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Wavefront-optimized excimer laser in situ keratomileusis for myopia and myopic astigmatism: Refractive outcomes and corneal densitometry
ARTICLE
Wavefront-optimized excimer laser in situ keratomileusis for myopia and myopic astigmatism: Refractive outcomes and corneal densitometry Usama Fares, MD, Ahamd Muneer Otri, MD, Mouhamed Ali Al-Aqaba, MB BCh, PhD, Lana Faraj, MB BCh, Harminder S. Dua, MD, PhD
PURPOSE: To determine the refractive outcomes of wavefront-optimized laser in situ keratomileusis (LASIK) treatments, in particular to measure corneal densitometry after LASIK using the densitometry function of the Pentacam Scheimpflug system. SETTING: Division of Ophthalmology and Visual Sciences, University of Nottingham, United Kingdom. DESIGN: Cohort study. METHODS: Changes in postoperative visual acuity, refraction, and contrast sensitivity were evaluated after wavefront-optimized laser treatment. Corneal densitometry was evaluated with the Scheimpflug system before and after LASIK. RESULTS: One year postoperatively, the uncorrected distance visual acuity was 6/6 or better in 92% of eyes and 6/9 or better in all eyes. Eighty-six percent of eyes had no change in the corrected distance visual acuity (CDVA); 4% gained 1 or more lines. Wavefront-optimized LASIK was stable over 1 year postoperatively. Eighty-nine percent of eyes were within G0.50 diopter (D) and 100% were within G1.00 D of the intended correction 1 year postoperatively. Contrast sensitivity showed a nonsignificant improvement (1.55 G 0.10 [SD] preoperatively to 1.57 G 0.09 12 months postoperatively) (P > .05). There was a nonsignificant increase in corneal densitometry 1 year postoperatively (from 12.72 G 2.43 to 13.04 G 2.58) (P > .05). No correlation was found between corneal densitometry and contrast sensitivity or CDVA. CONCLUSIONS: Wavefront-optimized LASIK gave excellent refractive and visual outcomes and did not seem to affect corneal densitometry significantly 1 year postoperatively. However, larger studies may show a masked effect on corneal densitometry. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2012; 38:2131–2138 Q 2012 ASCRS and ESCRS
Conventional laser in situ keratomileusis (LASIK) ablation profiles have been reported to be effective, safe, and predictable in the management of spherical and cylindrical refractive errors, known as lowerorder aberrations (LOAs).1,2 Nevertheless, these profiles based on Munnerlyn formula3 tend to change the average prolate shape of the cornea toward a more oblate profile. This may leave patients with night-vision problems, such as glare, halos, or starburst.3 The development of wavefront technology and its implications in the field of visual sciences Q 2012 ASCRS and ESCRS Published by Elsevier Inc.
and refractive surgery procedures have helped in measuring ocular aberrations, known as higherorder aberrations (HOAs). Many studies found a correlation between reduced retinal image quality and these aberrations, especially spherical aberrations and coma.4 New strategies have been used to develop excimer laser profiles and improve postoperative visual quality. These include wavefront-guided and wavefrontoptimized profiles. Wavefront-guided laser treatment is a custom ablation based on HOAs unique to the 0886-3350/$ - see front matter http://dx.doi.org/10.1016/j.jcrs.2012.07.041
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treated eye. In contrast, wavefront-optimized profiles do not take into account the unique HOAs in each eye but use certain preprogrammed profiles based on population analysis.4,5 Wavefront-optimized laser ablations are designed to obtain the prolate shape of the cornea and limit the tendency toward creating oblate surfaces. This can be achieved by removing more tissue in the periphery of the ablation zone.6 Most studies report post-keratorefractive surgery haze using slitlamp observation and subjective scales. However, the objective measurement of corneal haze is better than subjective assessments in terms of accuracy and repeatability.7,8 Recently, subepithelial haze and stromal scarring after laser surgery have been quantified and assessed objectively using a Scheimpflug-imaging technique incorporated into different commercial units.8–10 The Pentacam (Oculus Optikger€ ate GmbH) is a noninvasive optical system that uses the Scheimpflug principle to take up to 50 cross-sectional images of the entire anterior segment within 2 seconds.11 The ability to measure changes in corneal transparency noninvasively and objectively may enable more precise monitoring of progression and may improve patient management. In the literature, only 2 studies used Pentacam Scheimpflug imaging to report corneal densitometry 12 months after photorefractive keratectomy (PRK)9,12 and 1 study reported corneal densitometry 12 weeks after LASIK.10 The latter study was published in Japanese, and we could not ascertain which technique was used to report corneal densitometry. Corneal densitometry is being regarded as a useful objective indicator of corneal health.13 The objective of this prospective study was to assess the refractive outcomes of wavefront-optimized LASIK treatments to correct myopia and myopic astigmatism, in particular to measure corneal densitometry 1 year after LASIK using the densitometry function of the Pentacam system. To our knowledge, this is the first study to report corneal densitometry 1 year after LASIK.
Submitted: March 24, 2012. Final revision submitted: July 17, 2012. Accepted: July 31, 2012. From the Division of Ophthalmology and Visual Sciences, University of Nottingham, Nottingham, United Kingdom. Corresponding author: Harminder S. Dua, MD, PhD, Division of Ophthalmology and Visual Sciences, B Floor, Eye Ear Nose Throat Centre, University Hospital, Queens Medical Centre, Nottingham, NG7 2UH, United Kingdom. E-mail: harminder.dua@nottingham. ac.uk.
PATIENTS AND METHODS This prospective study comprised eyes having LASIK for the correction of myopia and/or myopic astigmatism.
Preoperative Examination The patients had a complete preoperative assessment. This included refraction, uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), applanation tonometry, corneal topography (Pentacam), ultrasound pachymetry (SP 3000, Tomey Corp.), pupil size (Colvard infrared pupillometer, Oasis Medical Inc.), and a full fundus examination.
