LASIK Techniques

Figure 12.1 (A) Mickrokeratome head inspection. (B) Suction ring inspection. (C) Microkeratome testing.

(Fig. 12.2). The use of preoperative prophylactic antibiotics is a valuable adjunct, but not a replacement, for proper sterile technique.

1. Preoperative Topical Anesthetics/Pachymetry

Many surgeons give drops of topical anesthetic while the patients are in the pre-op area. However, we have observed that prior instillation of topical anesthetic does not add to the overall anesthetic effect and may predispose to epithelial defects. Accordingly, we prefer to delay the application of topical anesthetic until the patient is in the operating room. After the patient is positioned on the laser bed, a drop of topical anesthetic is applied. One of us (DTA) routinely obtains 2 or 3 measurements of central pachymetry in succession and records the lowest of these. The patient is instructed to keep the eyes gently closed after anesthetic has been applied.

2. Skin and Eyelash Scrubbing

Either in the preoperative holding area or in the operating room, betadine is applied over the skin surface in preparation for LASIK (Fig. 2A). We do not recommend application of betadine in the fornices in LASIK surgery, since it may weaken epithelial integrity and may also cause subsequent pain and irritation, which may lead to ocular movement. To avoid this problem, we recommend limiting the betadine scrubs to the eyelid surface and recommend the use of preoperative broad-spectrum antibiotics beginning one-half hour before

surgery. Irrigation of the ocular surface may be necessary after eyelash scrubbing if the patient did not follow the preoperative instructions to clean the lids and remove makeup. However, this irrigation should not be done routinely because it can flush meibomian secretions into the tear film and therefore predispose to their becoming trapped in the interface after flap creation.

3. Sterile Drape

The betadine may be dabbed with sterile gauze and allowed to air dry for approximately 1 minute. A sterile drape is then applied over the lids to ensure an aseptic field (Fig. 12.2B). Several kinds of drapes are available for this purpose. When choosing a drape, the surgeon should consider the ability of the drape to cover the eyelashes, ease of application without break of sterile technique, ease of subsequent speculum insertion, and ease of removal at the end of the procedure. If the drape is applied on iodine-soaked skin, the drape may slip and the lashes may not be completely covered.

The lashes should be isolated not only for sterility but also to prevent them from jamming the microkeratome (Fig. 12.2B). Several alternatives to the surgical drape exist, including the use of 3M Tegaderm adhesive or a closed-blade speculum. One of us (DDK) uses the 3M Tegaderm adhesive to isolate the lashes and a blank drape to cover the forehead and temporal region. A similar drape is applied over the control panel of the laser microscope, ensuring that the surgeon’s gloved hands do not touch unsterile surfaces.

Figure 12.2 (A) Skin and eyelash scrubbing. (B) Sterile drape placement. (C) Wire speculum application. (D) Proparacaine instilled in inferior fornix.

4. Speculum

After the drape is placed, a lid speculum is introduced and opened slowly to avoid excessive eyelid pain (Fig. 12.2C). The aim of this step is to allow maximal exposure for the suction ring and ensure a clear path for the microkeratome. The choice of eyelid speculum often depends on the anatomical configuration of the patient’s eyelids and orbits. For larger palpebral fissures, a long speculum blade may be appropriate. For narrower fissures a smaller speculum blade may be useful. A hexagonal eyelid opening after speculum application is ideal, for it allows for maximal surface area. A square opening is less good but may be better than a rectangular one. A solid blade speculum with rounded blades may be useful in such patients by providing a more rounded lid opening capable of accepting the suction ring and the microkeratome. It may be difficult to predict the ideal speculum design and size for an individual patient with narrow palpebral fissures. Switching from one design to another with suction ring placement rehearsals may be necessary to optimize this step if the initial suction application is unsuccessful.

5. Proparacaine in Fornix

If insertion or widening of the lid speculum causes eye pain or discomfort, the speculum should be narrowed, the patient instructed to look sideways or upwards, and topical anesthetic applied in the fornix (Fig. 12.2D). The patient is asked to look in the opposite direction and another drop of anesthetic is applied, avoiding corneal application. After a short waiting period, the speculum is widened, and the patient is reassured that the surgeon is aware of possible discomfort and that this step may be the most painful part of the surgery.

