Christina M. Pecnik Malgorzata Roos Daniel Muff Ralph Spolenak Irena Sailer

In vitro color evaluation of esthetic coatings for metallic dental implants and implant prosthetic appliances

Authors’ affiliations: Christina M. Pecnik, Daniel Muff, Ralph Spolenak, Department of Materials, Laboratory for Nanometallurgy, ETH Zurich, Z€ urich, Switzerland Malgorzata Roos, Division of Biostatistics, Institute of Social and Preventive Medicine, University of Z€ urich, Z€ urich, Switzerland Irena Sailer, Clinic for Fixed Prosthodontics and Biomaterials, University of Geneva, Geneva, Switzerland

Key words: biomaterials, material sciences, Prosthodontics

Corresponding author: Prof. Dr. med. dent. Irena Sailer Clinic for Fixed Prosthodontics and Biomaterials Center of Dental Medicine University of Geneva 19 rue Barth elemy-Menn CH-1205 Geneva, Switzerland Tel.: +41 22 379 40 51 Fax: +41 22 379 40 52 e-mail: [email protected]

deposited on titanium substrates, which exhibited different roughness (polished, machined, and

Abstract Objectives: The aim of this study was to characterize the optical properties of newly developed esthetic coatings for metallic implants and components for an improved peri-implant soft tissue appearance. Material and methods: Pig maxillae (n = 6) were used for the in vitro color evaluation of coated and uncoated samples. Three different coating systems (Ti–ZrO2, Ti–Al–ZrO2, and Ti–Ag–ZrO2) were sand-blasted) and interference colors (pink, yellow, and white). Spectrophotometric measurements were made of samples below three different mucosa thicknesses (1 mm, 2 mm, and 3 mm) and titanium served as negative control. Color difference DE was calculated using DL, Da, and Db values for each sample (in total 30 samples). Results: DE values were significantly above the threshold value of 3.70 for sand-blasted Ti and Ti–ZrO2 samples when tested below 1 mm thick soft tissue, hence resulted in a dark appearance of the soft tissues. In contrast, Ti–Al–ZrO2 and Ti–Ag–ZrO2 samples showed significant DL values below 1 mm, which indicates a brightening of the covering tissue. In general, DE values decreased with increasing thickness of the tissue. At 3 mm thick tissue, DE values were significantly below 3.70 for Ti–Al–ZrO2 and Ti–Ag–ZrO2 samples. The preferable substrate surface should be machined due increased color brightness, good soft tissue integration and improved adhesion between coating and substrates. Improvement of the optical appearance of the metal was achieved with the coating systems Ti–Al–ZrO2 and Ti–Ag–ZrO2. Darkening effects could not be observed for these systems, and partially light brightening of the tissue was observed. Advantageous colors were suggested to be pink and yellow.

Date: Accepted 3 June 2014 To cite this article: Pecnik CM, Roos M, Muff D, Spolenak R, Sailer I. In vitro color evaluation of esthetic coatings for metallic dental implants and implant prosthetic appliances. Clin. Oral Impl. Res. 26, 2015, 563–571 doi: 10.1111/clr.12443

Implant restorations have to fulfill certain requirements in terms of mechanical, biocompatible, and esthetic properties. Titanium (Ti) and its alloys meet these demands as clinical studies showed that Ti implants and abutments reach excellent survival and success rates (Andersson 1995; Scheller et al. 1998; Buser et al. 2012). The long-term success of reconstructions made of Ti is mainly due to its appropriate mechanical properties (fracture toughness approximately 100 MPam1/2 and flexural strength between 480 and 550 MPa (Niinomi 1998)) and integration to tissue (Albrektsson et al. 1981; Niinomi 1998). However, further improvement for metallic reconstructions should be realized in their esthetic appearance below soft tissue: metallic abutments and implants can be an esthetic disadvantage at gingival recession or due to a gray discoloration of the

© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

peri-implant mucosa (McCartney et al. 1993; Park et al. 2007; van Brakel et al. 2011), resulting in decreased patient satisfaction. Alternatively, ceramic materials made of zirconia (ZrO2) can be used to achieve esthetically pleasing results in this region. It was shown that restorations with ZrO2 abutments matched the color of the peri-implant soft tissue better than with metallic abutments (Tan & Dunne 2004; Watkin & Kerstein 2008). Clinical studies reported excellent survival rates (Sailer et al. 2009; Zembic et al. 2009, 2013) and observed no difference in soft tissue integration for Ti and ZrO2 implant abutments (Abrahamsson et al. 1998; Kohal et al. 2009; van Brakel et al. 2012). Moreover, ZrO2 surfaces significantly reduced the presence of bacteria than Ti surfaces, and they presumably also lead to a healthier peri-implant soft tissue (Scarano

