Ocular higher-order aberrations features analysis after corneal refractive surgery

Refractive surgery was designed to optimize the optical performance of the eyes and has gained professional and patient acceptance in recent years. However, surgically induced increases in optical aberrations of the cornea have been reported for RK, photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK), demonstrating a major increase in coma and spherical-like aberrations.^1-3 Total and corneal aberrations (third-order and higher), mainly spherical aberration, showed a significant increase after LASIK myopia surgery. The main contribution was due to the increase of spherical aberration. Radial keratotomy and photorefractve keratectomy shift the distribution of aberrations from third order dominance (coma-like aberrations) to fourth order dominance (spherical-like aberrations).

To our knowledge, no study has compared the optical aberrations of the whole eye after PRK and LASIK surgery.⁴ There is only one published quantitative study on the changes in corneal wavefront aberration.⁵ In that particular study procedures did not used same amount of the transition ablation zone, which may have influenced the amount of wavefront aberration.⁶ However, some studies have shown that the first surface of the cornea and the internal optical system partially compensate for each other.⁷ We compared the results of PRK and LASIK using the same treatment algorithms, the same optical ablation zone and the same transition zone based on the whole optical system.

Previous studies on the human optical aberration showed that factors such as age and refractive state before surgery could interfere with aberrations.⁸ Therefore, we excluded these possible influences. All of the subjects chosen were under 35 years old due to the fact that wavefront aberrations of the eye increases with age.⁹ Subjects with refractive powers between -5.0 D and -6.0 D were chosen because previous reports showed the increase of optical aberration was more pronounced in patients with a higher magnitude of refractive correction.¹⁰

Patients
This prospective study included 50 consecutive patients who were scheduled for excimer laser operations from May to December of 2003 in the Tianjin Eye Hospital. Patients were eligible for inclusion if they had a best corrected visual acuity of 20/20, a refractive error correction between -5.0 diopters (D) and -6.0 D of sphere and less than -1 D of cylinder. Thirty-two eyes met the inclusion criteria in the study. The mean patients age was (24.254.91) years, ranging from 18 to 33 years old. The mean follow-up time was (3.460.81) months, ranging from 3 to 6 months. Written informed consent was obtained from each patient after a thorough discussion of benefits and known risks of the procedure.

Surgical technique
All of the surgical procedures were performed by the same surgeon under topical anesthesia (0.5% proparacaine), using the NIDK EC-5000 (NIDEK, Gamagori, Japan). The laser had an emission wavefront of 193 nm, a fixed pulse repletion rate of 30 Hz, and a radiant exposure of 100-140 mJ/cm² in the treatment plane. The ablation zone was 6.0 mm and the transition zone was 7.0 mm in both the LASIK and PRK eyes for corrections between -5 D and -6 D. Mechanically scraping the epithelium and an excimer laser was used to perform the Photorefractive keratectomy. The Automated Corneal Shaper (Chiron Vision ALK, USA) was used in the LASIK procedure. Surgical steps included creating an 8.0 mm diameter flap with a thickness of 160 µm with a nasal hinge, ablating the stromal bed and then repositioning the flap. Postoperatively, topical tobramycin 0.3% combined with fluorometholone 0.1% was applied four times daily for one month and then tapered over four months in the PRK group. In the LASIK group, it was applied four times daily for one week and then tapered over for one month.

Wave-front aberrations
Wave aberrations were measured by means of a subjective aberrometer (Su Zhou BriteEye Model WFA 1000, China) that has been described¹¹ and based on the principle of laser ray tracing that has been changed to a computer-monitor version.¹² The apparatus is comprised of a test channel, pupil-monitoring channel and record channel. The testing channel provides a green cross target on the retina via a movable aperture with 1 mm diameter. As the image of the aperture is moved from trial to trial among 37 locations within the subjects natural pupil, the cross shifts its retinal location as a function of the aberrations of the eye. The cross shifts were tracked by the subject with a cursor on a computer monitor in the recording channel. During the experiment a CCD camera monitored the position of the subjects pupil with a video monitor. The experimenter who moved a 3D translator, on which the subjects head rested, compensated any eye displacement relative to the optical axis of the system. Within the system a movable stage with two mirrors on the common path of a Badal system compensated for the subjects defocus. An active eye-tracking technique was used to align the center of the pupil to the optical axis.

Wavefront aberrations were measured with natural pupil. The pupil diameter was almost always greater than 6 mm due to the screen dimness (retinal illumination was less than 2 log td) and the room light was off during the experiment. Data from the few subjects whose pupil size was less than 6 mm in diameter were excluded from the analysis. Aberrations were measured preoperatively and 3 months postoperatively following PRK and LASIK. Each measurement was repeated three times per eye.

