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Background and Aims Sub-Tenon’s blocks are the most efficient fascial plain blocks, providing stand-alone analgesia for eyes. Presumably, many lessons could be drawn to improve fascial sheath blocks efficiency, especially about fluid dynamics. To visualize LA-distribution in STB, coloured mepivacaine was injected, differentiating between bulging bulk flow and viscous fingering- an arbitrating distribution pattern. Its incidence, correlation with bulk flow and its ultrasound-biomicroscopy (USBM) correlates are unknown.Methods 20 fresh pigeyes from abattoirs underwent USBM (50 MHz probe, axial resolution 30 μm) measurements of sclera, Tenon’s fascia and conjunctiva. Distribution patterns of colored LA were documented during injection. USBM measured increase of height of the gap between sclera and Tenon’s after injection-the eye’s interfacial sheath. Injection pressures were recorded via an X-Metra analog pressure gauge (range: 0–25 kPa, ±0.5 kPa accuracy) on the syringe. Distribution dynamics were categorized as: Bulk flow (B), Viscous fingering (VF ), Diffusion (D) Parametric and non-parametric methods based on distribution normality (Shapiro-Wilk test) applied. Continuous variables were reported as mean±SD or median[IQR]. Paired t-tests for normally distributed anatomical measurements, Wilcoxon tests for non-normal pressure curve data, and Pearson/Spearman correlations for linear/non-linear relationships were used. Analyses were performed in RStudio (v4.3.1) with significance at p<0.05.Results Initial VF followed by bulk flow was observed in 14 of 20 specimens (75%). Absence of VF (5 eyes) correlated with chemosis and septation, at mean height increase of 2.984 mm vs 1.834 mm in VF-positive eyes (p=0.0096). Injection pressure analysis showed no difference between groups. Initial VF cases robustly showed less chemosis (1.67 mm vs 3.14 mm, p=0.0096) and late VFI cases demonstrating a similar though less pronounced effect (1.81 mm vs 3.05 mm, p=0.047) in normality verification.Abstract EP111 Figure 1Statistical correlation of chemosis-thickness with viscous fingeringAbstract EP111 Figure 2STB fluid motion dominated by bulging bulk flow (chemosis) with microfingering at it’s edgesAbstract EP111 Figure 3STB fluid motion initiated by VFConclusions Viscous Fingering is a third fluid dynamic in FSB along with bulk flow and diffusion and should be studied for it’s potential to improve FSB