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Wavefront metrics poorly predict subjective post-LASIK visual quality
J Cataract Refract Surg. 2009;35(5):846-855.
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Post-LASIK wavefront error measurements showed a weak relationship with subjective quality of vision.
Low theoretical visual quality did not always correlate with low subjective visual quality, the study authors said.
“These findings suggest that there are different individual tolerance thresholds for wavefront aberrations, causing visual disturbances or simply ‘bad vision,’” the authors said. “Potential reasons are different benchmarks and expectations of the patients. … These results … imply that post-LASIK wavefront aberrations should be kept as low as possible to yield a high subjective quality of vision.”
The prospective study included 56 eyes of 29 patients who underwent myopic LASIK. The group included 51 eyes of 26 patients who had uneventful LASIK, and five eyes of three patients who had re-treatment for post-LASIK symptoms. At 1-month postop, patients underwent aberrometry and wavefront sensing and were administered a questionnaire to assess subjective visual quality under photopic, high mesopic and low mesopic lighting conditions.
The predictive power of all metrics among the three lighting conditions showed no significant discrepancies. The accuracy of wavefront metrics in predicting subjective quality of vision ranged from 0% to 24%, the authors said.
Low subjective vision ratings stemmed from residual refractive error and coma, they said. Postoperative retinal image quality should be maximized to yield good subjective visual quality.
The take-home message of the paper is that good subjective results for patients also can be measured by objective measures of optical quality; in this case, VSOTF (visual Strehl ratio based on optical transfer function), which is a “metric” or way of objectively measuring optical quality (in this case, “retinal image quality”). This seems intuitive, but there have been few studies looking at this in depth. The study simply stated, “good (higher and lower order) optics = good subjective vision.”
The study found that the newer optical quality metrics such as VSOTF are better predictors of visual satisfaction than the commonly used metric of RMS, which does not take into account aberration interaction or the stronger effect of certain aberration, namely coma, compared to other less disruptive aberrations such as pentafoil. Although metrics such as VSOTF are helpful in predicting visual quality and performance, they are not perfect predictors of visual satisfaction.
Another take-home point is that lower-order aberrations, namely residual sphere and cylinder, are important, but they become even more important when one looks at the effect of these aberrations in conjunction with the higher-order aberrations, such as coma and spherical aberration. Some argue that customized correction is not important, but this article stresses the importance of attempting to correct sphere, astigmatism and higher-order aberrations, namely customized or “wavefront driven” ablation. This affects clinical practice by demonstrating the practical importance of a newly developed visual metrics such as VSOTF, which can be validated subjectively.
– Scott M. MacRae, MD
OSN Optics Board
Member