Corneal surface asphericity, roughness, and transverse contraction after uniform scanning excimer laser ablation.
OBJECTIVE
To examine the interaction between the excimer laser and residual tissue.


METHODS
Ten cadaveric porcine eyes with exposed corneal stroma and plastic test spheres underwent uniform 6-mm ablation with a scanning excimer laser. Corneal profilometry of the central 3 mm was measured with submicrometer resolution optical interferometry, before and after uniform excimer ablation. Eleven surface-marked eyes were photomicrographed before and after excimer ablation. Images were superimposed, and mark positional changes were measured.


RESULTS
Uniform scanning excimer laser ablation of the corneal stroma produces a significant central steepening and peripheral flattening in the central 3-mm of the diameter. The central 1-mm corneal curvature radius (r) decreased from r = 10.07 ± 0.44 (95% CI) to 7.22 ± 0.30 mm, and the central 2-mm radius decreased from r = 10.16 ± 0.44 to 8.10 ± 0.43 mm. Q values, measuring asphericity in the 2-mm radius of the central cornea, were significantly lower before than after ablation (-5.03 ± 4.01 vs. -52.4 ± 18.7). Surface roughness increased significantly from 0.65 ± 0.06 to 1.75 ± 0.32 μm after ablation. The central 2 mm of the stromal surface contracted by 2.21% ± 0.80% at a sustained temperature of 5°C. Ablation of plastic spheres produced no significant change.


CONCLUSIONS
The excimer laser interacts with the nonablated residual stromal surface in a characteristic fashion not seen with isotropic, inorganic material. Increases in asphericity, surface roughness, surface contraction, and stromal morphologic changes are supportive of this interaction. The surface changes demonstrated may be indicative of temperature-induced transverse collagen fibril contraction and stress redistribution, or the ablation threshold of the stromal surface may be altered. This phenomenon may be of increased importance using lasers with increased thermal load.
Department of Ophthalmology, University of Arizona, 1630 East University Boulevard, Tucson, AZ 85721, USA. sjmccafferty66@hotmail.com
Sean J
SJ
McCafferty
Jim T
JT
Schwiegerling
Eniko T
ET
Enikov
D000818:Animals; D003315:Cornea / Q000473:pathology* / Q000601:surgery; D003319:Corneal Stroma / Q000473:pathology / Q000601:surgery; D004195:Disease Models, Animal; D020731:Keratomileusis, Laser In Situ; D008954:Models, Biological; D009216:Myopia / Q000473:pathology / Q000601:surgery*; D011184:Postoperative Period; D013552:Swine