A couple more unobvious things that you might not be told before laser vision correction
- пятница, 8 мая 2020 г. в 00:13:18
geektimes
https://habr.com/en/company/klinika_shilovoy/blog/500894/
Today, without the «tin», as you asked
There is already a post about how the laser cuts by creating millions of cavitation bubbles in the cornea layer of the eye, and analyzing telemetry from the real operation in seconds with comments of the surgeon's actions.
— If I look away while the laser is running, what will happen?
You simply will not work. In fact, immediately after anesthesia, the eye is pressed against a special pneumocapture. To blink at you too will not leave because of fixing (it is not long and not for long). The only moment where it is possible to seriously disrupt the course of the operation is to pull the head down strongly, pulling it out of the headrest by a serious willed effort. In this case, the operation will instantly stop. More precisely, it will stop even before the loss of capture (details below).
— How should an operating room be prepared?
In general — as a normal operating room, that is, a room with a clean area (air filtration, overpressure to prevent contamination from the outside after cleaning). It is important for the procedure that microparticles of dust flying in the air do not fall between the laser lens and the eye.
— What are the chances that I pull my head during an operation?
You have little chance, but about once every 2,000 operations the patient still manages to do this because of an external event, such as a flood or an earthquake. In serious clinics the process of operation is controlled by several people, the display is lit, the door is fixed, and in general everything is done to prevent this from happening. In the laser «Visumaks» there is a special function that automatically blocks the opening of the door during operation of the laser. For example, before the operation, the surgeon and the nurse alter the algorithm of actions with the patient several times. And yet, so that you are not nervous, the surgeon is constantly talking to you during the operation (surprise: most likely, when he is silent, the operation is in fact already over, and you just start to think that you need to concentrate). In your hands you have a special soft toy that distracts motor control. The head lies on a special lodgement on which it is difficult to move without a conscious strong effort. In general, almost everything is thought out. Yes, you take a toy with you as a souvenir of a successful operation.
— What happens in this unlikely event if the pneumatic grip is broken?
Pneumatic gripping is designed to keep the eye soft enough, because a strong pressure on the surface changes the pressure in the eye and the geometry of the tissues. In addition, the smart bed on which you lie, adapts to the micro-oscillations and slightly presses you up. If fixing disappears, the laser will stop the operation within a few milliseconds. This happens as follows: in the pneumatic holding there is a pressure gauge that controls, in fact, the fixation of the eye. If you pull your head, then a lot of things happen in the microcosm. First, pressure begins to fall, and then the eye loses contact with the grip. The sensor has time to turn off the laser before the eye finally loses contact and remembers the control breakpoint.
— I'm paranoid. Suppose I shook my head, the laser turned off. What's next with the eye?
Depends on where exactly the stop occurred.
In general, in the case of a complicated stop, we recommend waiting 2 hours before the gas leaves the cavitation bubbles — this is necessary not only for the surgeon to make an informed decision and make the diagnosis again, not on the fly, but also for the patient to calm down.
— And if I shake my head not under the laser, and when will the surgeon take out the lenticle?
One such case really was during the first 500 operations in Germany: the patient became nervous and jerked, and it was at that moment that there was a sharp spatula inside the eye (the fifth generation of lasers required sharper tools — you had to cut bridges and spikes a little, now spatulas are blunt). Since then, the patient is not in danger, since the mandatory part of the technique is to grip the eye with tweezers to prevent this. Yes, by the way, in this patient, nothing terrible happened — the surgeon a little differently exfoliated the lenticle, this did not affect the quality of vision. The worst thing that can happen is that the incision will expand. It naturally increases in the radial direction — it is not very scary, just as if the surgeon made a slightly larger area to gain access to the lenticule.
— What is energy protection used?
The standard femtolaser VisuMax is equipped with built-in backup power batteries and has a built-in power filter. Other lasers may not have their own batteries, but have standard inputs for medical UPSs. Even if the entire clinic turns off the electricity, the laser will be able to complete the operation. New, however, will not start. In case of problems, there is another level of insurance — VisuMax remembers the place of the stop and is able to find it later…
— Does the laser have a seismic protection?
For 10 years of practice, we have never heard that extraneous vibrations interfere at the stage of performing a laser incision. At the stage of the surgeon's handwork, they could really hinder, but the special emphasis under the doctor’s elbow is designed so that the patient and the surgeon’s hand become one rigidly connected system. In earthquake-prone regions there are special structures of the building in which the operating units are located.
