Article Date: 4/1/2009

Dawn of a New Era?

Dawn of a New Era?

Panelists discuss how new capabilities, such as ultra high-speed cutting and duty cycle control, are affecting their surgical approaches.

Kirk H. Packo, MD: Today's discussion focuses on a new platform in vitreous surgery. I would like to move from theory to reality. In addition to discussing our opinions about the concepts of the technology, I would like to share our real-world experiences with it.

The history of vitreous surgery began in the 1950s, but in the modern era, we saw a major transition from Machemer to O'Malley. Machemer introduced the concept of performing closed vitrectomy through the pars plana, and he gave us the vitreous infusion suction cutter (VISC) to accomplish that.1 O'Malley continued the charge of removing vitreous from the eye, and he gave us a new machine, the Ocutome.2 There were major differences between the VISC and the Ocutome. The functions of suction and cutting were separated from infusion; the illumination was separated off axis to improve vitreous illumination; and most notably, the entire system got much smaller, heralding the dawn of the 20-gauge instrument. Are we at a similar point in the history of vitreous surgery? We will explore the relevant features of the new technology to try to answer this question.

One concept that has developed over the past 20 years is how the speed of the cutter affects vitreous removal. Let's begin with a discussion of cutting speeds.

ULTRA HIGH-SPEED CUTTING

Dr. Packo: The maximum cut rate for the previous-generation technology, the Accurus vitrectomy system (Alcon Laboratories Inc., Fort Worth, Texas), was 2500 cuts per minute (cpm) with 20- and 23-gauge systems, and 1500 cpm with the 25-gauge probe. With the new-generation machine, the Constellation (Alcon Laboratories Inc., Fort Worth, Texas), the maximum speed is 5000 cpm for all gauges. The pneumatic probe of the Millennium Vitrectomy Enhancer (MVE) (Bausch & Lomb [B&L], Rochester, NY) has a maximum speed of 2500 cpm, but it is capable of being driven to 5000 cpm, as well (C. Wang, PhD, MID Labs Inc., oral communication, 2009).

With an open-bias duty cycle, is there truly a difference between 2500 cpm and 5000 cpm?

Timothy G. Murray, MD, MBA: I believe the ultra-high cut rate technology is putting 23-gauge and 25-gauge surgery on par with 20-gauge surgery with regard to safety, efficiency and outcomes, particularly when we are manipulating fluids within the eye and cutting vitreous. When dealing with tissues on the retinal surface, I want to be able to use very high cut rates, but I also want to minimize manipulation of the retinal structures. That is where the advantages of 5000 cpm vitrectomy probes coupled with alterations in duty cycle become incredibly important.

"… in the modern era, we saw a major transition from Machemer to O'Malley. … Are we at a similar point in the history of vitreous surgery? We will explore the relevant features of the new technology to try to answer this question."

Kirk H. Packo, MD

IMPACT OF CUTTER SPEED ON FLOW RATE

Dr. Packo: Hypothetically, if you remove vitreous at a rate of 5cc per minute, you can remove it in one cut and you have one 5cc ball of vitreous; or you can remove it in thousands of cuts and you have millions of pieces of vitreous. The difference is what happens to the retina. Pulling out the vitreous in one (or just a few) pieces transmits traction to whatever the vitreous was attached to, namely the retina. Pulling out the vitreous in minute pieces transmits only minute amounts of traction. That is where cut rate comes into play.

"With duty cycle changes, flow characteristic changes and port optimization, I think proportional vacuum with a high cut rate also has a role in doing core vitrectomy or in shaving vitreous near the retina."

Allen C. Ho, MD

With the spring-return pneumatic probe of the Accurus or the Millennium system, duty cycle is variable. As the speed increases, the amount of time the probe is open decreases and so does the flow. Thus, the faster the cutting speed, the slower the flow rate. At 2500 cuts, that is fairly slow, particularly if the probe shaft has a smaller internal diameter. All commercially available spring-return probes, regardless of manufacturer, share this fluidics principle. The Constellation probe is not spring-returned and can allow slightly higher dwell open times at the very high cut rates. Thus far in your experience, were flow rates at 5000 cpm extremely low, or were they comparable to flow rates at 2500 cpm?

