A Review of the Parameters Affecting Flow During Vitrectomy
Controlling the settings of the vitrectomy cutter is key to good outcomes.
DAVID S. LIAO, MD, PhD · DAVID S. BOYER, MD
|David S. Liao, MD, PhD, and David S. Boyer, MD, are both in private practice with Retina-Vitreous Associates Medical Group in Beverly Hills, CA. Dr. Boyer reports moderate financial interest in Alcon and minimal financial interest in Bausch + Lomb. Dr. Liao reports no financial interest in any of the products mentioned in this article. Dr. Boyer can be reached via e-mail at firstname.lastname@example.org.|
It has been said that vitreous is like the appendix of the eye: it is not needed and often causes problems. Despite this seemingly mundane reputation, surgical removal of the vitreous is a procedure that has undergone rapid technological advancement and that continues to evolve.
From the first development of vitrectomy1 to traditional 20-gauge three-port pars plana vitrectomy techniques2 and to the microincisional vitrectomy surgery systems available today, the vitreoretinal surgeon now has a large armamentarium of devices with which to accomplish the task at hand.
Vitrectomy cutters come in a variety of sizes, including 23-, 25-, and 27-gauge. In addition, newer generation vitrectomy machines are capable of driving cut rates up to 5,000 to 7,500 cuts per minute (cpm).
Despite the myriad instrumentation options, the goal continues to be the safe and efficient removal of vitreous. To this end, reminding oneself of the dynamics of vitreous flow through the cutter is always pertinent. The performance of each vitrectomy system will rely on factors such as cut rate, duty cycle, vacuum, and the diameter of the cutter’s inner lumen.3,4
One of the most important characteristics contributing to the performance of a vitrectomy system is the cut rate of the handpiece. Traditional teaching holds that higher cut rates lead to safer vitreoretinal surgery.
When vitreous flows into the cutter port, traction is transmitted to the retina through the attached collagen strands. The amount of traction is related to the length of collagen pulled and the distance to the retina.
Increased cut rate results in the cutter taking smaller bites of the vitreous, transmitting less traction, and theoretically decreasing downstream effects, such as iatrogenic retinal breaks or increasing the motion of mobile retina.5
Large gains in cutter speed have been attributable to advances in the technology responsible for driving the guillotine at the cutter port. Older-generation systems were either the electric or pneumatic-spring type.6
Electric cutters drive the cutter back and forth using a motor. In contrast, pneumatic-spring cutters drive the guillotine down by delivering a pneumatic pulse on one side of a diaphragm. Conversely, the force for port opening is provided by a counter-spring on the opposite side of the diaphragm.
The maximum cut rate is determined by how fast the cycles of pneumatic pulse and spring recoil can occur. The mechanical properties of the spring in part limit the cut rate. Maximum cut rates of 2,500 cpm were typical. The development of dual pneumatic cutters has mitigated these limitations.
Synergetics’ (O’Fallon, MO) vitrectomy systems offer vitrectomy cutters produced by Mid Labs (San Leandro, CA).
Duel Pneumatic Cutters
In a dual pneumatic cutter, the mechanical spring is replaced with a second pneumatic pulse. Alternating pneumatic pulses are placed on each side of the diaphragm, allowing the guillotine to open and close at much higher rates. Modern dual pneumatic devices boast cut rates of 5,000 to 7,500 cpm.
In addition to increasing safety by decreasing retinal traction and mobility, increased cut rates also increase efficiency by enhancing flow through the cutter. Vitreous is a viscous substance, and the act of cutting it decreases that viscosity,7 likely decreasing turbulent flow at the cutter and allowing for greater observed rates of throughput.8
Another integral component contributing to the dynamics of vitreous flow into the cutter is the duty cycle. Duty cycle is defined as the proportion of time the port stays open during the entire length of the open-close cycle. Not surprisingly, increasing the duty cycle increases flow because an open-biased duty cycle allows the port to be open a greater percentage of the time.