— Can I come already “caught” or will I be immersed right in you in a drug sleep?
In theory, this is possible, but in practice we need a patient with responsive eyes for precise centering (more precisely, its control). So forget about medication sleep (even light) — your 20-25 seconds of fear and discomfort are not worth the increased risk of surgery. Accordingly, for the same reason, we do not recommend using psychoactive substances before surgery. Well, do not come in altered states.
— What will I feel during the operation?
Before the operation, you will pass a triple briefing and three times listen to the story about the «green light bulb». The operation is performed on an outpatient basis, on both eyes it takes an average of 20 minutes. If you are a little nervous and nervous, this is perfectly normal. If you experience too much anxiety, you can ask for a sedative pill. Before the operation, we will once again examine all the data, after which the doctor will drop painkillers into the operated eye.
To prevent blinking during surgery, the eyelid is fixed with a special dilator, the second eye is closed. In order not to create risks of the appearance of external objects in the laser optical zone, the patient may wash the surface of the eye and trim the protruding eyelash. Then the patient is transferred to the working position of the laser by moving the table and asked to look at the green LED that moves and becomes clearly visible. At this point, the surgeon centers the optical surface of the lens with respect to the optical axis of the eye, seeking contact between the lens and the surface of the cornea. Grabbing and fixation is performed using a special lens, adjustable in accordance with the individual characteristics of the patient's eye. Then the laser works like this .on telemetry. At the end of the laser operation, the patient is transferred to a safe position on the table and transferred under the microscope. The doctor selects the lenticule with a spatula and removes it. In this case, the patient usually sees a bright point and feels light vibrations.
Then the patient may cry a little for a couple of hours. But not only for joy, but because of the reflex that causes tears when the eye is damaged. For some time, some people may feel a slight burning sensation in their eyes, which feels much weaker than the smoke when cooking kebabs. Most often, this stage takes place without any special emotions. The next day, the discomfort disappears, the visual load is not limited, but it is better, in spite of this, not to strain your eyes too much with reading, telephone or movies. You can not visit the sauna for some time, so that water does not fall into a 2.5-mm incision, which was used to access the lenticule.
— How is the laser operating mode and cut shape calculated?
Firstly, one of the most important things in the preparation of an operation is accurate diagnosis and accurate measurement of the eye. Comprehensive primary examination before correction in our clinic is 16 examinations on the most modern specialized equipment under a single European protocol. This is followed by a detailed consultation with one of the surgeons-professors to discuss the correction option that is optimal for the patient, taking into account his visual needs, lifestyle and anatomy of the eye. Examination and postoperative observation of patients is performed only by ophthalmologists with experience in the field of refractive surgery. All this is extremely important for correct calculation of laser correction parameters. Based on them, a model for laser automation is set. About this entire stage, I will tell you more later. Secondly, The parameters and shape of the cut depend on the size of your myopia and astigmatism, the size and shape of the cornea, even the size of the palpebral fissure and the nose. In the case of complex cases of astigmatism, a great deal depends on the correctness of capture and alignment, but how this is done and how the methods evolved is a separate story.
Thirdly, the working mode depends on the preferences of the surgeon, on his skills and skill — we change the porosity and spot diameter to obtain the optimal quality of cuts and optimal conditions for the extraction of lenticules. Fourth, on a specific VisuMax laser machine: you need to adapt a little to your particular laser, they are slightly different in the series.
— What to do if the center of view does not coincide with the center of the pupil?
This is infrequent (about 10%), but a possible case, when the angle between the visual and anatomical axes of the eye (Kapp angle) is large. It is detected before the operation and requires a special change in the model. Here the qualification of the surgeon is extremely important. Also a topic for a separate story.
— How about the additional corrections after SMILE?
In our practice (German and Russian), not a single patient had a need for additional correction. But technologically there are special methods that allow for additional correction. How exactly — to decide the doctor. At the European Congress in Copenhagen this year, Dr. Dan Renstein gave a report in which he listed 5 different methods of additional correction in different cases. So, if the patient suddenly needs, there is plenty to choose from. After SMILE, the quality of optics on 6th generation lasers is consistently higher than after LASIK or femtoLASIK, the probability of side optical effects is significantly less. When we are planning a laser correction, we expect that this procedure is done only once in a lifetime. The percentage of the correction for the generalized international statistics for SMILE is the lowest today — about 0.5%. In specialized clinics with experienced doctors, it is significantly less. I repeat, we didn’t have them specifically, but the probability cannot be ruled out, so this is, rather, a matter of experience and luck.
