Bond strengths of ceramics to biocompatible

dental metals; substitutes for gold alloys

Young won Cho

The Graduate School

Yonsei University

Department of Prosthodontics

Bond strengths of ceramics to biocompatible

dental metals; substitutes for gold alloys

A Dissertation

Submitted to the Department of Prosthodontics

and the Graduate School of Yonsei University

in partial fulfillment of the

requirements for the degree of

Master of Dental Science

Young won Cho

June 2011

This certifies that the dissertation of Young won Cho is

approved.

________________________

Thesis supervisor : [Hong-seok Moon]

________________________

[Keun-woo Lee : Thesis Committee Member #1]

________________________

[Jae-hoon Lee : Thesis Committee Member #2]

The Graduate School

Yonsei University

June 2011

감사의 글

이 논문이 완성되기까지 따뜻한 배려와 함께 세심한 지도와 격려를

아끼지 않으신 문홍석 지도 교수님께 먼저 깊은 감사를 드립니다.

또한 귀중한 시간을 내주시어 부족한 논문을 살펴주시고 조언과 격려를

해주신 이근우 교수님, 이재훈 교수님, 김성태 교수님께 감사 드립니다.

보철학을 공부할 수 있도록 기회를 주시고 제가 이 자리까지 올 수

있도록 항상 따뜻한 관심과 조언으로 인도해주신 정문규 교수님,

한동후 교수님, 심준성 교수님, 박영범 교수님, 김지환 교수님께도 감사

드립니다.

2011 년 6 월

조영원 드림

i

Table of Contents

List of Tables and Figures ······················································································ii

Abstract ·············································································································· iii

Ⅰ. Introduction ······································································································ 1

Ⅱ. Materials and Methods ···················································································· 5

1. Preparation of metal discs ··········································································· 5

2. Surface treatment ························································································· 6

3. Porcelain application ··················································································· 7

4. Shear bond testing ······················································································· 8

5. Statistical analysis ····················································································· 10

6. Scanning electron microscopy ··································································· 10

Ⅲ. Results ··········································································································· 11

Ⅳ. Discussion ····································································································· 14

Ⅴ. Conclusion ···································································································· 17

Reference ············································································································ 18

Korean abstract ···································································································· 20

ii

List of Tables

Table I. Experimental groups ················································································· 5

Table II. Materials used ·························································································· 7

Table III. One-way ANOVA results for bond strength ········································· 11

Table IV. Results of Tukey’s test ······································································· 12

List of Figures

Figure 1. Dimensions of the metal specimens and ceramics ··································· 8

Figure 2. Specimen embedded in a resin matrix ····················································· 9

Figure 3. Prepared specimen for testing with the universal testing machine ·········· 9

Figure 4. SEM and clinical images of the fractured specimens ···························· 13

iii

ABSTRACT

Bond strengths of ceramics to biocompatible dental metals;

substitutes for gold alloys

Objectives: This study evaluated the bond strengths of ceramics to 3

biocompatible dental metals that may substitute for gold alloys and compared

them with the bond strengths of ceramics to a conventional gold alloy.

Methods: Four groups of dental metals (conventional gold alloy, milled

titanium alloy, cast commercially pure titanium, and milled palladium-silver alloy

fabricated into discs with dimensions of 9 × 3 mm were included in this study (n =

40). All specimens were air-abraded with 50-μm alumina. Before application of

porcelain discs with a dimension of 5 × 2 mm at the center of the metal specimens,

the metal surfaces were surface-treated according to the manufacturers’

suggestions. The specimens were embedded in an acrylic resin matrix, and

bonding strengths were tested using the parallel shear bond test. The load at

fracture was recorded, and the fractured surfaces were observed under a scanning

electron microscope. One-way ANOVA and Tukey’s tests were used to analyze

the data at a 5% probability level.

Results: The average bond strengths were 25.9, 18.86, 21.95, and 20.33 MPa

for groups PG, TP, CT, and IN, respectively. Significant differences in bond

iv

strengths were only seen between groups PG and TP (p < 0.05). The fractured

surfaces showed a combination of cohesive and adhesive failures for all testing

groups with the exception of group PG, which showed mainly cohesive fractures

within the porcelain body.

Keywords: Palladium-Silver, Pd-Ag, Innovium, Porcelain bond strength, Shear

bond strength, Ceramic metal bonds

1

I. Introduction

Esthetic dental restorations have become possible since the introduction of

porcelain-fused-to-gold (PFG) restorations in the 1950s. However, because of the

persistent rise in the value of gold since the 1970s, the cost of fabrication of PFG

restorations has become a considerable factor, spurring studies to focus on

substituting gold alloys with other materials such as nonprecious metals,

zirconium, titanium, etc. [1-3].

