INTRODUCTION

Relapse is considered to be any change in tooth position or arch relationship that occurs during the initial posttreatment period.1 The exact causes of relapse are difficult to identify, but there are thought to be four factors that are responsible for its occurrence: the elastic recoil of the periodontal tissues, the pressures exerted from the facial and oral soft tissues, occlusal forces, and posttreatment facial growth and development.2 Clinicians have little or no control over the influence of growth and soft tissue factors on relapse, which adds to the difficulty of managing the problem.

Over the past 30 years, several studies have evaluated long-term dental stability following different orthodontic treatment modalities.3–7 All of these studies have had a minimum follow-up period of 10 years, extending to 20 years. Most of the treatment modalities—which included non-extraction, first premolar extraction, second premolar extraction, serial extraction, and expansion treatment—showed long-term relapse rates of up to 70%.

Vacuum-formed retainers (VFRs) are removable, clear, thermoplastic retainers. Ponitz8 first described their use for the purpose of orthodontic retention in 1971. Since being introduced into the UK National Health Service fee structure in 1996, the rate of increase in the use of VFRs has been approximately nine times greater than that of Hawley retainers.9 A vacuum machine adapts heat-softened plastic by negative pressure, creating a vacuum, and pulls the plastic onto a working study cast. The two most common materials used for VFRs are polyethylene co-polymers and polypropylene polymers.

Polyethylene polymers have the advantage of allowing acrylic to be bonded to the material and, thus, are the plastic of choice when bite planes need to be incorporated into the appliance. The material is also considered more esthetic because it is virtually transparent. Polypropylene polymers are considered to be more durable and flexible, but esthetically they are inferior to polyethylene because the material is translucent. Acrylic cannot be added to the plastic.

The advantages of VFRs include low cost, ease of fabrication, and patient acceptability due to minimal bulk and thickness.10 One randomized controlled trial has concluded that VFRs hold corrections of lower anterior teeth better and are more cost-effective than Hawley retainers. The majority of patients appear to prefer VFRs over Hawley retainers, and VFRs are less likely to be broken.9,11

However, recent studies have found that there appear to be some problems with the physical properties of VFRs, in particular their resistance to wear. Campbell et al.12 looked into the reasons for replacement of VFRs over a 1-year period. For 38% of the retainers that needed replacing (13 out of 34), the reason was because the retainer had worn away excessively. Lindauer and Shoff13 noted that a number of Essix retainers became perforated and cracked in the 6–18 months following placement.

As it is vital for VFRs to continue to be effective for many years because of the risk of relapse, there is a need to investigate which VFR materials available commercially have the best resistance to wear. Wear is defined as the removal of material from solid surfaces as a result of mechanical interaction between two or more relatively moving surfaces.14 Clinical wear is a complex process and involves several mechanisms interacting at once.

At the time of writing, the authors are only aware of one published study that has investigated the wear properties of VFRs. In that study, Gardner et al.15 assessed three different VFR products for their resistance to wear: two of the materials were polypropylene polymers, whereas the third was a polyethylene polymer. The samples underwent 1000 cycles of wear under a load of 25 kg using an Instron machine, with steatite balls as the antagonist material. The authors found that the polyethylene material had significantly less wear than the two polypropylene materials. There was no statistically significant difference found between the means of the two polypropylene products.

The material used as an antagonist has also been the subject of much debate in the literature. A broad variety of antagonist materials, such as human enamel, stainless steel, steatite, and dental porcelain, have been used, yet there appears to be no consensus on which of these materials is the most appropriate for use during wear testing.16–19

The aim of this study was to investigate the resistance to wear of four different commercially available VFR materials—Essix C+, Essix ACE, Duran, and Tru-Tain. Resistance to wear of the materials was evaluated by measuring the maximum wear depth of the samples using noncontact, three-dimensional surface profilometry. The wear depth was given in micrometers.

