RGP contact lens correction remains the optical correction of choice for keratoconic eyes. RGP corrections are effective at improving visual performance in keratoconic patients, as seen here with all eyes in this study having monocular entrance acuity of 0.30 logMAR (20/40 Snellen equivalent) or better. This level of visual acuity allows an individual with keratoconus to perform routine visual tasks such as read a newspaper or work on a computer.
While it is tempting to examine the elevated RMS levels directly to assess visual performance loss, previous studies have shown that the manner in which these aberrations interact is crucial to understanding resultant retinal image quality.14 Here, this is studied through the use of metrics. As seen in Figures 2 and 3, retinal image quality metrics determined from residual aberrations are correlated to visual performance measures.
Visual performance and higher order aberrations in RGP-wearing keratoconic eyes are compared to values for normal eyes. This is done in an attempt to examine the visual potential of keratoconic eyes. Given that a keratoconic eye generally has many years of normal vision before any significant loss, it is reasonable to assume that correcting ocular aberrations will improve visual performance. One clinical method that could be employed to compensate for residual low order aberrations would be to continue to fit different RGP contact lenses until the residual spherocylindrical corrections do not improve visual performance. Spectacles might also be worn over the RGP contact lenses. Either of these low order correcting techniques might be employed here with success for eyes K1 and K3, both of which display improved logMAR VA in the presence of an over-correction. However, Table 3 demonstrates that with either mode of correction (RGP alone or RGP + spectacle over-correction), three of the seven eyes still had reduced logMAR VA as compared to normals. Table 4 demonstrates that RGP performance alone leaves behind important high order optical aberrations in keratoconus that cannot be corrected by an over-refraction.
It is possible that high order aberrations induced by the anterior surface of the cornea are partly corrected by index matching. However, a traditional RGP would provide no such index-matching correction for higher order aberrations originating from the posterior surface of the cornea or the crystalline lens. Correction of any higher order posterior corneal or crystalline lens aberrations would require the specific introduction of a compensating aberration structure in the RGP correction. This type of wavefront-guided RGP correction is currently not available.
These data suggest methods should be sought to further reduce the impact of uncorrected aberration in the keratoconic RGP-eye system. Possible methods for achieving better correction of aberrations include customized contact lenses incorporating a wavefront correction. Such technology is under development at several centers worldwide and has been eye to a number of patent applications. Wavefront guided spectacles over the RGP correction may also become an option in some cases.
While the present study reports on only a small sample of keratoconic eyes (six moderate, one severe), the sample illustrates the large differences in visual performance and higher order aberrations that exist between RGP-corrected keratoconic eyes and normals and the relationships between visual performance and optical quality metrics. Metrics relating residual ocular aberration to visual performance may become increasingly useful in understanding the impact of aberration on visual performance for complicated clinical eyes as well as prospectively designing corrections. These results support the growing evidence that highly aberrated eyes will benefit from a custom correction that accounts for the aberrations of that individual eye.31–37
This work was supported by NIH/NEI T32 EY07024 (to JDM), NIH/NEI RO1 EY05280 (to RAA), NIH/NEI P30 EY007551 (to UHCO), NHMRC NHF Fellowship 0061 (to KP), NIH Loan Repayment Plan (to KEP).
The authors would like to thank Dr. Maija Mantyjarvi for making the raw data for normal PRCS available, the UHCO clinic and The Texas Eye Research and Technology Center (TERTC) for their assistance in recruiting eyes, Dr. Larry Thibos for providing the computer code for calculation of the optical quality metrics, and Kim Thompson for assistance.
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