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On the friction and wear behaviour of human tooth enamel and dentin
Wear 255 (2003) 967–974
On the friction and wear behaviour of human tooth enamel and dentin夽 J. Zheng, Z.R. Zhou∗ , J. Zhang, H. Li, H.Y. Yu Tribology Research Institute, Southwest Jiaotong University, Chengdu 610031, China
Abstract The differences in the friction and wear behaviour of different human tooth structures against titanium have been investigated using a reciprocating apparatus containing an artificial saliva solution. The effects of enamel, enamel rod orientation, and dentin were of particular interest. A normal load of 20 N, reciprocating amplitude of 500 m, and frequency of 2 Hz were used. Tests lasting up to 5000 cycles were conducted. The micro-indentation hardness of worn teeth specimens was measured. The enamel layer exhibits relatively low friction and good wear resistance. The number of cycles of low friction coefficient, prior to transition to a higher value, depends on enamel thickness and enamel rod orientation. Wear mechanisms differ between layers in the tooth structure. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Human tooth; Friction and wear; Titanium
1. Introduction Human teeth possess an unique structure composing of enamel, dentin-enamel junction (DEJ), dentin and pulp (shown in Fig. 1), and each zone is anisotropic due to the enamel rod orientation, dentinal tubule, etc. [1]. Enamel is the hardest tissue in the human body because of the existence of enamel rods and is a composite consisting of both a mineral and an organic phase. Compared with enamel, dentin is widely considered to be elastic and soft. Attrition of human tooth is a natural and unavoidable process [2,3]. For young people, the enamel of a thickness of 2–3 mm is served as an occlusal surface. However, with ageing and various pathologic factors, enamel will be gradually ground down by mastication. As a result, superficial dentin will be exposed and worn partially. The study of Xu et al. [4] has indicated that the microcracks formed on enamel interact with the DEJ and the enamel rods strongly, and that the mechanical properties of teeth are functions of microstructural orientations [4]. Mechanical properties, chemistry and microstructure of enamel in a maxillary second molar (M2 ) have also been characterised as a function of location on an axial cross-section [5]. Based on the previous results, it 夽 This paper was unable to be presented by the author at the 14th International Conference on Wear of Materials due to the prevailing political situation at the time. ∗ Corresponding author. Tel.: +86-28-87600-715; fax: +86-28-87600-971. E-mail address: [email protected] (Z.R. Zhou).
is reasonable to deduce that the tribological behaviours of human teeth also interact strongly with both microstructure orientation and location. Understanding of the friction and wear behaviour of human teeth would help the clinical treatment for teeth and develop new dental restorative materials [2–12]. Many studies on the wear of restorative materials, such as ceramic, composites, and metallic alloys, have been carried out mostly by means of traditional pin-disc apparatus. However, fewer studies on the friction and wear behaviour of human teeth were conducted, and previous studies have hardly attempted to investigate the wear-microstructure interaction of teeth in detail. It is also reported that the choice of testing machine and parameters influences the reliability of in vitro wear tests of human tooth to a great extent [7]. To better simulate mastication pattern, friction and wear tests on human teeth have been performed in a reciprocating sliding mode, instead of unidirectional sliding pion/disc mode, in our previous work [10], in which only the friction and wear behaviours in the dentin axial section of human teeth against two different materials were studied. The results show that the friction pair composed of pure titanium and human teeth has better tribological performance. In this paper, more detailed research was performed on the friction and wear behaviour of human tooth enamel and dentin using a reciprocating apparatus containing an artificial saliva solution (see Fig. 2). The wear-microstructure interactions were examined by evaluating wear behaviours in both axial and occlusal orientations and in different positions.
0043-1648/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0043-1648(03)00079-6
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(10 mm × 10 mm × 20 mm in size). One half of each tooth was ground and polished down to 0.5 mm on the occlusal surface (this orientation is referred to as “occlusal section”), see sub-part (a) of Fig. 3A, while another half of each tooth was ground and polished on a plane perpendicular to the occlusal surface, removing approximately one-fourth of the tooth, so that the polished surface intersected the cusp tips and exhibited the maximised enamel thickness (see Fig. 4A). This orientation is referred to as “axial section”. Samples were ground, using abrasive papers, from 360, 500, 800, 1200 to 1500 grit, followed by diamond paste polishing from 10, 5 to 1 m. After polishing, the samples were stored in distilled water at 4 ◦ C. The roughness of the flat samples was measured by means of high sensitive profilometer with Ra = 0.20. There were no significant differences between the two types of samples. The teeth were dehydrated partly during preparation, but efforts were made both to shorten the dry time and to keep the preparation time approximately the same for each sample. High-purity titanium (C 0.10%; Si 0.15%; H 0.02%; O 0.20%; N 0.05%; Fe 0.30%; Ti the balance) with a hardness of 240 HV and 40 mm in diameter was used as a ball sample. All samples were cleaned by alcohol before testing.
Fig. 1. Profile of the natural tooth.
2.2. Microhardness characterization
Fig. 2. Schema of friction and wear test rig.
