Associations Between Lid Wiper Microvascular Responses, Lens Fit, and Comfort After One Day of Contact Lens Adaptation by Neophytes.

PURPOSE: To determine associations between lid wiper microvascular responses, lens fit, and comfort after 1 day of contact lens adaptation by neophytes. METHODS: Functional slitlamp biomicroscopy was used to image the microvascular network of the upper tarsal conjunctiva, lid wiper, and bulbar conjunctiva. Fractal dimension was obtained to represent vessel density. Ultra-high-resolution optical coherence tomography was used to image the lens edge and fitting characteristics, including lens movement and lens-induced conjunctival indentation. Ocular comfort was rated using a 50-point visual analogue scale (VAS). Forty-nine healthy subjects without a history of contact lens wear were recruited. A contact lens was then fitted in the right eye. Imaging was taken at baseline and 6 hr after lens wear. RESULTS: The changes of VAS comfort score were negatively related to the changes of the vessel density in the lid-wiper (R 2 =0.18, P =0.002) and bulbar conjunctiva (R 2 =0.13, P =0.009). However, the changes of VAS were positively related to the changes in vessel density of the tarsal conjunctiva (R 2 =0.11, P =0.02). The changes of ocular microvasculature were not related to the objective metrics of the lens-fitting characteristics ( P >0.05). Similarly, the changes in the VAS comfort score were not related to the objective metrics of the lens-fitting characteristics ( P >0.05). CONCLUSION: Contact lens discomfort seemed to relate to lid wiper microvascular responses rather than fitting characteristics after 1 day of contact lens adaptation by neophytes.

Objective refraction from monochromatic wavefront aberrations via Zernike power polynomials.

With the increased popularity of wavefront sensor devices there has been significant interest in relating the subjectively measured sphero-cylindrical refractive error to the objectively measured wavefront aberrations. We investigate the applicability of four different measures for objective refraction assessment that are derived from a set of estimated Zernike wavefront coefficients. Two of the measures are based on the second- and higher-order Zernike polynomials while the other two measures are based on the estimated dioptric power representations. For the latter, closed-form expressions for dioptric powers based on the estimated focal length and the wavefront curvature are derived. We call these two new representations the 'refractive Zernike power polynomials' and 'curvature Zernike power polynomials'. Data from 120 eyes from 60 normal subjects were used to assess the correlation between the subjective and objective refractions. The results indicate that the objective sphero-cylindrical refraction calculated from the estimated refractive power map via the Zernike power polynomials is superior to the other considered representations and achieves best correlation with subjective sphero-cylindrical refraction.

Computer simulation of visual outcomes of wavefront-only corneal ablation.

PURPOSE: To evaluate the effectiveness, predicted visual outcome, and limitations of a corneal ablation algorithm that uses wavefront aberration measurement alone without the need for corneal shape information. SETTING: Contact Lens and Visual Optical Laboratory, School of Optometry, Queensland University of Technology, Brisbane, Australia. METHODS: Corneal topography and wavefront error data from 22 eyes of 11 potential refractive surgery candidates were used. A computer simulation of the corneal ablation was performed, and the predicted postoperative visual outcome was assessed by calculating the resulting wavefront root-mean-square (RMS) values and visual Strehl ratios. Additionally, the effect of ablation alignment error was examined. Finally, the visual outcomes of the wavefront-only corneal ablations were compared to those in an age-matched group of 20 emmetropic patients. RESULTS: Significant improvement in total and higher-order wavefront RMS was achieved postoperatively in both an ideal setting and in the case of ablation alignment errors. The predicted improvement in visual Strehl ratio in the potential refractive surgery candidates was significantly better than that in the untreated emmetropes. After additional simulated decentration of the pupil center by 150 microm, the result was slightly worse, but the change was found to not be significant when compared to the retinal image quality of emmetropes. CONCLUSIONS: Wavefront-only corneal ablation algorithms could potentially lead to significantly better visual outcomes than those normally encountered in untreated emmetropes, provided that the alignment error is not large. The presented methodology may be used as a screening tool to predict patients' visual outcomes before the surgery.

The topography of the central and peripheral cornea.

