September 2009 Advertorial Return to Homepage

-Allen C. Ho, MD Chief Medical Editor, Retina Today

  • Share
  • E-mail
  • Print

The main goal of every retina surgeon when removing vitreous is to not tear the retina. Vitrectomy and vitreous loss are not synonymous. There are two scenarios in removing vitreous: in the first, the surgery is controlled and the result is an eye with less vitreous; in the second, the surgery in uncontrolled and the result is a torn retina.

There are four principles to removing vitreous in a controlled fashion. First, do not pull the vitreous too close to the probe. Second, do not pull vitreous too far up the probe. Third, remove the vitreous in tiny pieces. Fourth, use as low a flow as possible. Regardless of the cut rate, vacuum, or duty cycle selected, it is important to maintain these four principles to avoid putting traction on the retina and cutting unintentionally.

THE RELATIONSHIP OF DUTY CYCLE AND CUTTING SPEED
Poiseuille’s law explains the relationship between viscosity and length, but it only addresses a tube that is open all the time. In vitreoretinal surgery, we are operating with a tube that is opening and closing continuously. Duty cycle is essentially the ratio of open time to total time. A closed cycle is when the port is mainly closed and open for a short time, and conversely, an open cycle is when the port is open for most of the time. A 50/50 cycle is when the port is open in direct proportion to the time that it is closed.

The standard probes that have been available to us include an electric probe (ie, MILLENNIUM, Bausch & Lomb, Rochester, NY), which operates on a constant duty cycle, and a spring-pneumatic probe (ie, ACCURUS, Alcon Laboratories, Inc., Fort Worth, TX), which has a variable duty cycle. At a slower cut rate, the electric probe slows the time between the time port open and port closed, but the ratio between the two is equal, which means that the flow at fast cut rates vs high cut rates is the same. The only thing that changes between low and high cut rates is that the size of the pieces taken at higher rates is smaller. The pneumatic probe with the spring return has a variable duty cycle, meaning that the dwell time between each cut decreases at higher cut rates. The relationships between duty cycle, flow rates, and cut rates are seen in Figure 1.

NEW PNEUMATIC PROBE
The CONSTELLATION Vision System (Alcon Laboratories, Inc.), has a newly designed ULTRAVIT pneumatic probe that operates on a variable duty cycle, but adds the ability to control the bias of the cycle to keep it primarily open or primarily closed.

Although it has been a goal for many years to create faster probes, it has been established that this is only significant for pneumatic probes because, as earlier stated, the electric probes merely cut smaller pieces at higher speeds with no change in flow.

Viewing the movement of particles in slow motion to the probe at various cut rates allowed us to see how fast tissue is moving to the probe and make a frame-byframe assessment. At 5 cc’s of isotonic flow, we looked at 1000 cpm vs 2500 cpm with the vacuum at various levels to ensure constant flow. In fact, we discovered that tissue was moving twice as quickly into the port at the slower cut rate (Figure 2). The action is all at the port—there is a significant difference in speed and the way that material is pulled.

Viewing the movement of particles in slow motion to the probe at various cut rates allowed us to see how fast tissue is moving to the probe and make a frame-byframe assessment. At 5 cc’s of isotonic flow, we looked at 1000 cpm vs 2500 cpm with the vacuum at various levels to ensure constant flow. In fact, we discovered that tissue was moving twice as quickly into the port at the slower cut rate (Figure 2). The action is all at the port—there is a significant difference in speed and the way that material is pulled.

Flow patterns are much more complex in real vitreous. Individual vitreous fibers are pulled “on stretch” to the port. When cutting occurs, the fiber snaps away, redistributing the remaining fibers. At ultraslow motion, the stretch of the vitreous fiber is apparent (Figure 3). The vitreous cuts cleanly, allowing remaining vitreous to snap away. When looking in slow motion with the same parameters as in the isotonic model (1000 cpm vs 2500, same flow and vacuum) there is far more turbulence in the frame showing 1000 cpm than in the frame showing 2500 cpm (Figure 4). There is a large amount of diffuse movement around the port at the slower cut rate, while the vitreous movement is primarily at the port at the faster cut rate.

Low cutting rates on a standard pneumatic probe result in a lengthy dwell open time for the port, resulting in high flow. If you cut at a higher rate, however, you are essentially stealing from the dwell time. The cutting time is the same, but the flow is even lower. Eventually, the machine will cap out as flow continues to decrease, minimizing the ability to cut effectively at higher speeds.

The new pneumatic cutter design removes the spring release (Figure 5) and replaces it with a pulse mechanism, which operates by using air to pulse the port open and pulse the port closed (Figure 6). The most important result of this new design is that dwell time is adjustable because either side of the pulse equation is changeable.

The advantages to the ULTRAVIT probe include the ability to cut at much higher rates and that duty cycles are easily controllable. The surgeon can keep the port biased open, biased closed, or even biased.

The working area with the ULTRAVIT probe is larger because of the ability to adjust duty cycle for a given cut rate and as a result, the surgeon has the ability to operate in any location efficiently.

SUMMARY
Duty cycle is an innovation that allows surgeons to control flow. The open-bias duty cycle, like our current ACCURUS probe, decreases flow with increasing cut rate. The closed-bias duty cycle on the ULTRAVIT cutter is new technology that allows surgeons to increase flow with increasing cut rate; once the cut rates reach high levels, such as 5000 cpm, the cut and flow become sinusoidal. Duty cycle is an inseparable parameter from cut rate and the improvements to the ULTRAVIT cutter have increased surgeons’ capability to efficiently control flow during surgery.

Kirk H. Packo, MD, is Professor and Chairman of Ophthalmology at Rush Medical College, Assistant Professor and Chairman of Ophthalmology at Illinois Eye and Ear Infirmary in Chicago, and a member of the Retina Today Editorial Board. He is a consultant for Alcon Laboratories, Inc. Dr. Packo can be reached via e-mail at Kirk_Packo@rush.edu.

You must be logged in to leave a comment.