Particular attention was paid to the effects of enamel, enamel rod orientation, and dentin on tribological behaviour of teeth.
2. Materials and methods 2.1. Sample preparation Flat samples used in this study were freshly extracted human teeth without caries. The teeth were then placed in distilled water at 4 ◦ C to avoid dehydration before sample preparation. Two mandibular second molars (M2 ) and one mandibular third molar (M3 ), which were obtained from different young males, were used in this study. Before testing, each of the three teeth was cut into two halves using a diamond saw, with the cut lying perpendicular to the buccolingual division line. The cutting was conducted under a water-cooling condition, aiming at mitigating local overheat which can result in dehydration and changes in microstructure and chemistry of human teeth. Each half of tooth was embedded into a steel-made mold with self-setting plastic
Microhardness of each contact surface of human teeth was tested before wear test. Ten to 20 indentations under a load of 50 g were produced for each polished surface, using a Vickers diamond indenter in a microhardness tester (MVK–H21, Japan). The lengths of indentation diagonals were measured under optical microscopy immediately after indenting and converted to microhardness automatically. The whole process of indentations were completed within 40 min at 25 ◦ C. Effort was also made to keep the indentation time approximately the same for each sample. 2.3. Friction and wear test Two-body in vitro wear tests were conducted in a ball-on-flat configuration (Fig. 2) using a reciprocating horizontal tribometer. Variations of tangential force versus reciprocating displacement were recorded automatically as a function of cycles. A normal force of 20 N, reciprocating amplitude of 500 m, and frequency of 2 Hz were used for all the wear tests. Tests up to 5000 cycles were conducted with artificial saliva bath lubrication. The artificial saliva (its composition is listed in Table 1) was prepared according to Fusayama’s guidelines with Holland’s modification [13,14]. To better simulate real wear condition of human
Table 1 Composition of artificial saliva NaCl
KCl
CaCl2 ·2H2 O
NaH2 PO4 ·2H2 O
Na2 S·9H2 O
Urea
Distilled water
0.4 g
0.4 g
0.795 g
0.78 g
0.005 g
1g
1000 ml
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Fig. 3. Friction behaviour of the natural tooth parallel to the occlusal section. (A) Contact position for different wear tests. (B) Variation of tangential force vs. reciprocating amplitude as a function of test cycles: (I) corresponding to the sub-part (a) of A, (II) corresponding to the sub-part (d) of A, (III) corresponding to the sub-part (e) of A. (C) Variations of the friction coefficient at different contact positions.
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Fig. 4. Friction behaviour of the natural tooth in the axial section. (A) Contact position for different wear tests. (B) Variations of the friction coefficient at different contact positions.
teeth, the choice of these parameters was based on clinical experience and literatures. During the chewing process of human beings, the magnitude of masticatory force in the oral cavity ranges from 3 to 36 N [3]. For occlusal section of each human tooth, wear test was first performed on sample surface. After microscopic examinations on morphology of wear scar and measurement of wear depth, the worn surface was removed by about 0.5 mm through the grinding and polishing process as mentioned before. A new surface was obtained as a result, and ready for second wear test. According to this method, wear tests were performed gradually from the outer enamel to dentin in every sample. And for axial section, wear tests were conducted in enamel, DEJ and dentin, respectively, with the three wear scars aligned. Wear scars were examined both by optical microscope and by laser scanning microscope (OLYMPUS OLS1100, Japan). Wear depth was measured by a profilometer.
3. Results The results of the wear tests for a mandibular M2 were described as follows. 3.1. Friction behaviour 3.1.1. Contact surface parallel to the occlusal section In the occlusal section, from the enamel to the dentin (Fig. 3A), friction logs describing variations of tangential
force versus displacement amplitude as a function of number of cycles, and corresponding coefficient of friction calculated from the amplitude extremities are shown in Fig. 3B and C. Near the occlusal surface, enamel layer of about 2 mm on tooth crown has shown excellent friction behaviour. Lower tangential force and the friction coefficient are observed at the early stage. The coefficient is about 0.10 and remains constant up to 1600 cycles (Fig. 3C). Then the coefficient increases rapidly to 0.80, and finally remains at about 0.85. With the enamel layer being thinned by mechanical polishing, the evolution of the coefficient was found to be similar to each other, while the number of cycles of low friction coefficient, prior to transition to a higher value, is reduced (Fig. 3C). Fig. 3C also shows the evolution of the coefficient of friction near the DEJ, where the lower value only lasts for about 50–100 cycles. When friction occurs on the superficial dentin, the coefficient increases rapidly from the first cycle. 3.1.2. Contact surface in the axial section In order to explore the effect of microstructural orientation on tribological behaviour of human tooth, wear tests were also conducted with same experimental parameters in axial section of the mandibular M2 (Fig. 4A). Three contact zones, located at the enamel, DEJ and dentin, respectively, were chosen. The variations of friction coefficient in the three contact zones are shown in Fig. 4B. For the enamel zone, the friction coefficient is about 0.26 before 10 cycles and increases rapidly to 0.77 between 10 and 50 cycles, and then increases slowly to a saturation value of 0.87 after 1200
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cycles (Fig. 4B). In the DEJ zone, the coefficient increases rapidly at the early stage and then increases gradually to 0.97 from 10 cycles to 5000 cycles (Fig. 4B). However, for the dentin zone, the variation of the coefficient fluctuates between 0.3 and 0.5 before 500 cycles, and between 0.5 and 0.85 after 500 cycles (Fig. 4B). The fluctuations may be attributed to the special dentin structure with lacuna. There are more fluctuations in the axial section than in the occlusal section. 3.2. Wear behaviour In the occlusal section, laser scanning microscope examinations after tests show three typical wear scars from the enamel to the dentin. There are many particles attached to worn surface of enamel (Fig. 5a), while a strong plough effect (Fig. 5b) due to abrasive wear along motion direction is observed on the worn surface of dentin with many dentinal tubule (black dots in Fig. 5b). In the DEJ, two different regions are observed clearly on the contact surface (Fig. 5c). The region on the left side is covered by many particles and the region on the right side is full of ploughs. Profile measurement shows significant increases both in depth and in area of wear mark from the enamel to the dentin, as shown in Fig. 6, but the results of the layer-to-layer variation in
Fig. 6. Profile measurement on wear scars parallel to the occlusal section from the enamel to dentin zone.
