In Vitro Evaluation of Sorption and Solubility in Orthodontic Composite Resins

Figure 1: Sorption and solubility measurements

Solid petroleum jelly was applied to the base of the mold using a microbrush (KG Sorensen, Cotia, SP, Brazil), and a polyester strip was then positioned in contact with the mold base. After isolation of the mold, the composite resins were inserted using a No. 1 titanium resin spatula (Indusbelo, Salvador, BA, Brazil) until complete filling of the mold was achieved. Subsequently, a glass slide was positioned over the
resin surface, and the material was light-cured using an LED based light-curing unit for 40 seconds with a tip irradiance of 1000 mW/cm² (VALO, Ultradent Products, South Jordan, UT, USA). The adopted radiant exposure (4.8 J/cm²) allowed specimen removal from the mold without deformation. Finally, the specimens were carefully removed using a Lecron spatula (SS White, Vasco da Gama, Rio de Janeiro, RJ, Brazil) to eliminate excess resin material (Figure 1).

Subsequently, the specimens were sanded with water sandpaper sequentially 600-800 and 1000-1200 to perform the finishing and flattening for subsequent testing.

The ISO 4049 specification employed in the determination water sorption (S) and solubility (SL) [15,17,18]. Immediately after polymerization, the specimens placed in silica gel dissectors, which transferred to a preconditioned oven at 37°C and left for 7 days. After that, they washed at 24h intervals until the mass variation between two consecutive days was less than 0.2 μg. A digital caliper (Figure 2) was used to measure the thickness and diameter of the specimens, with an accuracy of 0.01 mm, to calculate the volume (V in mm³).

Each specimen was weighed preliminarily (m1), then each specimen was placed in a micro tube (sealed Eppendorf) with 10 mL of distilled water (pH 7.2) at 37ºC. After intervals of 1, 8 and 24 h and 7, 14 and 28 days of storage time, each specimen removed from the oven and left at room temperature for 30 min. Then, they were washed in running water, gently cleaned with soft and absorbent paper, and weighed on an analytical balance (m2), returning to the micro tubes with 10 mL of fresh distilled water.

After the storage time of 28 days, the specimens were dried in desiccators containing silica gel at 37 °C, which remained at rest for 10 days or until the mass variations were less than 0.2 μg, obtaining a constant mass (m3). Once the weightings m1, m2 and m3 were obtained at the respective times, the S and SL were measured with the following formulas:¹⁶ S = (m² - m³) / V and SL = (m¹ - m³) / V  

RESULTS
Sorption mean values (um/mm³) and standard deviations are presented in Table 1. Sorption was different for the tested materials (p<0.05).

Table 1. Means and standard deviation of the obtained sorption (ug/mm³) data

(*) Equal letters indicate statistically similar results between columns (Tukey's test, p<0.05). Symbol (†) represents statistically different results compared to the 1 h (initial) sorption time. 

Figure 1 shows the results obtained for water sorption at the six different times. The Transbond XT resin group was the one that obtained the lowest sorption values at all times (ranging from 0.10 ± 0.04 to 0.15 ± 0.03 μg/mm³) when compared to the other resins, as well as no significant differences in values over the evaluated period. Orthobond Plus and Ortholink VLC resins reached similar values among themselves (peaking at 0.30 ± 0.10 and 0.35 ± 0.05 μg/mm³, respectively, at 24 hours), and did not undergo significant changes in mass over time. Orthocem and Orthocem UV Trace resins showed the highest values of water sorption over time.

Figure 1. Sorption data averages (ug/mm³).

Table 2 shows the solubility results of the tested materials. It can be observed that Ortholink VLC resin reached the highest value (0.35 ± 0.03 μg/mm³) when compared to Transbond XT, which on the other hand, obtained the lowest rate (0.12 ± 0.05 μg/mm³). The other resins, Orthocem (0.28 ± 0.09 μg/ mm³), Orthobond Plus (0.21 ± 0.08 μg/mm³), and Orthocem UV-Trace (0.18 ± 0.08 μg/mm³), demonstrated intermediate solubility values.

 
 Notes: (*) Equal letters indicate statistically similar results (Tukey's test, p<0.05).

Figure 2 shows the solubility variations between the different resins tested, where it is evident that the Transbond XT resin presents the lowest solubility.

Figure 2. Solubility of the orthodontic resins tested.

Statistical analysis
To detect normality between the groups the Shapiro Wilk test was applied, and the homogeneity of variances was tested using Levene's test. Sorption was analyzed by ANOVA twoway (time and resin). Solubility was analyzed by one-way ANOVA (resin). In order to observe the differences between the groups evaluated, for sorption and solubility, the Tukey's test performed afterwards. All calculations performed using the statistical software SPSS® (Statistical Package for the Social Science) version 20.0 (IBM, Inc Chigago, Illinois USA).