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et al. 2004). Despite the brittle behavior of ceramics, ZrO2 has adequate mechanical properties (fracture toughness ≤ 10 MPam1/2 and flexural strength between 900 and 1400 MPa), which allow the alternative use of an one-piece ZrO2 dental implant to eliminate the discoloration of the soft tissue (Christel et al. 1989). However, the number of long-term studies of one-piece ZrO2 implants is limited, and therefore, its use is not as common and established as it is for the Ti implant (Wenz et al. 2008; Depprich et al. 2014). Furthermore, its low fracture toughness compared to its Ti counterpart always bears an increased risk of failure in service. The combination of both restorative materials, Ti and ZrO2, would enhance the optical properties of Ti implants. An inert and colored ceramic coating in the transgingival part of the implant could contribute to improved natural soft tissue esthetics. In addition to the esthetically relevant issues, the mechanical properties of the coating have to be considered. Thin films demonstrated improved mechanical properties compared with properties of bulk materials (Volinsky et al. 2003; Kraft et al. 2010). Additionally, the roughness of the substrate material not only strongly influences the mechanical, but also the biocompatibility and optical properties. An appropriate assessment of the esthetic appearance of restorative materials was utilized in an in vitro study by Jung et al. (2007). In contrast to Ti samples, it was observed that bulk ZrO2 specimens caused color differences of porcine

mucosa below 3.70, which is a previously reported threshold for intraoral distinction of color differences (Johnston & Kao 1989). In this study, thin films of ZrO2, silver (Ag), and aluminum (Al) were deposited on Ti platelets with different surface roughness to brighten the underlying Ti by a combination of a mirror and interference effect as tunable by the coating thickness. The aim of the present study was to investigate the influence of the coated metal on the color of the soft tissues in vitro.

Material and methods Coating technique and sample preparation

The materials and subsequent coatings steps for this study are summarized in Fig. 1 and described as follows: Three coating systems were deposited by magnetron sputtering (PVD Products, Inc., Wilmington, MA, USA) (George 1992) at room temperature on Ti (Grade 4, ThyssenKrupp) platelets with a diameter of 15 mm and thickness of 0.5 mm. The base pressure in the deposition chamber was lower than 2
106 torr. The thickness of the reactively deposited ZrO2 layer (Zr target: 99.5%, MaTecK GmbH) was different for each interference color: 185 nm for pink, 150 nm for yellow, and 110 nm for white. The metallic intermediate layers, Al (99.9%, Sindlhauser Materials GmbH) or Ag (99.99%, Kurt J. Lesker), exhibited a thickness of 50 nm. The surface of the substrate material was modified

Fig. 1. Flowchart illustrating the different substrate roughnesses ((p) for polished, (m) for machined, (s) for sandblasted), and coating systems investigated in this study. The colors resulted from thin film interference behavior of the ZrO2 layer. The thickness of the metallic interlayers, Al and Ag, was 50 nm.

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(polished (p) with average area roughness Sa = 0.005 lm, machined (m) with Sa = 0.099 lm, and sand-blasted (s) with Sa = 0.247 lm) to investigate the influence of surface roughness on the optical properties in the coated state. For polished substrates, the platelets were ground on a grinding and polishing machine (Rotopol-2, Struers) using SiC papers (up to 4000 grit) and subsequently polished with a colloidal silica suspension (MasterMet 2, Buehler) with 10 vol% hydrogen peroxide (≥ 35%, Sigma–Aldrich). Machined samples were provided by the industrial partner (Institut Straumann AG, Switzerland). Sand-blasted substrates were achieved using alumina with a mean particle size of 22 lm (Biloxit no. 360, Sandmaster) at a pressure of 3.5 bar. Evaluation method

The effect of the specimens on the color of the mucosa was determined according to the model based on (Jung et al. 2007). Pig maxillae (n = 6) were used to simulate different thicknesses of mucosa. The investigations here were not classified as an animal study as these pigs were raised for food production only. In total, 30 samples were tested in this study. Mucosal flaps with a thickness of 1 mm ( 0.14 mm) were prepared in the palatal region for each pig maxillae (Fig. 2). Additionally, tissue grafts with a thickness of 1 mm ( 0.15 mm) and 2 mm ( 0.13 mm) were prepared to receive a total mucosa thickness of 2 mm, respectively, 3 mm beneath the mucosal flap. Prior to in vitro tests, the samples were measured without tissue using a spectrophotometer CM 2600-d (Konica Minolta) to characterize the reflectivity behavior of the coating systems. For the in vitro tests, the baseline spectrophotometric measurement was taken of each tissue thickness (1 mm, 2 mm, and 3 mm) on bone without the test samples. Afterward, each sample was placed beneath the tissue of one thickness, and spectrophotometric measurements were taken in the same region. The resulting laboratory color values were examined with the manufacturer’s software SpectraMagic NX (Konica Minolta). The observer angle was set to 2°, and the D65 standard light source with 100% UV with specular reflection included (SCI) was used. For the measurement of diffused illumination, the device uses an integrating sphere with apertures of 3 mm in diameter (SAV – small area view). The spectrophotometer was zero–calibrated with a black box (CM-A 32) and then calibrated with a white plate (CM-A 142). The mean reflectance and laboratory color values were automatically averaged from three measurements of each specimen. The

© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

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In vitro color evaluation of esthetic coatings for implants

(a)

Fig. 2. Pig maxilla preparation for the optical evaluation: a palatal mucosal flap with a thickness of 1 mm was prepared.

