Ординатура / Офтальмология / Английские материалы / Wavefront Customized Visual Correction The Quest for Super Vision II_Krueger, Applegate, MacRae_2003

OPTIMIZING OUTCOMES FOR

TOPOGRAPHY AND WAVEFRONT

When a LASIK flap is made on the eye, changes in corneal curvature and thickness occur outside of the ablation zone.24,25 This biomechanical response causes a FE in the treatment zone, which enhances a myopic procedure but counteracts a hyperopic ablation.24,25

Creating a flap on the eye changes the structure of the cornea. Applying principles of spheric corneal topography may account for the differences in relative effect of creating a flap through an elliptical or conic section. It enables differences in the relative effect of laser treatment in the presence of corneal cylinder that is either greater or less than the refractive cylinder to be incorporated into the treatment plan.

Optimizing outcomes for topography and wavefront examinations takes into account both the corneal structure and refractive function. For individual patients this means that any discrepancy between refractive and corneal astigmatism can be taken into account in the treatment plan to avoid an excess residual amount of corneal astigmatism. Changes in corneal structure induced by LASIK surgery particularly can affect the lowerorder aberrations of the eye.

Guiding the TIA, the meridian of maximum ablation, closer to the principal flat corneal meridian would result in less corneal astigmatism. The process of vector planning would reduce the excess amount of corneal astigmatism remaining demonstrated in this (2.81 versus 1.32) and other studies.3 This needs to be further investigated and then incorporated into the treatment plan if a favorable influence is identified.

The potential benefits of vector planning of preoperative corneal and refractive astigmatism values are:

1.Corneal values are included in the treatment plan

2.The treatment can be optimized and balanced between corneal and refractive parameters

3.The potential for less “off-axis” effect on principal corneal meridia and a greater reduction in corneal astigmatism

4.There is the potential to bias the treatment to a more favorable with-the-rule astigmatism outcome (which usually has the least adverse impact on distance vision) or to any other orientation preferred by the surgeon

5.Reduced corneal astigmatism is likely to provide less loworder (second) aberrations and even some high-order (third and fourth) aberrations and enhanced contrast sensitivity

CONCLUSION

Wavefront-guided laser refractive surgery holds future promise for correcting higher-order aberrations of the eye. However, we must not lose focus of the importance of correcting both the spherical and cylindrical components of refraction quantified by the lower-order (second) aberrations. Nomogram adjustment of laser treatments remains an important factor in the surgical planning process. Vector planning would potentially be a valuable adjunct to treatments that are guided by wavefront aberrometry measurements. Optimizing outcomes by incorporating both corneal and refractive values into the treatment plan are necessary challenges for the imminent future of refractive surgery.

Editor’s note:

Although the content of this chapter has been primarily focused on vector analysis and planning of wavefront vs refraction vs topographic cylinder, the possibility does exist to analyze the magnitude and axis of other higher-order aberrations as well. We reserve this topic for a later time, as the complexity of vector planning and astigmatism is sufficient for now. Further practical experience is needed to bring about more widespread acceptance of this method of analysis and planning.

R. Krueger, MD, MSE

DEFINITIONS

Astigmatism Analysis

angle of error (AE): The angle described by the SIA and TIA vectors. The AE is positive if the achieved correction was counterclockwise to that intended and is negative if the achieved correction was clockwise to the intended axis.

coefficient of adjustment (CA): The coefficient required to adjust future treatments of astigmatism magnitude. It is calculated by dividing the TIA by the SIA, and is preferably 1.00. correction index (CI): The CI is calculated by dividing the SIA by the TIA. The CI is ideally equal to 1.00. It is greater than 1.00 for an overcorrection and less than 1.00 if an undercorrection

has occurred.

difference vector (DV): The induced astigmatic change needed for the initial surgery to have achieved its intended target outcome. The DV is preferably 0.

flattening effect (FE): The amount of astigmatism reduction achieved by the effective proportion of the SIA at the intended meridian.

flattening index (FI): The FI is calculated by dividing the FE by the TIA. The FI is ideally 1.00.

