Inter-arch elastics are considered one of the most important auxiliaries in orthodontics, supporting the correction of molar relationship, spaces closure, and anchorage management.

Despite the widespread use of elastics in the orthodontic community, there are conflicting data on their mechanical properties. Applied forces are dependent on materials, sizes, and application sites. Anecdotally, the declared force is obtained when the elastic is stretched out three times its original diameter, but evidence supporting this suggestion is still lacking (1–3).

The sizes and declared forces of elastics may vary considerably among manufacturing companies. Ormco^® (Sybron Dental Specialties, Glendora, CA, USA), produces 24 different elastics, while there are 19 different elastics produced by American Orthodontics^® (Sheboyagan, WI, USA) and 21 by 3M Unitek^® (Monrovia, CA, USA).

The weakness of elastics is represented by their rapid deterioration and loss of elasticity into the oral environment (4, 5). A review of the existing literature in the field showed that most of the studies examined the applied force of inter-arch elastics, at standard distances, and in a static environment (6, 7). Only few authors used study models to determine the real elongation distance (8, 9). However, average real inter-arch distances at different anchorage sites obtained from Class II malocclusion patients have not been reported in reducing the validity of force measurements in previous studies.

According to the existing literature, the number of adults seeking orthodontic treatment is rising. The American Association of Orthodontists estimates that 27% of all the United States and Canadian orthodontic patients are adults (10). A similar percentage (26.52%) was obtained from a Brazilian study for patients between 20 and 40 years of age (11). A survey from the British Orthodontic Society indicated the increasing number of adult patients treated by United Kingdom orthodontists (12).

The aims of this study were to assess the average inter-arch distances characterizing Class II malocclusions in adults and to analyze the applied forces at those different distances by Class II inter-arch elastics of different manufacturing companies. Furthermore, the study aimed to compare measured forces with those declared by manufacturers of orthodontic elastics used for the correction of malocclusion class II, both in dry and wet environments.

This study should help clinicians to know the effectively applied forces by elastics, based on average inter-arch distances, to improve the clinical efficiency of Class II treatments.

Regarding Class II models, the average distances measured between the considered FA points are as follows (mean ± SD):

The force values declared by the manufacturer and the force values measured at T0 (dry elastics), T1, T2, and T3 (respectively, after 1, 6, and 12 h of artificial saliva immersion) for each elastic type and for every FA points distance considered in the study are shown in Supplementary Table 1.

Supplementary Table 2 shows the mean deviation (MD) between the manufacturer declared force value and the T0 measured value, for every considered FA points distance. Most of the elastics showed a significant difference between the declared force value and the measured one (p < 0.05).

Results of the statistical analysis comparing the force values measured at T0 with those measured after artificial saliva immersion showed significant results. The force released by most of the elastics at T1, T2, and T3 had a significant decay with respect to T0 (p < 0.05). Supplementary Table 3 reports the MD among the force values, for every considered FA points distance.

Despite the widespread use of inter-arch elastics in orthodontics, the real force released during their clinical applications is still unknown. In this experimental study, clinical application distances were obtained from 167 Class II patients. During the treatment of this malocclusion, the typical inter-arch elastics configuration is represented by anchoring them on the lower first molar and the upper canine (8, 9).

In the analyzed sample, the average distance between those two force application points was 24.6 mm, which is larger than three times the lumen of either 1/4” (18.7 mm), or 3/16” (14 mm) elastics.

The mean distance from the lower 1st molar to the upper 1st premolar was 16.3 mm, which is slightly lower than the three times the lumen of a 1/4” elastic, and slightly higher than that of a 3/16” elastic. Therefore, it may be advisable to use these reference average measurements to select the proper elastic diameter and force in adult patients requiring the use of Class II elastics. To our knowledge, this is the first study in which similar data are provided.

The existing literature related to Class II treatment with inter-arch elastics highlighted optimal elastics forces ranging from 2 (56.69 g) to 6.5 oz (184.27 g) (3, 7, 16). In the present study, we tested 8 oz (226.79 g) force elastics, considering their spreading in the clinical setting (17).

The mechanical testing was initially performed on dry elastics at room temperature (18). The force values obtained by most of the dry elastics (T0) showed significant differences when compared to the force values declared by the manufacturers. These results are important since the differences were measured for all the possible Class II elastics of the considered manufacturers and not only for those suggested by a pool of clinical orthodontists (2, 3, 6, 8, 9, 19).

