Ординатура / Офтальмология / Английские материалы / Bimanual Phaco Mastering the Phakonit MICS Technique_Agarwal_2004

3.Sugar A, Schertzer RM. Clinical course of phacoemulsification wound burns. J Cataract Refract Surg. 1999;25:688-692.

4.Fine IH, Packer M, Hoffman RS. Use of power modulations in phacoemulsification. J Cataract Refract Surg. 2001;27:188-197.

5.Fine IH. The choo choo chop and flip phacoemulsification technique. Operative Techniques in Cataract and Refractive Surgery. 1998;1:61-65.

6.Fine IH, Hoffman RS, Packer M. The STAAR Wave. In Kohnen T, ed. Modern cataract surgery update. Dev Ophthalmol. 2002;34 (in press).

7.Fine IH, Packer M, Hoffman RS. Use of power modulations in phacoemulsification. J Cataract Refract Surg. 2001;27:188-197.

8.Olson RJ, Soscia WL. Safety and efficacy of bimanual phaco chop through two stab incisions with the Sovereign. Presented at: XIII Congress of the European Society of Ophthalmology; June 7, 2001; Istanbul, Turkey.

9.Packard R. Evaluation of a new approach to phacoemulsification: bimanual phaco with the Sovereign system rapid pulse software. Presented at: XIII Congress of the European Society of Ophthalmology; June 3-7, 2001; Istanbul, Turkey.

10.Nishi O, Nishi K. Accommodation amplitude after lens refilling with injectable silicone by sealing the capsule with a plug in primates. Arch Ophthalmol. 1998;116(10):1358-61.

Chapter

11

WHITESTAR “COLD PHACO”

TECHNOLOGY

David F. Chang, MD

INTRODUCTION

WhiteStar technology for the Advanced Medical Optics (AMO, Santa Ana, Calif) Sovereign phaco system debuted at the 2001 ASCRS meeting. This is a revolutionary new power modulation, in which a digitally driven phaco handpiece can be programmed to deliver energy in a surprisingly precise fashion. Most importantly, WhiteStar is still ultrasound technology, which is deployed with the same standard handpiece and phaco needle as with conventional phaco. Similar technology has been incorporated into the Alcon Infiniti (Fort Worth, Tex) system as well.

WhiteStar technology changes the way ultrasound energy is delivered in two important ways. First, the duration of each phaco pulse can be shortened to as little as 6 msec. This allows surgeons to vary and increase the pulse rate to frequencies as high as 50 to 100 pulses per second (pps) (Figure 11-1). Second, the technology allows one to change the phaco duty cycle by prolonging and varying the amount of time that ultrasound is cycled off. The practical significance of these innovations is best understood by comparing continuous and pulse mode phaco.

CONVENTIONAL POWER MODULATIONS

With first generation phaco machines, ultrasound could only be delivered in continuous mode and at a 100% phaco power level. The advent of linear foot pedal control of power was a significant advance. As the surgeon gradually depresses the foot pedal in position three, the axial stroke length of the vibrating needle increases until the maximal programmed power setting has been reached. If the surgeon presets the maximum phaco power at 60%, the needle never vibrates at more than 60% of its maximum stroke length. Nevertheless, with continuous mode, ultrasound is always “ON” in foot pedal position 3.

In pulse mode phaco, ultrasound is delivered for a brief period (ON time) followed by a rest period where it is automatically shut off (OFF time). The ON and OFF periods are of equal duration. With a setting of 4 pps, each ON pulse is 125 msec long.

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Figure 11-1. Schematic diagram showing frequent brief 6 msec pulses (red) with prolonged rest periods, compared with continuous mode ultrasound delivery (blue) (courtesy of Advanced Medical Optics).

Each cycle of an ON pulse followed by an OFF rest period is 250 msec long. In pulse mode one can change the frequency of pulses, and shortening the duration of each ON pulse results in more pps. As an example, decreasing the ON time to 62.5 msec produces a rate of 8 pps.

The duty cycle refers to the percentage of time that ultrasound is ON while in foot pedal position three. It is equal to ON time divided by (ON + OFF time). Therefore, in continuous mode, the duty cycle is 100% because the phaco power is always ON. In conventional pulse mode, the duty cycle is always 50%.

