Article Date: 11/1/2008

Cutting on the Edge of Progress

Cutting on the Edge of Progress

Experts discuss advances in high-speed cutting, duty cycle control, MIVS technologies and other innovations in vitreoretinal surgery.

Stanley Chang, MD: Today, we've assembled a panel of international experts in vitreoretinal surgery whose experiences span decades and thousands of cases. These outstanding surgeons have contributed to the development of many of the surgical and pharmacologic advances we currently use.

The vitreoretinal surgery system has evolved from an instrument that simply cuts vitreous into a sophisticated machine with integrated multifunctional capabilities. These features include vitreous cutting, illumination, fluid and gas management, laser photocoagulation and adjunctive instruments such as scissors and forceps. The next generation surgical system will optimize the performance and safety of each of these components.

Our expert panel has had a glimpse of the next generation vitreoretinal surgical system — the Constellation Vision System (Alcon Laboratories, Inc.). We will discuss key new features, advances in high-speed and micro-incisional vitreous cutting, fluidics and intraocular pressure (IOP) control, illumination and laser photocoagulation that soon will be available.

HIGH-SPEED CUTTING

Dr. Chang: We have all used vitreous cutting probes with speeds ranging from 400 cuts per minute initially to 2500 cuts per minute. Future options for substantially higher cut speeds will increase the rate to 5000 cuts per minute or above. Dr. McCuen, could you describe the advantages in high-speed cutting?

Brooks McCuen, MD: The higher the cutting speed, the lower the traction that is transmitted to the retina,1 allowing you to work closer to the mobile retina or in the vitreous base area. For instance, 2500 cpm is better than 1200 cpm, which is better than 400 cpm. I believe 5000 cpm is better than 2500 cpm.

The vitreoretinal surgery system has evolved from an instrument that simply cuts vitreous into a sophisticated machine with integrated multifunctional capabilities.

Stanley Chang, MD

Dr. Chang: What speeds are you currently using?

Dr. McCuen: Usually 2500 cpm, especially when I'm working near mobile retina or in the vitreous base. I increase the speed to the maximum level.

Dr. Chang: Do you prefer the proportional setting or the 3D setting?

Dr. McCuen: I prefer the proportional setting. I learned to use it as a young surgeon and, over the years, I have become very comfortable with the feel of that setting.

Dr. Chang: Let's talk about cutting speeds and settings. I frequently use the 3D setting. I like the ability to control speed when I am working close to the retina. I also prefer having a lower vacuum and cutting at a faster rate. If I am simply debulking the vitreous, I may increase the suction. Dr. Flynn, what are your thoughts on this?

Harry Flynn, MD: I like the 3D, but I think both settings are excellent. I have grown up with the Accurus, using the 3D mode when cutting over mobile retina.

Dr. Chang: Dr. Charles, you like proportional cutting. Can you tell us why you prefer it over 3D?

Steve Charles, MD: There are two important reasons to use high cutting rates, in my opinion. One is to produce port-based flow limiting. The other is to reduce uncut vitreous from traveling through the port, and this is particularly crucial as you move the probe quickly around the vitreous cavity. I always use proportional vacuum and the highest cutting rate.

By controlling duty cycle and using speeds up to 5000 cpm, [the new machine] will deliver more precision and will prevent unnoticed or inadvertent traction to the peripheral retina.

Professor Yasuo Tano

Kirk Packo, MD: The high cutting rate has evolved over the past 20 years. As Dr. McCuen pointed out, you want to increase speed for safety reasons. Whether you are using proportional or 3D, I think we are using these modalities to overcome some of the disadvantages in the existing high-speed technology. The disadvantages cause flow to drop.

You could say that lower flow provides greater safety, which is true, but there is a diminishing return here. If we had the technology to increase to 20,000 cpm, the flow would be very slow unless you were controlling duty cycle. We have evolved from 400 cpm to 800 cpm to 1500 cpm to 2500 cpm — and now to 5000 cpm. For safety reasons, we want to keep cut speed high, but we need new technology to help us control flow when working at high speeds.

DUTY CYCLE CONTROL

Dr. Chang: The Constellation is making the Duty Cycle Control (percent time that the port is open during a complete cutting cycle) concept a reality. It offers more than one setting, including variable biased open, variable biased closed and a 50/50 option. How will this change cut speed and vacuum settings?

Dr. Packo: The biggest advantages are that we can maintain high cut speeds, avoiding the threat of uncut vitreous getting in the way, and we also can manage flow. We can decrease the flow, using the biased closed-duty cycle. Or we can increase the flow, using an open-biased duty cycle and still cut at the same rate. We can stay at 5000 cpm throughout a case and remove the vitreous in the core area by using a high duty cycle with excellent flow. On the retinal surface, we can stay at 2500 cpm or 5000 cpm with a low duty cycle and lower managed flow. This is a whole new concept in controlling flow, and it adds that next degree of safety and control for the surgeon.

Dr. Chang: The smoothness of the cutting action also is important. Has anyone noticed that the new cutter on the Constellation seems to vibrate less than spring-driven cutters?

George Williams, MD: I noticed that the device feels much more stable in my hand. I have found that it allows very precise cuts and that I am literally able to shave down tissue to a very fine tolerance. I think this represents a breakthrough in precision for cases that require meticulous dissection. This capability presents an opportunity to take things to the next level in terms of our ability to manipulate the tissue.

Dr. Chang: Professor Tano, what were your impressions about the higher cut speed and the variable duty cycle in the Constellation?

