Purpose
To evaluate the clinical outcomes of laser-assisted in situ keratomileusis (LASIK) surgery for the correction of high myopia using a new generation of excimer laser (500-Hz repetition rate) and optimized aspherical profiles.
Design
Retrospective interventional case series.
Methods
Retrospective study including 51 eyes of 32 patients (age range 23-61 years) with high levels of myopia or myopic astigmatism (spherical equivalent ≥8.5 diopters [D]). All cases underwent uneventful LASIK surgery using the sixth-generation excimer laser Amaris from Schwind and a femtosecond platform for flap creation. Postoperative changes in visual acuity and refraction were recorded and analyzed during a 6-month follow-up.
Results
A significant improvement of about 15 logMAR lines was observed in uncorrected distance visual acuity (UCDVA) at 3 months after surgery ( P < .01), with no significant changes afterwards ( P = .61). This improvement was consistent with a significant reduction of manifest refraction ( P < .01). Best corrected distance visual acuity (BCDVA) remained unchanged or improved in 98% of eyes at 3 months postoperatively, with only 1 eye losing 1 logMAR line of BCDVA. A similar distribution of BCDVA data was observed at 6 months postoperatively. A total of 84.3% of eyes had a postoperative spherical equivalent within ±0.50 D of emmetropia. A limited but significant induction of primary spherical aberration and coma was also found ( P < .01). LASIK enhancement was required during the follow-up in only 4 eyes (7.8%).
Conclusions
LASIK for high myopia using optimized aspherical profiles and the Amaris excimer laser is a safe, effective, and predictable procedure.
Twenty years ago the first LASIK (laser-assisted in situ keratomileusis) procedure was described and reported by Pallikaris in Greece. Nowadays, LASIK is one of the most common ophthalmic procedures performed all over the world because of its high rate of predictability and excellent visual outcomes. However, LASIK for the treatment of high levels of myopia is still currently a topic under discussion. The term “high myopia” is used by the American Academy of Ophthalmology to describe those myopic refractive errors that are above 6 diopters (D) of myopia. There are several reasons accounting for this controversy about the use of LASIK for high myopia, such as the very significant induction of higher-order aberrations attributable to the use of nonoptimized ablation profiles with the first generation of laser platforms or the relevant corneal biomechanical changes induced secondarily because of absence of a security criterion for tissue ablation and the uncontrolled levels of laser energy on the cornea surface. These limitations in the earliest stage of LASIK led to poor outcomes in terms of predictability and quality of vision in eyes with high myopia. In addition, long-term studies on cases operated on by LASIK with the first-generation excimer laser platforms demonstrated a limitation in the stability of this procedure over time for this type of refractive error, but keeping adequate levels of refractive correction.
The implantation of phakic intraocular lenses (pIOL) and clear lens extraction have also been described as alternatives for refractive correction in high myopia. Clear lens extraction was abandoned early because of the rate of intraocular complications in patients younger than 40 years old. Therefore, pIOLs and LASIK are currently the only 2 options widely accepted for the treatment of high levels of myopia. The current LASIK procedures for the correction of high myopia using sixth-generation excimer lasers are aimed at equalizing the refractive outcomes achieved with pIOLs, but with the advantage of avoiding intraocular surgical complications.
The aim of the current study was to evaluate the visual outcomes and the predictability of LASIK surgery in high levels of myopia using a sixth-generation laser platform and optimized aspherical ablation profile, and to demonstrate at the same time the positive effect of evolution in lasers and ablation profiles on the refractive outcomes achieved after LASIK in high myopia.
Patients and Methods
Patients
This is a retrospective consecutive noncomparative study evaluating 51 eyes of 31 patients with ages ranging from 23 to 61 years (mean age of 36.6 ± 10.2 years) that underwent uneventful primary LASIK surgery between March 2008 and October 2010 and with a postoperative follow-up period of 6 months. Inclusion criteria were eyes with high myopia discontinuing contact lens wear for at least 4 weeks before surgery, stable refractive error for 12 months before surgery, normal peripheral retina or treated with photocoagulation when necessary, a calculated postoperative corneal stromal bed thickness of more than 250 μm, and no previous ocular surgery, corneal disease, glaucoma, or history of ocular trauma. Exclusion criteria were keratoconus or keratectasia, active ocular or systemic disease that could affect corneal wound healing, and pregnancy.
