Ординатура / Офтальмология / Английские материалы / LASIK and Beyond LASIK Wavefront Analysis and Customized Ablation_Boyd_2001
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MICROKERATOMES
Fig 5-3. A- Diagram of the blade with 26 degrees of angular attack. B- Blade with 0 degree of angular attack.
Degrees (Fig 5-3). The angle attack of the blade in 0 degree doesn’t have good results.
Motor
The motor generates an oscillating movement to the blade in order to make the cut. Initially work was begun with electric motors, which only moved the blade through an eccentrical in its tip, nowadays there have been adapted reductors and gear system that automatically allow it to advance itself while it cuts. There also exist turbines and systems that can generate this oscillating movement to the blade.
Suction Ring (Fig 5-4)
It has the shape of a small cylinder with a central hole through which the cornea comes out.
Its superior face has a channel that allows the guiding of the Microkeratome. The inferior face is concave and it has an ample groove in which the suction is established in order to fix the ring to the ocular globe.
The suction reaches the cavity of the ring through a hole in the handle that is connected to an suction pump by a plastic tube.
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Fig 5-4. Original Barraquer Suction Ring. Section 6
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The suction ring has different functions depending on the type of keratome used: a) to settle the globe; b) to give a plane to the displacement of the Microkeratome; c) to regulate and to keep the intraocular tension; d) to control the resection diameter; e) to serve as brake to obtain the hinge; f) to serve as final coupling to the automatic advance
In the initial nomenclature used by Barraquer, the pneumatic ring is labeled with two numbers. The first one is the scleral radius, and the second, the height, which means the distance. (For example: 125- 4 tenths of millimeters). This ring can be adapted to almost any ocular globe and obtain a corneal flap of 8.5 to 9 millimeters of diameter.
There exists an inverse proportion between the corneal radius and the resection diameter, with the same ring; the flatter corneas will produce smaller resection diameters.
Subjects Index
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LASIK AND BEYOND LASIK 79
Chapter 5
The corneal flap that is obtained through Microkeratome setting of 26 degrees has beveled edges. The cutting blade edge in small diameter resections has an oblique incidence over the cornea, and in bigger diameter the incidence is more oblique, so the bevel changes with the disc size. This feature allows the positive lenticules resection if the resections are planned in 5 millimeters diameter or smaller.
Cables, Pedals and Power Source
Tonometer (Fig 6 )
It is use to measure the intraocular pressure. It is of plastic, and is based in the Maklakow principle. It has a constant weight, what varies is the applanation area.
These regulate and control the energy imparted to the motor and to the suction ring. They are different in size in all microkeratomes.
Applanation Lenses ( Fig 5-5)
They are transparent plastic lenses, labeled with different circles with known diameters.
The applanation lenses are used to know beforehand the diameter, which will be obtained in the patient’s cornea and avoid unexpected small diameter, it is clear that the differences will be kept in the order of tenths of millimeters, but nowadays a resection of 8.25 millimeters makes a difference if we are expecting 9 millimeters. The use of the applanation lens also helps in the planning of the hinge in general with 500 of curve longitude, allows an adequate manipulation of the flap and ensures an easy reposition of it.
Fig 5-5. Diagram of the applanation lens.
Fig 5-6. Diagram of the Barraquer tonometer.
General Steps for the Microkeratome Use
How to assemble the Microkeratome: Includes mounting the blade, placing the plate that is going to be used, or select it depending on the system that the instrument uses.
To verify the adequate functioning of the Microkeratome: The sound of the instrument is a guide of the function itself. The adequate movement of the blade and the advance system should be seen. It’s very important to check the vacuum with and without occlusion.
Selection of the suction ring that will be used with the patient: Due to the fact that esklerotomes are not used in the refractive surgery equipment, the ring is clinically chosen according to the exposed diameter of the ocular globe. If we are before a clinically big eye, a high radius ring should be used and vice versa. To use the same ring for all eyes is dangerous as there will always be eyes which are too far from average parameters and they will couple inadequately.
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80 SECTION II
To regulate the microkeratome brake: The brake of the microkeratome is a system that is used to avoid totally cutting the flap, in that way leaving a hinge, which allows guiding the flap to its original position.
Some microkeratomes have labeled brakes, others are regulable in the micro and/or in the ring, and others are adjustable from the power unit. In all cases it is very important to verify, to regulate, and to choose it before the surgery in order to avoid a complete section of the flap.
