Hi, I'm Chris Hood. Clinical Instructor and cornea and external disease. Cataract and refracted surgery at the University of Michigan Kellogg Eye Center. And today, we will be speaking about phacodynamics. The term phacodymanic was first coined by Dr. Barry Seibel more than 10 years ago. And he used the term to describe the physics of closed-system phacoemulsification cataract surgery and how it translates into clinical performance. In other words, it's a safe and efficient removal of a cataract through a small incision with minimal disturbance to the eye. As a subset of phacodymanics is the term fluidics, which describes a balance of fluid inflow and outflow of fluid in the eye of the type of cataract surgery. So to first understand phacodymanics, we have to understand phacoemulsification. And this was a principle that was borrowed by Dr. Charles Kelman in the 1960s from dentistry. The principles to use longitudinally oscillating tip in the ultrasonic range to emulsify and remove the nuclear fragments from the eye through a small initiation. And the frequency used today is between 28 and 45,000 hertz or cycles per second. The reason we use that range is that less frequency is inefficient and more frequency, higher than that leads to potentially bloom burn and heat build up. Now the actual mechanism of phacoemulsification is not entirely clear, and there's two possible mechanism by which it works. One is what we call the jackhammer effect, which is the effect of the direct mechanical impact of the needle that impacts and physically strikes the fragment that's trying to be removed. The other effect is called cavitation. In this effect, the needles moving in such a high frequency that are actually pulls this all gases out of solution. And then their frequency moving in the opposite direction are actually compresses and then causes implosion of those microbubbles. Which then creates a wave of heat and pressure that radiate from the bevel of the phacoemulsification tip. And these two likely probably acting combination in phacoemulsification. I want to introduce a term here called followability. This is defined as the combination of the forces that attract the fragments to the tip of the phacoemulsification hand piece against the repulsive action of the ultrasound. Because again that needle is actually pushing things away physically. So in other words, it's how well things are brought in tip of the ultrasound instrument and stay there. The term stroke length is another important term to understand, and it's basically the physical length that the needle moves within the phaco hand piece. And it's between two and 4,000 of an inch. And actually, the power setting we set on the phaco machine is a reflection of the stroke length. A higher stroke length is set at a higher power. Now it goes without saying that the efficient phacosurgeon removes the nucleus using the minimal amount of required power and energy in the eye. And this is because of a couple of reasons. One is that the heat buildup from the phaco can actually cause a wound burn, which can be devastating in its consequences. Secondly, the ultrasound energy has been shown to decrease the number of endothelial cells which can lead to post op corneal edema. And then lastly, there is the breakdown of the blood-aqueous barrier which has also been shown to be linked to the amount of energy used. So we can possibly decrease our post op inflammation and possibly even the rate of cystoid macular edema by using lower energy to remove the nucleus. So the savvy phaco surgeon will actually alter the amplitude, duration, and delivery of the phaco energy in order to efficiently remove the cataract from the eye. But it requires a knowledge of phacodynamics. So at this point we have a question. In what form of power modulation for phacoemulsification does the depression of the foot pedal in position to be progressively increase the duty cycle? And we see your choices there. A, continuous phaco. B, phaco pulse. C, phaco burst. D, phacodynamics. Or E, surge. So the answer is C, phaco burst. Let's first talk about different ways to modify the application of phaco energy. And the first of these is called continuous phaco, which is just what it sounds like. It's energy delivered continuously with no off period for the phaco energy. And in this setting, the surgeon sets the maximum preset power. And with the pressure of the foot pedal in position 3 the stroke length increases up to that maximum preset power that's been set by the surgeon. So an example will help to clarify, if we look at the bottom left graph. We can see that the power has been pre-set to a maximum of a 100%. And with the depression of the foot pedal into position three, so the linear rise in the power applied up to the pre-set maximum of a 100%. On the right of the screen we can see at the pre-set power has been set to maximum only 50%. And even with complete depression of the foot pedal in position 3. The energy is capped at 50%, but does increase up to that point in a linear manner. The other forms of phaco modification are