In their recent article, Toto et al.1 evaluated and compared the wavefront error, among other parameters, in patients with 2 diffractive multifocal intraocular lenses (DMIOLs). However, many elements suggest that the Hartmann-Shack (H-S)-based wavefront sensing method used by these authors cannot accurately represent the ocular aberrations induced by DMIOLs using Zernike decomposition.
An H-S sensor divides the incoming beam into subbeams, dividing the wavefront into separate facets, each focused by a microlens onto a subarray of pixels of a charge-coupled device camera. It is then possible to determine the local wavefront inclination (or tilt), which depends on where the focal spot of each facet strikes its subarray of pixels. Subsequent analysis of all facets together leads to determination of the overall wavefront shape. This shape carries phase information that can be used to calculate metrics such as the point spread function and the modulation transfer function. Hence, accurate phase estimation is mandatory to permit the relevant calculation of these metrics. The main drawback of H-S wavefront sensing methods is the lack of information about higher-order aberrations and scattering because of the limitation imposed by the lens sampling.
Bifocal IOLs such as the AcrySof ReSTOR (Alcon Laboratories, Inc.) and Tecnis ZM900 (Advanced Medical Optics, Inc.) multifocal IOLs investigated by Toto et al.1 use diffractive zones to create 2 focal points, one at distance and one at near. To achieve this effect, these IOLs use concentric stepped zones that induce discrete repetitive phase jumps to make the light interfere constructively at more than one foci. The H-S sampling of such locally distorted wavefronts may result in the apparition of some additional centroids straying inside or outside their pixel subarrays. Since the IOL diffractive zones are arranged in a circular concentric manner whereas the H-S uses a square microlens array, the spatial distribution of these additional centroids would be difficult to predict. Eventually, the rapid phase variations caused by the diffractive IOL zones may be under-sampled and/or inadequately reconstructed using conventional H-S technology.
Moreover, H-S sensors are not designed to capture the scattering incurred by the discrete junctions between the diffractive zones, and looking at the wavefront error only may lead to significant overestimation of the optical quality of eyes with DMIOLs. These inaccuracies may be more pronounced after implantation of a DMIOL with a full diffractive surface, such as the Tecnis ZM900, than one with a central 3.6 mm diffractive surface, such as the AcrySof ReSTOR. It must be emphasized that even in the hypothetical case of proper phase sampling, the fit of Zernike polynomials may fail to capture the highly detailed information of a diffracted wavefront.2
In consideration of these remarks, I do not think it is possible to accurately estimate and thus compare ocular wavefront errors after the insertion of DMIOLs with the H-S technology as described in the study by Toto et al.1 Despite the absence of direct phase information, double-pass techniques, which are sensitive to all the optical defects involved in retinal image degradation, such as diffraction, aberration, and scattering, may provide more accurate estimates of the eye's image quality after diffractive IOL implantation.3 I recommend that further investigations be performed to identify better methods to accurately measure the complex wavefront aberrations in eyes with diffractive optics.
1. Toto L, Falconio G, Vecchiarino L, et al. Visual performance and biocompatibility of 2 multifocal diffractive IOLs; six-month comparative study. J Cataract Refract Surg. 2007;33:1419-1425.
2. Klyce SD, Karon MD, Smolek MK. Advantages and disadvantages of the Zernike expansion for representing wave aberration of the normal and aberrated eye. J Refract Surg. 2004;20:S537-S541.
3. Díaz-Doutón F, Benito A, Pujol J, et al. Comparison of the retinal image quality with a Hartmann-Shack wavefront sensor and a double-pass instrument. Invest Ophthalmol Vis Sci. 47. 2006. 1710-1716. Available at: www.iovs.org/cgi/reprint/47/4/1710.pdf
. Accessed January 5, 2008.