Surgical Technique All cases were performed by the same surgeon (H.S.D.) using the Wavelight Allegretto excimer laser system (Wavelight AG) and the following parameters: 193 nm with a 0.95 mm spot size, fluence 200 mJ/cm2, and repetition rate 200 Hz. A nasal hinged flap was created in all eyes with a single-use microkeratome (One-Use-Plus, Moria). The postoperative refraction target was emmetropia in all cases.
Postoperative Examination Patients were seen on the first postoperative day and then at 1, 3, 6, and 12 months. The postoperative data collected included refraction, UDVA, CDVA, corneal topography, contrast sensitivity, and Scheimpflug imaging (Pentacam). Contrast sensitivity measurements were performed at the 1-year follow-up using the Pelli-Robson chart.
Densitometry The automatic release mode of the Pentacam Scheimpflug system was used to determine when correct focus and alignment with the corneal apex had been achieved. This reduced operator-dependent variables associated with manual scanning. Imaging was performed using the 50-scans setting and was completed in 2 seconds. The Scheimpflug images were used to calculate corneal densitometry preoperatively and postoperatively using the caliper tools provided by the system's software. The density (densitometry) was measured. This allowed the operator to mark a point on the cross-sectional scan of the cornea. The central 7.0 mm area of the cross-section of the cornea along each of 4 axes (0 to 180 degrees, 45 to 225 degrees, 90 to 270 degrees, and 135 to 315 degrees) was defined manually. Densitometry was then measured automatically for the entire cornea (Figure 1, red line) and the anterior half of the cornea (Figure 1, purple line). The density was measured using the caliper tools by assigning the specified area on the cross-sectional scan of the cornea; the software calculated the densitometry value of that defined area. The mean and the standard deviation readings were then recorded. Details of the method of ascertaining densitometry and the normal densitometry values of human corneas have been published.13
Statistical Analysis Statistical analysis was performed with the SPSS for Windows software (version 19.0, SPSS, Inc.). Datagraph-med for
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There were no flap creation complications and no postoperative complications. Efficacy All eyes had 6/15 or worse UDVA preoperatively. At the 6-month follow-up visit, the UDVA was 6/5 or better in 23 eyes (62%) and 6/7.5 or better in all eyes. At 12 months, the UDVA was 6/7.5 or better in 36 eyes (97%) and 6/9 or better in all eyes. The UDVA was 6/6 in 34 eyes (92%) at 6 months and 12 months (Figure 2). The efficacy index was 0.95.
Figure 1. Scheimpflug image showing the densitometry of the total cross-sectional area (red line) and the anterior half of the cornea (purple line) at given axis.
Safety
Microsoft Office Professional software (version 3.90b, Datagraph-med) was used to analyze refractive and other visual outcome data and to generate standard graphs. The efficacy index (ratio of mean postoperative UDVA to mean preoperative CDVA) and the safety index (ratio of mean postoperative CDVA to mean preoperative CDVA) were also calculated. The correlation between the following parameters was performed using the Pearson productmoment correlation coefficient parametric test: (1) contrast sensitivity and entire corneal densitometry, (2) contrast sensitivity and anterior corneal densitometry, (3) entire corneal densitometry and mean CDVA, and (4) anterior corneal densitometry and mean CDVA.
No patient lost 2 or more lines of Snellen CDVA. At the 6-month follow-up visit, 32 eyes (86%) had no change in CDVA and 3 eyes (8%) gained 1 line or more. At 1 year, 32 eyes (86%) had no change in CDVA, 2 eyes (5%) gained 1 line, 1 eye (3%) gained 2 lines, and 2 eyes (5%) lost 1 line (Figure 3). The safety index was 1.02. Stability Figure 4 shows the change in the mean SE refraction before LASIK and 3, 6, and 12 months after LASIK. There was a general tendency to form a stable plateau over time.
RESULTS Twenty-two patients (43 eyes) were enrolled in this study. The mean age at surgery of the 14 women (63.6%) and 8 men (36.4%) was 37 years (from 20 to 70 years). Table 1 shows the preoperative and postoperative visual and refractive outcomes. The mean preoperative spherical equivalent (SE) decreased significantly 6 months and 12 months postoperatively (P ! .05, paired t test). At the 3-month follow-up visit, 4 patients (6 eyes) had unsatisfactory CDVA due to undercorrection. All other patients (37 eyes) completed the 6-month and 12-month follow-up visits. All 6 undercorrected eyes had LASIK enhancement. Data from these patients were used in the baseline statistics and dismissed when retreatment was indicated.
Predictability The postoperative SE in all eyes was within G1.00 D at the 6-month and 12-month follow-up visits. The SE refractive outcome was within G0.50 D in 34 eyes (92%) at 6 months and in 33 eyes (89%) at 12 months (Figure 5). Figure 6 shows the attempted versus the achieved SE 1 year postoperatively. Defocus Equivalent The defocus equivalent was used to eliminate the inequity between eyes that had similar SE refractions but different amounts of astigmatism. One year
Table 1. Visual and refractive outcomes over time. Postoperative Preoperative Parameter Sphere (D) Cylinder (D) SE (D) UDVA (logMAR) CDVA (LogMAR)
Mean G SD 2.97 G 1.34 0.67 G 0.56 3.31 G 1.39 0.92 G 0.15 0.09 G 0.07
3 Months
Range 6.02, 0.75 1.78, 0.00 6.27, 0.88 d d
Mean G SD 0.02 G 0.38 0.35 G 0.31 0.15 G 0.38 0.00 G 0.18 0.10 G 0.06
Range 1.00, 1.00 1.25, 0.00 1.13, 0.87 d d
6 Months Mean G SD 0.05 G 0.31 0.28 G 0.27 0.09 G 0.29 0.07 G 0.08 0.10 G 0.06
Range 0.50, 0.75 1.25, 0.00 0.63, 0.50 d d
CDVA Z corrected distance visual acuity; SE Z spherical equivalent; UDVA Z uncorrected distance visual acuity
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12 Months Mean G SD 0.07 G 0.33 0.33 G 0.29 0.09 G 0.27 0.07 G 0.09 0.10 G 0.07
Range 0.50, 0.75 1.25, 0.00 0.63, 0.62 d d
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Figure 2. Preoperative CDVA versus postoperative cumulative UDVA (Snellen) at 3, 6, and 12 months (CDVA Z corrected distance visual acuity; UDVA Z uncorrected distance visual acuity).
