Microkeratomes and Laser Settings

eter suction ring is chosen then a larger diameter flap will result. The preoperative keratometry values play a significant role in the diameter of the flap (1,2). As the cornea becomes steeper, more of the cornea will pass through the ring, producing a larger diameter flap. Conversely, a flat cornea will not protrude into a given suction ring as much, and would produce a flap of reduced diameter.

In a patient with a very steep cornea, use of a large diameter suction ring, such as a 9.5 mm ring, could produce a flap that exceeds the clear cornea diameter, potentially injuring the limbus and causing unnecessary bleeding. In patients with a history of long-term soft contact lens wear and peripheral corneal neovascularization, it is possible to cause bleeding by cutting into these vessels. Steep corneas are also more likely to produce but- ton-hole complications during the microkeratome pass. This is most likely due to buckling of the cornea against the microkeratome footplate. A larger diameter ring size may increase the potential for this phenomenon. For this reason we recommend using a smaller diameter ring size for significantly steep corneas. Using the Hansatome we have had success using 46 D as a cutoff point: we use the 8.5 mm ring for corneas equal to or steeper than 46 D, when performing myopic or small optical zone corrections, and use the 9.5 mm ring for corneas flatter than 46 D provided there is adequate corneal diameter. For hyperopic, or large optical zone ablations, we use a K reading of 49 D as the cutoff point, using the 8.5 mm ring for 49 D and higher Ks and the 9.5 mm ring for lower Ks.

Patients with flatter corneas present less tissue through a given ring size, and produce smaller diameter flaps. Corneas that are very flat are at higher risk for producing a free cap during the creation of the flap. This occurs when relatively less of the corneal dome protrudes above the level of the ring, and subsequently a flap is produced which is so small that it is completely excised. For these reasons, when performing myopic ablations, we recommend a larger ring size, a 9.5 mm or 10.0 mm ring, for corneas with Ks below 46 D. A small flap, even if it is not a free cap, can be a problem for other reasons as well. Patients with flatter corneas are more commonly presenting for hyperopic correction, which requires a larger ablation zone. When attempting large ablations for hyperopia or mixed astigmatism, we utilize the 9.5 mm suction ring in patients with K readings below 49 D.

The above guidelines will typically allow creation of a flap that is significantly larger than the diameter needed for the ablation zone. This is beneficial for several reasons. Having a larger flap gives the surgeon a larger area for ablation, especially with newer, larger transition zones. A larger stromal bed makes decentration of the microkeratome cut less likely to interfere with the ablation area. The flap hinge is moved farther away with larger flaps, reducing the likelihood of accidental treatment of the back of the flap, and reducing the effects of fluid collection at the flap edge. Moving the flap further away from the ablation also prevents the surgeon from having to protect the flap or having to dry the fluid collecting at the flap edge. These extra maneuvers may slow the case, increasing the variability of the treatment effect. Having a larger flap is also probably advantageous when performing LASIK following radial keratotomy, astigmatic keratotomy, and penetrating keratoplasty. This allows a greater flap size to reduce the potential separation of the RK, AK, or penetrating keratoplasty incisions. In the event of an overcorrection leading to consecutive hyperopia, it is convenient to have a larger flap to perform the hyperopic treatment (usually requiring a larger ablation zone) rather than having to cut a new, larger flap. It has not been demonstrated in a controlled study that larger flaps are any more, or less, likely to be dislocated by late trauma, but larger flaps clearly have a larger area of adherence (a 9.5 mm flap has 84% more surface area than a 7.0 mm flap)

which may stabilize the flap. Larger flaps have edges closer to the limbus, in closer proximity to the limbal stem cells, which may speed the healing of the epithelial wound (3). Larger flaps will become increasingly important as we move to larger ablation zones and even larger transition zones.

Disadvantages of larger flaps include more fluid beneath the flap, which may require greater time at surgery to allow adherence of the flap. They may also require more stretching at the time of surgery to remove the extra fluid under the flap to reduce the incidence of striae. The cornea also is deformed to a greater extent, which can increase the incidence of epithelial defects. Placing the edge of the flap closer to the stem cells also may increase epithelial ingrowth rate, although if it were to occur it would be further from the visual axis.

