In vitro comparison of orthodontic band cements☆☆☆★★★♢♢♢♦

Material and methods 

To assess retentive strength, we collected 90 extracted human third molars and stored them for 4 months in distilled water after decontamination in 0.5% chloramine. The teeth were randomly divided into 3 groups of 30 teeth, with each group comprising 15 maxillary and 15 mandibular third molars. The apical third of each tooth was notched with a diamond bur, and the tooth was mounted to below the amelocementum junction in a block of self-curing acrylic resin, with its long axis vertical. The acrylic blocks were color-coded, with a different color for each group of teeth.

The teeth were then cleaned with a pumice slurry, washed in distilled water, and dried in a stream of air. Because bands do not exist for third molars, a stainless steel first permanent molar band (3M Unitek) was selected and adapted for best fit to the crown of each tooth. Bands with microetched fitting surfaces were used. Thirty bands were cemented with each band cement; 3 cements were assessed. Transbond Plus band adhesive (3M Unitek) is a single-component modified composite (compomer) formed by combining a composite resin with glass ionomer particles. Supplied in sealed capsules from which it is applied to the band- fitting surface, it hardens only through photopolymerization. Fuji Ortho LC is a powder/liquid-based resin-modified glass ionomer cement that is marketed in encapsulated form. After trituration for 10 seconds, the capsule is loaded into a customized gun to allow dispensing the cement to the band-fitting surface. Setting is via a tricure reaction comprising an acid-base reaction of the glass ionomer components, a free radical addition polymerization reaction promoted by visible blue light, and self-curing of the resin monomer. Ketac-Cem is a powder/liquid-based cement that sets initially by an acid-base reaction when the components are mixed and later by a cross-linking reaction.

Each cement was prepared according to the manufacturer's instructions. Band selection and band cementation for each specimen were carried out by 1 operator (D.M.) to eliminate the influence of operator variability.

Two bands were cemented with each mix of Fuji Ortho LC or Ketac-Cem, and Transbond Plus was applied directly to the band-fitting surface. Once the bands had been positioned accurately on the tooth surface and pressed firmly into place, excess cement was removed with dry cotton wool rolls.

Transbond Plus and Fuji Ortho LC were light cured with a dental curing light (3M Unitek) for 20 seconds from the occlusal aspect of the band, as directed by the manufacturers. Bands cemented with Ketac-Cem were allowed to bench cure for 5 minutes. All specimens were then transferred to a humidor set at 37° C for 24 hours; they were then tested with a Nene M3000 testing machine (Nene Instruments Ltd, Wellingborough, United Kingdom) with a crosshead speed of 1 mm/min. Each banded specimen was loaded into the jig by means of a 0.9-mm stainless steel loop that engaged fully under the buccal tube and the lingual cleat of each band. Testing proceeded for each specimen until the band was removed from the tooth. The maximum force recorded during debonding was chosen from the stress-strain curve18 for each specimen and then used to calculate bond strength with band surface area data provided from the manufacturers. Pulling a cemented band off a tooth by means of a vertically oriented force is not pure shear testing. By virtue of the convex shape of the crown and the approximate fit of a band, a vertical load will generate a combination of shear, compressive, and tensile forces. For that reason, the term retentive strength has been used for the bond strength data recorded.

After band removal, the site of failure was assessed at the enamel-cement or cement-band interface. The amount of cement remaining on the tooth surface at deband was assessed visually by 1 operator (D.M.) and coded with a modification of the adhesive remnant index (ARI) of Årtun and Bergland16 as follows: 0, no cement remains on the tooth surface; 1, less than half the crown surface under the band is covered by cement; 2, more than half the crown surface under the band is covered by cement; 3, all of the crown surface under the band is covered by cement.

To assess survival time, another group of 30 extracted human third molars was collected. These teeth were treated and stored in a manner identical to that used for the assessment of bond strength. They were divided into 3 groups, each with 5 maxillary and 5 mandibular third molars. The root surface of each tooth to be banded with each cement was coded with a diamond bur to allow identification later. Ten bands were cemented with each band cement, in accordance with the manufacturer's instructions as described previously, and the specimens were then stored in a humidor at 37°C for 24 hours. The specimens were then subjected to mechanical stress in a ball mill containing 470 g of ceramic spheres and 250 mL of distilled water at 37°C, rotating at 100 revolutions per minute.17 Under these operating conditions, reproducible results have been shown to be forthcoming within a reasonable period of time for this specimen type.13, 17, 18 After each hour of testing, failed specimens (those with loose bands) were removed from the mill. Bands were assessed manually as being loose if movement was detected when upward followed by downward force was applied to the buccal tube by a scaler. After replacement of the distilled water with a fresh sample at 37°C, testing recommenced and continued until all specimens had failed.