Figure 3. Lines of Snellen CDVA gained or lost over time (CDVA Z corrected distance visual acuity).
postoperatively, all eyes had a defocus equivalent refraction of less than 1.00 D and 31 eyes (84%) had a defocus equivalent refraction of 0.50 D (Figure 7).
contrast sensitivity or with CDVA. Table 2 shows the correlation data.
Contrast Sensitivity and Corneal Densitometry Although contrast sensitivity showed slight improvement 12 months postoperatively (mean 1.55 G 0.10 versus 1.57 G 0.09), the change was not statistically significant (P O .05, paired t test). There was no statically significant difference in the entire corneal densitometry between preoperatively (mean 12.72 G 2.43) and 12 months postoperatively (mean 13.04 G 2.58) (P O .05, paired t test). Also, there was no statistically significant difference between the preoperative anterior corneal densitometry (mean 14.71 G 2.21) and the 12-month postoperative anterior corneal densitometry (mean 15.08 G 2.13) (P O .05, paired t test). Corneal densitometry did not correlate with
DISCUSSION The Wavelight Allegretto is a multiple function system. It can be used to perform conventional, wavefront-optimized, or wavefront-guided LASIK treatments. The wavefront-optimized profile tends to preserve the asphericity of the cornea by ablating more corneal tissue in the periphery. This minimizes the induction of spherical aberrations.5,14 Several studies15–17 compared wavefront-guided treatments and wavefront-optimized laser treatments and found no significant difference in efficacy, safety, or predictability. In a metaanalysis, Fares et al.18 found no clear evidence of a benefit of wavefront-guided ablations over nonwavefront-guided ablations but suggested that wavefront-guided LASIK may be a better
Figure 4. Stability of the achieved SE over time (solid line). The dashed lines indicate G1 standard deviation of the distribution of the individual values.
Figure 5. Postoperative SE. The bars relate to the percentage of patients who achieved the SE indicated in the x-axis at 3 months, 6 months, and 12 months.
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Figure 6. Predictability. Attempted versus achieved SE 1 year postoperatively (solid line). The dashed lines indicate the G1.00 D range.
Figure 7. Preoperative refractive astigmatism versus 1-year postoperative refractive astigmatism.
choice in eyes with a root-mean-square value of 0.3 mm or higher. Furthermore, a recent metaanalysis19 concluded that both wavefront-guided LASIK and wavefront-optimized LASIK have shown excellent efficacy, safety, and predictability. However, wavefront-guided profiles tend to induce fewer HOAs than wavefront-optimized profiles postoperatively. In a study by Tran and Shah,17 100% in the wavefront-optimized group achieved a UDVA of 6/6 at 3 months. Stonecipher and Kezirian16 found that after wavefront-optimized treatment, 67% of eyes had a UDVA of 6/4.8 and 93% of eyes had a UDVA of 6/6. Our safety results are consistent with those of others, who report a good safety profile with wavefront-optimized laser ablations.15–17,20 To our knowledge, no study reported a loss of 2 or more lines of Snellen visual acuity. However, more attention should be paid to reporting changes in Snellen visual
acuity lines, as recommended by Reinstein and Waring21 and Waring22 and further noted by Fares et al.18 in their metaanalysis. Specific information on the gain and loss of CDVA Snellen lines would be a more accurate way of indicating safety. In our series, 5% of eyes lost only 1 line of CDVA; this loss is considered to be within he limits of normal biological variability.21 The mean preoperative SE decreased from 3.31 G 1.39 D to 0.15 G 0.38 D 3 months postoperatively and remained stable at 0.09 G 0.23 D 1 year after LASIK. Furthermore, 1 year after surgery, 100% of the eyes in our study were within G1.00 D of the intended correction and 89% were within G0.50 D. This is also consistent with results reported in other studies15,16,18 that found wavefront-optimized LASIK ablations have high stability and predictability profiles. The most important difference between wavefrontoptimized laser ablations and wavefront-guided laser ablations is that the former tend to address HOAs generated during surgery (especial spherical aberrations) and the latter deal with the preoperative HOAs. Mrochen et al.6 suggest combining the algorithm of wavefront-optimized treatments and individual wavefront-guided treatments. This would treat both preoperative HOAs and those generated during laser surgery. Results in a study that compared wavefront-optimized alone with a combined algorithm treatment14 found that eyes with preoperative HOAs of 0.2 mm could benefit from the combined treatment. Wavefront-guided profiles require ablation of more corneal tissue and create larger optical zone treatments. This may affect the corneal integrity and biomechanics. In addition, the time of the ablation
Table 2. Corneal densitometry, contrast sensitivity, and CDVA correlation analysis.
