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Laboratory science

Visualization of fluid turbulence and acoustic cavitation during phacoemulsification

Tognetto, Daniele MD; Sanguinetti, Giorgia MD; Sirotti, Paolo PhD; Brezar, Edoardo; Ravalico, Giuseppe MD

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Journal of Cataract & Refractive Surgery: February 2005 - Volume 31 - Issue 2 - p 406-411
doi: 10.1016/j.jcrs.2004.04.042
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Cataract extraction by phacoemulsification was introduced into surgical practice in the 1980s and became popular in the 1990s. Kelman1–3 was the first to report the use of ultrasound and aspiration to emulsify cataracts and describe a technique using a low-frequency (sonic–ultrasonic) needle combined with controlled irrigation and suction.

In ultrasonic phacoemulsification, the electrical energy of piezoelectric crystals is converted into mechanical energy and results in tip vibration at ultrasonic frequencies from approximately 25 to 62 kHz.4 Each machine is set to work at a fixed frequency, with the most commonly used setting around 40 kHz. Adjusting the machine's power setting does not affect the frequency at which the tip operates but does affect the stroke length; that is, the distance the tip travels during 1 cycle.

The mechanisms of lens fragmentation using phacoemulsification are not completely clear. Fragmentation of lens material is thought to be caused by a combination of mechanisms. The mechanical impact of the phaco tip on the lens surface (jackhammer effect of tip vibration) is directly related to stroke and tip angle.5 Other mechanisms include acoustic waves transmitted from the tip to the lens, the impact of particles and liquids on the lens surface and, possibly, cavitation.6

Acoustic cavitation is defined as the sonically induced activity of gas-filled cavities. Inertial cavitation occurs when a gas-filled cavity in a liquid expands during part of an acoustic cycle and then contracts rapidly (“collapses”) to a small fraction of its original volume. After it reaches minimum volume, the bubble rebounds, emitting a pressure pulse into the liquid.7 Cavitation has been recognized as an extremely destructive process when it occurs in a liquid environment system in which metal objects are moving very rapidly.8

The application of high-intensity ultrasound energy in aqueous media can generate acoustic cavitation with the concomitant generation of free radicals9–10 and sonoluminescence, a phenomenon in which electronically excited species cause the emission of a light flash.11 It can also cause localized high pressure and temperature elevation.12

Clinical studies show that cavitation may play a role in the decrease in endothelial cell density after phacoemulsification of cataracts.13,14 In addition, it is not clear whether low-frequency ultrasound radiates from the phaco tip and the possible effect on the lens and other eye structures.

The advent of new phacoemulsification techniques led to the development of new concepts in technology, giving rise to great interest in high-vacuum fluidics and low ultrasound rates.15,16 A better understanding of the mechanisms involved in phacoemulsification would enable new phaco technology that can be used more profitably. Thus, in addition to intuitive notions and purely physical and mathematical descriptions, it would be useful if this complex phenomenon could be visualized.

It is not easy to describe the cataract phacoemulsification procedure in terms of images. In an attempt to achieve a better understanding of phaco-related phenomena, we developed an optoelectronic system that is able to visualize the fluidics during the activation of different phaco settings. Using this system, we identified the possible presence of acoustic cavitation and ultrasound radiation related to phaco tip vibration.

Materials and Methods

Irrigation, irrigation combined with aspiration, irrigation/aspiration, and activation of tip vibration with related effects (phacosonication)11 are commonly combined during phacoemulsification and are therefore indistinguishable. They give rise to velocity fields in fluids, pressure waves, and bubble formation. Fluid turbulence and phenomena related to tip vibration produce local alterations of the refractive index of the medium and constitute a good example of phase images.

A usual image, called an amplitude image, can be detected by the eye or by a photo detector as it absorbs the light that shines on it if it is transparent. If it is not transparent, the image reflects light in different proportions, with the dark parts reflecting or transmitting the least and the light portions reflecting the most.

Phase images are transparent objects, altering only the phase of a light field passing across them. Phase images cannot be detected by the eye, television cameras, or photo detectors. Therefore, they cannot be seen or processed by a conventional image-processing system.

Optical image processing can be used to translate a phase image into something visible, turning it into an amplitude image.17–19 Optical image processing was made possible by the introduction of the laser technology of coherent optical image processing. In this system, an expanded, collimated laser beam transilluminates the input image. A convergent lens focuses the light on its focal plane, producing the optical bidimensional Fourier transform of the image. The focal plane assumes the meaning of the spatial frequency plane, and it is on this plane that the frequency spectrum of the image is shaped. The Fourier transform of the image can be frequency filtered and again transformed by a second convergent lens to give the processed image. The filters are positioned on the transform plane. These are objects whose amplitude and phase transmittance are projected to give the required results in terms of the processed image.

