Preoperative Simulation of Outcomes Using Adaptive Optics

Experimental validations of a tunable-lens-based visual demonstrator of multifocal corrections

The Simultaneous Vision simulator (SimVis) is a visual demonstrator of multifocal lens designs for prospective intraocular lens replacement surgery patients and contact lens wearers. This programmable device employs a fast tunable lens and works on the principle of temporal multiplexing. The SimVis input signal is tailored to mimic the optical quality of the multifocal lens using the theoretical SimVis temporal profile, which is evaluated from the through-focus Visual Strehl ratio metric of the multifocal lens. In this paper, for the first time, focimeter-verified on-bench validations of multifocal simulations using SimVis are presented. Two steps are identified as being critical to accurate SimVis simulations. Firstly, a new iterative approach is presented that improves the accuracy of the theoretical SimVis temporal profile for three different multifocal intraocular lens designs - diffractive trifocal, refractive segmented bifocal, and refractive extended depth of focus, while retaining a low sampling. Secondly, a fast focimeter is used to measure the step response of the tunable lens, and the input signal is corrected to include the effects of the transient behavior of the tunable lens. It was found that the root-mean-square of the difference between the estimated through-focus Visual Strehl ratio of the multifocal lens and SimVis is not greater than 0.02 for all the tested multifocal designs.

[Wavefront analysis in ophthalmologic diagnostics]

Modern aberrometry measures standard and so-called higher-order refractory aberrations. Ophthalmology and optometry use Zernike polynomials to describe aberrations of the retina and lens causing refractory errors. Aberrations of a higher order sometimes follow successful laser surgery, causing reduced vision and inducing patient dissatisfaction; enhanced wavefront data can help to avoid this. Aberrometry is used also for objective measurement of refractory changes. Wavefront techniques and their clinical application enable many options for understanding the delicate balance of eye optics. The future of refractive surgery lies in increasingly individualized treatment to suppress higher degrees of aberration and thus improve clinical results. Patients will continue placing greater demand on individualized intraocular lenses that correct higher-order aberrations.

High-resolution, in vivo retinal imaging using adaptive optics and its future role in ophthalmology

Until recently it was impossible to fully realize the optical resolution afforded by the human eye due to the inherent optical aberrations. These aberrations limit the ability to see fine structure in the retinal layers and visual perception of the outside world. A conventional spectacle or contact lens refraction only provides a static amelioration of the lowest order aberrations, namely defocus and astigmatism. In addition, all of these distortions are constantly evolving due to changes in accommodation and head/eye movements. The technique of adaptive optics not only corrects all of the static spatial modes but also measures and corrects any dynamic changes. Such systems have allowed for routine in vivo cellular imaging, the classification of individual photoreceptor cells and have enabled psychophysical testing of human visual function at the neural level. This review introduces the principle of adaptive optics and the key hardware required to implement such a scheme. The integration of adaptive optics into different imaging modalities is presented along with descriptions of current systems in use today and the experimental results obtained to date. Finally, the review concludes by discussing future technology and gives the author's prediction of how the field will evolve over the coming years.

Effect of intraocular lens implantation on visual acuity, contrast sensitivity, and depth of focus

PURPOSE

To determine the role of spherical and irregular aberrations in the optics of the natural eye and after intraocular lens (IOL) implantation in terms of visual acuity, contrast sensitivity, and depth of focus.

SETTING

Laboratory of Experimental Ophthalmology, University of Groningen, Groningen, The Netherlands.

METHODS

Visual acuity and defocus-specific contrast sensitivity in 11 pseudophakic patients (IOL group) and 27 age-matched phakic subjects were compared. The results were obtained psychophysically. Spherical and irregular aberrations were subsequently estimated by comparing the measured myopic shift (optimum focus of contrast sensitivity at 4 cycles per degree [cpd] compared to that at 16 cpd) and depth of focus with those of theoretical eye models with varying amounts of irregular and spherical aberrations.

RESULTS

The best corrected visual acuity and best corrected contrast sensitivity in the IOL group did not significantly differ from that in the phakic group. The depth of focus was larger in the IOL group at a pupil diameter of 6.0 mm (P<.05). Comparison with theoretical eye models suggested a higher amount of spherical aberration in the IOL group; irregular aberration was almost the same in both groups.

CONCLUSIONS

There was a higher amount of spherical aberration in the IOL group, related to a larger depth of focus, without loss of contrast sensitivity at optimum focus or loss of visual acuity. This might contribute to better quality of vision in pseudophakic subjects than in presbyopic phakic subjects.

Effect of methods of myopia correction on visual acuity, contrast sensitivity, and depth of focus

PURPOSE

To psychophysically measure spherical and irregular aberrations in patients with various types of myopia correction.

SETTING

Laboratory of Experimental Ophthalmology, University of Groningen, Groningen, The Netherlands.

METHODS

Three groups of patients with low myopia correction (spectacles, soft contact lens, and Intacs) and 4 groups with high myopia correction (spectacles, rigid contact lens, Artisan claw lens, and laser in situ keratomileusis [LASIK]) had through-focus contrast sensitivity measurements to establish the myopic shift and depth of focus. From these 2 parameters, spherical and irregular aberrations were determined using theoretical eye models and geometric optics. Visual acuity, stray light, and predictability were also studied.

