Shimizu et al.

Int Chin J Dent 2005; 5: 35-38.

Fracture strength of metal-based complete maxillary dentureswith a newly designed metal framework
Hiroshi Shimizu, DDS, PhD,a Fumitake Tsue, DDS,a Motofumi Obukuro, DDS,aHirofumi Kido, DDS, PhD,b Yutaka Takahashi, DDS, PhD,a and Hiroyuki Ohmura, DTcab

Division of Removable Prosthodontics, Department of Oral Rehabilitation, Division of Oral
Implantology, Department of Oral Rehabilitation, Fukuoka Dental College, Fukuoka, Japan, andcFukuoka Branch Office, Dental Laboratory Aishi, Fukuoka, Japan
Purpose: The purpose of the present study was to evaluate the fracture strength of a metal-based complete
maxillary denture with a newly designed metal framework that did not extend to the residual ridge crest.
Materials and Methods: Metal-based complete maxillary dentures with a conventionally designed metalframework (group A) and those with a newly designed metal framework (group B) were cast in cobalt-chromium
alloy. Resin-based complete maxillary dentures (group C) were also fabricated as a control. The maximum
fracture strengths were measured using a universal testing machine at a crosshead speed of 5.0 mm/minute. The
differences in the mean values were compared by means of one-way ANOVA and the Student-Newman-Keuls
post-hoc comparison test.
Results: The fracture strength of group A (2.01 kN) was significantly higher than that of group B (1.49 kN)(p<0.05). The fracture strength of group B was significantly higher than that of group C (0.79 kN) (p<0.05).
Conclusion: The fracture strength of the metal-based complete maxillary denture with a newly designed metalframework that did not extend to the residual ridge crest was approximately two times greater than that of theresin-based denture, but its strength was lower than the strength of the conventional denture.
(Int Chin J Dent 2005; 5: 35-38.)
Clinical Significance: The metal-based complete maxillary denture with a newly designed metal framework canbe used as an alternative to a conventional metal-based complete maxillary denture.
Key Words: cobalt-chromium alloy, fracture strength, metal-based complete maxillary denture, metalframework, type of failure.

Introduction
The most commonly used material to make complete dentures in clinical prosthodontic practice is acrylic
resin.1 However, the fracture of acrylic denture bases is occasionally an unavoidable complication because themechanical properties of acrylic resin may not be sufficient to withstand masticatory stress.2-4 Jagger et al.4
reported that, despite the popularity of acrylic at satisfying aesthetic demands, it is still far from ideal in fulfillingthe mechanical requirements of a prosthesis. There is a great risk of fracture, particularly if the thickness of thedenture base is minimal.
One of the ways to overcome this risk is to combine the acrylic denture base with a cast metal base.5 ElGhazali et al.6 proposed using cobalt-chromium bases in maxillary dentures to reduce the functional deformationand thrust to the supporting tissues that occur in the anterior part of the maxilla. The metal-base framework for acomplete maxillary denture mainly covers the palate and residual ridges, with the borders made of acrylic resin.5However, it is difficult to arrange the artificial teeth in this design when there is not enough vertical distance
between the upper residual ridge crest and the antagonist. The purpose of the present study was to evaluate thefracture strengths of a metal-based complete maxillary denture with a newly designed metal framework that did
not extend to the residual ridge crest.

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Materials and Methods
Table 1 lists the materials used in this study. Combined precision impressions of the maxillary edentulous
model with no undercuts or irregularities (G1-402, Nissin Dental Products Inc., Kyoto, Japan) were made withelastomeric silicone materials (Examix Fine injection/putty type, GC Corp., Tokyo, Japan); thirty identicalmaster casts were poured using a high-strength plaster stone (Fujirock, GC Corp.). The stone casts wererandomly assigned to the following groups (n=10 each): group A, metal-based complete maxillary dentures witha conventional metal framework; group B, metal-based complete maxillary dentures with the new metalframework that did not extend to the residual ridge crest; and group C, acrylic resin-based complete maxillarydentures (control group). For groups A and B, precision impressions of each master cast were made withanother silicone elastomeric material (Deguform, Dentsply-Sankin k.k., Tokyo, Japan).

Twenty identical

duplicate and refractory casts were formed using a rapid setting investment material (Rema Exact, Dentaurum
Inc., Newtown, PA, USA) for model casting preparations in the cobalt-chromium model casting technique.
Table 1.

Materials used.

