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G-Y Yoon, J Porter, S MacRae, IG Cox, CJ Roberts, DR Williams; Changes In The Eye's Aberrations After Cutting A Corneal Flap . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2087.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To study the changes in the monochromatic wave aberration induced by the process of cutting a corneal flap in a sample of normal human eyes. A better understanding of how the corneal biomechanics are altered by the microkeratome incision could improve the predictability of conventional and customized LASIK procedures. Methods: A corneal flap was cut using a 180 µm Hansatome in only one eye of 17 patients and no subsequent laser ablation was performed for two months. Patients had a spherical refraction between -0.75D and -6.75D, a refractive astigmatism ≤ -2.50D, and ranged in age from 30 to 50 years. A wavefront sensor (B&L Zywave) was used to measure the wave aberration for each patient before the flap cut, immediately after the cut, and 1-day, 1-week, 1-month and 2-months post-flap cut (6 mm pupil). Anterior and posterior corneal surfaces were measured using a corneal topographer (B&L Orbscan II). Zernike coefficients for the cornea were calculated based on each corneal surface's elevation data. Each patient then received a conventional LASIK ablation in both eyes and the same measurements were performed at 1-day, 1-week, 1-month and 3-months post-LASIK. The average attempted correction was -2.46D sph, -0.69D cyl. Results: On average, the rms of the higher order aberrations in the delayed ablation eyes increased at the 2-month post-flap cut visit and again at 3-months post-LASIK. Patients showed a slight tendency toward a hyperopic shift and spherical aberration had a small, systematic increase, on average, at 2 months post-flap and 3 months post-LASIK. However, there was an extremely wide variation in the response of individual Zernike modes across patients after cutting a flap. Conclusion: Cutting a corneal flap with a microkeratome increases the eye's higher order aberrations in a non-systematic fashion. Though there is a slight tendency for spherical aberration to increase across patients, the vast majority of spherical aberration induced in post-LASIK patients is due to the laser ablation and not the flap cut. In addition, some compensatory aberration changes were observed in the posterior cornea. However, most aberrations induced by the flap cut arise from changes in the anterior cornea, due to the large refractive index difference between the cornea and air. These changes limit the accuracy of a higher order correction that could be obtained with a customized LASIK procedure.
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