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Journal of Orthodontics, Vol. 31, No. 4, 323-328, December 2004 doi:10.1179/146531204225020697
© 2004 British Orthodontic Society
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Article

An ex vivo evaluation of resin-modified glass polyalkenoates and polyacid-modified composite resins as orthodontic band cements

J. Knox

University of Wales College of Medicine, UK

K. Y. Chye

Co. Westmeath, Eire

P. Durning

Cardiff & Vale NHS trust, UK

Address for correspondence: Jeremy Knox, Department of Dental Health and Development, University Of Wales College Of Medicine, Dental School, Heath Park, Cardiff CF4 4XY, Wales, UK. Email: Knoxj{at}cardiff.ac.uk

Received November 27, 2003; accepted April 28, 2004


   Abstract
Top
 Abstract
Introduction
 Materials and method
 Results
 Discussion
 Conclusions
 Contributors
 References
 
Objectives: The objective of this ex vivo study was to assess the use of resin-modified glass polyalkenoates and polyacid-modified composite resins, as orthodontic band cements.

Materials and method: Plain stainless steel bands were cemented to 350 human extracted third molar teeth using 1 of 7 different cements. Following complete cement cure, half of each sample group was exposed to mechanical stress in a ball mill. Stressed and unstressed samples were tested in tension and the stress at which initial cement failure recorded. The mode of failure was recorded using an adhesive remnant evaluation.

Results: The mean band retention stresses offered by the cements studied ranged from 0.96 to 1.56 MPa. Fuji OrthoTM provided the highest mean band retention stress in ‘stressed’ (1.56 MPa) and ‘unstressed’ (1.45 MPa) states. Exposure to mechanical stress did not appear to significantly influence band retention or mode of cement failure for most cements. Fuji OrthoTM cement recorded the highest Weibull modulus for all cements tested. Virtually all samples failed at either the cement/enamel or cement band interface.

Conclusions: Significant differences in band displacement stress values and mode of failure were demonstrated between the cements studied. However, generic comparisons were difficult to make.

Key words: Bands, bond strength, cements, orthodontics


   Introduction
Top
 Abstract
 Introduction
Materials and method
 Results
 Discussion
 Conclusions
 Contributors
 References
 
Orthodontic bands are still routinely employed in contemporary orthodontics, particularly in areas of high occlusal stress. The materials of choice for cementation are commonly the glass polyalkenoate cements, as these materials adhere to stainless steel and enamel, and also act as a fluoride reservoir helping to prevent decalcification. Conventional glass polyalkenoate cements have been demonstrated to offer superior retention for stainless steel molar bands in laboratory and clinical trials.1–Go6Go However, there has been some confusion in the terminology associated with the more recently developed glass polyalkenoate materials7Go and it is often unclear whether these materials offer the same band retention characteristics as the original glass polyalkenoate material.

McLean8Go has defined a conventional glass polyalkenoate (glass ionomer) cement as ‘a cement that consists of a basic glass and an acidic polymer, which sets by an acid-base reaction between these components’. A resin-modified glass polyalkenoate cement is defined as ‘a hybrid material that retains a significant acid-base reaction as part of their overall reaction process’. These cements set partly by an acid-base reaction and partly by a photochemical polymerization of the resin part of the cement. These cements are never in paste form, but in powder-liquid form as water is required for the acid-base setting reaction. According to McLean8Go these cements set at a slower rate in the dark, or in the absence of visible light, than conventional glass-polyalkenoate. Finally, polyacid-modified composite resins ‘may contain either or both of the essential components of glass-polyalkenoate cement but at levels insufficient to promote the acid-base cure reaction in the dark i.e. in the absence of visible light’. These cements contain the ingredients of a glass-polyalkenoate (acid decomposable glass and perhaps some polyacid), but in insufficient amounts to promote dark setting (acid-base reaction). The acid-base reaction will only occur once water is absorbed into the set material.

The range of glass polyalkenoate materials has been explained by Burgess9Go as a continuum with conventional glass polyalkenoate cements at one end and fluoride-releasing resins (polyacid-modified composite resins) at the other end of the continuum.

It was the objective of this study to evaluate 2 resin-modified glass polyalkenoates and 3 polyacid-modified composite resins, as orthodontic band cements with reference to a conventional glass ionomer cement and a zinc phosphate cement.