— Why do I need to get lenticular, and not evaporate it inside the eye?
In theory, of course, you can try to evaporate the layer after layer of lenticular inside the cornea, without “opening” it at all. The difficulty is that it will require the removal of decay products from the eye, and the exchange of fluids there is very, very slow. According to preliminary calculations, in theory, it is possible to do 0.25 diopters per week, plus a perfect positioning system will not be needed yet to continue the work started last time. Well, either technical tags right in the eye, which is also not the best idea. In practice, the restriction is even simpler: no patient will pay 10–20 times per operation (and the price will be slightly lower than with the usual ReLEx) and go to it for half a year every week. Therefore, while this is an idea for the future.
— What will happen when nanolasers appear?
Now nanosecond lasers are undergoing clinical trials. Their use will allow working at higher frequencies with a slightly lower heat transfer into the fabric, but before the first practical operations “into combat” there will be at least 5–7 years at least, rather, all 10. This is a quantitative growth of several percent, rather than qualitative, in the technology of operations in the eyes now, little will change fundamentally.
— Why now you can often find proposals for laser vision correction at very low prices, on what they save?
Now there are still a lot of clinics that work on morally and physically outdated equipment for diagnosis and surgery. And sometimes without the necessary diagnostic equipment. There are even such clinics that call themselves «premium», and the equipment of the past and before last generations. Hence the complaints of patients in the postoperative period on the quality of vision and other problems. Old laser systems are not capable of solving problems for which modern lasers are designed. Therefore, when choosing a clinic, ask what laser device will be used and what year it is manufactured.
— How much stronger is the cornea weakened during the LASIK procedure than with ReLEx?
While understanding is intuitive: ReLEx is better. A practical report on a multi-year study on biomechanics will be published in 7 months. Then we give the numbers and facts. In general, you can no less intuitively compare the results on the loss of innervation and other side effects such as keratoconus — in the case of LASIK / FLEX-methods there are many times more. But, I repeat, it is better to discuss this in numbers after the publication of 10-year studies, which are simply not there yet. Quite briefly — the FDA has the most stringent requirements and always insists on conducting its own independent research, but it found SMILE suitable for use in US clinics as one of the main methods. In Russian Wikipedia there are statistics on the side effects of various operations with confirming links.
— Why is ReLEx made only by femtosecond lasers?
In Lasik, two lasers are used: femtosecond to remove the “cap”, and then excimer to evaporate a wide lens. For precise minimally invasive (and transmitting little heat to the bowman membrane and below) operations, lasers with a frequency of about 500 kHz and very precise focusing are needed — traditional excimer ones are not suitable. Currently, only Zeiss optics provide the necessary accuracy, that is, lasers of the VisuMax series (the old models are most likely familiar to you). 13 microns thickness is one diopter. This accuracy was achieved by combining the actual laser tube from the industry (initially, seemingly cosmic) with Zeiss lenses. It was the latter who created focusing optics for medicine.
— What you need to know about the cornea and what features of the regeneration of each layer?
The cornea is the part of the eye that you can easily see: a convex transparent part that contacts the air. The usual diameter is 10–12 mm for people of any race. In the center, the thickness of this convex-concave lens is 520–560 microns, with an edge of about 1 millimeter (all sizes are average, there are thin and very thin corneas). The cornea has 5 layers. Here is the concept:
But in scale from Wikimedia Commons:
1. corneal epithelium; 2. Bowman's membrane; 3. corneal stroma; 4. Descemetov shell; 5. corneal endothelium.
Epithelium The cornea is a multi-layered flat tissue and accounts for about 10% of the entire thickness of the cornea. Corneal epithelium cells are located in 5-7 rows. The corneal epithelium performs a mechanical protective function, as it prevents microorganisms and foreign bodies from penetrating inside the eye; biological protective function, as it contains cells that are involved in the immune response, the optical function — the mucin of the tear film fills all the uneven elements in the surface layer, which provides a smooth, transparent surface for the passage and refraction of light rays; membrane function — the corneal epithelium is a biological membrane through which certain substances can penetrate. Like the normal epithelium of the skin, it regenerates well, and during PRK surgery it is removed to gain access deeper.
The second layer is the Bowman's membrane. This is an incredibly thin and important layer immediately under the epithelium. Bowman's membrane is located under the basement membrane, has a thickness of 12 microns and does not contain cells. The Bowman's membrane consists of randomly arranged collagen fibrils. It has a smooth front surface and a back surface for smoothing the non-uniform relief of the stroma, which ensures the transparency of the cornea.
