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

Table 39-4. Preoperative and postoperative BCVA in the keratoconus group after 31.6 months of follow-up, treated with the C-CAP method.

tiple visions. Improvement can be subtracted from these numbers: in the postsurgical group, with a follow-up of 36 months, no patient has returned to contact lens wear, 35% have returned to glasses, and 65% use no correction at all. In the ectasia group with the same 36 months of follow-up, 23% have returned satisfactorily to contact lens wear and another 45% wear glasses. The rest of the patients do not use any device for correction.

Safety

The C-CAP method has been demonstrated to be safe when lost and gained lines of BCVA are analyzed in both groups. In the postsurgical group, not one eye lost lines of BCVA. In fact, 42.5% gained one line, 18.5% gained two lines, and 7.4% gained more than two lines of BCVA.

In the ectasia group, only one eye lost one line of BCVA. This eye was treated with a BCVA of 20/400 and remained stable with a BCVA of 20/70 for 2 years. In the last 3 months, an activation of the ectatic problem was observed. This eye is ready for a corneal transplant. In terms of improvement, 52% gained one line of BCVA, 11.1% gained two lines, and 11.1% gained more than two lines.

Effectiveness

The C-CAP method is effective in the treatment of irregular astigmatism. In the postsurgical group, the mean preoperative UCVA was 20/80 while the mean postoperative UCVA was 20/40. Mean BCVA went from 20/40 preoperative to 20/25 postoperative (Figure 39-6).

In the ectasia group, mean preoperative UCVA was 20/300 and mean postoperative UCVA was 20/54. The BCVA went from 20/38 preoperatively to 20/29 postoperatively.

Stability was achieved earlier (1.2 months) in the postsurgical group than in the keratoconus group because 85% of the cases were treated with the LASIK CAP method. All patients reported subjective improvement in visual symptoms in different degrees. Satisfaction was generally rated high, although 25% of the patients reported “less improvement than expected.” No patient complained of worsening of symptoms, and all of them would repeat the surgery if needed.

Figure 39-5. Preoperative (left) and postoperative (right) elevation and axial curvature maps.

Figure 39-6. Preoperative (left) and postoperative (right) Orbscan maps using C-CAP to treat irregular astigmatism. The patient's BCVA went from 20/40 preoperatively to 20/20 postoperatively.

UNITED STATES CLINICAL EXPERIENCE

The recent approval by the FDA of the C-CAP method for the treatment of postexcimer surgery decentrations has given the US ophthalmologists this tool to correct those complicated cases. As a courtesy from Dr. Edward Manche from Stanford University School of Medicine, this is his experience with seven eyes treated with the C-CAP method and the Vision pro simulation software after a decentration from a previous LVC. The follow-up is only 3 months (see Table 39-5, a comparison of BCVA pre-LVC, preCAP treatment, and post-C-CAP ablation).

Not one eye lost any line of BCVA. Six out of seven eyes (86%) gained two or more lines of BCVA, and six eyes (86%) gained or maintained UCVA. Only one eye (14%) lost one line of UCVA. All patients felt a marked improvement of the debilitating symptoms from the decentration and the topography map showed the improvement in centration.

SUMMARY

The experience from the authors, of more than 5 years treating irregular astigmatism, has been reproduced by an independent ophthalmologist. This demonstrates that this method is safe and effective in the correction of these irregular corneas.

Table 39-5. US C-CAP method clinical experience. A comparison of pre LVC, pre C-CAP and post C-CAP treatment. No eye lost lines of BCVA. (Courtesy of Dr. Edward Manche.)

THE FUTURE

Improvement in the treatment of irregular astigmatism will derive from improvement in diagnostic tools such as wavefront technology and improvement in laser systems.

Wavefront technology, because it is not an analysis of shape but instead analysis of optical properties, is the best diagnostic tool to make the “link” with the excimer laser unit.13 Wavefront is able to make functional analysis transform the analysis into a treatment table, which when inserted into the laser dictates a change in shape to restore “normality” in optical functions. This is a real link and, of course, the future of any treatment of irregular astigmatism—provided there is enough tissue to remove from the cornea.