6. Corneal Preplaced Marks

Preplaced corneal marks help to ensure proper flap alignment at the completion of the surgery and are particularly useful in the unlikely event of a flap complication, such as a free cap. In this situation, asymmetric application of the marks assists in preventing inadvertent replacement of the cap with the epithelial surface facing down. Several markers are available, including existing radical keratotomy markers and specialized LASIK markers. Excessive ink on the marks should be avoided (Fig. 12.3A). The marks should be applied lightly to avoid epithelial toxicity or physical damage, and razor-sharp markers should be avoided. One option is to use rose bengal dye, since it is not toxic to the epithelium and therefore does not result in punctate epithelial staining that can sometimes be seen for several days after the use of gential violet. To prepare this, rose bengal strips are moistened in a pool of artificial tears (the authors’ preference is Computer Tears, which seem to give the most consistent results). The strip is then brushed over the surface of the marker, which is allowed to air dry.

If epithelial loosening occurs during LASIK, the epithelial marks can shift and are therefore no longer a reliable guide for flap replacement. As a result, the surgeon should determine flap position by the gutter symmetry, which is in all cases the best measure of flap realignment.

7. Laser/Microkeratome Readiness

Suction time should be as short as possible to minimize optic nerve head and retinal ischemia during LASIK. Therefore it is critical that the surgeon confirm microkeratome readiness prior to suction ring application (Fig. 12.1A). Choice of suction ring and

Figure 12.3 (A) Corneal preplaced marks. (B) Suction ring application. (C) Intraocular pressure check with pneumatonometer. (D) Microkeratome advance and reverse.

microkeratome settings should be individualized for each patient, taking into account ablation size, surgical eye (OD vs. OS), keratometry, refractive error, and pachymetry. Readiness of the microkeratome and integrity of the microkeratome blade should be checked before suction is applied. Laser readiness, including refractive error to be treated, should be confirmed as well. Laser recalibration should be performed prior to opening the eyelids for speculum application because, if performed at this stage, recalibration may be time consuming and may result in epithelial desiccation and stromal drying and thinning.

8. Suction Ring Application

In patients with adequate exposure, suction ring placement is relatively straightforward: close to the limbus, with approximately 0.5 mm decentration towards the flap hinge. In deep orbits, placement of the suction ring is facilitated by gentle pressure on the speculum to proptose the globe (Fig. 12.3B).

One of the most demanding steps for perfecting the LASIK surgical technique is ensuring suction ring centration. The suction ring should be intentionally decentered towards the hinge by approximately 0.5 mm to ensure flap centration relative to the pupil center. For certain microkeratomes, the suction ring may drift as suction builds up, more commonly in microkeratomes that have a relatively slow buildup of suction; the direction of the drift is toward the vacuum port (e.g., nasally with the Hansatome). This may not be always

obvious. The surgeon should have a relatively low threshold to release suction, to inspect, and to fix any decentration. One or two diametrically opposite corrections may be necessary for recentration. One approach to minimize the likelihood and magnitude of drift is to push firmly on both sides of the suction ring for 2 to 5 seconds before initiating suction. This seats the suction ring in the conjunctiva and minimizes its tendency to migrate.

Small lid fissures, lid squeezing, and Bell’s phenomenon may limit adequate placement and centration of the suction ring. If the problem is excessive lid squeezing, the surgeon should reassure the patient and repeat the instructions. A lid block may be necessary. For small orbits, options include (1) switching to a different lid speculum, particularly one that opens more widely and is less bulky; (2) applying the suction ring without a lid speculum, which warrants extra precaution to insure that lashes or lid skin are not in the path of the microkeratome; it is helpful to have an assistant watch the surgical field from the side and retract either lid as necessary, and (3) lateral canthotomy, which requires a local skin block and can be repaired with a single suture. It may be advisable to discuss this with the patient in advance. If the patient has already received a sedative, surgery may need to be postponed to obtain proper informed consent.