The Bowman's membrane cannot be restored after damage, therefore, after corneal damage in this part, scars are formed at the site of defects and the transparency of the cornea in these areas is disturbed — turbidity is formed. When laser correction lenses are formed deeper. However, at any cut through the bowman's membrane, we cut the nerve endings. During PRK surgery, it is removed to gain access to the stroma.
Accordingly, with minimally invasive removal of lenticules with ReLEx, there is only one short cut of 2.5 mm, and not nearly the entire circumference, as in the case of LASIK / FLEX-like methods. It is the injuries of the Bowman's membrane that violate the epithelialization of the eye, innervation, and give other side effects.
The next part of the cornea is the stroma. This is where the main work goes. Fabric — collagen filaments impregnated with hyaluronic acid. When magnified, they resemble ropes, such as in section:
The stroma is the main part of the cornea and occupies approximately 90% of its thickness. The cornea stroma consists of parallel plates. Plates are formed from collagen fibrils. Collagen provides transparency of the cornea and its strength. In the stroma of the cornea there are two main parts: the anterior stroma of the cornea and the posterior stroma of the cornea. The anterior stroma is looser and consists of thinner plates; the posterior stroma has a denser and more compact structure.
Stroma regeneration is carried out by keratocyte cells, which are capable of synthesizing collagen and, due to this, maintain an optimal level of collagen fibers and extracellular matrix.
These same ropes can perfectly coalesce, if they are stuck in one another (with the formation of nodes-adhesions, which interferes with visual acuity), but at the same time, being put on each other overlap (that is, from different angles), they do not form these nodes, but simply interlock. During the laser correction ReLEx SMILE we cut the lens in this layer and pull it out. After the operation, the cavity in the cornea closes — the “ropes” lie on top of each other, but at the places of the incisions a clear boundary does not form from the joints, that is, everything remains transparent (splices of single collegan filaments occur at the border of the lens, that is, in outer diameter). The frame is supported as usual — the bowman membrane stretched from above and the lower layers.
The next two layers are Descemet's membrane and endothelium. — we are almost not interested in laser correction, because we are not affected by these operations. This is actually a soft and rigid boundary of the body, a kind of standard “casing” for the body.
— What will happen after the operation?
On the day of surgery, most often after a couple of hours, you hardly feel any discomfort and it disappears completely the next morning. The main limitation after surgery is not to rub the eyes with your hands. During the week after the surgery, you need to bury the eye drops obtained in the clinic. If necessary, we will also provide preparations of artificial tears.
Visual acuity in the first days may vary somewhat. In order for the surfaces to come together after extracting the lenticules, a slightly longer process of visual rehabilitation is needed than with methods that imply “full opening” of the cornea of the eye. It takes time for the eye to learn to focus on close objects in order to make the right muscles work. We conduct examinations to check the results of the correction every other day, a few days later and a few more weeks after the operation.
After two or three days, you can get back to work and play sports. In general, the next day after the operation, you can quietly lead a normal life (ride a bicycle, bend, etc.) — there is no risk of a flap-flap moving as with other techniques. The visual load from the day after the operation is not limited, but it is better not to abuse the laptop, books and tablet during the first week. However, swimming and visiting the sauna should be abandoned for a couple of weeks in order to reduce the risk of infection through an incision in the cornea.
If we compare it with LASIK, then the recovery of vision is more gradual, within a week, but the restrictions on physical exertion are much less. I will say that this is probably one of the most frequent objections of patients — not everyone is ready to sacrifice a week without active visual load for the sake of good biomechanics of the cornea. As a doctor who is concerned about safety, I do not really approve of such an approach, but if the patient has a very high quality of vision the very next day, we usually meet them, explaining the risks. Seen under a microscope, a week after SMILE, it is very difficult to find the place of lenticular removal and the junction of the tissues when observing the patient's eye. After the LASIK / FLEX-methods, the correction zone is very clearly located on the damage at the edges, sometimes in the wake of displacements.
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Today, without the «tin», as you asked
There is already a post about how the laser cuts by creating millions of cavitation bubbles in the cornea layer of the eye, and analyzing telemetry from the real operation in seconds with comments of the surgeon's actions.
Now FAQ about various related things
— If I look away while the laser is running, what will happen?
