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Simultaneous laser in situ keratomileusis on the stromal bed and undersurface of the flap in eyes with high myopia and thin corneas

Joo, Myung-Jin MD; Kim, Ye-Ni MD; Hong, Hyo-Chang MD; Ryu, Dong-Kyu OD; Kim, Jae-Ho MD

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Journal of Cataract & Refractive Surgery: October 2005 - Volume 31 - Issue 10 - p 1921-1927
doi: 10.1016/j.jcrs.2004.09.055
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Abstract

Laser in situ keratomileusis (LASIK), combined lamellar corneal surgery with excimer laser photoablation, has become the procedure of choice for correcting refractive errors. In the LASIK era of refractive surgery, surgeons as well as patients are faced with certain complications. There have been reports of keratectasia after LASIK for high myopia1 (≥8.0 diopters [D]) and low myopia2 appearing as progressive central or inferior corneal steepening.

Laser in situ keratomileusis to correct myopia is performed by partially resecting a prescribed thickness of stroma and then removing corneal tissue from the exposed stromal bed using an excimer laser. This results in a substantial reduction of the biomechanically effective stress-bearing thickness of the cornea provided by the residual stromal bed. There is concern that at some point, the tensile strength of the cornea may be reduced to the degree that progressive ectasia occurs, resulting in steepening of the cornea, irregular astigmatism, and progressive myopia. Management of keratectasia after lamellar refractive surgery is difficult. The method of choice is a rigid gas-permeable contact lens; in patients who cannot tolerate contact lens, penetrating keratoplasty and Intacs (Addition Technology, Inc.) implantation are alternatives.

Because the stromal laser keratectomy depth is linearly related to the correction attempted,3 the biomechanical stability of the residual stromal lamella is of concern. No generally accepted limits for the minimum thickness of the residual stroma are found in the literature. Late-onset keratectasia after LASIK, considered a postoperative complication, has limited the range of myopia correction in conventional LASIK surgery.

Based on biomechanical consideration, Seiler and coauthors1 recommend a residual corneal thickness greater than 250 mm after LASIK to prevent corneal ectasia in a normal cornea and that at least 50% of the corneal thickness should be left untouched for biochemical reasons. Barraquer4 suggests a 300 μm thickness of stress-bearing cornea. By comparing the biomechanical properties of keratoconic corneas with normal corneas, Andreassen and coauthors5 estimate that for the normal cornea, a residual stromal bed thickness of less than 250 μm might produce a cornea with a tangential elastic modulus comparable to that of a keratoconic cornea. These recommendations have consequences for the upper limit of myopic corrections that can be safely achieved with LASIK.

Although LASIK is an effective and safe procedure, in some circumstances the stromal bed is not thick enough to qualify for ablation of high myopia. An increasing number of reports describe keratectasia after conventional LASIK surgery.1,2,6–13

The purpose of this study was to develop a novel technique for LASIK in eyes with high myopia with insufficient corneal thickness for conventional LASIK ablation. With the aid of intraoperative pachymetry, the actual stromal bed thickness was evaluated, and in eyes deemed suitable, simultaneous LASIK on the stromal bed and the undersurface of the flap was performed to extend the range of myopia correction by LASIK. We hypothesized that LASIK in eyes with high myopia and thin corneas should be considered only when a minimum thickness of at least 300 μm remains in the corneal stromal bed after stromal laser ablation.

PATIENTS AND METHODS

Patient Selection and Characteristics

Patients included in the study had high myopia with insufficient corneal thickness for conventional LASIK ablation (ie, the remaining stromal bed thickness would be less than 300 μm after conventional LASIK). Mean patient age was 29.6 years ± 7.9 SD (range 20 to 51 years). All surgeries (43 eyes of 27 patients) were performed by 1 surgeon (J.H.K.) from August 2002 through April 2003. The patients had a preoperative spherical equivalent (SE) refraction ranging from −12.5 D to −3.75 D, stable refraction for at least 12 months (defined as change in SE manifest refraction of 0.5 D or less), and a corneal thickness of at least 480 μm. All surgical candidates were fully informed of the potential risks and benefits of the procedure, and all provided informed consent. Patients were excluded if they had active ocular disease; previous intraocular or corneal surgery in the eye having surgery; history of herpes keratitis; or a diagnosed autoimmune disease, systemic connective tissue disease, or atopic syndrome.