Parameter Entire corneal densitometry and contrast sensitivity Entire corneal densitometry and CDVA Anterior corneal densitometry and contrast sensitivity Anterior corneal densitometry and CDVA
Pearson Correlation Variance (R2) P Value 0.07
0.004
.70
0.10
0.009
.57
0.08
0.006
.65
0.02
0.001
.90
CDVA Z corrected distance visual acuity
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treatment in wavefront-guided procedures is longer than that in wavefront-optimized procedures, which may lead to inaccurate refractive outcomes due to changes in corneal hydration and eye movements during laser treatment.17 At present, it is accepted that contrast sensitivity gives a better indication of visual function than visual acuity measurements and can be of help in evaluating patients having refractive surgery.23 Currently, sinewave gratings are the gold standard for evaluating contrast sensitivity due to their sensitivity, specificity, and reproducibility. On the other hand, they are expensive and the test takes more time to complete.23,24 Pelli-Robson, which is a letter optotypes-based contrast sensitivity test, has been reported to be a quick and reliable method in clinical settings.25 Our results agree with those in other studies16,26,27 that did not find a statistical difference in the contrast sensitivity between before and after wavefront-optimized laser treatment. Flap creation is another factor that can induce further LOAs, HOAs, or both. Studies28–30 have found that femtosecond laser flaps induce less HOAs and provide better visual outcomes than mechanical flaps. However, Mu~ noz et al.31 found no difference between mechanical microkeratome and femtosecond laser flap creation 4 years postoperatively. In our practice, a femtosecond laser is not available; therefore, all flaps were created with a single-use microkeratome. This provides sharp and smooth flap cutting, minimizing the risk for creating folds or wrinkles in the flap. We did not measure preoperative and postoperative HOAs, which is a limitation of the study but does not materially affect the inferences drawn. The Pentacam system simultaneously provides complete corneal pachymetry, corneal topography, and other metrics, such as densitometry.11,13,32 Corneal densitometry has been assessed, and the degree of subepithelial corneal haze and stromal scarring in refractive surgery patients has been quantified.9,10,12 Most studies used slitlamp observation and subjective assessment to report haze after keratorefractive surgery. However, objective assessments are more accurate and more repeatable than subjective ones.8 The ability to measure changes in corneal transparency noninvasively and objectively may enable more precise monitoring of progression and improve patient management. The transparency of the cornea can be indicated by corneal densitometry because it provides information about the clarity of the cornea.13 The densitometry function of the Pentacam system provides an objective quantitative evaluation of light scatter in the cornea. It is standardized from 0 to 100, where 0 means the cornea had no clouding and 100 means the cornea is
completely opaque.11 We optimized the method of measuring and recording densitometry of the cornea using the Pentacam system.13 This method is reliable and reproducible and was used in the present study. To our knowledge, this is the first report using the Pentacam Scheimpflug image system to evaluate changes in the optical density 12 months after LASIK. In our previous paper,13 we found that the mean normal corneal densitometry was 12.99 G 2.58 when it was measured along cross-sectional areas; this was not significantly different from values recorded at specific points (mean 12.30 G 2.40). Our preoperative densitometry values (mean 12.72 G 2.43) were comparable to those in healthy controls reported in the crosssectional areas and specific points techniques. Cennamo et al.9 found a significant early increase in the mean anterior density (from 27.71 G 4.39 to 37.812 G 12.31) 3 months after PRK; this was followed by a significant decrease (26.291 G 4.93) 12 month postoperatively. They selected scans centered on the pupil and recorded the highest peak shown on the green bars at the right of the Scheimpflug image of the Pentacam screen. They analyzed the highest corneal densitometry peak values corresponding to the anterior cornea on the grounds that PRK is a surface ablation treatment. However, the green bars give information about corneal (and lens) densitometry along a line that intersects the cornea at a specific location. The peak value corresponds to 1 point of the anterior cornea only, which may or may not be representative of the area treated. For example, if the cursor is moved along the area of cornea under evaluation, the peak value changes. To extrapolate data obtained from only 1 central point of the cornea as representative of the entire area treated can be considered a weakness of their study. We therefore recommend performing enhanced densitometry analysis using the cross-section densitometry function, by which the mean densitometry values for the full thickness of the cornea or the delineated anterior half can be obtained and analyzed. In our study, it was more appropriate to measure the densitometry along cross-sectional areas covering the ablated zone (entire cornea and anterior cornea) because LASIK is a stromal ablation procedure. Although Bhatt et al.33 have reported corneal densitometry along the graft–host interface in patients who had deep anterior lamellar keratoplasty (DALK), we could not obtain accurate measurement of corneal densitometry along the flap interface only. This is because the LASIK flap interface is not as easily detected as the DALK interface on Scheimpflug imaging. One Japanese study10 found no statistical difference between LASIK and epithelial LASIK 4 weeks and 12 weeks after surgery. Unfortunately, a translation of this study was not available, so we are unsure about
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the technique they used to measure their corneal densitometry. Furthermore, normal preoperative and postoperative densitometry values are likely to be indicative of minimal scarring or haze. In our patients, there was no significant correlation between corneal densitometry and contrast sensitivity or CDVA. This could be due to the small number of patients or because we did not detect visible haze in any eye. The densitometry program of the Pentacam system can provide a useful objective measure of postoperative corneal haze and keratocyte response. The latter is likely to be reversible and will be reflected in the densitometry values over time. Permanent haze resulting from scarring is likely to be longer lasting. Thus, densitometry measurements will give refractive surgeons an objective measure of the corneal response and haze and enable monitoring of these over time. Further studies comparing corneal densitometry between wavefront-optimized laser ablations and wavefrontguided laser ablations are highly encouraged.
WHAT WAS KNOWN Wavefront-optimized LASIK has good efficacy and safety profiles. Only 2 studies used Pentacam Scheimpflug imaging to report corneal densitometry 12 months after PRK, and 1 study reported corneal densitometry 12 weeks after LASIK. WHAT THIS PAPER ADDS This is the first study to report corneal densitometry 1 year after LASIK. There was no significant change in entire and anterior corneal densitometry 12 months after LASIK. Corneal densitometry did not correlate with contrast sensitivity or CDVA postoperatively.