index of success (IOS): The IOS is calculated by dividing the DV by the TIA. The IOS is a measure of success and is preferably zero.

magnitude of error (ME): The difference between the magnitudes of the SIA and the TIA. The ME is positive for overcorrections and negative for undercorrections.

ocular residual astigmatism (ORA): The ORA represents the calculated minimum amount possible of astigmatism remaining in the system after treatment. It is the vector difference between the corneal and refractive astigmatisms.

surgically-induced astigmatism vector (SIA): The astigmatic change actually induced by the surgery.

target-induced astigmatism vector (TIA): The astigmatic change the surgery was intended to induce.

torque effect: The amount of astigmatic change induced by the SIA that has been ineffective in reducing astigmatism at the intended meridian but has caused rotation and a small increase in the existing astigmatism. Torque lies 45 degrees counter clockwise to the SIA if positive and 45 degrees clockwise to the SIA if negative.2

Spherical Analysis

spherical correction index (S.CI): The spherical equivalent correction achieved divided by the targeted spherical correction. spherical difference (S.Diff): The absolute difference between the targeted spherical equivalent correction and the achieved

spherical equivalent correction.

spherical index of success (S.IOS): The absolute difference between the achieved and targeted spherical equivalents divided by the targeted spherical equivalent correction.

ACKNOWLEDGMENTS

The authors wish to thank Drs. Peter Heiner and Darryl Gregor for permission to analyze their data. Thanks also go to Mr. George Stamatelatos for performing the data analysis.

Financial Interest: Dr. Noel Alpins has a financial interest in the ASSORT vector planning and outcomes software used in the analysis of surgeries and results in this paper.

REFERENCES

1.Alpins NA. A new method of analyzing vectors for changes in astigmatism. J Cataract Refract Surg. 1993;19:524-533.

2.Alpins NA. Vector analysis of astigmatism changes by flattening, steepening, and torque. J Cataract Refract Surg. 1997;23:1503-1514.

3.Alpins NA. Astigmatism analysis by the Alpins method. J Cataract Refract Surg. 2001;27:31-49.

4.Williams D, Yoon GY, Porter J, Guirao A, Hofer H, Cox I. Visual benefit of correcting higher-order aberrations of the eye. J Refract Surg. 2000;16:S554-S559.

5.Thibos LN, Hong X. Clinical applications of the Shack-Hartmann aberrometer. Optom Vis Sci. 1999;76:817-825.

6.Seiler T, Kaemmerer M, Mierdel P, Krinke HE. Ocular optical aberrations after photorefractive keratectomy for myopia and myopic astigmatism. Arch Ophthalmol. 2000;118:17-21.

7.Marcos S. Aberrations and visual performance following standard laser vision correction. J Refract Surg. 2001;17:S596-S601.

8.Moreno-Barriuso E, Lloves JM, Marcos S, Navarro R, Llorente L, Barbero S. Ocular aberrations before and after myopic corneal refractive surgery: LASIK-induced changes measured with laser ray tracing. Invest Ophthalmol Vis Sci. 2001;42:1396-1403.

9.Oshika T, Miyata K, Tokunaga T, et al. Higher-order wavefront aberrations of cornea and magnitude of refractive correction in laser in situ keratomileusis. Ophthalmology. 2002;109:1154-1158.

10.Marcos S, Barbero S, Llorente L, Merayo-Lloves J. Optical response to LASIK surgery for myopia from total and corneal aberration measurements. Invest Ophthalmol Vis Sci. 2001;42:3349-3356.

11.Applegate RA, Thibos LN, Hilmantel G. Optics of aberroscopy and supervision. J Cataract Refract Surg. 2001;27:1093-1107.

12.Holladay JT, Dudeja DR, Chang J. Functional vision and corneal changes after laser in situ keratomileusis determined by contrast sensitivity, glare testing, and corneal topography. J Cataract Refract Surg. 1999;25:663-669.

