Advanced Vitreous Cutter Offers Duty Cycle Control
Unique design gives surgeons an important new parameter to manage.
PETER K. KAISER, MD
Until now, retinal surgeons could manage flow through a vitrectomy system by controlling 3 surgical parameters: infusion pressure, cutting speed, and aspiration. For the most part, a surgeon could increase flow by decreasing cut rates or increasing aspiration rates. This has changed with the new CONSTELLATION® Vision System (Alcon Laboratories Inc., Fort Worth, TX), which offers substantial improvements in these areas, but also offers control over a very important new parameter; namely, duty cycle.
Duty cycle is an engineering term, usually expressed as a percentage, that describes the ratio of time that a device or component is operating against the total cycle time.
The duty cycle of a vitrectomy probe indicates the percentage of time the port is open measured against the total time of the cut cycle. All other existing vitrectomy cutters, whether they are pneumatic or electric, have a duty cycle that cannot be adjusted. Some cutters have duty cycles that favor a more open port, while some favor a more closed port. Only the CONSTELLATION® Vision System has a duty cycle that can be adjusted to the needs and actions of the surgeon. The added ability to vary duty cycle, as well as cut speed and aspiration rate, has an enormous impact on the safety and efficacy of the new vitrectomy probes and will likely improve our ability to treat vitreoretinal pathologies.
|Peter K. Kaiser, MD, is on the professional staff of the Cole Eye Institute and is the founding director of the Digital Optical Coherence Tomography Reading Center, Cleveland Clinic, Cleveland, OH. Dr. Kaiser receives research grant support and is a consultant for Alcon.|
A conventional vitrectomy probe consists of a hollow inner tube surrounded by a hollow outer tube arranged coaxially. Vitreous is drawn by aspiration into a port near the distal end of the outer tube. Then the inner tube slides forward, closing the port and shearing off the vitreous. The cut material is aspirated out of the eye through the inner tube.
The cutting mechanism of these probes may be guillotine-style or rotational. In the former, the inner tube moves along the longitudinal axis. In the latter, the inner tube moves around the longitudinal axis.
There are various ways to create the movement of the inner tube. Two common methods are pneumatic and electrical. Existing pneumatic guillotine vitrectomy cutters are powered by a pneumatic drive pulse that acts upon a rubber diaphragm. The diaphragm pushes the cutter against a return spring, closing the port and dissecting any tissue drawn into the port. When the air pulse is vented, the return spring pushes the diaphragm back, reopening the cutter port. The repetition of this single drive sequence defines the cut rate.
There is a limit to how fast this sort of probe can cut. Drive pulse rising edge, the time it takes to vent the pressure pulse, and the characteristics of the mechanical spring all limit the velocity of the cut rate. The maximum speed of these probes is currently 2500 cuts per minute (cpm).
Electrically operated vitrectomy cutters are another option. They come in two varieties: rotational electric motor and solenoid drive. Rotating electrical motor cutters mechanically generate the cutter motion through a fixed mechanical coupling. Because the cutter translation is fixed to the rotary motion of the electric motor, the duty cycle is not adjustable and the shear velocity changes with the cut rate. At low cut rates, the slow shearing action diminishes membrane dissection and single-cut control. We do not encounter this problem with solenoid electric cutters because they use the same basic cutting mechanism as pneumatic devices. However, solenoid cutters overheat at higher cut rates.
The ULTRAVIT® probe used with the CONSTELLATION® Vision System is an advanced pneumatic dual-drive guillotine cutter. Similar to single-drive probes, the guillotine cutter of the ULTRAVIT® probe is driven closed with a pneumatic drive pulse that acts upon a diaphragm. However, instead of a return spring, a second pneumatic drive pulse acts against the diaphragm in the opposite direction to reopen the cutter. Because of this unique design, the ULTRAVIT® probe can operate at 5000 cpm, double the maximum cutting rate of other currently available probes. (Figure 1 shows the design of a conventional vitrectomy probe; the ULTRAVIT® probe is shown in Figure 2.)
Figures 1 (left) and 2 (above). Shown at left in Figure 1 is a cross section of a conventional vitrectomy probe. Drive pulse rising edge, the time it takes to vent the pressure, and the characteristics of the mechanical spring all limit the velocity of the cut rate. As shown in Figure 2 above, pneumatic pulse technology replaces spring operation in the ULTRAVIT® probe.
