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Corneal wavefront–guided customized laser in situ keratomileusis after penetrating keratoplasty

Imamoglu, Serhat MD*; Kaya, Vedat MD; Oral, Deniz MD; Perente, Irfan MD; Basarir, Berna MD; Yilmaz, Omer Faruk MD

Author Information
Journal of Cataract & Refractive Surgery: May 2014 - Volume 40 - Issue 5 - p 785-792
doi: 10.1016/j.jcrs.2013.10.042
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Abstract

Visual outcomes after penetrating keratoplasty (PKP) are unpredictable because the surgery often induces significant astigmatism. Astigmatism after keratoplasty is typically irregular. In addition, high levels of spherical refractive errors and higher-order aberrations (HOAs) are often present.1 On the other hand, patients with good corrected distance visual acuity (CDVA) often have problems related to anisometropia. When irregular astigmatism or anisometropia is present, spectacles provide limited visual rehabilitation while rigid gas-permeable (RGP) contact lenses typically give better results. However, the abnormal corneal shape and eyelid problems that are often present in eyes after PKP usually limit the use of contact lenses and prompt patients to seek surgical treatment.

Until excimer lasers became available, incisional refractive surgical procedures, such as astigmatic keratotomy and wedge resection, were used with limited success to correct astigmatism after keratoplasty.2 Photorefractive keratectomy (PRK) has been used successfully to correct spherical and cylindrical refractive errors; however, haze formation and loss of CDVA are frequent postoperative complications.3 Although laser in situ keratomileusis (LASIK) brings the added risk for flap-related and wound-related complications in eyes with corneal grafts, it has largely replaced PRK for the correction of refractive errors after keratoplasty.4,5

Although standard excimer laser ablation patterns can address spherocylindrical refractive errors only, the advent of topography-guided ablation systems made it possible to perform custom treatments and correct irregular astigmatism. The differences in curvature, thickness, and size between the graft and the recipient cornea might lead to misalignment at the graft–host interface. Also, variable tension of the sutures induces irregular astigmatism by increasing HOAs after PKP.6 After keratoplasty, eyes have nearly 5.5 times more HOAs than normal eyes. Trefoil is reportedly the most dominant HOA.1 By addressing both lower-order aberrations (LOAs) and HOAs, wavefront-guided LASIK has the potential to effectively correct irregular astigmatism and maximize visual improvement after keratoplasty.

The corneal wavefront is derived from corneal topography. Corneal wavefront–guided treatment is basically advanced topography-guided ablation. The technique gives the surgeon the ability to selectively correct corneal HOAs along with refractive errors. After PKP, anterior and posterior corneal HOAs are reported to be significantly higher than in normal eyes.7 In this study, we assessed the results of corneal wavefront–guided custom LASIK for the correction of LOAs and HOAs after PKP.

Patients and methods

This retrospective noncomparative clinical case series comprised consecutive patients who had corneal wavefront–guided custom LASIK between December 2005 and February 2008 to correct refractive errors and HOAs after PKP. All patients were unable to tolerate spectacle correction. Rigid gas-permeable contact lens correction was attempted in 4 eyes without success; the other patients refused to try the lenses.

The study inclusion criteria were a clear full-thickness corneal graft with an endothelial cell count (ECC) above 1500 cells/mm2, a stable refractive error for at least 12 months after all sutures had been removed, and the absence of ocular pathology other than the indication for PKP.

All patients received a detailed explanation about wavefront-guided LASIK, including potential risks and benefits, after which they provided informed consent. The same surgeon (V.K.) performed all LASIK and PKP procedures.

Surgical Planning

On the day of the surgery, corneal topography maps were taken and the best map was chosen with respect to reliability indices of the maps. The Placido-disk topography unit used in this study provides corneal elevation data using the arc-step method. The device compares the data obtained from the corneal elevation map with the reference ideal eye model with a Q-value of −0.25. The elevation difference between the 2 surfaces creates an optical path difference, which is expressed by Zernike polynomials (up to the 7th order) to represent the corneal wavefront.

The Optimized Refractive Keratectomy-Custom Ablation Manager software was used to plan the treatment with the Schwind Esiris excimer laser platform (both Schwind eye-tech-solutions GmbH and Co. KG). This platform uses a 0.8 mm para-Gaussian profile flying-spot laser at 200 Hz and a 330 Hz eye tracker. The software uses topographic data (obtained with the Placido-disk topography device) and other clinical parameters (manifest refraction, pachymetry, and flap thickness) to determine the most appropriate custom ablation profile to correct LOAs and HOAs. Using the software, the surgeon can change the optical zone diameter and select the aberrations to be treated. The ablation zone diameter was determined according to the scotopic pupil size, and full correction of LOAs and HOAs were targeted in all eyes. The ablations were planned to leave a minimum postoperative stromal bed thickness of 250 μm or 50% of the preoperative thickness. The aim of the treatment was to decrease HOAs and anisometropia primarily.