There are instances when a smaller diameter ring, and thus flap diameter, is advantageous. If the flap is placed too close to the limbus, problems with bleeding can occur owing to transection of the limbal vessels. This is more of a problem with larger flaps. Longterm soft contact lens wearers can have a significant peripheral neovascular ingrowth, and in these patients a smaller ring size may be more prudent to avoid the nuisance of bleeding that can occur if the flap extends into these vessels.

2. Flap Thickness

Most microkeratomes designed for LASIK on the market today offer multiple footplate depths to produce a range of flap thicknesses. When deciding which one to choose, several factors must be considered. The two most important patient factors to be considered are the corneal thickness and the preoperative refractive error. There are also advantages and disadvantages of thicker and thinner flaps. Another important factor to consider when choosing the head plate depth is that the actual flap depth produced by the same microkeratome with the same head can vary substantially (4–7). Preoperative pachymetry can be one of the biggest determinants of flap thickness (2,5,7). Thinner corneas tend to produce thinner flaps, and thicker corneas yield thicker flaps. As the blade on the microkeratome gets duller, the flaps cut will be thinner and more likely to be irregular (5,6,8). Increasing translational speed (forward progress during the cut) results in thinner flaps, and this can cause problems with flap depth consistency in manually advanced microkeratomes (9). Intraocular pressure can affect flap thickness as well, with lower pressures resulting in thinner flaps. In microkeratomes with replaceable depth plates, the plate can be inserted incorrectly, resulting in a deeper than intended cut with possible entry into the anterior chamber (Figure 13.1). Newer microkeratomes with a fixed depth plate are less likely to cause this complication.

One of the most important factors to consider when choosing flap depth is the limitation imposed by the patient’s central corneal thickness and level of ametropia. It is not known exactly how much residual posterior stromal thickness (RPST) is required to prevent consecutive ectasia following LASIK. It has become common at most institutions that 250 microns represents a minimum for RPST (10). This is clearly a controversial area, and few data are available to provide a definite answer. An RPST of 250 microns is probably more than necessary in most cases, but there probably will be rare cases where ectasia can occur even with a RPST of greater than 250 microns (11). From the patient’s minimal central thickness we must subtract this 250 microns and the flap thickness to determine how much ablation can be performed. The ablation depth can be calculated using the Munner-

Figure 13.1 Inadvertent corneal penetration from failure to assemble microkeratome footplate. An eight bite 10-0 nylon suture was used to repair the wound.

lyn formula (depth of ablation diopters of correction (ablation diameter)2/3) (12). In some cases of high refractive errors, thin central corneal thickness, or both, it may be necessary to choose a thinner flap to allow for more ablation. It should also be noted that thinner corneas tend to yield thinner flaps, and this may be advantageous in cases where residual stromal thickness is an issue (5). In any case in which the computation is close, the safest maneuver is to measure the residual thickness with ultrasonic pachymetry after the flap is reflected back. This will eliminate the unknown factor of flap thickness and alert the surgeon to a flap that may be thicker than expected.

While the ideal flap thickness has not been determined in any controlled studies, we believe that in most cases a flap thickness of around 180 microns is the best choice. This thicker flap has numerous advantages: it is easier to manipulate during surgery, has a lower incidence of folds and striae and a lower incidence of irregular astigmatism, and it is more likely to resist injury during manipulation and future enhancements if needed. In addition, a problem during the microkeratome cut resulting in a thinner flap is less potentially devastating if a thicker flap was initially targeted. The chief disadvantage of a thicker flap is that less residual stroma remains for ablation; this could become an issue in cases with high refractive errors, thinner corneas, or both.

The decision to use a lower footplate setting, and thus a thinner flap, is probably only advantageous in a setting where there is a risk of exceeding the minimum safe residual stromal bed thickness of 250 microns. In these cases, the disadvantages of the thinner flap (higher risk of folds, higher risk of irregular astigmatism, more difficulty handling thinner flaps, more difficulty lifting flap for enhancements, and a smaller margin of error when cutting the flap) must be weighed against the disadvantages of decreasing the treatment zone diameter to preserve tissue, or using an alternative technology such as a phakic intraocular lens. This decision should be made on a case-by-case basis, and patient factors that must be considered include patient’s pupillometry and special visual needs (driving at night, etc.). In our experience it is probably better to preserve residual bed thickness by using a thinner flap than decreasing treatment zone size, because smaller treatment zones are more likely to result in optical distortions. In cases where using a thinner flap does not allow at least a

vary from unit to unit, and poor quality blades should uniformly be rejected and replaced. Blades should never be reused unless expressly designed for such use by the manufacturer. It has been shown that dull blades produce thinner flaps and increase the risk of irregular flaps (5,6). Many centers allow for the same blade to be used on both eyes of the same patient, but not more than this. Even in this usage it has been demonstrated that the second flap cut is somewhat thinner than the first (6).