Placement of the suction ring over the ocular globe of the patient: Previous to the placement of the suction ring a corneal references mark must be make in order to guide the flap reposition in the eventual case of a total section. The placement of the suction ring over the ocular globe should be always go together with a previous verification of an adequate suction. The ring should be oriented according to each cutting system of the Microkeratome and should be suitably centered to come out of the cornea with the center of the hole. The best way to verify an adequate coupling of the ring over an eye, is pulling upward on it, and watching for its adequate grip . If the ring loosens, another ring of higher or lower radius should be tried according to the case, or the suction system verified.
Measuring intraocular pressure: At this moment, the best Tonometer that lets us verify the intraocular pressure is the Barraquer tonometer; it has to be approximately of 65 mmHg. If applanation inside the tonometer circle is not obtained it means we don’t have an adequate pressure and there is the risk of an irregular cut or a perforation of the flap. This can happen when there is not an adequate coupling of the ring with the ocular globe, which makes it primordial to change the suction ring for another one of a more adequate radius, or to look for the cause of the low pressure obtained .
Placement of the applanation lens: To place the applanation lens it is ideal to put it over the cornea coming in from the upper side so that the appla-
MICROKERATOMES
nation will be uniform to avoid to friction to the epithelium, and for corneal the distribution to be adequate. This lens is transparent and when place over the cornea it allows the observation of the diameter of the flap that is going to be cut before cutting it, and in this way modify the size of the hinge that is going to be leaf, or to change the suction ring if a flap of another size is wanted.
Coupling the guiding groove or grooves of the Microkeratome with the guides of the ring: This coupling must be done softly and uniformly without using leverage. When there is a double guide it has to be verified that both are inside their homonyms before activating the motor.
Once the Microkeratome is in the advance position, the cornea must be wet in order to reduce the heat that the blade generates to the corneal stroma during the cut, and for a better displacement of the applanation surfaces of the Microkeratome.
Activate the motor and make the cut: Once, the motor is activated, if the microkeratome is manual, a slow and uniform translation movement should be applied in order to avoid the irregular surfaces generated by not uniform movements, or the shallowness of the cut that can happen if the movement is too fast.
If the Microkeratome is automatic and of exposed gears, an adequate initial coupling of the advance gear with the first tooth of the track must be verified and you must make sure that there are no structures that will interfere with its path.
Remove the microkeratome from the ring:
Once the cut is made, the microkeratome should be put in the initial position manually or with the back pedal in automatic cases. If the microkeratome gets stuck on the way a maneuver must be done that includes stopping the suction and uncoupling softly the ring with the microkeratome as if they were a single unit, taking it in the opposite direction of the advance path. This movement allows the cut flap to slide between the plate and the blade and reposition itself on its original bed.
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LASIK AND BEYOND LASIK 81
Chapter 5
Release the suction and remove the ring from the ocular globe: the ideal is to uncouple the head of the microkeratome from the ring ,and then release the suction and remove the ring from the ocular globe.
Frequent mistakes are inherent to the inadequate use of the microkeratome and /or sur-
gical technique.
Microkeratome Stopping
In manual Microkeratomes, the most frequent stop observed is one of transitory type, which is overcome by increasing the forward force given by the surgeon’s hand. This stop is usually caused by a bad distribution in the direction of the force.
To impart a not uniform velocity to the cut, will generate scaled irregularities in the surfaces. When the advance of the Microkeratome is too fast, it generates flap of less thickness and it increases the risk making the cut too shallow and to break the flap.
The stopping problems of the microkeratomes that advance by gears over a rail track have different causes: ( Fig 5-7)
The engaging gear of the microkeratome should fit perfectly in the first tooth of the rail track,
in order to allow the advance of it. If the tooth of the gear doesn’t fit with the rail track, the motor stops from the beginning, therefore it has to perform and an uncoupling and recoupling in order to achieve an adequate engagement.. If there is not an adequate approach of the gears to the rail track at the moment the motor is activated, it will turn in the air and it will not advance. This is solved advancing the microkeratome adequately; in these last cases there isn´t a real complication as the blade has not made the cut.