Coherent optical processing allows visualization of phase images. Several methods are used, with the best being phase contrast,20 strioscopy, schlieren, and shadowgraph. In general, these methods consist of performing a filtering operation on the transform plane. The filter used in strioscopy is a small, black dot centered on the optical axis on the transform plane. Because it suppresses the background, it highlights the phase component of the image. The schlieren method involves cutting off half the Fourier transform, creating light and shade around the variations of the phase and producing a characteristic relief effect. It then becomes possible to derive the characteristics of the system and the material from the visualized phase image and to analyze phenomena and processes.

In this study, an experimental optical bench was developed for optical processing of images to gain a better understanding of the fluidics during phacoemulsification. In an initial version, a 632.8 nm helium–neon laser light source was expanded by a telescope system into a coherent, collimated beam of light with a diameter of approximately 50.0 mm (Figure 1). In a second version, a more compact laser diode was directly inserted inside the beam expander, significantly reducing the dimensions of the optical system and rendering it steady and reliable. The coherent laser light source transilluminates a rectangular transparent glass tube containing a liquid medium.

Figure 1.
Figure 1.:
Schematic representation of the optical test bench.

All experiments were conducted in an ambient temperature of 27°C and used the handpiece of the AMO Prestige phacoemulsifier. The handpiece works at a fixed frequency of 38 kHz. Forty-five degree titanium tips were used with the sleeves provided. Phacosonication was activated in the continuous mode.

In the first part of the experiment, fortified balanced salt solution (BSS Plus) was placed in a square, transparent glass tube. The tip of the phaco handpiece, angled at 45 degrees, was immersed in the BSS. The tip was activated in the medium, separating the setting options for irrigation, aspiration, and phacosonication. The expanded beam then read the images (almost all phase images) formed during the different portions of the experiment.

In the second part of the experiment, the BSS was replaced by a high-viscosity ophthalmic viscosurgical device (sodium hyaluronate 1.4% [MicroVisc Plus]) placed in a rectangular transparent glass tube. The irrigation and the aspiration options were excluded, and only phacosonication was activated.

The laser beam emerging from the test tube was focused by an optical collimator, producing the bidimensional Fourier transform of the image on the posterior focal plane of the collimator. The Fourier transform of the image was filtered by an optical filter positioned on the focal plane of the lens. The phase images were essentially visualized by strioscopy with different filter diameters and by the schlieren method using a filtering blade. The Fourier transform was transformed once again by a second optical collimator, yielding the processed image. The processed images were acquired by the charge-coupled device of a television camera, visualized on a television monitor, and recorded on a tape recorder.

Results

With the system, phase images could be visualized during the activation of the phaco tip immersed in the liquid medium. The images were black-and-red movies from which single frames were chosen to describe each step of the experiments.

Generalized turbulence around the phaco tip immersed in the BSS was seen, and the different functions were indistinguishable (Figure 2). Figure 3 shows a high-magnification view of a frame taken from the side, where the irrigation holes overlap. Irrigation from the phaco tip can be seen. When the irrigation option was activated independently, excluding aspiration and phacosonication, the laminar flow from the irrigation holes of the phaco sleeves was visible.

Figure 2.
Figure 2.:
Phaco tip, seen from the front, immersed in BSS with all phaco functions activated. The 2 holes in the sleeve are on either side, and the flow is seen emerging.
Figure 3.
Figure 3.:
Phaco tip immersed in BSS, seen from the side, with only the irrigation function activated. One of the 2 holes of the phaco sleeve is at the front and the other behind. Only 1 laminar flow is seen emerging from the irrigation holes as the flow from the front hole covers the flow from the hole behind.

When irrigation was combined with aspiration, the laminar flow appeared to be disturbed by aspiration as the liquids flowed out through the lumen of the phaco tip. Figure 4 shows a side view of this phenomenon. The laminar flow was still detectable when the aspiration did not interfere with its movement.

Figure 4.
Figure 4.:
Phaco tip immersed in the BSS, seen from the side, with the irrigation and aspiration function activated.

When phacosonication was activated in addition to irrigation and aspiration, generalized fluid turbulence surrounded the phaco tip (Figure 5). No laminar flows were detectable.

Figure 5.
Figure 5.:
Phaco tip immersed in the BSS, seen from the side, with the phacosonication and irrigation/aspiration functions activated.