RESULTS

There were no differences in best corrected visual acuity (BCVA) or best corrected contrast sensitivity between the low myopia groups. The Intacs group had a significantly larger depth of focus (P<.05). The results in the soft contact lens group were comparable to those in a human eye model with an average amount of spherical and irregular aberrations. The LASIK group had worse uncorrected visual acuity (UCVA) and best corrected contrast sensitivity than the spectacles, rigid contact lens, and Artisan claw lens groups (P<.05) due to the amount of spherical and irregular aberrations present after LASIK. The low and high myopia spectacles groups had average amounts of spherical and irregular aberrations.

CONCLUSIONS

Neither surgical techniques nor contact lenses resulted in BCVA or best corrected contrast sensitivity that surpassed the values measured in the best corrected spectacles groups. The Artisan claw lens performed better than LASIK in UCVA, predictability, and best corrected contrast sensitivity.

Wavefront-guided versus standard laser in situ keratomileusis to correct low to moderate myopia

To evaluate the 6-month refractive outcomes of wavefront-guided laser in situ keratomileusis (LASIK) (Zyoptix, Bausch & Lomb) versus standard LASIK (PlanoScan, Bausch & Lomb). Department of Ophthalmology, University Hospital Maastricht, Maastricht, The Netherlands. In a prospective randomized study, 12 patients with myopia had Zyoptix wavefront-guided LASIK in 1 eye and PlanoScan LASIK in the contralateral eye. The safety, efficacy, predictability, stability, optical zone size, and ablation depth were evaluated. The mean preoperative spherical equivalent (SE) of the subjective manifest refraction was -3.88 diopters (D) +/- 1.92 (SD) (Zyoptix) and -4.35 +/- 2.11 D (PlanoScan). Six months postoperatively, 8% of PlanoScan patients and 16% of Zyoptix patients gained at least 2 lines of best corrected visual acuity; the safety index was 1.12 in the Zyoptix group and 1.08 in the PlanoScan group. An SE of +/-1.00 D and +/-0.50 D was achieved by 100% and 92%, respectively, in both groups. There were 2 undercorrections in the Zyoptix group and 1 undercorrection in the PlanoScan group. In the Zyoptix group, 100% had a UCVA of 20/40 and 67% of 20/20 and in the PlanoScan group, 100% and 83%, respectively. The efficacy index was 0.87 and 0.93 in the Zyoptix group and PlanoScan group, respectively. The mean optical zone 6 months postoperatively was 6.16 +/- 0.34 mm in the PlanoScan group and 6.23 +/- 0.41 mm in the Zyoptix group (P =.67). The ablation depth per diopter of defocus equivalent was 13.5 +/- 4.6 microm/D and 8.6 +/- 4.4 microm/D, respectively (P =.01).An excellent safety index was achieved with the Zyoptix and PlanoScan treatments. The efficacy index was marginally lower for Zyoptix treatments as a result of 2 undercorrections. The ablation depth in the Zyoptix group per diopter of defocus equivalent was significantly lower than in the PlanoScan group. Further refinements in defining the ablation algorithms may increase the efficacy index.

Spherical and irregular aberrations are important for the optimal performance of the human eye

Contrast sensitivity measured psychophysically at different levels of defocus can be used to evaluate the eye optics. Possible parameters of spherical and irregular aberrations, e.g. relative modulation transfer (RMT), myopic shift, and depth of focus, can be determined from these measurements. The present paper compares measured results of RMT, myopic shift, and depth of focus with the theoretical results found in the two eye models described by Jansonius and Kooijman (1998). The RMT data in the present study agree with those found in other studies, e.g. Campbell and Green (1965) and Jansonius and Kooijman (1997). A new theoretical eye model using a spherical aberration intermediate between those of the eye models described by Jansonius and Kooijman (1998) and an irregular aberration with a typical S.D. of 0.3-0.5 D could adequately explain the measured RMT, myopic shift, and depth of focus data. Both spherical and irregular aberrations increased the depth of focus, but decreased the modulation transfer (MT) at high spatial frequencies at optimum focus. These aberrations, therefore, play an important role in the balance between acuity and depth of focus.

Highlights of the 1st International Congress of Wavefront Sensing and Aberration-free Refractive Correction

As we review the many new and evolving techniques for treating patients with customized ablation, it is obvious that there is a rapid evolution of technology and thought. Newly refined diagnostic technology, such as wavefront sensing, and more sophisticated spot laser delivery systems with eye tracking gives the refractive surgical team greater flexibility in tackling challenging optical abnormalities. These highlights of the 2000 Congress now set the stage for further development, outlined in the following selected papers from the 2001 Congress.

Wavefront technology in ophthalmology

Wavefront sensing is an emerging technology that can measure irregular astigmatism as a higher-order wavefront aberration. The application of wavefront sensing in ophthalmology might enable the non-invasive observation of living retinal cone cells; the measurement of detailed visual function of the central nervous system by eliminating higher-order aberrations during examinations by adaptive optics; the correction of irregular astigmatism; and the prevention of iatrogenic irregular astigmatism induced by conventional refractive surgical procedures. In addition, it will be theoretically possible to obtain supernormal vision by wavefront-guided refractive surgery.

Optics of aberroscopy and super vision

This paper (1) reviews the fundamental limits to visual performance imposed by optical imaging and photoreceptor sampling to determine the limits to the potential gains offered by ideal corrections; (2) examines the predicted losses in vision induced by chromatic aberration, phase shifts, typical ocular aberrations, and the gains possible by correcting the monochromatic aberrations of the eye; (3) discusses the principles of aberration measurement in the eye; and (4) presents methods for measuring and classifying monochromatic aberrations of the eye.