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ORIGINAL ARTICLE Table of Contents   
Year : 2006  |  Volume : 9  |  Issue : 1  |  Page : 43-47
Fracture resistance of molars with bonded class II Amalgam restorations - an in- vitro study

Dept of Conservative Dentistry & Endodontics A.B Shetty Memorial Institute of Dental Sciences, Derlakatte, Mangalore, India

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The aim of this study was to determine whether amalgam bonded to tooth structure with sixth generation adhesives can increase the fracture resistance of restored teeth. Fifty- two extracted mandibular molars were randomly divided into six groups, which included two control groups of six teeth each and four experimental groups of ten teeth each.
Group I consisted of six intact teeth which served as negative control. Standardized class 2 (mesio­occlusal / disto-occlusal) preparations were made in groups 2-6. The preparations were lined with: group 3. varnish; group 4, Excite DSC; group 5, AdheSE; group 6, Clearfil liner Bond 2V and then restored with high copper amalgam. Group 2 preparations were not restored and served as positive control. The bonded amalgam technique using sixth generation adhesives also increase the fracture resistance of mandibular molars and could be used as time saving alternatives to the conventional multistep adhesive systems

Keywords: Bonded amalgam restoration, class 2 restorations, resistance form

How to cite this article:
Mathew BP, Hegde MN, Hegde P. Fracture resistance of molars with bonded class II Amalgam restorations - an in- vitro study. J Conserv Dent 2006;9:43-7

How to cite this URL:
Mathew BP, Hegde MN, Hegde P. Fracture resistance of molars with bonded class II Amalgam restorations - an in- vitro study. J Conserv Dent [serial online] 2006 [cited 2021 May 7];9:43-7. Available from:

   Introduction Top

Studies [1],[2],[3] have shown that teeth with cavity preparations become weaker as the occlusal isthmus is widened, and they fracture more easily than intact teeth. Among direct restorative materials, silver amalgam is still frequently chosen for posterior restorations since it resists masticatory stress and can achieve adequate proximal contacts in proximo occlusal cavities. However lack of adhesiveness to dental structures requires cavity design with mechanical retention at the expense of healthy tooth structure, which increases fracture susceptibility.

With the development of adhesive systems, bonding of dental amalgam to tooth structure became possible. This technique reduces the need for mechanical retention and allows more conservative cavity preparations. Several studies [4],[5] using bonding system have suggested that the use of an adhesive resin liner beneath metal restorations might increase the fracture strength of the restored tooth.

Hence the objective of this study was to investigate the influence of fifth and sixth generation dentin bonding agents on the fracture strength of mandibular molars restored with mesio-occlusal\disto-occlusal silver amalgam restorations and to determine whether self etching primers are suitable alternatives to the conventional multistep fifth generation bonding systems.

   Materials and Methods Top

A comparative in vitro study to evaluate the fracture resistance of mandibular molars with class It preparations restored with bonded amalgam restorations using different adhesive systems was undertaken at the Department of Conservative Dentistry and Endodontics, A. B. Shetty Memorial Institute of Dental Sciences in collaboration with 3M India Ltd., Bangalore, Karnataka.

Fifty two freshly extracted, intact, non-carious and unrestored human mandibular molars collected from the Department of Oral and Maxillofacial Surgery, A. B. Shetty Memorial Institute of Dental Sciences, Mangalore were stored , disinfected and handled as per the recommendations and guidelines laid down by Occupational Safety and Healthy Administration (OSHA) and Centre for Disease Control and Prevention (CDC). All the collected teeth were cleared of blood, calculus and surface deposits and stored in isotonic saline. The selected specimens were utilized for the study within one month of extraction and storage.

   Preparation and Grouping of the Specimen Top

The specimen teeth were embedded 2mm apical to the cemento enamel junction in autopolymerising acrylic resin held in a hollow cylindrical metallic mold of height, 1 inch and diameter 0.75 inch.