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First author: Usama Fares, MD Division of Ophthalmology and Visual Sciences, University of Nottingham, Nottingham, United Kingdom
Wavefront-optimized excimer laser in situ keratomileusis for myopia and myopic astigmatism: Refractive outcomes and corneal densitometry Usama Fares, MD, Ahamd Muneer Otri, MD, Mouhamed Ali Al-Aqaba, MB BCh, PhD, Lana Faraj, MB BCh, Harminder S. Dua, MD, PhD
PURPOSE: To determine the refractive outcomes of wavefront-optimized laser in situ keratomileusis (LASIK) treatments, in particular to measure corneal densitometry after LASIK using the densitometry function of the Pentacam Scheimpflug system. SETTING: Division of Ophthalmology and Visual Sciences, University of Nottingham, United Kingdom. DESIGN: Cohort study. METHODS: Changes in postoperative visual acuity, refraction, and contrast sensitivity were evaluated after wavefront-optimized laser treatment. Corneal densitometry was evaluated with the Scheimpflug system before and after LASIK. RESULTS: One year postoperatively, the uncorrected distance visual acuity was 6/6 or better in 92% of eyes and 6/9 or better in all eyes. Eighty-six percent of eyes had no change in the corrected distance visual acuity (CDVA); 4% gained 1 or more lines. Wavefront-optimized LASIK was stable over 1 year postoperatively. Eighty-nine percent of eyes were within G0.50 diopter (D) and 100% were within G1.00 D of the intended correction 1 year postoperatively. Contrast sensitivity showed a nonsignificant improvement (1.55 G 0.10 [SD] preoperatively to 1.57 G 0.09 12 months postoperatively) (P > .05). There was a nonsignificant increase in corneal densitometry 1 year postoperatively (from 12.72 G 2.43 to 13.04 G 2.58) (P > .05). No correlation was found between corneal densitometry and contrast sensitivity or CDVA. CONCLUSIONS: Wavefront-optimized LASIK gave excellent refractive and visual outcomes and did not seem to affect corneal densitometry significantly 1 year postoperatively. However, larger studies may show a masked effect on corneal densitometry. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2012; 38:2131–2138 Q 2012 ASCRS and ESCRS
Conventional laser in situ keratomileusis (LASIK) ablation profiles have been reported to be effective, safe, and predictable in the management of spherical and cylindrical refractive errors, known as lowerorder aberrations (LOAs).1,2 Nevertheless, these profiles based on Munnerlyn formula3 tend to change the average prolate shape of the cornea toward a more oblate profile. This may leave patients with night-vision problems, such as glare, halos, or starburst.3 The development of wavefront technology and its implications in the field of visual sciences Q 2012 ASCRS and ESCRS Published by Elsevier Inc.
and refractive surgery procedures have helped in measuring ocular aberrations, known as higherorder aberrations (HOAs). Many studies found a correlation between reduced retinal image quality and these aberrations, especially spherical aberrations and coma.4 New strategies have been used to develop excimer laser profiles and improve postoperative visual quality. These include wavefront-guided and wavefrontoptimized profiles. Wavefront-guided laser treatment is a custom ablation based on HOAs unique to the 0886-3350/$ - see front matter http://dx.doi.org/10.1016/j.jcrs.2012.07.041
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treated eye. In contrast, wavefront-optimized profiles do not take into account the unique HOAs in each eye but use certain preprogrammed profiles based on population analysis.4,5 Wavefront-optimized laser ablations are designed to obtain the prolate shape of the cornea and limit the tendency toward creating oblate surfaces. This can be achieved by removing more tissue in the periphery of the ablation zone.6 Most studies report post-keratorefractive surgery haze using slitlamp observation and subjective scales. However, the objective measurement of corneal haze is better than subjective assessments in terms of accuracy and repeatability.7,8 Recently, subepithelial haze and stromal scarring after laser surgery have been quantified and assessed objectively using a Scheimpflug-imaging technique incorporated into different commercial units.8–10 The Pentacam (Oculus Optikger€ ate GmbH) is a noninvasive optical system that uses the Scheimpflug principle to take up to 50 cross-sectional images of the entire anterior segment within 2 seconds.11 The ability to measure changes in corneal transparency noninvasively and objectively may enable more precise monitoring of progression and may improve patient management. In the literature, only 2 studies used Pentacam Scheimpflug imaging to report corneal densitometry 12 months after photorefractive keratectomy (PRK)9,12 and 1 study reported corneal densitometry 12 weeks after LASIK.10 The latter study was published in Japanese, and we could not ascertain which technique was used to report corneal densitometry. Corneal densitometry is being regarded as a useful objective indicator of corneal health.13 The objective of this prospective study was to assess the refractive outcomes of wavefront-optimized LASIK treatments to correct myopia and myopic astigmatism, in particular to measure corneal densitometry 1 year after LASIK using the densitometry function of the Pentacam system. To our knowledge, this is the first study to report corneal densitometry 1 year after LASIK.
Submitted: March 24, 2012. Final revision submitted: July 17, 2012. Accepted: July 31, 2012. From the Division of Ophthalmology and Visual Sciences, University of Nottingham, Nottingham, United Kingdom. Corresponding author: Harminder S. Dua, MD, PhD, Division of Ophthalmology and Visual Sciences, B Floor, Eye Ear Nose Throat Centre, University Hospital, Queens Medical Centre, Nottingham, NG7 2UH, United Kingdom. E-mail: harminder.dua@nottingham. ac.uk.
PATIENTS AND METHODS This prospective study comprised eyes having LASIK for the correction of myopia and/or myopic astigmatism.
Preoperative Examination The patients had a complete preoperative assessment. This included refraction, uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), applanation tonometry, corneal topography (Pentacam), ultrasound pachymetry (SP 3000, Tomey Corp.), pupil size (Colvard infrared pupillometer, Oasis Medical Inc.), and a full fundus examination.