(b) differences DL, Da, and Db were then calculated by subtracting the baseline measurement (tissue on bone) from the measurement with the sample below the tissue. The overall color change DE was calculated by the following equation: DE ¼ ½ðDLÞ2 þ ðDaÞ2 þ ðDbÞ2 1=2

ð1Þ

In literature, a threshold value of DE > 3.70 was considered to be a clinically distinguishable color difference in the intraoral environment (Ruyter et al. 1987; Johnston & Kao 1989). Statistical analysis

The received data were coded in Excel and analyzed with SPSS version 20. The assumption of the normality of the data was investigated with Kolmogorov–Smirnov and Shapiro–Wilk tests. Descriptive statistics such as mean, standard deviation (SD), median (MD), and interquartile range (IQR) were computed. One sample Wilcoxon test was used for DE at 1 mm, 2 mm, and 3 mm thick soft tissue to find whether the sample mean is significantly different from the cutoff 3.70 (Ruyter et al. 1987; Johnston & Kao 1989) and for DL, Da, and Db against 0.00. The Kruskal– Wallis test was applied together with Bonferroni–Holm corrected Mann–Whitney post hoc tests with respect to substrate treatment for DE at 1 mm, 2 mm, and 3 mm, separately split for coating system and color. For multiple comparisons, the Bonferroni–Holm method was applied. It consists of multiplying the smallest P-value by the number of individual tests (k), the second smallest by k1 and so on (with the restrictions that the initial order of P-values is kept and all Pvalues < 1), and evaluation at the initial level a (typically a = 5%). Mathematically, the

Fig. 3. The reflectance curves of the investigated samples were measured before the in vitro evaluation: (a) Ti as a substrate with three different roughnesses, (b) coatings deposited on polished Ti substrates. The reflectance curves for yellow and white Ti–Al–ZrO2, respectively, Ti–Ag–ZrO2 samples show a similar maximum reflectance values as the pink-coating systems with metallic interlayers.

Bonferroni–Holm procedure can be described as follows (Holm 1979): 1. Ordered P-values: p1 ≤ p2 ≤. . . ≤ pk 2. Adjusted P-values: piadj = maxj≤i [(kj + 1) pj]1 3. Where [x]1 = min(x,1) and i = 1,2,. . ., Otherwise P-values of the analysis, which were smaller than 0.05, were considered to be statistically significant.

region of the spectrum. Similarly, there was absorption for the yellow Ti–ZrO2 sample as well, but shifted to approximately 390 nm (violet region). The white Ti–ZrO2 sample covered a larger part of the spectrum but not entirely and exhibited therefore a rather gray than pure white color. All interference colors of the ZrO2 coatings reached a similar level of maximum reflectance values. Coating systems with an intermediate metallic layer, such as Al or Ag, revealed very high reflectance values.

Results Reflectivity behavior of coating systems without tissue

Fig. 3 (a) shows that the reflectance value decreased with increasing surface roughness (from polished down to sand-blasted). The reflectance curves of the coated samples are depicted in Fig. 3(b). The pink Ti–ZrO2 sample showed an absorption peak at approximately 510 nm (green), whereas the maximum reflectance was reached in the red

© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

Influence of the thickness of soft tissue on optical appearance

In the esthetic evaluation, first focus should be placed on spectrophotometric tests below 1 mm thick tissue as thin soft tissue is prone to be translucent and could most likely lead to a dark discoloration (Chang et al. 1999; Park et al. 2007). The mean overall color change for 1 mm thick tissue is presented in Fig. 4 (a–c). No DE value was significantly below the threshold value of 3.70. Significant

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In the following, the results of samples below thicker soft tissue (2 mm and 3 mm) are presented, particularly for Ti–Al–ZrO2 and Ti–Ag–ZrO2 coating systems which behaved esthetically favorable in the case of 1 mm thick soft tissue. In general, the DE values decreased with increasing tissue thickness. In Fig. 5 (a–b), the specimens showed mean values below 3.70, except for sandblasted specimens. A significant difference in DE below 3.70 was observed for yellow and white Ti–Ag–ZrO2 on machined substrates. The mean DE values of Ti–Al–ZrO2, Ti–Ag– ZrO2, and the negative control were all below 3.70 when tested below 3-mm-thick tissue (Fig. 5c,d). Not all DE values were significantly below the threshold, but mainly for white Ti–Al–ZrO2, respectively, Ti–Ag–ZrO2 samples. Significantly positive Da values were observed for polished and machined Ti–Ag–ZrO2 samples, resulting in more yellowness of the covering mucosa (Table 1). The Kruskal–Wallis test (with Bonferroni– Holm corrected Mann–Whitney post hoc tests) results for DE demonstrated that only one interaction was significant, namely between polished and sand-blasted Ti–ZrO2 samples with pink color measured below 1 mm thick tissue (pcorr = 0.045). At 2 mm thick tissue, DE values showed a tendency toward statistical significance for pink Ti–ZrO2 between machined and sand-blasted samples (pcorr = 0.075) and for white Ti–Ag– ZrO2 between machined and sand-blasted samples (pcorr = 0.075). All other interactions did not significantly influence DE measured at 2 and 3 mm thick tissue. Partial delaminations of the coatings were observed for polished samples, especially at the edges of the specimens.