C. GUIDELINES FOR LASER SETTINGS

It is a popular misconception that lasers are computer-controlled devices that are incapable of producing a less than perfect outcome. In reality these devices, while capable of great accuracy, must be carefully maintained and calibrated to achieve proper performance. When setting up the excimer laser the following parameters merit special attention: laser room environment, fluence testing, and beam homogeneity.

The laser room environment is an important variable that must be controlled. The room should be kept cool for optimal laser performance. This varies from laser to laser, but a temperature of 18 to 24 degrees Celsius should be maintained. The ambient humidity should be kept low (40 to 50% relative humidity) and as steady as possible. Humidity can significantly affect the ablation rate of corneal tissue, with overcorrections more likely in very dry conditions and undercorrections more likely in more humid conditions (13). The room should be kept free from particulate debris, which can adversely affect ablation regularity if it is deposited on the optical system of the laser. This can be achieved with an active filtration system using a HEPA (high efficiency particulate air) filter. These systems are capable of removing particles as small as 0.1 micron, including bacteria. The system should be in operation continuously, as shutdown periods will allow particulate matter to coat the optics of the laser system.

Laser fluence is a measure of the energy density and is described as the amount of energy applied per unit area with each pulse. This is measured in millijoules per centimeter squared (mJ/cm2). The minimum fluence necessary for proper photoablation of the cornea is approximately 50 to 60 mJ/cm2. The fluence of the laser should be checked before every ablation, which is usually automatically performed in most lasers. If the fluence is low, this is an indication that the gas concentrations should be raised to prevent undercorrections, which may occur due to inadequate tissue ablation. Gas levels that are too high can cause higher fluence and overcorrections.

Beam homogeneity is a measure of the consistency of the distribution of energy applied over the ablation area. Homogeneity is an important parameter in broad beam lasers, and if it is poor, it can lead to an irregular ablation and potential loss of best spectacle corrected visual acuity. It is of much less significance in small scanning spot/slit lasers, as any inhomogeneity present would be spread out evenly over the ablation area. Manual verification of beam homogeneity is essential in broad beam delivery systems. This is usually accomplished by test ablations into appropriate substrate materials as provided by the individual laser manufacturer. Visual evidence of poor homogeneity is reason to cancel the procedure until the problem can be remedied.

1. Adjustments to Standard Laser Nomograms

It should come as no surprise to any experienced surgeon that the laser is very much like any other surgical instrument in that an adjustment must be included in the treatment plan

to allow for different surgical variables. While the primary determinant of the tissue ablation pattern and final visual outcome is the treatment design that is included in the laser software, this does not lessen the importance of developing personal nomograms. Today’s surgeon has the advantage of using ablation software that has evolved through several generations and provides for advanced features such as wider optical zones, multiple treatment zones, transition zones, and software to reduce surface irregularities such as central islands. However, each laser is used in different settings, and surgeons have individual variations in technique and operating environment. To allow for these differences, individual nomograms should be developed to improve the accuracy and predictability of treatments. Increasing the standardization of the technique will improve the standard deviation of the achieved effect, and adjusting the nomogram by the average achieved effect will move the mean towards the desired result, which is typically emmetropia.

There are many factors that can contribute to the final visual and refractive outcome after LASIK. These include patient age, gender, preoperative keratometry, preoperative pachymetry, degree of planned correction, laser type, software version, room temperature, room humidity, facility altitude, accuracy in laser calibration, depth of keratectomy, degree of corneal hydration used by the surgeon, time that the flap is raised and the total procedure duration, whether or not forced air (or vacuum) is used across the stromal bed during ablation, postoperative inflammation (e.g., diffuse lamellar keratitis), and postoperative medications. It is the job of the surgeon to control as many of these factors as possible (e.g., temperature, humidity, technique, time of surgery, postoperative medications), as this is the first step in achieving precise outcomes. For the factors that are not controllable, it is important to examine how they affect refractive outcomes using statistical methods so that the accuracy of the ablation can be improved, decreasing the rate of overand undercorrections.