hardness (H) exhibit opposite trend: H decreases by 17% from the outer layer of enamel to the DEJ and further by 77% from the DEJ to the dentin (Fig. 7). Each value of H is the average of 10–20 indentations under same load in one layer. The standard deviation for these average values ranges from 2 HV, for the dentin, to 20 HV, for the vicinity of DEJ. It is noted that very few micro-cracks were observed at the cross-section perpendicular to the occlusal surface.
Fig. 5. Three types of wear scars observation parallel to the occlusal section at: (a) enamel zone; (b) dentin zone; (c) DEJ zone.
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Fig. 7. Variations of wear depth and hardness as function of the distance from the enamel to dentin zone in the occlusal section.
In the axial section, microscopic examinations have also been performed on wear scars from the enamel to the dentin. Three different types of scars were also obtained, as shown in Fig. 8. It can be seen that the wear morphologies are similar to those of the scars observed in occlusal section. The main difference is that the wear depth of the scars in the axial section is bigger than that of the scars in occlusal section (Fig. 9). In addition, the wear of titanium sample was examined after testing. A few particles are observed both on the wear
Fig. 9. A comparison of wear depth between different contact zones for two different orientations.
scar of titanium ball and on the worn enamel surface. Also found is that few particles are observed on the worn dentin surface. So it is deduced that a transfer layer forms only on the enamel. To determine whether the intra-tooth variations reported previously are common to other molars, a second mandibular M2 and a mandibular M3 (each from different individuals) were also examined, on which similar friction and wear behaviours are observed.
Fig. 8. Three types of wear scars observation in the axial section at: (a) enamel zone; (b) dentin zone; (c) DEJ zone.
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4. Discussion The laser scanning micrographs in Figs. 5 and 8 show significant difference in morphology of wear scar between the enamel and the dentin, which appears both in the occlusal section and axial section. There are many particles attached to the worn enamel surface, while plenty of strong ploughs along motion direction appears on the worn dentin surface. Based on the fact that enamel is more fragile than dentin, it could be inferred that the particles obtained at the worn enamel are mainly the products of microcracking induced in the process of wear, and the ploughs are consequences of plastic deformation. The inference is also supported by the result of Xu et al. [4]. Xu et al. [4] found that during indentation of human teeth, cracks were observed in enamel with low indentation energy, whilst no crack was observed for dentin with high indentation energy. This behaviour was explained by a different cutting mechanism [4]. That is, the enamel is removed by a microfracture process, whereas the dentin is probably removed by a ductile chip formation process. Variations in the friction coefficient as well as the depth and area of wear mark between the outer and the interior of enamel in the occlusal section are also evident in our study. It is observed in Fig. 3 that the number of cycles of low friction coefficient, prior to transition to a higher value, decreases from the outer layer of enamel to DEJ, although the evolution of coefficient is similar. In addition, the curves in Fig. 6 exhibit the increases in depth and area of wear mark from the outer enamel to DEJ. The observations suggest that the friction and wear behaviour changes from location to location for the same tooth. Furthermore, an obvious decrease in H from about 360 HV in the outer enamel to 60 HV in the dentin is observed (Fig. 7). It implies that the H value is different not only between enamel and dentin but also between different occlusal layers of the enamel for a tooth. Cuy et al. [5] also suggested that such mechanical properties as H and elastic modulus (E) of human enamel in a maxillary M2 were the function of position in an axial cross-section [5]. The difference in H of different layers of a tooth clearly illustrates an increase in wear depth and the different wear mechanisms from the enamel to the dentin since wear behaviour is closely associated with mechanical properties such as H and E of materials tested. Xu et al. [4] reported a difference in H, toughness and E of the enamel between occlusal section and axial section [4]. However, none of previous studies has made on the effect of tooth orientation on tribological behaviour. The plots in Figs. 3 and 4 and the bars in Fig. 9 demonstrate a significant difference in friction and wear behaviours between the two orientations. Firstly, friction coefficient of the enamel generally appears to remain a low value for a longer time before increasing in the occlusal section than in the axial section, as shown in Figs. 3 and 4. Secondly, both depth and area of the wear scars are bigger in the axial section than in the occlusal