(a)

(b)

(c)

Discussion

Fig. 4. The overall color change DE (mean,  SD) of the investigated samples measured below 1 mm thick tissue for (a) pink, (b) yellow and (c) white coating on three different substrate roughnesses. The coated samples are compared to Ti, the negative control group. The dotted line corresponds to the critical threshold value of 3.70 for intraoral distinction by the naked eye (Johnston & Kao 1989).

changes in DL were noted for polished specimens coated with Ti–Al–ZrO2 (yellow and white) and Ti–Ag–ZrO2 (yellow), increasing the brightness of the covering mucosa. Moreover, Da and Db were significantly positive for yellow Ti–Ag–ZrO2 with a polished substrate, which shifts the color to the preferable

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red–yellow region of the spectrum. Sandblasted Ti and Ti–ZrO2 specimens (in all three colors) indicated a darkening with DE values significantly above 3.70. For white Ti–ZrO2 specimens, the darkening was significant for polished respectively machined substrate.

In this study, significant differences were found in the optical appearance of the soft tissue depending on the esthetic coating system for the metal, its color, and substrate treatment. Furthermore, the color change decreased with increasing soft tissue thickness. Color measurement and shade determination in dentistry are important tasks for the production of prosthetic materials. Darkening or excessive brightening of the covering soft tissue is not desired and should be therefore considered in developing an improved prosthesis. In the present work, it was observed that the coating system Ti–ZrO2 appeared noticeably dark below 1 mm thick mucosa

© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

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In vitro color evaluation of esthetic coatings for implants

(a)

(b)

(c)

(d)

Fig. 5. The overall color change DE (mean,  SD) of (a), (b) yellow and (c), (d) white Ti–Al–ZrO2 and Ti–Ag–ZrO2 samples measured below (a), (c) 2 mm and (b), (d) 3 mm thick tissue. The coatings were deposited on Ti substrates with three different roughnesses and compared with the negative control group Ti. The dotted line corresponds to the critical threshold value of 3.70 (Johnston & Kao 1989).

for all three colors and substrate treatments. This darkening of mucosa might lead to esthetic complications as previously mentioned. The spectrophotometric curves already revealed that the reflectance of this coating system did not reach high enough values compared with the substrate material. Both curves showed similar maximum values at approximately 60%. As the ZrO2 thin film is transparent in this study, the dark color of the substrate is still able to shine through it, decreasing the final color of a ZrO2 coated Ti. Therefore, it was assumed that the system Ti–ZrO2 was not able to improve the esthetics of dental implants to a sufficient extent, especially when implanted in anterior teeth area. An improvement of the optical properties was found for Ti–Al–ZrO2 and Ti–Ag–ZrO2. Figure 6 shows that the metallic interlayers enhance the lightness of the resulting color below 1 mm thick tissue, showing significantly positive DL values for yellow and white Ti–Al–ZrO2, respectively,

Ti–Ag–ZrO2. This indicated a brightening of the covering tissue. It is evident that the reflectance values for Ti–Al–ZrO2 and Ti–Ag–ZrO2 samples are higher than for uncoated Ti, as Al and Ag are highly reflecting materials in the visible spectrum (Foiles 1982). For very thin soft tissue, however, this noticeable brightening might negatively affect the esthetic appearance, as well. Suitably, DE values below or close to the critical threshold value might still behave advantageously in practice, such as pink and yellow Ti–Al–ZrO2 on machined Ti (Table 2a and b). This then further implies the question of the appropriate color of the coating for the future application. The color of healthy gingival tissue varies ranges from pale pink to purple (Jones & McFall 1977; Heydecke et al. 2005; Huang et al. 2011). Pink and yellow colored coatings show positive and partially even significant Da and Db values, especially for the Ti–Ag–ZrO2 system. Similar findings were found in (Ishikawa-Nagai et al. 2007), where

© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

light pink and light orange behaved advantageously. Negative values of Da and Db shift the final color into the green and blue region of the color space and do only contribute to the risk of darkening the peri-implant soft tissue. However, the color of peri-implant mucosa is different for every patient, therefore the patient’s mucosa color could be measured and the thickness respectively the color of the final coating adjusted accordingly. Mainly all coating systems deposited on sand-blasted Ti was shone through the overlying tissue. A significant difference in DE (tested at 1 mm) was found between polished and sand-blasted Ti–ZrO2 samples. The behavior of sand-blasted samples can be explained by the poor reflectance in the visible spectrum as these samples mostly reflect the incident light diffusely rather than specularly. As machined substrates partly exhibit a mirror-like surface, they reflect a higher amount of the incident light. Although