Dr. Murray: Based on where my duty cycle was positioned, I felt the flow rates at 5000 cpm were significantly better. To me, the evolutionary step with this technology is that I am not compromising flow while maintaining high cut rates. I think we now can manipulate the tissue of interest without altering collateral tissues.

SURGICAL MODES

Dr. Packo: With the Accurus, we can use proportional cutting or dual dynamic drive (3D) technology to vary the cut rate and the vacuum at the same time. Did you use the 3D mode in the Constellation? Does this mode still have advantages?

Allen C. Ho, MD: I am accustomed to using the 3D mode, and that is what I used with the Ultra Vit cutter with the Constellation Vision System. Using the 3D mode, we can shave vitreous near the retina with low vacuum and high-speed cutting and do efficient core vitrectomy work with high vacuum and slower cut speeds. With duty cycle changes, flow characteristic changes and port optimization, I think proportional vacuum with a high cut rate also has a role in doing core vitrectomy or in shaving vitreous near the retina.

I agree that we are better able to manipulate tissues, which we could not do as readily with older systems, especially for surface retinal work against a stable structure, such as attached retina. These changes in duty cycle, flow control and cutter port optimization also allow us to manipulate tissue and vitreous gel or fluids when we have mobile tissue. The system is not focused solely on cut speed, which, in isolation, I think adds only incrementally, but if you combine that with the other new features, such as aperture change, port optimization and duty cycle change, you get more utility.

Dr. Packo: Dr. Pollack, do you use 3D control with the Accurus?

John S. Pollack, MD: Yes, I use 3D to achieve my desired flow rates during core vitrectomy by reducing the maximum cut rate from 2500 to 1800 and increasing the vacuum to 500. The net result is higher flow levels when the pedal is fully depressed. When I come off the pedal, I return to higher cut rates, lower vacuum and resulting lower flow, which gives me excellent control with a mobile retina.

One of the advantages I found with the Constellation's 5000 cpm cut rate is that I have greater control without sacrificing flow and efficiency. I keep the cut rate constant at 5000 cpm and vary the vacuum. By setting my maximum vacuum to 450 mmHg, with full depression of the pedal, I have access to a high flow rate of 10cc/min with the safety and control of the 5000 cpm constant cut rate. Even at 10 cc/min, the probe produces 500 cuts/cc of vitreous passing through the tip. This is a significant improvement over the Accurus vitrectomy probe, which produces a maximum 200 cuts/cc of vitreous at a flow rate of 9 cc/min. For even greater control and safety with the newer technology, I simply decrease flow by coming off the pedal. As flow decreases the cuts/cc of vitreous passing through the cutter tip increases, reaching 1250 cuts/cc of vitreous at 4 cc/min. This is a doubling of the 625 cuts/cc of vitreous attained by the Accurus at the same flow rate.

Dr. Packo: Dr. Rizzo, do you use the vitrectomy probe primarily to remove tissue on the retinal surface, or do you rely more on scissors?

Stanislao Rizzo, MD: I use the 25-gauge probe for membranectomy because we now can control the vacuum and flow at high cut rates. With the 25-gauge probe, we also have the smallest probe possible near the retina. I was surprised to see how much flow you can have with such a small probe.

I also use the 23- and 25-gauge Millennium systems. The B&L vitrector is a spring-return pneumatic cutter, which was the first cutter with a constant flow also at high cutting speeds. This is because it maintains an efficient duty cycle even at 2500 cpm, which is as high as B&L can ensure.3 Better duty cycle control guarantees small caliber flow values comparable to larger caliber ones.3 The new Alcon 25-gauge vitrector ensures a flow of 6.8 cc/min at 600 mmHg at 2500 cpm compared to the previous 25-gauge vitrector with a flow of 3.5 cc/min at 600 mmHg at 1500 cpm.

We are seeing a new development in vitreous surgery with ever smaller probes and performance that was unimaginable until a few months ago. I believe 25-gauge once again will be the preferred caliber for minimally invasive surgery.

Dr. Packo: Dr. Rizzo, what parameters do you choose when operating on the retinal surface of a diabetic patient?

