Describing the corneal shape after wavefront-optimized photorefractive keratectomy

Tim de Jong, Robert H J Wijdh, Steven A Koopmans, Nomdo M Jansonius
Optometry and Vision Science: Official Publication of the American Academy of Optometry 2014, 91 (10): 1231-7

PURPOSE: To develop a procedure for describing wavefront-optimized photorefractive keratectomy (PRK) corneas and to characterize PRK-induced changes in shape.

METHODS: We analyzed preoperative and postoperative corneal elevation data of 41 eyes of 41 patients (mean [±SD] age, 38 [±11] years) who underwent a myopia-correcting (preoperative spherical equivalent refraction -3.9 [±1.5] diopters) wavefront-optimized PRK, assessed with Scheimpflug imaging. Zernike polynomials were fitted to the elevation data. The diameter of the greatest corneal zone that could be analyzed and the number of Zernike orders needed to make a reliable fit were determined, based on the fitting error. The PRK-induced changes in corneal shape were studied.

RESULTS: All subjects could be included for analysis of zones up to 6 mm in diameter. This was 90, 83, and 44% for 7, 8, and 9 mm, respectively. The number of Zernike orders (terms) needed for an accurate fit ranged from four (15) for a 4-mm analyzed zone diameter to eight (45) for 8 mm. For the anterior surface, the spherical aberration term decreased significantly (p < 0.01) after PRK for the 4-mm analyzed zone diameter (-0.09 μm), remained unchanged for 5 mm (+0.01 μm; p = 0.56), and increased significantly for 6 mm (+0.40 μm) and above (p < 0.01). There was no significant change in any other higher-order Zernike term, but the square root of the sum of the squares of all higher-order terms increased significantly for all analyzed zone diameters. Photorefractive keratectomy did not influence the posterior corneal shape.

CONCLUSIONS: The cornea after wavefront-optimized PRK can be described as accurately as a healthy cornea and with a similar number of Zernike polynomials. The anterior corneal shape changes significantly after treatment, whereas the posterior corneal shape remains unchanged. The observed effect of wavefront-optimized ablation on the spherical aberration term depends on the diameter of the analyzed zone.

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