1. Correlation Between Corneal Biomechanics and Wavefront Aberrations Alejandro Rivera, MD Private Practice Mexico City The author has no financial interest in any material or method mentioned Alejandro Rivera, MD Private Practice Mexico City The author has no financial interest in any material or method mentioned

2. Purpose  The cornea is one of the main refractive structures of the eye and therefore its contribution to the presentation of wavefront aberrations is critical. There is no complete knowledge of the origin of higher-order aberrations. The influence of intraocular pressure (1), corneal thickness (2), eyelid pressure (3), tear film (4), age (5), etc. has been studied. Altered biomechanical parameters in the cornea as a result of refractive surgery can increase ocular image errors. The purpose of this study was to know if in the normal cornea there is a direct correlation between changes in the biomechanical parameters and the presentation of wavefront aberrations.

3. Methods  The design was an observational series of patients without history of ocular abnormalities. 230 eyes of 115 patients were retrospectively evaluated. The data were taken from files of patients evaluated for refractive surgery. The inclusion criteria were patients of age 18 to 35 years, without clinical signs of keratoconus or other ectatic diseases, no contact lens use, no ocular pathology. The root-mean-square (RMS) wavefront error of the combined coma-like aberrations was measured by an Optical Path Difference Scan II ARK-10000 (Nidek Corporation, Hiroishi-Cho, Japan). The Ocular Response Analyzer (Reichert Corporation, Depew, USA) was used to determine the corneal hysteresis (CH). The relationship between higher-order wavefront aberrations and a biomechanical parameter such as corneal hysteresis was statistically analyzed using the Analise-it for Microsoft Excel program (Version 2.07, Analise-it Software, Ltd. Leeds, UK).

4. Results  Using the Pearson Correlation Coefficient a negative correlation was found between RMS Coma and corneal hysteresis (r = - 0.27, 2-tailed p < 0.0001).

5. Conclusion  Anterior corneal aberrations vary greatly from patient to patient (6) and little of this is attributable to age, refraction, corneal thickness or intraocular pressure. Changes in higher-order aberrations after refractive or cataract surgery are well known and are believed to be a consequence of variations in factors such as corneal thickness (2) among other causes. Corneal hysteresis is a main biomechanical parameter and it could be considered as playing a role in the origin of wavefront aberrations. However, it was found in this study that in the normal eye there is a negative correlation between RMS coma and corneal hysteresis. The real sources of higher-order aberrations in the normal eye are still unknown (1).

6. References 1.Qu J, Lu F, Wu JX, Wang QM, Xu CC, Zhou XT, He JC. Wavefront aberration and its association with intraocular pressure and central corneal thickness in myopic eyes. J Cataract Refract Surg 2007; 33: 1447-1454. 2.Mierdel P, Krinke HE, Pollack K, Spoerl E. Diurnal fluctuation of Higher order ocular aberrations: Correlation with intraocular pressure and corneal thickness. J Cataract Refract Surg 2004; 20: 236-242. 3.Lieberman DM, Grierson JW, The lids influence on corneal shape. Cornea 2000; 19: 336-342 4.Montés-Micó R, Phil M. Role of the tear film in the optical quality of the human eye. J Cataract Refract Surg 2007; 33: 1631-1635. 5.Amano S, Amano Y, Yamagami S, Miyai T, Miyata K, Samejima T, Oshika T. Age-related changes in corneal and ocular higher-order wavefront aberrations. Am J Ophthalmol 2004; 137: 988-992. 6.Wang L, Dai E, Koch D, Nathoo A. Optical aberrations of the human anterior cornea. J Cataract Refract Surg 2003; 29: 1514-1521. 1.Qu J, Lu F, Wu JX, Wang QM, Xu CC, Zhou XT, He JC. Wavefront aberration and its association with intraocular pressure and central corneal thickness in myopic eyes. J Cataract Refract Surg 2007; 33: 1447-1454. 2.Mierdel P, Krinke HE, Pollack K, Spoerl E. Diurnal fluctuation of Higher order ocular aberrations: Correlation with intraocular pressure and corneal thickness. J Cataract Refract Surg 2004; 20: 236-242. 3.Lieberman DM, Grierson JW, The lids influence on corneal shape. Cornea 2000; 19: 336-342 4.Montés-Micó R, Phil M. Role of the tear film in the optical quality of the human eye. J Cataract Refract Surg 2007; 33: 1631-1635. 5.Amano S, Amano Y, Yamagami S, Miyai T, Miyata K, Samejima T, Oshika T. Age-related changes in corneal and ocular higher-order wavefront aberrations. Am J Ophthalmol 2004; 137: 988-992. 6.Wang L, Dai E, Koch D, Nathoo A. Optical aberrations of the human anterior cornea. J Cataract Refract Surg 2003; 29: 1514-1521.