Characterizing the Field of Fluidic Effect Around Vitrectomy Probes Using Particle Image Velocimetry

The shape and characteristics of the velocity fields around vitrectomy probes are unknown, resulting in an incomplete understanding of their surgical effects and behavior. The primary aim of this study was to define and measure with high spatial and temporal resolution the field of fluidic effect induced by vitrectomy probes, with assessment of both convective and temporal acceleration in a range of surgically relevant settings with an optimized particle image velocimetry (PIV) methodology.

This was a laboratory-based investigation using in vitro testing carried out at Newcastle University. Testing was conducted using a DORC EVA Nexus vitrectomy system with dual cutting action vitrectomy probes in three gauge sizes (23G, 25G and 27G). With the use of balanced salt solution (BSS), PIV measurements were conducted for various flow, vacuum, and cut rates. Experiments were conducted in a range of different probe orientations to reconstruct a three-dimensional representation of the flow fields.

Using a variety of improvements in the PIV setup, including choosing an optimal time delay (dt) between the image pairs, the field of fluidic effect was measured with high spatial resolution up to the probe port. Fluid acceleration in the vicinity of the cutter was accurately resolved with values significantly higher than previously reported values. Two clinically relevant zones, high-flow and low-flow, were identified and defined. The high-flow zone (HFZ) had an acceleration of up to 400 m/s2 and reached velocities of up to 1 m/s close to the port for the maximum aspiration setting. The low-flow zone (LFZ) extended approximately 2 to 2.5 mm from the port, or 3.5 to 4 times the diameter of the cutter (during maximum aspiration), with the mean velocity at the edge of the zone reaching 0.01 m/s. The extent of the HFZ was relatively constant with respect to gauge size and increased only marginally with both increased flow and vacuum settings. The LFZ increased in extent with both flow and vacuum. It varied in extent by gauge size with vacuum but was relatively unaltered by gauge size with fixed flow. Cut rate had no clinically relevant effect on the HFZ and LFZ extents and velocity fields.

Using optimized PIV measurements on an experimental in vitro vitrectomy setup, two clearly defined flow zones could be identified around the vitrectomy port. The study defined the shape of the two flow zones in BSS, outlined influencing factors, and correlated fluctuations with gauge, flow, and cut rates. These parameters enhance surgical understanding and provide a benchmark for future design evaluations. Further study using non-Newtonian fluids would be of interest to outline the differences in flow fields.

Understanding the flow patterns around the probe and the effect of key surgical parameters on their extent could assist in optimizing vitrectomy and reducing the risk of iatrogenic retinal trauma.