You simply will not work. In fact, immediately after anesthesia, the eye is pressed against a special pneumocapture. To blink at you too will not leave because of fixing (it is not long and not for long). The only moment where it is possible to seriously disrupt the course of the operation is to pull the head down strongly, pulling it out of the headrest by a serious willed effort. In this case, the operation will instantly stop. More precisely, it will stop even before the loss of capture (details below).
— How should an operating room be prepared?
In general — as a normal operating room, that is, a room with a clean area (air filtration, overpressure to prevent contamination from the outside after cleaning). It is important for the procedure that microparticles of dust flying in the air do not fall between the laser lens and the eye.
— What are the chances that I pull my head during an operation?
You have little chance, but about once every 2,000 operations the patient still manages to do this because of an external event, such as a flood or an earthquake. In serious clinics the process of operation is controlled by several people, the display is lit, the door is fixed, and in general everything is done to prevent this from happening. In the laser «Visumaks» there is a special function that automatically blocks the opening of the door during operation of the laser. For example, before the operation, the surgeon and the nurse alter the algorithm of actions with the patient several times. And yet, so that you are not nervous, the surgeon is constantly talking to you during the operation (surprise: most likely, when he is silent, the operation is in fact already over, and you just start to think that you need to concentrate). In your hands you have a special soft toy that distracts motor control. The head lies on a special lodgement on which it is difficult to move without a conscious strong effort. In general, almost everything is thought out. Yes, you take a toy with you as a souvenir of a successful operation.
— What happens in this unlikely event if the pneumatic grip is broken?
Pneumatic gripping is designed to keep the eye soft enough, because a strong pressure on the surface changes the pressure in the eye and the geometry of the tissues. In addition, the smart bed on which you lie, adapts to the micro-oscillations and slightly presses you up. If fixing disappears, the laser will stop the operation within a few milliseconds. This happens as follows: in the pneumatic holding there is a pressure gauge that controls, in fact, the fixation of the eye. If you pull your head, then a lot of things happen in the microcosm. First, pressure begins to fall, and then the eye loses contact with the grip. The sensor has time to turn off the laser before the eye finally loses contact and remembers the control breakpoint.
— I'm paranoid. Suppose I shook my head, the laser turned off. What's next with the eye?
Depends on where exactly the stop occurred.
- In approximately 50% of cases, it will be possible to continue normally, but the surgeon will have to make a small incision at the stopping point with a diamond scalpel (a small uncut bridge is formed there). I have 300 ReLEx operations per year, and so far there have been no cases. My partner and inventor of technology, Professor Sekundo spends more — about 8 years in large quantities. With thousands of vision correction operations with such a scalpel at the ready, he has never used it during ReLEx.
- In about 10% of cases, a different type of correction will be required. We always choose the best option and stop at the slightest doubt. A micro incision in the eye itself almost does not affect anything, the key change in mechanics is precisely the extraction of the cut-out lenticule lens and the collapse of the resulting cavity.
- ВIn some cases (about 40%), it will be necessary to make a wide cut, that is, to change the operation from SMILE to FLEX — it is more invasive, but the quality of the correction is the same. It was twice in 8 years in a German clinic, both patients did not experience complications.
In general, in the case of a complicated stop, we recommend waiting 2 hours before the gas leaves the cavitation bubbles — this is necessary not only for the surgeon to make an informed decision and make the diagnosis again, not on the fly, but also for the patient to calm down.
— And if I shake my head not under the laser, and when will the surgeon take out the lenticle?
One such case really was during the first 500 operations in Germany: the patient became nervous and jerked, and it was at that moment that there was a sharp spatula inside the eye (the fifth generation of lasers required sharper tools — you had to cut bridges and spikes a little, now spatulas are blunt). Since then, the patient is not in danger, since the mandatory part of the technique is to grip the eye with tweezers to prevent this. Yes, by the way, in this patient, nothing terrible happened — the surgeon a little differently exfoliated the lenticle, this did not affect the quality of vision. The worst thing that can happen is that the incision will expand. It naturally increases in the radial direction — it is not very scary, just as if the surgeon made a slightly larger area to gain access to the lenticule.
— What is energy protection used?
The standard femtolaser VisuMax is equipped with built-in backup power batteries and has a built-in power filter. Other lasers may not have their own batteries, but have standard inputs for medical UPSs. Even if the entire clinic turns off the electricity, the laser will be able to complete the operation. New, however, will not start. In case of problems, there is another level of insurance — VisuMax remembers the place of the stop and is able to find it later…
— Does the laser have a seismic protection?