Surgical Technique

In all eyes, a Carriazo-Pendular microkeratome (Schwind eye-tech-solutions GmbH and Co. KG) was used to create a superior, hinged 160 μm thickness after the flap had been lifted. After excimer laser ablation, the flap was placed back on the stromal bed and a plano soft contact lens was placed over the cornea until the first postoperative visit. Prednisolone acetate 1% and tobramycin 0.3% eyedrops 4 times a day and preservative-free artificial tears every hour were prescribed. Tobramycin drops were used for 10 days, and the prednisolone acetate drops tapered over 6 weeks.

Patient Evaluation

The uncorrected distance visual acuity (UDVA), manifest refraction, CDVA, and corneal wavefront aberrations as well as subjective complaints were evaluated preoperatively and at the last postoperative visit. The preoperative workup also included tonometry, slitlamp biomicroscopy, fundus examination, pupillometry (Colvard pupillometer, Oasis Medical, Inc.), scanning-slit topography (Orbscan IIz, Orbtek, Inc.), and Placido-disk topography (Keratron Scout, Optikon 2000 SpA). Ultrasonic pachymetry (Tomey AL-3000, Tomey Corp.) measurements were taken at 9 points over the corneal graft including the center and 4 points in 3.0 mm and 7.0 mm zones. Endothelial cell counts were performed with a noncontact specular microscope (Noncon Robo FA-3509, Konan Medical).

The change in astigmatism achieved at the last follow-up was evaluated using Alpins vectorial analysis.8 The following vectors were determined and evaluated: target-induced astigmatism (TIA) as the vector of the intended change in cylinder for each treatment and surgically induced astigmatism (SIA) as the vector of the actual change achieved.

Statistical Analysis

Preoperative and postoperative data were compared by using paired t test, and P values less than 0.05 were considered statistically significant. Snellen visual acuity scores were used for statistical analysis.

Results

The study included 11 eyes of 10 patients. Table 1 shows the patients’ demographics. The mean interval from PKP to LASIK was 66.8 months ± 30.2 (SD) (range 23 to 96 months). One patient had bilateral treatment. The mean follow-up after LASIK was 42.9 ± 21.8 months (range 10 to 72 months).

Table 1
Table 1:
Patient characteristics.

In 6 of the 11 eyes, the theoretical ablation depth exceeded the thickness of corneal lenticule available for ablation due to a high amount of aberrations. In these eyes, the ablation zone diameter was decreased or undercorrection was targeted to maintain the minimum residual stromal bed (RSB) thickness. The mean preoperative graft thickness was 552 ± 42 μm (range 518 to 660 μm). The mean laser ablation depth was 71 ± 11 μm (range 47.9 to 85.3 μm), and the mean postoperative theoretical RSB thickness was 320 ± 43 μm (range 279 to 431.2 μm).

Visual Acuity and Refraction

Table 2 shows the visual and refractive outcomes. Preoperatively, all eyes had a UDVA of worse than 20/40 and 5 (45%) eyes had a UDVA of 20/200 or worse. After LASIK, 1 (9%) eye had a UDVA of worse than 20/200 and 7 (63%) eyes had a UDVA of 20/40 or better (Figure 1). The mean postoperative UDVA was statistically significantly better than the mean preoperative UDVA (P=.002). After LASIK, the UDVA improved in 7 (63%) eyes and remained unchanged in 4 eyes (36%). One eye (9%) gained 1 line, 1 eye (9%) gained 4 lines, 2 eyes (18%) gained 5 lines, 1 eye (9%) gained 6 lines, 1 eye (9%) gained 7 lines, and 1 eye (9%) gained 8 lines of UDVA.

Figure 1
Figure 1:
Percentage of eyes with a certain level of cumulative preoperative Snellen CDVA and of postoperative UDVA at last follow-up (CDVA = corrected distance visual acuity; UDVA = uncorrected distance visual acuity).
Table 2
Table 2:
Visual and refractive outcomes.

There was also a statistically significant decrease in the mean manifest refraction spherical equivalent (MRSE) (P=.002) and in the mean refractive astigmatism (P=.011) between preoperatively and postoperatively. The attempted astigmatic correction was not totally achieved in all cases (Table 3). The decrease in the mean refractive astigmatism was statistically significant.

