Abstract
Purpose :
Hypersonic vitrectomy mechanism of action is not fully understood and, to-date, the dependency of frequency on vitreous flow rate has not been reported. We propose a model based on harmonic oscillator’s quadratic force-frequency relation to explain the non-linear increase in vitreous flow rates measured at a higher frequency operation. We also performed a computational fluid dynamics (CFD) simulation and extracted the shear rate at the port’s longitudinal and transverse walls (parallel and perpendicular walls to the vibration direction, respectively) to support this hypothesis.
Methods :
Vitreous flow rates were measured for 30 seconds and normalized using 18 fresh pig eyes in an open-sky technique, using a 25 gauge ultrasonic vitrectomy needle with a tear-drop shaped side port. Two frequencies were compared; 31 kHz and 41 kHz between 50 and 300 mmHg vacuum settings.
CFD simulations: 25 gauge oscillating needle simulated with ANSYS (CFX v19.1) was used to calculate the shear rate at the port walls with the following parameters: No-slip boundary conditions, saline solution parameters (density ρ=1005.3 kg/m^3, viscosity μ=1.02×10^-3 Pa.s), adiabatic and isothermal process assumption, and without Buoyancy effects. The transient timesteps was set to 10^-6 sec.
Results :
Flow measurements:
Increase of ~32% in frequency, to 41kHz, shows 78% higher flow rates (above 50 mmHg vacuum) on average, compared to the same needle at 31kHz.
CFD simulations:
Maximum shear force was achieved at the transverse port walls, with more than five times the shear rate compared to the longitudinal walls. Green: Transverse walls shear rate (upper and bottom walls). Blue: 10 degrees longitudinally angled wall shear rate. Yellow, and magenta: Longitudinal (parallel oscillation to the needle axis) walls at different heights (port’s side walls locations).
Conclusions :
The additional factor contributing to the non-linear increase in vitreous flow rate is the increase in cutting efficiency. This efficiency increase is due to the stronger force applied in each oscillation, arising from the higher frequency. The simulation showed the crucial effect of port geometry on the distribution of shear forces and agrees with the theory of shear mechanics and motion. This approach enforces the hypothesis that the transverse walls oscillations are the major contributor to the shear force.
This is a 2020 ARVO Annual Meeting abstract.