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section, as shown in Fig. 9. The observations imply that it is more difficult for brittle failure of enamel to occur in the occlusal section than in the axial section, which is consistent with the research conducted by Xu et al. [4]. According to Xu et al., for the occlusal section, most of the enamel rods appeared nearly perpendicular to the surface, while in the axial section, about half of the enamel rods appeared nearly parallel to the surface, and the other half appeared angled to the surface [4]. Therefore, the anisotropic friction and wear behaviour of enamel could be attributed to the enamel rod orientation. Similar phenomenon is observed at the dentin zone due to the effect of the dentinal tubule orientation. Although the number of samples examined is limited, these results, to the best of our knowledge, represent a detailed analysis of variations in friction and wear behaviour within an individual tooth. And our future study will explore the interaction of other factors, such as tooth-to-tooth variation, different conditions of saliva lubrication, and “the third body”, for example, food slurry, on wear behaviour. 5. Conclusion This study shows that the friction and wear behaviours of human teeth are functions of orientation and position. Within the limits of our research, the following conclusions were drawn: 1. Lower friction coefficient and better wear resistance are observed in the enamel zone than in the dentin, which appears both on the occlusal section and axial section. The number of cycles of low friction coefficient, prior to transition to a higher value, depends on enamel thickness and enamel rod orientation. Wear mechanisms differ between layers in the tooth structure due to a significant variation in hardness. 2. Tribological behaviours of tooth strongly interact with microstructure orientations. Wear mechanisms differ between the occlusal section and the axial section in a tooth. The orientation of the enamel rods and the dentinal tubule plays an important role in wear resistance. 3. Human enamel in occlusal section will be ground down gradually with ageing, and the dentin might become exposed, causing high sensitivity to normal irritations. The present research shows that tribological behaviour of human tooth may be closely correlated with the thickness of enamel. It suggests that high-quality restorative materials should possess microstructures and mechanical properties similar to those of teeth in living condition in the bio-tribological point of view. Otherwise pathologic overwear will occur on the surface of teeth.
Acknowledgements This work was supported by National Natural Science Foundation of China (No. 50275126).
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References [1] F.Z. Yu, et al., Pathology of Oral Structure, People Hygiene Publishing, Beijing, PRC, 2000 (in Chinese). [2] R. Koczorowski, S. Wloch, Evaluation of wear of selected prosthetic materials in contact with enamel and dentin, J. Prosthet. Dent. 81 (4) (1999) 453–459. [3] D. Dowson, History of Tribology, Professional Engineering Publishing Limited, London, UK, 1998, p. 577. [4] H.H.K. Xu, D.T. Smith, E. Romberg, et al., Indentation damage and mechanical properties of human enamel and dentin, J. Dent. Res. 77 (3) (1998) 472–480. [5] J.L. Cuy, A.B. Mann, K.J. Livi, et al., Nanoindentation mapping of the mechanical properties of human molar tooth enamel, Arch. Oral Biol. 47 (2002) 281–291. [6] K.W. Hemmings, U.R. Darbar, S. Vaughan, Tooth wear treated with direct composite restorations at an increased vertical dimension: result at 30 months, J. Prosthet. Dent. 83 (3) (2000) 287–293. [7] P. Magne, Oh. Won-Suck, M.R. Pintado, Wear of enamel and veneering ceramics after laboratory and chairside finishing procedures, J. Prosthet. Dent. 82 (6) (1999) 669–679.
[8] V.S. Nagarajan, B.J. Hockey, S. Jahanmir, Contact wear mechanisms of a dental composite with high filler content, J. Mater. Sci. 35 (2000) 487–496. [9] X.Q. Hu, P.M. Marquis, A.C. Shortall, Two-body in vitro wear study of some current dental composites and amalgams, J. Prosthet. Dent. 82 (2) (1999) 214–220. [10] H. Li, Z.R. Zhou, Wear behavior of human teeth in dry and artificial saliva conditions, Wear 249 (2002) 980–984. [11] P. Derand, P. Vereby, Wear of low-fusing dental porcelains, J. Prosthet. Dent. 81 (4) (1999) 460–463. [12] F.J. Gil, A. Espias, L.A. Sánchez, et al., Comparison of the abrasive wear resistance between amalgams, hybrid composite material and different dental cements, Int. Dent. J. 49 (6) (1999) 337– 342. [13] A. Embong, J. Glyn-Jones, A. Harrison, The wear effects of selected composites on restorative materials and enamel, Dent. Mater. 3 (1987) 236–240. [14] R.I. Holland, Galvanic currents between gold and amalgam, Scand. J. Dent. Res. 88 (1980) 269–272.
