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. 2026 Feb 20;27:5. doi: 10.1186/s40510-025-00603-0

Influence of storage conditions on the mechanical properties of intermaxillary elastics

Wanda Urbanova 1,2,6,7, Iva Voborná 3,4, Tereza Šrolerová 5, Dávid Sluka 1,6, Petra Poláčková 1,2,6,7, Ivana Dubovská 3,✉
PMCID: PMC12923697  PMID: 41718992

Abstract

Background

Latex intermaxillary elastics (IE) are common orthodontic auxiliaries, but their generated force degrades over time. This study investigated the impact of light and heat exposure during storage on the IE force.

Materials and methods

From 500 3/16” Medium IE ten test groups were formed. For one month, four groups were exposed to natural daylight and four to a temperature of 50° C in three different packaging types (original semi-opaque, transparent, and closed orthodontic box) or stored loosely. The control groups were stored in the dark at 23° C. The initial extension force (F0), the residual force after 2 h of tensile loading (F2h) at three times original diameter and maximal force at break (Fmax) were measured. Visual evaluation using a stereomicroscope and light microscope was conducted, and shape, visible damage or defects, and colour changes were documented.

Results

After light exposure, IE stored loosely showed the lowest mean forces (F₀, F₂h, Fmax), all lower than those of the control, while IE stored in a box maintained values closest to the control. Significant differences in force magnitudes were observed between different packaging types, especially in F2h and Fmax. Heat exposure had minimal effect on F₀ values, and reductions were limited mainly to F₂h for IE stored in transparent packaging and in the box, and Fmax in the loosely stored group. Shape variability was observed in 21% of IE, with major defects in 6% and minor defects in 31%. Light exposure resulted in visible discoloration.

Conclusion

One month of light exposure reduced IE force even when stored in original packaging. Heat was less harmful, though residual force still declined. Storage in a box yielded the best results. Structural defects or shape irregularities may contribute to the differences in force degradation. Patient guidelines should advise keeping IE in a light-proof container at a stable room temperature.

Keywords: Intermaxillary elastics, Force, Storage conditions, Light exposure, Heat exposure

Background

Orthodontic intermaxillary elastics (IE) are widely used in aligner and fixed appliance therapy to generate auxiliary forces for tooth movement and anchorage control [1, 2]. The force generated upon stretching on a defined diameter is one of the key characteristics of IE and, ideally, should remain stable throughout the entire duration of clinical use. However, force degradation in IE is a well-documented phenomenon, driven by an interplay of intrinsic material properties and mechanical stress induced by elongation in clinical settings [3]. Numerous in vitro and in vivo studies have aimed to identify and elucidate the factors contributing to force degradation over time, however limited number have focused specifically on the material properties themselves or considered the influence of storage conditions [4–18].

IE are typically composed of either natural latex or synthetic non-latex elastomers, this study focuses strictly on latex IE. The primary component of latex IE is a natural polymer derived from rubber trees, recognized for its high elasticity, flexibility, and cost-effectiveness [19]. During IE fabrication, latex undergoes a process called vulcanization, in which cross-linking occurs in the presence of sulfur and other compounds, improving its flexibility and strength [19, 20]. As natural latex is highly susceptible to degradation from external environmental conditions, stabilizers, antioxidants, and anti-ozone agents are incorporated to enhance IE durability [20].

Nevertheless, IE have an expiration date, as their mechanical integrity and durability deteriorate over time [21]. Furthermore, the safety data sheet for IE specifies, under the “Conditions for Safe Storage” section, that IE should be protected from light while storing [22]. However, this instruction lacks clarity regarding whether individual elastics or the entire packaging requires protection. Also, no storage recommendations are usually provided to patients when given the IE during treatment.

Many clinicians have noticed that patients sometimes report frequent ruptures of IE during use in some of the batches, even though these IE are within a reasonable expiration date. This raises the question of whether these problems could be due to suboptimal storage conditions both during transport and handling, as emerging evidence suggests these may alter the properties of IE material [5, 6]. However, this issue has not been encountered much in the literature yet.