13.Applegate RA, Hilmantel G, Howland HC, Tu EY, Starck T, Zayac J. Corneal first surface optical aberrations and visual performance. J Refract Surg. 2000;16:507-514.

14.Applegate RA, Howland HC. Refractive surgery, optical aberrations, and visual performance. J Refract Surg. 1997;13:295-299.

15.Liang J, Williams DR, Miller D. Supernormal vision and high-reso- lution retinal imaging through adaptive optics. J Opt Soc Am A. 1997;14:2882-2892.

16.Mrochen M, Kaemmerer M, Seiler T. Wavefront-guided laser in situ keratomileusis: early results in three eyes. J Refract Surg. 2000;16:116121.

17.Seiler T, Mrochen M, Kaemmerer M. Operative correction of ocular aberrations to improve visual acuity. J Refract Surg. 2000;16:S619-S622.

18.Arbelaez MC. Super vision: dream or reality. J Refract Surg. 2001;17:S211-S218.

19.Panagopoulou SI, Pallikaris IG. Wavefront customized ablations with the WASCA Asclepion workstation. J Refract Surg. 2001;17:S608-S612.

20.Vongthongsri A, Phusitphoykai N, Naripthapan P. Comparison of wavefront-guided customized ablations vs conventional ablation in laser in situ keratomileusis. J Refract Surg. 2002;18:S332-S335.

21.Mrochen M, Kaemmerer M, Seiler T. Clinical results of wavefrontguided laser in situ keratomileusis 3 months after surgery. J Cataract Refract Surg. 2001;27:201-207.

22.Alpins NA. New method of targeting vectors to treat astigmatism.

J Cataract Refract Surg. 1997;23:65-75.

INTRODUCTION

After the official release of the contoured ablation pattern (CAP method) by VISX (Santa Clara, Calif) in 1999, the technique and the method improved and evolved until its approval by the US Food and Drug Administration for use in December 2001, under a Humanitarian Device Exemption (HDE) protocol. The approval was limited to the treatment of irregular astigmatism produced by a previous refractive excimer laser surgery, and it is linked to the use of the CAP ablation planner from the Humphrey topography system (Carl Zeiss Meditec, Dublin, Calif).

The FDA exemption reflects the growing concern of a very severe problem, namely irregular astigmatism. The custom-con- toured ablation pattern (C-CAP method) is the only topographylinked system with proven and safe results for the treatment of irregular astigmatism.

The method is easy to use. It allows the surgeon to manipulate the excimer laser system and set the treatment parameters, such as shape, depth, location, size, and number of ablations, based on the topographic map and the clinical judgment.

DEFINITION OF CUSTOM-CONTOURED

ABLATION METHOD

C-CAP is software developed by VISX to use in conjunction with the VISX STAR S4 Excimer Laser System. It is extremely flexible and gives the surgeon a tool to produce any type of ablation, in any shape, any size, any depth, and in any location, to treat all refractive defects from regular to irregular. The software allows the user to program 20 different sequential ablations, with or without a pause between them.

The size of the beam can be varied from 0.60 millimeter (mm) (minimum) to 6.5 mm (maximum). Three different shapes can be utilized: sphere, cylinder, and ellipse. Hyperopic sphere or hyperopic cylinder can also be added as part of the treatment. Depth may go from 1 micron (µm) to 150 µm, and the beam may be directed to the center of the pupil or decentered up to 4 mm horizontally or vertically as measured from the center of the pupil, utilizing the X or Y coordinates in the laser computer (Figure 39-1).

CUSTOM-CONTOURED

ABLATION PATTERN BACKGROUND

After the publication of a paper from Gibralter and Trokel in 19941 about the treatment of irregularities in the cornea with manual decentration of the excimer beam, we started to move the beam to treat the steepest areas of the cornea without good results. However, in 1997, the topographic difference between the steepest area and the most elevated area was noted2 (Figure 39-2) and the beam was then deviated to ablate the areas that were elevated the most. Results improved dramatically. That was the beginning of the “manual” CAP method.3 Several treatments were performed with promising results (Figure 39-3) until March 30, 1999 when the three first eyes were treated with the CAP method software, launched then by VISX in April 1999 during an American Society of Cataract and Refractive Surgery (ASCRS) meeting in Seattle, Wash.