Dr. Rizzo: I used both the 3D and the proportional vacuum system. With the 3D system, the pedal gives me ultra high-cutting speed and lower vacuum at the start and then, by depressing the pedal more, a gradual decrease of cutting rate and increase of vacuum, thus obtaining different aspiration and cutting parameters without changing the program. When I used proportional vacuum, I kept the same cutting rate and had linear aspiration control on the pedal. With the 25-gauge during peripheral vitrectomy, I prefer the 3D system as I need more cutting or aspiration, depending on the distance from the retinal surface.

Dr. Packo: Dr. Ho, do you still use the momentary mode during surgery?

Dr. Ho: I use the momentary mode in certain situations, for example, for retained lens fragments and in certain cases of proliferative diabetic retinopathy with tabletop detachments. I started using it to engage a cutting action and then stop it quickly so I could use the MPC scissors.

"Better duty cycle control guarantees small caliber flow values comparable to large caliber ones. … We are seeing a new development in vitreous surgery with ever smaller probes and performance that was unimaginable until a few months ago."

Stanislao Rizzo, MD


"Duty cycle is the percentage of time the port is open in a given cut cycle. Being able to control duty cycle is an elegant way for us to change the flow rate without changing the cut rate or the vacuum."

Pravin U. Dugel, MD

NEW LEVEL OF CONTROL: DUTY CYCLE

Dr. Packo: Dr. Dugel, you have spent a great deal of time thinking about and studying duty cycle. Please describe your experience and your findings.

Pravin U. Dugel, MD: Duty cycle is the percentage of time the port is open in a given cut cycle. Being able to control duty cycle is an elegant way for us to change the flow rate without changing the cut rate or the vacuum.

Regarding your original question about whether or not this is a defining moment in vitreous surgery, I believe it is. The crux of that comes from the dual pneumatic line design of the Ultra Vit cutter. I believe we had reached the limit of the spring design as it relates to duty cycle, and the reason for that is fairly clear. Although spring-return probes can and have been driven to rates of 5000 cpm in the lab, the commercially available probes have maxed out at 2500 cpm. Driving them past that rate simply reduced the flow too much. (See "Spring-design vs Pneumatic Probes.")

Dr. Packo: The relationship of probe cut rate to flow depends on the style of duty cycle the system provides. High cut rates with the Accurus and the Millennium give us small pieces of vitreous and lower flow. As surgeons, we are conditioned to equate low flow with safety on the retinal surface. The Millennium system offers us a choice of either a spring-return pneumatic probe, the Millennium Vitrectomy Enhancer, or a motor-driven electric probe, the Lightning. Unlike pneumatic probes, the Millennium Lightning electric probe operates at a 50% constant duty cycle that matches the on-time and off-time. In reality, the cycle is slightly less than 50%, but it is constant, regardless of the cut rate. Thus, the flow with the Millennium electric probe is always constant, even at high speeds. As soon as you pass that point, however, and get into a priority closed duty cycle, which is what the shave mode gives us, the relationships switch, and the faster we go, the higher our flow will be.

When using the Constellation, what were your settings in the shave mode? What was the lowest cut rate you tried?

"When I started performing vitreous surgery in my fellowship, a cut rate of 600 cpm was blazingly fast, and now we are considering 3000 cpm potentially slow, but there's no reason to stop there. … We may want to explore the effects of extremely low cut rates and biased closed duty cycle, since this would give us very low flow."

Kirk H. Packo, MD

Dr. Dugel: If we are operating close to the retina, and especially if the retina is detached, we want to decrease the flow, but we do not want to compromise our cut rate. So in shave mode, I will keep my cut rate as fast as possible, which is 5000 cpm. I will have a closed-bias duty cycle, which will minimize my flow, and my vacuum will remain at 400 to 450. In core vitrectomy mode, I will have an open-bias duty cycle, which will increase my flow. At that point, I will be comfortable with my cut rate at 3000 cpm or 4000 cpm and my vacuum will stay at 400 to 450.

Dr. Packo: This is where I think our conditioning is still getting in our way. In shave mode, you still want your cut rate high?

Dr. Dugel: That is correct.

Dr. Ho: As we gain experience with this new technology, I think we will find some of the concepts are counterintuitive because, as Dr. Packo says, we have been conditioned otherwise. For example, in actuality, you can have very fine shaving with a very low cut rate if you bias your port to a more closed position. I can understand that conceptually, but at this time, it does not resonate with me because for years, we have been told that faster cutting probes are better, but for years, we have never been able to create a closed-bias port.