The aim of this study was to simulate and verify the effect of daylight and high temperature on the generated force of IE in different packaging options, and to check the IE for defects and shape anomalies.

Materials and methods

Experimental groups

The 3/16” medium Dentaurum® 1.3 N (128 g), with a diameter 4.8 mm (Dentaurum GmbH & Co. KG, Ispringen, Germany) IE were selected for investigation based on previous research, as they had demonstrated to deliver an initial force most consistent with the manufacturer’s specified value when prestretched and extended to three times its diameter [23]. An initial evaluation of two different batches of latex IE was performed by measuring extension force of ten randomly selected IE from each batch. The batch with the lower variability in initial force values was selected for further study (LOT PO354331; expiration date 2027-07-18).

IE from the selected batch (five packages, each containing 100 elastics) were combined, and from the total of 500 elastics, 10 test groups (2 control groups and 8 experimental groups) of 30 specimens each were randomly formed. Eight experimental groups were then exposed to different conditions involving light (L) and heat (T) in 4 different packaging options: in original semi-opaque plastic packaging (Dentaurum, Ispringen, Germany) (Lorig, Torig), in transparent packaging acquired from IE of different manufacturer (JMU Dental, USA) (Ltransp, Ttransp), in a closed black box (part of Invisalign® System, Align™, USA) (Lbox, Tbox) and stored loosely on a Petri dish (Lfree, Tfree). Detailed descriptions and flowchart of the research are provided in Fig. 1. These four different packaging options were selected as the clinically most relevant methods by which patients typically store the given IE (Fig. 2). Two test groups were stored in the dark at room temperature (23 ± 2 °C), to mimic storage conditions advised by the manufacturer, served as controls, one stored loosely on a Petri dish and the other one in the original semi-opaque plastic packaging (Dentaurum, Ispringen, Germany) (Corig, Cfree).

Fig. 1.

Fig. 1

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A flowchart of the research

Fig. 2.

Fig. 2

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IE samples from left to right: loosely placed on a glass Petri dish, stored in original semi-opaque plastic packaging, in non-original transparent packaging, and in a box

Simulation of storage conditions

  1. Light exposure.

Four experimental groups in different packaging were placed in a light box Just 521 PJC/CVL 5E (Just Normlicht, Weilheim/Teck, Germany; see Fig. 3a, b) equipped with light sources simulating the spectral power distribution of natural daylight. The chamber contained two D65 fluorescent tubes (colour temperature 6500 K, 2 × 18 W, wavelength range according to manufacturer: 300–830 nm) and one UV-A source (T8 tube-shaped lamp, 315–380 nm with peak emission at 350 nm, 8 W). These experimental groups were exposed to the light continuously for one month. The distance between the tested IE and the light sources was approximately 50 cm (bottom of the light chamber). Spectral wavelength and irradiance at the level of the experimental group’s location were measured using a USB2000 + spectrometer equipped with a cosine corrector (Ocean Optics, Orlando, FL, USA). The irradiance was approximately 1.45 mW/cm² in the whole spectrum (λ = 350–800 nm) and between 0.2 and 0.3 mW/cm² in the UV range (λ = 350–400 nm).

Fig. 3.

Fig. 3

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Light box Just 521 PJC/CVL 5E used for light exposure (a); spectral distribution of the applied light (daylight + UV) (b)

One month period of daylight exposure was chosen as one package of IE lasts for approximately 25–50 days when worn daily, therefore, when stored in a bright spot as a windowsill, the last pair can be exposed to light more than 700 h (two months for 12 h e.g. 1 month for 24 h).

  • b)

    Heat exposure.

Four experimental groups in different packaging options were stored in darkness at 50 °C in a Binder BD 53 incubator (Binder, Tuttlingen, Germany) continuously for a period of one month.

The temperature of 50 °C was selected based on reports indicating that in some tropical countries without air conditioning, in-clinic temperatures can rise to as high as 50 °C and in transport containers, they may reach up to 60 °C during transit [6, 24, 25].