Except for six intact molars which comprised one of the control groups, standardized class II cavity preparations (MO/DO) were prepared in all the teeth using a No. 245 tungsten carbide bur with a high speed airotor hand piece under air/water spray. The size of the preparation was made proportional to the dimensions of the tooth to minimize variations resulting from tooth size. Standard preparation widths were to the following specifications:

  • The occlusal isthmus had a facial-lingual width of 1/3 of the distance between cusp tips.
  • The occlusal box floor depth was 2\3 of the mean height of the cusps above the cementoenamel junction
  • The facial-lingual width of the proximal boxes was l\3 of the maximum facial-lingual width of the tooth.
  • The floor of the proximal box was established 1 mm above the cementoenamel junction.
  • The proximal box depth (mesial-distal) was 1.0mm.
  • All internal line angles were rounded, and no additional mechanical retention was cut
[Figure 1]

The specimens were randomly divided into groups with specimens in each group and labeled with colored adhesive tape for identification. Two groups were coded as the control groups and the other four as experimental groups.

   Groups Top

Control Groups

Group 1:
Intact mandibular molars

Group 2 : Prepared teeth without amalgam

Experimental groups

Group 3:
Prepared specimens coated with cavity varnish, prior to commencement of condensation of amalgam.

Group 4: Prepared specimens, coated with single bottle total etch adhesive, Excite DSC according to manufacturer's instructions, prior to restoration with amalgam

Group 5: Prepared specimens, coated with self etching primer Adhese according to manufacturer's instructions, prior to restoration with amalgam.

Group 6: Prepared specimens, coated with self etching primer Clearfil liner Bond 2V according to manufacturer's instructions, prior to restoration with amalgam.

   Results Top

The compressive loads required to fracture the specimens of the control groups (group 1 and group 2) and the experimental group (group 3 to group 6) obtained in this study were subjected to Kruskal­Wallis test, for group comparison, which revealed significant difference among the groups at p<0.05 (p=0.031)

According to the results analyzed group I (intact Mandibular molars) required the highest mean compressive load to fracture at 102.7017 Kgf and group 2 (prepared molars not restored) required the lowest mean compressive load to fracture at 46.8483Kgf intergroup comparison among the groups was done using the Mann -Whitney 'U' test.

Results showed significance among the following groups:

  • group 1(intact molars) and group3 (varnish +amalgam)
  • group 1(intact molars) and group 4 (Excite +amalgam)
  • group 2(unrestored molars) and group 4 (Excite +amalgam)
  • group 2(unrestored molars) and group 5 (Adhese +amalgam)
  • group 2(unrestored molars) and group 6 (Clearfil Liner Bond 2V +amalgam)
Results were highly significant. in the comparison of the following groups

  • group 1 (intact molars) and group 2 (unrestored molars)
  • Results of the intercomparison of the other groups were not significant
[Table 1],[Table 2],[Table 3],[Figure 1],[Figure 2]

   Discussion Top

As a restorative material, amalgam strengthens teeth only by distributing the stresses of mastication over a broad occlusal surface. Hence in large class II preparations wide isthmuses or sharp internal line angles weaken the cusps of posterior teeth, and it is not uncommon for patients to experience cuspal fracture adjacent to mesio-occlusal-distal amalgam restorations. Clinical studies have shown that tooth fractures are the main reason for replacing 11-13% of amalgam restorations.

Amalgam bonding was first suggested in 1986 by Varga [4] in the U.S. and Shimizu in Japan, in an effort to reduce the incidence of cuspal fracture, to improve marginal seal and to encourage the use of a more conservative design.

In the present study, a varnish lined group and representatives from the fifth and sixth generation bonding systems were utilized in comparison to the intact and unrestored control groups. Selected mandibular first molars with standardized mesio­occlusal or disto-occlusal preparations were utilized. The results of the present study are in agreement with the results of other studies [5],[6] in that the bonding of amalgam restorations significantly increased the strength of the teeth. The three experimental groups utilizing adhesive systems resulted in statistically significant strength values when compared to the unrestored group. However comparatively higher fracture resistance values were obtained with group 6 (Clearfil liner Bond 2V group) than with either group 5 (Adhese group) or group4 (Excite group). As noted before [7],[8] , group 3 (varnish control group) did not exhibit a significant difference as compared to group 2 (unrestored group).

