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The influence of the framework thickness on surface strain of the
3-unit zirconia resin-bonded fixed dental prostheses under thefunctional loading
Kyoshi Matsukawa, DDS (1), Reina Nemoto, DDS, PhD (1), Kosuke Nozaki, DDS, PhD(2), Mariko Kubo, DDS (1), Tasuku Inagaki, DDS, PhD (1), Keiichi Yoshida, DDS, PhD(1), and Hiroyuki Miura, DDS, PhD (1)(1) Fixed Prosthodontics, Oral Health Sciences, Graduate School of Medical and Dental Sciences, and(2) Department of Inorganic Biomaterials, Institute of Biomaterials and Bioengineering, Tokyo Medical
and Dental University, Tokyo, Japan
Purpose: The purpose of the present study was to optimize the form of a resin-bonded fixed dental prosthesis(RBFDP) frame by preparing various framework designs using ceria-stabilized tetragonal zirconia/alumina nanocomposite (Ce-TZP/Al) materials.
Materials and Methods: Six types of frameworks were fabricated as follows: 0.8-mm-thick metal (0.8M),
0.8-mm- and 0.5-mm-thick yttria-stabilized tetragonal zirconia polycrystal frameworks (0.8Y, 0.5Y) and 0.8mm-, 0.5 -mm- and 0.3-mm- thick Ce-TZP/Al frameworks (0.8C, 0.5C, 0.3C). A load up to 200 N crosshead
speed of 1.0 mm/min was applied to center of the palatal surfaces of the pontic of the RBFDPs by a universal
testing machine. The strains of the RBFDPs during loading were measured and recorded by sensor interfaces.
Then, the magnitudes of the maximum and minimum principal strain were calculated. A two-way analysis ofvariance (ANOVA) and a t-test with Bonferroni correction were used for the statistical analysis of strain, with asigniﬁcance level at α = 0.05.
Results: In the canine, the magnitudes of the maximum principal strain of 0.8M were significantly higher thanthose in group 0.8Y. There were no signiﬁcant differences among the 0.3C, 0.5Y, and 0.8M groups.
Conclusion: This report suggested that Ce-TZP/Al can improve the long-term prognosis of resin bonded fixed
dental prostheses in the oral cavity due to its desirable mechanical and hydrothermal degradation-resistantcharacteristics, enabling the design of a resin bonded fixed dental prosthesis frame with a thickness of 0.3 mm.
(Asian Pac J Dent 2017; 17: 1-7.)
Key Words: resin-bonded fixed dental prosthesis, strain gage, zirconia ceramics

Introduction
Fixed dental prostheses are widely used in clinical settings to provide prosthetic dental treatment to patients withanterior tooth loss because they restore functionality, provide good esthetics, and are comfortable. Resin-bonded
fixed dental prostheses (RBFDPs), which in principle limit plasia to enamel formation [1,2] and preserve healthy
tooth substances, are widely utilized since minimal intervention concept [3] is required. However, current
RBFDPs using gold-silver-palladium and chromium-cobalt system alloys have limitations. For example, theycannot achieve sufficient esthetics [4,5] and may cause metal-induced allergy due to elution of metal ions.
Since yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) is superior in bend strength and esthetics, anddoes not cause metal-induced allergy, its clinical application has expanded. In addition, owing to improvementsin the precision of abutment tooth scanning, design, and milling, a framework for RBFDPs can be made. To
investigate the clinical application of RBFDPs with Y-TZP, we measured the distortion of Y-TZP using a strain
gage and reported excellent stress-relaxation [6].
However, one study has stated that exposure of Y-TZP to the oral cavity is not preferable since it causes thephenomenon of hydrothermal degradation, in that a lattice constant varies due to an invasion of OH- ions into
the oxygen vacancy of zirconia and hydrolysis [7,8]. This leads to phase transition from a tetragonal tomonoclinic phase, and reduced physical strength. In contrast, a ceria-stabilized tetragonal zirconia/alumina nanocomposite (Ce-TZP/Al) eliminates the oxygen vacancy using CeO2 as a stabilizer of ZrO2 that drastically1Matsukawa et al.