The null hypothesis for this study was that there is no difference in maximum wear depth between the four VFR materials after the materials have been subjected to a standardized wear test.

MATERIALS AND METHODS

The study was a controlled laboratory study that did not involve patients; hence, ethical approval was not needed. Samples were prepared at St Luke's Hospital, Bradford, UK, and the wear test and analysis of specimens were done at the Leeds Dental Institute, Leeds, UK.

Four well-known materials were chosen for inclusion in the study: Essix C+, Essix ACE, Duran, and Tru-Tain (Table 1). The methodology was standardized for all the specimens, and each material was used as supplied by the manufacturers. The specimens were all 1.0 mm thick (0.040 inches) and had dimensions of 125 mm × 125 mm before vacuum forming. The manufacturer's instructions for each product were followed in detail when vacuum forming.

Table 1. VFR Materials Investigated in This Study^a

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Wear Testing

An acrylic template block was manufactured to be used in the Pro-Form vacuum forming machine for the creation of the samples (Figure 1). The final specimens were flat and rectangular in shape and placed in the custom-made sample holder prior to wear testing (Figure 2).

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The wear machine used in this study has previously been described in detail by Harrison and Lewis,20 with some minor modifications made for this study. The sample holder is secured to the base-plate, which is moved horizontally by the electrical motor. Above this are 10 steel rods that hold the antagonist/abrader material—steatite balls 8 mm in diameter (PE Hines & Sons Ltd, Staffordshire, UK). At the superior end of the rods are weights that apply a load of 460 g as the steatite antagonist contacts the VFR specimen.

In this study, a cycle was defined as the horizontal cycle of the base plate, which moves 16 mm horizontally to the right and then 16 mm to the left, back to its start position. A complete horizontal cycle takes 47 seconds. Simultaneously, the steel rods that secure the antagonist have a vertical drop of 3 mm that results in contact of the steatite balls with the VFR sample. The duration of contact is 0.2 seconds before the steel rods move upward again. Figure 3 is a diagram that describes the major component parts of the wear machine and the movements caused by the camshaft.

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Each specimen was abraded for 1000 cycles of the wear machine, which took 13 hours and 4 minutes to complete. A linear wear trough was clearly created in the samples during this process. When the next set of specimens was tested, the wear machine was cleaned and lubricated and new abraders were used.

The specimens were thoroughly cleaned with distilled water and dried with air from a 3-in-1 dental air compressor to remove and clear any debris prior to scanning. The profilometer used for the analysis of samples was the Proscan 2000 (Scantron, Taunton, UK), which is a noncontact, three-dimensional surface profilometer that uses confocal multiplex sensors. The S5/03 sensor used to scan all the samples has a resolution of 0.01 µm and a spot size of 4 µm, thereby allowing detailed analysis. The central 4 mm of the wear trough was the area determined for analysis.

The software affiliated with the Proscan 2000 produced a series of two- and three-dimensional images. An example of a scan from the Essix C+ group is shown in Figure 4. The wear depth of the trough was measured using a three-area wear-depth analysis. The deepest area of the wear trough was compared against the average depth of the two “normal height” areas on either side. The output measure was drop in height (in micrometers). The analysis was repeated, and an average of the two readings was taken to be the wear depth of the trough for that sample.

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RESULTS

Table 2 shows the descriptive statistics for the wear depth in the four treatment groups. The median wear depth for the Essix C+ group was 63.20 µm, much higher than the median wear depth in the other groups. The median wear depth was 7.88 µm in the Essix ACE group, 9.75 µm in the Duran group, and 12.08 µm in the Tru-Tain group.

Table 2. Median Wear Depth and Other Numerical Descriptive Statistics of the Groups^a

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The box-and-whisker plot in Figure 5 shows that Essix C+ had a much greater median wear depth and more variation than the other three materials. The box plot demonstrates that the data was skewed.

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The intrarater agreement was good, as demonstrated by the Bland-Altman plot (Figure 6). The mean of the differences (bias estimate) was 0.02 µm, and no measurements were outside the 95% limits of agreement (0.31, −0.27).