In 1997, the topographic difference between the steepest area and the most elevated area was noted2 and the beam was then deviated to ablate the areas that were elevated the most. Results improved dramatically.

Since then, thousands of eyes around the world have been treated with this method, and several investigational studies have been performed.4 The technique was improved over the years and the advantages of beam changes such as the seven beam split, which makes the beam smaller like scanning and rotational, was then applied. The CAP method was incorporated into all the hardware and software changes. The eye tracker— one of the most prominent features of the VISX STAR S3 sys- tem—added much more precision to the decentration properties of the beam, making the CAP software more effective and safe in ablating the most elevated areas.

At the same time, elevation topography has improved and has become more precise and more exact in its calculations. It gives information about location, shape, size, and depth of the most elevated areas, allowing the surgeon to choose the best treatment for every zone. The term topography-assisted excimer laser ablation was created, reflecting the ability to topographically detect the irregularities and then treat them. Topography systems, such as the

332 Chapter 39

A B

Figure 39-1. (A) C-CAP ablation on cornea demonstrating sphere and eliptical treatment. (B) Seven beam array emerges from the homogenizing and beam splitting module.

Humphrey and the Dicon, developed simulation software topographies that allow the surgeon to see the simulated topography result on the screen of the computer before the treatment is actually carried out. They have been called CAP planners. A section of this chapter will be dedicated to this subject (see p. 334).

CUSTOM-CONTOURED

ABLATION PATTERN APPLICATIONS

For the purpose of this book, we will concentrate on the treatment of irregular astigmatism with the VISX CAP method. However, the software also has other treatment capabilities.

Regular Defects

1.Myopia

2.Myopic astigmatism

3.Hyperopia

4.Hyperopic astigmatism

5.Mixed astigmatism with or without crosscylinder technique

6.Phototherapeutic keratectomy (PTK)

7.Presbyopia (discontinued with the use of variable spot software [VSS])

Irregular Defects

These are one of the most important indications of the C-CAP method and the reason for its approval in the United States by the FDA. They are also known as irregular astigmatism and will be the focus of this chapter. Irregular astigmatism is a very disabling problem because of the symptoms it produces (eg, glare, halos, phantom images, multiple images). It can be produced by two main sources: nonacquired or naturally occurring, and acquired.5

1.Nonacquired or naturally occurring

a.Keratoconus

b.Pellucid marginal degeneration

c.Congenital asymmetrical astigmatism

d.Corneal dystrophies

2.Acquired

a.Previous refractive treatments –Incisional surgery

Figure 39-2. C-CAP corneal topography and elevation map.

Figure 39-3. Preand postoperative corneal topography.

–Laser in-situ keratomileusis (LASIK) surgery: decentered flap complications

b.Other types of corneal surgery

c.Corneal trauma

CORRECTION OF IRREGULAR ASTIGMATISM WITH THE

CUSTOM-CONTOURED ABLATION PATTERN METHOD:

TOPOGRAPHY-ASSISTED EXCIMER LASER ABLATION

The treatment of irregular astigmatism with the excimer laser is based on resection of the elevated areas of the cornea that produce the irregularities. Lasers cannot add tissue to elevate the depressed zones. The method is essentially a topography-assist- ed ablation system; therefore, several rules must be followed in order to obtain an acceptable result.

It is extremely important to remember that the goal of the treatment is to regulate the corneal contour and not to get a complete refractive correction to allow patients to get rid of glasses.6 Such a correction can be intended once the corneal anterior surface has been brought back to normal, provided there is enough tissue to ablate and more than 270 µm of corneal tissue remains untreated.

The C-CAP method can be used with photorefractive keratectomy (PRK) or LASIK. The election of the technique should be based on experience, thickness of the cornea, amount of tissue needed to be resected, availability for follow-up, etc.