For 10 years of practice, we have never heard that extraneous vibrations interfere at the stage of performing a laser incision. At the stage of the surgeon's handwork, they could really hinder, but the special emphasis under the doctor’s elbow is designed so that the patient and the surgeon’s hand become one rigidly connected system. In earthquake-prone regions there are special structures of the building in which the operating units are located.
— Can I come already “caught” or will I be immersed right in you in a drug sleep?
In theory, this is possible, but in practice we need a patient with responsive eyes for precise centering (more precisely, its control). So forget about medication sleep (even light) — your 20-25 seconds of fear and discomfort are not worth the increased risk of surgery. Accordingly, for the same reason, we do not recommend using psychoactive substances before surgery. Well, do not come in altered states.
— What will I feel during the operation?
Before the operation, you will pass a triple briefing and three times listen to the story about the «green light bulb». The operation is performed on an outpatient basis, on both eyes it takes an average of 20 minutes. If you are a little nervous and nervous, this is perfectly normal. If you experience too much anxiety, you can ask for a sedative pill. Before the operation, we will once again examine all the data, after which the doctor will drop painkillers into the operated eye.
To prevent blinking during surgery, the eyelid is fixed with a special dilator, the second eye is closed. In order not to create risks of the appearance of external objects in the laser optical zone, the patient may wash the surface of the eye and trim the protruding eyelash. Then the patient is transferred to the working position of the laser by moving the table and asked to look at the green LED that moves and becomes clearly visible. At this point, the surgeon centers the optical surface of the lens with respect to the optical axis of the eye, seeking contact between the lens and the surface of the cornea. Grabbing and fixation is performed using a special lens, adjustable in accordance with the individual characteristics of the patient's eye. Then the laser works like this .on telemetry. At the end of the laser operation, the patient is transferred to a safe position on the table and transferred under the microscope. The doctor selects the lenticule with a spatula and removes it. In this case, the patient usually sees a bright point and feels light vibrations.
Then the patient may cry a little for a couple of hours. But not only for joy, but because of the reflex that causes tears when the eye is damaged. For some time, some people may feel a slight burning sensation in their eyes, which feels much weaker than the smoke when cooking kebabs. Most often, this stage takes place without any special emotions. The next day, the discomfort disappears, the visual load is not limited, but it is better, in spite of this, not to strain your eyes too much with reading, telephone or movies. You can not visit the sauna for some time, so that water does not fall into a 2.5-mm incision, which was used to access the lenticule.
— How is the laser operating mode and cut shape calculated?
Firstly, one of the most important things in the preparation of an operation is accurate diagnosis and accurate measurement of the eye. Comprehensive primary examination before correction in our clinic is 16 examinations on the most modern specialized equipment under a single European protocol. This is followed by a detailed consultation with one of the surgeons-professors to discuss the correction option that is optimal for the patient, taking into account his visual needs, lifestyle and anatomy of the eye. Examination and postoperative observation of patients is performed only by ophthalmologists with experience in the field of refractive surgery. All this is extremely important for correct calculation of laser correction parameters. Based on them, a model for laser automation is set. About this entire stage, I will tell you more later. Secondly, The parameters and shape of the cut depend on the size of your myopia and astigmatism, the size and shape of the cornea, even the size of the palpebral fissure and the nose. In the case of complex cases of astigmatism, a great deal depends on the correctness of capture and alignment, but how this is done and how the methods evolved is a separate story.
Thirdly, the working mode depends on the preferences of the surgeon, on his skills and skill — we change the porosity and spot diameter to obtain the optimal quality of cuts and optimal conditions for the extraction of lenticules. Fourth, on a specific VisuMax laser machine: you need to adapt a little to your particular laser, they are slightly different in the series.
— What to do if the center of view does not coincide with the center of the pupil?
This is infrequent (about 10%), but a possible case, when the angle between the visual and anatomical axes of the eye (Kapp angle) is large. It is detected before the operation and requires a special change in the model. Here the qualification of the surgeon is extremely important. Also a topic for a separate story.
— How about the additional corrections after SMILE?