all going to involve a period of off power alternating with the period of phaco on power. And this is beneficial because it actually allows a vacuum only to be applied during the period when no power, phaco power is applied. This allows the emulsate of the nuclear fragments to be removed and allows heat to dissipate when there's no phaco energy being applied. So let's talk about phaco pulse, which is one of these ways where phaco energy can be alternated with on and off periods. So in phaco pulse, as the foot pedal is depressed, the phaco power increases. And you'll see the phaco on and off alternating in the graph down below. Let's talk some more about that. So a few terms to find for phaco pulse, the first is pulse rate, this is how many pulses are delivered per second or pps. Secondly, we have what we call the duty cycle which is the ratio of the ultrasound on time to the total interval of time that you're applying the pedal. So the benefit of phaco pulse is that it actually decreases your overall phaco time. It increases the followability so pieces are drawn to the tip and stay there better. And there's less energy used, so there's likely some thermal protection that occurs with this power modification. Let's look at the two graphs below. The graph on the left, you can see that the energy applied and there's an energy off period followed by each of those pulses. As the foot pedal depress, the ultrasound energy increases up to the preset maximum. In this case, it's 100%. And the periods of on and off stay alternating as we increased our power applied up to a 100%. In the right graph we have a slower pulse rate, so it's less pulses applied per second. This is to be preset by the surgeon, but we could still see that applying the foot pedal or to pressing it into position 3 all the way down still increases the power applied up to the preset maximum. In this case, a 100%. Let's talk now about another form of power modification with phaco, this one is called phaco burst. In phaco burst there is again a period of phaco on followed by a period of phaco off. In this case, it's actually very well duration of period between the burst, but the phaco power actually remains set. So the phaco power is set beforehand by the surgeon. But it's a way to frequency of burst that increases the foot pedal excursion. So essentially if you think of the extreme of foot pedal depression in this case. We actually approach continuous power of as the foot pedal is depressed. So essentially we have linear control on the number of burst applied per second up to the point where it's continuously applied ultra sound. So in other words, you can think about it like the rest interval between the bursts, progressively decreases as the foot petad is depressed. But again in this case the pressing the foot pedal does not increase the power applied, power is applied at a preset limit by the surgeon. So let's talk about some other ways that phaco energy can be modified. What we've been talking about so far is so-called longitudinal or traditional phaco. Where there's really a forward and back jackhammer style movement, axial movement of the phaco needle in the handpiece. But another way to modify this movement is to actually apply it with a torsional or so-called Ozil movement. Which is actually a lateral movement that we can see this plate in the lower right hand picture here. And we can see the picture looking at the handpiece with this lateral movement going back and forth. Now the benefit of this is that there's better followability. Because the ultrasound handpiece needle is no longer pushing our fragment away when we're trying to actually aspirate it into the handpiece because only that lateral movement. So our lumen actually occludes more easily. We have minimal chatter, which is the movement of that nuclear piece against the phaco handpiece. And again, we have improved followability to remove efficiently our nuclear fragments. And these can actually be used in combination together. So you can actually have a traditional jackhammer movement, as well as an oscillatory or torsional movement, combined together to different degrees. So you can get the benefits of both of these when applying phaco. We can also modify the shape of the phaco needle, which can influence the fluidics and the power delivered. The details of these are beyond this lecture, but we can see on the picture on the right, the needle is actually bent. And this is the so called Calman style needle which helps to maximize the traditional movement of the hand piece. So that's the so called Calman needle. So let's talk now about fluidics, and we have to find this previously as the balance of fluid inflow and outflow from the eye. And our goal of fluid increase is to maintain a constant intraocular volume for the stable and deep anterior chambers. So we can officially remove the nucleus in the cataract. So it follows from this, that if we remove fluid out an increase rate from the eye. We have to balance it by an increase inflow to maintain the steady state system which is the goal of