It is extremely important to remember that the goal of the treatment is to regulate the corneal contour and not to get a complete refractive correction to allow patients to get rid of glasses.6

Surface Ablation: PRK or LASEK

Epithelium can be removed with the laser if the treatment fits into the central 6.50 mm optical zone. If it does not, a manual removal is selected: epithelial brush such as the Amoils brush (Innovative Excimer Solutions Inc, Toronto, Canada) or 20% or 30% alcohol. It is beyond the purpose of this chapter to explain and discuss the technique called LASEK. However, it has been used by the authors in more than 170 cases, with excellent results very similar to the PRK results.

In both surface ablation techniques, when selected, mitomycin 0.02% is applied to the exposed stroma7 for 1 minute, as soon as the ablation is terminated, with a wet sponge. The application of this antimetabolite helps to prevent the formation of haze. It has been used very successfully by the authors in the last 1.5 years, avoiding haze in those corneas prone to scarring, postLASIK patients treated with surface ablation, corneas with multiple incisions, etc. Mitomycin should be carefully removed from the cornea, avoiding contact with other ocular surfaces. Then the cornea should be washed thoroughly and a bandage contact lens applied and left in place for 1 week or until the epithelium is fully recovered.

For the first 2 weeks after surgery, a combination of a strong corticosteroid drop (prednisolone acetate) and an antibiotic (ofloxacin or ciprofloxacin) is used. For the next 4 weeks, the patient is switched to a mild steroid treatment such as fluorometholone and tear substitutes as needed. In the last 4 months after treatment, the patient is kept on a nonsteroidal anti-inflam- matory (NSAID) drop.

The correction of the residual refractive defect, if chosen by patient and doctor, should not be attempted before 4 months after the initial treatment and the same protocol of the surface treatment must be followed.

•Wounds from old incisional cuts can open during the flap creation, producing unexpected astigmatism that is very difficult to resolve

•When a new flap is created in a cornea with a previous flap, some complications may take place, such as cutting part of the old flap, rolling it over, etc. Those complications can cause unexpected astigmatism or undercorrection of the irregularity

•A previously treated cornea can have iatrogenic ectasia that is not recognized by the surgeon because the keratometry may no longer be a factor. Extreme care should be taken when deciding to perform LASIK in those corneas

•In corneal ectasias—iatrogenic or naturally occurring— LASIK is no longer an option; surface treatment is a must

•In some treatments, the beam must reach distant areas from the center of the pupil. Be careful to not touch the flap. To do this, it must be centered according to the proposed treatment plan

CUSTOM-CONTOURED ABLATION PATTERN METHOD:

THE TECHNIQUE

The surgical plan for a custom ablation with C-CAP is usually subjective and individual. Every cornea with irregular astigmatism has a unique and personal condition. As opposed to regular astigmatism, generalization of the treatment when irregularities are present is not possible. There is no fixed nomogram. A rationale behind every single treatment has to be applied.

The C-CAP method from VISX is a topography-assisted ablation method of treatment of irregular astigmatism. It is a link between the topography map and the VISX STAR excimer laser through the analysis and decision of the surgeon. The excimer laser beam can only remove tissue from the corneal surface; it cannot add tissue to it. Therefore, regularization of the corneal contour is based on decreasing the heights of the “bumps” of the corneal surface. Understanding a topography map is crucial in the decision-making process of these types of treatments.

LASIK

In the correction of irregular astigmatism, this is always the first option for treatment because patient recovery is almost immediate, there is no pain after surgery, and less postoperative care is needed. However, several considerations should be taken into account. The flap does debilitate the cornea and by no means gives strength to it. It cannot be counted in the residual nontouched stromal bed, which must be at least 270 µm. The equation (central pachymetry – flap thickness + microns of ablation) should equal 270 µm or more. The following are other considerations:

•Creating a flap in a highly irregular cornea is a challenge. The corneal curvature has been altered by the previous surgery, but the scleral curvature remains unchanged. Therefore, the ring size cannot be chosen based on the actual keratometry but on the one measured before the first surgery

