The frictional coefficients for human parotid (HPS) and human submandibular/sublingual (HSMSL) salivas and water were measured for five subjects. Twenty different biomechanical conditions were used which approximated the range of force and movement of the human mandible. The experimental set-up comprised a bruxing motion under servohydraulic control; a microchamber with a small recirculating volume (3 ml minimum) under physiologic temperature control; and a maxillary element capable of measuring frictional forces at the enamel interface. The relative movement of enamel-on-enamel demonstrated the slip-stick phenomenon which was consistent with the negative velocity gradient of the coefficient of friction under all conditions. Studies on the variation of the frictional forces as a function of vertical load confirmed that the mechanism of lubrication was thin film, under the experimental conditions. HSMSL saliva had the greatest potential to vary the lubrication mechanism. At low speeds and higher force, it is suggested that boundary lubrication may offer the greatest potential in the development of enhanced lubrication of the hard tissues in the oral cavity.
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