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Table 1. Mean, standard deviation (SD), median (MD), and interquartile range (IQR) of DE values for the investigated samples with different substrate treatments below mucosa of different thicknesses. 1 mm

2 mm

3 mm

Coating system

Color

Substrate treatment

Mean

SD

MD

IQR

Mean

SD

MD

IQR

Mean

SD

MD

IQR

Ti

–

Ti–ZrO2

Pink

Polished Machined Sand-blasted Polished Machined Sand-blasted Polished Machined Sand-blasted Polished Machined Sand-blasted Polished Machined Sand-blasted Polished Machined Sand-blasted Polished Machined Sand-blasted Polished Machined Sand-blasted Polished Machined Sand-blasted Polished Machined Sand-blasted

3.88 5.29** 6.30** 3.86 5.22 8.31** 5.47 5.52 7.90** 5.43** 5.80** 7.98** 4.36 3.89 5.98 5.14 3.35 5.21 5.04 4.83 5.35** 4.73* 4.31 4.38 7.03** 5.24 4.16 4.70 5.32 5.10

0.88 1.89 2.34 1.23 2.75 3.11 2.21 1.77 2.65 1.01 1.67 3.50 1.54 2.46 2.20 2.09 2.26 2.62 2.69 2.93 1.91 3.47 1.86 1.98 3.04 2.07 1.72 3.28 2.34 1.95

3.75 5.04 6.25 3.79 4.64 8.96 5.14 5.76 7.71 5.65 6.05 7.76 4.30 3.15 6.85 5.08 2.60 5.16 4.90 5.41 5.00 3.73 3.96 4.88 5.83 5.18 3.95 5.03 5.69 4.36

0.93 2.16 3.79 2.30 5.47 5.76 3.80 3.07 4.83 1.40 2.26 6.99 3.11 4.76 4.51 4.37 3.19 5.38 4.69 5.79 2.70 4.83 2.30 3.54 5.35 3.25 2.59 5.55 4.07 3.66

2.56 3.23 4.48 2.42** 2.04** 4.60 3.19** 4.10 4.33 3.28 3.69 3.41 1.92** 2.30** 3.00 3.00 2.66 3.92 3.25 2.82 3.52 2.81 2.23** 2.48** 3.25 2.40** 2.90 2.57 1.70** 4.03

1.64 0.71 1.71 0.93 0.98 1.96 0.15 1.60 1.34 1.57 1.36 1.26 1.02 0.89 1.64 1.72 1.13 2.02 2.42 2.45 1.34 1.31 1.03 1.12 1.18 0.43 1.22 1.25 1.29 1.73

2.19 3.17 4.69 2.10 1.86 4.47 3.25 4.08 5.67 3.18 3.62 3.35 1.46 2.60 2.19 2.38 2.46 3.14 2.32 2.07 3.67 2.98 2.10 2.52 2.70 2.37 2.95 2.41 1.38 4.17

3.07 1.44 2.31 1.55 1.72 3.56 0.75 2.86 4.62 2.78 2.43 2.29 2.00 1.32 2.53 3.35 2.29 3.96 3.78 3.79 2.08 2.58 1.85 2.27 2.01 0.79 1.94 2.13 2.07 3.13

2.27** 3.41 3.31 2.61 2.04** 3.11 2.72 2.20** 2.52 2.60 3.41 2.99 2.43** 2.29** 3.02 2.75 2.69 3.36 2.28** 2.16** 3.00* 2.74 2.44** 3.02* 3.11 2.47 2.40** 2.43** 2.76* 2.49**

1.00 2.33 2.39 0.94 1.05 1.40 1.63 0.59 1.35 1.14 1.65 1.30 1.09 1.06 1.08 2.62 0.83 1.61 1.22 0.89 1.08 1.80 1.31 2.31 1.99 1.44 0.44 0.75 1.66 0.93

2.23 2.73 2.36 3.60 1.91 3.20 2.64 2.08 1.97 4.46 3.43 2.82 2.42 2.09 3.30 1.88 2.38 3.95 1.99 1.98 3.57 2.51 2.79 2.27 2.47 2.08 2.49 2.36 2.28 2.38

1.41 3.51 3.46 5.47 1.93 2.47 3.22 0.95 1.67 1.18 2.79 2.29 1.38 1.72 1.99 3.01 1.68 2.88 2.42 1.43 1.90 3.27 2.10 2.44 2.52 1.68 0.78 1.33 1.69 1.85