However, today wavefront cannot measure eyes with big irregularities and the development of a treatment table is difficult and still in its infancy. We need to learn much more about aberrations and tissue interactions before we are able to develop an algorithm for treatment with the laser. A lot of work is being carried out in this direction. VISX is working with a ShackHartmann device (WaveScan) (VISX, Santa Clara, Calif) and with a ray tracing device (Tracey Technologies, Houston, Tex). In mild cases of irregular astigmatism, treatment with waveprint has been done with successful and encouraging results (Figure 39-7). Although this use of new technology is still investigational, the future can be foreseen with the application of wavefront for mild cases of irregularities. In more severe irregularities, a topogra- phy-assisted treatment such as the CAP method will be used first to regulate the corneal contour. A second procedure with waveprint-guided ablation will refine the result.

The CAP method is still the first choice to correct the irregularities of a cornea with the help of topography.

Figure 39-7. A mild case of irregular astigmatism that is being treated with customized ablation using the WaveScan (ShackHartmann) wavefront sensor.

REFERENCES

1.Gibralter R, Trokel SL. Correction of irregular astigmatism with the excimer laser. Ophthalmology. 1994;101:1310-1315.

2.Snook R. Pachymetry and true topography using the Orbscan system. In: Gills J, Sanders D, Thornton S, eds. Corneal Topography: The State of the Art. Thorofare, NJ: SLACK Incorporated; 1995:89-103.

3.Tamayo G, Serrano M. Grid corneal marker. A new surgical approach for the treatment of irregular astigmatism. Poster presented at: the Panamerican Congress; May 1997; Cancun, Mexico .

4.Tamayo G, Serrano M. Early clinical experience using custom excimer laser ablations to treat irregular astigmatism. J Cataract Refract Surg. 2000;26:1442-1450.

5.Alio JL, Artola A, Claramonte PJ, et al. Complications of photorefractive keratectomy for myopia: two year follow up of 3000 cases.

J Cataract Refract Surg. 2000;24:619-626.

6.Buzard KA, Fundingsland BR. Treatment of irregular astigmatism with a broad beam excimer laser. J Cataract Refract Surg. 1997;13:624636.

7.Maeda N, Klyce SD, Tano Y. Detection and classification of mild irregular astigmatism in patients with good visual acuity. Surv Ophthalmol. 1998;43:53-58.

8.Friedlander MH, Granet MS. Non-placido disk corneal topography. In: Elander R, Rich L, Robin J, eds. Principles and Practice of Refractive Surgery. Philadelphia, Pa: WB Saunders; 1997.

9.Belin M, Litoff D, Stroods S, et al. The PAR technology corneal topography system. Refract Corneal Surg. 1992;8:88-96.

10.Sborgia C. Usefulness of the Orbscan machine in the management of photorefractive keratectomy. Paper presented at: ISRS/AAO Orbscan Clinical Topography Symposium; October 24-26, 1996; Chicago, Ill.

11.Andreassen T, Simonsen A, Oxlund H. Biomechanical properties of keratoconus and normal corneas. Exp Eye Res. 1980;31:435-444.

12.Bilgihan K, Ozdec SC, Konu KO, et al. Results of photorefractive keratectomy in keratoconus suspects at 4 years. J Refract Surg. 2000;16:438-443.

13.Bille J, Freischlad K, Jahn G, et al. Image restoration by adaptive optical phase conjugation. Paper presented at: the Proceedings of the 6th International Conference of Pattern Recognition; 1982; Munich, Germany.

Corneal surface irregularities, decentration of the treatment zones, or inadequate treatment zone diameters are possible complications of excimer laser refractive surgery (laser in-situ keratomileusis [LASIK] or photorefractive keratectomy [PRK]). Such inadequate ablations are a source of irregular astigmatism and degrade the optical performance of the cornea. Consequences are loss of best-corrected vision, halos, glare, monocular diplopia, or residual ametropia.1-9 In the future, new customized ablation technologies (topo-link- or wavefrontbased) will probably be available to correct those types of complications. However, such technologies are not likely to be widely available for several years. Patients who are experiencing significant vision difficulties after an initial excimer laser surgery are usually motivated by a short-term solution to their problem. With the current laser technology, it is possible to improve the quality of vision for several of these patients.

