Ординатура / Офтальмология / Английские материалы / LASIK and Beyond LASIK Wavefront Analysis and Customized Ablation_Boyd_2001

Microjet ( Fig 5-38 )

It is defined as a coherent, high-speed waterjet beam with a beam diameter of under 50 um. Although many of the properties of the microjet have been described in other literature they are reviewed briefly here. (Figure 5-38) schematically illustrates the elements of microjet beam information.

The microjet system operates at a stagnation pressure of about 20 to 25 Kpsi (Kilopounds per square inch ) (1360 to 1700 atmospheres). The highpressure tubing is flexible and thin, yet strong enough to support the high pressure fluid. The shape and size of the stagnation volume geometry plays a key role in producing a coherent beam. The fabrication of the circular orifice, made of ruby, is critical to proper beam formation. The resulting beam diameter is about 87% of the orifice diameter. A typical beam diameter is 33 um have been used. There is a trade-off between stagnation pressure and beam diameter. The smaller the beam diameter, the greater the required stagnation pressure of 25 Kpsi is relatively straightforward. Higher pressures increase the size and cost of the system. The output nozzle plays a role in stabilizing the beam but is not essential.

Although the beam travels in a plane, the cut boundary is not in the same plane. Unambiguously, the beam moved tissue separation. Hence, it cut along interfaces between layers rather than across layers until at the last instant the tissue resisted further displacement and then the beam cut across a layer. Clearly, the beam exerts transverse force on the tissue and moves it.

The origins of the transverse force are small angle collisions of the high speed water with the tissue at the perimetric boundary is deflected to the trailing edge of the beam and this produces a reactive transverse force on the tissue. A detailed experimental study of beam forces during the cut has been carried out using an elegant experimental technique.

As a result of the collisions, the water breaks section of intact lamella layers away from the stroma as demonstrated later. Hence, cutting is usually accompanied by ablation or erosion. Ablation never involves loss of partial thickness lamellar layers or

Fig 5-38. Microjet elements.

keratocytes. The lamellar layers are carried away by the spent water.

By virtue of the applanation, internal hoop forces develop in the cornea and tend to push the cornea back to its normal position. Thus, there is a tendency for the cornea to move upward toward the microjet beam in the stromal bed beneath the beam, which tends to move away from the beam. Thus the stromal bed experiences more erosion than the underside of the flap. The important conclusion from this model is that the greater the scan speed of the microjet beam, the less time there is for erosion to occur and the smaller the amount of eroded tissue. Another important conclusion is that the greater the amount of applanation, the greater the rate of erosion. These two characteristics allow shaping merely by waterjet cutting.

The design of scleral chuck for use in a microjet keratome is unexceptional. The goal is to achieve adequate holding and resistance to motion of the globe without increasing the IOP. Hence, one must avoid distortion of the globe. Typical base IOP values are about 25 mm Hg. The increase in IOP during the cut is about 10 mm Hg. An essential element of the scleral chuck is the beam block shown schematically. The edge of the beam block intercepts the microjet beam and directs it away from the cornea. Hence, the tissue is the shadow region of the beam block is not cut, leaving a hinge. Note that the geometry guarantees the position of the hinge relative to the boundary of the flap. The hinge is positioned superiorly.

Contents

Section 1

Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Subjects Index

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Contents

Section 1

Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Subjects Index

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(Note from the Editor in Chief: Chiron’s Automated Corneal Shaper is still an excellent microkeratome that creates a very fine lamellar cut.

Nevertheless, it has certain limitations when compared with newer equipments. Among them stands out its lack of versatility in which area to make the corneal flap hinge. As with many other microkeratomes, the flap is always cut only in the nasal (horizontal) sector.

Some surgeons prefer the new models available from Chiron and from Moria that have important advantages in comparison with the Automatic

Corneal Shaper. For example, the flap hinge can be located in any meridian, horizontally or vertically.

The superior area is the most popular since it allows a more comfortable placement of the flap at the end of the surgery and it also makes it easier for the flap to stay homogeneous and wrinkle free.)

Introduction

The Automatic Corneal Shaper (ACS) is an excellent microkeratome manufactured by Chiron Vision (Bausch & Lomb). It allows for cutting of a precise corneal disc of pre-selected thickness & diameter. When Lasik was started it was not that successful. Later on three important techniques improved and changed the success of Lasik. They were an automated microkeratome, the development of a hinged flap technique and the development of the sutureless technique. For all three, the Automatic Corneal shaper was excellent and the results of Lasik improved drastically with this microkeratome.

