Effects of photorefractive keratectomy and cataract surgery on ocular optical errors of higher order.

BACKGROUND

This pilot study was carried out to assess the effects of photorefractive keratectomy (PRK) for myopia and myopic astigmatism and cataract surgery on the ocular optical aberrations of higher degrees.

METHODS

The optical aberrations were measured in 12 patients before and after PRK and in 10 patients after cataract surgery with a video aberroscope for clinical use (based on Tscherning's aberroscope) designed by the authors. To characterize the optical performance of the eye the deviation of the wavefront of a foveal image point from its ideal (spherical) shape (wavefront aberration) was determined. The wavefront aberration is represented mathematically in Zernike polynomials. The first 14 Zernike coefficients K(i) were determined and compared with data from normal eyes with full visual acuity.

RESULTS

Most Zernike coefficients were considerably greater after PRK than before surgery. These changes differed significantly from the variability of repeated individual measurements. In particular, coefficients corresponding to astigmatism, spherical aberration or coma were highly significantly increased (P<0.001). After cataract surgery, the averaged Zernike coefficients exhibited no significant differences from normal values, except the coefficient K(5) (astigmatism at 0 degrees or 90 degrees ). However, coefficients showed a significant high variability, especially the coefficients for spherical aberration or astigmatism.

CONCLUSION

Both PRK and cataract surgery are operations which may considerably increase the ocular optical errors of higher order. These aberrations are not predictable and can affect the visual acuity despite optimal sphero-cylindrical correction, in particular under mesopic conditions.