Preoperative Evaluation

Preoperative examinations were performed by the operating surgeon with assistance from trained technical staff. Evaluation included slitlamp biomicroscopy, uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BSCVA, Snellen), manifest and cycloplegic refractions, intraocular pressure (IOP), keratometry, photokeratoscopy, corneal topography (Oculus), contrast sensitivity, optical and ultrasound pachymetry of the central cornea, specular photography of the central endothelium, fundus examination, detailed medical and ophthalmic history, and discussion of postsurgical occupational and recreational visual requirements. Contact lens wearers were required to discontinue hard lens use for 2 weeks and soft lens use for 3 days before the preoperative evaluation.

Surgical Technique for Simultaneous Bed and Flap LASIK

Laser in situ keratomileusis procedures were performed using topical proparacaine hydrochloride 0.5% (Alcaine). The eye to be treated was draped in standard sterile fashion, with care taken to keep the lashes out of the surgical field. The corneal center of the entrance pupil was carefully marked with a Gulani LASIK marker (model ET9179, Bausch & Lomb Surgical) and a Slade LASIK spatula (Stephens Instruments), stained with a gentian violet solution (Figure 1, A and B). The peripheral cornea was also stained with a Gulani LASIK marker. The suction ring was centered, and suction was applied. In this series, a Moria M2 automated microkeratome was used to create a superiorly hinged corneal flap 8.5 μm in diameter. Selected plate thickness was 160 μm or less.

Figure 1.
Figure 1.:
Demonstration of the steps in simultaneous LASIK on stromal bed and undersurface of the flap. A: Using Gulani LASIK marker, a circle is created around the entrance pupil. B: The optical axis is marked with a Slade LASIK spatula inked with gentian violet solution. C: A special light blue rigid contact lens (diameter, 8.8 mm; front base curve, 6.5 mm; hind base curve, 9.00 mm) supports the flap. It has a 5.0 mm cord that is in contact with corneal hinge under the flap with a ragged edge to prevent slippage during ablation of the undersurface of the flap. D: The patient is instructed to look down, and the flap is reflected onto the rigid contact lens. The excimer laser is focused and centered on the marked optical axis, while a wet Merocel spear aids in controlling the position and smoothness of the reflected flap.

The hinged anterior corneal flap was temporarily elevated, and the thickness of the actual residual stromal bed was measured using a pachymeter. Using the approximate expected laser ablation depth provided by the manufacturer, the amount of laser ablation on the stromal bed and undersurface of the flap from the total attempted amount was calculated. Spherical and toric correction was carried out on the stromal bed; only spherical correction was done on the undersurface of the corneal flap, taking care to avoid possible dehydration of the flap because this could induce overablation and result in hyperopic shift. Correction of astigmatism was not attempted on the flap. All eyes received laser ablation without compromising the posterior 300 μm of the residual stromal bed after the stromal portion of the laser ablation.

All procedures were performed using a Visx Star S4 laser with active track, calibrated with a fluence of 160 mJ/cm2, a frequency of 10 Hz, a 6.0 mm ablation optical zone, and an 8.0 mm blend zone. After the stromal bed was ablated, the flap was reflected on a specially designed rigid contact lens and extended with a wet Merocel spear (Xomed). A special light blue rigid contact lens (diameter, 8.8 mm; front base curve, 6.5 mm; hind base curve, 9.0 mm) to support the flap (Figure 1, C). This allowed the surgeon better control over centration of the ablation of the undersurface of the flap and ultimately maximized visual improvement. During the ablative procedure, surgical spears were used to spread and position the reflected flap (Figure 1, D). Adequate exposure of the flap stroma was achieved by requesting the patient to look down. After accurate centration of the helium-neon laser beam on the marked visual axis, the laser program was executed and the photoablation was performed with microscope surveillance.

The laser-treated corneal flap was then folded back onto the cornea with a cannula. The interface was irrigated with balanced saline solution. The flap was applied with a Johnston LASIK flap applanator (Rhein Medical) to remove fluid from the interface and enhance seating of the flap on the stromal bed; it was then painted into position with a soft, wet Merocel sponge. The flap was allowed to settle for at least 3 minutes to ensure good adhesion between it and the stromal bed.