In our practice (German and Russian), not a single patient had a need for additional correction. But technologically there are special methods that allow for additional correction. How exactly — to decide the doctor. At the European Congress in Copenhagen this year, Dr. Dan Renstein gave a report in which he listed 5 different methods of additional correction in different cases. So, if the patient suddenly needs, there is plenty to choose from. After SMILE, the quality of optics on 6th generation lasers is consistently higher than after LASIK or femtoLASIK, the probability of side optical effects is significantly less. When we are planning a laser correction, we expect that this procedure is done only once in a lifetime. The percentage of the correction for the generalized international statistics for SMILE is the lowest today — about 0.5%. In specialized clinics with experienced doctors, it is significantly less. I repeat, we didn’t have them specifically, but the probability cannot be ruled out, so this is, rather, a matter of experience and luck.
— Why do I need to get lenticular, and not evaporate it inside the eye?
In theory, of course, you can try to evaporate the layer after layer of lenticular inside the cornea, without “opening” it at all. The difficulty is that it will require the removal of decay products from the eye, and the exchange of fluids there is very, very slow. According to preliminary calculations, in theory, it is possible to do 0.25 diopters per week, plus a perfect positioning system will not be needed yet to continue the work started last time. Well, either technical tags right in the eye, which is also not the best idea. In practice, the restriction is even simpler: no patient will pay 10–20 times per operation (and the price will be slightly lower than with the usual ReLEx) and go to it for half a year every week. Therefore, while this is an idea for the future.
— What will happen when nanolasers appear?
Now nanosecond lasers are undergoing clinical trials. Their use will allow working at higher frequencies with a slightly lower heat transfer into the fabric, but before the first practical operations “into combat” there will be at least 5–7 years at least, rather, all 10. This is a quantitative growth of several percent, rather than qualitative, in the technology of operations in the eyes now, little will change fundamentally.
— Why now you can often find proposals for laser vision correction at very low prices, on what they save?
Now there are still a lot of clinics that work on morally and physically outdated equipment for diagnosis and surgery. And sometimes without the necessary diagnostic equipment. There are even such clinics that call themselves «premium», and the equipment of the past and before last generations. Hence the complaints of patients in the postoperative period on the quality of vision and other problems. Old laser systems are not capable of solving problems for which modern lasers are designed. Therefore, when choosing a clinic, ask what laser device will be used and what year it is manufactured.
— How much stronger is the cornea weakened during the LASIK procedure than with ReLEx?
While understanding is intuitive: ReLEx is better. A practical report on a multi-year study on biomechanics will be published in 7 months. Then we give the numbers and facts. In general, you can no less intuitively compare the results on the loss of innervation and other side effects such as keratoconus — in the case of LASIK / FLEX-methods there are many times more. But, I repeat, it is better to discuss this in numbers after the publication of 10-year studies, which are simply not there yet. Quite briefly — the FDA has the most stringent requirements and always insists on conducting its own independent research, but it found SMILE suitable for use in US clinics as one of the main methods. In Russian Wikipedia there are statistics on the side effects of various operations with confirming links.
— Why is ReLEx made only by femtosecond lasers?
In Lasik, two lasers are used: femtosecond to remove the “cap”, and then excimer to evaporate a wide lens. For precise minimally invasive (and transmitting little heat to the bowman membrane and below) operations, lasers with a frequency of about 500 kHz and very precise focusing are needed — traditional excimer ones are not suitable. Currently, only Zeiss optics provide the necessary accuracy, that is, lasers of the VisuMax series (the old models are most likely familiar to you). 13 microns thickness is one diopter. This accuracy was achieved by combining the actual laser tube from the industry (initially, seemingly cosmic) with Zeiss lenses. It was the latter who created focusing optics for medicine.
— What you need to know about the cornea and what features of the regeneration of each layer?
The cornea is the part of the eye that you can easily see: a convex transparent part that contacts the air. The usual diameter is 10–12 mm for people of any race. In the center, the thickness of this convex-concave lens is 520–560 microns, with an edge of about 1 millimeter (all sizes are average, there are thin and very thin corneas). The cornea has 5 layers. Here is the concept:
But in scale from Wikimedia Commons:
1. corneal epithelium; 2. Bowman's membrane; 3. corneal stroma; 4. Descemetov shell; 5. corneal endothelium.
Epithelium The cornea is a multi-layered flat tissue and accounts for about 10% of the entire thickness of the cornea. Corneal epithelium cells are located in 5-7 rows. The corneal epithelium performs a mechanical protective function, as it prevents microorganisms and foreign bodies from penetrating inside the eye; biological protective function, as it contains cells that are involved in the immune response, the optical function — the mucin of the tear film fills all the uneven elements in the surface layer, which provides a smooth, transparent surface for the passage and refraction of light rays; membrane function — the corneal epithelium is a biological membrane through which certain substances can penetrate. Like the normal epithelium of the skin, it regenerates well, and during PRK surgery it is removed to gain access deeper.
