Ultrasonic-generated fluid velocity with Sovereign WhiteStar micropulse and continuous phacoemulsification

J CATARACT REFRACT SURG - VOL 32, FEBRUARY 2006

Ultrasonic-generated fluid velocity with Sovereign WhiteStar micropulse and continuous phacoemulsification Roger F. Steinert, MD, Mark E. Schafer, PhD

PURPOSE: To evaluate and compare ultrasonic turbulence created by conventional and micropulse ultrasound technology. SETTING: Sonora Medical Systems, Longmont, Colorado, USA. METHODS: A high-resolution digital ultrasound probe imaged the zone around a phacoemulsification tip. Doppler analysis allowed determination of flow. The fluid velocity was measured at 4 levels of ultrasound power at a constant flow, comparing the ultrasonic conditions of continuous energy to WhiteStar micropulses. RESULTS: In addition to the normal baseline irrigation and aspiration, fluid movement was detected directly below the phaco tip, produced by a nonlinear effect known as acoustic streaming. Acoustic streaming increased with increased phacoemulsification power for both conditions. At each of the 4 levels of power, fluid velocity away from the tip was less with micropulse technology than with continuous phacoemulsification. CONCLUSIONS: The demonstrated decrease in acoustic streaming flow away from the phaco tip with Sovereign WhiteStar micropulse technology compared to conventional ultrasound provides an objective explanation for clinical observations of increased stability of nuclear fragments at the tip and less turbulence in the anterior chamber during phacoemulsification. This methodology can be used to examine and compare fluid flow and turbulence under a variety of clinically relevant conditions. J Cataract Refract Surg 2006; 32:284–287 Q 2006 ASCRS and ESCRS

Phacoemulsification creates significant turbulences that expand in all directions from the phaco tip. The extent of the turbulence spread is related to ultrasound intensity. Excessive turbulence can damage corneal endothelial cells as well as increase the risk that small particles will disperse from the site of emulsification toward the endothelium.1,2 WhiteStar power control technology on the Sovereign phacoemulsification system introduced a new way of modulating the delivery of ultrasound to the eye. Ultrasound energy is delivered in microbursts as short as 4 msec. In addition, the proportion of ultrasound burst time to rest time can be varied (Figure 1). Compared to conventional burst mode ultrasound delivery, microbursts have higher peak power, shorter duration, the potential for more rapid pulsing, a completely quiet energy-free period between pulses, and overall less total energy delivery to the eye. The ultrasound micropulses, therefore, have the potential to decrease the amount of ultrasound energy delivered to the eye without sacrificing cutting efficiency.3 Anecdotally, some surgeons have observed less turbulence and Q 2006 ASCRS and ESCRS Published by Elsevier Inc.

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improved followability of nuclear fragments when using Sovereign WhiteStar micropulses compared to using traditional continuous phacoemulsification (W.F. Fishkind, MD, ‘‘Multisite Comparative Study of the Current Sovereign Power Control System with the WhiteStar Control System,’’ presented at the ASCRS Symposium on Cataract, IOL and Refractive Surgery, Philadelphia, Pennsylvania, USA, June 2002). In an attempt to find an objective measure that may correlate with the anecdotal reports, we used high-resolution digital ultrasound imaging to record fluid velocity with conventional continuous phacoemulsification and the Sovereign WhiteStar power delivery system. MATERIALS AND METHODS A high-resolution digital ultrasound imaging system (ATL HDI 3000 Ultrasound Scanner, Phillips Medical Systems) was used to form an image of the fluid motion in the region of the vibrating tip. This system is designed to produce images of moving bloodflow within the body, using Doppler ultrasound techniques to form a color flow image of moving structures or fluid.4,5 For 0886-3350/06/$-see front matter doi:10.1016/j.jcrs.2005.09.025

LABORATORY SCIENCE: ULTRASOUND FLUID VELOCITY

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Figure 1. Ultrasound on (red) and off (white) times for different duty cycles with Sovereign WhiteStar; % indicates percentage of time ultrasound was ‘‘on’’ in different duty cycles.

