Structural Stability of Silicone-Based Elastodontic Appliances After Clinical Use: Insights from FTIR Spectroscopy

<i>Background and Objectives:</i> Elastodontic appliances made of medical-grade silicone are increasingly used in interceptive orthodontics, but prolonged intraoral exposure may affect their stability. This study evaluated structural changes in LM-Activator^TM 2 appliances after clinical use, using Fourier-transform infrared (FTIR) spectroscopy. <i>Materials and Methods:</i> Eight appliances (one unused control and seven worn for 3-24 months) were analyzed by FTIR-ATR in the 4000-650 cm^-1 range. Absorption bands characteristic of polydimethylsiloxane (PDMS) were quantified, and indices reflecting backbone crosslinking, side-group retention, hydrophilicity, and relative reduction in methyl-related spectral contributions were calculated. <i>Results:</i> The PDMS backbone remained chemically intact across all samples. However, progressive molecular reorganization was detected with wear duration. The Backbone Dominance Index increased significantly from control to 24 months, while side-group indices decreased, confirming apparent depletion of methyl-related FTIR bands. Hydrophilicity and crosslinking indices rose over time, particularly after 12 months, indicating increased surface polarity and network densification. <i>Conclusions:</i> LM-Activator^TM 2 appliances undergo gradual intraoral aging, marked by backbone crosslinking and apparent reduction in methyl-associated vibrational contributions inferred from FTIR ratio side-groups. These changes, while not compromising the polymer identity, may influence surface properties, biofilm retention, and long-term mechanical behavior. Periodic replacement is recommended to ensure optimal clinical performance.