Asian Pac J Dent 2017; 17: 1-7

improves its tolerance to hydrothermal degradation compared with Y-TZP [9-11]. In addition, since the physicalstrength resulting from the phase transformation mechanism is not expected to improve with Ce-TZP alone,
Ce-TZP/Al contains and scatters approximately 20% of alumina across the polycrystalline substance, whichleads to improvement in physical strength [10].
Given the above, Ce-TZP/Al causes no hydrothermal degradation and has superior long-term stability in theoral cavity compared with Y-TZP. Furthermore, the fracture toughness of Ce-TZP/A is approximately from 2 to
3 times compared with that of Y-TZP. When Ce-TZP/Al has been used as zirconia crown framework in previous
studies, even a framework thickness of 0.3 mm, which is thinner than the conventional metal crown framework,
has been suggested to provide sufficient strength for use in a clinical setting [12]. Therefore, it is expected that
Ce-TZP/Al materials have application in RBFDPs frameworks. The aim of the present study was to optimize theform of a RBFDPs framework by preparing various framework designs using Ce-TZP/Al materials, and measure
the surface distortion.

Materials and Methods
Master model fabrication
A human dry maxilla was used in this study. It had been donated for scientific research to Maxillofacial
Anatomy, Department of Maxillofacial Biology, Maxillofacial/Neck Reconstruction, Graduate School, Tokyo
Medical and Dental University (TMDU). It does not have the defect of the bone, and the teeth remain in ahealthy state and the personal information cannot be identified. This study was approved by the ethics review
committee of TMDU (No. 1173).
To obtain the Digital Imaging and Communication in Medicine (DICOM) data of the maxilla, micro
computed tomography (µCT) imaging taken by Micro focus X-ray computer tomography system (inspeXio
SMX-100CT, Shimadzu Co., Ltd, Kyoto, Japan) were used. The data were in a range of the maxillary left central
incisor to the maxillary left canine including their surrounding alveolar bone. The DICOM data was converted
into the Standard Triangulated Language (STL) data, and then trim the maxillary left lateral incisor.
Three-dimensional printer (Objet500 Connex, Stratasys, Minneapolis, MN, USA) with the STL data provided
the model that the teeth can be removed from the alveolar bone.
The maxillary left central incisor and the canine were prepared for the application of RBFDPs. The palatal
surfaces were prepared with a diamond football-shaped point (117, Shofu Inc., Kyoto, Japan) at the regulated
thickness (0.3 mm and 0.5 mm). Cervical preparations were performed by a diamond chamfer point (K1, GC Co.,Tokyo, Japan) within its 1.0 mm from the anatomical cervical lines. The finish lines were designed as chamfer
having the same thickness as the palatal surfaces. Two vertical grooves (0.5 mm depth) were prepared both sides
of proximal surfaces and a hole (1.0 mm diameter and 0.5mm depth) were prepared on the cingulum areas with adiamond round point (MI-2R, Shofu Inc.).
To duplicate the abutment teeth, impressions of the abutment teeth were made with vinyl polysiloxane
impression material (Memosil2, Heraeus-Kulzer GmbH, Hanau, Germany). Then an auto-mix resin composite(Clearfil DC Core Automix One, Kuraray Noritake Dental Inc., Tokyo, Japan) was injected into the impressions.
Fabrication of RBFDPs
The 0.8-mm-thick gold-silver-palladium alloy (Castwell M.C.12%Gold, GC Co.) frameworks (0.8M), 0.5-mmand 0.8-mm-thick Y-TZP (C-Pro HTZR, Panasonic Healthcare Co., Ltd, Tokyo, Japan) frameworks (0.5Y,

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0.8Y) and 0.3- mm-, 0.5 -mm- and 0.8- mm- thick Ce-TZP/Al (C-Pro Nanozr, Panasonic Healthcare Co., Ltd,)
frameworks (0.3C, 0.5C, 0.8C) were defined as the experimental groups. Each groups consisted of 5 specimens.
To design and fabricate RBFDPs using zirconia (0.5Y, 0.8Y, 0.3C, 0.5C, 0.8C), the resin abutment teeth were
scanned by a dental laboratory scanner (3Shape D700, 3Shape, Copenhagen, Denmark). The RBFDPs using
zirconia were designed respectively using the three-dimensional data of the corresponding resin abutment teeth.
The cross-sectional area of the connectors was defined as 6.0 mm2, and the design of the shape was elliptical.
The cement layer thickness was determined to 40 µm in the range from the margin to 1.0 mm, in the other sites
20 µm. The RBFDPs using zirconia were designed uniform thickness according to the rule of the experimentalgroup (Fig. 1).