Yellow

White

Ti–Al–ZrO2

Pink

Yellow

White

Ti–Ag–ZrO2

Pink

Yellow

White

Results marked with * were found to be non-normally distributed and marked with ** were significantly above, respectively, below the threshold value of 3.70. In total, 30 samples were tested in this study.

substrate roughness plays an important role for esthetically satisfying results, the soft tissue integration might depend on the topography of the Ti surface. In vitro (Eisenbarth et al. 1996) and in vivo (Chehroudi et al. 1990, 1991) studies presented that the soft tissue is influenced by the surface roughness of Ti. The growth of gingival tissue cells might be favored on groove-like surfaces compared with sand-blasted and/or acid-

etched surfaces. However, the in vivo study by (Abrahamsson et al. 2002) showed that the soft tissue attachment was observed to be similar on machined and acid-etched Ti surfaces. Moreover, the adhesion of thin films also depends on the surface roughness. Mechanical interlocking of a coating to a rough substrate presumably provides an increased interfacial adhesion (Yang et al. 2003). In the present study, it can be assumed

Fig. 6. Individual DL values (mean  SD) of polished samples tested below 1 mm thick soft tissue.

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that the developed coating systems might adhere better on sand-blasted or machined rather than on polished Ti surfaces. During in vitro evaluation, it was observed that the coatings partially delaminated at the edges of polished Ti platelets. As a sand-blasted surface is esthetically seen not favored for a dental coating, the choice might come to a machined surface, which is nowadays commonly applied to commercially available Ti implants. Our observed decrease in mean DE values with increasing thickness of mucosa corresponds well with previous studies (Jung et al. 2007; van Brakel et al. 2011; Jun et al. 2013). Mucosa thicknesses thicker than 2 mm may prevent a visible discoloration of the periimplant mucosa. Accordingly, the results of Ti–Al–ZrO2 and Ti–Ag–ZrO2 samples indicated that the shine-through effect diminished with increasing thickness. As expected, coating systems with Al or Ag layers were significantly not visible when tested below 1 mm thick tissue. Furthermore, it can be observed that the standard deviation decreases as well when testing with 2 and 3 mm thick tissue. It is speculated that differences in mucosa thicknesses and densities

© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

Pink

Ti–Al–ZrO2

© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

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White

Yellow

Pink

White

Polished Machined Sand-blasted Polished Machined Sand-blasted Polished Machined Sand-blasted Polished Machined Sand-blasted Polished Machined Sand-blasted Polished Machined Sand-blasted Polished Machined Sandblasted

Polished Machined Sand-blasted Polished Machined Sand-blasted Polished Machined Sand-blasted Polished Machined Sand-blasted Polished Machined Sand-blasted Polished Machined Sand-blasted Polished Machined Sand-blasted

Substrate treatment

2.28 4.19 2.79 5.82 4.81 3.77 5.35 5.71 5.45 5.19 6.18 6.76 7.09 4.36 4.66 6.72 5.90 3.71 5.50 8.52 5.26

IQR

MD 2.27** 1.47 3.36** 0.27 0.43 3.37** 2.42** 0.36 1.11 2.60** 3.33 2.43 2.36 2.23** 2.42 3.45** 2.27 1.07 2.28** 2.09 1.65

1.56 2.72 1.68 3.55 2.58 2.21 2.93 3.45 3.44 3.07 3.39 3.51 4.08 2.54 2.83 3.35 3.39 2.04 3.51 4.97 3.22

2.64** 0.81 3.57** 0.26 0.47 3.92** 3.12** 1.54 1.31 3.71** 3.33 0.73 2.07 2.61** 2.05 4.06** 2.87 1.03 3.16** 2.13 1.31

1.06** 2.26** 2.35** 1.98** 0.71 1.71** 0.82 0.70 1.88** 0.47 1.11 2.31** 0.80 1.41** 0.12 1.15** 0.59 0.63 0.25 0.84 1.47**

MD

1.11** 2.69** 2.77** 1.81** 0.21 1.85** 0.63 0.43 2.27** 0.20 1.46 2.53** 1.30 1.55** 0.36 1.31** 0.78 1.28 0.06 0.60 1.79**

2.04 3.62 2.63 1.41 2.62 2.32 1.58 2.75 2.42 1.54 3.02 2.39 3.19 1.96 2.06 1.79 2.22 3.38 2.51 2.65 3.18

IQR

1.02 1.89 1.52 0.83 2.35 1.39 1.28 1.39 1.45 1.01 1.84 1.61 1.94 0.69 1.15 1.23 1.44 1.72 1.97 1.68 1.55

SD

1.37 4.60** 2.69** 1.52** 0.50 3.55** 2.29** 0.67 2.47 1.68** 0.46 1.62** 1.70 2.23** 0.70 3.78** 2.32** 1.31 1.61 1.03 1.82

MD

0.71 3.06** 3.81** 1.98** 0.64 3.26** 2.40** 0.27 2.73 2.73** 0.49 2.44** 1.89 1.97** 0.43 4.69** 2.88** 1.38 1.57 0.35 2.33

