Ординатура / Офтальмология / Английские материалы / The Art of Phacoemulsification_Mehta, Alpar_2001

42 THE ART OF PHACOEMULSIFICATION

Fig. 3.7: Multifunciton foot switch

Foot pedals are no longer simple car accelerator-type pedals. Many machines now offer the opportunity to adjust the proportion of the total range of movement that is used for the three different “positions”, and save these in memory as part of the settings for a particular surgeon. All pedals have had some form of additional toggle switch (usually to the side) that switches on the reflux function. Now, however, it is common to offer even more toggle or rocker switches (Fig. 3.7). These can be used for activating the in-built diathermy on many machines, or for switching to different memory settings or “modes“. Some pedals can be set to activate an “emergency”change (for example rapid lowering of the inflow bottle height in the event of a posterior capsule rupture).

While many see the proliferation of switches and rockers on newer foot pedals as a yet another complicating factor, they do undoubtedly offer the surgeon a greater degree of control and independence from support staff if that is desired. They do not have to be used if not desired!

Major innovation in foot pedal design leading to even greater control over the procedure has been the introduction of a simultaneous dual linear foot pedal by Bausch and Lomb. This pedal allows the surgeon (again only if desired) to have linear control in position 2 while simultaneously having linear control of position 3. Standard pedals only have travel in one plane (pitch). The dual linear pedal also has the ability to rotate sideways (yaw). The surgeon can select whether ultrasound or flow/vacuum is the function controlled by yaw. If ultrasound is chosen for yaw, then this means that position 2 can be used to linearly control flow rate (say between 15 cc/min and 35 cc/min). At any point in the travel (i.e. at any flow rate) the surgeon can then add yaw and begin phaco (Figs 3.5B and 3.8). The phaco is also linearly controlled in yaw, and so at any flow rate there is also a range of phaco—

NEW PHACOMACHINES OFFER MORE CONTROL 43

Figs 3.8A and B: Yaw movement of pedal

say ranging from 0 to 30 percent. In addition to the added control this gives it also allows a greater range of pedal travel in position 2 (i.e. the whole range normally allocated to two and three are available in position 2).

It is clear that by using this dual linear function there is a much reduced need for multiple preset modes for different types of nucleus or for different stages in the procedure. The surgeon can continuously adjust all the parameters in real time in response to changes in the characteristics of the lens being dealt with.

Sophisticated Microprocessors

The single most important factor in the development of the sophisticated equipment now available to the cataract surgeon has been the development of microprocessor control of the functions of the machine. Initially electronics allowed control over the power functions of the handpiece—pulse mode and “auto-tuning” being significant developments. More recently the processors have enabled sophisticated ways of enabling postocclusion surge to be controlled and minimized. The surgical design ocusystem was the first machine to introduce active ways of controlling postocclusion surge. This machine has a pressure (vacuum) sensor in the aspiration line. When it detects a significant and rapid drop in vacuum (as when occlusion breaks) fluid is immediately bled into the aspiration line, and this temporarily increases the pressure in the aspiration line, neutralizing and temporarily reversing the postocclusion surge. Later machines have employed a variety of methods to try to reduce postocclusion surge—some of which also employ sensitive pressure sensing devices and rapid electronic responses.

The ability to program an adjustable rise time has already been mentioned. The AMO “Diplomax” and more recently “Sovereign” machines have made particular use of sophisticated microprocessors to enable their “occlusion mode phaco”. The

basic concept here is that the surgeon may wish to use one set of phaco parameters while the tip is unoccluded (e.g. continuous phaco at a relatively low flow while sculpting). Once the tip is occluded (which the computer detects by rapid rise in vacuum level) the surgeon may want different parameters to consume a nuclear fragment (e.g. pulse phaco, at lower power, but at a high flow). Exactly how the various parameters change, and at what threshold vacuum limit this occurs is set according to the surgeon’s preferences, and stored in that surgeon’s individual settings file within the computer.

Allusion has already been made in earlier sections of this chapter to the ability of the modern computers that control the generation of the currents that power the phaco probe itself. Control circuits within the computer continuously monitor the working of the crystals or magnet stacks in the handpiece and fine tune the frequency and power with which the needle is driven. Ability to adjust the foot pedal settings, range of travel in different modes, assignment of different functions to certain switches are particularly well developed in the unique dual linear foot pedal of the Bausch and Lamb “Millennium“ machine, which has three additional switches. One of these additional controls is a rocker switch to which the surgeon can assign various functions. For example it can be set to increase or decrease flow rate, vacuum limit, bottle height, etc. or be set to move backwards and forwards between program modes.