On the friction and wear behaviour of human tooth enamel and dentin夽 J. Zheng, Z.R. Zhou∗ , J. Zhang, H. Li, H.Y. Yu Tribology Research Institute, Southwest Jiaotong University, Chengdu 610031, China
Abstract The differences in the friction and wear behaviour of different human tooth structures against titanium have been investigated using a reciprocating apparatus containing an artificial saliva solution. The effects of enamel, enamel rod orientation, and dentin were of particular interest. A normal load of 20 N, reciprocating amplitude of 500 m, and frequency of 2 Hz were used. Tests lasting up to 5000 cycles were conducted. The micro-indentation hardness of worn teeth specimens was measured. The enamel layer exhibits relatively low friction and good wear resistance. The number of cycles of low friction coefficient, prior to transition to a higher value, depends on enamel thickness and enamel rod orientation. Wear mechanisms differ between layers in the tooth structure. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Human tooth; Friction and wear; Titanium
1. Introduction Human teeth possess an unique structure composing of enamel, dentin-enamel junction (DEJ), dentin and pulp (shown in Fig. 1), and each zone is anisotropic due to the enamel rod orientation, dentinal tubule, etc. [1]. Enamel is the hardest tissue in the human body because of the existence of enamel rods and is a composite consisting of both a mineral and an organic phase. Compared with enamel, dentin is widely considered to be elastic and soft. Attrition of human tooth is a natural and unavoidable process [2,3]. For young people, the enamel of a thickness of 2–3 mm is served as an occlusal surface. However, with ageing and various pathologic factors, enamel will be gradually ground down by mastication. As a result, superficial dentin will be exposed and worn partially. The study of Xu et al. [4] has indicated that the microcracks formed on enamel interact with the DEJ and the enamel rods strongly, and that the mechanical properties of teeth are functions of microstructural orientations [4]. Mechanical properties, chemistry and microstructure of enamel in a maxillary second molar (M2 ) have also been characterised as a function of location on an axial cross-section [5]. Based on the previous results, it 夽 This paper was unable to be presented by the author at the 14th International Conference on Wear of Materials due to the prevailing political situation at the time. ∗ Corresponding author. Tel.: +86-28-87600-715; fax: +86-28-87600-971. E-mail address: [email protected] (Z.R. Zhou).
is reasonable to deduce that the tribological behaviours of human teeth also interact strongly with both microstructure orientation and location. Understanding of the friction and wear behaviour of human teeth would help the clinical treatment for teeth and develop new dental restorative materials [2–12]. Many studies on the wear of restorative materials, such as ceramic, composites, and metallic alloys, have been carried out mostly by means of traditional pin-disc apparatus. However, fewer studies on the friction and wear behaviour of human teeth were conducted, and previous studies have hardly attempted to investigate the wear-microstructure interaction of teeth in detail. It is also reported that the choice of testing machine and parameters influences the reliability of in vitro wear tests of human tooth to a great extent [7]. To better simulate mastication pattern, friction and wear tests on human teeth have been performed in a reciprocating sliding mode, instead of unidirectional sliding pion/disc mode, in our previous work [10], in which only the friction and wear behaviours in the dentin axial section of human teeth against two different materials were studied. The results show that the friction pair composed of pure titanium and human teeth has better tribological performance. In this paper, more detailed research was performed on the friction and wear behaviour of human tooth enamel and dentin using a reciprocating apparatus containing an artificial saliva solution (see Fig. 2). The wear-microstructure interactions were examined by evaluating wear behaviours in both axial and occlusal orientations and in different positions.
0043-1648/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0043-1648(03)00079-6
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(10 mm × 10 mm × 20 mm in size). One half of each tooth was ground and polished down to 0.5 mm on the occlusal surface (this orientation is referred to as “occlusal section”), see sub-part (a) of Fig. 3A, while another half of each tooth was ground and polished on a plane perpendicular to the occlusal surface, removing approximately one-fourth of the tooth, so that the polished surface intersected the cusp tips and exhibited the maximised enamel thickness (see Fig. 4A). This orientation is referred to as “axial section”. Samples were ground, using abrasive papers, from 360, 500, 800, 1200 to 1500 grit, followed by diamond paste polishing from 10, 5 to 1 m. After polishing, the samples were stored in distilled water at 4 ◦ C. The roughness of the flat samples was measured by means of high sensitive profilometer with Ra = 0.20. There were no significant differences between the two types of samples. The teeth were dehydrated partly during preparation, but efforts were made both to shorten the dry time and to keep the preparation time approximately the same for each sample. High-purity titanium (C 0.10%; Si 0.15%; H 0.02%; O 0.20%; N 0.05%; Fe 0.30%; Ti the balance) with a hardness of 240 HV and 40 mm in diameter was used as a ball sample. All samples were cleaned by alcohol before testing.
Fig. 1. Profile of the natural tooth.
2.2. Microhardness characterization
Fig. 2. Schema of friction and wear test rig.
Particular attention was paid to the effects of enamel, enamel rod orientation, and dentin on tribological behaviour of teeth.