Pulse mode provides several important advantages over continuous mode. First, the total amount of energy output in pedal position 3 is cut in half. Second, the interspersed OFF rest periods reduce the mechanical repelling force of the axially vibrating phaco needle. Clinically, this is seen as a reduction in chatter of nuclear fragments at the phaco tip. This jiggling or momentary bouncing of pieces off of the vibrating tip indicates that the tip repelling force is momentarily exceeding the attracting force of fluidic suction. Since increasing phaco power means increasing the axial stroke length of the vibrating tip, excessive power levels will also exacerbate chatter.

HOW DOES WHITESTAR WORK?

By virtue of its ability to further shorten the pulse duration and to precisely vary the duty cycle, WhiteStar represents the next major innovation in power modulation. Being able to shorten the ON pulse to 6 msec permits a higher rate of 80 pps with a 50% duty cycle. In addition, WhiteStar allows us to vary the duty cycle by lengthening or shortening the rest periods. For example, following a 6-msec phaco pulse with a 12-msec rest period creates a 33% duty cycle and 55 pps. A 24-msec rest period would create a 20% duty cycle and 33 pps. A 3-msec rest period creates a 66% duty cycle and over 100 pps.

This combination of features provides three distinct clinical benefits. First, using a lower duty cycle further reduces the amount of ultrasound energy delivered. This, in combination with interrupting ultrasound delivery more frequently, significantly decreases heat buildup from the vibrating phaco needle. Finally, both factors substantially reduce the tip repelling forces. This noticeably enhances nuclear followability and reduces turbulence of particles at the tip.

Figure 11-2. Composite representation of temperature studies of WhiteStar technology (courtesy of Advanced Medical Optics).

In 2001, in one of the first articles on WhiteStar, I coined the term “hyperpulse” to help doctors conceptualize the clinical benefits of using WhiteStar.1 If one understands the advantages of pulse mode over continuous mode, hyperpulse implies a many-fold amplification of these benefits as a result of a lower duty cycle and more frequent interruption of phaco energy. Let’s analyze each of these three major benefits to WhiteStar hyperpulse technology in greater detail.

REDUCTION IN PHACO ENERGY

By enabling us to use a lower duty cycle, WhiteStar can dramatically lessen phaco time. Recording and assessing the effective phaco time (EPT) can readily quantify this reduction. The EPT is a composite value that factors both the ON/OFF time and the percentage of phaco power used, and converts this to an equivalent amount of time hypothetically spent in continuous mode at 100% phaco power.

Unfortunately, various manufacturers measure and define stroke length in different ways. Lacking a standard methodology, it is not possible to meaningfully compare EPTs between different manufacturer’s machines. However, EPTs can provide valuable comparisons within the same phaco system, such as the varying energy requirements for different nuclear grades. One can also compare EPTs for different techniques, such as divide and conquer and chopping. Finally, using the same technique and machine for the same grade of nucleus, one can assess the energy reduction from using different power modulations such as pulse or WhiteStar hyperpulse modes.

REDUCTION IN THERMAL HEAT

Cadaver studies conducted by Dr. Olson and Dr. Donnenfeld have confirmed a dramatic reduction in thermal heat compared to conventional non-WhiteStar ultra- sound2-4 (Figure 11-2). In their studies, it was virtually impossible to create an incisional burn. This confers an enormous advantage in terms of decreasing the risk of wound burn or thermal injury to the cornea. This is especially important when one is removing brunescent cataracts via a clear corneal incision.

Over the years, there have been numerous and extensive efforts to find a superior alternative to ultrasound. Other “cold phaco” technologies have included laser phaco (Erbium or Nd:YAG laser source), STAAR Surgical’s (Monrovia, Calif) sonics which uses a much lower frequency of tip vibration, and Alcon’s Aqualase (Fort Worth, Tex), which uses fluid jets instead of ultrasound. Like WhiteStar, each of these technologies will prevent wound burns and, by obviating the need for a cooling fluid leak, the incisions can be made tighter. In addition, these technologies are all adaptable to bimanual microincisional surgery in that they do not require a coaxial infusion source.