Differences in technique can make a large difference in the final ablation. Even small variables should be sought out and eliminated if possible. For example, lifting the flap with nontoothed forceps instead of an irrigating cannula removes the risk of accidental introduction of fluid into the interface. Even a small amount of fluid can hydrate the stroma, resulting in decreased tissue ablation and undercorrection. The dryer the stromal tissue is, the more tissue is ablated per pulse of the laser (13). Increasing the amount of time that the flap is raised increases the evaporation and decreases the hydration of the bed. This can result in significantly increased ablation and thus overcorrection. The time to perform the entire technique from start to finish, and especially the time while the flap is lifted, should be consistent from case to case. If the surgeon chooses to wipe the bed, then the same technique should be performed in every case, including the dampness/dryness of the sponge and the number of wipes during the ablation. When choosing an initial nomogram one should choose one from a surgeon who ideally has experience on the same laser, at the same facility, and who uses a similar technique. This will give the beginning surgeon a starting point.

Patient age is a very important consideration when designing nomograms. The same laser will typically have more effect in an older patient. Older patients also have less accommodative amplitude, which will make them less tolerant of hyperopia. We are less aggressive when treating myopia in patients over 40 for these reasons. Some surgeons will include an adjustment for mild residual myopia in older patients where the nondominant eye treatment is reduced by 5 to 10%. Younger patients show a greater tendency towards regression and can tolerate small overcorrections owing to their increased accommodative

amplitudes. For these reasons it is appropriate to be somewhat more aggressive in younger patients.

The development of a personal nomogram begins with the collection of data including the patient’s age, refractive error, gender, pachymetry, keratometry, and postoperative manifest refractions, including data with the longest follow-up time possible; typically 6 months to 1 year is ideal. While in most cases the patient’s refractive error will stabilize at 3 months, higher refractive errors may take longer, sometimes 6 to 12 months. It is important to collect and include this data, if it is available. The data is then analyzed using multiple regression analyses, and the resulting information is used to create a personal nomogram. There are also commercial programs on the market that will assist in nomogram development (Table 1).

Other factors should also be considered. As was mentioned above, it may be desirable to attempt slight hyperopia to achieve fewer myopic undercorrections in younger patients, and mild myopia on overage in older patients to reduce undesirable hyperopia. It must be remembered that treatments for overcorrection of myopia can be technically more difficult than the initial surgery, especially if a small flap was originally used, and the retreatment requires the cutting of a new flap. Because of the relative difficulty and poorer predictability in hyperopic treatments, some surgeons structure their nomograms so that the rate of enhancement for an overcorrection is less than the rate of enhancement for an undercorrection.

The nomograms should also be altered to allow for differences in corneal response following PRK, LASIK, radial keratotomy, or penetrating keratoplasty. Concerning retreatments, several trends can be observed. Patients who have shown large amounts of regression may be more likely to underrespond to retreatments as well. For this reason, some surgeons are more aggressive when treating an initial undercorrection. We tend to use the same nomogram for retreatment of initial undercorrections, unless it is clear why the patient originally underresponded, attempting to avoid problems with overtreatment. When treating overcorrections after previous LASIK, most surgeons reduce the correction, because these eyes have a tendency to overrespond to the second treatment.

The frequency of updating nomograms is a matter of personal choice. Some surgeons use a more rapid cycle when recalculating their nomograms and change nomograms every few months. This can allow for a more rapid adjustment to a new technique, new instruments, or other factors. Other surgeons prefer to make fewer adjustments on their nomograms, opting to refine their nomogram only when larger patient numbers and more followup data indicate that a change is necessary. Beginning refractive surgeons should begin data collection and analysis in a timely manner but should avoid the tendency to start changing the nomogram until an adequate amount of data has been collected at a stable time point postoperatively.

Table 1 Commercially Available Outcomes and Data Analysis Software

Columns to far right are for consecutive hyperopia. Use the above adjustment to spherical equivalent (patient’s spherical equivalent is the horizontal axis and age is the vertical); enter full cylinder without correction.

D. CONCLUSION

LASIK continues to evolve into a very safe and effective technique. The proper selection of microkeratome settings and the development of personal laser nomograms are important elements of a successful LASIK procedure. It is only by diligent attention to details and the continuous analysis of variables that we can continue to advance the state of the art, while providing the greatest accuracy and best possible vision for our patients.

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