Introduction
Chewing turns food into small particles with a large surface, so that digestion can work on it. The engine is the force of mastication: the force made by the masticatory muscles during the physiological act of chewing. Every force has a direction and a magnitude, and the teeth, periodontium and bone have to carry both.
Direction of the forces
No functional movement is purely vertical or purely horizontal. The inclines of the cusps and of the articular eminence tilt every force, so the supporting tissues are always loaded by oblique (non-axial) forces. Axial forces are well tolerated; oblique ones tip the tooth and load the bone unevenly.
Magnitude of the forces
The force a muscle can make depends on its cross-section. Weber (1936) found that each 1 cm² of masticatory muscle gives about 98 N (10 kg).
The mandible as a lever
The fulcrum is the TMJ, the effort is applied at the angle of the mandible and the load sits on the teeth. The further a tooth is from the angle, the less force it receives, so Ps is greatest on the molars and smallest on the incisors:
where L is the distance from the tooth to the angle of the mandible.