Abstract
Abstract: :
Purpose: To characterize changes in lower and higher order optical aberrations following phototherapeutic keratectomy (PTK), and to assess the relative contribution of laser ablation efficiency and corneal biomechanics to these aberrations. Methods: Three normal cats (felis cattus) underwent PTK 100 microns in depth, over a 6mm optical zone, using a VISX laser. A PMMA rod with a radius of curvature similar to that of the cat corneas received the same ablation. Each cat's optical aberrations were measured using a modified Shack–Hartmann wavefront sensor in the awake behaving state 2, 4, 8 and 12 weeks post surgery. Individual Zernike terms were computed through the 5th order over a 6 mm pupil for the PMMA rod and 6–9 mm for cat corneas. The amount of material removed from the PMMA rod was precisely measured to calculate the loss of laser ablation efficiency and the optical aberrations induced by this loss. Results: In cats, PTK induced a significant change in defocus, coma and spherical aberration (SA), but left all other aberrations unaffected. Coma decreased by ∼50% when corrected for ablation decentration. Measurements in plastic revealed an 8.7% loss of laser ablation efficiency from center to periphery of the ablation. Theoretical modeling predicted that this loss of efficiency would cause a 1.15 D hyperopic shift and ∼ 0.01 micron decrease in SA for a 6mm pupil. Cats showed a 3.23+1.61 D hyperopic shift, but no significant change in SA for a 6 mm pupil. However, there was a massive increase in positive SA to 1.5 + 0.88 microns for a 9 mm pupil, accompanied by a further increase in hyperopic defocus to 4.88 + 2.41D. Conclusions: While PTK induces changes in defocus and SA in both materials, corneas exhibit more than double the defocus change seen in plastic over the ablation area. In addition, corneas show increased coma, which is partially due to decentration of the ablation. Outside the ablation zone, corneas exhibi a further increase in hyperopic defocus and a massive increase in positive SA. The selective increase in SA outside the ablation zone may be due to the presence of an inflection point at the edge of the ablation profile. Taken together, these observations suggest a central flattening and peripheral steepening of the cat cornea in response to PTK.These changes are significantly different from those predicted solely by a loss of laser ablation efficiency.
Keywords: refractive surgery: comparative studies • refractive surgery: optical quality • refractive surgery: complications