Postoperative Management

After LASIK, a protective clear shield was placed on the operative eye. The shield was removed on the first postoperative day. Levofloxacin 0.5% (Cravit) and fluorometholone 0.1% (Fluorometholon) were prescribed 4 times a day for 7 days, and nonpreserved artificial tears were prescribed for 6 months.

Simultaneous Bed and Flap LASIK Ablation Parameters

The following formula was used:

where F = full correction amount, S(S+T) = stromal portion (spherical correction + toric correction), and F(S) = flap portion (spherical correction). Toric ablation was performed only on the stromal bed, and the ablation on the undersurface of the flap was only for spherical ablation. The application of this formula did not lead to a change in toric ablations axis on the stromal portion. The stromal ablation pattern (spherical with or without toric ablation) and the undersurface of the flap ablation pattern (spherical ablation) were identical to those in conventional LASIK with the Visx laser.

Main Outcome Measure

Complications were defined as loss of BSCVA, flap complications (striae, wrinkling, epithelial ingrowth), infection, or other events requiring medical or surgical intervention.

Refractive predictability was defined as the difference between the attempted and achieved refractive change. Refractive predictability was the evaluated difference between the mean postoperative SE and the targeted SE using the manifest refraction.

Stability of the manifest refraction was measured using the difference in SE measured at each visit. Refractive stability was defined as a change in the manifest SE of 0.5 D or less on 2 observations at least 1 month apart.14

Data Analysis

Results are presented as mean ± standard deviation. Mean values were compared using the paired t test or repeated-measures analysis of the variance. Analysis was performed using SPSS, version 10.0. A P value less than 0.05 was considered statistically significant.

RESULTS

Spherical Equivalent Refraction

Mean SE refraction before LASIK was −8.74 ± 2.32 D (range −12.5 to −3.75 D). After LASIK treatment, mean SE refraction was −0.13 ± 0.66 D (range −1.75 to +1.25 D) at 1 month, −0.18 ± 0.63 D (range −1.50 to +1.25 D) at 3 months, and −0.13 ± 0.58 D (range −1.50 to +1.25 D) at 6 months (Figure 2).

Figure 2.
Figure 2.:
The SE refraction outcome over time after simultaneous LASIK on the stromal bed and undersurface of the flap. Each time point shows the mean SE refraction; the error bars indicate 1 SD at various intervals.

Visual Acuity

Before treatment, the BSCVA was 20/20 or better in 26 eyes (60.5%) and 20/30 or better in 42 eyes (97.7%). At 1 month, the UCVA was 20/20 in 15 of 43 eyes (34.9%) and 20/30 or better in 35 of 43 eyes (81.4%) at 1 month. At 3 months, it was 20/20 in 16 eyes (37.2%) and 20/30 or better in 36 eyes (83.7%).

At 6 months, the UCVA was 20/15 or better in 5 eyes (11.6%), 20/20 or better in 16 eyes (37.2%), 20/25 or better in 30 eyes (69.8%), 20/30 or better in 34 eyes (79.1%), and 20/40 or better in 42 eyes (97.7%) (Figures 3 and 4).

Figure 3.
Figure 3.:
Cumulative uncorrected Snellen visual acuity 6 months after simultaneous LASIK on the stromal bed and undersurface of the flap.
Figure 4.
Figure 4.:
Scattergram of attempted versus achieved SE refraction in simultaneous LASIK on the stromal bed and the undersurface of the flap at 6 months.

Astigmatism

The mean pre-LASIK refractive astigmatism was −1.19 ± 0.81 D (range −4.00 D to 0 D). The mean astigmatism was −0.19 ± 0.40 D (range −1.50 to 0 D) at 1 month, −0.18 ± 0.35 D (range −1.00 to +0.25 D) at 3 months, and −0.13 ± 0.28 D (range −1.00 to +0.25 D) at 6 months. With respect to the pre-LASIK cylinder, the postoperative examination showed a statistically significant reduction in the 1-, 3-, and 6-month follow-up visits (P<.0001). Twenty-eight eyes (65.1%) had astigmatism of at least 1.00 D before treatment; 2 eyes (4.7%) had an astigmatism equal to 1.00 D at the 6-month follow-up visit.