2. Materials and methods 2.1. Sample preparation Flat samples used in this study were freshly extracted human teeth without caries. The teeth were then placed in distilled water at 4 ◦ C to avoid dehydration before sample preparation. Two mandibular second molars (M2 ) and one mandibular third molar (M3 ), which were obtained from different young males, were used in this study. Before testing, each of the three teeth was cut into two halves using a diamond saw, with the cut lying perpendicular to the buccolingual division line. The cutting was conducted under a water-cooling condition, aiming at mitigating local overheat which can result in dehydration and changes in microstructure and chemistry of human teeth. Each half of tooth was embedded into a steel-made mold with self-setting plastic
Microhardness of each contact surface of human teeth was tested before wear test. Ten to 20 indentations under a load of 50 g were produced for each polished surface, using a Vickers diamond indenter in a microhardness tester (MVK–H21, Japan). The lengths of indentation diagonals were measured under optical microscopy immediately after indenting and converted to microhardness automatically. The whole process of indentations were completed within 40 min at 25 ◦ C. Effort was also made to keep the indentation time approximately the same for each sample. 2.3. Friction and wear test Two-body in vitro wear tests were conducted in a ball-on-flat configuration (Fig. 2) using a reciprocating horizontal tribometer. Variations of tangential force versus reciprocating displacement were recorded automatically as a function of cycles. A normal force of 20 N, reciprocating amplitude of 500 m, and frequency of 2 Hz were used for all the wear tests. Tests up to 5000 cycles were conducted with artificial saliva bath lubrication. The artificial saliva (its composition is listed in Table 1) was prepared according to Fusayama’s guidelines with Holland’s modification [13,14]. To better simulate real wear condition of human
Table 1 Composition of artificial saliva NaCl
KCl
CaCl2 ·2H2 O
NaH2 PO4 ·2H2 O
Na2 S·9H2 O
Urea
Distilled water
0.4 g
0.4 g
0.795 g
0.78 g
0.005 g
1g
1000 ml
J. Zheng et al. / Wear 255 (2003) 967–974
969
Fig. 3. Friction behaviour of the natural tooth parallel to the occlusal section. (A) Contact position for different wear tests. (B) Variation of tangential force vs. reciprocating amplitude as a function of test cycles: (I) corresponding to the sub-part (a) of A, (II) corresponding to the sub-part (d) of A, (III) corresponding to the sub-part (e) of A. (C) Variations of the friction coefficient at different contact positions.
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Fig. 4. Friction behaviour of the natural tooth in the axial section. (A) Contact position for different wear tests. (B) Variations of the friction coefficient at different contact positions.
teeth, the choice of these parameters was based on clinical experience and literatures. During the chewing process of human beings, the magnitude of masticatory force in the oral cavity ranges from 3 to 36 N [3]. For occlusal section of each human tooth, wear test was first performed on sample surface. After microscopic examinations on morphology of wear scar and measurement of wear depth, the worn surface was removed by about 0.5 mm through the grinding and polishing process as mentioned before. A new surface was obtained as a result, and ready for second wear test. According to this method, wear tests were performed gradually from the outer enamel to dentin in every sample. And for axial section, wear tests were conducted in enamel, DEJ and dentin, respectively, with the three wear scars aligned. Wear scars were examined both by optical microscope and by laser scanning microscope (OLYMPUS OLS1100, Japan). Wear depth was measured by a profilometer.
3. Results The results of the wear tests for a mandibular M2 were described as follows. 3.1. Friction behaviour 3.1.1. Contact surface parallel to the occlusal section In the occlusal section, from the enamel to the dentin (Fig. 3A), friction logs describing variations of tangential
force versus displacement amplitude as a function of number of cycles, and corresponding coefficient of friction calculated from the amplitude extremities are shown in Fig. 3B and C. Near the occlusal surface, enamel layer of about 2 mm on tooth crown has shown excellent friction behaviour. Lower tangential force and the friction coefficient are observed at the early stage. The coefficient is about 0.10 and remains constant up to 1600 cycles (Fig. 3C). Then the coefficient increases rapidly to 0.80, and finally remains at about 0.85. With the enamel layer being thinned by mechanical polishing, the evolution of the coefficient was found to be similar to each other, while the number of cycles of low friction coefficient, prior to transition to a higher value, is reduced (Fig. 3C). Fig. 3C also shows the evolution of the coefficient of friction near the DEJ, where the lower value only lasts for about 50–100 cycles. When friction occurs on the superficial dentin, the coefficient increases rapidly from the first cycle. 3.1.2. Contact surface in the axial section In order to explore the effect of microstructural orientation on tribological behaviour of human tooth, wear tests were also conducted with same experimental parameters in axial section of the mandibular M2 (Fig. 4A). Three contact zones, located at the enamel, DEJ and dentin, respectively, were chosen. The variations of friction coefficient in the three contact zones are shown in Fig. 4B. For the enamel zone, the friction coefficient is about 0.26 before 10 cycles and increases rapidly to 0.77 between 10 and 50 cycles, and then increases slowly to a saturation value of 0.87 after 1200
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cycles (Fig. 4B). In the DEJ zone, the coefficient increases rapidly at the early stage and then increases gradually to 0.97 from 10 cycles to 5000 cycles (Fig. 4B). However, for the dentin zone, the variation of the coefficient fluctuates between 0.3 and 0.5 before 500 cycles, and between 0.5 and 0.85 after 500 cycles (Fig. 4B). The fluctuations may be attributed to the special dentin structure with lacuna. There are more fluctuations in the axial section than in the occlusal section. 3.2. Wear behaviour In the occlusal section, laser scanning microscope examinations after tests show three typical wear scars from the enamel to the dentin. There are many particles attached to worn surface of enamel (Fig. 5a), while a strong plough effect (Fig. 5b) due to abrasive wear along motion direction is observed on the worn surface of dentin with many dentinal tubule (black dots in Fig. 5b). In the DEJ, two different regions are observed clearly on the contact surface (Fig. 5c). The region on the left side is covered by many particles and the region on the right side is full of ploughs. Profile measurement shows significant increases both in depth and in area of wear mark from the enamel to the dentin, as shown in Fig. 6, but the results of the layer-to-layer variation in
Fig. 6. Profile measurement on wear scars parallel to the occlusal section from the enamel to dentin zone.