Fig. 1 Photographs of framework; Metal (a), Y-TZP (b), and Ce-TZP (c)Impressions of 0.8Y were made with vinyl polysiloxane impression material (Exafine putty type, GC Co.)and improved dental stone (New Fujirock, GC Co.). Acrylic resin (Pattern Resin, GC Co.) was injected among
the putty and the dental stone to obtain the resin pattern of RBFDPs using metal. The resin pattern of 0.8M were
invested with an investment material (Cristobalite PF, Shofu Inc.) and cast in gold-silver-palladium alloy(12%Au-Ag-Pd alloy) with vacuum casting machine (KDF Cascom, Denken Co., Ltd. Kyoto, Japan).
Testing procedure
The frameworks and the master dies were abraded with 70 µm Al2O3 airborne particles (0.2 MPa air pressure for10 s) and ultrasonically cleaned in deionized water for 10 min. The surfaces of the resin abutment teeth andframeworks using zirconia were cleaned with alcohol and applied with a ceramic primer (Clearfil Ceramic
Primer plus, Kuraray Noritake Dental Inc.) and the surfaces of the 0.8M were cleaned with alcohol and appliedwith a metal primer (Alloy Primer, Kuraray Noritake Dental Inc.). Then, the RBFDPs were cemented to the resin
abutment teeth with resin-based luting cement (Panavia F2.0 TC, Kuraray Noritake Dental Inc.) according to the
manufacturer’s instructions.
The buccal surfaces of the RBFDPs were sandblasted and ultrasonically cleaned with the same way as
previous process. Rosette gages (KFG-1-120-D17-11 N30C2, Kyowa Electronic Instruments Co., Tokyo, Japan)
were bonded to the center of the buccal surfaces of the maxillary left central incisor and the canine of the
RBFDPs with strain gage cement (cc33-A, Kyowa Electronic Instruments Co.) using finger pressure for 1 minon polyethylene film (Fig. 2).
The resin abutment teeth were embedded in acrylic resin base (Palapress Vario, Heraeus-Kulzer GmbH).
Each root was surrounded with a layer of vinyl polysiloxane impression material (Correct Plus Bite, Pentron Co.,
Wallingford, CT, USA) as the artificial periodontal ligament (approximately 0.25 mm), and the angles between
the roots and the horizontal plane were determined to 45˚.

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Fig. 2The palatal aspect of framework with two strain gages
bonded to the palatal surface of each retainer

Fig. 3Stress strain test scenario
Specimens were fixed and loaded at 45˚ to long axis
with the universal testing machine up to 200 N.

Stress strain test
A load up to 200 N with crosshead speed of 1.0 mm/min was applied to center of the palatal surfaces of the
pontic of the RBFDPs by a universal testing machine (Autograph AGS-H, Shimadzu Co., Ltd, Kyoto, Japan)
with a stainless rod with ball-end (φ2.0mm) (6x6x40, KDA Co., Tokyo, Japan) (Fig. 3). The loading direction
was at 45˚ to the long axis of the abutment teeth. The strains of the RBFDPs during load were measured andrecorded by sensor interfaces (PCD-300B, PCD-330B-F and PCD-300A, Kyowa Electronic Instruments Co.).
Then, the magnitudes of the maximum and minimum principal strain (εmax, εmin) were calculated as follows:
max=

+ )+

min=

+ )

εa, εb and εc were the strains of each gage component.
Statistical analyses
A two-way analysis of variance (ANOVA) and a t-test with Bonferroni correction were used for the statisticalanalysis of strain, with a signiﬁcance level at α = 0.05.

Results
To clarify the effect of the framework materials and the thickness on the surface strain, the magnitude of theprincipal strains of the metal frameworks (0.8M), the Y-TZP frameworks (0.8Y, 0.5Y), and the Ce-TZP/Al
frameworks (0.8C, 0.5C, 0.3C) were calculated from the strains of each gage (Fig. 4). In the canine, the meanvalue for the metal frameworks (0.8M), the Y-TZP frameworks (0.8Y, 0.5Y) and the Ce-TZP/Al frameworks(0.8C, 0.5C, 0.3C) were 261.6 ± 55.1 µε, 141.5 ± 40.7 µε, 205.5 ± 83.0 µε, 180.7 ± 41.6 µε, 202.5 ± 50.5 µε, and
216.8 ± 35.2 µε respectively. The magnitudes of the maximum principal strain in group 0.8M were significantly4Matsukawa et al.

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higher than those in group 0.8 Y-TZP frameworks. There were no signiﬁcant differences among the 0.3N, 0.5Y,
and 0.8M groups.
In the central incisor, the magnitude of the principal strains of the metal frameworks (0.8M), the Y-TZP
frameworks (0.8Y, 0.5Y), and the Ce-TZP/Al frameworks (0.8C, 0.5C, 0.3C) were 52.8 ± 45.3 µε, 44.1 ± 42.8
µε, 33.8 ± 64.4 µε, 55.7 ± 35.7 µε, 0.1 ± 39.8 µε, and −21.7 ± 70.8 µε respectively. In all the groups, there wasno significant difference between the groups. In all groups, the framework of the canine showed a larger
distortion than that of the central incisor.

