Corneal asphericity and visual function after radial keratotomy.

Corneal asphericity was measured before and after surgery in 81 radial keratotomy eyes. Asphericity was calculated between keratoscope rings 2 and 9, representing corneal diameters of 2.3 mm and 7.6 mm. Statistical methods were used to assess the relationship between postoperative asphericity and undilated, uncorrected visual acuity. Smaller radial keratotomy clear zones resulted in greater negative asphericity after surgery (R = 0.74, p < 0.0001). For eyes with smaller clear zones of < 4.0 mm, postoperative negative asphericity was high (mean -6.44). Eyes in this smaller clear zone range had better uncorrected visual acuity if they possessed higher levels of negative asphericity (Mann-Whitney U test, p < 0.0001). The study suggests that eyes with smaller clear zones gain a beneficial increase in depth of field as the degree of negative asphericity increases. The study offers this as one reason that radial keratotomy patients with residual refractive error often have better uncorrected visual acuity than expected.