Table 3
Table 3:
Values of refractive astigmatism in each eye.

Predictability

The postoperative achieved correction was within ±1.00 D of the intended correction in 7 eyes (64%). The mean magnitude of the TIA was 5.73 ± 2.52 D (range 1.01 to 8.24 D), and the mean magnitude of the SIA at last follow-up was 3.96 ± 3.32 D (range 0.07 to 11.1 D). The difference between the TIA and the SIA were statistically significant (P<.05). Therefore, there was a trend toward undercorrection of the refractive astigmatism after wavefront-guided LASIK for irregular astigmatism (Figure 2). In an ideal complete correction, the SIA and the TIA would be identical.

Figure 2
Figure 2:
Relationship between the achieved postoperative spherical equivalent (SE) correction and the attempted SE correction.

Safety

Postoperatively, no eye lost CDVA lines. All eyes had a CDVA better than 20/40, and 9 eyes (82%) had a CDVA of 20/25 or better. After corneal wavefront-guided LASIK, the CDVA improved in all eyes. Two eyes (18%) gained 1 line, 4 eyes (36%) gained 3 lines, and 5 eyes (45%) gained 6 lines of CDVA. There was a statistically significant improvement in the mean CDVA between preoperatively and postoperatively (P<.001). No eye had corneal ectasia or recurrent keratoconus on topographic examination.

Higher-Order Aberrations

Figure 3 shows the changes in the mean corneal HOAs. The mean preoperative total root mean square (RMS) was 4.65 ± 1.14 μm (range 2.26 to 5.94 μm). The postoperative mean total RMS decreased to 2.71 ± 1.31 μm (range 1.22 to 5.33 μm), showing a statistically significant improvement (P=.001).

Figure 3
Figure 3:
Higher-order corneal aberrations in eyes that had corneal wavefront-guided custom LASIK after PKP (* = P<.05; HOA = higher-order aberrations; SA = spherical aberration).

The mean preoperative coma was 0.77 ± 0.23 μm (range 0.48 to 1.20 μm). The postoperative mean coma was 0.43 ± 0.21 μm (range 0.04 to 0.79 μm). This represented a statistically significant reduction (P=.002).

The mean preoperative trefoil was 1.18 ± 0.52 μm (range 0.36 to 2.19 μm). The postoperative mean trefoil was 0.92 ± 0.32 μm (range 0.41 to 1.34 μm). The change was not statistically significant (P > .05).

The mean preoperative spherical aberration was 0.39 ± 0.23 μm (range 0.01 to 0.77 μm). The postoperative mean spherical aberration was 0.40 ± 0.25 μm (range 0.11 to 0.94 μm). The change was not statistically significant (P > .05).

Complications and Enhancement

Limited epithelial ingrowth that did not require intervention was observed at the flap border in 2 eyes (18%). No enhancements were performed.

Case Report

A 25-year-old man had PKP in the right eye for keratoconus 7 years previously. The patient presented with contact lens intolerance and 4.00 D of mixed astigmatism. He reported reduced vision, halos, glare, and sunburst at night. The preoperative UDVA was 0.1, and the manifest refraction was −1.75 −4.00 with a CDVA of 0.4. Corneal wavefront-guided custom LASIK was performed to correct both HOA-related symptoms and anisometropia. The preoperative corneal pachymetry of the graft was 660 μm. The 160 mm flap was lifted, and a 68.8 mm ablation was performed in a 5.5 mm optical zone. The total HOA RMS decreased from 5.31 μm preoperatively to 2.46 μm postoperatively. Postoperatively, the UDVA in the eye improved to 0.8 and the CDVA to 1.0 with −0.25 −2.75 manifest refraction.

Table 4 shows the patients’ individual preoperative and postoperative data.

Table 4
Table 4:
Comparison of preoperative and postoperative data in 11 eyes with corneal wavefront-guided LASIK.

Discussion

Irregular astigmatism, which is almost always present after keratoplasty, translates into very high levels of HOAs that are difficult to quantify with total wavefront analysis. Aberrations in eyes after keratoplasty might even exceed the capabilities of currently available wavefront analyzers or lead to faulty measurements.9 Limited data are available in the current literature on quantification of wavefront aberrations in eyes with corneal grafts. Although various studies have used different devices for wavefront measurements, the common finding is the presence of high levels of HOAs compared with the levels in normal eyes.1,10 Using a large-dynamic-range Hartmann-Shack wavefront sensor, Pantanelli et al.1 reported 5.5 times more higher-order RMS (2.25 ± 0.75 μm) in postkeratoplasty eyes than normal eyes over a 6.0 mm pupil. The most dominant HOA was trefoil followed by coma and spherical aberration. Shah et al.10 compared wavefront measurements in normal, keratoconic, and PKP eyes and reported higher RMS, coma, and spherical aberration in eyes with keratoconus and PKP. However McLaren et al.11 did not find a correlation between corneal higher-order wavefront errors and visual function in postkeratoplasty eyes.