Nonprecious metal alloys provide durable and esthetic metal-ceramic

restorations at a comparatively reasonable cost [1]. For these metals to chemically

bond with porcelain, they must contain certain amounts of Ni and Cr (60%–80%),

which, according to previous studies, may induce various allergic reactions such

as urticaria, pruritus, xerostomia, eczema, lichen planus, or vesicular eruptions [4-

6]. Studies have reported that up to 4.0% and 31.9% of the female population

experience chromium and nickel sensitivity, respectively; therefore, the clinical

application of these metals as dental restorations has been under debate [4].

The introduction of metal-free zirconium restorations eliminated the

controversy of applying nonprecious metals in vivo while maintaining low

fabrication costs. However, challenges of applying zirconium as dental

restorations have been reported in previous studies: delamination of the veneering

porcelain, low temperature degradation, and lower fractural strength when

2

compared with metal restorations [7-9]. Authors have reported on the aging

phenomenon of zirconium in the oral environment and of their considerably lower

fracture strength than those of titanium restorations and have questioned their use

in patients with parafunction and in areas with high occlusal loading [8, 10].

The biohazardous potential of nonprecious metals and questions concerning the

long-term reliability of zirconium restorations have resulted in studies that focus

on titanium as a replacement for precious metals in PFG restorations. Titanium is

a useful dental material because it possesses low thermal conductivity, low density

(4.5 g/cm3), and a low corrosion rate while offering excellent biocompatibility [11,

12]. The biocompatibility of titanium is a result of a stable oxide layer that forms

on its surface after exposure to air [13, 14].

Conventional ceramics have CTE and firing temperatures that are compatible

with those of gold alloys, and are not well-suited with titanium. The excessive

difference in CTE between conventional ceramics and titanium as well as the

thick oxide layer (α-case layer) that forms on the surface of titanium at firing

temperatures not only interfere with the ceramic-metal bond, but also decrease the

mechanical properties of the titanium itself [6, 15-17]. In solution, low-fusing

ceramics that are fired at temperatures that decrease the formation of the α-case

layer (<883°C) and provide compatible CTE with titanium have been introduced

[14, 18, 19]. Although these lower firing temperatures have greatly decreased the

formation of the oxide layer, acquisition of an ideal thickness has yet to be

3

achieved. Previous studies have reported bond strengths between titanium and

low-fusing ceramics of 14–24 MPa, which are significantly lower than the bond

strengths of gold alloys to porcelain [1, 20].

The potential biohazards of nonprecious metals, delamination of veneering

porcelain, potential fracture of zirconia restorations, and low bonding strengths of

ceramics to titanium have generated continuous interest in methods to increase bond

strengths and utilize new metals that provide durable bonds with porcelain without

compromising biocompatibility. A new palladium-silver-based milling alloy was

recently introduced. Palladium-based alloys have been used since the 1970s;

however, most of these alloys contained copper, thereby reducing biocompatibility

and increasing corrosion when used as dental restorations [21]. Studies on

palladium alloys have reported that alloys with high palladium content cause in vivo

reactions similar to those caused by 22-carat gold, affirming the biocompatibility of

alloys with high palladium content [22]. This newly introduced palladium-silver

alloy is free of potentially biohazardous elements, including Cu, is high in

palladium content, possesses sufficient strength (yield strength 390 MPa), has low

density (10.93 g/cm3), is a warm yellowish color, and has a CTE and oxide layer

that are compatible with those of conventionally used porcelain. In addition to these

characteristics, this alloy is millable, allowing for one visit dentistry, using the

CAD/CAM milling system. However, there are limited studies reporting the

appropriateness of the application of this new alloy in dental restorations.

4

The purpose of this study was to evaluate the shear bond strengths of ceramics

to 3 biocompatible dental metals that may be used as substitutes for gold alloys

and compare them with the bond strengths of ceramics to a conventionally used

gold alloy. The null hypothesis of this study was that the bond strengths of

ceramics to the test metals will be lower than those of ceramics fused to

conventional gold alloys.

5

II. Materials and Methods

In the present study, 3 types of biocompatible metals (milled titanium alloy,

cast CP Ti, and a milled palladium-silver alloy) bonded to their corresponding

ceramics were used as the experimental groups, and a conventional gold alloy

bonded to a conventional ceramic was used as the control. The 4 groups and type

of porcelain used for each group are summarized in Table I.

