The principal theory of laser refractive surgery is that the optical power of the eye can be changed by modifying the corneal curvature. Flattening a myopic cornea corrects nearsightedness, whereas steepening a hyperopic cornea corrects farsightedness.
The majority of human corneas are aspherical in shape. Changing the shape of the corneal surface through laser refractive surgery also alters its asphericity. The degree of asphericity in a cornea can be described using a conic constant, Q. A negative conic constant is referred to as a prolate surface with the highest curvature occurring at the apex, like at the tip of an egg. A positive conic constant is referred to as an oblate surface and resembles the bottom of an egg with the top of the curvature being flatter than the rest. The Q-value is similar to the shape factor used in corneal topography, but the sign convention used for the Q-value is the opposite of that used for the shape factor.
After conventional LASIK procedures, many previously myopic eyes experience increased spherical aberration. It has been suggested that this increase is the cause of reduced visual performance. According to this theory, the increase in spherical aberration arises from flattening the cornea, and because most natural corneas are prolate, the postsurgical increase in spherical aberration results from altering the natural prolate surface. Therefore, this theory continues that good-quality vision can best be achieved with a theoretically optimal conic shape on the anterior cornea. This hypothesis is at odds with the theory that the best vision can be accomplished by correcting defocus and higher-order aberrations of the entire eye on an individual basis with a customized shape derived from the patient’s wavefront measurement.
DISCUSSION
This study did not find a correlation between corneal Q-value and visual performance. There was no evidence that preserving preoperative corneal Q-value guaranteed better visual outcomes. An oblate cornea is as likely to produce high-quality vision after surgery as a prolate cornea. In addition, the amount and direction of change in corneal asphericity had no influence on visual acuity or contrast sensitivity. Most important, amount and direction of change in corneal asphericity were not predictors of improvement in visual functions.
The theory that managing Q-value during refractive surgery can affect outcome was based on the assumption that Q-value is closely related to spherical aberration.
Mathematical modeling of the relationship among corneal asphericity, Q-value, and corneal spherical aberration using a corneal refractive index of 1.377 and corneal curvatures ranging from 41.25 to 45.25 D. Q-value is the best fit conic shape of a cornea. It does not take the central curvature of the cornea into account. With a constant Q-value, one can find a range of magnitude of spherical aberration, depending on central corneal curvature. In addition, prolate cornea (negative Q-value) only relates to negative spherical aberration when the value is more negative than -0.53. Because alteration of the corneal curvature is inherent to refractive surgery, i.e., the curvature is steepened or flattened to correct vision, constant Q-value and spherical aberration are not likely to occur simultaneously. Furthermore, other published mathematical modeling has established that no single conic shape could maximize retinal image quality.
Whereas a correlation exists between Q-value and corneal spherical aberration, it is not a perfect correlation. To further demonstrate, imagine two glass eggs, one small and one large. Both have the same asphericity (Q-value), but the difference in the radius of their curvatures causes them to have different amounts of spherical aberration. That means that even if Q-value were successfully preserved after LASIK surgery, because the corneal curvature is modified, the alteration of corneal spherical aberration is inevitable. Furthermore, retinal image quality is the cumulative result of various ocular components, which also include the posterior corneal surface and the crystalline lens. Therefore, it is not surprising to find that correlation between corneal Q-value and visual performance is nil. This finding does not contradict the findings of theoretical and experimental studies on the impact of ocular spherical aberration on visual performance. On the contrary, our findings agree with previously published findings that spherical aberration significantly affects contrast sensitivity performance. However, one should not overlook the fact that spherical aberration is just one of many aberration terms and that any of these, whether alone or in combination, affect the quality of the retinal image.
Our study did not intend to examine the individual contribution of each HOA to visual performance. It is known that RMS evaluations of the impact of HOAs on vision do not correlate well with visual performance, especially when RMS is of low magnitude. It is also known that aberrations interact to increase or decrease visual performance. Because real eyes do not exhibit isolated HOAs, and because the possible relevant combinations are extensive, we did not attempt to study the impact of each different aberration on visual performance in normal populations. However, HOA variables were applied in multivariate analysis to isolate the effect of Q-value from spherical aberrations.
In theory, ocular aberrations have more direct impact on retinal image quality than corneal aberrations, but our visual performance data showed stronger correlation of visual performance with corneal HOA than ocular HOA. In a normal population, HOAs from different optical components tend to neutralize each other, which results in much smaller overall HOAs. In addition, wavefront-guided refractive surgical technology attempts to minimize postoperative ocular aberration. Consequently, the ocular HOAs are lower both before and after surgery than the corneal HOAs. The result is a data distribution for the ocular HOAs that is approximately half that of the corneal HOAs. To analyze this limited distribution of ocular HOAs statistically would require a larger sample size than that used to analyze corneal HOAs. A larger study population than ours would be likely to show more solid effects with the ocular than with the corneal aberrations.
The role of HOAs in visual performance has been studied extensively, and they have been shown to influence visual outcomes. Current outcomes of wavefront-guided LASIK procedures reinforce these published conclusions. In addition to enhanced best-corrected visual acuity, wavefront-guided LASIK treatments have produced improved contrast sensitivity and reduced night vision complaints in some patients after surgery, all of which further uphold the findings of this study, i.e., correcting the overall optical system is likely to provide better visual outcomes.
CONCLUSIONS
Findings presented in this article reinforce that better visual outcomes are most likely to be achieved with a customized shape that corrects the overall optical system. Corneal conic constant alone does not appear to be a determining factor in postoperative quality of vision. The human eye is a complex optical system, and the quality of the retinal image depends on the combined optical effects of many elements, including, but not limited to, the anterior and posterior of cornea and crystalline lens. Based on our population data and modeling, corneal surface curvature, which is expressed as Q-value, did not by itself influence the postoperative outcome on retinal image quality in a significant way.
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