Preoperative Examination
The preoperative examination included the following tests: uncorrected distance visual acuity (UCDVA); best corrected distance visual acuity (BCDVA); refraction (manifest and cycloplegic); slit-lamp biomicroscopy; applanation tonometry; ultrasonic pachymetry (OcuScan RxP; Alcon, Fort Worth, Texas, USA); scotopic, low mesopic, and high mesopic pupillometry (Procyon Pupillometer P2000SA, Procyon Instruments Ltd, London, UK); corneal topography (CSO, Firenze, Italy); and fundus evaluation. Corneal aberrometry was also recorded and analyzed with the CSO topography system, because this device has the capability to calculate directly this specific information. The software of this topographic system, the EyeTop 2005 (CSO, Firenze, Italy), automatically performs the conversion of the corneal elevation profile into corneal wavefront data using the Zernike polynomials with an expansion up to the seventh order. The primary coma root mean square (RMS) (computed for the Zernike terms Z 3 ±1 ) and the corresponding Zernike coefficient for primary spherical aberration (Z 4 0 ) were calculated for a 6-mm pupil in all cases.
Excimer Laser and Ablation Profile
LASIK procedures were performed using the Amaris excimer laser (Schwind Eye-Tech-Solutions, Kleinostheim, Germany). This system has a fast repetition rate of 500 Hz and incorporates 2 levels of fluence. A high fluence level is used in the first 80% of the treatment to speed up the procedure and for the remaining 20% of the treatment a low fluence level is used to ensure the smoothness of the ablated surface. A small beam size of 0.54 mm with a Gaussian ablative spot profile is delivered in a randomized flying spot pattern to reduce the successive overlapping of the laser spot and minimize the thermal load to the cornea. This laser platform has incorporated a 5-dimension high-speed eye tracker with an acquisition speed of 1050 Hz that tracks both the limbus and the pupil simultaneously with a reaction time of less than 3 ms. This platform also includes a high-resolution online pachymetry that provides information about the thickness of the cornea throughout the entire duration of the treatment. This allows the surgeon to know the amount of ablated cornea for a specific treatment at any time point of the surgical procedure and, therefore, the thickness of the stromal residual bed.
All treatments performed for patients included in the current series were based on optimized aspherical ablation profiles and calculated using the commercially available software ORK-CAM (Schwind Eye-Tech-Solutions). These optimized aspherical aberration-free profiles consider a focus-shift balance due to tissue removal and a compensation factor for the loss of efficiency when the laser hits the cornea in non-normal incidence, a phenomenon that is often referred to as the “cosine effect,” in order to avoid the induction of aberrations and to balance the aberrations that are present in the treated eye. With the same purpose, a multidynamic aspherical transition zone is always created.
Surgical Technique
All surgical procedures were performed at Vissum Instituto Oftalmológico de Alicante by the same experienced surgeon (J.L.A.). First, the designed treatment with the ORK-CAM software was loaded into the excimer laser computer and reviewed by the surgeon to confirm the data. As previously commented, the Amaris excimer laser (Schwind Eye-Tech Solutions) was used to perform all the LASIK treatments. The corneal flap was created with the IntraLase femtosecond laser (60-kHz IntraLase femtosecond system; IntraLase Corp, Irvine, California, USA), using the following parameters: superior hinge, 9.5 mm of diameter, line and spot spacing of 7 μm pocket function in status “ON,” and flap thickness of 100 μm.