Surgical Technique All cases were performed by the same surgeon (H.S.D.) using the Wavelight Allegretto excimer laser system (Wavelight AG) and the following parameters: 193 nm with a 0.95 mm spot size, fluence 200 mJ/cm2, and repetition rate 200 Hz. A nasal hinged flap was created in all eyes with a single-use microkeratome (One-Use-Plus, Moria). The postoperative refraction target was emmetropia in all cases.
Postoperative Examination Patients were seen on the first postoperative day and then at 1, 3, 6, and 12 months. The postoperative data collected included refraction, UDVA, CDVA, corneal topography, contrast sensitivity, and Scheimpflug imaging (Pentacam). Contrast sensitivity measurements were performed at the 1-year follow-up using the Pelli-Robson chart.
Densitometry The automatic release mode of the Pentacam Scheimpflug system was used to determine when correct focus and alignment with the corneal apex had been achieved. This reduced operator-dependent variables associated with manual scanning. Imaging was performed using the 50-scans setting and was completed in 2 seconds. The Scheimpflug images were used to calculate corneal densitometry preoperatively and postoperatively using the caliper tools provided by the system's software. The density (densitometry) was measured. This allowed the operator to mark a point on the cross-sectional scan of the cornea. The central 7.0 mm area of the cross-section of the cornea along each of 4 axes (0 to 180 degrees, 45 to 225 degrees, 90 to 270 degrees, and 135 to 315 degrees) was defined manually. Densitometry was then measured automatically for the entire cornea (Figure 1, red line) and the anterior half of the cornea (Figure 1, purple line). The density was measured using the caliper tools by assigning the specified area on the cross-sectional scan of the cornea; the software calculated the densitometry value of that defined area. The mean and the standard deviation readings were then recorded. Details of the method of ascertaining densitometry and the normal densitometry values of human corneas have been published.13
Statistical Analysis Statistical analysis was performed with the SPSS for Windows software (version 19.0, SPSS, Inc.). Datagraph-med for
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There were no flap creation complications and no postoperative complications. Efficacy All eyes had 6/15 or worse UDVA preoperatively. At the 6-month follow-up visit, the UDVA was 6/5 or better in 23 eyes (62%) and 6/7.5 or better in all eyes. At 12 months, the UDVA was 6/7.5 or better in 36 eyes (97%) and 6/9 or better in all eyes. The UDVA was 6/6 in 34 eyes (92%) at 6 months and 12 months (Figure 2). The efficacy index was 0.95.
Figure 1. Scheimpflug image showing the densitometry of the total cross-sectional area (red line) and the anterior half of the cornea (purple line) at given axis.
Safety
Microsoft Office Professional software (version 3.90b, Datagraph-med) was used to analyze refractive and other visual outcome data and to generate standard graphs. The efficacy index (ratio of mean postoperative UDVA to mean preoperative CDVA) and the safety index (ratio of mean postoperative CDVA to mean preoperative CDVA) were also calculated. The correlation between the following parameters was performed using the Pearson productmoment correlation coefficient parametric test: (1) contrast sensitivity and entire corneal densitometry, (2) contrast sensitivity and anterior corneal densitometry, (3) entire corneal densitometry and mean CDVA, and (4) anterior corneal densitometry and mean CDVA.
No patient lost 2 or more lines of Snellen CDVA. At the 6-month follow-up visit, 32 eyes (86%) had no change in CDVA and 3 eyes (8%) gained 1 line or more. At 1 year, 32 eyes (86%) had no change in CDVA, 2 eyes (5%) gained 1 line, 1 eye (3%) gained 2 lines, and 2 eyes (5%) lost 1 line (Figure 3). The safety index was 1.02. Stability Figure 4 shows the change in the mean SE refraction before LASIK and 3, 6, and 12 months after LASIK. There was a general tendency to form a stable plateau over time.
RESULTS Twenty-two patients (43 eyes) were enrolled in this study. The mean age at surgery of the 14 women (63.6%) and 8 men (36.4%) was 37 years (from 20 to 70 years). Table 1 shows the preoperative and postoperative visual and refractive outcomes. The mean preoperative spherical equivalent (SE) decreased significantly 6 months and 12 months postoperatively (P ! .05, paired t test). At the 3-month follow-up visit, 4 patients (6 eyes) had unsatisfactory CDVA due to undercorrection. All other patients (37 eyes) completed the 6-month and 12-month follow-up visits. All 6 undercorrected eyes had LASIK enhancement. Data from these patients were used in the baseline statistics and dismissed when retreatment was indicated.
Predictability The postoperative SE in all eyes was within G1.00 D at the 6-month and 12-month follow-up visits. The SE refractive outcome was within G0.50 D in 34 eyes (92%) at 6 months and in 33 eyes (89%) at 12 months (Figure 5). Figure 6 shows the attempted versus the achieved SE 1 year postoperatively. Defocus Equivalent The defocus equivalent was used to eliminate the inequity between eyes that had similar SE refractions but different amounts of astigmatism. One year
Table 1. Visual and refractive outcomes over time. Postoperative Preoperative Parameter Sphere (D) Cylinder (D) SE (D) UDVA (logMAR) CDVA (LogMAR)
Mean G SD 2.97 G 1.34 0.67 G 0.56 3.31 G 1.39 0.92 G 0.15 0.09 G 0.07
3 Months
Range 6.02, 0.75 1.78, 0.00 6.27, 0.88 d d
Mean G SD 0.02 G 0.38 0.35 G 0.31 0.15 G 0.38 0.00 G 0.18 0.10 G 0.06
Range 1.00, 1.00 1.25, 0.00 1.13, 0.87 d d
6 Months Mean G SD 0.05 G 0.31 0.28 G 0.27 0.09 G 0.29 0.07 G 0.08 0.10 G 0.06
Range 0.50, 0.75 1.25, 0.00 0.63, 0.50 d d
CDVA Z corrected distance visual acuity; SE Z spherical equivalent; UDVA Z uncorrected distance visual acuity
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12 Months Mean G SD 0.07 G 0.33 0.33 G 0.29 0.09 G 0.27 0.07 G 0.09 0.10 G 0.07
Range 0.50, 0.75 1.25, 0.00 0.63, 0.62 d d
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Figure 2. Preoperative CDVA versus postoperative cumulative UDVA (Snellen) at 3, 6, and 12 months (CDVA Z corrected distance visual acuity; UDVA Z uncorrected distance visual acuity).