PURPOSE: To investigate the topography of the central and peripheral cornea in a group of young adult subjects with a range of normal refractive errors. METHODS: Corneal topography data were acquired for 100 young adult subjects by a method that allows central and peripheral maps to be combined to produce one large, extended corneal topography map. This computer-based method involves matching the common topographical features in the overlapping maps. Corneal height, axial radius of curvature, and axial power data were analyzed. The corneal height data were also fit with Zernike polynomials. RESULTS: Conic fitting to the corneal height data revealed the average apical radius (Ro) was 7.77 +/- 0.2-mm and asphericity (Q) was -0.19 +/- 0.1 for a 6-mm corneal diameter. The conic fit parameters were both found to change significantly for increasing corneal diameters. For a 10-mm corneal diameter, Ro was 7.72 +/- 0.2 mm and Q was -0.36 +/- 0.1. A slight but significant meridional variation was found in Q, with the steepest principal corneal meridian found to flatten at a slightly greater rate than the flattest meridian. The RMS fit error for the conic section was found to increase markedly for larger corneal diameters. Higher-order polynomial fits were needed to fit the peripheral corneal data adequately. Analysis of the axial power data revealed highly significant changes occurring in the corneal best-fit spherocylinder with increasing distance from the corneal center. The peripheral cornea was found to become significantly flatter and to decrease slightly in its toricity. Individual subjects exhibited a range of different patterns of central and peripheral corneal topography. Several of the higher order corneal surface Zernike coefficients were found to change significantly with increasing corneal diameter. CONCLUSIONS: Highly significant changes occur in the shape of the cornea in the periphery. On average, the peripheral cornea becomes significantly flatter and slightly less astigmatic than the central cornea. A conic section is a poor estimator of the peripheral cornea.

Combining central and peripheral videokeratoscope maps to investigate total corneal topography.

PURPOSE: To extend the area of standard corneal topography maps, one central map is combined with six peripheral maps after correlating them in a custom written computer program. METHODS: The point corresponding to the vertex normal of the central map is found in each of the peripheral maps. Data from the peripheral maps can then be added on to the edges of the central map to create a topography map that extends from limbus to limbus horizontally and vertically. RESULTS: The average size of the combined maps from 15 subjects was 11.3 +/- 0.3 mm horizontally and 10.3 +/- 0.3 mm vertically, compared to 9.2 +/- 0.4 mm horizontally and 7.5 +/- 0.7 mm vertically for the standard single maps. These values represent an increase in surface area of approximately 70%. CONCLUSIONS: The topography of the entire cornea can be represented by combining multiple measurements from a Placido videokeratoscope. Conic fits based on central topography data are a poor representation of the total corneal shape.

Optical considerations in the contact lens correction of infant aphakia.

BACKGROUND: Correction of infant aphakia with conventional soft and rigid designs induces significant amounts of positive spherical aberration. METHODS: Different levels of positive spherical aberration were induced by rigid lenses positioned close to the eye. Visual acuity and depth of focus were measured for twelve young adult subjects. RESULTS: Positive spherical aberration acts to produce an increase in relative depth of focus, but because of the reduction of best visual acuity, there is a concurrent decrease in absolute depth of focus. As the spherical aberration increases, there is a progressive loss of best-corrected visual acuity. CONCLUSIONS: The correction of spherical aberration in contact lenses for infant aphakia provides the best theoretical optical conditions for normal ocular growth and visual development. We provide examples of the optimal aspheric surfaces to minimize spherical aberration in contact lenses for infant aphakes.

Automatic estimation of the corneal limbus in videokeratoscopy.

An algorithm for estimating the corneal limbus from videokeratoscopic images is proposed. After the image is transformed to a polar grid, a novel edge-detection procedure, suitable for the detection of the soft edge produced by the limbus, is used to locate the limbus. Outliers due to the eyelids, eyelashes, and videokeratoscopic rings are removed by taking advantage of the approximate circularity of the cornea. An ellipse which minimizes the sum of the squared algebraic errors is fitted to the remaining edge points. Comparisons between the proposed algorithm, a manual computer-based technique and an algorithm which uses conventional edge-detection techniques demonstrate the accuracy of the proposed algorithm.