Abstract: PURPOSE: To summarize the various values of asphericity in different notations and present how corneal asphericity, corneal curvature, and entrance pupil diameter influence the longitudinal spherical aberration of the anterior corneal surface. METHODS: After the conversion factors between the different asphericity notations were described, finite ray tracing through a conic section that models the anterior cornea profile was performed. The anterior cornea was given a range of curvatures and asphericities and a range of entrance pupil diameters. RESULTS: If the value of asphericity remains constant, longitudinal spherical aberration increases with the square of the entrance pupil diameter. If the pupil diameter remains fixed, the spherical aberration becomes a function of the value of asphericity, the refractive index, and the radius of curvature. If the refractive index, pupil diameter and asphericity are considered constant, the spherical aberration will decrease if the corneal surface flattens and increase as the cornea becomes steeper. In this way, with the same shape factor and with the same starting apical radius, longitudinal spherical aberration became a function of the surgically induced refractive change. With equal curvature, the longitudinal spherical aberration becomes negative if the surface is more prolate than perfect Cartesian oval; it will become positive if it is less prolate, spherical, or oblate. CONCLUSIONS: A conversion chart for corneal asphericity notations with the corresponding spherical aberration and a diagram reporting values of asphericity necessary to maintain the physiological value of the corneal spherical aberration after refractive procedures may be useful tools in corneal surgery.

Abstract: PURPOSE: When calculating the power of an intraocular lens (IOL) with conventional methods in eyes that have previously undergone refractive surgery, in most cases the power is inaccurate. To minimize these errors, a new IOL power calculation formula was developed. METHODS: A theoretical formula empirically adjusted two variables: 1) the corneal power and 2) the anterior chamber depth (ACD). From the average curvature of the entrance pupil area, weighted according to the Stiles-Crawford effect, the corneal power is calculated by using a relative keratometric index that is a function of the actual corneal curvature, type of keratorefractive surgery, and induced refractive change. Anterior chamber depth is a function of the preoperative ACD, lens thickness, axial length, and the ACD constant. We used our formula in 20 eyes that previously underwent refractive surgery (photorefractive keratectomy [n = 6], laser subepithelial keratomileusis [n = 3], laser in situ keratomileusis [n = 6], and radial keratotomy [n = 5]) and compared our results to other formulas. RESULTS: Mean postoperative spherical equivalent refraction was +0.26 diopters (D) (standard deviation [SD] 0.73, range: -1.25 to +/- 1.58 D) using our formula, +2.76 D (SD 1.03, range: +0.94 to +4.47 D) using the SRK II, +1.44 D (SD 0.97, range: +0.05 to +4.01 D) with Binkhorst, 1.83 D (SD 1.00, range: -0.26 to +4.21 D) with Holladay I, and -2.04 D (SD 2.19, range: -7.29 to +1.62 D) with Rosa's method. With our formula, 60% of absolute refractive prediction errors were within 0.50 D, 80% within 1.00 D, and 93% within 1.50 D. CONCLUSIONS: In this first series of patients, we obtained encouraging results. With a greater number of cases, all statistical adjustments related to the different types of surgery should be improved.

Abstract: PURPOSE: To determine corneal aberrometric values in a normal population. METHODS: Corneal topography from a group of 4340 patients was retrospectively evaluated. Exclusion criteria were applied in order to select a population with good visual acuity, no previous ocular surgery, no contact lens influence, and no corneal disease. Aberrometric analysis of the corneal wavefront derived from the topographical data of the selected population was performed. Topography from 500 patients (1000 eyes) was selected for Zernike analysis of the corneal surface, with coefficients derived up to the tenth order. RESULTS: Corneal surface aberration values related to different pupil diameter were determined. CONCLUSION: This preliminary study provided reference values for corneal aberrations in the normal population.