Null hypotheses


   Materials and method
Top
 Abstract
 Introduction
 Materials and method
Results
 Discussion
 Conclusions
 Contributors
 References
 
Three-hundred-and-fifty extracted human third molar teeth were stored in 70% alcohol, according to local cross infection policies. The teeth were randomly divided into 7 groups of 50 teeth. Each tooth was cleaned with a pumice slurry, washed and dried. Plain molar bands were chosen for each tooth and adapted. A strip of stainless steel band (110 x 3.5 mm) was spot welded onto each band on its lingual and buccal aspect to form a loop. These loops aided the attachment of the specimen onto a specially designed jig (Figure 1Go).



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  		Figure 1 Test jig

 
The bands were then readapted and cemented with one of the cements to be evaluated (Table 1Go). All cements were mixed according to the manufacturers’ recommendations at ambient temperature. Once the bands were loaded with cement they were then cemented onto the tooth with a band seater under hand pressure. Excess cement was removed from around the band with cotton wool roll and a Mitchell’s trimmer. The light curable cements were cured with a conventional light-curing machine with the duration of exposure recommended by each manufacturer. The efficiency of the light-curing machine was tested with a Demetron Model 100 curing radiometer. All specimens were then stored in distilled water at 37° C for 7 days.


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  		Table 1 Cements used in the study
 
Seven days after cementation, 25 samples were randomly selected from each group and subjected to mechanical stress in a ball mill machine containing 470 g of ceramic spheres of various sizes and 0.5 l of distilled water at 37° C. The ball mill machine was allowed to run at 250 rpm. Each ‘stressed’ sample was exposed to 1 minute of mechanical stress.

The roots of all specimens (stressed and unstressed) were invested in cold cure acrylic and tested under tensile load in a Lloyds LR-10K. A specially designed jig (Figure 1Go), incorporating universal joints, was used to reduce lateral and torsional stresses during testing.

The cross-head speed of the Lloyds LR-10K machine was set at 1 mm/minute. All of the specimens were tested until total dislodgement of the band from the crown of the tooth had occurred. A load-extension graph was printed for each specimen (Figure 2Go). The load, T1, at which point the curve initially deviated from linearity was noted as the initial point where the cement failed.



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  		Figure 2 Load/extension graph (as printed for each specimen) (T1 point of initial failure, T2 point of complete failure)

 
After testing, the band fitting surface and the enamel surface of all the specimens were assessed to determine the band surface area and the site of cement failure. The failure sites were graded as: 0=primarily cohesive failure within the cement, i.e. equal volumes of cement on enamel and stainless steel surfaces; 1=the majority failure is at the cement-metal junction; 2=the majority failure is at the cement-enamel junction.


   Results
Top
 Abstract
 Introduction
 Materials and method
 Results
Discussion
 Conclusions
 Contributors
 References
 
Band retention
The mean band displacement stress values for all unstressed and stressed cement samples are presented in Figure 3Go and Table 2Go. Several specimens failed for reasons that were judged to be sporadic and do not imply any deficiency in the performance of the cement.



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  		Figure 3 Band displacement stresses for stressed and unstressed cements

 

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  		Table 2 Mean band displacement stress values (MPa)
 
A one-way analysis of variance with Tukey post hoc tests was used to compare the mean band displacement values for unstressed and stressed samples (Tables 3Go and 4Go).


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  		Table 3 Comparison of unstressed cements (p values)
 

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  		Table 4 Comparison of stressed cements (p values)
 
Exposure to mechanical stress did not appear to significantly influence band retention for all cements other than Ultra BandlokTM and 3M Unitek MulticureTM cements (p≤0.04). Independent t-tests demonstrated that Ultra BandlokTM offered a significantly stronger band retention in the unstressed state, whereas 3M Unitek MulticureTM recorded a higher bond strength following exposure to mechanical stress.

The Weibull probability of failure for band displacement was plotted for both the unstressed and stressed specimens. The Weibull analysis allowed the fracture probability to be calculated or predicted as a function of applied band displacement stress.10,Go11Go The Weibull modulus illustrates the dependability of the cement (Table 5Go).