The second layer is the Bowman's membrane. This is an incredibly thin and important layer immediately under the epithelium. Bowman's membrane is located under the basement membrane, has a thickness of 12 microns and does not contain cells. The Bowman's membrane consists of randomly arranged collagen fibrils. It has a smooth front surface and a back surface for smoothing the non-uniform relief of the stroma, which ensures the transparency of the cornea.
The Bowman's membrane cannot be restored after damage, therefore, after corneal damage in this part, scars are formed at the site of defects and the transparency of the cornea in these areas is disturbed — turbidity is formed. When laser correction lenses are formed deeper. However, at any cut through the bowman's membrane, we cut the nerve endings. During PRK surgery, it is removed to gain access to the stroma.
Accordingly, with minimally invasive removal of lenticules with ReLEx, there is only one short cut of 2.5 mm, and not nearly the entire circumference, as in the case of LASIK / FLEX-like methods. It is the injuries of the Bowman's membrane that violate the epithelialization of the eye, innervation, and give other side effects.
The next part of the cornea is the stroma. This is where the main work goes. Fabric — collagen filaments impregnated with hyaluronic acid. When magnified, they resemble ropes, such as in section:
The stroma is the main part of the cornea and occupies approximately 90% of its thickness. The cornea stroma consists of parallel plates. Plates are formed from collagen fibrils. Collagen provides transparency of the cornea and its strength. In the stroma of the cornea there are two main parts: the anterior stroma of the cornea and the posterior stroma of the cornea. The anterior stroma is looser and consists of thinner plates; the posterior stroma has a denser and more compact structure.
Stroma regeneration is carried out by keratocyte cells, which are capable of synthesizing collagen and, due to this, maintain an optimal level of collagen fibers and extracellular matrix.
These same ropes can perfectly coalesce, if they are stuck in one another (with the formation of nodes-adhesions, which interferes with visual acuity), but at the same time, being put on each other overlap (that is, from different angles), they do not form these nodes, but simply interlock. During the laser correction ReLEx SMILE we cut the lens in this layer and pull it out. After the operation, the cavity in the cornea closes — the “ropes” lie on top of each other, but at the places of the incisions a clear boundary does not form from the joints, that is, everything remains transparent (splices of single collegan filaments occur at the border of the lens, that is, in outer diameter). The frame is supported as usual — the bowman membrane stretched from above and the lower layers.
The next two layers are Descemet's membrane and endothelium. — we are almost not interested in laser correction, because we are not affected by these operations. This is actually a soft and rigid boundary of the body, a kind of standard “casing” for the body.
— What will happen after the operation?
On the day of surgery, most often after a couple of hours, you hardly feel any discomfort and it disappears completely the next morning. The main limitation after surgery is not to rub the eyes with your hands. During the week after the surgery, you need to bury the eye drops obtained in the clinic. If necessary, we will also provide preparations of artificial tears.
Visual acuity in the first days may vary somewhat. In order for the surfaces to come together after extracting the lenticules, a slightly longer process of visual rehabilitation is needed than with methods that imply “full opening” of the cornea of the eye. It takes time for the eye to learn to focus on close objects in order to make the right muscles work. We conduct examinations to check the results of the correction every other day, a few days later and a few more weeks after the operation.
After two or three days, you can get back to work and play sports. In general, the next day after the operation, you can quietly lead a normal life (ride a bicycle, bend, etc.) — there is no risk of a flap-flap moving as with other techniques. The visual load from the day after the operation is not limited, but it is better not to abuse the laptop, books and tablet during the first week. However, swimming and visiting the sauna should be abandoned for a couple of weeks in order to reduce the risk of infection through an incision in the cornea.
If we compare it with LASIK, then the recovery of vision is more gradual, within a week, but the restrictions on physical exertion are much less. I will say that this is probably one of the most frequent objections of patients — not everyone is ready to sacrifice a week without active visual load for the sake of good biomechanics of the cornea. As a doctor who is concerned about safety, I do not really approve of such an approach, but if the patient has a very high quality of vision the very next day, we usually meet them, explaining the risks. Seen under a microscope, a week after SMILE, it is very difficult to find the place of lenticular removal and the junction of the tissues when observing the patient's eye. After the LASIK / FLEX-methods, the correction zone is very clearly located on the damage at the edges, sometimes in the wake of displacements.