Some surgeons are intimidated by the added sophistication of the latest generation of phaco machines. However they do offer user the ability to control much of the procedure in a way which was not possible with earlier simpler machines, yet they can be used in a “simple” way if that is the surgeon’s wish. Notwithstanding these reservations, the ability to take more control is welcomed by many.

Peter L Davis

INTRODUCTION

Our ophthalmic literature has frequently presented inaccurate ideas concerning the basic physics of the energy source that emulsifies cataract. Included among them are the following

•Cataracts are emulsified by phaco needles moving back and forth at ultrasonic speed, jack hammering and cutting lens nuclei into an emulsate.

•A 45-degree angle phaco needle emulsifies hard lens nuclei more readily because a sharper tip cuts best into the nucleus.

The above statements have been repeated many times since the early days of phaco. A senior executive of a pioneering manufacturer of phacoemulsification equipment related to me told that his company did not wish to confuse eye surgeons with information about ultrasonic energy. Therefore, the marketing section of his company decided that it would be best for salesmen to inform surgeons the phaco needle was “cutting” thus making the new phaco method a simple extension of scalpel surgery. The knowledge that ultrasonically activated phaco needles are creating massive shock waves the same as those that have been used to scale teeth and clean surgical instruments is often met with skepticism. This chapter reviews library and basic research that shows the shock waves energy that emulsifies cataract are generated by imploding microbubbles that form and collapse in fluid when the titanium needle is ultrasonically activated.

History

The basic physics study of the massive energy created by imploding bubbles started when the British Royal Navy began using steam turbine engines in the 1890s. These

46 THE ART OF PHACOEMULSIFICATION

Fig. 4.1: Charles Parsons’ ship powered by a steam turbine engine

rotary engines had been engineered by Charles Parsons. He fabricated an engine and installed it in a small ship that he then demonstrated to the British Admiralty (Fig. 4.1).

These new engines doubled propeller speed but to the consternation of the Navy the torpedo boat “Daring” that was first equipped with the new engines fractured its propellers. Lord Raleigh1 led the research that revealed the rapid propeller revolutions were hydrodynamically creating bubbles that imploded under water generating massive shock waves that destroyed the ship’s propellers. Research has since led us to the current situation when imploding shock waves are generated hydrodynamically, by whistles and by ultrasonic transducers that convert electrical to ultrasonic energy.

KS Suslick2 has published a review of the basics of ultrasound and its applications. In the introduction to his text he states the use of ultrasound in industry and clinical medicine is common place but “there had been almost a complete lack of review material on the underlying principles from which such effects originate.”

Reviewing Suslick’s 1988 multiauthor book and his article in Scientific American3 and going through the physics literature leads one to the conclusion that our phaco needles are not oscillating chisels.4

Basics of Phaco Transducers

A transducer is a device that converts one energy form to another (Fig. 4.2). Handheld phaco transducers are converting electrical energy to ultrasonic (beyond 16,000 cycles/sec) acoustic waves. In the past, vibrating metal wafers were used in magnetostrictive devices notorious for generating and releasing heat that caused corneal thermal injury. Modern designs use piezo (Greek—to press) ceramic crystals to make the conversion. The piezo designs initially incorporated two crystals that were less durable than the magnetostrictive designs but were more efficient and generated less heat. In recent times, the transducers are manufactured with four crystals that makes them even more efficient (Figs 4.3 and 4.4).

CAVITATING MICROBUBBLES CREATE SHOCK WAVES THAT EMULSIFY CATARACT 47

Fig. 4.2: A transducer converts one form of energy to another

Fig. 4.3: Piezoelectric transducers have replaced magnetostrictive designs

Cavitation (Transient)

Eye surgeons dislike the gas bubbles that are released during phaco procedures to collect inside the corneal dome making it difficult to view the cataract. These annoying bubbles are air that was dissolved in the balanced salt solution (BSS) and was released from the BSS by ultrasonic wave activity in the eye (degassing fluids with ultrasound has been used in industry for decades).2 Unfortunately, these air-bubbles have been labeled “cavitation bubbles”. In contrast, ultrasound scientists discuss micron-sized bubbles formed by transient cavitation which is the formation of tiny invisible gas bubbles that implode creating energy at the end of acoustic horns in fluid (in ophthalmology—at the end of phaco needles) (Fig. 4.5). Ultrasonic waves released in fluid cause 100 to 150 micron size bubbles to form and then implode within a few acoustic cycles (current phaco transducers are vibrating at 27 to 55 kilohertz). The energy released by this microbubble implosion is in the form of shock and fluid waves. These activities are listed in Table 4.1.