Data analysis
We used Zernike polynomials to process 37 wavefront data points obtained with the aberrometer. The Zernike polynomials were those recommended by Vision Science and its Application Standards Taskforce team.¹³ For each condition we have used the mean of three measurements by MatLab mathematics software to calculate 35 Zernike coefficients and the root mean squared wavefront errors (RMS) for each order aberration. The RMS is calculated simply by taking the square root of the sum of the squares of the coefficients for the chosen set of Zernike aberration. The coefficients for the first-order Zernike functions (C₁^-1 and C₁¹) and the defocus (C₂⁰) were excluded when calculating the RMS of the wavefront aberrations. Two-tailed t tests were used to analyze Zernike coefficients and RMS preoperatively, postoperative and for control groups. A P value less than 0.05 was considered statistically significant.

RESULTS

Visual acuity and refractive results
The mean uncorrected visual acuity (UCVA) before surgery was 0.110.86 and was raised to 1.070.27 after surgery. Best-corrected visual acuity was increased to at least 20/20 (1.0). There was only one exception, a patient who had undergone PRK surgery and had a BCVA of 20/30 due to epithelial haze. The mean preoperative spherical equivalent (SEQ) of (-5.410.41) D was significantly reduced to (-0.230.74) D after surgery, (-0.561.08) D in PRK (n=10) and (-0.080.48) D in LASIK (n=22).

Higher order aberrations
At 3 months, the mean RMS value of higher-order (3rd to 6th) aberrations was significantly increased compared with the corresponding preoperative value. Overall higher order aberrations increased from (0.550.26) µm preoperatively to (0.930.37) µm for PRK group (t=3.95, P=0.0002; 1.69 fold) and to (0.790.38) µm in the LASIK group (t=2.60, P=0.01; 1.43 fold) postoperatively. The differences were statistically significant. The RMS of each order also increased after surgery. S₃-S₆ aberrations increased significantly from preoperative to post-PRK and post-LASIK. S₃-S₆ was 0.48, 0.39, 0.38, 0.20 µm (by 1.82, 2.64, 2.14, 1.80 fold) post-PRK and increased to 0.37, 0.34, 0.29, 0.19 µm (by 1.42, 2.31, 1.65, 1.74 fold) post-LASIK. Compared with preoperative values, the differences were of statistical significance (Post-PRK t=4.91, 5.9, 4.37, 3.17, P<0.002; Post-LASIK: t=1.98, 5.25, 3.28, 3.49, P<0.05).

Zernike coefficients
Preoperative and postoperative Zernike coefficient values are given in Table. The statistically significant differences were C₄⁰, C₄⁺⁴, C₅⁺¹, C₅⁺³, C₅⁺⁵ and C₆⁺² for the PRK group and C₃^-3, C₄⁰, C₅^-5, C₅⁺⁵ and C₆^-2 for the LASIK group.

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Table. Preoperative and postoperative Zernike coefficient values

Comparison between Post-PRK and Post-LASIK
RMS values of high-order aberrations in the PRK group were higher than those in the LASIK group (Fig.), but these differences are not of statistical significance. After further analysis, we found that the higher the order of aberration is, the smaller the RMS values are. Zernike coefficients 6, 18, and 19 (C₃^-3, C₅⁺¹, C₅⁺³ and C₅⁺⁵) were statistically significant when comparing the PRK and LASIK groups. C₃^-3 in LASIK was higher (t=-2.11, P<0.05) and C₅⁺¹, C₅⁺³ was lower (t=2.07, P=0.04; t=2.77, P=0.009 respectively) than those in the PRK group.

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Fig. Comparison of 3rd and higher order aberration post PRK and LASIK.

DISCUSSION

The success of modern refractive surgery should not only be the operation itself, but also the optimal visual performance after the surgery. Patients should obtain clear vision under various intensity of illumination and various contrasts after surgery. In addition, customized ablation and obtainment of supernormal vision is dependent on the knowledge of wavefront aberrations.

