Dental School
N. Burak, J. A. Kaidonis, L. C. Richards, G. C. Townsend.
Dental School, The University of Adelaide, South
Australia 5005
Summary
Previous in vitro studies have described the relationship between rates of enamel wear and variables such as applied load and lubricant pH. The aim of this study was to extend our understanding of tooth wear processes by considering the rate of wear in human dentine. Enamel was removed from extracted third molar teeth that had been sectioned mesiodistally. Moisture fluctuation within dentine was minimised by conducting all procedures under copious irrigation or in sealed containers of lubricant at pH=7.0. Specimens were subjected to wear using a purpose built apparatus at loads of 6.2 kg, 9.95 kg and 13.2 kg. All experiments were conducted with a unidirectional wear stroke of 3 mm at a rate of 80 cycles/minute for 75 minutes and repeated for 75 minutes. Dentine wear was assessed by specimen weight loss. At pH 7.0, wear rates ranged from 0.50 mg/103 cycles at a load of 6.2 kg to 0.77 mg/103 cycles when a load of 13.2 kg was applied. At higher loads, dentine wear rates were similar to enamel wear rates. Increasing load is associated with a progressive increase in the rate of dentine wear. This relationship differs significantly for enamel, reflecting fundamental differences in the composition and structure of these tissues.
The diagnosis and appropriate management of tooth wear is a significant clinical problem but our understanding of the basic processes involved in human tooth wear is far from complete. Clinical studies have demonstrated that enamel wear usually proceeds very slowly (Lambrechts et al, 1989; Pintado et al, 1997). Although we might expect dentine wear characteristics to vary from enamel due to structural differences, little is known about the nature of human dentine wear. This experimental study extends our knowledge of tooth wear by comparing data from dentine wear experiments with directly comparable information reported previously for human enamel (Kaidonis et al., 1998).
Specimens for this study were freshly extracted, non-carious third molar teeth. Approval for the tooth collection protocol was obtained from the appropriate ethics committee of The University of Adelaide (approval H/27/90). The specimens were stored and prepared in physiological Krebs’ solution. Teeth were fractured into halves mesiodistally along a predetermined fracture line. Enamel, pulpal and periodontal tissues were removed from each specimen and the paired dentine specimens were mounted in an electromechanical tooth wear machine which produced wear facets while load and pH variables were controlled (Kaidonis et al., 1998). An external lubricating system directed a constant volume of water (pH 7.0) over the wear facets to act as a lubricant.
Initial wear facets with standardised surface areas were produced by wearing paired specimens for 60 minutes at a load of 9.95 kg and pH of 7.0. Specimens were then worn in the wear machine for 75 minutes with a unidirectional wear stroke of 3 mm at a rate of 80 cycles/minute. Wear was assessed from the weight loss of specimens which was determined using an electronic analytical balance (A&D, R-182A). Hydration of the specimens was maintained by conducting the weighing procedure with specimens placed in sealed containers of lubricating fluid. After weighing, the experiment was then continued for a further 75 minutes before the specimens were re-weighed. Where the first and second estimates of the wear rate differed, the data for the specimen were rejected. The average weight loss and wear rate were calculated from five measurements for each specimen in experiments conducted at loads of 6.2 kg, 9.95 kg and 13.2 kg under pH 7.0 conditions.
There was a direct relationship between load and dentine wear rates at pH 7.0. Table 1 includes the wear rate data for dentine compared with previously published data for enamel. Enamel wear rates were less than dentine at the lighter loads of 6.2 kg (p<0.02) and 9.95 kg (p<0.05), whereas similar rates of wear were demonstrated for the two tissues at the highest load. Wear rates for enamel and dentine are summarised in Figure 1.
The observed differences between enamel and dentine wear rates at the lower loads supports the premise that structural factors affect human tooth wear processes. Within enamel, a load threshold of the order of 8 kg distinguishes two different rates of wear. At lower loads the high mineral content of enamel and its corresponding hardness result in relatively low wear rates when compared with dentine with its higher organic content and relative softness (Boyde, 1984; Beynon et al, 1991; Khera et al, 1990; Macho and Berner, 1993). However, the brittle nature of enamel results in relatively high wear rates at higher loads, whereas the connective tissue matrix of dentine makes it less prone to fracture under these conditions.
This study has provided information about basic differences in the mechanisms of wear in enamel and dentine with lubricant at pH=7 and over the load range 6.2 - 13.2 kg. Future investigation of the pattern of wear of dentine under other conditions is now a high priority.
This research was supported in part by the National Health and Medical Research Council of Australia.
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