The corneal wavefront is derived from corneal topography and reflects only the aberrations resulting from the anterior corneal surface, excluding the aberrations originating from internal structures. Unlike total wavefront, pupil dilation is not necessary and accommodation does not influence the measurements. Reliable corneal wavefront measurements can be obtained in eyes with corneal irregularity and high astigmatism. In normal eyes, the cornea is the main source of the ocular aberrations that degrade the retinal image. With the presence of a graft and surgically induced changes, the cornea becomes a much more significant source of aberrations in postkeratoplasty eyes. Corneal wavefront–guided treatment is well suited to eyes with highly irregular corneas because the aberrations are corrected exactly where they originate. In our study, although significant improvements were seen in RMS and coma values after corneal wavefront–guided LASIK, the slight improvement achieved in trefoil values did not reach statistical significance and spherical aberration remained essentially unchanged. The wavefront errors located close to the center of the Zernike pyramid (coma and spherical aberration) have a more adverse effect on visual acuity than errors located near the edge (trefoil and tetrafoil).12 We speculate that the reduction in coma might have been the reason for the significant improvement in CDVA in all study eyes. In our study, we used the Optimized Refractive Keratectomy-Custom Ablation Manager system. By treating corneal HOAs, the system has proved effective in improving the sight of patients who have different corneal alterations.13

In our study, corneal wavefront-guided LASIK effectively reduced spherical and astigmatic refractive errors and significantly improved UDVA. There was a gain of 1 to 8 lines of UDVA in 63% of the eyes. In addition, there was a statistically significant improvement in the postoperative mean CDVA over preoperative levels. Two previous studies5,14 report a loss of CDVA of 2 lines or more in 6.5% of eyes and 15.0% of eyes, respectively, when LASIK was performed to correct refractive errors after keratoplasty. There was no loss of CDVA lines in our series; the CDVA improved in all eyes.

In 64% of the eyes in our series, the achieved correction postoperatively was within ±1.00 D of the intended correction. Similar to our results, Barraquer and Rodriquez-Barraquer5 found 67% of cases and Alió et al.14 72% cases were within ±1.00 D of emmetropia after LASIK in eyes with corneal grafts.

We reduced the ablation zone diameter when the theoretical ablation depth exceeded the thickness of the corneal lenticule available for ablation. The ablation zone diameters in our patients ranged from 4.0 mm to 6.0 mm to 9.0 mm. Although ablation diameters were often smaller than the patients’ scotopic pupil size, no patients reported optical side effects. Pupil-related optical aberrations might have been overshadowed by the visual degradation caused by the irregularities in the periphery of the corneal grafts.

Formation of fluid-filled cysts in the interface, poor flap adherence, and postoperative flap displacement have been reported after LASIK in grafts with low endothelial cell function.4,15 Although LASIK has no significant effect on corneal endothelial density in the long term,16 transplanted corneal grafts are known to lose endothelial cells at a faster rate than normal eyes.17 To avoid potential flap complications, we did not perform LASIK in grafts with an ECC less than 1500/mm2. Moreover, expected survival of the grafts with limited endothelial reserve are short and regrafting will most likely be necessary; this might be complicated by the changes caused by LASIK in the host corneal rim.

Correction of refractive errors after keratoplasty with surface laser ablation dates back to the early 1990s.18 However, haze formation is common and may be accompanied by regression in eyes with corneal grafts.3 The risk for haze formation is reportedly correlated with the ablation depth, as in normal corneas.5 High spherical and cylindrical refractive errors frequently present after keratoplasty require deep ablations, increasing the risk for haze formation. Intraoperative mitomycin-C has been used to decrease the risk for haze formation after surface ablation in postkeratoplasty eyes.19,20

Although haze formation is usually not a concern with LASIK, Malecha and Holland21 report stromal haze formation leading to a loss of 7 lines of CDVA in a postkeratoplasty eye; the cause was persistent diffuse lamellar keratitis (DLK). However, the patient had systemic lupus erythematosus and was on systemic immunosuppressive therapy. Although DLK was encountered in 3 of the 20 eyes in the same study, DLK is not a common complication of LASIK after keratoplasty. In our series, there were no cases of DLK or interface haze formation. In addition to the risk for flap-related and graft-related complications, another concern about LASIK in postkeratoplasty eyes is the extension of the microkeratome cut into the host cornea, which could lead to problems should regrafting be necessary.