In the second part of the experiment, in which the BSS was replaced with OVD (Figure 6) and the irrigation and the aspiration options were excluded, bubble formation from the phaco tip became evident when the phacosonication was activated with the tip immersed in the OVD (Figure 7). Bubble formation was revealed to be a threshold phenomenon. The threshold above which bubbles were noted was at 30% of the power setting in the continuous mode. When the phaco power was set at higher levels, bubble formation increased (Figure 8). Bubbles of different size were noted, and 2 epicenters from which they emerged were observed at the phaco tip angularities (Figure 9).

Figure 6.
Figure 6.:
Phaco tip immersed in OVD.
Figure 7.
Figure 7.:
Phaco tip in OVD with only the phacosonication function activated.
Figure 8.
Figure 8.:
Phaco tip immersed in OVD, with increased bubble formation at higher power levels.
Figure 9.
Figure 9.:
Phaco tip in OVD and the 2 epicenters from which the bubbles emerged.

No radiation of ultrasound from the phaco tip was detectable at any time.

Discussion

Kelman1–3 was the first to suggest using phacoemulsification to break the crystalline lens into small pieces and emulsify the lens material. He developed the idea after noticing that some dentists used an ultrasound tool to help descale teeth.

Technological progress, new surgical techniques (eg, continuous curvilinear capsulorhexis),21 and the development of foldable IOLs22 led to improvements in phacoemulsification, leading to more effective and less traumatic surgery.4 Greater stress is now given to reducing energy production within the eye, with the aim of decreasing trauma and inflammation to achieve more rapid visual rehabilitation and better visual outcomes. Different types of power modulation have been introduced and have led to an overall reduction in the amount of ultrasound energy used in phacoemulsification.23

The physical mechanisms that break up nuclear material when the phaco tip is used are complex, and the relative importance of the various factors is not yet entirely clear.5,24 Several theories involving tip vibration and cavitation have been developed to explain lens emulsification. The most frequently reported are the mechanical effects of the impact of the tip on the nucleus, acoustic shock waves propagated by and emitted from the tip, acoustic cavitation, and the mechanical effects of fluid and small-particle streams.8 It has been speculated that cavitation is involved in the fragmentation of the nucleus.23 As the phaco tip withdraws in the second phase of its cycle during phaco activation, it seems to create a void, or bubble, in the fluid just in front of the retreating tip. As the tip retreats farther, the bubble increases in size until the forces acting on it cause it to implode.8

Cavitation has been assumed to play a role in the lens fragmentation process as well as in endothelial cell damage during cataract surgery. However, the presence and extent of cavitation at the phaco tip extremity have been questioned. Bond and Cimino postulate that the most important factor in tissue fragmentation is the stroke amplitude of the tip vibration (jackhammer effect) associated with the aspiration.25–27

In our experiments, cavitation was present and clearly identifiable when the tip was activated in the OVD medium without the presence of irrigation and aspiration. It appeared as a threshold phenomenon; that is, no bubble formation was noticed at a power setting lower than 30% with the phaco machine we used.

Setting the power to higher levels increases the length of the stroke of the tip. This increases the distance the tip travels during each cycle; thus, the tip reaches greater acceleration and a higher maximum speed. In our experiments, when the power of the machine was increased from 0% to 30%, bubbles began to appear, emerging from the phaco tip extremity. At a power greater than 30%, bubble production increased, culminating in the appearance of 2 epicenters at the extremity of the phaco tip (Figure 9).

In our study, when phacosonication was activated with irrigation and aspiration in a water medium, we observed considerable turbulence in the fluid that dispersed forces away from the phaco tip. This turbulence can be interpreted as an effect of all forms of energy produced during activation of the phaco tip; therefore, cavitational energy can also be emitted forward from the phaco tip, disturbing the laminar flow.

Cavitation is caused by pressure waves emitted from the tip in all directions during phacoemulsification. Thus, the tip generates cavitational energy in all directions. Turbulence might convey the cavitational energy toward the eye tissues and is thus a possible factor in surgical trauma. Although increased cavitational energy may be necessary to emulsify cataracts, especially hard cataracts, it can also damage the corneal endothelium. Reducing the average phaco power and effective phaco time might be crucial to minimizing surgical stress.

The system we describe provides a possible means of evaluating the power setting of a single phaco apparatus and comparing the power modulation and cavitation production of different phaco machines. Further studies are necessary to test different phaco tip shapes, taking into consideration the tip's diameter, the thickness of its walls, the diameter of the lumen, and the angle of the tip extremity.

References

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