In this chapter, we describe techniques to retreat cases of initial corneal surface irregularities or inadequate ablations by using combinations of small diameter or decentered ablations. The first technique describes the use of decentered small diameter myopic ablations to decrease irregular astigmatism due to corneal irregularities. The second technique involves a combination of decentered myopic and hyperopic standard wide diameter ablations to correct cases of previous decentered treatment. The third technique, used to enlarge previous small diameter ablation zones, also involves a combination of myopic and hyperopic ablations but without decentration. A summary of these three retreatment techniques is presented in Table 40-1.

The retreatment parameters, dioptric value, diameter, axis, and distance of decentration were determined using measurements from topographical maps taken after the initial surgery. Defining optimal retreatment parameters is difficult due to the multiple variables that must be considered: initial and residual ametropia, diameter, elevation of the irregular zone, distance and axis of decentration, etc. Therefore, each case has to be analyzed individually. Some cases need more than one retreatment session to achieve significant visual improvement. In these more complex cases, an initial conservative retreatment with limited parameters is probably more appropriate than an aggressive retreatment with results that are difficult to predict, although this course of action implies several retreatment sessions. This multi-step approach is easier with LASIK cases because of the short healing time and the little pain associated with this technique. For this reason, LASIK was preferred for the retreatment

of decentration and for the enlargement of an initial small ablation treatment zone, even if PRK was used for the initial surgery. However, cases of irregular astigmatism due to localized small irregular zones were retreated with the same technique that was used in the initial surgery. It seemed logical to correct these irregularities where they are located, either on the stromal surface or under the flap.

The laser used for all these retreatments was the Technolas 217Z (Bausch & Lomb, Rochester, NY), which is a scanning laser using a 2.00 millimeter (mm) flying spot and an active tracking system. This instrument is equipped with two diode lasers: one is the fixation light, while the other coincides with the center of the 2.00 mm spot of the excimer beam. After the tracker has been activated and locked on the eye’s visual axis, it is possible to decenter the treatment beam from the fixation light by using the compatible laser program. The swivel scale integrated into the ocular of the laser microscope was used to align the treatment at the planned axis and distance from the eye’s optical center.

Editor’s note:

This chapter reviews the well-planned efforts of correcting surgically-induced aberrations without a wavefront or topographic uplink. First, irregular steep areas are corrected with well-placed, decentered, small zone myopic ablations. Second, previous decentrations are corrected with a combination of decentered or inversely decentered hyperopic and myopic ablations. Finally, small zone ablations are corrected (enlarged) by a combination of myopic and hyperopic ablations without decentration.

R. Krueger, MD, MSE

RETREATMENT OF CORNEAL SURFACE IRREGULARITIES

Irregular corneal surface, after excimer laser refractive surgery, can be the consequence of nonhomogeneous laser ablation; of a flap complication in LASIK surgery; or of a postoperative complication with resulting corneal tissue loss, such as infection, diffuse lamellar keratitis (DLK) or epithelial ingrowth.10-14

A small diameter myopic ablation can be used to flatten irregular steep areas to reduce the irregular astigmatism and aberrations that result as a complication after an initial excimer laser procedure. This technique is similar to a technique we have used to correct central islands.15 However, for the correction of irreg-

ular astigmatism, this previous technique was modified by decentering the retreatment from the visual axis. It was decentered to correspond to the location of the irregularity to be corrected. In our experience, this retreatment technique achieved a better corneal surface regularity and better vision in most of the retreated cases, although the irregular astigmatism correction was only partial in several cases. In some cases, a standard myopic or hyperopic large diameter ablation was added to the small diameter ablation in order to correct some residual ametropia. The following examples describe the retreatment parameters used and illustrate the results obtained.

A small diameter myopic ablation can be used to flatten irregular steep areas to reduce the irregular astigmatism and aberrations that result as a complication after an initial excimer laser procedure.