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  		Table 5 Weibull modulus of unstressed and stressed specimens
 
Cement failure
Figure 4Go demonstrates that virtually all samples failed at either the cement/enamel or cement/band interface. Very few samples in this study failed cohesively. A test for unpaired difference12Go demonstrated that the application of mechanical stress before testing did not influence the mode of failure.



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  		Figure 4 Mode of failure for all cements (‘metal’=cement/band interface; ‘enamel’=cement/enamel interface)

 
A Chi-squared test demonstrated a significant difference in the failure mode of the different cements (Table 6Go). Ketac CemTM, 3M Unitek MulticureTM and Fuji OrthoTM cements all tended to fail at the band/cement interface, whereas Band LokTM, Diamond Bond VLCTM and Ormco GoldTM tended to fail at the cement/enamel interface. Ultra Band-LokTM failed predominantly at the band/cement interface, but a significant number of samples (38%) failed at the cement/enamel interface.


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  		Table 6 Chi-square test for the failure mode of different cements
 

   Discussion
Top
 Abstract
 Introduction
 Materials and method
 Results
 Discussion
Conclusions
 Contributors
 References
 
It was the objective of this study to evaluate 2 resin-modified glass polyalkenoates and 3 polyacid-modified composite resins, as orthodontic band cements with reference to a conventional glass ionomer cement and a zinc phosphate cement. Band retention is affected by cement mechanical properties,13Go adhesion at the cement/enamel and cement/band interfaces14–Go16Go and, possibly, the influence of repeated mechanical stress on the cement adhesion and cohesion.17Go

The mean band retention stresses recorded in this study compare well with those published by Millett18Go who evaluated the mean band retentive strength of a modified composite, a resin-modified glass ionomer cement and a conventionally cured glass ionomer cement. Ketac CemTM and Fuji OrthoTM were common to both studies and Millett18Go recorded a slightly higher mean retentive strength for both cements. However, Millett18Go used micro-etched bands, which have been demonstrated to offer increased bond strength,19,Go20Go and maximum bond strength was recorded, rather than the initial point of cement failure.

Significant differences were demonstrated between the strength of attachment offered by the materials included in this study but generic comparisons were not possible. Peutzfeldt21Go and Meyer22Go have reported an improved tensile and compressive strength of the newer glass polyalkenoate cements, resin-modified glass polyalkenoate cements and polyacid-modified composite resins. However, these physical properties only influence the cohesive strength of the cement, and not the strength of the interface of the cement with enamel and the orthodontic band, which may be more important.

For the materials tested in this study, virtually all samples failed at either the cement/enamel or cement/band interface. The zinc phosphate and polyacid modified composite materials tended to fail at the cement/enamel interface whereas the conventional and resin-modified polyalkenoate materials failed largely at the band/cement interface. These results do not concur with Millett18Go who reported failure predominantly at the cement enamel interface for Fuji OrthoTM. However, Millett’s study18Go evaluated micro-etched bands.

Exposure to mechanical stress did not appear to significantly influence the strength of band retention or mode of cement failure for most cements in this study. However, the strength of attachment offered by a particular cement in a laboratory study is probably less important to the orthodontic clinician than the clinical reliability of the attachment.18Go Fuji OrthoTM provided both the highest band retention stresses and highest Weibull modulus indicating that this material would provide greater bond reliability than the other cements studied. Interestingly, the Weibull moduli recorded for Ketac CemTM and Fuji OrthoTM in this study were higher than those recorded by Millett18Go for the same materials.

This study clearly suggested that the use of Fuji OrthoTM, rather than Ketac CemTM may offer a significantly more robust method of orthodontic band cementation. Fuji OrthoTM bonds well to tooth tissue and the quality of attachment offered is not significantly influenced by exposure to mechanical stress. However, other polyacid modified glass polyalkenoate materials studied did not perform as well, and it is clear that individual materials and products need to be evaluated rather than making generic assumptions.


   Conclusions
Top
 Abstract
 Introduction
 Materials and method
 Results
 Discussion
 Conclusions
Contributors
 References
 


   Contributors
Top
 Abstract
 Introduction
 Materials and method
 Results
 Discussion
 Conclusions
 Contributors
References
 
J. Knox and P. Durning were responsible for the concept and design of the study. K. Y. Chye was responsible for the acquisition of the data. J. Knox, P. Durning and K. Y. Chye were responsible for the analysis and interpretation of the data. J. Knox has drafted the article and all authors have approved the final version of the manuscript. J. Knox is the guarantor.