When the Microkeratome it well engaged the |
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advance is uniform and an excellent regularity of the |
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cut is obtained. |
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These gear systems and exposed rail tracks |
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present the following problems: |
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They depend on a perfect state of the teeth of |
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the rail track and of the gear, and the damage of these |
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generate an imminent obstruction of the machine. |
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The motor that imparts the advance has the |
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adequate potency in order to move the system while |
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the system is adjusted. The back weight of the motor |
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added to the leverage, which transmit a unilateral |
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and superior traction to the contact floor, generates |
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an effect of torque that can stop the movement of the |
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instrument producing an incomplete flap. ( Fig 5-8) |
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Fig 5-7. Diagram of different possibilities of the microkera- Fig 5-8. Corneal disc with incomplete cut. tomes engagement that uses external gears.
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Fig 5-9. Corneal disc with irregular cut.
Irregular Cuts of the Flap: (Fig 5-9)
This complication is observed when a defective blade is used or there is a transitory or a permanent suction loss.
When this complication presents itself you must abort the procedure and replace the flap as anatomically as possible drying its edges.
After this complication you must wait three months as a minimum to perform the procedure again.
Flap Perforation: (Fig 5-10)
The perforation of the corneal flap is more frequent in curved corneas or with high astigmatisms and it is the result of the inadequate use of the ring or of a microkeratome with an inadequate dimension for these cases.
The phenomenon that occasionally presents itself at the moment of flattening a cornea that is too curved is a central folding of the cornea and a superficial cut in the center of the flap. Is due to the fact that in the middle of the cut you are in the area of larger diameter of the flap. This types of corneas need a bigger space to expand to the sides; when the internal lateral walls of the microkeratome don’t allow its lateral extension the cornea has no other chance different than to fold downwards (towards
Fig 5-10. Corneal disc with button hole.
the anterior chamber of the eye) as it is been flattened by the applanation plate. The handling of this complication is the same as that of irregular cut of the flap.
Irregularity in the Interface
In the cases of manual microkeratomes it presents a not uniform translation movement as a result of an inadequate push and in the cases of automatic Microkeratomes it presents a transitory stopping, which can produce a surface of irregular cut. It also can be seen when the cutting edge of the blade is imperfect. In these cases when the flap is not perforated or ripped, the surgical procedure can be performed keeping in mind to make an adequate repositioning of the disc so that the irregular surfaces a
replaced in their exact position.
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Transitory and/or Permanent Suction Loss
Promoting microkeratomes with only one suction ring is to subtract importance to the morphology of the ocular globe. And generally, the observed complications are because of this ignorance
All eyes are not equal and therefore they don’t have equal curvature scleral radius. We cannot gen-
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Chapter 5
Fig 5-11. Color diagram of adequate coupling: A- Eye with small scleral curvature radius (11 mm) and ring with the same curvature radius (11 mm) B- Eye with bigger scleral curvature radius (12 mm) and ring with the same curvature radius (12 mm ).
eralize either saying that myopic eyes have high curvature radius or saying that hypermetropics have low curvature radius, as there exist eyes of high axial length and low scleral radius and vice versa.
When we have a ring with an adequate curvature radius for an eye with the same curvature radius, the coupling generated in the vacuum chamber is perfect and it translates in an adequate grip of the ocular globe. As the adjustment is made at the level of corneal limbus, which is the site of highest adherence of the conjunctive. ( Fig 5-11)
When a suction ring of a higher radius than the scleral radius is used, its grip is achieved at the expense of the conjunctive and therefore you don’t get an adequate intraocular pressure during the cut. In the opposite case when you use a suction ring with a lesser radius than the scleral radius of the eye, the grip is achieved at the expense of diminishing the ocular diameter, increasing the risk of loss of suction during the cut, as it is subject to tension and will tend to get loose. ( Fig 5-12 and 5-13)
The explanation given in the numeral “perforation in the flaps” is also valid in the transitory suction loss, because of the point at which the cornea is held with more strength is in the periphery, where the suction is applied , and the most vulnerable is the corneal center. An inadequate tension because of suction loss or by an inadequate adaptation of the
Fig 5-12. Color diagram of inadequate coupling before to activating the suction: A- Eye with big scleral curvature radius (12 mm ) and ring with smaller curvature radius (11 mm) B- Eye with small scleral curvature radius (11 mm) and ring with bigger curvature radius (12 mm ).
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ring with the globe, leaves the corneal center as the
most hypotense point of the entire cornea and its Section 2
behavior is similar to the mentioned fold.
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Fig 5-13. Color diagram of inadequate coupling after to activating the suction: A- Eye with big scleral curvature radius (12 mm) and ring with smaller curvature radius (11 mm) B- Eye with small scleral curvature radius (11 mm) and ring with bigger curvature radius (12 mm).