Table I. Experimental groups

Group Metals N Surface treatment Ceramic

PG Gold alloy 10 Sandblasting GC InitialTM

TP Ti6Al-4V ELI 10 Sandblasting + titanium bonder

Triceram®

CT CP Ti 10 Sandblasting + titanium bonder

Triceram®

IN Palladium-silver alloy

10 Sandblasting GC InitialTM

1. Preparation of metal discs

10 specimens were fabricated for each group. Conventional gold alloys were cast

using acrylic resin patterns and conventional firing techniques, grade 23 titanium

discs (Ti6Al-4V ELI, Grandis Titanium, CA, USA) were milled, grade 1 titanium

6

discs (EUTITAN, EUKAMED, Essen, Germany) were cast, and palladium-silver

alloy discs (Innovium, Ceragem Biosys, Ilsan, Korea) were milled, each with the

following dimensions: diameter of 9 mm and height of 3 mm. The cast titanium discs

were cast using wax patterns invested in magnesium-based investment material

(Biotan® Titanium Investment Material MG, Schütz Dental GmbH, Rosbach,

Germany) and cast. The milling titanium specimens were fabricated using grade 23

titanium alloys (Ti 6Al-4V ELI, Grandis Titanium, CA, USA). Gold alloys were cast

using acrylic resin patterns and conventional firing techniques, and the Innovium

discs were milled. The cast commercially pure titanium discs were fabricated with

dimensions that were slightly larger than the desired ones to grind away the α-case

layer that may have formed on the surface of the cast titanium specimens. The

materials used in this study are summarized in Table II.

2. Surface treatment

All specimens were air-abraded for 20 seconds at 2 bar with 50-μm alumina

using an airborne particle abrasion device at a 45° angle and distance of 20 mm

from the metal strip to increase the surface area, enhance porcelain wetting, and

increase bond strength.

The cast gold alloys were degassed and oxidized using conventional methods.

Degassing to eliminate the penetrated gasses during casting was not required for

7

the milled Innovium discs. The specimens were oxidized at 600°C before

application of the ceramics.

Table II. Materials used

Material Manufacturer Composition

Milled titanium

(Ti6Al-4V ELI)

Grandis Titanium, CA,

USA

Grade 23: Ti >89%, Al 6.0%, V 4.0%

Cast titanium

(EUTITAN)

EUKAMED, Essen,

Germany

Grade 1: Ti >99.0%

Pd-Ag alloy

(Innovium)

Ceragem Biosys, Ilsan,

Korea

Au 3%, Pd 36.77%, Ag 33%, Ir + Zn

+ In 27.23%

Gold alloy

(Argedent 52)

Argenⓒ, CA, USA Au 52.5%, Pd 26.9%, Ag 16%, In

2.5%, Sn 2%, Ru <1%

Triceram® Dentaurum, Ispringen,

Germany

Low-fusing ceramic power

GC InitialTM GC, Tokyo, Japan Conventional ceramic powder

3. Porcelain application

For all testing groups, the corresponding porcelain was applied in a disc shape

with a diameter of 5 mm and a height of 2 mm at the center of the metal disc. For

groups TP and CT, a porcelain bonder was first applied to the sandblasted surface.

A thin layer of opaque porcelain was then applied to each specimen followed by

8

low-fusing dentin body porcelain (Triceram®, Dentaurum, Ispringen, Germany)

using a pattern resin index and vibrator. Because of the firing shrinkage of

ceramics, a compensatory second layer of porcelain was applied for the desired

ceramic dimension. A conventional gold alloy porcelain (GC InitialTM, GC, Tokyo,

Japan) was applied to groups IN and PG in the same manner. All ceramics were

fired according to each of the manufacturer’s recommendations.

Figure 1. Dimension of the metal specimens and ceramics

4. Shear bond testing

The specimens were embedded in an acrylic resin matrix with a length of 20

mm and a diameter of 15 mm (Figure 2) and stored in distilled water for 24 hours.

A universal testing machine (Instron® 3366, Instron Co., Ltd., USA) with a 1-mm

diameter stainless steel stylus was placed at the junction of the metal disc and

ceramics, and load was applied at a constant crosshead speed of 0.5 mm/min until

9

fracture (Figure 3). The load at fracture was recorded in newtons (N), and the

shear bond strengths (MPa) were calculated using the following equation:

MPa = F/A (N/mm2)

F = force in newtons

A = cross-sectional area (mm2)

Figure 2. Specimen embedded in a resin matrix. C, ceramic; M, metal specimen;

R, acrylic resin matrix.