Refractive Predictability and Stability

The achieved SE refraction was within ±0.50 D of the targeted SE refraction; in 25 eyes (58.1%) and within ±1.00 D in 39 eyes (90.7%) at 1 month (Figure 5). Two eyes (4.7%) changed more than 1.0 D during the postoperative period, and 4 eyes (9.3%) between 0.5 D and 1.0 D (Figure 6).

Figure 5.
Figure 5.:
Refractive predictability of the refraction after simultaneous LASIK on the stromal bed and the undersurface of the flap.
Figure 6.
Figure 6.:
Refractive stability of the refraction after simultaneous LASIK on the stromal bed and the undersurface of the flap.

Two eyes (4.7%) were overcorrected between +1.00 D and +2.00 D and 2 eyes (4.7%) were undercorrected between −1.00 D and −2.00 D at 1 month.

Two eyes (4.7%) were overcorrected between +1.00 D and +2.00 D and 4 eyes (9.3%) were undercorrected between −1.00 D and −2.00 D at 3 months. Two eyes (4.7%) were overcorrected between +1.00 D and +2.00 D and 2 eyes (4.7%) were undercorrected between −1.00 D and −2.00 D at 6 months.

Pachymetry

Mean central corneal thickness decreased from 523 ± 23 μm (range 475 to 563 μm) before treatment to 439 ± 17 μm (range 398 to 470 μm) at 3 months. Intraoperative mean central corneal stromal thickness after reflecting the flap was 367 ± 24 μm (range 312 to 413 μm).

Mean flap thickness was 156 ± 4 μm (range 150 to 165 μm). Attempted myopic correction on the undersurface of the flap was −3.13 ± 1.38 D (range −0.75 to −6.00 D), and the amount of flap ablation was 41 ± 18 μm (range 10 to 77 μm). Estimated final flap thickness after treatment was 116 ± 18 μm (range 81 to 148 μm).

Complications

During surgery and the follow-up period, no vision-threatening complications occurred. Fine microstriae was present in 1 eye (2.3%). Interface debris was present in 1 eye (2.3%) after LASIK. During the 6-month follow-up period, serial topographic examinations did not detect keratectasia.

DISCUSSION

In recent years, LASIK has become the preferred choice for vision correction because of reduced postoperative discomfort and improved immediate acuity. Unfortunately, there have been increasing reports of corneal keratectasia as a complication of LASIK,1,2,6–13,15 especially in patients with thin corneas and high corrections.7,10,11,12 The risk for creating keratectasia by removing excessive stromal tissue during LASIK has recently been documented.1,6,12 At this point, most refractive surgeons use a 130 to 160 μm flap to minimize irregular astigmatism in a 6.0 mm treatment zone to maximize the quality of vision and recommend that 250 to 300 μm of residual posterior stroma be left untouched to ensure adequate biomechanical corneal strength to minimize the risk for keratectasia.1,16,17

In their study of surgically induced keratectasia, Pallikaris and coauthors11 suggest that the residual corneal bed thickness should be taken into consideration. In this study, most eyes (n = 13, 68%) had less than 250 μm of residual stroma after creation of the flap and application of the ablation correction. In addition, 6 eyes (32%) retained a residual stromal bed greater than 250 μm. It was unlikely that these patients had preexisting keratoconus or forme fruste keratoconus based on normal preoperative topography, corneal pachymetry, and normal keratometry. In these patients, a statistically significant positive correlation with age was observed. It is possible that in these patients, the development of ectasia could be attributed to mechanisms other than corneal weakening through tissue ablation. These mechanisms could be the result of aging, changes in endothelial cells, chemical factors (ie, epithelial growth factor, fibronectin), or intracellular links.18 The development of keratoectasia in eye with a residual stromal bed thinner than 250 μm after LASIK is not surprising; however, late-onset keratectasia in eyes with a residual stromal bed thicker than 250 μm after LASIK has been reported. This suggests that in some corneas, even 250 μm may not be an adequate stromal bed thickness to prevent progressive keratectasia. Based on these reports and our personal experience with lamellar refractive surgery, we believe that a minimum thickness of 300 μm of stress-bearing corneal stroma is necessary to prevent keratectasia after LASIK.