The following five examples (see p. 342) describe cases of irregular astigmatism due to a localized steeper or flatter corneal area, in relation to the surrounding ablated cornea. These areas were decentered in relation to the central papillary axis. Except for the last example, the retreatments consisted of a small diameter myopic ablation delivered to the localized steep cornea area. Cases initially treated with PRK were retreated with PRK, and LASIK cases were retreated with LASIK. For PRK cases, the epithelium was removed with the laser. This transepithelial approach with the laser has the advantage of using the epithelium as a masking agent, which helps to decrease the underlying stromal surface irregularities. For LASIK cases, the surgical approach was always to lift the previous flap. The diameter, curvature, distance, and axis of decentration of the steep corneal zone to be treated were evaluated on topographical maps taken prior to the retreatment. Immediately before the laser ablation, an ink mark was placed under slit lamp visualization at 6:00 and 12:00 to rule out a possible cyclotorsion during the laser treat-

ment when the patient is lying down.

In our experience of 10 initially myopic eyes retreated with this technique, best-corrected visual acuity (BCVA) lines were recovered in nine cases and significant subjective improvement was also described by the patients in these nine cases. These results showed that this technique is effective in reducing the irregular astigmatism present as a complication of a previous excimer laser myopic treatment. Also, this technique was found to be safe: no complications were encountered, no eyes lost lines of vision, and no patients experienced any subjective worsening of vision. The main difficulty with this technique was aligning the retreatment exactly over the area to be retreated; there is risk that a misalignment could worsen the corneal surface regularity instead of improving it. The same technique was also used in four cases of irregular ablation present after initial excimer treatment to correct hyperopia. A good result was obtained in only two cases (Example 4). In the other two cases, no significant improvement was noted; however, no deterioration was induced.

Defining optimal parameters is difficult, as our experience is limited to several cases and there are multiple variables to consider. These variables are the initial and residual ametropia, distance from the optical corneal center, and the axis of the steep zone to be retreated (ie, its curvature, its diameter, and its regular or irregular contour). Therefore, each case is different from the other and has to be analyzed individually. In general, wider ablation diameters and higher dioptric values were used for the retreatment of wider and steeper corneal irregularities. The retreatment diameters used for small localized irregularities, as in Examples 1 and 2, were between 2.30 and 3 mm. For a wider irregular zone, as in Example 3, a 3.50 mm zone was preferred. Retreatment dioptric value was usually between 2 and 3 D. Total ablation thicknesses were between 9 and 14 microns (µm). One point to emphasize is that the laser used for these retreatments, the Technolas 217Z laser, is a scanning laser that automatically creates a transition zone. In contrast to the same treatment diameter without such transition zone, the ablation thickness is

greater if a transition zone is added. Consequently, if this small diameter ablation technique is performed with a broadbeam laser without transition zones, slightly larger diameter or higher dioptric values should be used to respect this range of 9 to 14 µm of localized tissue removal. We have no experience with an initially more aggressive retreatment. Considering our limited experience and the inherent difficulties of this type of retreatment, we believe that in more complicated cases it is probably safer to use limited retreatment parameters. Although doing so may imply more than one retreatment session, this conservative approach decreases the risk of inducing undesirable effects resulting from excessive tissue removal.

One point to emphasize is that the laser used for these retreatments, the Technolas 217Z laser, is a scanning laser that automatically creates a transition zone. In contrast to the same treatment diameter without such transition zone, the ablation thickness is greater if a transition zone is added.

We found that these very small diameter myopic ablations induced some myopic correction, but this correction is significantly less than the retreatment dioptric value (ie, less than the same myopic ablation value delivered on a standard wide diameter). In our experience, the average refractive correction obtained per diopter of retreatment, delivered at a mean 3 mm diameter zone, was -0.27 D. For example, a -3 D ablation delivered on this 3 mm diameter zone should induce a final refractive change of -0.81 D. Therefore, when a minimal residual myopia is present prior to the retreatment of a localized irregular steeper area, this retreatment by itself should correct this myopia (Examples 3 and 4). However, when some hyperopia is present, the retreatment should increase this hyperopia. To correct for the residual ametropia one might expect after a small diameter myopic treatment, a standard large diameter ablation (myopic or hyperopic) can be added in the retreatment process (Examples 1 and 2).