Figure 3. Prepared specimen for testing with the universal testing machine

(Instron® 3366, Instron Co., Ltd., CA, USA)

10

5. Statistical analysis

The statistical software SPSS 12.0 for Windows (Release 12.0.0, 4 Sep

2003, SPSS Inc., IL, USA) was used to generate descriptive statistics and

perform inferential tests. One-way ANOVA and Tukey’s test were used to

determine the statistical significance of the mean differences between the 4

groups (p < 0.05).

6. Scanning electron microscopy

The fractured specimens were ultrasonically cleaned in distilled water for

5 minutes, and fracture patterns were observed under a scanning electron

microscope (SEM) (S-3000H, Hitachi Science and Technology, Berkshire,

UK).

11

III. Results

The average shear bond strengths of groups PG, TP, CT, and IN were 25.9

MPa (SD 5.01), 18.86 MPa (SD 4.05), 21.95 MPa (SD 6.46), and 20.33 MPa (SD

2.97), respectively. One-way ANOVA results (Table III) showed no significant

differences between bond strengths within any single testing group. However,

there was a significant difference in bond strength between the groups (p < 0.015).

Tukey’s test (Table IV) showed a significant difference in bond strengths between

PG and TP (p < 0.012). Examination of the fractured surfaces revealed that all

testing groups showed a combination of cohesive and adhesive failures with the

exception of group PG, which showed mainly cohesive fractures within the

porcelain body (Figure 4).

Table III. One-way ANOVA results for bond strength

Source Df Sum of squares Mean square F Sig

Between 3 276.243 92.081 3.996 0.015*

Within 36 829.595 23.044

Total 39 1105.838

*Significant levels between the groups

12

Table IV. Results of Tukey’s test

Group Bond strength (MPa) Homogeneous groups

Mean Standard deviation

PG 25.9 5.01 A

TP 18.86 4.05 B

CT 21.95 6.46 A B

IN 20.33 2.97 A B

Different letters in the right column represent statistically significant differences

(p < 0.05)

Comparison between groups Significance level

PG vs TP 0.012*

PG vs CT 0.273

PG vs IN 0.063

TP vs CT 0.482

CT vs IN 0.873

IN vs TP 0.902

* Significant levels between the testing groups

13

A B

C D

Figure 4. SEM and clinical images of the fractured specimens. Left: Clinical

photos of the fractured surfaces of the metal specimens. Right: SEM images of the

fractured surfaces of the metal specimens. A) Group PG: The fracture pattern

mainly consisted of cohesive fractures within the ceramic in group PG. B) Group

TP, C) Group CT, D) Group IN; The fracture patterns were mainly adhesive

between the metal and ceramic in groups TP, CT, and IN.

C, Ceramic- representing cohesive failure within the ceramic. M, Metal-

representing adhesive fracture between the metal and ceramic.

14

IV. Discussion

This study analyzed the bond strengths of ceramics to biocompatible dental

metals and compared them with porcelain-fused-to-gold alloys using the parallel

shear bond test. Statistically significant differences in bond strength were only seen

between groups TP and PG; thus, within the limitations of this study, the null

hypothesis that the bond strengths of ceramics to 3 biocompatible dental alloys are

lower than those of ceramics to conventional gold alloys was only partially accepted.

In the present study, the shear bond test was used to measure the ceramic-

metal bond. A study that evaluated the efficiency of bond tests concluded that the

values acquired from in vitro bond tests were useful only in making comparative

analyses because the stresses applied in bond testing do not resemble the forces

applied to metal-ceramic restorations in clinical situations [15, 23]. Compared

with conventional gold alloy, all testing groups with the exception of group TP

showed similar bond strengths; therefore, within the limitations of this study, it

can be assumed that metal-ceramic restorations using casting grade 1 titanium and

newly introduced palladium-silver alloy will produce clinically acceptable results.

However, other factors, such as marginal fit and color stability, must be evaluated

before they are utilized as dental restorations.

As previously mentioned, at the titanium casting temperature, formation of

the α-case layer interferes with ceramic bonding [24, 25]. According to various

15

studies, when titanium is cast in traditionally used phosphate-bonded SiO2

investment material, the thickness of the α-case layer is 50–500 μm. Based on a

recent study, MgO-based investment material minimized α-case formation [16].