9. IOP Check

Most microkeratomes provide an auditory or visual indicator of adequate occlusion and the sound of the pump changes. As suction increases, the surgeon should ensure that scleral engagement is achieved as opposed to conjunctival occlusion of the suction ring and development of pseudosuction. Indicators of good suction include pupillary dilation, dimming of patient vision, and sympathetic forward movement of the globe upon the lifting of the suction ring. The IOP can be measured qualitatively with digital pressure on the globe or quantitatively with a fixed tonometer (e.g., Barraquer tonometer) or a device that measures the pressure such as a pneumotonometer (Fig. 12.3C). IOP measurement with a tonometer is advisable especially for beginning surgeons until they are comfortable relying upon the other signs of adequate suction. During the cutting of the flap, the suction ring can again be gently lifted to enhance unobstructed passage of the microkeratome.

B.LASIK STAGE II. MICROKERATOME ADVANCEMENT AND FLAP LIFTING

1. Preoperative Familiarity with Equipment

As discussed in Chap. 4, several microkeratomes may be used for LASIK, but none is ideal. As certain features are incorporated, additional limitations may be introduced. Furthermore, the LASIK microkeratome changes may require a different surgical technique and approach. The surgeon may thus have constantly to ensure preoperative familiarity with equipment prior to engaging the microkeratome head. Fortunately, most newer microkeratomes do not require a prolonged learning curve and result in far fewer complications, given surgeon familiarity and experience with the equipment.

2. Patient Concerns

The surgeon may engender good cooperation by anticipating and paying attention to patient concerns. Preoperatively and at the beginning of surgery, we reassure the patient that the

feelings of pressure and pain in the eye are not unexpected and that the vision may fade and disappear after suction application. The patient is told to expect a “cold sensation” during the following step of wetting the cornea.

3. Microkeratome Lubrication and Inspection

Several solutions may be used to wet the corneal surface. Proparacaine is recommended by several microkeratome manufacturers because it contains glycerin, which lubricates the ocular surface and therefore may minimize initial resistance and subsequent friction during the passage of the microkeratome head. This lubrication is an essential step that is often overlooked by the surgeon, who may be eager to pass the microkeratome, and one that requires attention on the part of the surgical assistant. The surgeon is handed the previously inspected microkeratome head for engagement on the suction ring. The surgeon and the assistant should ensure that there are no potential impediments such as eyelids, drape, or speculum in the path of the microkeratome head.

4. Advance/Reverse Microkeratome

Several microkeratomes are available; they are described in detail in Chapter 4. For horizontal microkeratomes, the head is engaged, and once a clear path is ensured, the foot pedal is used to advance the microkeratome in a temporal-to-nasal direction. Once the microkeratome cut is completed, the surgeon reverses the microkeratome.

For vertical microkeratomes, the head is dropped on the pivot prior to engagement of the microkeratome (Fig. 12.3D). It is sometimes helpful to rotate the eye slightly superiorly to minimize the risk of engaging the lower lid. The head is activated by depressing the foot pedal and rotates to create a superiorly based hinge. For manual vertical microkeratomes, the surgeon’s index finger may provide assistance to ensure a constant speed of operation. The auditory signal indicating completion of the flap allows the surgeon to reverse the direction of the microkeratome head prior to disengagement. If the microkeratome stops prematurely, an attempt at continued forward advancement is acceptable after clearing any possible obstructions, as long as the microkeratome is not reversed. If the jam requires reversal, the microkeratome should be removed without attempting to go forward.

In most microkeratomes, the blade oscillates on the backward pass. Some surgeons elect to release suction prior to the automated backward pass. However, this can predispose to flap injury: if the patient moves the eye in any direction other than nasal for a horizontal microkeratome or superiorly for a vertical microkeratome, the flap can be lacerated by the oscillating blade. An alternative approach is to stop both suction and blade oscillation and simply slide the microkeratome off the eye after the forward pass. This shortens suction time and reduces friction between the flap and microkeratome during the reversal step; this may reduce the incidence and/or magnitude of epithelial injury. This approach is routinely used by one of us (DDK) when using the Hansatome.

In patients with deep-set orbits and prominent brows, a chin-up position may be helpful. In cases of prominent lower cheek and overhanging skin over the lower blade of the speculum, the assistant may have to retract the skin to ensure unobstructed passage of the microkeratome. It is important to release suction as soon as the reverse pass is completed in order to reduce the duration of the elevated IOP and minimize potential retinal and optic nerve injury.