Figure 3. Lines of Snellen CDVA gained or lost over time (CDVA Z corrected distance visual acuity).
postoperatively, all eyes had a defocus equivalent refraction of less than 1.00 D and 31 eyes (84%) had a defocus equivalent refraction of 0.50 D (Figure 7).
contrast sensitivity or with CDVA. Table 2 shows the correlation data.
Contrast Sensitivity and Corneal Densitometry Although contrast sensitivity showed slight improvement 12 months postoperatively (mean 1.55 G 0.10 versus 1.57 G 0.09), the change was not statistically significant (P O .05, paired t test). There was no statically significant difference in the entire corneal densitometry between preoperatively (mean 12.72 G 2.43) and 12 months postoperatively (mean 13.04 G 2.58) (P O .05, paired t test). Also, there was no statistically significant difference between the preoperative anterior corneal densitometry (mean 14.71 G 2.21) and the 12-month postoperative anterior corneal densitometry (mean 15.08 G 2.13) (P O .05, paired t test). Corneal densitometry did not correlate with
DISCUSSION The Wavelight Allegretto is a multiple function system. It can be used to perform conventional, wavefront-optimized, or wavefront-guided LASIK treatments. The wavefront-optimized profile tends to preserve the asphericity of the cornea by ablating more corneal tissue in the periphery. This minimizes the induction of spherical aberrations.5,14 Several studies15–17 compared wavefront-guided treatments and wavefront-optimized laser treatments and found no significant difference in efficacy, safety, or predictability. In a metaanalysis, Fares et al.18 found no clear evidence of a benefit of wavefront-guided ablations over nonwavefront-guided ablations but suggested that wavefront-guided LASIK may be a better
Figure 4. Stability of the achieved SE over time (solid line). The dashed lines indicate G1 standard deviation of the distribution of the individual values.
Figure 5. Postoperative SE. The bars relate to the percentage of patients who achieved the SE indicated in the x-axis at 3 months, 6 months, and 12 months.
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Figure 6. Predictability. Attempted versus achieved SE 1 year postoperatively (solid line). The dashed lines indicate the G1.00 D range.
Figure 7. Preoperative refractive astigmatism versus 1-year postoperative refractive astigmatism.
choice in eyes with a root-mean-square value of 0.3 mm or higher. Furthermore, a recent metaanalysis19 concluded that both wavefront-guided LASIK and wavefront-optimized LASIK have shown excellent efficacy, safety, and predictability. However, wavefront-guided profiles tend to induce fewer HOAs than wavefront-optimized profiles postoperatively. In a study by Tran and Shah,17 100% in the wavefront-optimized group achieved a UDVA of 6/6 at 3 months. Stonecipher and Kezirian16 found that after wavefront-optimized treatment, 67% of eyes had a UDVA of 6/4.8 and 93% of eyes had a UDVA of 6/6. Our safety results are consistent with those of others, who report a good safety profile with wavefront-optimized laser ablations.15–17,20 To our knowledge, no study reported a loss of 2 or more lines of Snellen visual acuity. However, more attention should be paid to reporting changes in Snellen visual
acuity lines, as recommended by Reinstein and Waring21 and Waring22 and further noted by Fares et al.18 in their metaanalysis. Specific information on the gain and loss of CDVA Snellen lines would be a more accurate way of indicating safety. In our series, 5% of eyes lost only 1 line of CDVA; this loss is considered to be within he limits of normal biological variability.21 The mean preoperative SE decreased from 3.31 G 1.39 D to 0.15 G 0.38 D 3 months postoperatively and remained stable at 0.09 G 0.23 D 1 year after LASIK. Furthermore, 1 year after surgery, 100% of the eyes in our study were within G1.00 D of the intended correction and 89% were within G0.50 D. This is also consistent with results reported in other studies15,16,18 that found wavefront-optimized LASIK ablations have high stability and predictability profiles. The most important difference between wavefrontoptimized laser ablations and wavefront-guided laser ablations is that the former tend to address HOAs generated during surgery (especial spherical aberrations) and the latter deal with the preoperative HOAs. Mrochen et al.6 suggest combining the algorithm of wavefront-optimized treatments and individual wavefront-guided treatments. This would treat both preoperative HOAs and those generated during laser surgery. Results in a study that compared wavefront-optimized alone with a combined algorithm treatment14 found that eyes with preoperative HOAs of 0.2 mm could benefit from the combined treatment. Wavefront-guided profiles require ablation of more corneal tissue and create larger optical zone treatments. This may affect the corneal integrity and biomechanics. In addition, the time of the ablation
Table 2. Corneal densitometry, contrast sensitivity, and CDVA correlation analysis.