One of the reasons among others, for bonded amalgam restorations not being very popular with clinicians is the increase of clinical time. With the newer 6 th generation bonding systems, the clinical steps are substantially reduced. The mean fracture strengths of the Group 5 (AdheSE group) and Group 6 (Clearfil Liner Bond 2 V group) showed no statistical differences when compared to the Group 1 (intact teeth group). This could indicate that sixth generation bonding agents may improve the strength of amalgam restored teeth and may be used as alternatives to the routine multistep bonding systems

Although the bonded amalgam technique looks very promising, the use of natural teeth for the study, the type of force used to load the specimens (which is not typical of the type of loading that occurs clinically) and the questionable longevity of bonded amalgam restorations are some of the limitations of laboratory studies that must be recognized.

Thus these studies and recent clinical applications have, in the short-term, demonstrated that amalgam can be bonded to tooth structure and can thereby produce cuspal reinforcement.

   Conclusion Top

The present in-vitro investigation was conducted to evaluate comparative fracture resistance of intact teeth, prepared but unrestored teeth, teeth restored with amalgam and varnish, Excite, Adhese and Clearfil liner Bond 2V.

Within the limitations of the study the following conclusions could be drawn

  1. All teeth restored with bonded amalgam restorations significantly improved the fracture resistance.
  2. Teeth restored with bonded amalgam restorations using Clearfil liner Bond fractured at a higher mean compressive load but was not significantly different from the other experimental groups

Thus using sixth generation adhesives for the bonded amalgam technique can also increase the fracture resistance of molars and could be used as time saving alternatives to the conventional multistep adhesive systems.[9]

   References Top

1.M. H. El-Sherif, M. N. Halhoul, A. A. Kamar, A. Nour El- Din. Fracture strength of premolars with class 2 silver amalgam restorations Operative Dentistry 1988; 13: 50-53.  Back to cited text no. 1    
2.Jose Mondelli, Lincoln Steagall, Aquira Ishikiriama, Maria Fidela De Lima Navarro, Francisco Bruno Soares. Fracture strength of human teeth with cavity preparations. Journal of Prosthetic Dentistry 1980;43(4):419-422.  Back to cited text no. 2    
3.Mithra B., Sureshchandra B. fracture resisitance of intact and endodontically prepared human mandibular molars restored with three different combinations of restorative materials- an in vitro study. Endodontology 1993;5;15-211  Back to cited text no. 3    
4.Denhey GE, Torney DL. Internal reinforcement through micromechanical bonding. Journal of Prosthetic Dentistry 1976;36;171-175  Back to cited text no. 4    
5.Varga J, Matsmara H, Masuhara E. Bonding of amalgam filling to tooth cavity with adhesive resin. Dental materials 1986;5;225-232  Back to cited text no. 5    
6.J.P. Oliviera, M.A. Cochran, B.K. Moore. Influence of bonded amalgam restorations on the fracture strength of teeth. Operative Dentistry 1996;21:110-115  Back to cited text no. 6    
7.G. M. Diaz De Souza, G. D. S. Pereira, C. T. S. Dias, L. A. M. S. Paulillo. Fracture resistance of premolars with bonded class II amalgams. Operative Dentistry 2002; 27: 349-353.  Back to cited text no. 7    
8.R.Pilo, T.Brosh, H.Chweidan. Cusp reinforcement by bonding of amalgam restorations. Journal of Dentistry 1998; 26:467-472.  Back to cited text no. 8    
9.R. S. Chen, C. C. Liu, M. R. Cheng, C. P. Lin. Bonded amalgam restorations: using a glass­ionomer as an adhesive liner. Operative Dentistry 2000;25:411-417.  Back to cited text no. 9    

Correspondence Address:
Beena Philip Mathew
Dept of Conservative Dentistry & Endodontics A.B Shetty Memorial Institute of Dental Sciences, Derlakatte, Mangalore
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0972-0707.41308

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  [Figure 1], [Figure 2]

  [Table 1], [Table 2], [Table 3]


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