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Figure 6-1: Power pack for the automatic corneal shaper

Figure 6-2: Suction ring handle. Note the track for the gears in the suction ring

place the ACS Power Pack on a firm, level surface. In the Chiron 217 Excimer laser (Lasik Laser) there is no need of a separate Power Pack. The power pack is inbuilt in the Excimer Laser itself. Also, the tubing of the suction ring and the cords for the microkeratome are protruding out of the Chiron 217.

Suction Ring

The Suction pump is connected to the suction ring handle (Figure 6-2) by a disposable tubing. This comes in a sterile package. On opening this package we will find not only the tubing but also a disposable blade for the microkeratome. The Suction ring consists of the suction handle, the ring and the Track on which the gears of the microkeratome move (See Figure 6-2). The suction handle screws onto the suction ring. The handle is hollow and is in turn connected to the power pack via the disposable suction tubing. The handle is hollow so that the suction can be applied once the foot switch of the suction pump is switched on. Once the suction ring is

Figure 6-3: Corneal shaper head exposing the blade holder. The blade is adjacent to it. Note the magnet holding the balde so that one does not damage the sharp edge of the blade

edge of the blade. Gently close the shaper head and replace the locking nut with the knurled portion facing out. The head should close flush without force. Test the movement of the blade by placing the test shaft into the rear of the shaper head. Rotate the test shaft to feel the oscillation of the blade, which should be absolutely smooth & free of resistance. While rotating the test shaft, visually confirm that the blade is moving side to side to verify that the blade holder has been installed properly. If the blade movement is restricted, carefully remove the blade and confirm that all surfaces of the shaper head and blade holder are clean and perfectly dry. Replace the blade and check again. If resistance is still detected, install another blade and repeat the testing process until you feel absolutely no resistance. Do not proceed with a blade with restricted movement, as this could cause the unit to jam or create a less than optimal cut.

Remove the desired thickness plate from the tray by placing the locking wrench onto the hex nut of the plate and tilting back. Place your forefinger on the plate to keep it from falling off the wrench and lift out of the tray. The thickness plates can be of 130 microns or 160 microns. This means if we use the 130 microns thickness plate the flap will be of 130 microns. If we use the 160-micron thickness plate

the flap created will be of 160-micron thickness.

Always confirm the engraved number on the plate with your surgical plan. If the corneal pachymetry is not much or the refractive error to correct is a lot one might use the 130-micron thickness plate so that we get an extra 30 microns to ablate. But by and large in our hospital, we prefer to use the 160-mi- cron thickness plate, as the flap created by this is better. By using the 130-micron thickness plate we found the flap a bit thin and so we shifted to the 160micron thickness plate.

The plate has a nut and washer, which must be slightly loosened to allow the plate to be installed into the shaper head. Stabilize the plate with your forefinger on the front and slide the plate completely into the notch on the front of the shaper head. You should feel and hear the plate snap into place so that it is perfectly flush to the front of the shaper head. Secure the plate by rotating the nut clockwise with the wrench. The plate must be perfectly flush to the shaper head and locked firmly in place so that it will not come loose during a surgical procedure. If the thickness plate is not fixed properly or if we forget to fix the thickness plate we will create a perforation in the anterior chamber.

Once the microkeratome is fully installed (Figure 6-4) it has to be fixed to the motor cord which

Contents

Section 1

Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Subjects Index

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Figure 6-5: Automatic corneal shaper head with permanent stopper for use with fixed suction ring

in turn is fixed to the power pack. Insert the motor cord into the keyed receptacle at the back of the motor. A click will indicate a positive lock has been achieved. With the motor foot switch, run the motor in each direction and listen for any irregular sounds from the operation of the motor. Place the shaper head on the motor shaft and carefully rotate the shaper head until it is firmly attached.

Check All Details

Before starting the procedure on the patient, check if everything is working. Check the gears, check the movement of the blades etc. Most important, install the microkeratome (shaper head) onto the suction ring (Figure 6-5). Engage the dovetail side first then rotate the shaper flat against the ring. The dovetail of the head should be kept parallel to the track. Gently advance the shaper until the gears are engaged. Press the forward switch of the motor with your foot. Promptly release the foot switch when the permanent stopper abuts the suction ring and stops the forward progress of the shaper. Press the reverse foot switch to retract the shaper head and remove it from the suction ring. The forward and reverse travel of the shaper head across the suction ring must be completely smooth. If binding or resistance is encountered, do not attempt the surgical procedure. Repeat the cleaning process and once again the functional test. If everything has been checked and found right the ACS is ready for use in a surgical procedure.

Figure 6-7: Pre-surgical tonometer

occurred. On the other hand it solved problems of patients being uncooperative and getting decentered ablations.

