Application of Fourier Transform for Analysis of Surface Topographic Properties of Dental Zirconia

2.2. Fracture surface The block of 3 mm high, 50 mm long with the upper surface of 2 mm width and the lower surface of 1.8 mm width was cut from zirconia cylinder and sintered at 1350◦C as recommended by Product Company. The size of cross-section of zirconia block was selected to fit to the clinical fracture case. It was fractured manually through bending by loading from upper to lower surface without any treatment on the sintered surface. Clinically derived fracture zirconia was also used as sample. 2.3. Observation Variously treated surfaces and fractured cross-sections were observed by FE-SEM (S4000: Hitachi, Tokyo, Japan). 2.4. Contact angle measurement The contact angles with water were measured for the surface of zirconia after silicone wheel, sandblast and tribochemical treatments with DMs-200 (Drop Master S series: Kyowa Interface Science Co.Ltd, Saitama, Japan) Measurements were performed 10times for each. All experimental results were evaluated by Non-repeated Measures ANOVA (n = 10) (p < 0.001). 2.5. Surface roughness measurement The surface roughness was measured using SURFCOM 1400A (Tokyo Seimitu, Japan) after the treatments with silicone wheel, sandblast, tribochemical treatments, mirror polishing and porcelain layering. 3. Results Fig. 1 shows SEM observation of smoothed zirconia surface. Fig. 1a is silicone wheel polishing, Fig. 1b is mirror polishing, and Fig. 1c is porcelain layering. The surfaces after mirror polishing were smooth except mechanically formed grooves during silicone wheel polishing. Porcelain layering was smooth except large formed grooves by bubbles. Both mirror polishing and porcelain layering look similarly smooth. Fig. 2 shows SEM observation of roughened zirconia surface, for silicone wheel polishing Fig. 2a, sandblast treatment Fig. 2b and tribochemical treatment Fig. 2c. After sandblast and tribochemical treatments, the surfaces showed several micron-sized caving with micron to submicron level irregularities. When Fig. 2b and c were compared, the latter was slightly rougher. Fig. 3 is the enlargement of roughened zirconia surface by SEM observation. Fig. 3a is tribochemical treatment, and Fig. 3b is 24-hacid treatment. 24-h acid treatment induced much more surface roughening with the 50–100 nm particulate roughness than tribochemical treatments. Fig. 4 shows the specimens of dental zirconia. Fig. 4a shows zirconia cylinder supplied for dental CAD/CAM machining and a fabricated bridge. Fig. 4b is the fractured block and matchstick behind for reference of size. Block was obtained by milling from zirconia cylinder. Fig. 4c is fractured zirconia bridge used in clinical case and d is enlargement of pontic with fracture cross-section seen as white plane in right side. Fracture occurred nearly in the center of bridge c. Fig. 5 is SEM observation of fracture cross-section, Fig. 5a clinically fractured, Fig. 5b enlargement of Fig. 5a, Fig. 5c experimentally fractured, Fig. 5d enlargement of Fig. 5b. Clinically occurred fracture exhibited the scab-like surface. For experimentally attained fracture surface cracks were formed straight in most part and in some places curved or twisted into S or Ωform. Figs. 6 and 7 show the photographs and the corresponding mean and standard deviation of the contact angle with water, respectively. Fig. 6a is silicone wheel polishing, Fig. 6b is sandblast treatment, and Fig. 6c is tribochemical treatment. In Fig. 7 all these treatments showed a significant difference each other (p<0.001). The average contact angles were 91.6 ± 4.8◦ (SW), 65.6 ± 8.0◦ (SB), and 51.4 ± 4.8◦ (TC). Sandblast and tribochemical treatments had the smaller water contact angle than silicone wheel polishing. Fig. 8 shows the surface roughness of zirconia with various treatments. The measured values of mean surface roughness Ra were 0.12μm for silicone wheel polishing, 0.26 μm for sandblast treatment, 0.32μm for tribochemical treatment, 0.11 μm for mirror polishing and 1.45μm for Porcelain layering. Porcelain layering showed the largest roughness. Mirror polishing roughness had the smoothest surface. 