclination of the mandibular canine, which was found to follow the same principle (mean: 90.98 AE1.13). A significant smaller variance for the mandibular canine inclination than for the mandibular incisor suggests that the canine may play a key role in occlusal stability.Conclusion: The orthogonal principle of the "tangent law" could constitute a reference value for the inclination of the mandibular canine when planning orthodontic, prosthodontic or implantology procedures.

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“…These values are 15 for a third molar, 20 for a second molar, 25 for a first molar, 28 for a second premolar and the mean differential angle was close to 0 for incisors, and a recent paper showed that the mandibular canine inclination follow the same rule [7]. On the posterior teeth, the differential angle leads to horizontal components of force tending to tilt the posterior teeth forward.…”