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Fig 5-14. Nasal hinge Vs. Superior hinge. A-B-C. Surgical technique sequence and adequate post-operative result. D-E-F-G- Mechanical influence of the upper lid in a case of traumatic displacement of the disc.
Folds, Displacements, and
Detachments of the Corneal Flap
Within the complications inherent described to the corneal flap you find folds, displacements, and detachment of the corneal flap, being the last one very infrequent.
The folds and displacements of the flap are cause by a twist or wrinkle of the corneal flap of direct traumatic origin (inadequate technique, strokes, friction,) or by the effect of the patient’s blinking . There exists another type of fold less perceptible and it is observes after the flap replacement in deep ablations ( in high myopias). This is due to distribution folds of the flap that are generated when you reposition it upon a stromal bed very different to its original.
Many times, these findings pass unnoticed and they are observed more frequently as the surgeon has more experience in the technique and perform a better and more detailed post-surgical examination of the flap.
The tinting with fluorescein in post surgery easily lets you evaluate if there exists some degree of displacement of the flap. When this presents itself a larger area of tinting is observed in the superior edges of the flap, since when the hinge is nasal by the blinking effect the tendency to displacement
is toward the inferior quadrant. This phenomenon also could be because of an inadequate replacement of the corneal flap during the surgery. But it is less frequent.
These displacements are not generally significant in the patient’s visual recovery unless they are accompanied by folds in the corneal flap.
In 1995 in our research work on rabbits13, we find a significant statistical difference in favor of the hinge that is located in the eyelid quadrant that realizes the active movement.
The advantages observed with the “ superior hinge ” technique are: (Fig 5-14)
It is more physiological since it is located favoring the blinking.
Statistically less flap displacement was presented.
More comfort for patient since the superior epithelium stays intact. Less bleeding in patients with corneal pannus by contact lens, in general located superiorly; less contact of the stromal face of the flap with the eye’s circulating liquid.
Less ablation signs of the hinge as it stays above and the majority of astigmatisms are carved in the horizontal meridian.
In case of superior hinge ablation less optical aberration is produced since it stays partially covered by the superior eyelid.
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Chapter 5
Fig 5-15. Color diagram and Topography of a Negative Hinge Syndrome.
Astigmatism Inductions by Hinge Ablation
When we perform an intrastromal ablation with laser, the ablation diameter must be less than the flap diameter in order for all the ablation to be done in the stromal bed.
I recommend to orientate the hinge in a quadrant 90 degrees away from the astigmatism axis to avoid a negative hinge syndrome (presented at the American Academy of Ophthalmology, Atlanta, GA, USA 1995 , by C. Carriazo). This occurs when there is ablation of the base of the hinge that induces an additional ablation effect that appears once the flap is repositioned over the cornea; topographically, it is seen as a peripheral ablated area at the base of the hinge (Fig 5-15) and induces irregular astigmatism that is difficult to resolve.
Initially, to solve this complication, we began to cover the base of the hinge with sponges or spatulas until we observed a few cases of induced astigmatism that showed a topographic image of elevation in red at the hinge’s base. We called it a positive hinge syndrome (Fig 5-16).
Finally, we recommend orienting the hinge 90 degree apart from the astigmatism axis or to program a wider diameter flap in those nasal hinge’ Microkeratome (Fig 5-17 and 5-18).
Fig 5-16. Color diagram and Topography of a Positive Hinge Syndrome in comparison with the Negative one in Hyperopic Astigmatism Correction.
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Fig 5-17. Color diagram of the mechanism of Hinge Ablation Subjects Index in Myopic Astigmatism.
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Fig 5-18. Color diagram of the advantage of Superior Flaps to orient the hinge 90° from the astigmatic axis.
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Fig 5-19. Clasification of microkeratome systems.
Free Cap
This is not a true complication since its adequate repositioning does not generate problems. The complete flap section is produced when there is no way to confirm the hinge size and/or when the applanation lenses that show the diameter of the flap that is going to be cut are not used.
When a total flap cut is presented it is usually found between the blade and the applanation plate of the Microkeratome and must be taken off slowly always keeping the stromal face and its orientation identified.
“ The simplification of surgical techniques has made us forget some very important principles in the use of the Microkeratome. To consider all ocular globes as having the same size is a big mistake as those eyes that deviate from established parameters set as standards, are at great risk in some Microkeratome “
We have classified the different types of Microkeratome according to the movement of the cutting blade and the advance system.