48 THE ART OF PHACOEMULSIFICATION

Fig. 4.4: Drawing of both piezoelectric and magnetostrictive design transducers

Fig. 4.5: Svensson’s photo of microbubbles at the end of phaco needles

This energy has been used for many industrial purposes including the cleaning of surgical instruments and contact lenses (the tiny flame at 1500 degree C that forms as the microbubbles implode is too small to raise the temperature of the fluid).3

Activity Around Phaco Needles

Research done on phaco needles in vitro has demonstrated the same activity as has been shown in physics laboratories. The first papers using photography to demonstrate the microbubbles and flame were presented in 1991 at the American

Table 4.1: Activities of imploding microbubbles (transient cavitation)

1.Shock waves of 500 atmospheres (enormous force)

2.Fluid waves of 400 km/h

3.A flame at 1500 degree C within the microbubble

4.Radical ions

CAVITATING MICROBUBBLES CREATE SHOCK WAVES THAT EMULSIFY CATARACT 49

Society of Cataract and Refractive Surgery (ASCRS) Meeting. Table 4.2 outlines the initial presentations of re-phaco needle shock wave physics.

Table 4.2: Outline of two presentations at ASCRS 1991 showing evidence of transient cavitation with phaco needles

In more recent times, the demonstration of shock waves in fluid that have a different index of refraction (Schlieren activity) has been shown by optical and video recording (Fig. 4.6). Table 4.3 summarizes this research.

Table 4.3: Documentation of Schlieren waves at the end of phaco needles

Fig. 4.6: Photo from Fishkind’s video recording of shock waves anterior to a 45-degree phaco needle

In recent years, phaco needles have had their ends enlarged or recessed to increase their surface area and allow the formation of more microbubbles that implode and release more shock wave energy (Fig. 4.7). I demonstrate this idea with an illustration prepared in 1991.

Phaco surgeons often notice cataract tissue breakdown anterior to their phaco needles without the tip touching the cataract. This is because the shock waves are

50 THE ART OF PHACOEMULSIFICATION

Fig. 4.7: A 0.4-mm phaco needle may have more microbubble formation

focused in front of the phaco needle as shown by Schlieren imaging. The presentation that phaco needles are simply vibrating chisels was a convenient way to market phaco technique when it was introduced. However, a reading of the basics of ultrasonic energy and more recent ophthalmic in vitro and cadaver eye studies outlined in this chapter, reveals the forces that emulsify cataract are massive shock waves formed at the end of phaco needles by imploding microbubbles.

REFERENCES

1.Lord Raleigh: Phil Mag Serv 6:34-94, 1917.

2.Suslick KS (Ed): Ultrasound: It’s Chemical, Physical and Biologic Effects VCH Pub Inc: New York, 1988.

3.Suslick KS: The chemical effects of ultrasound. Scientific American 80-86, 1989.

4.Davis PL: Phaco transducers, basic principles and corneal thermal injury. Eur J Implant Ref Surg 5: 109-12, 1993.

KR Murthy

I N T R O D U C T I O N

Surgical procedures in ophthalmology are usually performed under local anesthesia. General anesthesia is employed when the procedure is expected to be time consuming or if the patient cooperation is not possible due to young age, mental status or extreme apprehension. Even among regional blocks, there has been a change, towards topical anesthesia, as the technique of surgery has been constantly altered. Knowledge of the principles of mechanism of drug action, as well as the administration techniques, recognition of complications, and their management, is essential for the success of the surgical procedure.

Mechanism of Action

Local anesthetics must be, water soluble as well as lipid soluble in order to traverse the nerve membrane and bind with the phospholipid membrane that surrounds the inner openings of sodium channels. This impedes the access of sodium, to the axons and results in a reversible blockage of the nerve.1 The pH of the solution has an effect, on the onset, and spread of the anesthetic effect.2 Buffered solutions are less painful during administration.3 The duration of the effect depends, on the length of time, the agent is bound to the nerve membrane. The anesthetic drug and its concentration, and its rate of removal have an effect on the duration of action. It is to be remembered that these agents can be, readily absorbed through mucous membranes, and can cause toxic reactions.4 Hence, a surface anesthetic should never be injected. Cardiorespiratory resuscitation facilities should always be ready at hand. The body can metabolize the drug when toxic reaction