•Production of irregular flaps, incomplete cuts, or buttonholes has a higher rate in these cases. The reason is the irregular protrusion of the cornea through the ring selected. When the head of the microkeratome comes to make the cut, the irregular protrusion produces an irregular flap

First Step: Elevation Topography

Elevation topography is mandatory. Vector analysis topography, also known as curvature topography, cannot be used to decide and to make a treatment plan for irregular astigmatism. In fact, there is a strong difference between the steepest area of the cornea and the most elevated zone. They are different not only in conception and origin but also in location.8

The steepest area in curvature topography denotes the area where the greatest difference in dioptric power of two separate points is detected (ie, the slope of the mountain). The most elevated area is the peak of the mountain, the highest area of the cornea—in millimeters not in power—when compared to an “ideal” best fit curvature. Removing tissue from the steepest area may increase the slope of the elevation and compromise rather than improve the visual quality of that particular cornea.

There are two types of elevation topography:9,10

1.Placido’s disk-based elevation topography systems take advantage of the reflecting properties of the cornea. They use various proprietary algorithms to produce graphic representations of corneal topography. The elevation topography from a Placido’s disk unit is based on the specific program used for the adaptation of contact lenses. A mathe-

Figure 39-4. C-CAP corneal and elevation map.

matical algorithm calculates the elevation from the reflection of the concentric rings on the anterior surface of the cornea as compared to an “ideal corneal curvature,” which leaves areas that are under this best fit curvature (depressed areas: cold colors) and elevated ones related to it (elevated areas: hot colors). Units with elevation topography based on Placido’s disk topography are the Humphrey Atlas Unit and the Dicon-Paradigm topography unit (Paradigm Medical Industries Inc, Salt Lake City, Utah). They produce extremely accurate and reliable maps, although they are dependent on the reflecting properties of the anterior surface of the cornea, and they also depend on the quality of the tear film. Without any doubt, their great advantage is the CAP planner software present in these two systems, which is a very helpful tool in designing a treatment. The Humphrey CAP planner is the only one commercially available and approved by the FDA for its use in conjunction with the C-CAP Method and the VISX STAR S3 system

2.Slit-scan elevation topography. The only unit available with this technology is the Orbscan (Bausch & Lomb, Rochester, NY). This system produces very reliable and accurate maps, calculates the posterior surface of the cornea, and does not depend on the tear film quality. However, its great disadvantage is the lack of a computer program to help the surgeon to plan the treatment. Therefore, calculations have to be made manually

Second Step: Program of the Ablation

Ablation is based on the analysis of elevation topography. The map has to be taken very carefully and the pupil marker must be activated. There may be more than one elevated area, and it is the surgeon who decides which one should be treated first and the order of the sequential treatments. The following parameters must be determined to plan the treatment: shape, location, axis, and depth of each ablation.

Shape

The shape of the elevated area is decided by looking at the elevation map. There are three options to choose: sphere, cylinder, or ellipse.

Location

of the elevated area to be ablated. Remember that in some cases the pupil center (line of sight) and corneal vertex do not match. Some topography systems show the measurements as the cursor moves along the axes. Other systems, like Orbscan, have a scale of marks that starts at 1 mm from the center in each meridian and concentric circles that measure 3, 5, 7, and 9 mm in diameter.

Axis

This must be determined any time the shape is a cylinder or an ellipse. The long axis of the irregularity is found and then translated to the center of the pupil. This is the real axis of the zone to ablate.

Depth

Elevation maps give the depth in a scale. Elevated zones are above the surface reference and are presented in hot colors (eg, red).

Third Step: The Treatment

Given the facts mentioned above, the surgeon has to decide whether the best option for the treatment of irregular astigmatism is PRK, LASEK, or LASIK. In any type of surgery, the surgeon has to remember when he or she looks through the laser microscope, he or she will find the coordinates are visually (not mathematically) rotated 180 degrees as compared to the supine position of the topography examination. When the program is made manually in the Orbscan map, this should be taken into account to make sure the treatment goes to the right location. When using the Humphrey CAP planner, this is automatically transferred to the laser.