fluidics. So looking at our model here, where we can see this yellow circle as a representation of the eye in the phaco handpiece, of course is in sort of into the eye. We can see in the side of the phaco handpiece, there's a white circle that's faces us. There'd be another circle on the other side that represents the irrigation port of the phaco handpiece. We can follow this back and see that there's a white box in this other box. And this white box actually represents the pump which we'll talk about more. And this is the way that the aspiration force is created. And we can see that there's a bottle represented here that has bound salt solution that's going to be flowing into the eye by a gravity fed system. And it simply is a gravity fed system. So the higher the bottle height, the more fluid inflow there is into the eye. So if we have a steady state system where there is no fluid being removed, it follows up that if we raise that bottle height that we actually deepen the interior chambers. This is the modification we can make if we're trying to work at the time of cataract surgery. So let's talk now about vacuum sources. And there's a few different ways that vacuums can be created. And the goal of vacuum is to create a fluid outflow from the eye. So the more common system to use is the so called peristaltic or flow pump, which is pictured on the left side here. And this is delivered by a series of rollers which press the tubing and move fluid through the tubing in such a manner. We can see that the direction of the arrow indicates the direction of fluid flow as the rollers are spinning. So the faster these wheels are spinning, the higher the flow will be through the tubing. On the right side here, we can see a picture of a so called vacuum pump. In this pump, there is gas is shown by the red arrow that flows through the housing. And as the gas is pumped through the housing, there is a force that is directly transmitted to box D, and box D is a rigid cassette. And we can see that the black arrow on the right side here is the direction that fluid will then be pulled into the cassette. From the forces of the gas being pumped through the so called vacuum pump. And these have different ways of functioning at the time of cataract surgery, so we'll talk about those shortly. So now we have a question, so what modification would you make on phaco machine to slow down events in the anterior chamber if they're happening too rapidly? A, decrease the aspiration flow rate. B, increase the aspiration flow rate. C, decrease the vacuum level. D, raise the irrigating bottle height. E, lower the irrigating bottle height. Or F, change from a peristaltic pump to a vacuum pump. And the answer is A, decrease the aspiration flow rate. So let's now talk about fluidics, outflow. And this is more complicated than fluid inflow to the eye. We have a few different factors that we have to take into account. And let's define a few terms here. So we'll first then define the aspiration rate. This is actually the flow of fluid in volume or cubic centimeters per minute through the aspiration tubing. And we can see in our diagram picture here, the blue arrows are being drawn into the phaco hand piece. And the aspiration flow rate or just called the flow rate determines how well particles are actually attracted and how quickly they're attracted to the phaco tip. So if we increase the flow rate, then we actually increase the speed of events that are happening in the anterior chamber of the eye. So events will happen more quickly, fragments will be drawn to the phaco handpiece with more speed. That's what's governed by the aspiration flow rate. We also define what we call the vacuum or the aspiration level. And this is defined as the magnitude of the negative pressure in millimeters of mercury just inside the tubing of the phaco handpiece. The vacuum determines how well once the tip is occluded, particles stay on the tip. So in this case, it's how well do we have a holding power of a nuclear fragment on the phaco tip. And relation to this we have it called the rise time and it's relates specifically to flow pumps. And the rise time is the amount of time that's required to reach a given vacuum. And so what happens is vacuum actually has to build up to our preset maximum. So we actually set the maximum, and we have to have this time required for the vacuum to build and once the tip is occluded. And the tip has to be completely occluded for the vacuum to build. That's very important for the phaco surgeon to understand. So this set of pictures will help us to understand how the vacuum actually rises when occlusion is achieved. So we can see in the picture on the right here. The yellow blob represents a nuclear piece that's attracted to the phaco tip, it fully occludes it. At time 0.2 seconds, our flow rate's 20. Our pre-set maximum vacuum is 400, but our actual vacuum just inside the handpiece is not yet risen. It's still just about 0. A couple seconds later with the tip fully occluded still, our vacuum is now rising and it's 200. So we're halfway up to our pre-set