Effect of saliva viscosity on tribological behaviour of tooth enamel Eugeniusz Sajewicz à Faculty of Mechanical Engineering, Bialystok Technical University, Wiejska 45c,15-351 Bia!ystok, Poland article info Article history: Received 28 December 2007 Received in revised form 24 June 2008 Accepted 3 July 2008 Available online 21 August 2008 Keywords: Enamel Whole saliva Viscosity Lubrication Friction coefficient Wear abstract Employing a special device simulating conditions of the masticatory process, two series tests were conducted involving studies of the tribological behaviour of enamel subjected to two- and three-body tests at different saliva viscosities. The studies showed no significant influences of saliva viscosity on the mean of the friction coefficient. However, below 2.08 mPa s the coefficient was unstable, i.e. scatter was observed. Both higher wear rate and higher scatter were observed for viscosities below of 1.68 mPa s. Comparing the wear results of the three- and two-body tests, no significant differences were obtained for viscosities above 1.68 mPa s, although for lower viscosities the wear rate in the presence of abrasive particles was higher than that obtained with pure saliva. Two regions with different tribological behaviour of enamel were observed in relation to saliva viscosity; the region with stable and low values of the tribological parameters beyond some threshold value, and the other with higher mean values of tribological parameters and their scattering. The author suggests that different concentrations of proteins are possible at the same low saliva viscosity, thus there are qualitatively different interactions between enamel surface and saliva components resulting in different lubrication regimes. & 2008 Elsevier Ltd. All rights reserved. 1. Introduction One of the important functions of saliva is to provide lubrication between hard tooth and soft (mucosal) tissues. Good lubricity of saliva decreases the friction at the tooth–tooth or tooth–food contact, as well as between teeth and tongue or oral mucosa [1–3]. As a result it substantially prevents lesions of soft tissues and reduces tooth wear. It is obvious that appropriate intra- oral lubrication is of a great significance for maintaining functions such as mastication, swallowing or the faculty of speech. It is accepted that the viscosity of saliva is one of the main factors affecting the friction force arising between lubricated surfaces during relative movement. Hence, many efforts have been concentrated on the determining of dependences between the rheology of native as well as artificial saliva and their lubricating performance [4–10]. However, the obtained results were ambig- uous and showed rather the complexity of lubricating mechanisms than simple correlations. It is clear that further investigations in this area are needed. It is commonly assumed that the decrease of the friction accompanying good saliva’s lubricity is correlated to the tooth wear decrease. Hence, the loss of tissues subjected to the frictional force action is not considered in the context of saliva viscosity. However, it is well known from the engineering field that the dependence between wear and friction has a very complex character, as there are many factors involved in the wear process, e.g. regime of lubrication. To the author’s knowledge, the relation between rheological properties of saliva and wear of tooth enamel has not been reported in the literature. The objective of this study was to investigate friction and wear behaviour of human enamel at different viscosities of whole human saliva, using a friction (wear)-testing device developed to characterize tribological properties of enamel at conditions close to those existing during mastication. 2. Materials and methods Fourteen healthy voluntaries (seven males and seven females) aged between 24 and 53 were donors of saliva. Approximately 6 ml of unstimulated whole saliva was collected from a single subject prior to each experiment during 10–15 min periods between 6 and 7 am. The subjects had refrained from eating, drinking and smoking at least a one-hour period prior to collection. The enamel samples were prepared from premolars freshly extracted for orthodontic reasons. 2.1. Viscosity measurements It is well known that saliva is a viscoelastic medium whose elastic and viscous components are dependent, among other things, on its aging [11]. Recently it has been shown that the ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/triboint Tribology International 0301-679X/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2008.07.001 à Tel.: +48 85 746 92 53; fax: +48 85746 9210. E-mail addresses: hsaj985@gazeta.pl, esaj@pb.bialystok.pl (E. Sajewicz). Tribology International 42 (2009) 327–332