After the exposure period, all experimental groups were stored under the same conditions (in the dark at room temperature 23 ± 2 °C, without direct exposure to moisture) as the control groups until mechanical testing was performed.

Force measurements

Testing procedures were based on the requirements of the technical standard EN ISO 21606:2022 — Dentistry — Elastomeric auxiliaries for use in orthodontics [22]. Initial extension force (F₀) and residual force after extension three times the diameter were measured. However, residual force was assessed after 2 h (F₂h), rather than after 24 h as specified by the standard, to capture the most significant initial decrease in extension force.

Half of the IE in each experimental group (n = 15) was tested on the Universal Testing Machine Instron 3336 (Instron Corporation, Norwood, MA, USA; Fig. 4a, b) equipped with a 500 N load cell, at a crosshead speed of 100 mm/min. Accordingly to the requirements of the technical norm, after insertion, each IE was extended to four times its original diameter (4 × 4.8 mm = 19.2 mm) and held for 5 s. The extension was then reduced to three times the original diameter (3 × 4.8 mm = 14.4 mm), and the acting initial force F₀ (in N) was recorded after 30 ± 2 s 100 mm/min. Force measurements were performed at the laboratory temperature of 23 ± 2 °C.

Fig. 4.

Fig. 4

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Universal testing machine Instron 3366 (a); detail of IE fixation during mechanical testing (b)

Subsequently, the IE were extended on a custom-made support plate with pins of 1 mm diameter set at an appropriate distance (Fig. 5) and stored there for 2 h in a simulated oral environment (37 °C, artificial saliva). The composition of artificial saliva was: NaCl (0.8 g/L), KCl (1.2 g/L), CaCl₂·2 H₂O (0.1 g/L), K₂HPO₄·3 H₂O (0.3 g/L), and MgCl₂·6 H₂O (0.1 g/L), dissolved in distilled water and adjusted to pH = 7.0 using 0.1 N NaOH. Residual force (F2h) was then measured after 2 h using the same procedure as for F0. The time intervals between the onset of extension and the measurement were identical for all IE, as during handling, consistent timing was maintained to ensure that for each IE F2h was measured after the same time delay.

Fig. 5.

Fig. 5

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Support plate with pins designed especially for this research with IE extended to three times their original diameter

An additional 15 IE from each test group without any prior deformation history were used to measure maximal force achieved at break (Fmax) and maximal elongation at break (Emax).

Statistical analysis of measured data

Statistical analysis was performed using Statistica 14 software (StatSoft, Tulsa, Oklahoma, USA) with a significance level of p = 0.05 for all applied statistical tests. Normality was assessed using the Shapiro-Wilk W-test, and homogeneity of variances was tested using Levene’s test. One-way (Fmax) and two-way ANOVA (F₀ and F₂h), with factors including packaging type and duration of loading, followed by Scheffé post-hoc tests to identify significant differences between test groups were performed.

Visual image analysis

In 150 IE from the control and experimental groups, after the F₀ and F₂h were measured, visual evaluation using a SZX 10 stereomicroscope (Olympus, Tokyo, Japan) and an ECLIPSE E600 transmitted light microscope (Nikon, Tokyo, Japan), both equipped with QuickPHOTO INDUSTRIAL 3.1 software (PROMICRA, Prague, Czech Republic) was conducted. The shape of each IE, visible damage or defects, and colour changes were documented.

Results

Force measurement

Table 1 summarizes the measured average values of F0,F2h, Fmax, and Emax with standard deviations (SD) for the two control groups. As no significant differences were observed between the results of the two control groups (p>0.05); all experimental groups were statistically compared to a single control group Corig. Table 2 presents the results of the F0,F2h,Fmax and Emax for the experimental groups of IE exposed to light and heat. Significant differences were found between some experimental groups and the control group, as well as between the IE packaging types in the mean values of all measured force magnitudes. Lower case letters (a, b) represent statistically significant differences within measured forces for each group, while upper case letters (A, B, C) indicate significant differences within the groups, i.e., between different packaging types and the control group separately for light-exposed and heat-exposed experimental groups.