hardness (H) exhibit opposite trend: H decreases by 17% from the outer layer of enamel to the DEJ and further by 77% from the DEJ to the dentin (Fig. 7). Each value of H is the average of 10–20 indentations under same load in one layer. The standard deviation for these average values ranges from 2 HV, for the dentin, to 20 HV, for the vicinity of DEJ. It is noted that very few micro-cracks were observed at the cross-section perpendicular to the occlusal surface.
Fig. 5. Three types of wear scars observation parallel to the occlusal section at: (a) enamel zone; (b) dentin zone; (c) DEJ zone.
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Fig. 7. Variations of wear depth and hardness as function of the distance from the enamel to dentin zone in the occlusal section.
In the axial section, microscopic examinations have also been performed on wear scars from the enamel to the dentin. Three different types of scars were also obtained, as shown in Fig. 8. It can be seen that the wear morphologies are similar to those of the scars observed in occlusal section. The main difference is that the wear depth of the scars in the axial section is bigger than that of the scars in occlusal section (Fig. 9). In addition, the wear of titanium sample was examined after testing. A few particles are observed both on the wear
Fig. 9. A comparison of wear depth between different contact zones for two different orientations.
scar of titanium ball and on the worn enamel surface. Also found is that few particles are observed on the worn dentin surface. So it is deduced that a transfer layer forms only on the enamel. To determine whether the intra-tooth variations reported previously are common to other molars, a second mandibular M2 and a mandibular M3 (each from different individuals) were also examined, on which similar friction and wear behaviours are observed.
Fig. 8. Three types of wear scars observation in the axial section at: (a) enamel zone; (b) dentin zone; (c) DEJ zone.
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4. Discussion The laser scanning micrographs in Figs. 5 and 8 show significant difference in morphology of wear scar between the enamel and the dentin, which appears both in the occlusal section and axial section. There are many particles attached to the worn enamel surface, while plenty of strong ploughs along motion direction appears on the worn dentin surface. Based on the fact that enamel is more fragile than dentin, it could be inferred that the particles obtained at the worn enamel are mainly the products of microcracking induced in the process of wear, and the ploughs are consequences of plastic deformation. The inference is also supported by the result of Xu et al. [4]. Xu et al. [4] found that during indentation of human teeth, cracks were observed in enamel with low indentation energy, whilst no crack was observed for dentin with high indentation energy. This behaviour was explained by a different cutting mechanism [4]. That is, the enamel is removed by a microfracture process, whereas the dentin is probably removed by a ductile chip formation process. Variations in the friction coefficient as well as the depth and area of wear mark between the outer and the interior of enamel in the occlusal section are also evident in our study. It is observed in Fig. 3 that the number of cycles of low friction coefficient, prior to transition to a higher value, decreases from the outer layer of enamel to DEJ, although the evolution of coefficient is similar. In addition, the curves in Fig. 6 exhibit the increases in depth and area of wear mark from the outer enamel to DEJ. The observations suggest that the friction and wear behaviour changes from location to location for the same tooth. Furthermore, an obvious decrease in H from about 360 HV in the outer enamel to 60 HV in the dentin is observed (Fig. 7). It implies that the H value is different not only between enamel and dentin but also between different occlusal layers of the enamel for a tooth. Cuy et al. [5] also suggested that such mechanical properties as H and elastic modulus (E) of human enamel in a maxillary M2 were the function of position in an axial cross-section [5]. The difference in H of different layers of a tooth clearly illustrates an increase in wear depth and the different wear mechanisms from the enamel to the dentin since wear behaviour is closely associated with mechanical properties such as H and E of materials tested. Xu et al. [4] reported a difference in H, toughness and E of the enamel between occlusal section and axial section [4]. However, none of previous studies has made on the effect of tooth orientation on tribological behaviour. The plots in Figs. 3 and 4 and the bars in Fig. 9 demonstrate a significant difference in friction and wear behaviours between the two orientations. Firstly, friction coefficient of the enamel generally appears to remain a low value for a longer time before increasing in the occlusal section than in the axial section, as shown in Figs. 3 and 4. Secondly, both depth and area of the wear scars are bigger in the axial section than in the occlusal
