Role of clearance and treatment zones in contrast sensitivity: Significance in refractive surgery

Purpose: To evaluate the relationship between contrast sensitivity, surgica l treatment zone, and clearance (ablation or clear zone - pupil diameter) i n photorefractive keratectomy (PRK) and radial keratotomy (RK). Setting: Saint Louis University Eye Institute, St. Louis, Missouri. and Hun keler Eye Center, Kansas City, Missouri, USA. Methods: Thirteen patients had PRK and 20, RK. Contrast sensitivity was mea sured with the Stereo Optical F.A.C.T. (F.A.C.T.) and VectorVision CSV-1000 (VV) charts. Pupils were measured with the Rosenbaum card. Results: In the PRK group, VV contrast sensitivity at 6 and 12 cycles per d egree (cpd) correlated with the ablation zone (r(2) = 0.18 and 0.22, respec tively), while visual acuity and F.A.C.T, contrast sensitivity did not corr elate, in the RK group, both VV and F.A.C.T. contrast sensitivity at 6 cpd correlated with clearance (r2 = 0.29 and 0.12, respectively). Pupils were l arger with the VV test than with the F.A.C.T. chart because ambient chart l uminance was less in the former. Conclusion: Contrast sensitivity is likely a more sensitive indicator of vi sual function than acuity in refractive surgery. The VV system unmasks aber rations from the transition zone of ablated and unablated cornea in PRK. La rger samples are needed to determine the critical ablation clearance of the pupil to avoid loss of visual function.