Table 1.

Force measurements and maximal elongation at break of IE for the control groups

Control group
F0 ± SD [N] F2h ± SD [N] Fmax ± SD [N] Emax ± SD [mm]
C free 1.18 ± 0.09 1.06 ± 0.08 37.4 ± 3.8 70.0 ± 1.5
C orig 1.16 ± 0.07 1.08 ± 0.06 40.9 ± 4.1 71.5 ± 1.9
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SD - standard deviation; F₀ - initial force; F2h– residual force; Fmax– maximal force at break; Emax– maximal elongation at break; Cfree – control group stored loosely; Corig – control group stored in original packaging; N-newton; mm-millimetres.

Table 2.

Force and maximal elongation measurements for control and experimental groups after light and heat exposure

Control group
Measured variables F0 ± SD [N] F2h ± SD [N] Fmax ± SD [N] Emax ± SD [mm]
C orig 1.16 ± 0.07aA 1.08 ± 0.06aA 40.9 ± 4.1A 71.5 ± 1.9A
LIGHT-EXPOSED GROUP
L free 0.90 ± 0.09aB 0.76 ± 0.08bB 18.8 ± 2.9C 63.4 ± 2.1C
L orig 1.07 ± 0.04aA 0.93 ± 0.06bC 28.1 ± 2.4B 68.2 ± 2.2B
L transp 1.04 ± 0.10aA 0.86 ± 0.09bBC 26.8 ± 3.0B 68.1 ± 2.3B
L box 1.05 ± 0.12aA 0.98 ± 0.10aAC 40.0 ± 4.6A 71.9 ± 1.4A
HEAT-EXPOSED GROUP
T free 1.14 ± 0.11aA 0.97 ± 0.09bAB 36.8 ± 2.9C 71.1 ± 3.4A
T orig 1.17 ± 0.05aA 0.99 ± 0.05bAB 37.2 ± 4.6A 69.2 ± 2.5A
T transp 1.10 ± 0.08aA 0.93 ± 0.09bB 35.6 ± 6.8 A 69.4 ± 2.6A
T box 1.11 ± 0.07aA 0.96 ± 0.07bB 37.9 ± 5.7 A 70.4 ± 1.7A
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SD -standard deviation; F₀ - initial force; F2h – residual force; Fmax– maximal force at break; Emax – maximal elongation at break; Corig – control group stored in original packaging; L/Tfree – loosely stored IE exposed by light/heat; L/Torig - IE exposed by light/heat stored in the original packaging; L/Ttransp - IE exposed by light/heat stored in the original packaging; L/Tbox - IE exposed by light/heat stored in the box. N-newton; mm-millimetres.

Following light exposure, the lowest mean forces were recorded in IE stored loosely on a Petri dish. In these IE all measured forces differed significantly from the control group; the opposite was true for the IE stored in the box. The values of F0 for experimental groups exposed to light didn´t significantly differ from the control group, except for Lfree. The F0 values observed in the IE stored in the Lorig and in the Lbox were lower than those of the control group, however, the difference was not significant. For F2h all experimental groups except Lbox differed significantly from Corig; in addition, differences were also observed between Lbox and the other two packaging options and IE stored loosely. For Fmax, only the experimental group Lbox did not differ significantly from the control, whereas Fmax values were significantly lower in the Lorig and Ltransp groups. When stored loosely, the Fmax results differed from those of all other experimental or control groups. Except for Lbox, the Emax distances were significantly lower than those of Corig. The Emax values for the Lfree group were also significantly different from the Emax values of the other experimental groups. The difference between the F0 and F2h values of each group was always significant except for the Lbox elastics.