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section, as shown in Fig. 9. The observations imply that it is more difficult for brittle failure of enamel to occur in the occlusal section than in the axial section, which is consistent with the research conducted by Xu et al. [4]. According to Xu et al., for the occlusal section, most of the enamel rods appeared nearly perpendicular to the surface, while in the axial section, about half of the enamel rods appeared nearly parallel to the surface, and the other half appeared angled to the surface [4]. Therefore, the anisotropic friction and wear behaviour of enamel could be attributed to the enamel rod orientation. Similar phenomenon is observed at the dentin zone due to the effect of the dentinal tubule orientation. Although the number of samples examined is limited, these results, to the best of our knowledge, represent a detailed analysis of variations in friction and wear behaviour within an individual tooth. And our future study will explore the interaction of other factors, such as tooth-to-tooth variation, different conditions of saliva lubrication, and “the third body”, for example, food slurry, on wear behaviour. 5. Conclusion This study shows that the friction and wear behaviours of human teeth are functions of orientation and position. Within the limits of our research, the following conclusions were drawn: 1. Lower friction coefficient and better wear resistance are observed in the enamel zone than in the dentin, which appears both on the occlusal section and axial section. The number of cycles of low friction coefficient, prior to transition to a higher value, depends on enamel thickness and enamel rod orientation. Wear mechanisms differ between layers in the tooth structure due to a significant variation in hardness. 2. Tribological behaviours of tooth strongly interact with microstructure orientations. Wear mechanisms differ between the occlusal section and the axial section in a tooth. The orientation of the enamel rods and the dentinal tubule plays an important role in wear resistance. 3. Human enamel in occlusal section will be ground down gradually with ageing, and the dentin might become exposed, causing high sensitivity to normal irritations. The present research shows that tribological behaviour of human tooth may be closely correlated with the thickness of enamel. It suggests that high-quality restorative materials should possess microstructures and mechanical properties similar to those of teeth in living condition in the bio-tribological point of view. Otherwise pathologic overwear will occur on the surface of teeth.
Acknowledgements This work was supported by National Natural Science Foundation of China (No. 50275126).
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References [1] F.Z. Yu, et al., Pathology of Oral Structure, People Hygiene Publishing, Beijing, PRC, 2000 (in Chinese). [2] R. Koczorowski, S. Wloch, Evaluation of wear of selected prosthetic materials in contact with enamel and dentin, J. Prosthet. Dent. 81 (4) (1999) 453–459. [3] D. Dowson, History of Tribology, Professional Engineering Publishing Limited, London, UK, 1998, p. 577. [4] H.H.K. Xu, D.T. Smith, E. Romberg, et al., Indentation damage and mechanical properties of human enamel and dentin, J. Dent. Res. 77 (3) (1998) 472–480. [5] J.L. Cuy, A.B. Mann, K.J. Livi, et al., Nanoindentation mapping of the mechanical properties of human molar tooth enamel, Arch. Oral Biol. 47 (2002) 281–291. [6] K.W. Hemmings, U.R. Darbar, S. Vaughan, Tooth wear treated with direct composite restorations at an increased vertical dimension: result at 30 months, J. Prosthet. Dent. 83 (3) (2000) 287–293. [7] P. Magne, Oh. Won-Suck, M.R. Pintado, Wear of enamel and veneering ceramics after laboratory and chairside finishing procedures, J. Prosthet. Dent. 82 (6) (1999) 669–679.
[8] V.S. Nagarajan, B.J. Hockey, S. Jahanmir, Contact wear mechanisms of a dental composite with high filler content, J. Mater. Sci. 35 (2000) 487–496. [9] X.Q. Hu, P.M. Marquis, A.C. Shortall, Two-body in vitro wear study of some current dental composites and amalgams, J. Prosthet. Dent. 82 (2) (1999) 214–220. [10] H. Li, Z.R. Zhou, Wear behavior of human teeth in dry and artificial saliva conditions, Wear 249 (2002) 980–984. [11] P. Derand, P. Vereby, Wear of low-fusing dental porcelains, J. Prosthet. Dent. 81 (4) (1999) 460–463. [12] F.J. Gil, A. Espias, L.A. Sánchez, et al., Comparison of the abrasive wear resistance between amalgams, hybrid composite material and different dental cements, Int. Dent. J. 49 (6) (1999) 337– 342. [13] A. Embong, J. Glyn-Jones, A. Harrison, The wear effects of selected composites on restorative materials and enamel, Dent. Mater. 3 (1987) 236–240. [14] R.I. Holland, Galvanic currents between gold and amalgam, Scand. J. Dent. Res. 88 (1980) 269–272.