   References
Top
 Abstract
 Introduction
 Materials and method
 Results
 Discussion
 Conclusions
 Contributors
 References
 
1 Fricker JP, McLachlan MD. Clinical studies on glass ionomer cements. Part 2. A two year clinical study comparing glass ionomer cement with zinc phosphate cement. Aus Orthod J 1987; 10: 12–14.

2 Norris DS, McInnes-Ledoux P, Schwaninger B. Weinberg R. Retention of orthodontic bands with new fluoride-releasing cements. Am J Orthod 1986; 89: 206–11.[Medline]

3 Mizrahi, E. Glass ionomer cements in orthodontics—an update, Am J Orthod Dentofac Orthop 1988; 93: 505–507.[Medline]

4 Durning P. Clinical and laboratory studies of cements used to retain orthodontic bands. MDS Thesis, University of Newcastle upon Tyne, 1989.

5 Stirrups DR. A comparative clinical trial of a glass ionomer and a zinc phosphate cement for securing orthodontic bands. Br J Orthod 1991; 18, 15–20.[Abstract]

6 Durning P, McCabe JF. Gordon PH. A Laboratory Investigation into cements used to retain orthodontic bands. Br J Orthod 1994; 21: 27–32.[Abstract]

7 Sidhu SK, Watson TF. Fluid permeability and other interfacial characteristics of light-cured glass ionomer restorations. J Dent Res 1994; 73: 183 (Abstract 651).

8 McLean JW, Nicholson JW, Wilson AD. Proposed nomenculture for glass-ionomer dental cements and related materials. Quint Int 1994; 25(9): 587–589.

9 Burgess J, Norling B, Summit J. Resin ionomer restorative materials: the new generation. J Esthet Dent 1994; 6(5): 207–215.[Medline]

10 Weibull W. A statistical distribution function of wide applicability. J Appl Mech 1951; 18: 293–7.

11 McCabe JF, Carrick TE. A statistical approach to the mechanical testing of dental materials. Dent Mater 1986; 2: 139–42.[CrossRef][Medline]

12 Newcombe RG. Unpaired difference. Stat Med 1998; 17: 873–90, Method 10.[CrossRef][Medline]

13 Williams JD, Swartz ML, Phillips RW. Retention of orthodontic bands as influenced by the cementing media. Angle Orthod 1965; 35: 278–85.[Medline]

14 Bills RC, Yates JL, McKnight JP. Retention of stainless steel bands cemented with four dental cements. J Pedod 1980; 4: 273–86.[Medline]

15 Brown D. Orthodontic materials update—orthodontic band cements. Br J Orthod 1989; 16: 127–31.[Medline]

16 Hotz P, McLean JW, Sced I, Wilson AD. The bonding of glass ionomer cements to metal and tooth substrate. Br Dent J 1977; 142: 41–7.[CrossRef][Medline]

17 Abu Kasim NH, Millett DT, McCabe JF, The ball mill as a means of investigating the mechanical failure of dental materials. J Dent 1996; 24: 117–24.[CrossRef][Medline]

18 Millett DT, Duff S, Morrison L, Cummings A, Gilmour WH. In vitro comparison of band cements. Am J Orthod Dentofac Orthop 2003; 123: 15–20.[Medline]

19 Mennemeyer VA, Neuman P, Powers JM. Bonding of hybrid ionomers and resin cements to modified band materials. Am J Orthod Dentofac Orthop 1999; 115: 143–7.[Medline]

20 Aggarwal M, Foley TF, Rix D. A comparison of sheer peel band strengths of 5 orthodontic cements. Angle Orthod 2000; 70: 308–16.[Medline]

21 Peutzfeldt A. Compomers and glass ionomers: bond strength to dentin and mechanical properties. Am J Dent 1996; 9(6): 259–63.[Medline]

22 Meyer JM, Cattani-Lorente MA, Dupuis V. Compomers: between glass-ionomer cements and composites. Biomaterials 1998: 19: 529–39.[CrossRef][Medline]





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