5. Forniceal Drying

Drying of the fornices after removal of the microkeratome and suction ring is important to minimize the movement of meibomian and other tear debris into the flap interface. This can be done by using dry Murocel sponges, initiating vacuum with an aspirating speculum, or reapplying the suction ring lightly into the upper and lower fornices. Drying the ocular surface may also provide better visualization of the gutter around the flap and may reveal any epithelial irregularities.

6. Chayet Ring Application

Prior to lifting the flap, the application of a conventional or a modified Chayet ring (Fig. 12.4A) serves two purposes: (1) providing an elevated step for the everted flap to rest on, which will minimize tear fluid migration from the fornix to the undersurface of the flap, and

(2) providing a relative barrier to the centripetal migration of debris onto the bed upon irrigation in preparation for and during flap repositioning.

7. Proper Centration of the Laser

The dehydration rate of the cornea increases after the flap is elevated. This is believed to alter corneal ablation rate, which may decrease the predictability of the procedure. It is therefore important to ensure proper centration of the laser prior to flap lifting. This will not

Figure 12.4 (A) Chayet ring application. (B) Lifting the flap. (C) Wiping off blood from ruptured limbal vessels. (D) Pupil centration and laser ablation.

only minimize the interval between lifting and ablation but also provide the patient with an opportunity to visualize the blinking target while the flap is in place. The patient should be instructed that the target may fade and change color as the flap is lifted.

8. Lifting the Flap

Several instruments may aid in lifting the flap, including an irrigating cannula, a modified hook, or a modified curved tying forceps (Fig. 12.4B). The advantage of the forceps is that as the flap is lifted the surgeon can separate the tips of the forceps by releasing the grasp on the forceps, thus preventing folding of the flap. The flap is then positioned on the Chayet ring. Gentle sweeps with a dry forceps on the undersurface of the flap may be necessary to prevent flap wrinkling while it dries on the ring. The flap can also be folded in half in the “taco” fold. This shields the undersurface of the flap from the laser in instances in which the ablation might slightly overlie the hinge.

A dry Murocel sponge may be necessary to wipe any fluid or blood pooling straddling the flap edge, especially at the hinge (Fig. 12.4C). This process may have to be repeated prior to laser ablation, but the time from flap turning to ablation should be minimized to avoid excessive stromal dehydration.

9. Ablation Depth and Pachymetry

The calculated ablation depth is reconfirmed prior to laser ablation. In patients in whom the calculated residual bed thickness approaches 250 m, pachymetry is obtained intraoperatively. Because the flap thickness chosen for a particular procedure provided by the microkeratome manufacturer may vary dramatically from actual flap thickness, it may be risky to use the microkeratome thickness setting for calculating residual bed thickness. Optical pachymetry avoids touching the cornea and may underestimate ultrasound pachymetry by 15 to 20 m. The use of an ultrasound pachymeter requires precalibration to measure pachymetry readings below 200 m. Furthermore, the alcohol used to sterilize the tip of the pachymeter must be air dried prior to touching the cornea to avoid altering corneal ablation rates. This may be confirmed by sweeping the pachymeter tip in the surgeon’s sterile glove prior to touching the stromal bed.

10. Residual Bed Calculations

Once the pachymetry is measured, residual bed calculations are performed and the laser parameters may be reconfirmed or altered to suit these calculations. If the flap is thinner than expected, the treatment diameter may be increased in patients with larger pupils. If the residual bed is smaller than expected, the surgeon should consider undercorrection to leave an adequate untreated bed and reduce the risk of subsequent ectasia. In cases of astigmatism 1 D, prior to finalizing the modified laser setting, if needed, the surgeon should confirm alignment of the cylinder marks to avoid axis shift.

C. LASIK STAGE III. LASER TREATMENT AND FLAP REALIGNMENT

1. Dim Oblique Illumination

Direct illumination is helpful in the first two stages of LASIK, but laser treatment may sometimes be achieved with greater patient cooperation if only dim oblique illumination is utilized. This allows physiologic dilation of the pupil, decreased light sensitivity, and

greater ability to fixate on the blinking target. Alternatively, a ring light illuminator on its lowest setting may assist the patient in locating the fixation light and in some patients seems to increase the sharpness of the fixation target, perhaps because of the reduced pupil size.