The mean retentive strengths for the cements tested were compared with a 1-way analysis of variance (ANOVA) followed by a Tukey multiple comparison procedure. In addition, we used Weibull analysis19 to calculate the probability of failure at given values of applied force. This form of analysis might be more meaningful for evaluating bond strength data because it takes into account the bond strength values at the extremes of the distribution.20, 21 Weibull analysis has been used previously to assess fracture processes in chemically activated and light-activated composites19 and in the evaluation of bonding agents21 and band cements13, 18, 20 for orthodontic use. The probability of failure (Pf) is related to stress (δ) by the Weibull equation as follows: Pf = 1 − exp { − (δ − δu/δo)m} where δu, δo, and m are constants. δu is the lowest level of stress at which Pf approaches 0, and it is usual to assume that δu = 0.19 δo is generally referred to as the normalizing parameter, and m is the Weibull modulus. By generating a Weibull modulus for each band cement, numerical evaluation of its reliability is feasible. A low value of Weibull modulus indicates a wide scatter of bond strength values, whereas a high value of Weibull modulus indicates close grouping of bond strength values and better dependability of the cement.21 A χ2 test was used for comparison of site of failure between cements. For comparison of ARI scores, a chi-squared test followed by a multiple comparisons procedure with the Fisher exact test was used. For these analyses, score categories 0 and 1 as well as score categories 2 and 3 were combined. A log rank test was used to compare survival time distributions for banded specimens in the ball mill experiment.

Results 

The mean retentive strength of each band group is shown in Table I. The mean retentive strength of bands cemented with Transbond Plus (0.415 MPa) was significantly lower than that of bands cemented with either Fuji Ortho LC (1.715 MPa) or Ketac-Cem (1.454 MPa; P < .001). However, there was no significant difference in mean retentive strength values for bands cemented with either Fuji Ortho LC or Ketac-Cem. Weibull data are also shown in Table I and demonstrated graphically in Figure 1.

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Fig. 1. Weibull curves for 30 bands cemented with each cement.

Table I. Retentive strength values for 30 molar bands cemented with Transbond Plus, Fuji Ortho LC, or Ketac-Cem

	Cement	Mean retentive strength (MPa)	SD	Characteristic strength	Weibull modulus	Correlation coefficient
	Transbond Plus	0.415	0.264	0.470	1.391	0.996
	Fuji Ortho LC	1.715	0.438	1.876	3.764	0.993
	Ketac-Cem	1.454	0.627	1.636	2.414	0.996

Specimen failure occurred predominantly at the enamel-cement interface. The distribution of cement remnant scores for each cement is shown in Table II.

Table II. Distribution (number/%) of cement remnant scores for 30 bands cemented with each cement

	ARI score	Transbond Plus	Fuji Ortho LC	Ketac-Cem
	0	3 (10%)	0	7 (24%)
	1	25 (83%)	28 (93%)	22 (73%)
	2	2 (7%)	2 (7%)	1 (3%)
	3	0	0	0

The survival time distributions for bands cemented with each cement are shown in Figure 2.

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Fig. 2. Survival time distributions for 10 bands cemented with each cement.

Discussion 

This laboratory investigation assessed the mean retentive strength of orthodontic bands with microetched fitting surfaces cemented with a modified composite, a resin-modified glass ionomer cement, or a conventional chemically cured glass ionomer cement. Microetched bands were used because this surface treatment has been shown to improve bond strength14, 15 and to reduce clinical failure rate18 compared with untreated bands.

In this study, 30 third molars were used per cement group for bond strength testing. This sample size has been recommended as optimal for studies of this nature,21 and specimen storage before use complied with guidelines in the orthodontic literature.21 Other studies of orthodontic band cements have used a similar sample size and specimen storage regime.13, 15, 18 The molar tooth type, however, has not been specified by others.15 In another study, acrylic blocks, rather than teeth, were used, to which 6×6-mm strips of stainless steel band were attached with a cyanoacrylate adhesive, and only 10 specimens were bonded with each cement.

The mean retentive strength of molar bands cemented with the modified composite, Transbond Plus, was significantly less than for bands cemented with either the resin-modified or the conventional glass ionomer cement. No other study has compared these cements for band cementation, although similar products have been assessed.15 Comparisons can therefore be made only in the broadest sense.