We must gain more experience and learn the capabilities of the dual pneumatic cutter with port optimization and a variety of other functions that we can control. Not only can we control functions on the side of the cutter, but there are forces that change tissue response from the in-flow as well.

Dr. Dugel: We now have three factors to consider: cutting rate, vacuum and duty cycle. I have no doubt that in a few years, we will consider duty cycle as important as vacuum and cutting rate.

One advantage of duty cycle control, which I did not anticipate, was increased followability of tissue. In my early cases using the Constellation, I had thick membranes, and in one case, I also had retained lens material. I had no problems having that material follow into my port in an open-bias mode.

We also must be careful to define what we mean by lower cut rates. We are not talking about 400 cpm or 800 cpm any more. We are talking about cut rates of 3000 cpm or 4000 cpm, where we have a great deal of duty cycle play. We may need to refer to high and ultra high cut rates, rather than saying cut rates of 3000 cpm to 4000 cpm are low.

Dr. Packo: When I started performing vitreous surgery in my fellowship, a cut rate of 600 cpm was blazingly fast, and now we are considering 3000 cpm potentially slow, but there's no reason to stop there. Now that we have the technology to do so, we may want to explore the effects of extremely low cut rates and biased closed duty cycle, since this would give us very low flow. Whether or not this is more desirable than achieving low flow with high cut rates and an open-biased duty cycle remains to be seen. The new technology allows us to do either.

"We had to devise parameters to address limitations in the technology of previous platforms. The recently introduced technology does not appear to be limiting us. We now will be exploring ways to approach duty cycle that affect flow, and we will have to look conceptually at cut rates in a unique way."

Timothy G. Murray, MD, MBA


Spring-design vs Pneumatic Probes
With a spring-design probe, two contradictory forces must be matched. The spring must be weak enough for the diaphragm to push it and close the port, but strong enough to continue to open the port properly. If the spring force is too weak, the air valve will open, the pulse of air will push on the diaphragm and close the port, but the spring will not push back enough to allow it to open fully. On the other hand, if the spring force is too strong, you will have difficulty closing the port because it will not push enough against the diaphragm and allow the port to close. Either way, the cut will be mismatched. At a very high speed, that will be a poor cut and possibly a dangerous cut.
As far as duty cycle is concerned, as you increase cut rate in the single air line system, a pulse of air enters and pressure builds up enough to allow the diaphragm to close. There is a process whereby the air pressure needs to be vented, and the venting does not occur instantaneously. As you increase the cut rate, there is less venting time and the duty cycle decreases. That is the reason why, when we are cutting at 2500 cpm with the Accurus, for example, our duty cycle is somewhere between 30% and 40%, depending on the gauge we are using. The Ultra Vit probe has two air lines, eliminating the need for venting and the resultant drop in duty cycle, and basically, this is because there is no spring in that cutter. The dual pneumatic line cutter is a revolutionary new design.

Pravin U. Dugel, MD

REDISCOVERY

Dr. Murray: It is interesting that we had to devise parameters to address limitations in the technology of previous platforms. The recently introduced technology does not appear to be limiting us. We now will be exploring ways to approach duty cycle that affect flow, and we will have to look conceptually at cut rates in a unique way. To me, it is exciting to have the technology that allows us to determine what we want to do, rather than having the technology dictate what we are able to do.

Dr. Packo: This is an exciting time of rediscovery. Over the next few years, we all will learn from our own experience and from each other. Because some of these new concepts are subtle, however, we will need time to determine what settings will be the best for us. We need to keep learning. RP

References
  1. Machemer R, Buettner H, Parel JM. Vitrectomy, a pars plana approach. Instrumentation. Mod Probl Ophthalmol. 1972;10:172-177.
  2. O'Malley C, Heintz RM. Vitrectomy via the pars plana–a new instrument system. Trans Pac Coast Oto-Ophthalmol Soc Annu Meet. 1972;53:121-137.
  3. Fang SY, DeBoer CM, Humayun MS. Performance analysis of new-generation vitreous cutters. Graefes Arch Clin Exp Ophthalmol. 2008;246:61-67.


Retinal Physician, Issue: April 2009