Material/Trade name

Manufacturer

Batch number

Cobalt-chromium alloy
Remanium GM380

Dentaurum Inc., Newtown, PA, USA

9930

Denture base resin
Vertex Rapid Simplified Vertex-Dental B.V., Oldenbarneveltlaan, Netherlands

Powder YX113P01
Liquid YW144L01

Artificial anterior resin teeth
Gloria new ace
Yamahachi Dental Mfg., Co., Gamagori, Japan311109

Artificial posterior resin teeth
B.S.A. resin teeth
B.S.A. Sakurai, Co., Nagoya, Japan

LG010

The metal framework was constructed either with the new design (Fig. 1) or with a conventional design. The
metal framework, which was very thin (0.3–0.4 mm), had no tissue stop and did not extend to the residual ridge
crest. The retentive area of the framework had two metal plates (with a mucosal surface and a polished surface)to connect to the denture base resin (Fig. 2). Five wax rods (0.8 mm diameter) were placed vertically between
the two plates at almost equal intervals horizontally in order to reinforce the two metal framework plates and toensure the retention of the denture base resin. The denture base resin was sandwiched between the two plates,
whereas in the conventional denture, the metal framework was sandwiched by the denture base resin. The
conventional metal framework in this study had retention webbing that completely covered the residual ridge
crest (Fig. 3) and had four tissue stops touching the alveolar tissue on the master cast to maintain its position
during packing procedure (Fig. 4).
The wax patterns of both the new and the conventional metal frameworks of the metal-based complete
maxillary denture were fabricated on a duplicate cast, invested with the same rapid setting investment material
and burned out in the furnace according to the manufacturer’s instructions. The metal framework was then castfrom cobalt-chromium alloy (Remanium GM380; 64.6% Co, 29.0% Cr, 4.5% Mo, and 1.9% others; mass%)
using an argon-arc melting/pressurize casting machine (Cosmos CA-100S, Casting Okamoto Inc., Imadu, Shiga,Japan).

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All the cast metal frameworks were airborne-particle abraded with 50 µm grain-sized aluminum oxide
particles (Aluminous Powder WA 360, Pana Heraeus Dental Inc., Osaka, Japan) using a grit blaster (MicroBlaster MB102, Comco Inc., Burbank, CA, USA) to eliminate the investment material and to clean the castsurfaces; then electrochemical polishing was performed.

1234Fig. 1. New metal framework.
Fig. 2. Peripheral retention structure of new
metal framework with two plates andvertical rods to retain denture baseresin.
Fig. 3. Conventional metal framework.
Fig. 4. Mucosal view of complete maxillarydenture using conventional metalframework with four tissue stops.
Fig. 5. Fracture strength test using
compression load assembly.

5Thermoplastic sheets 4.0 mm in thickness (Erkodur, Erkodent Erick Kopp GmbH, Pfalzgrafenweiler,Germany) were used to form thermoplastic denture bases on a standard master cast by vacuum forming(Erkopress 2002, Erkodent Erick Kopp GmbH). The thickness of the thermoplastic denture bases was adjusted
to 2.5 mm by careful trimming and adding wax when necessary; they were then measured with a caliper toensure uniformity. After arrangement of the artificial teeth on the same plane using a jig custom made for thisstudy, the metal frameworks and base plates (groups A and B) or base plate only (group C) were invested withdental stone in denture flasks. After the thermoplastic sheets were discarded and the wax was eliminated, aheat-polymerized poly(methyl methacrylate) resin was used for the denture base material. The acrylic resin

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Int Chin J Dent 2005; 5: 35-38.

polymer and monomer were mixed according to the manufacturer's instructions. Conventional packing andpolymerizing procedures were performed. The dentures were processed for 15 minutes in water held at aconstant temperature of 100°C and deflasked.
All dentures were stored in 37°C distilled water for 50 hours before testing. The maximum fracture strengths
were measured using a universal testing machine (Autograph AGS-J, Shimadzu Corp., Kyoto, Japan) at a
crosshead speed of 5.0 mm/minute. A compressive load was applied to each complete maxillary denture throughan 11.5 mm ball attachment (Fig. 5). The application of the downward load along the midline of the tissue
surfaces of the denture was designed to be equivalent to the upward load on both sides combined with unyielding
support in the center of the plate, as described by Polyzois et al.7 All testing was performed under uniform
atmospheric conditions of 23˚C and 50% relative humidity.
The mean fracture strengths of each group were statistically analyzed using a one-way analysis of variance(ANOVA) with the group as the variable. The Newman-Keuls post-hoc comparison test was applied whenappropriate (95% confidence level). Types of fracture were observed in groups A and B. Types of fracture werecategorized as fracture or deformation of the metal framework, fracture at the metal framework outline, or
fracture of the denture base resin.

Results
The one-way ANOVA and Newman-Keuls post-hoc comparison test demonstrated that there were significant
differences in the fracture strengths among the three groups (p<0.05). The fracture strength of group A wassignificantly higher than that of group B (p<0.05), and the fracture strength of group B was significantly higher
than that of group C (p<0.05). The mean and standard deviations of the fracture strengths for each group withstatistical categories are summarized in Table 2.

The types of specimen failure after fracture testing arepresented in Table 3.
Table 2.

Fracture strengths for each group.