In keeping with the findings of the study reported here, Aggarwal et al15 also found a significantly lower mean retentive strength of Transbond Plus cement compared with 2 resin-modified glass ionomer cements. Factory microetched bands were also used in that study, but, in contrast to the present study, specimens were incubated at 37° C for 30 days after preparation. They were then thermocycled for 24 hours before shear-peel band strength was assessed with a customized band removal device. However, comparison with a conventional glass ionomer cement was not made in that study.

Mean tensile, rather than mean shear, bond strength of modified composites, resin-modified glass ionomer cements, and a conventional glass ionomer cement to microetched stainless steel specimens also have been evaluated.14 Interestingly, the mean tensile bond strength of Fuji Ortho LC was greater than that of the modified composite Ultra Band-Lok (Reliance Orthodontic Products, Inc, Itasca, Ill), but only for Maximum Retention photoetched band specimens, whereas the opposite was observed for standard band specimens. Although a different mode of testing and a different modified composite were used in our study, the findings reported by Mennemeyer et al14 for the photoetched band group lend broad support to the greater mean retentive strength of Fuji Ortho LC relative to Transbond Plus recorded in the present study. The conventional glass ionomer cement, Ketac-Cem, demonstrated a significantly lower tensile bond strength than the other products evaluated14; this is at variance with the findings reported here. However, enamel substrate was not used in their study, and this, together with other differences in methodology between that study and ours, might account for the different findings with respect to this cement.

Although the strength of a particular band cement is important to learn from a laboratory study, the clinician is most likely to be particularly concerned with whether this strength will be exhibited reliably.20 Application of Weibull analysis to bond strength data allows this information to be obtained readily.19, 20 The Weibull modulus of Fuji Ortho LC was greater than that of Ketac-Cem or Transbond Plus, indicating greater bond reliability with this resin-modified glass ionomer cement.

For specimens tested in this study, bond failure occurred predominantly at the enamel-cement interface. This concurs with the results of other studies in which microetched bands were used.18 The amount of cement remaining on the enamel after debanding differed significantly for bands cemented with Fuji Ortho LC or Ketac-Cem. Almost all bands cemented with Fuji Ortho LC had a cement remnant score of 1, indicating that less than half of the crown surface under the band was covered by cement. Although the percentage of cement remaining on the band after debanding has previously been recorded with this cement,14 banded tooth specimens were not used, so the results cannot be compared objectively with those of the present study. For 25% of the Ketac-Cem specimens, no cement remained on the enamel after debanding, and for the remaining specimens (with 1 exception), less than half of the crown surface was covered with cement. These findings confirm those of other studies that used this cement.18 In most of the Transbond Plus specimens, less than half of the crown surface was covered by cement, but 10% of specimens had no cement on the crown, and in 7% of specimens, more than half the crown was covered by cement. On the basis of the findings of this study, enamel cleanup after debanding might be slightly faster if bands are cemented with Ketac-Cem rather than either of the other cements.

Although the results of bond strength tests and Weibull analysis are of interest, laboratory testing should simulate the environmental stresses that orthodontic specimens are likely to encounter clinically.20 Banded specimens have been subjected to thermal15, 21 insult, but only 2 studies have exposed specimens to mechanical insult13, 18 in a ball mill. The study reported here found the mean survival time of bands cemented with Fuji Ortho LC to be significantly longer than for bands cemented with Ketac-Cem, but there was no significant difference between the mean survival times of bands cemented with Transbond Plus and those cemented with either Fuji Ortho LC or Ketac-Cem. Forces of varying diversity and magnitude operate in the ball mill,17 but band failure occurs within a reasonable time period. This is likely to proceed from slow crack propagation within the cement generated by the force of impact and mechanical action of the ceramic spheres on the banded specimens.13 As a predictor of the likely clinical performance of band cements, the ball mill has proved to be useful,12, 18 and, on that basis, one would expect bands cemented with Fuji Ortho LC to survive longer clinically than bands cemented with Ketac-Cem. This lends support to the findings of the Weibull analysis for both of these cements. Although bands cemented with Transbond Plus had the lowest Weibull modulus, specimens cemented with this luting agent did not exhibit the shortest mean survival time in the ball mill experiment. Disagreement between results obtained from bond strength testing and those after application of simulated mechanical stress in a ball mill have been reported previously when bands were cemented with either zinc phosphate or glass ionomer cement.20 The findings of the 2 experiments reported here (retentive strength and survival time) are of interest because they reflect the variation in response of the cements to different modes of testing. Further work is required to determine whether the findings of this in vitro study are witnessed in clinical practice.

Conclusions