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 aberrations22,23 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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