CAP PLANNERS

The Humphrey Atlas topography unit and the DiconParadigm topography unit have developed, independently from VISX, a software program that allows the surgeon to see the topographic changes the treatment will produce in the cornea before actually doing it. The Humphrey CAP planner is the only one commercially available and approved by the FDA to be used in conjunction with the C-CAP method to treat decentered LASIK patients under the HDE protocol.

The CAP planner is extremely user-friendly software and allows the surgeon to program several ablations in the computer and see how the topography map would look if the ablation were carried out. This “simulated map” is extremely reliable when compared to the real topography postoperative maps. The possibility to know the possible topography after the planned ablations allows the surgeon to develop several plans and to change them as many times as needed until the goal of the treatment is accomplished. This is a valuable safety feature for these rather difficult and sometimes unpredictable treatments. Once the surgeon is convinced about the ablation, the treatment can be printed, the paper taken into the laser, and the data entered into the computer (Figure 39-4).

For the use of a CAP planner, several steps must be taken:

1.If the depth of the ablation is more than 30 µm, it is advisable to divide it into two or three ablations, starting from smaller to larger until the total tissue removal is obtained

Table 39-1. Preoperative and postoperative UCVA in the postrefractive surgery group, treated with C-CAP method (36 months fol- low-up).

Table 39-2. Preoperative and postoperative BCVA in the postrefractive surgery group, treated with C-CAP method (36 months fol- low-up).

Table 39-3. Preoperative and postoperative UCVA in the keratoconus group after 31.6 months of treatment with the C-CAP method.

2.It is advisable not to have many tissue intersections among the different ablations. It is better to produce smaller ablations with less touch or less coupling between one ablation and the other

3.The least amount of tissue that is resected, the better the result. Therefore, the surgeon should try to produce a regular topographic pattern with the minimum amount of ablation

4.CAP planners can simulate the predicted corneal topography map. However, they cannot predict the final refractive result. Surgeon should be aware of “Barraquer’s law,” which states that any tissue resected at the periphery will produce steepening at the center of the cornea or myopic shift. Any tissue resected centrally will produce flattening at the center, or hyperopic shift

CLINICAL RESULTS

Once again, the treatment of irregular astigmatism is meant to restore the regularity of the anterior surface of the cornea, not to correct a refractive defect. Of course, some steps are taken to at least decrease the refraction of the patient without compromising the main goal of regularization of corneal contour. It is the irregularity that causes the bothersome and incapacitating symptoms of those patients. They are not produced by the refractive defect.

Patients were divided subjectively into two main groups: irregular astigmatism as a result of previous surgery and “primary” irregular astigmatism/ectasias. This division depicts the large differences between these two groups not only in origin but also in results.11,12 Ectasias are always treated by PRK and postsurgical cases are treated preferably with LASIK unless not enough tissue is found. The mean follow up is 36 months for both groups of patients. All eyes had complete preoperative and postoperative ophthalmologic evaluations. Uncorrected visual acuity (UCVA), best-corrected visual acuity (BCVA), keratometry readings, manifest refraction, and elevation topography maps were recorded.

Judging the success of this technique is very difficult, since the refraction is not what we aim to correct. In fact, it does not influence the patient’s decision to have the surgery. Eighty-seven percent of patients decide to have the surgery because of the presence of symptoms and only 13% wanted to have the surgery with the goal of getting rid of their glasses. Nevertheless, when the C- CAP method is analyzed against the conventional method, marked improvement is obtained, as can be seen in Tables 39-1 and 39-2 for the postrefractive surgery group and Tables 39-3 and 39-4 for the ectasia or keratoconus group.

Postoperative topography is probably one of the best ways to measure the results of the C-CAP method. In fact, more regular topography maps are obtained after our treatment, although improvement varies among patients (Figure 39-5). Another way to judge results is the subjective answer from patients to a questionnaire regarding symptoms. Again, all patients experienced improvement to different degrees in glare, halos, burst, and mul-