DISCUSSION
The observed sorption and solubility patterns in this study reveal marked inter-material disparities among the tested orthodontic resins. Transbond XT consistently demonstrated the lowest values (0.10–0.15 μg/mm³ across 1h to 28d; p>0.05), a finding that aligns with its established hydrophobic formulation and high inorganic filler loading (up to 80 wt%). This high filler content reduces the volume of the organic matrix available for water uptake, while the combination of Bis-GMA and UDMA monomers—which are less hydrophilic than other low-viscosity monomers—restricts water ingress via reduced hydrophilic domains [1,24]. This stability mirrors reports by Pelourde et al. (2018), who attributed such resilience to superior silane coupling that preserves filler-matrix integrity against hydrolytic assault [24].
Regarding the mechanisms of water sorption, several factors determine the diffusion coefficient, including resin composition, porosity, and the nature of the monomeric units. One of the most common monomers in composite resins is TEGDMA, which is a relatively hydrophilic compound with a high water sorption capacity due to its ethoxy groups [2, 10]. According to Venz and Dickens, the performance of common monomers regarding water uptake follows the order: TEGDMA > Bis-GMA > UDMA [10]. In this study, the resins with the highest water sorption indices were Orthocem and Orthocem UV-Trace (up to 0.48 μg/mm³ at 28d; p<0.05). This behavior is likely attributable to their lower inorganic content (55%) and greater residual monomer elution. These findings are consistent with recent analyses where UV tracers and higher TEGDMA concentrations correlated with accelerated porosity amplification and time-dependent mass gain [23,25].

Solubility profiles closely paralleled the sorption trends, as the same parameters affecting water uptake—such as the degree of conversion and monomer chemistry—interfere with the leaching of unreacted components. Transbond XT’s minimal mass loss (0.12 ± 0.05 μg/mm³) parallels recent in vivo saliva monitoring, which detected rapid tapering of monomer elution (HEMA/poly-EGDMA) within 24 hours [26]. Conversely, Ortholink VLC’s higher solubility (0.35 ± 0.03 μg/mm³) echoes studies linking high solubility to potential cytotoxicity and degradation of the bonding interface [27]. These variances decisively reject the null hypothesis, pinpointing compositional determinants— filler volume fraction, monomer hydrophilicity, and silane efficiency—as pivotal modulators of material longevity [2].

From a clinical perspective, an ideal orthodontic resin should exhibit low sorption and solubility to prevent oxidation and staining during the bonding of metal or ceramic brackets, which is crucial considering the high aesthetic demands and the risk of overtreatment in contemporary dentistry [28], as well as to avoid filler-matrix decohesion that leads to debonding. While all tested resins remained within the ISO 4049:2019 thresholds for restorative materials (<40 μg/mm³ for sorption and <7.5 μg/mm³ for solubility) [29], the elevated values in the Orthocem variants and Ortholink VLC signal a higher vulnerability to chronic degradation compared to the "gold standard" Transbond XT.

Recent innovations in self-etch systems (e.g., 10-MDP variants) have shown up to 35% reduction in leaching compared to conventional systems, yet commercial analogs often still exhibit formulation gaps that may impact long-term intraoral performance [26].

This investigation unequivocally rejects the null hypothesis, revealing significant inter-material disparities that underscore compositional hydrophilicity and filler-matrix interfaces as fundamental for long-term stability. The resilience of Transbond XT supports its preference for sustaining durable orthodontic bonding. Conversely, highsorption adhesives require caution or adjunctive protocols to mitigate porosity induced failures [24]. These outcomes advance material selection paradigms by linking in vitro metrics to real-world performance amidst evolving bioactive innovations [22].

Study limitations include the static immersion without thermal cycling, enzymatic challenge, or biofilm simulation. Nonetheless, the temporal granularity of this study (up to 28 days) enhances translational fidelity compared to shorter protocols [23]. Future research should integrate dynamic intraoral simulations—encompassing thermomechanical fatigue and biofilm interactions—to refine predictive models and catalyze the development of next-generation adhesives with sub-ISO degradation profiles [23]. Furthermore, the surface degradation and porosity of these resins could be thoroughly evaluated using advanced texture analysis methods [30].

CONCLUSION
The mechanical properties of resins used in orthodontics present differences according to their chemical composition. The resin that showed the best properties in sorption and solubility tests was Transbond XT, which demonstrated the lowest sorption values (ranging from 0.10 to 0.15 μg/mm³) and the lowest solubility (0.12 μg/mm³) across all evaluated periods. Conversely, the highest water sorption was observed in Orthocem (0.48 μg/mm³ at 28 days), while Ortholink VLC exhibited the highest solubility (0.35 μg/mm³).

CONFLICTS OF INTEREST
The authors declare no conflict of interest.

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