Any discussion of vitreoretinal surgery must include fluidics, specifically how the integrity of the eye is maintained while fluid flows through it during complex surgical maneuvers. With the introduction of ever smaller gauge instrumentation, the role of fluidics has become critical for safe and efficient surgeries.
“The most significant advances we have had over the last decade have been not only the implementation of smaller gauge vitrectomy instrumentation but also the engineering advances that have allowed for progressively improved control of fluidics,” says Yannek Leiderman, MD, PhD, Chicago. “Compared with previous generations of vitrectomy systems, we now have vastly improved abilities to precisely control the rate of fluid flow in the eye. Some platforms do that by modulating the vacuum and some by directly modulating flow.”
The Enhancing Visual Acuity (EVA) surgical system with VacuFlow valve timing intelligence (VTi) pump system (Dutch Ophthalmic Research Center) simulates the characteristics of either Venturi or peristaltic pumps, enabling surgeons to adjust settings to achieve either vacuum (Venturi) or flow (peristaltic) control. By combining the functionality of two types of pumps in one machine, surgeons can transition seamlessly between vacuum and flow mode without changing pumps, optimizing the fluidics according to each surgeon’s preference and surgical demands.
Fast Fluidics Facts
Fluidics refers to the properties of fluid flow, and applied fluidics in engineering usually refers to fluid flow in closed systems, Dr. Leiderman explains. In the context of vitreoretinal surgery, fluidics refers to the flow rate — how quickly surgeons can remove and replace fluid and other tissues in the eye — and the differential, meaning how quickly surgeons can start and stop fluid flow.
“In vitreoretinal surgery, we are always trying to balance the efficiency of removing vitreous while not disturbing the neurosensory tissues and other delicate structures that are intimately associated with the vitreous,” Dr. Leiderman says. “We aspirate in a controlled fashion, and we must be able to ‘apply the brake’ as close to instantaneously as possible.”
Dr. Leiderman notes a challenge all surgeons encounter frequently during retinal detachment repair is mobile retina, which is subject to turbulence created during surgery.
“When we are removing vitreous, aspirating with our instrument and simultaneously replacing fluid, turbulence is induced, causing the retina to flap around like a sail in the wind,” he says. “Ideally, what we want in terms of fluidics is a system that creates the least amount of turbulence in the eye, while maintaining flow at the probe port sufficient for efficient removal of vitreous.”
Versatility in the OR
Valve timing intelligence technology (Figure 1), a series of sensitive computer-controlled operating pistons and closure valves, is incorporated into the EVA surgical system and offers 2 distinctly different methods for surgeons to manage fluidics: flow control, whereby the fluid movement itself controls the amount of vacuum that is generated at the instrument tip, and vacuum control, whereby the flow is driven by vacuum.
“Vacuum control will make quick work of a vitrectomy, but if the tip of the instrument occludes, the vacuum builds up instantaneously so that the tissue is drawn into the port with great force,” says Colin A. McCannel, MD, Los Angeles. “Flow control is a slower, more controlled way of removing fluid from the eye. It can function at very low flow rates and still have aspiration.
“These two different fluidic controls offer different advantages in different situations,” Dr. McCannel continues. “Dutch Ophthalmic recognized that and built a machine that has both. Physicians who want to optimize surgery can use vacuum control for the portions of the surgery where vacuum control is most efficient, such as core vitrectomy, and they can switch to flow control when the instrument is getting closer to the retina, such as while shaving or trimming vitreous at a retinal break or working at the edge of a retinectomy. Working in flow control mode, we avoid the surges and turbulence that are possible in vacuum mode. The flow control is a gentle vacuum, and if tissue occludes the tip, you can quickly take your foot off the pedal and nothing bad happens. Whereas in the vacuum control situation, if the tissue occludes the tip, you almost always end up with a hole in the retina in that location.”
Benefits of Flow Control
Most U.S. surgeons are accustomed to using vitrectomy systems with vacuum-controlled fluidics, which have dominated the market here, while the trend among European vitrectomy machine manufacturers seems to be toward flow-controlled systems, Dr. McCannel says. As U.S. surgeons become more familiar with flow control, they are coming to appreciate its utility.
Brian C. Joondeph, MD, Denver, now performs flow-based vitrectomy exclusively. “The EVA is the only system that has a flow-based aspiration system, which is a big advantage, particularly when working near detached retina,” Dr. Joondeph says. “While I am aspirating vitreous, blood, and saline, the flow is constant. I do not experience sudden surges of fluid into the eye when I encounter less resistance, as is the case with vacuum mode.
“Other vitrectomy systems allow me to adjust the duty cycle, which helps somewhat, but still perpetuates the challenges and disadvantages of vacuum-based vitrectomy. I prefer the flow-based control, because the amount of material flowing into the cutter is the same, regardless of what the material is. That makes it safer to work near detached retina, which is quite mobile, and if one is not careful, it is easy to engage retina in the cutter.
“Although I have the option of using vacuum-based control with the EVA system, I never use it, because the flow control works so well. It is predictable, reliable, and efficient,” Dr. Joondeph says.
Surgeons consistently mention efficiency and safety when discussing the benefits of the EVA system.
“In flow mode, as opposed to vacuum mode, we can precisely control what we remove from the eye,” says Gaurav K. Shah, MD, St. Louis. “There is no danger of the eye collapsing, because the machine senses the pressure and automatically adjusts the flow to compensate. This is an important safety feature, and the manner in which it functions is unique to the EVA system. Safety leads to better outcomes and more efficient surgeries.”