We found that these very small diameter myopic ablations induced some myopic correction, but this correction is significantly less than the retreatment dioptric value. In our experience, the average refractive correction obtained per diopter of retreatment, delivered at a mean 3.00 mm diameter zone, was -0.27 D.

Another particular point to emphasize is the correction of residual or induced astigmatism. An irregular corneal surface resulting from the initial treatment is often the source of some astigmatism. The process of flattening the irregular steep area with the small diameter spherical myopic ablation should by itself decrease the astigmatism. Therefore, when some induced astigmatism is present after the initial surgery, the full correction

of this astigmatism with a standard wide diameter treatment zone should not be added to the small diameter myopic ablation. In Example 2, a partial correction of the induced astigmatism delivered at a standard wide diameter was included in the retreatment process. The residual astigmatism, with a 90 degree axis shift, shows that this astigmatism correction—although par- tial—was slightly excessive.

The process of flattening the irregular steep area with the small diameter spherical myopic ablation should by itself decrease the astigmatism. Therefore, when some induced astigmatism is present after the initial surgery, the full correction of this astigmatism with a standard wide diameter treatment zone should not be added to the small diameter myopic ablation.

This retreatment technique can improve some cases of irregular astigmatism that developed as a complication from a previous excimer laser procedure. The retreatment uses very small diameter regular myopic ablations. However, because corneal irregularities are obviously irregular, the retreatment cannot correspond exactly to the contour of the steep zone to be treated and can only decrease its overall elevation. Also, it is difficult to achieve a precise retreatment alignment on the area to be retreated. Therefore, this technique does have limitations and does not completely correct the irregular astigmatism. Although the correction can be only partial, it may significantly improve the visual symptoms of some patients.

Example 5 is completely different from the others. In this case, the irregular astigmatism is not due to a localized steep area, but is rather due to a localized flat area, being the consequence of a corneal melt after severe DLK. In association with the irregular astigmatism, a marked overcorrection was present. In this case, the retreatment was done with the use of a decentered hyperopic ablation. The treatment zone diameter was a standard wide diameter. We believe that there is probably a greater limitation in decentering a hyperopic ablation than a myopic ablation, and that hyperopic ablation should be used only with a standard wide diameter zone. The main reason for these concerns is the ablation profile in the hyperopic transition zone, which is the inverse of the central zone. The central treatment zone induces a steepening of the treated corneal area, whereas the transition zone induces a flattening of the cornea. The overlapping of the transition zone in the visual axis could induce an unpredictable result with an increase of the irregular astigmatism. Our experience with decentered hyperopic ablation for the retreatment of such cases of irregular astigmatism is limited to two eyes. In both cases, marked improvement was achieved. However, as described in the following pages, we have used more extensively decentered hyperopic ablations for the correction of previous treatment decentration.

EXAMPLE 1

A 37-year-old female had an initial myopic LASIK treatment of -3.25 + 1.00 x 170. This treatment induced an irregular corneal surface (map A) with a resulting three-line BCVA loss. Ablation over a wet corneal bed after the LASIK cut was believed by the referring surgeon to be the reason for this result. Residual ametropia was +0.50 + 0.25 x 65 for 20/40 vision. The initial LASIK flap was lifted and a -3.00 myopic retreatment was delivered within a 2.50 mm zone (transition zone extending to

5.50 mm) with a 1 mm decentration at 315 degrees. This resulted in a marked improvement of the surface regularity (map B), with two lines of vision recovery. However, this small diameter myopic ablation increased the hyperopia induced by the initial treatment, and the resulting refraction was +1.25. A second retreatment session was needed to correct this hyperopia. A standard wide diameter hyperopic ablation of +1.25 on a 5.80 mm zone with a transition zone extending to 9.30 mm was added 2 months after the first retreatment. Final refractive outcome was plano, resulting in 20/20 vision.