4. Discussion 4.1. Suitable surface treatments for dental zirconia To have the good adhesion of porcelain onto zirconia, the wettability is important. Sandblast and tribochemical treatments showed the contact angle smaller than silicone wheel treatment (Figs. 6 and 7), which provides the improved wettability. This maybe mainly due to the increased surface roughness granted by sandblast and tribochemical treatments (Fig. 8). Both treatments are effective to enhance the adhesion of zirconia with resin cement to fix on teeth and for bonding of porcelain [3]. Plaque tends to attachment more easily to the rougher surfaces [4]. In the surface roughness measurement, porcelain layering showed the largest mean roughness (Ra 1.45). Only porcelain layering treatment was done manually with the human hands. During the process of condensing of porcelain powders, they contain bubbles inside. By the following glazing process, bubbles go out then the large and gradually sloped grooves are formed. This may cause the larger roughness with the longer periodicity. Except there large grooves, however, the surface is quite smooth with the much less high frequency roughness with the small periodicity. On the other hand, although Ra is smaller, SB and TC had much more high frequency roughness, which leads to the easier plaque attachment. Thus the large Ra value of PL has nothing to do with plaque attachment. From the aspect of plaque attachment, zirconia surface with the high frequency roughness without porcelain layering like SB and TC is recommended to be polished. The 24-h acid treatment done in this study deteriorated the surface (Fig. 3b). The acid treatment is used to dissolve the porcelain to replace with the new one in such cases that the matching of color shade dose not fit well. When the amount of porcelain is large, the acid treatment needs a longer time. If the acid treatment time is less than 10 min, zirconia suffers little damage. To shorten acid treatment time, it is preferable to remove porcelain mechanically as much as possible before the acid treatment. When there are possibilities to choose treatments from various ones, it is also important to pay attention to health care in working condition. Some surface treatments produce abraded powders whose nano particles may induce the health risk to human body [5]. Most treatments shown in this study were done under water spray, which decreases the risk to much lower level. 4.2. Fracture of zirconia In the cross-section of experimentally fractured samples, cracks were formed straight in most part and in some places curved or twisted into S or Ω form. This suggests that crack propagation was obstructed by the fracture toughness of zirconia resulted from martensitic crystalline transformation [6,7].In clinics, complex stress is applied in mouth and it is not a simple bending deformation mode to one direction at one time. Various deformation mode of tension, bending, twist and fatigue in various directions are imposed many times for long time. Therefore fracture was progressed little by little in a long time. This resulted scab-like morphology in fracture surface. 5. Conclusions This study evaluated the surface morphology and wettability after various treatments done in the fabrication process of dental zirconia. There is the tendency that the larger surface roughness leads to better wettability, which may be one of the factors to con-tribute to the better bonding of porcelain onto the zirconia surface. References [1] M. Uo, G. Sjogren, A. Sundh, M. Goto, F. Watari, Dental Materials Journal 25 (3)(2006) 626–631. [2] P. Benetti, A.D. Della Bona, J.R. Kelly, Dental Materials 26 (2010) 743–750. [3] W.-S. Oh, C. Shen, B. Alegre, K.J. Anusavice, Journal of Prosthetic Dentistry 88 (6)(2002) 616–621 [4] M.I. Al-Marzok, H.J. Al-Azzawi, Journal of Contemporary Dental Practice 10 (6)(2009) 017–24. [5] F. Watari, N. Takashi, A. Yokoyama, M. Uo, T. Akasaka, Y. Sato, S. Abe, Y. Totsuka,K. Tohji, Journal of the Royal Society Interface 6 (2009) 371–388. [6]P. Christel, A. Meunier, M. Heller, J.P. Torre, C.N. Peille, Journal of BiomedicalMaterials Research 23 (1989) 45–61. [7] J. Tinschert, D. Zwez, R. Marx, K.J. Anusavice, Journal of Dentistry 28 (2000)529–535. Fig. 1. SEM observation of smoothed zirconia surface. (a) Silicone wheel polishing treatment, (b) mirror polishing, (c) porcelain layering. Fig. 2. SEM observation of roughened zirconia surface. (a) Silicone wheel polishing treatment, (b) sandblast treatment, (c) tribochemical treatment. Fig.3.Enlargement of roughened zirconia surface SEM. (a) Tribochemical treatment, (b) 24-h acid treatment. Fig. 4. (a) Zirconia cylinder for dental CAD/CAM and a fabricated bridge, (b) experimentally fractured block obtained by milling from zirconia cylinder, (c) fractured zirconia bridge used in clinical case, (d) enlargement of pontic with fracture cross-sections. Fig. 5. SEM observation of fracture cross-section. (a) Clinically fractured, (b) enlargement of (a), (c) experimentally fractured, (d) Enlargement of (c). Fig. 6. Contact angle measurements of zirconia surface with various treatments. (a) Silicone wheel (SW), (b) sandblast (SB), (c) tribochemical (TC). Fig. 7. Contact angle for the treatments. Fig. 8. Surface roughness of zirconia with various treatments. Application of Fourier transform for analysis of surface topographic properties of dental zirconia Naoyoshi TARUMI, Tsukasa AKASAKA, Fumio WATARI 1.Graduate School of Dental Medicine, Hokkaido University, Sapporo, Japan 2.Sapporo Dental Laboratory, Sapporo, Japan