(Fig 5-19).
Barraquer Microkeratome (Fig 5-20)
It is a 3-piece unit 10 to 12 centimeter long; it has an oscillatory motion, an inclined blade at 26 degrees, different height plates, a single shaft, and motor over the blade.
Fig 5-20. The third Barraquer microkeratome generation.
It has different suction rings with an aperture in the center of 11.5 millimeters for the cornea. Its anterior flat surfaces serves as a guide for the Microkeratome and its posterior concave surface has the appropriate size for it to fit in the anterior segment of the sclera. This last surface has a space that acts as a vacuum chamber that fixes itself to the ring over the anterior aspect of the sclera. The ring is designed with a handle to keep it in position, which also serves as vacuum duct. The plates to change the cutting thickness are numbered according to the tenths of millimeters of the cutting thickness that you wish to obtain. It has applanation lenses labeled with the cutting diameter that you wish to obtain.
Later the Barraquer Krumeich-Swinger appeared, which followed the same line as the original Barraquer. This Microkeratome used bases of Teflon, in order to support the cornea and for the first time, allowed the performance of keratomileusis without freezing.
Draeger Microkeratome (Fig 5-21)
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It was the first Automatic Microkeratome and the only one with rotary movement. This is a single
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Fig 5-21. The Draeger microkeratome.
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Chapter 5
Fig 5-22. The Automatic Corneal Shaper (A.C.S).
unit 30-cm long. The body of the instrument consists of one motor that runs with a double shaft at 500 rotations per minute and a periphery speed of 3.140 cm per minute using 6 volts d.c. One circular cutting blade located at the end of the instrument that moves mechanically to 00 with a rotary movement in order to cut the tissue at a speed between 0,06 and 0,08 mm/sec. Added to the body of the instrument there is a suction ring with a central opening of 12 mm and an anterior flat surface and a posterior concave surface. In order to obtain the needed application diameter it uses a plastic transparent plate with a small precision lattice labeled with an adjustable height of 0.5mm. The tissue should be irrigated constantly while the cut is made in order to avoid overheating. This instrument has a plastic tube for a constant irrigation.
Automatic Corneal Shaper (ACS) -
(Fig 5-22)
It is a 3-piece unit 12 to 15 cm long. It has a blade with oscillatory movement, inclined 26 degrees, and plates of different height. It is powered by an electric motor that provides both forward motion and blade oscillation of 8,000 oscillations/min. The blade travels along a geared track on top of the suction ring, cutting tissue at a rate of 3.7 mm/sec. The body of the instrument has a gear system that engages over a rail track located laterally in the ring
and that allows the displacement over it, conducted by the same motor that controls the cutting blade. It has two lateral guides designed to slide in the anterior surface to the fitting ring keeping up a constant cutting plane.
The Microkeratome head is composed of a main piece that contains the advancing gears and opens to accept the blade holder and blade. Once it is closed and secured by a cylindrical nut around its “neck” the applanation or gauge plate is placed and secured in its frontal aspect. The stopper can be mounted as a separate adjustable piece over the neck’s nut. In the newer models, it is incorporated as a fixed part of the head’s main piece.
The importance of adequate cleaning and maintenance of the Microkeratome parts cannot be overstated. This should include periodic polishing to avoid buildup of a layer of debris. Especially on the lower surface of the plate, this buildup leads to shallower cut.
The original ACS system included an adjustable pneumatic fixation (suction) ring with a maximum applanation diameter of 7.5 mm cut. Customs ring can be made to allow fee or larger cuts.
If this value is not reached, the vacuum pump should be checked for malfunction. However, if a conjunctiva fold (or any other obstruction along the tubing or at the console valve) blocks the vacuum line, pneumatic fixation will be poor in spite of a normal manometer reading.
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Carriazo-Barraquer (Fig 5-23)
(C-B Moria and Supratome Schwind)
It was the first Microkeratome designed in order to perform the Superior Hinge Technique,
developed by Dr. C. Carriazo and Dr. J.I. Barraquer Help ? M. in 1996.
Also, it was the first Microkeratome which gave to the surgeons both manual and automatic system.
The Microkeratome is composed of an original crown system located inside a tubular guide of the head, which engages on a pivot located on the ring, it avoids the use of external gears and tracks.
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