EXAMPLE 2

This 48-year-old male had LASIK for -6.00 D of myopia. Three months after this initial surgery, because of a minor undercorrection, the surgeon performed a touch-up. Instead of lifting the initial flap, a new cut was done with the microkeratome. The result was a corneal surface irregularity (map A), probably the consequence of the intersection of the two cuts with the resulting displacement or loss of microlayers of corneal tissue. In addition to a three-line BCVA loss, the patient was complaining of very disturbing monocular diplopia. Refractive result was +0.50 + 1.00 x 0.5 for 20/40 vision. When the initial LASIK flap was lifted for the retreatment, the two different cut layers were visible. The retreatment profile, used to flatten the steep irregular area, was -

2.00 D within a 3 mm diameter zone (tapering zone extending to 6 mm) with a 2 mm decentration at 35 degrees. Because this patient was already overcorrected after the initial treatment, it was expected that such further myopic ablation would increase this hyperopia. Therefore, during the same retreatment session, a standard wide diameter hyperopic ablation was also included. The value of this ablation was +1.00 + 0.75 x 0.5 delivered centrally within a 5.80 mm zone with a transition zone extending to 9.30 mm. Final refractive outcome was plano + 0.50 x 97 for a 20/25 vision. Although some surface irregularities are still present after retreatment, a significant improvement is visible (map B). Two BCVA lines were recovered, and the patient’s subjective symptoms were markedly reduced.

EXAMPLE 3

This 38-year-old female presented an incomplete ablation in the superior part of the treatment zone (map A) after an initial LASIK for -4.75 D of myopia. It was the source of irregular astigmatism, ghost images, and a one-line loss of BCVA. Initial myopia was -4.75 for 20/20 vision. Refraction after this initial LASIK was -1.00 + 0.25 x 105, resulting in 20/25 vision. Retreatment consisted of a -2.00 D myopic ablation delivered within a 3.50 mm zone (transition zone extending to 6.30 mm) with a superior 2 mm decentration. Topographical map taken after

retreatment (map B) shows a regular ablation zone. Final refractive outcome was plano + 0.25 x 90 resulting in 20/20 vision, and there was a complete regression of the patient’s subjective symptoms. Compared to the two previous examples, the irregular steeper area to be flattened was wider; consequently, a wider retreatment diameter was used. The retreatment diopter value exceeded the residual myopia measured after the initial treatment. However, because these retreatments were decentered and within a small diameter, their refractive effect was much less than their standard zone dioptric value.

EXAMPLE 4

A 49-year-old male had initial hyperopic PRK treatment of +3.25 + 1.00 x 170 D, which induced an inferior steepening (map A). Initial vision was 20/20. The refractive outcome was -1.25, resulting in 20/50 vision. At first, a -2.50 D retreatment was performed within a small 2.60 mm ablation zone (transition zone of

5.60 mm), with a 1.50 mm inferior decentration at 270 degrees. This was used to flatten the steep zone and resulted in an initially good result (map B). However, some of this irregularity recurred in the following months (map C). A second treatment, using identical ablation parameters, succeeded in producing a stable result with a regular corneal surface (map D). Final result was -1.25 + 1.00 x 140 for 20/25 vision.

EXAMPLE 5

Following LASIK for -4.50 D of myopia, this 47-year-old male developed severe diffuse lamellar keratitis (DLK) with corneal melting. Initial vision was 20/50-. The result 10 months postoperatively was approximately +2.50 D resulting in 20/50 vision. Marked irregular astigmatism was present. The topographical map (first map) showed an area of marked flattening, which corresponded to the zone of corneal melting. An initial retreatment was performed 10 months after the initial surgery. The surgical approach was to lift the initial flap followed by a retreatment of +1.75 D hyperopia within a 5.80 mm diameter with a transition

