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In this article, the authors present the highlights of their research into a new blade design for pars plana vitrectomy.
In fact, the blade progressively obstructs the entire cutting port at every cutting action, zeroing flow at the end of each cutting cycle. The variability of open port surface generates flow instability and continuous fluid acceleration and deceleration, leading to limited efficiency and retinal traction.1
We have been studying the fluidics of guillotine blade vitreous cutters by means of a technique named 'particle image velocimetry',1 showing that both Venturi and peristaltic pump equipped instruments determine macroscopic fluid acceleration which, in turn, results in retinal traction. The road towards a safer removal of the vitreous jelly passes through a better comprehension of its fluidics and the goal is to reduce drastically fluid velocity changes.
To partially overcome this problem, most available machines are used at increasingly higher aspiration values, as cut rate increases, because this is virtually the only way to aspirate enough vitreous through the port within the shorter available time window.
Available pneumatic valves, used to close the blade (and in some vitrectomy machines, also to open it up), take 0.03 seconds, while strains used to open the port need a little more time: 0.07 seconds. This time is 'incompressible', so that when cut rate is increased, this is done at the expense of 'open port' time because 'port opening' and 'port closing' times remain fixed and can not really be modified for technical reasons while the 'closed port' time is set to zero at higher cut rates.
Overall, opening and closing the blade takes between 6 and 10 thousands of a second, so if we make the 'closed port' time zero seconds, the theoretical maximum cut rate per minute with available valves is between 6000 (= 60 s/0.01 s) and 8571 (= 60 s/0.007).