State-of-the-Art Vitreous Cutting and Fluidics

With a top cutting rate of 5000 cpm, the Stellaris PC is optimized for efficiency, functionality and reliability.

By Tarek Hassan, MD

When we evaluate vitreoretinal surgical systems, what we're really doing is evaluating the four basic functions that are common to all vitrectomies, which are cutting, aspirating, infusing and illuminating. Among those functions, the fluidics, i.e., the ability to cut and aspirate, are the core competencies of any system. Taking that a step further, the aspects of fluidics important to surgeons are the comfort and ergonomics of the cutter, the cut speed and efficiency and the functionality and stability of the system. If all of those aspects work together well, we have a safe machine that is, importantly, both successful and ultimately reliable. The Stellaris PC, the new combined vitrectomy and phacoemulsification system from Bausch + Lomb, excels in these key areas.

COMFORTABLE AND ERGONOMIC

The Stellaris PC vitreous cutter is of a familiar size and shape. It has a visual and tactile port locator, which is easy to feel. The over-molded rubber grip is very comfortable, rotates easily in the hand, and is color-coded according to gauge. The cutter is ergonomic in its handling and ability to move inside the eye. It comes with an extension handle for surgeons who prefer a larger handpiece, though it is extension is removable for those who do not.

FASTER IS BETTER

A question that often arises in regard to vitreous cutters is whether the faster cutting speeds available in today's systems really offer an advantage over those in prior systems. The answer is unequivocally yes. Higher cutting speeds lead to better flow and more efficient vitrectomy—and this ultimately, translates into safer vitrectomy. This was shown years ago when the Lightning cutter from Bausch + Lomb was working at a much faster speed than others on the market at the time.

The Stellaris PC has a 5000-cpm probe. It can be set to cut at a fixed rate of 5000 cpm at all levels of aspiration, which I personally prefer. Alternatively, it can be set so that low suction corresponds to a high cut rate and higher suction corresponds to a lower cut rate. Using flow rate curves generated in vitreous, we've seen how higher speed cutting increases efficiency. Flow rate increases as cut rate increases. Some of the earlier work with flow rate curves was done in water; therefore, people naturally thought that increasing cut rate decreased flow because the port is closed so much more of the time at higher cut speeds. More recently, as tests have been performed in vitreous fluid, it has become clear that flow rates actually increase as we reach 5000 cpm. So, with increasing cut speed, flow increases in vitreous and decreases in water/fluid. Given that, at the highest cut speeds such as 5000 cpm, our vitrectomy is safer and the eye more stable as we move the cutter back and forth in the eye between pockets of vitreous and fluid. Flow rates aren't changing as dramatically as we move our cutter through different phases within the vitreous cavity.

This is the case because at higher cut speeds we are taking many smaller bites of vitreous which thus makes vitreous act more like a low viscosity fluid that has better laminar flow. Nonlaminar flow, which is seen with large vitreous bites, disrupts the movement of material through the tubing. Large bites drag on the inner lumen of the vitreous cutter. On the other hand, with small bites of vitreous that are chewed up, we get laminar flow with near zero velocity at the wall and increasing flow toward the middle of the lumen. Vitreous flow rate curves show that at the highest cutting speeds there is near equal flow through 25- 23- and 20-gauge Stellaris PC cutters.

When video of procedures utilizing the Stellaris PC cutter is viewed side-by-side with video of procedures with a competing product that cuts at 2500 cpm, we can see vitreous literally being pulled from across the eye to the Stellaris probe because its cutting characteristics and flow are so much better. This is also seen when the Stellaris PC cutter is compared with an earlier Bausch + Lomb cutter that works at 2500 cpm. Both have the same port size and similar duty cycles, yet the Stellaris PC obviously pulls vitreous preferentially to it.

It's important to point out that the Stellaris PC duty cycle is optimized for safety and performance throughout the procedure. It's preset duty cycle ensures that the open time is optimized for all cutting speeds. Regardless of cutting speed, the port is never open less than 5% of the time. Aspiration, therefore, is controlled with the foot pedal. The pedal is used like a car accelerator to vary the vacuum, which allows us to intuitively manage the flow. The surgeon can focus on the patient without worrying whether the duty cycle must be changed or is set correctly.

CUTTER DESIGN ENHANCES FUNCTIONALITY

In addition to its ultra-high speed, the Stellaris PC cutter is optimized in other ways that improve its functionality. The port has been moved farther down toward the end of the probe. It is now the same length from the tip as is seen in the Constellation® Vision System from Alcon. However, the Stellaris PC port has a larger surface area than the Constellation port, which also improves how the cutter functions. The difference has been apparent to surgeons in their early comparative experiences with the Stellaris PC.

Laboratory testing has shown that the combination of the cutter design and high cut speed reduces traction in the eye. The vitreous acts more like water, which means there's less pulling on the retina when vitreous is aspirated and cut. Turbulence is reduced at the high cut rates as well, again, because smaller bites of vitreous are moving through the tubing. When we look at varying levels of aspiration, we see the turbulence curves almost converging as the traction continues to decrease as cut speeds approach 5000 cpm. Furthermore, we know if we operate with less traction and the highest flow rate, we have the greatest level of safety for the eye.

ACCURATE AND PREDICTABLE VACUUM

Previously I mentioned how vacuum, the second key aspect of fluidics, is surgeon-controlled on the Stellaris PC via the foot pedal. An advanced algorithm drives the vacuum in this system. It is so accurate and predictable that it can discern changes in vacuum pressure down to 2 mmHg. It is a robust and responsive system, able to go from zero to 600 mmHg in 1.5 seconds. There is a second vacuum line dedicated to ancillary use, so the inline stop-cock has been removed in this new system, and that is an advantage. We can run aspiration from the soft tip and from the cutter by simply pressing a button, which is ideal.

SENSITIVE AND RESPONSIVE VENTED, AIR-FORCED INFUSION

Another feature that contributes to the excellent fluidics of the Stellaris PC is the use of vented air-forced infusion. This technology is as good and as time-proven as gravity-based infusion for ensuring stable, predictable fluidics throughout the vitrectomy. We all know about using a pressurized bottle and how that is more sensitive and more responsive to surgeons’ control of intraocular pressure than an automated IV pole. Vented air forced infusion actually can change pressures very quickly. It can go from 60 mmHg to 40 mmHg in 0.5 seconds. It has an independent pump that makes any type of infusion failure that could harm the eye highly unlikely. The surgeon is able to predictably control the intraocular pressure, even turning infusion off from the foot switch, without concern about the system trying to take over. Pressure control remains in the hands of the surgeon.

THE SURGEON'S CHOICE

Given the fluidics parameters of the Stellaris PC, including its ultra-high-speed cutting capability, vitreoretinal surgeons have the safety and effectiveness—and ultimately, the reliability—we need to move to the next level of advanced vitreoretinal surgery.

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Dr. Hassan is a partner at Associated Retinal Consultants in Royal Oak, Michigan. He is also clinical assistant professor of Biomedical Sciences at Oakland University in Rochester, Mich.