RETREATMENT OF PREVIOUS DECENTERED ABLATION

Decentration can be a complication of excimer laser refractive surgery.16-18 Causes for this complication are varied: poor patient fixation, inadequate laser beam centration on the cornea, poor performance of an eye tracker system, etc. Minor decentration can induce few symptoms, mostly halos in low light conditions. However, significant decentration can degrade the cornea’s optical performance and result in decreased vision, monocular diplopia, constant halos, or induced astigmatism.19-21 Several retreatment techniques have been proposed for decentration, including selective blocking of part of the retreatment to be delivered and decentration of a myopic ablation in the opposite direction from the first ablation.21-28 However, these techniques, by increasing the total myopic ablation, are only indicated for decentration associated with significant undercorrection. In cases of decentration associated with a near plano refractive result, additional myopic treatment will result in overcorrection. In our experience, we have retreated cases with significant undercor-

zone extending to 9.30 mm. This ablation was decentered 1.50 mm at 180 degrees. Topography taken after the retreatment (second map) showed a significant improvement, although some surface irregularities persisted. The refractive result was +1.00 with a marked decrease of the irregular astigmatism. Four months later, the LASIK flap was lifted again for a second retreatment. A +1 hyperopic ablation within a 5.80 mm diameter (transition zone extending to 9.30 mm) was added with a 1.00 mm decentration at 130 degrees. The final result was -0.75 + 0.50 x 170 D resulting in 20/25vision. Topographical map taken after this second retreatment (third map) showed a marked improvement of corneal surface.

rection by retreating with a decentration in the opposite direction of the initial decentered treatment (Example 6 [see p. 346]). However, the approach was different when the initial decentration is associated with a near plano or minimal residual ametropia (Examples 7 to 11 [see pp. 347 to 350]). Such cases are more complex, and the objective of a retreatment is to recenter the ablation zone without altering the refractive result obtained by the first surgery. This can be achieved with a combination of a decentered myopic and a decentered hyperopic ablation. In the same surgical session, the hyperopic ablation is first decentered in the same direction as the original off-center myopic treatment. A myopic ablation of a near equivalent dioptric power is then added, but in the opposite direction (ie, 180 degrees apart). These hyperopic and myopic ablations are of near equivalent dioptric value so that their respective refractive effects are neutralized, and the eye’s refractive status should not be altered significantly.

The initial decentration can be classified as mild, moderate, or marked. When, as measured on topographical maps, the distance between the center of the ablation and the optical center of the

When the initial decentration is associated with a near plano or minimal residual ametropia, the hyperopic ablation is first decentered in the same direction as the original off-center myopic treatment. A myopic ablation of a near equivalent dioptric power is then added, but in the opposite direction (ie, 180 degrees apart).

eye was less than 1 mm, it was classified as mild. It was classified as moderate when this distance was between 1 and 2 mm and as marked for distance greater than 2 mm. Example 6 illustrates the retreatment of a marked decentration case with marked undercorrection by using a decentered myopic ablation. Although our experience is limited to few such cases, this retreatment technique was simple and effective. The other examples illustrate moderate and marked decentrations, but without such undercorrection. The technique, using a combination of myopic and hyperopic ablations, was used in the retreatment of 16 such cases. Prior to retreatment, all of these patients complained of various symptoms such as halos, glare, or ghost images, and BCVA loss was present in five cases. This technique was very effective in recentering the ablation zone and has a predictable refractive outcome. The efficacy of this technique is readily visible after comparing topographical maps before and after retreatment in the following examples. After the retreatment, all but one patient acknowledged a reduction of the presenting symptoms. Five eyes with BCVA loss after the initial surgery recovered lines of vision. Results can be improved even further with additional retreatment as seen in Examples 10 and 11. In our experience, ablations used for the retreatment of these cases were delivered with decentration distances ranging from 1 to 2 mm from the visual axis, 2 mm being the maximum distance used. This 2 mm decentration was used for the retreatment of marked decentration. For cases of initial moderate decentration, retreatment decentration was between 1 and 1.50 mm. The hyperopic and myopic ablation values were usually between 1 and 2 D. However, in cases in which some residual ametropia was present, these values can be set smaller or higher to correct this ametropia. These ablations were performed within a large diameter. The hyperopic ablation diameter used for retreatment is usually 6 mm with a transition zone extending to 9.60 mm. The myopic ablation was usually 6 mm with transition zone to 9 mm. However, smaller diameters were used in cases of limited residual corneal thickness, or small LASIK flap diameters.