For many years, retinal surgeons have understood that faster cutting means increased safety. Some believe it is the single most important factor in surgical safety. Faster cutting speed means less vibration of the probe and less traction transmitted to the retina. This is because, as cut rate increases, the volume of each individual "bite" decreases, thus increasing fluidic resistance at the port. Better fluidic resistance translates to better fluidic stability, which in turn decreases vitreoretinal traction and the incidence of iatrogenic retinal breaks. It also allows the user to shave close to mobile retina without "chatter."
This effect has been termed "port-based flow limiting" by the pioneering retinal surgeon and engineer Steve Charles, MD, who played a major role in designing the CONSTELLATION® Vision System. Port-based flow limiting can be achieved by smaller diameter cutters. A 25-gauge cutter produces more fluidic resistance than a 23-gauge one because resistance is proportional to the 4th power of the diameter of the probe. Surgeons often incorrectly believe that 25-gauge vitrectomy is "inefficient" or produces insufficient flow rates, when in fact it is safer because it reduces vitreoretinal traction.
The graph presented in Figure 3 (below) demonstrates the benefit of ultra-high-speed cutting. It illustrates the maximum volume of vitreous that can be pulled into the port before being dissected by the guillotine cutter, assuming a constant flow rate of 8 cc per minute. At 1500 cpm, the discrete aspirated volume is 5.3 μl/cut (8/1500). At 5000 cpm, the discrete aspirated volume is reduced significantly, to 1.6 μl/cut (8/5000).
DUTY CYCLE CONTROL
In addition to high cutting speeds, the revolutionary design of the ULTRAVIT® probe offers, for the first time, duty cycle control. Now, surgeons can achieve beneficial port-based flow limiting by altering the duty cycle of the vitrectomy cutter.
High flow rates can be achieved with a "biased open" duty cycle, in which the port remains open during most of the cycle, producing greater flow of vitreous into the vitrector. Alternatively, the surgeon can achieve port-based flow limiting with a "biased closed" duty cycle. In this case, the port remains closed for a longer duration, with consequently lower flow. The surgeon controls duty cycle by adjusting the pneumatic drive pulses that hold the port open or closed.
The CONSTELLATION® Vision System currently offers 3 duty cycle settings:
• Core is the maximum port-open duty cycle control. It is best suited for core vitrectomy, in which higher flow rates are more efficient and desirable, and the risk of traction is not as great.
• Shave is the minimum port-open duty cycle. It is best suited for delicately removing tissue in situations where lower flow rates are desirable, such as near mobile retina or shaving in the periphery.
• 50/50 is the setting between core and shave, for those users who prefer that the cutter be open and closed for the same amount of time. This is a similar setting to older electric cutters.
This ability to select duty cycle makes it possible to shave the vitreous very close to the retina with ultra-high cutting rates, and with safer aspiration rates.
In addition to these advantages, Alcon® has moved the port of the ULTRAVIT® cutter 50% closer to the axial tip of the probe on every gauge of the new ULTRAVIT®. This is a crucial advantage in cases where the surgeon needs to perform meticulous dissection very close to the retina, for example, when delaminating a diabetic membrane. The new cutter is available in all standard gauge sizes: 20, 23, and 25.
ASPIRATION FLOW LIMIT
Another revolutionary innovation of the CONSTELLATION® Vision System is its ability to predict intraocular pressure (IOP) and flow rate while the procedure is in progress. This unique technology allows the machine to maintain stable IOP to +/-2 mmHg and intervene in sudden high flow situations.
With current technology, the surgeon is responsible for reacting in the face of a sudden port occlusion by mobile retina or a collapsing eye. The user must quickly assess the situation and control the foot pedal in hopes of avoiding a very negative outcome. With the CONSTELLATION® Vision System, no matter what the surgeon does with the vacuum — even if the lowest cutting rate is employed with the highest vacuum — the instrument's flow limit capability can assist to automatically reduce sudden bursts of fluid from entering the port.
Again, as with variable duty cycle and high-speed cutting, aspiration flow limit is valuable when working near the retinal surface in the presence of a mobile giant break, when there is a danger of the edge of the break entering the port.
Like anti-lock breaks on automobiles, the CONSTELLATION® Vision System has taken a reaction that was once subject to human error and automated it, performing a task better than humans can. The combination of these three innovations in the CONSTELLATION® Vision System — increased cutting speed, duty cycle control and aspiration flow limitation — offers surgeons the ability to perform more precise, more controlled, and more efficient vitrectomies. RP