When exposed to a temperature of 50 °C for one month, the F0 values were rather consistent for all four experimental groups and did not differ significantly from Corig. The F2h magnitudes were significantly different from Corig only for Ttransp and Tbox. Fmax and Emax values did not differ significantly between the experimental groups and the control, except for Fmax in the Tfree group, which was significantly lower compared with the control and the other three packaging options. Otherwise, there was no significant difference between the experimental groups themselves in Fmax and Emax. There was a significant decrease in force after two hours of tensile load in all four experimental groups exposed to heat.

Visual image analysis

Figures 6, 7, 8 and 9 illustrate representative examples of IE exhibiting typical defects, shape anomalies, and discolorations as observed under a stereomicroscope and light microscope in some of the 150 IE (30 from control groups, 120 from experimental groups). Variability in shape was noted in 21% of the 150 examined IE, which were unevenly cut, resulting in an oval shape and non-uniform thickness along the circumference (Fig. 6a, b). Major defects—such as pronounced tears or material loss—were observed in approximately 6% of the 150 IE, while minor defects, including small pores or slight chipping, were identified in around 31% of the same group (Fig. 7a, b, c). Additionally, IE with an increased diameter or an oval shape were detected in 2% and 5%, respectively (Fig. 8a, b). After exposure to light, pronounced bleaching and increased transparency of the IE were observed, resulting in a noticeable color change (Fig. 9). After heat exposure, no discoloration was found.

Fig. 6.

Fig. 6

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Standard shape of IE (a) and IE unevenly cut (b)

Fig. 7.

Fig. 7

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IE without defect (a); IE with a major defect (b) and corresponding detail (c)

Fig. 8.

Fig. 8

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IE with standard dimensions and no defects or anomalies (diameter 4.8 mm) (a); IE with enlarged diameter and oval shape (b) after light exposure

Fig. 9.

Fig. 9

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IE on the left after light exposure and on the right from the control, with a visible color change

Discussion

Previous studies have shown that the force generated by IE significantly decreases over time, depending on factors such as the amount and intensity of extension, in vitro or in vivo conditions, material composition, and the surrounding environment [7–19]. However, only two studies have specifically addressed the effects of storage conditions as temperature and humidity of latex elastics, while none have investigated the impact of direct light exposure [5, 24]. In the present study, following continuous light exposure over one month, a reduction in initial force was observed in IE stored loosely. Contrary to control specimens after two hours of IE tensile loading and storage in artificial saliva, all experimental groups—except for those IE stored in a box — demonstrated a significant decline in force with significant differences observed in both packet-stored specimens and in the IE stored loosely; the maximal extension before breakage was also reduced. These findings indicate that light exposure accelerates the degradation of the mechanical properties of IE, leading to a more rapid decline in force output. Long-term exposure to daylight and its UV components promotes the formation of reactive radicals, which initiate photo-oxidative degradation of the latex polymer chains [26]. Chain scission reduces the mechanical properties of IE, while radical attack on chromophoric structures may lead to IE bleaching, as demonstrated in our study. After the light exposure, IE not only generate a lower force that diminishes more rapidly, but they also exhibit a higher tendency to rupture when stretched beyond three times their original diameter. Patients undergoing orthodontic treatment therefore, should be advised to store the IE in their original packaging within a closed, light-impermeable container or drawer at home. This guideline should be incorporated into the general patient instructions.

After continuous thermal exposure at 50 °C for one month, no significant differences in initial force were found between the various IE packaging types and the control group. Comparable results were reported in two studies evaluating the effects of heat and cold storage conditions on latex IE. In the study by Piradhiba et al. [24]. IE were stored at temperatures ranging from 4 to 37 °C for one year, with no distinguished differences in initial force observed, whereas Gonzaga et al. [5] investigated IE stored exclusively under refrigeration. However, no measurements of force decrease and residual force were performed in these studies. In the present study, following a 2-hour tensile extension, the force magnitude was lower than the initial force in all specimens subjected to thermal exposure. Moreover, the residual force differed significantly in IE stored either in the transparent package or in the box compared with the control group. The transparent packaging and the box may have adversely affected the force generation of IE after thermal exposure, potentially due to chemical substances released during one month of continuous heating. These results indicate that storage of IE at ambient temperatures fluctuating up to 50 °C can indeed lead to clinically relevant alterations in their mechanical properties, particularly when elastics are used by the patients for periods exceeding two hours. Nevertheless, temperature fluctuations during storage appear to be less detrimental to preserving the mechanical properties and functional integrity of IE.