The results of this study demonstrated that both PRK and LASIK increased the total amount of wavefront aberrations especially for the higher order aberrations. The aim of refractive surgery is to help patients achieve ideal visual effect without the aid of spectacles or contact lens. Conventional refractive surgery may correct low-order aberrations such as defocus but it ignores high-order aberrations. Higher order wavefront errors (S₃-S₇) are considered to have a highly significant correlation with best-corrected visual acuity (BCVA) and the quality of vision.¹⁴

We used S₃ (third order component of the wavefront aberration) to represent coma-like aberration. S₄ represented spherical aberration and S₅ and S₆ were the fifth and sixth order components of wavefront aberrations respectively. Our results showed that the spherical aberration increased most significantly after PRK and LASIK surgery as other studies have reported.^2,4,15

The increase of aberrations after refractive surgery may be correlated with the change of corneal shape throughout the surgery. Normal cornea is non-spherical and can partially compensate for aberrations caused by other optical systems. Both PRK and LASIK reduce the central areas curvature by reshaping the front surface of the cornea. As a result, the corneal periphery is steeper than the central area and rays near the optical axis focus behind peripheral rays. Therefore aberrations especially spherical aberration increased. Liang et al¹⁶ found that aberrations greater than the fourth order did not seriously degrade image quality in normal eyes when the pupil diameter was small. However, such higher order aberrations had a remarkable effect on retinal image quality for dilated pupil and could influence visual performance as well as the resolution of the image on the retina. A number of patients complained about glare and poor night vision after refractive surgery despite the fact that visual acuity had been raised. The increase of aberrations after surgery could possibly explain this phenomenon. When the pupil is dilated, especially at night, an increase in aberrations can reduce the contrast sensitivity of images on the retina and influence their visual performance. Coma-like aberration is not sensitive to the change of pupil size. In addition, the increase of aberrations is perhaps correlated with the conversion of biodynamic and the healing of the corneal cut.^17,18

RMS corresponding to S₃ and S₅ in the two groups differed slightly, with those in the PRK group being higher than those in the LASIK group.

Our study showed that the creation of a lamellar flap after LASIK might affect contour and higher order aberrations. When we compared the LASIK with the PRK group, the difference of RMS was not statistically significant. However, we found a more significant increase in terms of Zernike coefficients 6, 7 and 8 in the LASIK group than in the PRK group, and a more significant increase in coefficient 18, 19 and 20 in the PRK group than in the LASIK group. In the standardized double-indexing scheme these coefficients are terms C₃^-3, C₃^-1, C₃¹, C₅⁺¹, C₅⁺³ and C₅⁺⁵.¹⁵ The reason for C₃^-3, C₃^-1, C₃¹ being higher in the LASIK group may be related to the edge or the root of the flap with LASIK or eccentricity caused by instability of observance, since the fixed light could be burred after the flap is left during the procedure. We postulated that this was caused by the placement of the hinge. The C₅⁺¹, C₅⁺³ and C₅⁺⁵ correspond to secondary coma, which appeared higher in the PRK group. Whether they were concerned with slight irregularity of corneal surface needs further researches although histological finding showed more wound healing activity and marked keratocyte responses to laser stromal ablation in cornea treated with PRK than in those treated with LASIK.¹⁹

The knowledge of wavefront aberration is of great clinical significance. The findings of this study showed that total wavefront aberrations after LASIK surgery were not higher than those after PRK surgery. Only some types of aberrations increased postoperatively. Previously it had been thought that the corneal flap created in LASIK might cause aberrations to increase. Therefore new operation designs had been proposed that first corrected aberrations after the flap was made then treated with an excimer laser. Further investigation is needed to determine whether the healing process or the corneal flap causes the increase of aberration after refractive surgery. There has been speculation that aberrations after PRK surgery were probably correlated with epithelial haze. Our research discovered that the change of aberrations in patients who had epithelial haze was not notable. However, further research is necessary.

In conclusion, excimer keratectomy (PRK and LASIK) can change the optical system of human eyes with the result of correcting refractive errors. But at the same time, it can increase the wavefront aberrations and influence visual performance because the compensations between various optical interfaces are disturbed during surgery. The change of aberrations induced by refractive surgery mainly consists of spherical aberration, but the increase of higher order aberrations differs with various surgeries. Our study showed PRK induced an optical aberration prominently in higher order aberration compared to LASIK, which may have been induced by wound healing.

It is exciting that people are beginning to design wavefront-guided techniques and non-spherical ablation. Presently this technique still needs further research and refinement.^20,21 There are many problems which need to be solved. For example, the extent of influence on accommodation and pupil size on aberrations^22,23 and the relation between wavefront aberrations and visual acuity.^24-26 Basic data regarding waterfront aberrations in relation with each procedure are useful for designing of customized waveguide ablation refractive surgery and to minimize the optical side effects.^27,28 We hope that refractive surgery will step into a new era with the advancement of research and the solutions of technical problems.

Acknowledgments: We thank Reshma Shah and Amita Patel OD (Nova Southeastern University, FL, USA) for attentive correction of the language and providing generous assistance.

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