Once the cornea is flapped and the ablations completed, the stroma and the flap are cleaned with the BSS fluid. Then a wet Merocel sponge is taken and the flap cleaned with both sides of the Merocel sponge. This helps to prevent epithelial ingrowths. Then the cannula attached to a syringe with BSS fluid is taken and placed under the flap. It is moved to the junction of the flap and the cornea and gradually the flap lifted with it so that the flap falls back into its original position over the cornea. This method appears to be an excellent method to reposit back the corneal flap. When the flap is in position one should check if the corneal reference markers are in apposition. Check if any foreign body is present in the interface. Wait for a couple of minutes and check if the flap has stuck or not. Then carefully take out the speculum.

Both eyes are done simultaneously by the authors. The patient is then seen on the slit lamp after about half an hour and sent home without any pad or shield. The patient is put on topical steroids and artificial tears for 2-3 weeks. The patient is seen the next day. On the first postoperative visit one should generally see only subconjunctival hemorrhages due to the suction ring and nothing on the cornea. The cornea should look like a normal cornea on the slit lamp. Check the vision, which should generally be 6/6 (20/20) without glasses. The authors

Chapter 6

running your thumb over the gears to purge the residue. If this does not help loosen up the gears, repeat the cleaning process with the toothbrush and cleaning solution, rinse completely with distilled water and dry before sterilizing again.

2. Blade Holder not Moving Freely

The blade holder may be restricted within the cavity of the shaper head because of a residue of cleaning solution, BSS or Cidex. Remove the blade holder and thoroughly clean it and the cavity of the shaper head with a toothbrush in the cleaning solution. Make sure to completely rinse the components with distilled water and dry before sterilizing again.

3. Restricted Balde Movement With Shaft

If the blade holder moved freely when tested by itself, but feels restricted with a blade installed and the shaper head closed, remove the thickness plate and open the shaper head to confirm that the blade is properly seated on the blade holder and that all surfaces are perfectly clean and dry. One may use a fiber-free surgical sponge to absorb any moisture in the cavity. Close the shaper head and test the blade again. If the movement still feels restricted, the blade may have been crimpled in the installation process. Replace the blade with another and check with the test shaft again. Proceed only if the blade movement is absolutely smooth and resistancefree.

4. No Blade Movement with Shaft or Motor

If the blade does not move at all, remove the test shaft or motor from the shaper head. Look into the back of the shaper head to see if the blade holder is inserted properly. If you do not see the groove, open the shaper head and remove the blade. Take the blade holder out and insert properly, then replace the blade, close the shaper head and re-test.

2. Motor Will Not Attach to the Shaper

Head

If the motor will not attach to the shaper head, check to see that the locking nut is installed with the knurled portion out and the smooth portion in. Correct as necessary. Also, confirm that the blade holder is properly installed with the notch facing out towards the back of the shaper head. If not, disassemble the shaper head and remove the blade so that the blade holder may be removed and properly installed.

AUTOMATIC CORNEAL SHAPER

lutely smooth & resistance-free. The ACS device must be cleaned thoroughly after each use. One should clean with a cleaning solution consisting of 2 parts green Palmolive dishwashing liquid to 100 parts warm water. The type of soap present in Palmolive helps provide lubrication for the device. All parts must be thoroughly rinsed with distilled water after cleaning with the solution. It is best not to perform any cleaning steps in or over a sink, as many of the components are small and could be lost in the drain. Use small plastic bowls or basins instead.

TABLE 1

STERILIZATION OF ACS & ITS COMPONENTS

Contents

Section 1

Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Subjects Index

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Introduction

The Hansatome(1,2) is the latest microkeratome from Chiron Vision (Bausch & Lomb) which performs the Down Up Lasik technique. Instead of creating a nasal hinge this creates a superior hinge. The basic advantage of the hinge being superior is that the eyelid when it moves downward presses on the flap in that direction thus creating less chances of displacement of the flap.

Setting up of the Hansatome

Figure 7-1: Power supply unit

Power Supply Unit

The Hansatome has a power supply unit (Figure 7-1), a suction ring and a motor head. First of all place the power supply unit on a firm surface. The authors place the Hansatome power supply on the Chiron 217 excimer laser machine. This is also called sometimes as the Lasik Laser. Attach the power cords to the power supply unit. Also attach the foot switch cords to the unit.

Suction Ring

Take the suction tubing and connect it to the appropriate receptacle in the front panel of the power supply unit. Insert the tubing connector until a positive latch is obtained. If the tubing connector is not fully latched, the vacuum panel connector will remain occluded and no suction will be available through the tubing to the patient. Take the suction

ring and the suction handle (Figure 7-2) from the Figure 7-2: Suction ring and handle

Contents

Section 1

Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Subjects Index

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