When ablating the undersurface of the flap, special care must be taken not to ablate through the disk into Bowman's layer. Otherwise, as reported by Buratto and coauthors,19 wrinkling of Bowman's layer with significant loss of BSCVA may occur.

Maldonado20 reports that undersurface ablation of the flap for LASIK retreatment for low residual refractive errors in eyes with an SE residual refraction between −0.75 D and −3.25 D and astigmatism between 0 D and −1.5 D after LASIK with sufficient flap stroma seems to be effective and may prevent future keratectasia.

Our results indicate that simultaneous LASIK treatment is safe. In our series of patients, less than 20% of the eyes lost 2 or more lines of BSCVA (Figure 7). No vision-threatening complications occurred. Although we had 1 case of fine striae in the early study stage, we did not experience postoperative striae after we started using the Johnston LASIK flap applanator. The Johnston LASIK flap applanator seems to reduce this complication. The predictability of simultaneous LASIK was good; 37 eyes (86.1%) were within ±1.0 D of the targeted SE refraction at 3 months, and 39 eyes (90.7%) were within ±1.00 D at 6 months. In addition, the stability of the correction was also satisfactory, with 2 eyes (4.7%) changing more than 1.0 D during the postoperative period. Their SEs were −5.75 D and −6.25 D preoperatively, −0.38 D and −1.50 D at 1 month, and −1.50 D and −0.25 D at 6 months. In our study, 34 of 43 eyes (79.1%) had an UCVA of 20/30 or better at the 6-month examination. This percentage reportedly ranges from 80.0% to 85.1% after conventional LASIK treatment.21,22 Our refractive results indicate that simultaneous LASIK might be a valuable tool for treatment of high myopia with or without astigmatism in eyes with insufficient stromal bed; the reduction of myopia and astigmatism was remarkable and accurate.

Figure 7.
Figure 7.:
The percentage of eyes that gained and lost visual acuity at 6 months.

As expected, no keratectasia or suspicious central corneal steepening was detected with serial tangential videokeratography (Figure 8). We left the posterior stromal base of 300 μm to ensure sufficient corneal integrity and avoid late-onset corneal keratectasia. We think our results suggest that this limit, a depth limit of 300 μm from the corneal endothelium, is effective.

Figure 8.
Figure 8.:
A: Preoperative corneal topographic map of the right eye demonstrating myopia with with-the-rule astigmatism. B: Postoperative corneal topographic map of the same eye demonstrating a homogeneously regular central corneal contour.

During the ablation of the undersurface of the flap, centration was not always easy to accomplish because patient had difficulty maintaining fixation in the deviated eye. The visual axis is marked on the cornea before making the flap. This mark is easily visible when centering the laser and allows creation of an ablation zone in the desired position on the reflected flap as long as the eye is kept steady. We also used a special light blue rigid contact lens to support the flap. Therefore, the surgeon was better able to control centration of the ablation on the undersurface of the flap and ultimately maximize visual improvement. At the same time, surgical spears were used to spread and position the reflected flap.

CONCLUSION

Simultaneous LASIK appears to be a useful surgical aid in LASIK treatment in eyes with high myopia with or without astigmatism when adequate residual stromal bed does not exist. The procedure left the residual posterior stromal more than 300 μm thickness, which supplies tectonic integrity to the cornea.23

Patients with thin corneas who have an SE less than 6.0 D (6 eyes in our study) may have standard photorefractive keratectomy (PRK). In addition, patients with higher levels of myopia may have PRK or laser-assisted subepithelial keratectomy (LASEK) with mitomycin-C application to minimize haze formation. We suggest that our modified LASIK procedure is an alternative for this group of patients. At this time in the evolution of refractive surgery, these cases might be managed in the future with the approval of implantable contact lenses.

Simultaneous LASIK may reduce the risk for future keratectasia. Because the 6-month follow-up period in our study was not sufficient to exclude post-LASIK corneal ectasia, a long-term follow-up study and comparison with conventional LASIK or LASEK should be conducted to confirm the validity of this procedure.

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