maximum. And if we advance another two seconds, our vacuum must continue to rise because our tip is still completely occluded. And now we're up to our maximum preset vacuum of 100%. And this is what occurs in a flow pump, when the vacuum has to rise after tip occlusion. So let's talk now about how these relate to the vacuum sources. So we talked about a peristaltic pump, where the rollers actually move fluid through the pump. And the speed of the rollers determine the flow rate. So the surgeon actually sets both the flow rate and the maximum vacuum on the machine. And the vacuum will rise when seclusion is achieved, up to the preset maximum. So the surgeon sets both the flow rate as well as the vacuum. In contrast the vacuum pump, the surgeon actually only sets the vacuum and the flow rate is determined by other parameters in the eye. So the vacuum is almost instantaneously achieved once occlusion is achieved on a nuclear piece. And so in this case the flow rate is sort of secondary and can be modified by a variety of other factors in the eye. It is not set by the surgeon directly. So a question, for which pump type does the surgeon have direct control of flow and indirect control of vacuum? And the answer is a peristaltical flow pump. So now I want to talk about another concept we call surge. And this can be a great utility to the phaco sort of to understand, because when it does occur, it can have devastating consequences. So what surge refers to is a temporary shallowing of the anterior chamber. When the fluids leaving the eye is greater than the fluid coming into the eye. And so in our picture on the left here, we have a nuclear fragment that's occluded the phaco tip. Our vacuum level has risen to the maximum that we've pre-set it at. And we can see that the actual tubing is starting to indent because the vacuum is so high and the phaco piece is fully occluding the nuclear fragment. As a phaco energy is applied, the fragment is then emulsified and gets drawn into the phaco handpiece. At that level, the vacuum drops instantaneously. However the expansion of the tubing and the rapid decrease in vacuum actually causes a surge of fluid to flux into the handpiece. This then causes a rapid shallowing of the anterior chamber because the inner flow of fluid cannot match the quickness of which the fluid was just removed. In this case, we can actually have trampolining of the posterior capsule. It can sort of bounce up, and at this point you can actually cause a posterior capsule tear, if you're not careful. The cornea can actually even collapse, if there is a large amount of surge. The surgeon has to understand this and try to modify factors to prevent it. So let's talk now about ways to reduce or prevent surge. And the easiest of these is actually just to lower your flow rate and your vacuum on your phaco machine. And by doing that, you'll lower your amount of fluid coming through the hand piece and you can actually reduce surge in that way. Manufacturers have also introduced tubing that has reduced compliance because it's actually the compliance of the tubing. The compression than the subsequent expansion that leads to the rapid influx of fluid in lose to surge. And so this is also helped to reduce the incidents of surge. And lastly, I want to mention what we call the aspiration bypass stabilizer, the ABS. You can see it pictured on the right side here of the screen. And on the bottom you see a small hole that's actually drilled in the shaft of the phaco needle. And what this does is actually allows some fluid to go into the phaco handpiece even when the tip is occluded, as it's supposed to be demonstrated in this picture here. So with some fluid flow, there's actually dampening of surge once the occlusion does break. And we can see at the right side here, the green line represents our maximum pre-set vacuum. And on the top we've reach our maximum pre-set vacuum by the red bar with the occlusion of the phaco tip. On the bottom we can actually see that because there's some fluid flow our pre-set vacuum is never reach by the actual vacuum which is represented by the red bar here. So you sometimes have to actually account for this and raise your maximum pre-set vacuum a little bit, in these cases when you have the ABS in place. But it should help to reduce the amount of surge, potentially reduce your complications. So in conclusion, the term phacodynamics refers to the physics of closis and phacoemulsification cataracts are great in the efficient removal of the nucleus in cataract lens. And the technology of phacoemulsification has advanced rapidly. So the savvy phaco surgeon has to really understand phacodynamics in order to be safe and to maximally remove and efficiently remove the nucleus in the cataract lens. So by understanding these concepts especially early in your career as a phaco surgeon. You're going to increase surgical efficiency and potentially decrease complications. So obviously, our goal is safe and efficient removal of cataracts through a small incision and with minimal disturbance to the eye. Thank you.