The same advances in cutter technology that have allowed for greater cut rates have also allowed for greater surgeon control of the duty cycle. In older pneumaticspring cutters, the duty cycle was greatly affected by cut rate. At low cut rates, the port-closing pneumatic pulses arrived relatively slowly, allowing sufficient time for the spring to provide a long open interval (an open-biased duty cycle).
However, as cut rates increased, the closing pneumatic pulses arrived more frequently, keeping the port closed most of the time. Thus, paradigms that kept cut rate low to facilitate higher rates of flow, such as those used during core vitrectomy, necessarily caused more retinal traction than intended.
Modern dual pneumatic cutters allow for better control of duty cycle by regulating the width of the opening pulse and doing away with the rate-limiting closing spring. A high cut rate can be sustained while keeping the port open longer, allowing for high flow with minimal creation of vitreous traction, thereby optimizing safety and efficiency.
In contrast, a closed-bias duty cycle can be used to decrease flow rate. In this scenario, the port is closed most of the time, and mobile tissue is less attracted, making work on detached areas of the retina safer.
Bausch + Lomb’s (Rochester, NY) Stellaris PC vitrectomy cutters.
COURTESY: BAUSCH + LOMB
While dual pneumatic systems help to control the duty cycle more efficiently, this control is limited because part of the cycle time must be devoted to physical movement of the guillotine. As cut rate increases, more of the cycle is wasted on movement, and less time can be devoted to keeping the port open. Thus, at high cut rates, duty cycle decreases for open-biased settings.
Dutch Ophthalmic USA’s (Exeter, NH) selection of small-gauge vitrectomy cutters.
Conversely, duty cycle increases for closed-biased, high cut rate settings. The decrease in duty cycle at high cut rates tends to result in a decreased flow of purely aqueous fluid through open-biased cutters.
Fortunately, that high cut rates increase flow in viscous substances, such as vitreous, counteracts this effect. These limitations notwithstanding, the ability to control cut rate and duty cycle give the surgeon an added dimension of control in the vitreous cavity that was not previously available.
Finally, the physical dimensions of the cutter itself help to determine the amount of flow. Poiseuille’s law states that the flow through a tube (such as the cutter) is proportional to the fourth power of the radius and is inversely proportional to the length. Thus, as the inner lumen diameter of the cutter decreases, the flow for a given vacuum pressure will decrease.
At typical system settings, flow may therefore be lower for smaller-gauge systems,3 which may impact surgeon efficiency. Port diameter also adds resistance to flow; however, this effect becomes negligible as port diameter approaches inner lumen diameter.
Alcon’s (Fort Worth, TX) Ultravit system continues to offer 20-gauge vitrectomy, as well as smaller gauges.
Another interesting factor to consider is the effect of instrument size on tissue attraction.9 In vitro, larger-gauge instruments cause greater pull on a larger area of surrounding tissue, but this area effect is less for smaller-gauge instruments. Certainly, minimizing the instrumentation’s effects on adjacent tissues has implications for overall safety.
Finally, instrumentation gauge has undeniable effects on rigidity. While smaller-gauge devices may be ideal for straightforward cases, more complex cases often require more durable instruments.
Advances in cutter design have changed the landscape of options available in the vitreoretinal operating room. The next-generation systems all feature increased cut rates and some degree of duty cycle control, and they are amenable to the use of multiple smaller-gauge instruments.
Regardless of which system is employed, the ultimate goal continues to be removal of vitreous in the safest, most efficient manner possible. Each vitrectomy system will have its own advantages: some may offer higher cut rates, and some may allow the surgeon to control the duty cycle, while others may optimize duty cycle and cut rates automatically.
Likewise, the gauge of the instruments will likely vary with the needs of the particular case at hand.
However, keeping in mind the principles that govern vitreous flow through the cutter and how they are related will allow the surgeon to get maximal use out of whatever equipment she or he employs. That should translate into better outcomes for our patients. RP
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