The critical difference, however, is that these other technologies all achieve their “cold” status by eliminating ultrasound as the source of energy. As a result, these other cold technologies are effective for soft nuclei, but become less efficient and less effective with increasing nuclear density. While proponents will point out that denser nuclei can be removed with these modalities, the procedure becomes so slow and inefficient that any advantages over standard ultrasound are greatly overshadowed. Finally, none of these competing technologies is effective or appropriate for brunescent nuclei.

Given the prior experience with laser phaco and sonics, a crucial question when WhiteStar was launched was whether it would be effective for dense lenses. In fact, it turns out that WhiteStar is actually superior to conventional phaco for brunescent nuclei. At the 2002 ASCRS meeting, I reported on 30 consecutive 4+ brunescent nuclei removed with WhiteStar.5 Using vertical phaco chop with no sculpting, the range in EPT was 6 to 30 seconds, with an average EPT of 9.1 seconds. Clearly, what differentiates WhiteStar from the other “cold” modalities is that this technology is still capable of delivering full ultrasound power. Instead of using a different energy source, it is the alternative delivery modulation that limits heat production. As a result, surgeons can still access maximal phaco power and tip stroke length for brunescent material, while minimizing heat and energy delivery at the same time.

Mark Shafer, PhD, an engineer specializing in ultrasound systems, has provided an explanation for this win-win solution. According to Dr. Shafer, there are two types of acoustic cavitational energy involved in phacoemulsification—transient and stable.6 Transient cavitation creates a momentary water jet that breaks up the nuclear tissue. This is immediately followed by a longer period of stable cavitation, which causes vibration of cavitational bubbles without energy release. This can therefore be considered wasted energy, compared to the transient cavitation that is effective at cutting the nucleus. By interspersing microbursts of ultrasound energy delivery with microrest periods, WhiteStar generates transient cavitation with each microburst, and limits stable cavitation by immediately cycling the phaco tip off. By selectively maximizing the effective transient cavitational energy, and minimizing the ineffective stable cavitational energy, WhiteStar is able to optimize cutting performance, while at the same time reducing overall heat and energy output.

REDUCTION IN PARTICLE CHATTER AND TURBULENCE

The third major advantage of WhiteStar is its ability to facilitate followability of mobile lens fragments. This refers to the degree to which particles stay on the phaco tip as ultrasound is delivered, instead of momentarily deflecting away. As mentioned earlier, this jiggling and bouncing of pieces is called chatter, and indicates momentary periods where the repelling force of the axially vibrating phaco tip exceeds the aspirating force of the pump.

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Figure 11-4. Improved fragment followability using AMO Sovereign with WhiteStar mode in a 4+ brunescent nucleus.

Chatter is less of a problem with soft nuclei because the “gummy” material readily molds into the mouth of the tip and the vacuum takes hold. However, with brunescent cataracts, the rigid fragment contours do not easily conform to the mouth of the phaco tip, rendering vacuum less effective. When you combine the fact that these nuclei require higher power settings and longer tip stroke lengths, the result is chatter and a greater tendency for particles to bounce away.

The frequent interspersed rest periods characteristic of hyperpulse dramatically reduce this repelling force and better enable the aspirating forces to hold pieces at the tip. The technical term for these repelling cavitational forces is acoustic streaming, and this contributes to fluid and particle turbulence that can be injurious to the endothelium.

Chatter also increases if the size of a dense nuclear fragment is much larger than the size of the phaco tip opening. Chopping a large fragment into smaller pieces is therefore one way to reduce chatter. However, using micro phaco tips, whose smaller diameter openings impede this molding process, exacerbates chatter. The need for 20-gauge phaco tips is another important reason why WhiteStar is particularly suitable for bimanual phaco.

BENEFITS FOR BRUNESCENT NUCLEI

In assessing the value of any new lens removal technology, the most important test is the brunescent nucleus. These cases take the longest time to phaco, and they challenge us with an increased risk of wound burn, posterior capsule rupture, and endothelial cell loss. While we don’t routinely do endothelial cell counts, the postoperative day one corneal clarity gives us immediate feedback about how traumatic our procedure was to the endothelium. While the corneas are generally clear following phaco of soft to medium density nuclei, we are certainly accustomed to seeing more edema in eyes with brunescent cataracts. Endothelial cell count studies have confirmed greater cell loss following surgery in these eyes.7