In a study reporting the results of LASIK after PKP, Barraquer and Rodriquez-Barraquer5 analyzed 30 eyes that had keratoplasty for keratoconus and found refractive results to be stable 5 years postoperatively. All but 2 eyes in our series had PKP for keratoconus. We have aimed to leave a postoperative stromal bed thickness of 50% of the preoperative pachymetric reading in all eyes. We did not encounter ectatic changes during our relatively short follow-up; however, a much longer follow-up should be performed to assess the safety of LASIK after PKP.

Performing LASIK as a 2-step procedure after keratoplasty has been advocated by some because cutting a lamellar flap may reduce astigmatism in the graft.22 In this study, our preference was to perform LASIK as a 1-step procedure because previous studies failed to show significantly better visual results with the 2-step approach. We also had concerns about potential flap and graft complications, especially the epithelial ingrowth that could result from relifting of the flap.

All complications associated with LASIK in normal eyes can also occur with LASIK after keratoplasty. Reported LASIK complications in eyes after keratoplasty include epithelial ingrowth, sterile interface inflammation, flap striae, recurrent herpes simplex keratitis, flap displacement, flap edema and retraction, corneal edema, punctate keratitis,4,15,21,23 graft rejection,24,25 stromal haze formation secondary to persistent DLK,20 buttonhole flap formation,20,26 and formation of fluid-filled pockets in the interface.4 As one of the most dreaded complications of keratoplasty, graft dehiscence can theoretically occur with LASIK in postkeratoplasty eyes secondary to increased intraocular pressure during suction. Although dehiscence of the graft–host junction has been reported in an eye that had LASIK 3 years after PKP, the dehiscence did not occur during LASIK but was caused by moderate eye rubbing 2 weeks after surgery.27 Most authors advocate waiting 3 to 6 months after the last suture removal or any other refractive procedure before performing LASIK in postkeratoplasty eyes to allow time to achieve adequate wound strength and refractive stability.26

We did not encounter significant flap- or graft-related complications other than limited epithelial ingrowth in 2 eyes (18%); the ingrowth did not require removal. Regarding corneal aberration, no different behavior was observed between a femtosecond laser and a microkeratome for flap creation.28 The pendular microkeratome functioned without problems in all eyes. The incidence of epithelial ingrowth after LASIK in postkeratoplasty eyes has been reported between 11% and 16%.4,25,26 To avoid the risk for wound dehiscence, we waited for at least 12 months after all sutures had been removed and LASIK was performed only in eyes with a visible fibrotic scar at the graft–host interface preoperatively. During surgery, we also tried to keep the suction time at a minimum.

This study is limited by the small sample and lack of follow-up data for all eyes at regular postoperative intervals. However, because the shortest time after LASIK was 9 months in our study, the refractive results would likely have stabilized by that time. Although refractive surgery in normal eyes aims to reduce the patient’s dependency on corrective lenses, refractive surgical procedures after PKP mainly aim to resolve anisometropia, thus making spectacle correction tolerable to patients. Insufficient stromal bed thickness did not allow full correction of refractive errors and HOAs in most eyes in our study. However, significant decreases in total RMS, coma, myopia, and astigmatism resulted in significant improvement in UDVA and CDVA.

Although visual results were good and patient satisfaction high with this method, the risk for long-term corneal biomechanical problems should be kept in mind. To minimize this risk, creating a LASIK flap that does not extend onto the host cornea with the use of a femtosecond laser instead of a mechanical keratome could be considered. Use of a femtosecond laser might decrease the risk for wound dehiscence as well. Our limited experience in this study indicates that corneal wavefront–guided LASIK is a surgical option for the correction of refractive errors and HOAs after keratoplasty.

What Was Known

  • Postoperative HOAs and refractive error after PKP are common findings.
  • Although LASIK and PRK after PKP to correct refractive error have been reported, wavefront-guided LASIK to correct HOAs after PKP has not been studied.

What This Paper Adds

  • Correcting both HOAs and refractive error with corneal wavefront–guided custom LASIK after PKP improved UDVA and CDVA without significant surgical complications, although the attempted astigmatic correction was not totally achieved.
  • This technique did not result in significant refractive regression in any eye during the long-term follow-up.

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