Mean

4.38 2.85 4.13 2.99 5.21 4.38 3.34 1.50 4.63 2.46 3.86 4.62 4.60 2.78 7.00 4.16 2.39 4.29 4.72 3.64 4.54

IQR

2.23 2.34 2.47 1.77 3.34 2.39 2.61 1.25 2.88 2.00 2.37 2.76 2.79 1.61 3.49 2.78 1.77 3.33 2.46 2.43 3.12

SD

0.66 1.13 3.56** 0.47 0.42 1.05 0.69 0.65 0.54 0.84* 0.03 1.31 1.24 1.07 1.20** 1.34 0.59 0.37 0.37 0.37 1.16

MD

0.94 1.14 3.27** 0.72 0.19 1.18 0.82 0.04 1.66 0.04* 0.17 0.71 0.68 1.10 1.64** 1.33 0.13 0.88 0.53 0.07 1.04

Mean

Mean

SD

Mean

2.14 3.22 3.31 2.22 2.44 2.48 3.47 3.03 4.78 2.45 3.20 2.74 4.37 2.49 2.19 3.12 3.76 4.19 2.37 2.32 5.43

IQR

1.69 1.81 1.75 1.36 1.73 1.88 3.05 2.06 2.87 2.30 2.02 1.88 2.64 1.69 1.08 2.24 1.84 2.69 1.53 1.43 2.74

SD

0.86** 1.66 1.65** 0.28 0.01 1.59** 0.38 0.10 1.85** 0.04 0.59** 1.58** 0.05 0.57** 1.04** 0.65** 0.64 0.84 0.25 0.00** 1.66**

MD

1.08** 1.06 1.49** 0.46 0.15 1.42** 0.39 0.13 1.87** 0.07 0.73** 1.68** 0.21 1.02** 0.68** 1.11** 0.57 0.75 0.03 0.51** 1.55**

Mean

Da

1.32 2.68 1.54 0.98 0.65 0.75 0.97 0.99 1.34 1.74 1.09 0.97 2.11 0.69 0.31 1.57 1.06 1.34 0.83 0.81 0.49

IQR

0.67 1.35 0.90 0.74 0.40 0.37 0.60 0.61 0.87 0.93 0.71 0.52 1.07 0.23 0.55 0.88 0.58 0.84 0.48 0.76 0.47

SD

0.50 1.31 1.65 0.67 1.13 1.31 0.33 0.00 0.85 1.73 0.63* 2.07 0.01 0.54 0.32 0.97 0.03 0.08 0.31 0.42 1.66

MD

1.27 0.91 1.26 0.58 0.87 1.60 0.05 0.40 0.80 1.74 1.34* 2.00 0.27 0.38 0.83 1.24 0.53 0.50 0.81 0.12 1.72

Mean

Db

3.45 3.48 3.91 2.53 3.00 3.47 3.24 3.95 3.97 3.83 3.39 2.47 1.98 2.28 2.45 3.00 3.11 1.82 4.65 1.83 5.18

IQR

1.68 1.99 2.39 1.33 1.74 1.62 1.78 2.23 2.29 2.96 2.87 2.02 1.56 1.29 1.44 1.68 1.62 1.34 2.41 1.47 2.66

SD

0.11 1.02 0.48 0.35 0.26 1.22 0.96 0.12 1.30 0.12 0.31 1.94** 0.24 0.25 0.97** 0.16 0.07 1.61 0.13 0.30 0.06

MD

0.24 0.59 0.85 0.24 0.15 1.41 0.07 1.53 0.68 0.08 0.02 1.57** 0.78 0.53 1.39** 0.48 0.84 0.57 0.05 0.46 0.17

Mean

DL

3 mm

2.31 4.29 2.70 2.33 2.39 2.34 2.61 2.49 3.42 2.51 2.13 2.11 4.16 3.26 1.80 2.56 2.66 2.64 2.33 3.04 1.79

IQR

1.25 2.31 1.65 1.33 1.32 1.21 1.41 1.84 1.17 1.41 1.17 1.27 2.21 1.64 1.61 2.10 1.57 1.48 1.58 1.55 1.62

SD

0.23 0.24 0.56** 0.52** 0.09 0.76 0.37 0.22 0.69** 0.37 0.22 0.69** 0.76 0.43** 0.66** 0.96** 0.36** 0.29 0.96 0.36 0.29**

MD

0.38 0.69 0.87** 0.44** 0.03 0.63 0.54 0.29 0.79** 0.04 0.38 0.88** 0.69 0.43** 0.82** 1.17** 0.52** 0.41 0.25 0.52 0.74**

Mean

Da

1.12 1.59 1.61 0.37 0.72 1.07 1.22 0.74 1.17 1.22 0.74 1.17 1.42 0.45 0.75 1.28 0.90 0.56 1.28 0.90 0.56

IQR

0.59 1.04 1.01 0.27 0.42 0.80 0.94 0.47 0.72 0.38 0.53 0.70 0.76 0.31 0.64 0.73 0.48 0.42 0.62 0.53 0.68