Ablations were centered on the corneal vertex using the pupillary offset; that is, the distance between the pupil center and the normal corneal vertex measured by videokeratoscopy (CSO, Firenze, Italy). The pupillary offset measurement was translated into the treatment plan as polar coordinates and then manually entered in the excimer laser computer. The optical zone of the treatment was selected according to the preoperative scotopic pupil size. Depending on the pachymetry, optical zones with at least the same diameter as the scotopic pupil were targeted in order to avoid uncomfortable optical effects. In the current series, the mean optical zone was 6.20 ± 0.23 mm (ranging from 6.0 to 6.8 mm) and mean ablation zone (automatically calculated by the system when the optical zone is selected) was 8.10 ± 0.28 mm (ranging between 7.5 and 8.7 mm). In all cases, the targeted postoperative refraction was emmetropia.
Retreatments
The criteria for retreatment included 1 of the following 3 parameters: 1) manifest spherical equivalent (SE) of −1.00 D or greater; 2) UCDVA of 0.3 or less; and 3) patient dissatisfaction with the visual result. Undercorrection was defined as an SE of −1.00 D or greater at the first postoperative visit. Regression was noted when a 0.50-D or greater myopic shift occurred between follow-up visits without the presence of a retreatment. LASIK retreatments were performed by lifting the flap and re-ablating the stromal bed with the Amaris excimer laser (Schwind Eye-Tech Solutions). After ablation, the flap was replaced to its original position and the interface was irrigated copiously. All enhancements were performed at least 3 months after the initial surgery. Postoperative visits after retreatment were excluded from the refractive analysis to avoid biased results.
Postoperative Follow-up
Patients were examined on the first postoperative day and at 1, 3, and 6 months after surgery. On the first postoperative day, a detailed slit-lamp examination was performed to evaluate the flap position and the integrity of the cornea. At the rest of the postoperative visits, UCDVA, manifest and cycloplegic refraction, BCDVA, anterior segment status (slit-lamp biomicroscopy), and corneal topography were evaluated. An independent observer performed all the postoperative examinations.
Statistical Analysis
Statistical analysis was performed with the SPSS statistical software package, version 10.1 for Windows (SPSS, Chicago, Illinois, USA). Normality of the data analyzed was confirmed by the Kolmogorov-Smirnov test. When parametric analysis could be applied, the Student t test for paired data was used for the comparison between preoperative and postoperative data, and also between postoperative consecutive visits. However, when nonparametric tests were needed, the Wilcoxon rank sum test was applied. Differences were considered statistically significant when the associated P value was less than .05. Correlation coefficients (Pearson or Spearman, depending on whether normality condition could be assumed) were used to assess the correlation between different variables.
The efficacy index was calculated as the ratio of the postoperative uncorrected distance visual acuity to the preoperative corrected distance visual acuity, and the safety index as the ratio of the postoperative corrected distance visual acuity to the preoperative distance corrected visual acuity.
Main Outcome Measures
Main outcome measures were UCDVA, BCDVA, manifest refraction, efficacy and safety indexes, and corneal aberrations.
Results
The Table summarizes the visual and refractive outcomes obtained in this study. Mean preoperative spherical equivalent was −8.66 D, ranging from −6.75 D to −13 D. Mean preoperative keratometry was 44.13 D, ranging from 39.96 D to 48.11 D. Fifty-one eyes were evaluated at 3 months after surgery and 29 eyes were evaluated at 6 months, with no statistically significant differences between the 3-month and 6-month results ( P ≥ .31). Four of the 51 eyes (7.8%) were retreated during the follow-up to eliminate the residual refractive error. All enhancement procedures were performed because of undercorrection. Preoperative SE in all retreated patients ranged between −8.00 and −12.5 D. Refractive outcomes of the preoperative and postoperative visits of the retreated eyes were excluded from the refractive analysis to avoid biased results. In addition, it should be considered that some 6-month dropout (26 eyes) visits were present as a consequence of irregular attendance of some patients to the postoperative visits. This is a common fact of retrospective studies without a strict program of visits, as the current series.