this experiment, the apparatus was set up so that the flow patterns in and around the area of the tip would be rendered as color patterns, while regions with little or no flow would be rendered black. The color patterns encode both the direction and intensity of fluid flow. The ultrasound transducer (L12-5 38 mm linear array, 5 to 12 MHz broadband, Phillips Medical Systems) was made watertight and immersed in a large (0.5 m  0.5 m  0.5 m) water tank, oriented to image toward the surface of the water. A Sovereign phacoemulsification handpiece with 20-gauge phaco tip was suspended above the tank so that the tip was immersed to a depth of approximately 1.0 cm, directly above the ultrasound imaging transducer. The experimental setup is shown in Figures 2 and 3. The Sovereign phacoemulsification system was run at continuous power or the WhiteStar CF duty cycle (6 msec on; 12 msec off). In 4 series of imaging studies, the ultrasound power was set to 25%, 40%, 55%, or 70%. Other machine settings are detailed in Table 1. The images of fluid flow were videotaped as the system was cycled on and off (footswitch depressed and released) for each condition listed above.

Figure 2. The Sovereign phacoemulsification handpiece suspended directly above the imaging ultrasound transducer.

RESULTS

The pseudocolor images showed an inverted V of highvelocity fluid flow from the phaco tip due to irrigation flow. As shown in the color scale on the left side of the images,

Accepted for publication September 8, 2005. From the Department of Ophthalmology (Steinert), University of California, Irvine, Irvine, California, and Sonic Tech, Inc. (Schafer), Ambler, Pennsylvania, USA. Presented in part at the ASCRS Symposium on Cataract, IOL and Refractive Surgery, San Francisco, California, USA, April 2004. Supported in part by a Departmental Challenge Grant from Research to Prevent Blindness and a grant from Advanced Medical Optics, Inc., Santa Ana, California, USA. Both authors are consultants to Advanced Medical Optics, Inc. Neither author has a financial or proprietary interest in any material or method mentioned. Reprint requests to Roger F. Steinert, MD, Department of Ophthalmology, University of California, Irvine, 118 MedSurge I, Irvine California 92651-4375, USA. E-mail: [email protected].

Figure 3. Close-up of the handpiece, tip, and ultrasound transducer.

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LABORATORY SCIENCE: ULTRASOUND FLUID VELOCITY

Table 1. Phacoemulsification parameters.

Aspiration flow rate Bottle height Vacuum

40 cc/min 3 inches above top of test chamber 200 mm Hg

the overall green indicates irrigant fluid motion downward, away from the tip. Fluid movement caused by the acoustic energy radiating from the phaco tip is shown extending vertically from the phaco tip (Figures 4 and 5). As ultrasound power levels increased, despite aspiration at the rate of 40 cc/minute toward the phaco tip, the ultrasound caused net flow away from the phaco tip. This fluid movement directly below the tip is known as acoustic streaming. At all levels of power, acoustic streaming was markedly reduced with micropulses compared to continuous phacoemulsification. Only at very high power did micropulse phaco show momentary net flow away from the phaco tip. In contrast, continuous phaco showed repulsive fluid flow away from the phaco tip at all power levels.

DISCUSSION

In this experiment, the direction and extent of fluid movement in the region around the tip was examined using Doppler color flow mapping techniques. In the absence of acoustic streaming6 (ie, fluid movement directly below the

tip), the images would have been expected to show fluid movement toward the tip because of the aspiration flow (set at a relatively high 40 cc/minute). However, the study indicates fluid movement away from the tip region due to this nonlinear acoustic phenomenon. Notably, less fluid moving directly away from the phaco tip occurs with WhiteStar CF ultrasound micropulses than with conventional continuous ultrasound. This decreased fluid flow may correlate to the anecdotal reports of increased followability with the micropulse technology. With less flow away from the phaco tip, nuclear fragments remain in the vicinity of the tip for more efficient emulsification. A decrease in fluid velocity and concomitant decrease in fragment movement away from the phaco tip may also lead to the lower rates of endothelial cell loss after surgery with micropulse phaco than with surgery using continuous power, consistent with prior interventions such as viscofluids that reduce turbulence and improve tissue protection.7 The hypothesis that reduced turbulence or other characteristics of micropulse phacoemulsification lead to reduced endothelial cell loss is speculative and requires further controlled investigation in clinical studies. Fluid turbulence has been shown to cause tissue damage through several mechanisms. In addition to the inertial impact of cavitation bubbles on cells,1 turbulence associated with bubble translation may cause cell lysis.2,8 In addition to traumatic cell death, moderate agitation can induce nonlethal responses affecting growth rate, metabolism, and product formation.9 The decrease in fluid velocity