SD

0.06 0.48 0.63 0.31 0.77 0.51 0.08 0.30 1.76 0.43 1.03 1.86 0.04 0.35 0.40 0.30 0.20 0.03 0.23 0.13 0.89

MD

0.11 1.74 1.26 0.39 0.38 0.06 1.23 0.31 1.24 0.98 0.77 1.40 0.20 0.00 0.54 0.63 0.75 0.09 0.08 0.98 0.73

Mean

Db

3.60 5.21 6.48 4.42 3.34 5.49 4.34 5.61 4.04 0.44 3.26 3.63 4.15 3.39 5.14 5.51 3.78 3.08 3.69 3.69 2.51

IQR

2.24 2.93 3.34 2.44 2.31 2.82 3.18 2.63 2.62 2.11 1.96 1.90 2.33 2.35 3.12 3.01 2.21 1.84 2.17 2.73 1.98

SD

Results marked with * were found to be non-normally distributed and marked with ** were significantly positive resp. negative with respect to the test value of 0.00. A positive resp. negative DL value results in a brighter resp. darker color; a positive resp. negative Da value shifts the color into the red resp. green region; a positive resp. negative Db value shifts the color into the yellow resp. blue region.

Ti–Ag–ZrO2

Pink

Ti–Al–ZrO2

Yellow

–

White

Yellow

Pink

White

(b) Ti

Ti–Ag–ZrO2

–

Yellow

Color

Coating system

(a) Ti

DL

Da

DL

Db

2 mm

1 mm

Table 2. (a) Mean, standard deviation (SD), (b) median (MD), and interquartile range (IQR) of DL, Da and Db values for Ti, Ti–Al–ZrO2 and Ti–Ag–ZrO2 samples with different substrate treatment below mucosa of different thicknesses.

Pecnik et al
In vitro color evaluation of esthetic coatings for implants

Clin. Oral Impl. Res. 26, 2015 / 563–571

Pecnik et al
In vitro color evaluation of esthetic coatings for implants

Fig. 7. Decrease of color difference DE with increasing soft tissue thickness for machined Ti with pink-coating systems and machined Ti. Curves of yellow- and white-coating systems are shown as insets. The decrease of DE is asymptotically fitted (y = a  b∙cx) and serves as a guide to the eye only. The arrows highlight the mucosa thickness for each sample, above which the sample might not be optically noticeable in the intraoral environment.

varied stronger for 1 mm flaps than for the thicker tissue grafts. The in vitro evaluation according to (Jung et al. 2007, 2008) provides a good estimation about possible outcomes compared with the clinical situation. The validity of our conclusions relies on the comparability of the optical properties of human and porcine mucosa (Kuytz et al. 2001; van Eyk & van der Bijl 2004) where the authors observed similarities in structure and permeability. However, in vivo studies on human gingiva (Bressan et al. 2011; Happe et al. 2013) reported that they still perceived a discoloration for 3 mm thick mucosa thicknesses. This may be due to poor tissue quality or the use of an extremely rough implant. In general, our results can be interpreted on a relative level where a significant improvement of a critical mucosa thickness

(see Fig. 7), above which the implant is not optically detectable, can be established. In our best-coating systems, this thickness was reduced by more than 50% compared with the uncoated Ti reference.

system Ti–Ag–ZrO2. This observation could be advantageous for thin and fragile gingival tissues to avoid a dark appearance and therefore poor esthetic of the peri-implant mucosa. 2. Colors that acted advantageously for an esthetic gingival tissue were suggested to be pink and yellow. The adjustment of the coating color to the color of the patient’s mucosa could be a possibility to elaborate impeccable soft tissue esthetics. 3. Preferentially, the coatings should be deposited on machined substrates, as these surfaces reflect enough light and increase the brightness of the color, showed good soft tissue integration in previous studies and may exhibit a better adhesion between coating and substrate than polished substrates.

1. The coating systems Ti–Al–ZrO2 and Ti– Ag–ZrO2 delivered improved optical properties compared with Ti below soft tissue which led to DE values either close or below the visually perceivable threshold of color distinction in the oral environment. Instead of dark shine-through effects light brightening of the tissue could be observed, particularly for the

Acknowledgements: The authors thank Beatrice Sener for her support with the spectrophotometer and helpful discussions. The present study was a part of a multidisciplinary research project granted by the Competence Centre for Material Science and Technology (CCMX, project no. 43) within the framework of the project ‘Colored Ceramic Surfaces for Metallic Dental Implants and Prosthetic Appliances.’ The research partners were the Swiss Federal Institute of Technology Zurich (ETHZ), the University of Zurich (UZH), the Swiss Federal Laboratories for Material Science and Technology (Empa), and the Institut Straumann AG. The authors thank the Institut Straumann AG for providing the machined samples and its financial support.

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Conclusion From this study, the following conclusions can be made:

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