The influence of molecular weight on the optical parameters of silicone oil endotamponades for vitreoretinal surgery.

Introduction Silicone oil, more specifically polydimethylsiloxane (PDMS) of different molecular weight, is an established endotamponade in vitreoretinal surgery. As emulsification is a feared complication of their intraocular use, high molecular weight compounds (HMWCs) may be added to modify their physicochemical properties lowering emulsification tendency. However, the optical properties of various HMWCs, as well as blends of silicone oils with different fractions of HMWC have never been studied in a controlled laboratory setting and compared with each other. Optical parameters, such as forward light scattering and turbidity, may indicate intermolecular interactions among PDMS fractions with varying molecular weights. Methods Blends of Siluron 1000 and different HMWCs in various ratios were created. Viscosities of the pure HMWC components were 60,000 mPas, 100,000 mPas and 2,500,000 mPas, respectively. Mixtures of Siluron 1000 with 10%, 30% and 90% HWMC component were created. Light scattering of these blends and the raw components was quantified using a custom-made C-Quant straylight meter and turbidity was quantified with an ISO-normed turbidity meter. Light scattering and turbidity were compared between the different mixtures and the raw materials. Additionally, measurement of Dynamic Light Scattering (DLS) using a Zetasizer was performed to quantify the formation of possible particle-like HMWC aggregates. Results Turbidity of PDMS showed a molecular weight-associated increase with the highest turbidity values appearing for pure 2,500,000 mPas HMWC PDMS. For mixtures of Siluron 1000 with 100,000 mPas and 2,500,000 mPas HMWC-PDMS, an increased turbidity was observed when the HMWC component exceeded 10%. Forward light scattering values were very low for all tested samples, slightly increased values were only detectable for mixtures with a proportion of 90% HMWC components. DLS revealed a concentration and molecular weight dependent appearance of particle-like phenomena. Conclusion Although there was no evidence of clinically relevant optical disturbances for all tested substances, the molecular-weight dependent optical properties of HMWC-blends may be attributed to the presence of optically active particles when mixtures exceed a 10% proportion of high-molecular weight components, specifically for 100,000 and 2,500,000 mPas HWMC PDMS. This finding may indicate suboptimal solubility and compromised intermolecular interaction.