Abstract: PURPOSE: We describe a case of corneal warpage caused by 14 years of rigid gas permeable (RGP) contact lens wear. METHODS: The patient was refit with daily wear high water content soft contact lenses. RESULTS: After 6 months there were significant changes in both refraction and keratometry; computerized videokeratography showed that the corneal contour of both eyes had normalized. CONCLUSIONS: Knowledge of contact lens induced corneal warpage is limited; it is therefore nor possible to follow a standard protocol for managing patients with this corneal deformation. In the case described, it was possible to reestablish a normal cornea without completely suspending contact lens wear by changing from RGP to hydrophilic material.

Abstract: INTRODUCTION. Emmetropization of the eye may be affected by visual experience. In many kinds of animal species it is possible to alter the refractive state of the eyes by manipulating the visual input during early periods of development. It is however doubtful whether the results obtained in experimental animals will help us explain the genesis of human myopia. MATERIALS AND METHODS. We selected a group of patients who suffered from unilateral visual deprivation in their early life. We examined 13 adult patients who had a unilateral traumatic infantile cataract. Seven of these patients were aphakic. RESULTS. In all of these patients we found a greater axial length in the affected eye as compared to the uninjured eye. The difference ranged from 0.1 to 11.5 mm (mean 2.48) and was statistically significant (p = 0.0015). CONCLUSIONS. Our data support the view that in humans as well as in animals a severe visual deprivation in early life induces an axial elongation of the eyeball and that the refractive errors experimentally induced in animals by visual deprivation have a clinical counterpart in the human population. These data suggest that the model of the system's loss of control on the elongation of the eye may provide valuable information and a precise direction for research on etiology and control of myopia.

Abstract: BACKGROUND: In monofocal intraocular lens (IOL) implants we can sometimes obtain a certain degree of pseudoaccommodation. The use of a slight myopic astigmatism is proposed for postoperative refraction to achieve a significant increase in the depth of field and to reestablish a certain degree of pseudoaccommodation in pseudophakic eyes. METHODS: Eighty-one eyes with against-the-rule myopic astigmatism were evaluated in 50 patients who underwent IOL implant surgery. We selected patients with corrected monocular visual acuity greater than or equal to 20/30 and with a refraction ranging from +0.50 to -1.25 diopters sph, from -0.50 to -2.75 D cyl, ax from 55 degrees to 140 degrees. RESULTS: Monocular uncorrected distance visual acuity in 68% of eyes was 20/40 or better, while binocular uncorrected distance visual acuity in 84% of eyes was 20/40 or better and in 66% of eyes was 20/30 or better. Monocular uncorrected near visual acuity in 67% of eyes was J3 or better, while binocular uncorrected near visual acuity in 82% of eyes was J3 or better and in 68% was J2 or better. Sixty-four percent of patients never wear glasses for distance, while 34% use them occasionally and only 2% always wear them. Fifty-four percent of patients never wear glasses for near, 32% use them occasionally, while 14% always wear them. CONCLUSION: These data show that with a low against-the-rule simple myopic astigmatism (about -1.50 D cyl x 90) we can often offer pseudophakic patients a rewarding independence from glasses both for distant and near vision.

Abstract: It is often necessary to know the solution of two spherocylinders having oblique axes. Resolving obliquely crossed cylinders involves tedious trigonometric calculation; for this reason we are presenting a simple BASIC program for the rapid and accurate calculation with the help of a personal computer or a programmable calculator.

Abstract: BACKGROUND: After refractive surgery, there may be changes in the spherical and the cylindrical components of the ametropia. In astigmatism, there may be variations in both the power and the axis. The best way to describe change in astigmatism caused by surgery is by vectorial analysis, which indicates the changes in power and the axis. However, in clinical practice, this procedure is limited by the tedious calculations required to establish the oblique cylinder combinations. METHODS: We propose a computer program for the calculation, by vectorial analysis, of the refractive changes induced by surgery. The program allows for reference to refractive data or keratometric readings. When the refractive data are considered, it is also possible to obtain the transposition of the correction values from the spectacle plane to that of the cornea. Finally, the program enables the user to calculate the coupling ratio. RESULTS: Some selected surgical cases are discussed by way of example. CONCLUSIONS: We think that this program will be a simple and accurate means of evaluating the results of refractive and corneal surgery.