According to Kersey et al., differences between measured and declared forces, varied from negative to positive values, independent of the application points (18). In the present study, for elastics applied at 24.6 mm distance (upper canine-lower first molar), the observed differences ranged from −169.5 cN (6.1 oz) of the GLORIA 3M^® elastics to 36.4 cN (1.3 oz) of the PUMA American Orthodontics^® elastics, while for elastics applied at 16.3 mm (upper first premolar-lower first molar) distance, the observed differences ranged from −117.9 cN (4.2 oz) of the GLORIA 3M^® elastics to 128.6 cN (4.6 oz) of the PUMA American Orthodontics^® elastics. The elastic diameter showing the lowest discrepancy between the tested and declared force is the 3/16”, at a distance of 16.3 mm (upper first premolar-lower first molar). However, for the 3/16” diameter, the lowest discrepancy was observed for the 6.5 oz American Orthodontics elastic band, for the 2 oz 3M^® elastic band and for 4.5 oz ORMCO^® elastics. So regardless of the brands, it seems advisable to carefully choose the size (inches) of elastics based on the inter-arch distance rather than on the strength needed.

Mansour et al. analyzing 3/16” and 1/4” elastic diameters at a distance of 14.6 mm, revealed no significant differences between tested and declared forces (9). In particular, the ORMCO^® 3/16” 4.5 oz showed the minimum discrepancy, as observed in our study.

In contrast, Kanchana et al., comparing elastics with different sizes, extended at a distance of 15 mm, observed that 3/16” Tomy^® (Tomy Inc., Tokyo, Japan) 4 oz elastics showed the greatest discrepancy, confirming that manufacturing processes have a huge impact on the applied force (19).

Analyzing the existing literature about inter-arch elastics, many studies focused on testing inter-arch distances >20 mm, to simulate real clinical conditions (5, 9, 16, 18, 19). In previous works, force-extension curves were provided with distances arbitrarily set at 5- or 10-mm intervals from 0 to 60 mm. In this study, we have provided force-extension curves based on real clinical distances, trying to provide more useful information from a clinical perspective. Based on our measurements, the forces exerted by 3/16” ORMCO^® elastics (Figure 3) showed the best performance. Kanchana et al. obtained the best force-extension curve for 1/4” elastics and their results in terms of measured strength for 3/16” elastics were greater with respect to our observations (18). However, according to Kanchana et al., tests were conducted in 2000 and in the last 20 years, some changes in the structure of the elastomer could explain the performance improvement detected in our study.

As previously reported, salivary pH, oral hygiene conditions, diet, and oral habits influence the elastics behavior in the oral environment (4, 20–22). Force degradation is the greatest disadvantage in using elastics, despite their application is expected to generate constant and optimal force for a specified time period.

In the present study, the wet test was conducted for a period of 12 h simulating a real clinical condition. In agreement with many previous studies, for almost all the elastics, a large force degradation was observed just after 1 h (3, 5, 23–25) of saliva immersion: at the 24.6 mm distance, the initial force loss of wet elastics was 14.12% for American Orthodontics^®, of the 7.92% for 3M^®, and of 14.61% for ORMCO^®. These data are slightly lower than those reported by Fernandes et al. (15.26–20.72%) that performed the test at a distance of 30 mm (26). Therefore, from a clinical point of view, the percentages reported above could be used to calculate a force level close to the declared one to compensate for the initial force loss of the elastics.

An increase of the force level was observed between the first and the 6th h of artificial saliva immersion, while another force loss was revealed between the 6th and the 12th h.

This trend is particularly evident for 3M^® 3/16” and 1/4” elastics, stretched between the upper canine and the lower first molar and between the upper first premolar and lower first molar (Figure 4).

A possible cause of this mechanical behavior could be the transitory hardening of the material in saliva immersion (26). Therefore, as suggested by Lopes Nitrini et al. (25), Andreasen and Bishara (27), and Wang et al. (4), elastics do not need to be replaced frequently.

Furthermore, 3/16” elastics showed the lowest discrepancy between the declared and the measured forces and the lowest percentage of force degradation in a wet environment, especially for higher force levels (4.5, 5.5, 6, and 8 oz), confirming the results of previous studies (22). Therefore, in the clinical setting, 3/16” and at least 4.5 oz elastic bands should be used in Class II mechanics.

To our knowledge, only Baty et al. reported a possible clinically significant impact when the difference between the measured and the declared force (ΔF) of elastomeric auxiliaries is >10% (28).

In the present study, almost all the tested elastics had ΔF > 10%.

In conclusion, orthodontists should choose carefully the size and force of the elastics, and this experimental setup provided some suggestions to possibly improve the clinical performances in Class II treatments of adults, using 3/16” and at least 4.5 oz elastics and asking the patients to change them every 6 h.

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

TC and GR contributed to conception and design of the study. FS organized the database. GR performed the statistical analysis. TC and AS wrote the manuscript. GC: substantial contributions to the analysis and interpretation of data for the work, drafting the work for important intellectual content, and agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All authors contributed to manuscript revision, read, and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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