Group

Fracture strength (kN)
MeanSDGroup AGroup B
Group C

Conventional metal-based denture
New metal-based denture
Resin-based denture

2.01
1.49
0.79

Newman-Keuls grouping

0.33
0.24
0.20

abcSD: standard deviation. Identical letters indicate that the values are not statistically different (p>0.05).

Table 3.

Type of fracture.

Type of fracture

Conventional metal framework

Fracture or deformation of metal framework
Fracture at metal framework outline
Fracture of base resin

90%
0%
10%

New metal framework
60%
40%
20%

Discussion
The main purpose of the present study was to evaluate the fracture strengths of a metal-based complete

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Int Chin J Dent 2005; 5: 35-38.

maxillary denture with a newly designed metal framework that did not extend to the residual ridge crest. The
new metal framework does not disturb the arrangement of artificial teeth when the vertical distance between theupper residual ridge crest and the antagonist is insufficient. An added advantage of the proposed design is that atissue stop,8,9 to be positioned at a predetermined, controlled position within the prosthesis on the master cast, isunnecessary.

Consequently, it is easy to adjust the mucosal surface of the denture base since the metalframework is not exposed in the denture base resin area.
The results of the fracture tests revealed that the fracture strengths of both groups A and B were significantly
higher than of group C (p<0.05). The fracture strength of B was approximately two times greater than that ofgroup C. This finding suggests that the new cobalt-chromium alloy metal framework may be acceptable, as wellas the conventional design, for fabricating metal-based complete maxillary dentures. The fracture strengths ofgroup B were sufficient compared with the maximal bite forces of the complete denture wearers.10,11 However,repetitive loads may finally lead to denture fracture even if the yield strength or the proportional limit are within
the acceptable range for the denture.
Fracture or deformation of the metal framework without fracture of the base resin (90% of group A and 60%of group B) indicated that the fracture strength of the framework-denture base resin connection was higher thanthe mean value shown in Table 2. On the other hand, fracture at the metal framework outline without fracture of
the base resin (40% of group B) indicated that the stress concentration against the compressive load in the centerof the plate tended to occur along this line.
Recently, the use of metal adhesive conditioners improved the bond strengths of a denture base resin to castcobalt-chromium alloy.12,13 It is hypothesized that good bonding of the metal framework to the denture baseresin should improve the fracture strength of the metal-based denture. Therefore, further in vitro studies andclinical research are necessary to evaluate the fracture strength of the bonding system.

Conclusion
This in vitro investigation evaluated the design of the metal frameworks of complete maxillary dentures bymeasuring the maximum fracture strengths of the dentures. Within the limitations of the present study, thefollowing conclusions were drawn: The strength of the metal-based complete maxillary denture with a newly
designed metal framework that did not extend to the residual ridge crest was approximately two times greaterthan that of the resin-based denture and was lower than the strength of the conventional denture.
Acknowledgment
The authors would like to thank Mrs. Jeanne Santa Cruz for editing the manuscript.

References
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Reprint request to:
Dr. Hiroshi Shimizu
Division of Removable Prosthodontics, Department of Oral Rehabilitation, Fukuoka Dental College
2-15-1, Tamura, Sawaraku, Fukuoka 814-0193, Japan
FAX: +81-92-801-0513 E-mail: simizuh1@college.fdcnet.ac.jp
Received May 2, 2005. Revised May 10, 2005. Accepted May 29, 2005.
Copyright ©2005 by the Editorial Council of the International Chinese Journal of Dentistry.

International Chinese Journal of Dentistry 2005 Outstanding Article AwardInternational Chinese Journal of Dentistry announces the fourth Outstanding Article Award.
Award:A cash prize of US$ 500 and a certificate of commendation are awarded for the most outstanding article published in theInternational Chinese Journal of Dentistry.
Criteria:
Original articles, clinical reports, and dental technology articles published in the Journal between issue 1 and issue 4 areconsidered for the award. The winning article will be selected by a committee on the basis of scientific impact on the dental
professional communities, and interest to the readers. The 2005 Award is sponsored by the International Chinese Journal ofDentistry and the following award sponsors.
Award Sponsors:
GC Corporation, Tokyo, Japan, http://www.gcdental.co.jp
Kuraray Medical Inc., Tokyo, Japan, http://www.kuraray.co.jp/dental
Shofu Inc., Kyoto, Japan, http://www.shofu.co.jp
Sun Medical Co., Ltd., Moriyama, Japan, http://www.sunmedical.co.jp
Toho Dental Products, Saitama, Japan
Tokuyama Dental Corp., Tokyo, Japan, http://www.tokuyama-dental.co.jp
3M Health Care Limited, Sagamihara, Japan, http://www.3m.com/espe/
2004 Award winner
Hiroshi Shimizu, Kouichi Watanabe, Toru Okabe, and Yutaka Takahashi
Mold filling of titanium castings using a 45° marginal edge angle
Int Chin J Dent 2004; 4(3): 67-71.

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