Fig. 4 Comparison of magnitude of the principal strains for six kinds of frameworks

Discussion
To optimize the framework design of RBFDPs manufactured using Ce-TZP/Al, we examined the transformationof RBFDPs framework via the in vitro strain gage method. The transformation of the framework under the deadload was analyzed using a finite element method, photoelasticity method, and strain gage method. In the strain
gage method, it is possible to attach a strain gage to the prosthesis, and measure its stress and distortion directly[13,14]. This allows the evaluation of the transformation during the functioning of the prosthesis used in clinicalsettings. Previously, we examined the surface distortion of crown and partial dentures in vitro and in vivo using
the strain gage, and reported no change in the transformation of the framework in each experimental model [15].
This highlighted the importance of the examination method for the design of the framework in vitro.
With regard to the in vitro experimental model, a human dry skull was scanned using µCT, and a model of an
abutment tooth and alveolar bone was created using a three-dimensional printer. This method is believed tomeasure the distortion of the framework more accurately because it precisely mimics the conditions such as theform of the alveolar bone and the planting direction of the abutment tooth. In this study, 200 N, assumed to be
the greatest biting force of the anterior teeth [16], was loaded on a pontic. Generally, for occlusal adjustment of
RBFDPs, the biting force applied is balanced between the pontic and the abutment tooth. However, in thisexperimental model, the greatest biting force of the anterior teeth was loaded on only the pontic in order to
conduct an analysis under a severe condition comparable to that in the oral cavity.

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A RBFDPs using Ce-TZP/Al can be designed with a thickness of 0.3 mm, based on the excellent mechanicalstrength of Ce-TZP/Al. Considering that Y-TZP must be designed with a thickness of at least 0.5 mm in order toprevent breakage, due to its mechanical strength and milling, a reduction in the abutment tooth substancedeletion can be expected using Ce-TZP/Al. The general enamel thickness is 0.5-0.8 mm [17], and dentin can be
exposed depending on its form. In exposed dentin, the adhesive strength may be reduced [18] and caries may
advance rapidly. Therefore, a reduction in deleted tooth substances using Ce-TZP/Al can improve the long-term
prognosis.
In the present study, based on the above conditions, various designs of Ce-TZP/Al were created and thesurface distortion was measured. Results indicated a tendency similar to the distortion of RBFDPs made using an
alloy, and suggested potential clinical application. Detachment of the prosthesis is a main factor contributing to
reduced survival rate of RBFDPs. When a functional force such as masticatory force acts on the prosthesis, theframework is transformed, applying detachment power to the adhesive interface [19,20]. Examination of thethird maximal distortion revealed that the 0.8 mm-thick framework created using Y-TZP reduced the surface
distortion more prominently compared with the framework made from an alloy. However, there was no
statistical significance between Y-TZP frameworks with a thickness of 0.5 mm and Ce-TZP/Al frameworks ofvarious thicknesses.
The framework of the canine tooth showed a larger distortion than that of the central incisor in all groups. The
distortion of the framework was determined based on the moment, elasticity coefficient, and section modulus.
This could be due to the fact that the model canine tooth, prepared based on CT scan data of a human
dry-toothed jawbone in order to reproduce the oral cavity more precisely, was slightly twisted. This resulted in atendency for moment to be produced easily when a load was applied to the pontic; the adhesion area was 103
mm2 and 75 mm2 at the central incisor and the canine tooth, respectively. Considering that surface distortion
may be reduced by providing supportive abutment pieces, such as a groove or hole to the abutment tooth [21-24],
it is thought to be desirable to provide such pieces to the abutment tooth when stress appears to be concentrated
in one abutment tooth, as in the case of the present study.
Based on the above findings, it is suggested that Ce-TZP/Al can improve the long-term prognosis of RBFDPs
in the oral cavity due to its desirable mechanical and hydrothermal degradation-resistant characteristics, enabling
the design of a RBFDPs framework with a thickness of 0.3 mm.
Acknowledgments
This work was supported in part by Grant-in-Aid for Young Scientists (B26861624) from the Japan Society for the
Promotion of Science (JSPS).

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Correspondence to:
Dr. Reina Nemoto
Fixed Prosthodontics, Oral Health Sciences, Graduate School of Medical and Dental Sciences,
Tokyo Medical and Dental University
1-5-45, Yushima, Bunkyo-ku, Tokyo 113-8549, Japan
Fax: +81-3-5803-0201 E-mail: r.nemoto.fpro@tmd.ac.jp
Accepted December 1, 2016.
Copyright ©2017 by the Asian Pacific Journal of Dentistry.
Online ISSN 2185-3487, Print ISSN 2185-3479

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