The patient is instructed to maintain fixation on the target while the eye is monitored for any movement (Fig. 12.4D). If the laser system has no tracker, the surgeon may consider using a suction ring on low suction to assist a patient who cannot maintain fixation, but this is rarely required. Verbal contact with the patient is initiated at this step and continued throughout the laser treatment.

2. Side View to Confirm Alignment

Additional confirmation of appropriate centration by an assistant or the surgeon may be necessary. This is facilitated by viewing the eye from the side to confirm alignment. Not infrequently, the patient may fixate on a target other than the fixation target. When this is suspected, the patient can be asked to describe the target (in order to correct fixation). The surgeon may have to dim all other microscope and room lights to ensure patient cooperation. Problems with proper fixation can usually be prevented by having the patient fixate on the target just before lifting the flap, which enables the patient to recognize the blurrier image of the fixation light as viewed through the exposed stromal bed.

3. Pupil Centration

The laser treatment is centered on the pupil. Laser manufacturers provide several methods of centration. Decentration relative to the flap or the geometric center of the cornea should be disregarded. Once the pupil and the target are aligned, target fixation is confirmed by the patient, and the patient’s head is held in place (Fig. 12.4D). Alternatively the laser joystick is used to track the eye. In lasers with active trackers, it may still be important to minimize eye and head movement to reduce postoperative aberrations.

4. Head Holding

Head holding is preferred even in lasers with a tracker. It helps not only with tracking and correcting involuntary eye movement but also with regard to patient reassurance. A disadvantage is the possibility of change of plane of focus. Accordingly, the surgeon should estimate the customary amount of induced defocus and take it into consideration when centering the laser prior to holding the patient’s head. Most often the surgeon’s hand brings the head away from the microscope; thus defocus can be prevented by prior focusing on the iris plane.

5. Updates During Surgery

As the laser procedure proceeds, the surgeon should keep updating the patient regarding the progress of laser ablation and reassuring the patient that he or she is fixating adequately. This coaching may help increase the patient’s ability to maintain fixation and avoid giving up fixation prior to termination of the laser ablation.

Instead of asking a patient who has lost fixation to refixate during surgery, the surgeon should stop the laser and recenter prior to continuing the treatment. In hyperopic patients, these tasks have to be performed while protecting the flap, but priority should be given to fixation and centration rather than flap protection.

6. Flap Repositioning and Interface Irrigation with BSS

After congratulating the patient at the end of the treatment, one of the authors (DTA) routinely uses a long BSS cannula to wet the stromal bed (Fig. 12.5A). Such a cannula may offer the advantages of high squirting pressure and low volume of irrigation. It is advisable to have a dedicated syringe and cannula for this purpose and avoid introducing any other instruments into the interface, particularly the instrument previously used to lift the flap, which may theoretically reduce the chance of introducing epithelial cells in the interface. During this step, the surgeon may consider explaining to the patient that there will be blurring and apparent movement of the fixation target.

7. Flap Refloating

The flap is repositioned using one of several devices, such as a spatula or the same forceps initially used to lift the flap (Fig. 12.5B) to refloat it onto the stromal bed. The techniques of flap refloating vary among surgeons, but the speed of replacing the flap should be commensurate with the speed of the free fall of the flap, which may depend on patient age and flap thickness. Fast replacement may stretch the flap; slow replacement may predispose to incongruent edges and flap folding.

Figure 12.5 (A) Interface irrigation. (B) Flap repositioning. (C) Flap refloating. (D) Murocel drying of gutter.

8. Interface Reirrigation

Some surgeons reirrigate the interface only if the initial flap replacement resulted in inadequate reapposition of the flap. Others, including the authors, routinely reirrigate the interface to loosen adherent debris and to assure technique consistency and presumed increased accuracy of outcomes (Fig. 12.5C). The fluid should be injected vigorously to flush debris from the interface without fluid pooling. If injecting from both sides of the flap, the direction of the injection should be centrifugal. If injecting from only one side, the injection should begin with the cannula tip just under the flap edge, moving the cannula centrally to flush material out of the opposite side of the flap. Irrigation should be continued while the cannula is being removed. Excess fluid irrigation should be avoided to minimize edema and may need to be aspirated with suction or with a Murocel sponge.