Our analysis of the results showed that final refractive outcomes were predictable. The final refractive results were within ±0.50 D (spherical equivalent) in 11 of these 16 eyes. No significant astigmatism was induced by the retreatment. A second retreatment session was needed to correct some residual ametropia in only three cases. Concerning the safety of this technique, the only complication encountered is illustrated in Example 6, where the retreatment induced an inadvertent new decentration in the opposite direction of the initial treated one, resulting in a one-line BCVA loss. However, a second retreatment session improved the centration and recovered the visual loss.

In most cases of decentration, some induced or residual astigmatism is present. Usually moderate decentrations induce minor astigmatism, but marked decentration can induced a significant amount of astigmatism, as observed in Examples 6, 9, 10, and 11. Recentering the ablation zone, with spherical ablation, by itself should correct some of the induced astigmatism. In Example 6, which illustrates a decentration with marked undercorrection

and induced astigmatism, less than half of the induced astigmatism value was included in the myopic ablation retreatment. The result was its complete correction. In Examples 7 and 9, no toric ablation at all was included in the retreatment protocol. Results show that the induced astigmatism was also corrected by the decentration of this combination of myopic and hyperopic ablations. In one of our first cases retreated with the technique of combined decentered ablations, the full value of the residual astigmatism was included in the retreatment parameters, and the final result was an unexpected residual astigmatism that shifted 60 degrees from its previous axis. Therefore these results suggest that, in the retreatment of decentrations, the retreatment protocol should not include the total amount of astigmatism induced by the initial decentered treatment.

In a decentration with marked undercorrection and induced astigmatism, less than half of the induced astigmatism value was included in the myopic ablation retreatment. The result was its complete correction.

What should be the optimal retreatment parameters? Each case of decentration differs from the others in the following ways: the initial correction delivered, the amplitude and axis of decentration, the regularity or irregularity of the initial ablation, and the presence or absence of a residual ametropia or an induced astigmatism. Therefore, ideal standard retreatment nomograms would be difficult to define precisely. In our experience of moderate to marked decentration retreatment, retreatment decentration ranged from 1 to 2 mm and retreatment dioptric values were relatively small, usually between 1 and 2 D. Those parameters proved adequate for most cases. However, for some severe cases, such as Example 10, these retreatment parameters were insufficient, and several retreatment sessions were needed. For such extreme cases, retreatment parameters are very difficult to estimate. Aggressive retreatment could induce unpredictable results. A conservative approach with limited parameters was preferred, knowing that probably more than one retreatment session would be needed to restore good vision quality.

There is probably a greater limitation in the use of a decentered hyperopic ablation than the equivalent decentered myopic ablation. The main reason for this concern is the ablation profile in the hyperopic transition zone. For example, with the laser used for these retreatments, the Technolas 217Z, a 6 mm myopic treatment has a transition zone extending to 9 mm. The myopic treatment profile is a convex ablation with a regular curve in the central 6 mm zone and then flattens progressively in the transition zone. A 6 mm hyperopic treatment has a transition zone extending to 9.60 mm. The ablation has a concave profile in the 6 mm central zone; however, in the transition zone, the ablation changes for a convex profile. A myopic ablation flattens the corneal curvature in the central zone and also in the transition zone, but to a lesser extent. A hyperopic ablation steepens the corneal curvature in the central zone, but flattens the corneal curvature in the transition zone. For example, using 6 mm myopic and hyperopic ablations, with each one decentered 1.50 mm from the eye’s optical center, but in opposite directions, the total distance between the centers of each of these ablations is 3 mm. Therefore, these two 6 mm ablations overlap in a central 3 mm zone and their respective refractive effects are neutralized. Outside this central 3 mm zone, on the side of the hyperopic ablation, 3 mm of the hyperopic central ablation zone juxtapose