Asheesh Tewari, MD, Ann Arbor, MI, also appreciates the level of safety afforded by flow control. “VacuFlow technology controls flow so well that it allows me to work very close to the surface of the retina to perform procedures such as vitreous shaving,” he says.
High-flow Infusion Line Adds Value to EVA System
Dutch Ophthalmic Research Center recently introduced an over-the-cannula, high-flow infusion line designed to manage the increased flow generated by the 2-dimensional cutting (TDC) and 92% open-biased duty cycle that are hallmarks of the EVA surgical system. It is available in 23-, 25-, and 27-gauge.
“This is another one of those inventions where you say, ‘Why didn’t I think of that?’” says Asheesh Tewari, MD, Ann Arbor, MI. “The high-flow infusion line works in tandem with the TDC vitrectomy probe. If you are cutting in both directions at up to 16,000 cuts per minute and your port is open 92% of the time, you are aspirating a fair volume of fluid. The high-flow silicone infusion line delivers fluid into the eye at an appropriate rate without it tapering down into a smaller cone of fluid, which happens with traditional infusion lines.”
Colin A. McCannel, MD, Los Angeles, considers the high-flow infusion line another positive step in the evolution of the Dutch technology. “I am particularly fond of this advancement,” he says. “Typically in small-gauge vitrectomy, the infusion cannula fits inside the trocar cannula, occupying more lumen and restricting flow at the tip of the infusion line. With the high-flow infusion line, the tubing covers the trocar cannula — I call it a ‘hoodie’ — so that the full diameter is available for fluid to flow into the eye.
The utility of the high-flow infusion line becomes immediately apparent during a typical procedure.
“Traditionally, when you are aspirating fluid at maximum vacuum during core vitrectomy, you almost always outstrip the inflow when working at a constant pressure,” Dr. McCannel says. “The EVA system has flow compensation, whereby the pressure automatically increases to compensate for that difference. With the new high-flow tubing, however, you can stay at the same pressure and the infusion will keep up at that same pressure without causing hypotony and scleral in-folding, even at very high vacuum levels. Amazing.
“With the traditional cannula placed inside the trocar cannula, a narrow stream of fluid shoots out at a high pressure,” Dr. McCannel says. “With the high-infusion cannula, a larger bore delivers fluid into the eye at an appropriate rate without this very narrow jet shooting in, and it keeps the eye inflated constantly regardless of how much aspiration is being used during vitrectomy. It’s a way to maintain intraocular volume without having a high-pressure fluid jet shooting into the eye and causing who knows what problems in some cases. For instance, in the past it has been speculated that temporal visual field deficits following vitrectomy may be due, at least in part, to nerve fiber layer trauma from fluid jets hitting the retina near the optic nerve head.”
Renewed Interest in 27-gauge
Dr. Tewari credits the EVA system, specifically the VacuFlow VTi pump, along with the 2-dimensional cutting (TDC) technology developed by DORC for increasing interest in 27-gauge vitrectomy.
“One of the early criticisms of small-gauge vitrectomy was that it was too slow, and the flow was inefficient,” Dr. Tewari says. “That is one of the reasons why many surgeons did not jump on the 27-gauge bandwagon. But with the EVA machine, we can use a 27-gauge TDC vitrectomy probe — with a maximum cut rate of 16,000 cuts per minute and an open port 92% of the time — and it is aspirating all of the time. When I use the TDC probe for 27-gauge surgery, the fluidics feels more like 23- or 25-gauge surgery than 27-gauge surgery. If not for the TDC vitrectomy probe and the fluidics of this system, I would not be interested in 27-gauge surgery.”
Summary
The idea of a surgical system with adaptable fluidics, one that combines a peristaltic-type pump for flow control with a Venturi vacuum pump, is a new concept in vitreoretinal surgery.
“The EVA system is predicated on peristaltic control, but the engineers have developed a novel way of inducing and controlling the flow of fluid,” Dr. Leiderman says. “A number of factors influence vitreous traction and turbulence during vitrectomy. Cutting rate, the frequency at which the guillotine blade is opening and closing on the cutter, is an important factor. The geometry of the port, the size as well as the shape of the opening in the vitrectomy probe needle, and duty cycle, the tempo of the opening and closing of the blade, all affect the dynamics of retina and vitreous during surgery.
EVA BENEFITS
- EVA VTi fluidics is the ideal partner for the 2-dimensional cutting technology
- Enhanced control with three times faster vacuum rise time than traditional Venturi
- Designed to reduce the traditional pulsatile characteristics of peristaltic systems
- TDC + 27-gauge vitrectomy delivers the performance of 25-gauge vitrectomy with all of the outcome benefits of minimally invasive 27-gauge surgery.
“As surgeons performing complex cases, it is advantageous for us to minimize the motion of retina and other tissues in the eye as we perform vitrectomy,” Dr. Leiderman says.
According to Dr. McCannel, the versatility of the EVA system creates opportunities for surgeons to tackle complex cases, knowing that they can adapt the fluidics to each individual case as well as their preferences.
“An increasing number of surgeons are recognizing the advantage of using the different fluidics for different situations, and the EVA machine makes this available to us, enabling us to choose the fluidic parameters that seem to be the safest and most efficacious for a given portion of the surgery,” Dr. McCannel says. “For me, that would be vacuum control for core vitrectomy and bulk vitreous removal. When I get close to the retina and I am dissecting vitreous from the retinal surface or performing a retinectomy, I value the opportunity to use flow control, because I find I have far more control than I do with the vacuum control in those situations.” ■
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