Figure 4. Pseudocolor images of fluid velocity at 25% and 40% power. Left: Pseudocolor scale for fluid velocity ranges from ÿ2.2 cm/sec to C2.2 cm/sec. Right: Open block arrows indicate fluid stream due to irrigation flow. Solid arrows indicate fluid flow due to the motion of the phaco tip (acoustic streaming). Fluid velocity increased with increased phaco power. At both power settings (25%, 40%), acoustic streaming was lower for WhiteStar CF than for continuous phacoemulsification.

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LABORATORY SCIENCE: ULTRASOUND FLUID VELOCITY

Figure 5. Pseudocolor images of fluid velocity at 55% and 70% power. Left: Pseudocolor scale for fluid velocity ranges from ÿ2.2 cm/sec to C2.2 cm/sec. Right: Open block arrows indicate fluid stream due to irrigation flow. Solid arrows indicate fluid flow due to the motion of the phaco tip (acoustic streaming). Acoustic streaming continued to increase as phaco power increased. At both power settings (55%, 70%), fluid velocity was lower for WhiteStar CF than for continuous phaco.

away from the phaco tip becomes even more important as cataract surgery moves to microincision surgery with bimanual phacoemulsification. With irrigation removed from the phaco handpiece, the direction of irrigation and aspiration can be aligned. The decrease in fluid flow away from the phaco tip demonstrated with micropulse technology contributes to the stabilization of nuclear fragments at the phaco tip for emulsification and may also enhance the maintenance of corneal integrity during cataract surgery, particularly in the presence of hard nuclei.10,11 REFERENCES 1. Topaz M, Motiei M, Assia E, et al. Acoustic cavitation in phacoemulsification: chemical effects, modes of action, and cavitation index. Ultrasound Med Biol 2002; 28:775–784 2. Miller MW, Miller DL, Brayman AA. A review of in vitro bioeffects of inertial ultrasonic cavitation from a mechanistic perspective. Ultrasound Med Biol 1996; 22:1131–1154

3. Oki K. Measuring rectilinear flow within the anterior chamber in phacoemulsification procedures. J Cataract Refract Surg 2004; 30:1759–1767 4. Hartley CJ. Characteristics of acoustic streaming created and measured by pulsed Doppler ultrasound. IEEE Trans Ultrasonics Ferroelectrics Frequency Control 1997; 44:1278–1285 5. Starritt HC, Duck FA, Humphrey VF. An experimental investigation of streaming in pulsed diagnostic ultrasound beams. Ultrasound Med Biol 1989; 15:363–373 6. Nyborg WL. Acoustic streaming. In: Mason WP, ed, Physical Acoustics, IIB. New York, NY, Academic Press, 1965; 265–331 7. Assia EI, Yehezkel M, Ezov N, et al. Experimental studies on viscofluids for intraocular surgery. J Cataract Refract Surg 1998; 24:78–83 8. Thomas CR, al-Rubeai M, Zhang Z. Prediction of mechanical damage to animal cells in turbulence. Cytotechnology 1994; 15:329–335 9. Cherry RS. Animal cells in turbulent fluids: details of the physical stimulus and biological response. Biotechnol Adv 1993; 11:279–299 10. Bourne RRA, Minassian DC, Dart JKG, et al. Effect of cataract surgery on the corneal endothelium; modern phacoemulsification compared with extracapsular cataract surgery. Ophthalmology 2004; 111:679–685 11. O’Brien PD, Fitzpatrick P, Kilmartin DJ, Beatty S. Risk factors for endothelial cell loss after phacoemulsification surgery by a junior resident. J Cataract Refract Surg 2004; 30:839–843

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