Professor Yasuo Tano: By controlling duty cycle and using speeds up to 5000 cpm, it will deliver more precision and will prevent unnoticed or inadvertent traction to the peripheral retina.

Dr. Charles: The port is closer to the tip on every gauge — 25, 23 and 20 — and that is an additional, crucial advantage. If you are going to be close to the retina, then you must use the highest cutting rates so that port-based flow limiting will reduce the chances of iatrogenic retinal breaks.

Dr. Chang: Dr. Corscostegui, we noticed some improvements in the 25-gauge cutter — the port is closer to the end of the probe and the probe has been made stiffer. Because of these types of improvements, will you use 25-gauge more often when the Constellation is available?

Borja Corcostegui, MD: I think the new Constellation will offer advantages for those using 25-gauge instruments. Maybe with this new device, I will try using 25-gauge again. I am encouraged by the port being larger and closer to the end of the probe. The increased stiffness of the probe and the increased efficacy of the vitreous cutter also add value to this new machine.

Dr. Chang: I hope this technology will allow us to move to 25-gauge more readily in all cases.

Dr. Packo: If you think about it, 23-gauge evolved because of dissatisfaction with 25-gauge. The early problems associated with 25-gauge, such as flexibility and lower flow, were addressed with the creation of 23-gauge. However, the new and improved 25+ gauge provides another alternative to consider. Why not make a smaller hole if a new generation 25-gauge probe is stiffer, the flow is better and you are able to work on the retinal surface? I think you are right — it does provide a rationale for trying 25-gauge again.

ASPIRATION FLOW LIMIT

Dr. Chang: Let's move ahead. Aspiration flow limit is a new technology that has been used frequently in anterior segment surgery and will be available on the Constellation Vision System. Dr. Charles, could you talk about the advantages of this feature and why it was added to the Constellation?

Dr. Charles: Yes, this application is different from the anterior segment use in the sense that it actually measures flow. In the anterior segment systems, peristaltic pumps control the rate that the pump rotates. Remember that the word peristaltic means "pulsatile," and that is not what you want. You want non-pulsatile vacuum. The Constellation controls flow better than any phaco system.

Flow control limits the flow through the port just like high-speed cutting, duty cycle control and small diameters do. The point is that no matter what you do with vacuum — let's say you used the lowest cutting rate and the highest vacuum, which is not a good idea — you still would have this flow limit capability. As a result, you will avoid a sudden burst of fluid flow through the port.

I think this feature is complementary to variable duty cycle and high speed cutting. It is not a separate feature. It is part of the whole story of advanced fluidics. We want a continuous state of fluidic stability to prevent pulsatile traction on detached or attached retina.

Imagine a surgeon removing a dislocated nucleus with a 20- or 23-gauge fragmatome, 25-gauge infusion and a high vacuum level. Aspiration Flow Limit technology will help prevent a sudden decrease in IOP after occlusion break.

Aspiration Flow Limit is also valuable when we are working near the retinal surface in the presence of a mobile giant break and we do not want the edge of the giant break to enter the port. There are always two considerations:

• We want steady-state fluidic stability to avoid pulsatile vitreoretinal traction.

• We want to avoid the issue of sudden elastic deformation of dense epiretinal membrane moving through the port when we are performing conformal cutting delamination. Flow limit addresses this need.

Dr. McCuen: We had the opportunity to experiment with some peristaltic pumps some years ago in vitrectomy systems and it proved unsatisfactory in the lab and in the operating room because of issues with residual suction and the other issues you just mentioned, Dr. Charles.

Professor Tano: What if you used it in the pediatric eye? Does it merit changes in flow because of very confined space?

Dr. Charles: In pediatric eyes, the challenge is not the confined space as much as the characteristics of Stage 4 retinopathy of prematurity (ROP), with attached vitreous and a thin mobile retina.2 The pre-equatorial retina is very thin and attenuated, presenting a high risk for a disastrous retinal break.3 Flow limiting is extremely helpful in these cases.

The flow limit is a feature that mirrors the way many other aspects of technology have come about. You want a machine that reacts better than the human body. … Flow limit is a great way to let the machine think for us, and to react more quickly than we, as humans, can react.

Kirk Packo, MD

Dr. Packo: The flow limit is a feature that mirrors the way many other aspects of technology have come about. You want a machine that reacts better than the human body. Currently, we can see when the retina suddenly starts surging in or the eye starts collapsing, and we react by taking our foot off the pedal quickly. That is how we have been dealing with flow limitation in the past, and it is only as good as our eye-to-foot coordination time. Just like automotive technology advances led to anti-lock brakes that keep our cars from skidding out of control, we now have a machine that does it better than we can do it ourselves. Flow limit is a great way to let the machine think for us, and to react more quickly than we, as humans, can react.

Dr. Charles: Human reaction time is 400 milliseconds; the machine reacts in less than 20 milliseconds and port-based flow limiting is instantaneous. RP

References
  1. Fang SY, Deboer CM, Humayun MS. Performance analysis of new-generation vitreous cutters. Graefes Arch Clin Exp Ophthalmol. 2008;246:61-67.
  2. Shah PK, Narendran V, Kalpana N, Tawansy KA. Anatomical and visual outcome of stages 4 and 5 retinopathy of prematurity. Eye. Aug. 3, 2007. (Epub ahead of print).
  3. Repka MX, Tung B, Good WV, et al. Outcome of eyes developing retinal detachment during the Early Treatment for Retinopathy of Prematurity Study (ETROP). Arch Ophthalmol. 2006;124:24-30.


Retinal Physician, Issue: November 2008