Previous studies have also reported no significant differences in porcelain bonding

strength between casting and milled titanium when the α-case layer was removed

[12, 24, 26] and no significant differences between commercially pure and alloyed

titanium alloys [18]. In this study, to remove any remnants of this layer, the

surfaces of the casted titanium were slightly ground using fine-grit burs.

The findings of this study concur with the results of previous studies in that

there were no significant differences in bond strength between the casted and

milled titanium groups. However, significant differences were found between

groups TP and PG. This may have been because the milled titanium specimens

used in this study were grade 23, which is a modification of the grade 5 alloys

used in previous studies. However, because grade 23 alloys are identical in

content to grade 5 alloys with the exception of the fact that they contain lower

levels of iron and oxygen, more studies on the effect of decreased iron and oxygen

content of titanium on their bond strengths to low-fusing porcelain are needed.

All specimens with the exception of the gold alloy group showed mostly

adhesive fractures between the ceramic and metal alloys. These results suggest

that the bond strengths between the ceramic and metals of the gold alloy group

16

exceeded the cohesive strengths within the porcelain layer. The adhesive fractures

in groups TP, CT, and IN suggest that the chemical and mechanical bonds formed

between the metal and ceramics were weaker than the bonds within the ceramic

particles. Therefore, although the fractural strength between groups CT, IN, and

PG were not statistically different, the bonds between ceramics to metals in these

testing groups may be improved.

The effect of thermal and mechanical cycling and surface modification

methods of metals were not evaluated in this study. Previous studies that

compared changes in shear bonding strengths of low-fusing porcelains to titanium

when thermal and mechanical cycling was applied to the testing specimens found

a significant decrease in the shear bond strengths when cycling was applied [2].

Studies have evaluated the effects of various surface treatments on the

bonding between titanium and ceramics [14, 27, 28]. This study was limited in

that only one type of surface treatment was applied to the metal specimens. In

addition, the effect of surface treatments on Innovium has yet to be evaluated.

Therefore, the effects of thermal, mechanical cycling and different surface

treatments of metal specimens have on bond strengths of porcelain-metal

specimen need to be evaluated.

17

V. Conclusion

Within the limitations of this study, the following conclusions were made:

1. Statistically significant difference in bond strengths were found between

conventional ceramics bonded to a gold alloy and low-fusing ceramic

bonded to milled titanium.

2. The fracture patterns of the milled, cast titanium, and palladium-silver

alloys consisted mainly of adhesive fractures between the metal and

ceramics, while those of the gold alloy group showed mainly cohesive

fractures within the ceramic.

18

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titanium-porcelain bonding. Dent Mater, 2008. 24(1): p. 28-33.

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국문요약

생체친화성을 지닌 치과용 금속과 도재간의 결합강도;

금합금 대체 금속에 관한 연구

<지도교수 문 홍 석>

연세대학교 대학원 치의학과

조 영 원

연구 목적: 본 연구의 목적은 도재-금속 수복물에서 주로 사용되는 금합금과 생체

친화성을 지닌 세가지의 금 대체 금속의 금속-도재간 결합 강도를 비교하는데 있다.

실험 방법: 금합금 (PG), 밀링용 티타니움 합금 (TP), 주조용 순수 티타니움 금속

(CT)과 밀링용 팔라디움-은 합금 (IN)을 9 mm x 3 mm 크기의 원반으로 제작한 후

(n=40), 제조사의 지시에 따라 표면 처리 하였다. 표면 처리 후 시편의 중앙

부위에 5 mm x 2 mm 의 크기로 도재 소성을 하였고 각 시편을 아크릴릭 레진

메트릭스에 포매 하였다. 각 시편의 전단 강도 측정을 위하여 universal testing

machine 을 이용하여 파절 시까지 힘을 가했고, 전자현미경을 이용하여 파절 양상을

관찰 하였다. 통계학적 분석을 위하여 one-way ANOVA 와 Tukey 검정을 사용

하였다.

결과: 각 군의 평균 결합 강도는 PG 25.9 MPa, TP 18.86 MPa, CT 21.95 MPa,

그리고 IN 20.33 MPa 로 측정 되었고, PG 과 TP 에서 통계학적인 차이를 보였다

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(p<0.05). 전자 현미경으로 파절 양상을 관찰한 결과, PG 에서 나타난 파절면들은

대부분 도재 내에서 발생한 cohesive 파절 양상을 보였으나, 나머지 군에서는

cohesive 와 adhesive 파절이 복합적으로 나타나는 양상을 보였다.

핵심 용어: 팔라디움-은, Pd-Ag, Innovium, 금속-도재 결합강도, 전단강도