Monson’s Spherical Theory of Occlusion:

G.S. Monson proposed this theory in 1918, based on the observations of natural teeth and skulls. According to this theory, the anteroposterior and mesiodistal inclines of the artificial teeth should be arranged in harmony with a spherical surface. This theory of occlusion, showed that, the lower teeth moved over the surface of the upper teeth as over the surface of a sphere with a diameter of 8 inches ( approximately 20 cm). The centre of the sphere was located in the region of glabella and the surface of the sphere passed through glenoid fossae concentric with the articulating eminences.

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Monson felt that the condylar path and the occlusal plane form a curve. Bonwill said that the two condyles and the incisors formed an equilateral triangle with sides of 4 inches. Monson associated with Bonwill’s triangle with his own observations and formulated his spherical theory.

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Bonwill’s Theory of Occlusion

This theory of occlusion proposed that the teeth move in relation to each other as guided by the condylar controls and the incisal point. It was known as the theory of equilateral triangle, in which each side of the triangle was of 4 inches (approximately 10cm). The distance between the two condyles and between the condyle and incisor point.

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Hall’s Conical Theory of Occlusion

This theory proposed that the lower teeth move over the surface of the upper teeth as over the surface of the cone, with a generating angle of 45º and with the central axis of the cone tipped at a 45º angle to the occlusal plane.

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To explain his theory of mandibular movement, Hall envisioned that if 2 equilateral triangles (constructed on Bonwill’s principles) were placed back to back, they would share a common base that represented the condylar axis. The vertex of the anterior triangle would be located at the incisor point, and the posterior vertex would be located in the region of the external occipital protuberance. Viewed on the horizontal plane, it represented a double-Bonwill equilateral triangle with a common base.