Parameter Entire corneal densitometry and contrast sensitivity Entire corneal densitometry and CDVA Anterior corneal densitometry and contrast sensitivity Anterior corneal densitometry and CDVA
Pearson Correlation Variance (R2) P Value 0.07
0.004
.70
0.10
0.009
.57
0.08
0.006
.65
0.02
0.001
.90
CDVA Z corrected distance visual acuity
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treatment in wavefront-guided procedures is longer than that in wavefront-optimized procedures, which may lead to inaccurate refractive outcomes due to changes in corneal hydration and eye movements during laser treatment.17 At present, it is accepted that contrast sensitivity gives a better indication of visual function than visual acuity measurements and can be of help in evaluating patients having refractive surgery.23 Currently, sinewave gratings are the gold standard for evaluating contrast sensitivity due to their sensitivity, specificity, and reproducibility. On the other hand, they are expensive and the test takes more time to complete.23,24 Pelli-Robson, which is a letter optotypes-based contrast sensitivity test, has been reported to be a quick and reliable method in clinical settings.25 Our results agree with those in other studies16,26,27 that did not find a statistical difference in the contrast sensitivity between before and after wavefront-optimized laser treatment. Flap creation is another factor that can induce further LOAs, HOAs, or both. Studies28–30 have found that femtosecond laser flaps induce less HOAs and provide better visual outcomes than mechanical flaps. However, Mu~ noz et al.31 found no difference between mechanical microkeratome and femtosecond laser flap creation 4 years postoperatively. In our practice, a femtosecond laser is not available; therefore, all flaps were created with a single-use microkeratome. This provides sharp and smooth flap cutting, minimizing the risk for creating folds or wrinkles in the flap. We did not measure preoperative and postoperative HOAs, which is a limitation of the study but does not materially affect the inferences drawn. The Pentacam system simultaneously provides complete corneal pachymetry, corneal topography, and other metrics, such as densitometry.11,13,32 Corneal densitometry has been assessed, and the degree of subepithelial corneal haze and stromal scarring in refractive surgery patients has been quantified.9,10,12 Most studies used slitlamp observation and subjective assessment to report haze after keratorefractive surgery. However, objective assessments are more accurate and more repeatable than subjective ones.8 The ability to measure changes in corneal transparency noninvasively and objectively may enable more precise monitoring of progression and improve patient management. The transparency of the cornea can be indicated by corneal densitometry because it provides information about the clarity of the cornea.13 The densitometry function of the Pentacam system provides an objective quantitative evaluation of light scatter in the cornea. It is standardized from 0 to 100, where 0 means the cornea had no clouding and 100 means the cornea is
completely opaque.11 We optimized the method of measuring and recording densitometry of the cornea using the Pentacam system.13 This method is reliable and reproducible and was used in the present study. To our knowledge, this is the first report using the Pentacam Scheimpflug image system to evaluate changes in the optical density 12 months after LASIK. In our previous paper,13 we found that the mean normal corneal densitometry was 12.99 G 2.58 when it was measured along cross-sectional areas; this was not significantly different from values recorded at specific points (mean 12.30 G 2.40). Our preoperative densitometry values (mean 12.72 G 2.43) were comparable to those in healthy controls reported in the crosssectional areas and specific points techniques. Cennamo et al.9 found a significant early increase in the mean anterior density (from 27.71 G 4.39 to 37.812 G 12.31) 3 months after PRK; this was followed by a significant decrease (26.291 G 4.93) 12 month postoperatively. They selected scans centered on the pupil and recorded the highest peak shown on the green bars at the right of the Scheimpflug image of the Pentacam screen. They analyzed the highest corneal densitometry peak values corresponding to the anterior cornea on the grounds that PRK is a surface ablation treatment. However, the green bars give information about corneal (and lens) densitometry along a line that intersects the cornea at a specific location. The peak value corresponds to 1 point of the anterior cornea only, which may or may not be representative of the area treated. For example, if the cursor is moved along the area of cornea under evaluation, the peak value changes. To extrapolate data obtained from only 1 central point of the cornea as representative of the entire area treated can be considered a weakness of their study. We therefore recommend performing enhanced densitometry analysis using the cross-section densitometry function, by which the mean densitometry values for the full thickness of the cornea or the delineated anterior half can be obtained and analyzed. In our study, it was more appropriate to measure the densitometry along cross-sectional areas covering the ablated zone (entire cornea and anterior cornea) because LASIK is a stromal ablation procedure. Although Bhatt et al.33 have reported corneal densitometry along the graft–host interface in patients who had deep anterior lamellar keratoplasty (DALK), we could not obtain accurate measurement of corneal densitometry along the flap interface only. This is because the LASIK flap interface is not as easily detected as the DALK interface on Scheimpflug imaging. One Japanese study10 found no statistical difference between LASIK and epithelial LASIK 4 weeks and 12 weeks after surgery. Unfortunately, a translation of this study was not available, so we are unsure about
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the technique they used to measure their corneal densitometry. Furthermore, normal preoperative and postoperative densitometry values are likely to be indicative of minimal scarring or haze. In our patients, there was no significant correlation between corneal densitometry and contrast sensitivity or CDVA. This could be due to the small number of patients or because we did not detect visible haze in any eye. The densitometry program of the Pentacam system can provide a useful objective measure of postoperative corneal haze and keratocyte response. The latter is likely to be reversible and will be reflected in the densitometry values over time. Permanent haze resulting from scarring is likely to be longer lasting. Thus, densitometry measurements will give refractive surgeons an objective measure of the corneal response and haze and enable monitoring of these over time. Further studies comparing corneal densitometry between wavefront-optimized laser ablations and wavefrontguided laser ablations are highly encouraged.
WHAT WAS KNOWN Wavefront-optimized LASIK has good efficacy and safety profiles. Only 2 studies used Pentacam Scheimpflug imaging to report corneal densitometry 12 months after PRK, and 1 study reported corneal densitometry 12 weeks after LASIK. WHAT THIS PAPER ADDS This is the first study to report corneal densitometry 1 year after LASIK. There was no significant change in entire and anterior corneal densitometry 12 months after LASIK. Corneal densitometry did not correlate with contrast sensitivity or CDVA postoperatively.
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First author: Usama Fares, MD Division of Ophthalmology and Visual Sciences, University of Nottingham, Nottingham, United Kingdom