| Mean ± SD (Range) | Preoperative | 3 Months After Surgery | 6 Months After Surgery | P Value (Preoperative-6 Months) |
|---|---|---|---|---|
| Sphere (D) | −8.05 ± 1.12 (−6.00 to −12.00) | −0.15 ± 0.62 (-3.00 to 0.75) | −0.27 ± 0.95 (-3.00 to +1.00) | <.01 |
| Cylinder (D) | −1.24 ± 0.85 (0.00 to −3.00) | −0.27 ± 0.38 (0.00 to −1.50) | −0.36 ± 0.47 (0.00 to −1.50) | <.01 |
| Spherical equivalent | −8.66 ± 1.13 (−6.75 to −13.00) | −0.29 ± 0.69 (-3.50 to +0.50) | −0.42 ± 0.82 (-3.50 to +0.63) | <.01 |
| Defocus equivalent | 9.27 ± 1.28 (7.25 to 14.00) | 0.53 ± 0.76 (0.00 to 4.00) | 0.75 ± 0.87 (0.00 to 4.00) | <.01 |
| logMAR UCDVA | 1.64 ± 0.18 (1.30 to 1.78) | 0.10 ± 0.19 (0.00 to 1.00) | 0.11 ± 0.26 (0.00 to 1.30) | <.01 |
| logMAR BCDVA | 0.04 ± 0.13 (-0.08 to 0.52) | 0.03 ± 0.07 (-0.08 to 0.34) | 0.01 ± 0.04 (-0.08 to 0.15) | .58 |
Predictability
A large and statistically significant reduction of the spherical equivalent (SE) was observed at 3 months after surgery (P < .01), with no additional significant changes during the remaining follow-up ( P = .31) ( Figure 1 ). At 3 months, 84.3% (43/51) and 90.2% (46/51) of eyes had an SE within ± 0.50 D and ±1.00 D of intended correction, respectively ( Figure 2 ) . Similar percentages of eyes with an SE within ±0.50 D were found at 6 months in spite of the smaller sample size (69.0% ±0.50 D, 89.6% ±1.00 D). Undercorrection (SE larger than −1 D) was detected in 9.8% (5/51) and 10.3% (3/29) of eyes at 3 and 6 months, respectively ( Figure 3 ).
Regarding sphere and cylinder, a significant reduction of both parameters was observed at 3 months after surgery ( P < .01). No significant changes in these 2 refractive parameters were observed afterwards ( P ≥ .42). Mean defocus equivalent (DE) decreased significantly at 3 months ( P < .01) and remained stable during the remaining follow-up ( P = .73). A total of 68.63% (35/51) and 65.52% (19/29) of eyes had a DE equal to or below 0.50 D at 3 and 6 months, respectively ( Figure 4 ). DE was equal to or below 1 D in 90.20% (46/51) and 79.31% (23/29) of eyes at 3 and 6 months, respectively ( Figure 4 ).
Visual Outcomes
UCDVA improved significantly at 3 months (P < .01), with an average improvement of 15 logMAR lines. No significant changes in this parameter were observed afterwards ( P = .60). A total of 88.2% (45/51) and 58.82% (30/51) of eyes presented at 3 months postoperatively an UCDVA of 0.3 logMAR or better and of 0.0 logMAR or better, respectively ( Figure 5 ). Regarding the BCDVA, it did not change significantly during the follow-up (preoperatively to 3 months, P = .58; 3–6 months, P = .69). A total of 92.1% (47/51) and 86.27% (44/51) of eyes presented at 3 months postoperatively a BCDVA of 0.3 logMAR or better and of 0.0 logMAR or better, respectively. At 3 months after surgery, no eye lost more than 2 lines of BCDVA, 11. 8% (6/51) of eyes gained 1 or more lines, 86.2% (44/51) of eyes remained with the same BCDVA, and just 1 eye lost 1 line of BCDVA ( Figure 6 ). Similar percentages were observed at 6 months postoperatively ( Figure 6 ).