When considering the maximal force at break after thermal exposure, a slight decrease was observed across the experimental groups compared with the control group. In contrast, a study by Sharma et al. [6] evaluating non-latex elastics demonstrated that prolonged exposure to temperatures between 35° C and 40° C can adversely affect the breaking force. In the present study, only light exposure resulted in a significant reduction in maximal force values for IE stored in original and transparent packets, and an even greater decrease for freely stored IE. Only IE stored in the box were comparable with the control group in terms of force at break and maximal elongation. Therefore, the recommended storage conditions for IE should include storage in a closed container protecting the IE from light exposure, and maintenance of a stable temperature around 23 °C without fluctuations to higher values.

Visual image analysis revealed shape and dimensional anomalies in approximately one-fifth of the IE, which may have contributed to the variability observed in force values in control and experimental samples. As IE are produced by cutting long tubes into individual units, this manufacturing process may introduce shape irregularities, indicating the need for enhanced quality control [27]. Visual inspection revealed that a proportion of IE exhibited structural defects, ranging from prominent flaws to minor irregularities. Similar defects were found on the IE in the study of Gurdán et al. [4] after tensile tests and cyclical tensile fatigue tests. However, it must be taken into account that in both studies, it was not verified that these abnormalities were not already present in the IE prior to exposure. Due to the need to maintain simulation conditions, it was not possible to inspect the IE to avoid exposing them to light from the microscope. The possibility exists that the observed light-induced changes were limited to colour alteration and that all other anomalies were already present in the purchased IE prior to visual testing.

It is challenging to compare the force degradation of various studies with the present findings, as the testing conditions differ slightly in each one of them. Although there is a clearly defined international standard for testing the generated force of IE, very few studies dhere strictly to this protocol.

Limitations

This study has some limitations that should be considered when interpreting the results, as it cannot be fully excluded that the IE had been influenced by suboptimal storage conditions prior to the study, despite efforts to minimize this risk by selecting IE from a batch with lower variability in initial force. Moreover, variations in shape and non-uniform thickness observed in some elastics may have introduced additional bias into the results of force measurements. Visual assessments of the sample were performed only once and by a single, albeit well-trained, examiner. In addition, the chemical composition of the plastic packaging was not analysed.

Conclusion

One month of light exposure markedly decreased the mechanical resistance and residual force of IE, even when the devices were kept in their original packaging. Thermal exposure at 50 °C for the same period produced comparatively less deterioration. The most favorable results were observed in IE stored in a closed box, whereas those stored freely were most susceptible to force degradation. Appropriate storage—particularly protection from light—is essential for maintaining mechanical integrity. Clinicians and staff should strictly follow the manufacturer’s storage recommendations, and patients should receive explicit instructions on proper home storage. Approximately one-fifth of the tested IE demonstrated shape irregularities, and nearly one-third exhibited structural defects.

Acknowledgements

Not applicable.

Author contributions

Conceptualization W.U. and I.D.; methodology W.U., T.Š. and I.D.; validation I.V. and P.P.; investigation T.Š. and D.S.; resources I.V. and P.P.; data curation T.Š. and D.S., I.D. and W.U.; writing —original draft preparation W.U., I.D. and D.S.; writing — review and editing W.U., I.D. and T.Š.; supervision I.V.; project administration and P.P. All authors have read and agreed to the published version of the manuscript.

Funding

Funding for this study was provided by grant IGA LF 2024 002.

Data availability

The datasets analysed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

Ethical approval was not required for this study.

Competing interests

The authors declare no competing interests.

Footnotes

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Availability Statement

The datasets analysed during the current study are available from the corresponding author on reasonable request.

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