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The Kruskal-Wallis test showed that there were significant differences in the median wear depths between the four groups (P < .001). The results of the group-wise comparisons using the Mann-Whitney U-test are shown in Table 3. These results confirmed that the Essix C+ group showed significantly more wear than the other three groups. The only other statistically significant difference was between the Essix ACE and Tru-Tain groups, with Essix ACE material shown to have less wear.

Table 3. Summary of the Results Obtained Using the MWU Test to Compare VFR Groups^a

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DISCUSSION

The laboratory study by Gardner et al.15 described in the introduction is the only published study the authors have found investigating the wear properties of VFRs. In that study, the authors tested three different VFR products, two of which were polypropylene polymers—Essix C+ and Invisacryl C—while the third was a polyethylene copolymer, TR. Essix C+ was, therefore, tested in both studies.

Gardner et al.15 reported that the mean wear in the Essix C+ group was 5.9 µm, in the Invisacryl C group it was 6.1 µm, and in the TR group it was 1.6 µm. The TR group (polyethylene copolymer) demonstrated significantly less wear than the two polypropylene groups. The results in that study indicated that the polypropylene materials had 3.7 to 3.8 times greater wear than the polyethylene material under their standardized wear trial.

The current study tested one polypropylene material, Essix C+, and three polyethylene copolymer based materials, Essix ACE, Duran, and Tru-Tain. The median was used to report the findings, as the data were found to be nonparametric. The difference in median wear depth between the Essix C+ group and the other three groups was statistically significant. Median wear depth in the Essix C+ group was eight times greater than that in the Essix ACE group, 6.4 times greater than that in the Duran group, and 4.7 times greater than that in the Tru-Tain group.

Though both studies used different summary measures, the results show a similar, statistically significant trend. The findings of this study and the study by Gardner et al.15 indicate that the polyethylene-based materials are at least 3.7 times more wear resistant than the polypropylene materials. In total, both studies have looked at six different widely available VFR materials. Sheridan et al.10 stated that Essix C+ was extremely resistant to wear without providing supporting evidence. However, two laboratory studies have now found that polyethylene copolymers are more resistant to wear than polypropylene materials like Essix C+.

It is noted that there is a big difference between the wear depths of the samples achieved between this study and those of the Gardner et al.15 study. Both trials used different wear-simulating machines. Their study had a load of 25 kg, whereas our study had a much lighter load of 460 g. Both trials had an experimental run of 1000 cycles and used the same material for the antagonist, but the mean wear depth achieved for Essix C+ in their study was 5.9 µm, and in our study it was 65.50 µm.

It may be reasonable to suggest that the greater load in the Gardner et al.15 study would have resulted in more wear of the VFR samples, but the opposite has been found. The effect of the wear simulation is, therefore, unpredictable, and different machines may produce different wear rates. However, it must be remembered that the general trend between polypropylene and polyethylene polymers is consistent between the two studies.

Wear is the net result of a number of fundamental processes: abrasion, adhesive effects of the contacting surfaces, fatigue, and corrosive effects, which act in various combinations depending upon the physical properties of the contacting surfaces and materials. Thus, due to the complex nature of clinical wear, any laboratory study designed to study wear is subject to criticism.

CONCLUSIONS

• The three polyethylene copolymer materials—Essix ACE, Duran and Tru-Tain—exhibited significantly less wear than the polypropylene material, Essix C+.

• Essix ACE had the lowest median wear depth of the three polyethylene copolymer materials, followed by the Duran group and then the Tru-Tain group.

• There was no statistically significant difference in median wear depth between the Essix ACE and Duran groups.

• Essix ACE had significantly less wear than the Tru-Tain group, but the difference between the Duran and Tru-Tain groups was not statistically significant.

• An appropriately designed, randomized, controlled clinical trial may be helpful to determine whether these findings are replicated in vivo.