9. Murocel Drying of Gutter and Flap Realignment

The endpoint is flap repositioning in a symmetrical gutter, realignment of the preplaced corneal marks, and absence of air, fibers, and debris in the interface Most surgeons use a noncellulose, nonfragmenting sponge (Murocel) to dry the gutter around the flap and to sweep the epithelial surface of the flap (Fig. 12.5D). Our approach is to apply a dry Murocel sponge just outside the flap margin adjacent to the inner margin of the Chayet ring. This allows centrifugal movement of fluid from the interface with minimal osmotic exchange of fluid from the tear film in the fornices and around the ring. This is followed by a gentle sweep with the moist expanded Murocel, starting at the hinge, and by multiple centrifugal pararadial sweeps with a (blotted) moist Murocel.

Even in the absence of basement epithelial membrane dystrophy, epithelial defects and loose epithelium may occur after flap dissection, especially if excessive proparacaine was used or if LASIK flaps were created with a rotating microkeratome. After repositioning of the flap stroma, loose epithelium is realigned. A moist Murocel is very helpful to achieve this goal without inducing striae.

10. Direct Illumination

At this point one of us (DTA) switches the light source to the direct illumination mode, preferably using the placido single-ring light source. To avoid a Bell’s phenomenon or other ocular movement, the patient is warned that the light will be brighter. The corneal reflex of the light source is valuable to identify striae and flap stretching (Fig. 12.6A). It will be helpful in deciding the need for and degree of sweeping of the flap once dried.

11. Pararadial/Radial Sweeping

For one of us (DTA), the endpoint of corneal sweeping should be observation of a round placido reflex in the case of ring illumination as in the VISX laser. The direction of the pararadial and radial sweeps is guided by the shape of the light reflex and should start at the center aiming at making it more or less circular and avoiding folds (Fig. 12.6B). The reflex obtained after wetting the surface often appears more circular and more regular than reality (Fig. 12.6C). Furthermore, the wet corneal surface may prevent detection of flap folds or stretch marks.

Figure 12.6 (A) Radial sweep. (B) Pararadial sweep. (C) Direct illumination and waiting period for adequate flap apposition. (D) Speculum removal.

12. Waiting Period for Adequate Flap Apposition

Once the sweeping with the moist sponge is complete, a 2 to 5 minute period of waiting ensues. The ocular surface should not be allowed to dry completely during the waiting period. Topical antibiotics, steroids, and nonsteroidal anti-inflammatory eyedrops are applied during the waiting period. After unintentional eye movement, observation of a circular light reflex should reassure the surgeon regarding the adequacy of flap apposition. Most surgeons use the waiting time to review postoperative precautions of avoiding eye rubbing and lid squeezing.

Alternatively, if the hinge is superior, one of us (DDK) does not wait once the flap is aligned. Antibiotic drops, preservative-free nonsteroidal drops, and preservative-free artificial tears are administered at 15 second intervals prior to removing the lid speculum.

13. Speculum Removal

At the conclusion of surgery, the lid speculum and drape are removed. This should be done with care to avoid flap dehiscence and folds (Fig. 12.6D). The speculum may be rotated superiorly prior to removal to minimize displacement or injury of the flap. Reinspection of the flap after removal of the speculum is valuable to ensure an adequate flap position.

14. Recheck Period and Discharge

Following surgery, the eye is inspected at the slit lamp to insure correct flap alignment and absence of clinically significant interface debris. The patient is then asked to wait 20 to 40 minutes with the eyelids gently closed. Following the waiting period, the surgeon can again examine the patient using slit lamp biomicroscopy to ensure flap and epithelial mark alignment, gutter symmetry, and clarity of the interface. In addition, flap folds and wrinkles may be seen. The flap may need to be re-elevated and irrigated and the patient brought back to the operating room to manage any abnormalities noted. Otherwise, the patient is discharged after receiving instructions and is followed 1 day after surgery.