In my personal experience with WhiteStar, postoperative day 1 corneal edema is significantly less in this subgroup of brunescent cataracts.5,6 While decreased energy delivery may play a role, I believe that the most important factor is the reduced particle turbulence at the tip (Figure 11-4). The brunescent lens tends to have the largest

Figure 11-5. Alcon Infiniti hyperpulse mode with overlay showing 50 pps and 30% duty cycle.

nuclear mass, and normally requires the longest phaco time and highest phaco power levels to remove. As stated earlier, the larger size and rigid contours of the fragments, and the longer tip stroke lengths combine to produce much greater amounts of particle chatter and turbulence. This undoubtedly increases endothelial cell loss, and explains why the improved nuclear followability seen with WhiteStar would be particularly beneficial for corneal safety with dense cataracts.

Although WhiteStar technology can be used with any phaco technique, it compliments phaco chop particularly well. This is because during phaco chop, the majority of ultrasound is used for removing chopped fragments, and enhanced tissue followability facilitates this step. By eliminating sculpting, chopping also further reduces overall use of ultrasound.8,9 The ability to chop dense nuclei into multiple small fragments, as opposed to dividing/cracking them into four large quadrants, also helps to reduce particle turbulence and chatter. Finally, chopping is the phaco technique that seems most suitable for bimanual microincisional phaco.10

ALCON INFINITI SYSTEM

In 2003, Alcon introduced the Infiniti phaco system. The Infiniti also offers the option of hyperpulse with the ability to vary both duty cycle and pps (Figure 11-5). The shortest duration pulse available is 5 msec. The duty cycle can be altered in 5% increments and the pulse rate can be set as high as 100 pps (5 msec pulse/50% DC). Both cadaver heat studies and clinical experience have shown that this delivers the same benefits as WhiteStar technology (Figure 11-6).10

SUMMARY

WhiteStar hyperpulse technology represents a quantum leap in phaco efficiency and safety. It eliminates the risk of wound burn without requiring an alternative energy source. Of all of the so-called “cold” phaco technologies, it is the only one that can efficiently handle the entire spectrum of nuclear density. Finally, it is truly superior to standard ultrasound for brunescent cataracts. By limiting particle turbulence and overall phaco energy delivery, it reduces the greater endothelial trauma associated with phacoemulsification of these lenses. That WhiteStar is able to do so with a standard phaco handpiece and without any alteration of surgical technique is even more satisfying.

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Figure 11-6. Bimanual horizontal phaco chop (Alcon Infiniti with hyperpulse, and Katena’s [Denville, NJ] Chang horizontal irrigating hydrochopper).

KEY POINTS

The WhiteStar is a revolutionary new power modulation, in which a digitally driven phaco handpiece can be programmed to deliver energy in a surprisingly precise fashion.

WhiteStar technology changes the way ultrasound energy is delivered in two important ways. First, the duration of each phaco pulse can be shortened to as little as 6 msec. This allows surgeons to vary and increase the pulse rate to frequencies as high as 50 to 100 pps. Second, the technology allows one to change the phaco duty cycle by prolonging and varying the amount of time that ultrasound is cycled off.

By virtue of its ability to further shorten the pulse duration and to precisely vary the duty cycle, WhiteStar represents the next major innovation in power modulation. Being able to shorten the ON pulse to 6 msec permits a higher rate of 80 pps with a 50% duty cycle.

By enabling us to use a lower duty cycle, WhiteStar can dramatically lessen phaco time.

The WhiteStar has an ability to facilitate followability of mobile lens fragments. This refers to the degree to which particles stay on the phaco tip as ultrasound is delivered, instead of momentarily deflecting away.

REFERENCES

1.Chang DF. Can cold phaco work for brunescent nuclei? Cataract and Refractive Surgery Today. 2001;1:20-23.

2.Soscia W, Howard JG, Olson RJ. Bimanual phacoemulsification through 2 stab incisions. A wound-temperature study. J Cataract Refract Surg. 2002;28:1039-1043.

3.Soscia W, Howard JG, Olson RJ. Microphacoemulsification with WhiteStar. A wound-temperature study. J Cataract Refract Surg. 2002;28:1044-1046.

4.Donnenfeld ED, Olson RJ, Solomon R, et al. Efficacy and wound-temperature gradient of WhiteStar phacoemulsification through a 1.2 mm incision. J Cataract Refract Surg. 2003;29:10971100.