Prolate lens design improves contrast sensitvity

Stefanie Petrou-Binder MD
in Nuernberg

A RECENT German study provides additional support for the use of aspherical intraocular lens (IOL) designs which compensate for spherical aberrations and improve contrast visual acuity and sensitivity.

"Modern IOLs are optically superior to the natural crystalline lens. Nonetheless, the outcome of cataract surgery with conventional IOL implantation often yields reduced contrast sensitivity in older patients, compared to young phakic eyes," said Ulrich Mester MD in a lecture at the annual Congress of the German Ophthalmic Surgeons (DOC).

He recounted his own experience with aspherical IOLs in a randomised study which included 44 patients with bilateral cataracts. He implanted the aspherical IOL, Tecnis Z9000 (Pfizer Ophthalmics) in one eye, and a conventional, spherical, bi-convex IOL, the SI-40, in the other eye.

The Tecnis Z9000 IOL has a modified, flattened, more elliptical, anterior surface design which compensates for positive corneal spherical aberrations. By contrast, the SI-40 IOL has a conventional spherical surface.

Dr Mester assessed the anterior and the posterior segments of the eye, including IOL decentration and tilt, pupil size, high and low contrast visual acuities (with the Functional Acuity Contrast Test, FACT, Ginsburg), photopic and mesopic contrast sensitivities and wavefront aberrations of the cornea and the eye, at one month, three months and six months postoperatively.

He reported that although the Z9000 eyes revealed significantly better BCVA three months postoperatively, the improved quality of vision was even more apparent when assessing low contrast visual acuity and contrast sensitivity.

The wavefront measurements revealed no significant spherical aberrations in the 44 Z9000 eyes, while a significantly positive spherical aberration could be measured in the partner eyes implanted with the SI-40 IOL.
The reduced spherical aberration seen in Z9000 eyes allowed a significantly better photopic and mesopic contrast sensitivity in those eyes. Control studies carried out six months postoperatively validated these results.

Dr Mester’s findings corroborate with the outcome of a similar trial by Mark Packer MD and his co-workers which was published at the end of 2002. Dr Packer also showed a significantly improved contrast visual acuity with the Z9000, when compared to visual results from the implantation of a biconvex spherical IOL, the AR40e.

Dr Packer implanted the spherical and aspherical lenses randomly in patients with one-sided cataracts. His results showed significantly better contrast sensitivity at 1.5 and 3 cycles per degree under mesopic conditions and 6, 12, and 18 cycles per degree under photopic conditions with the Z9000 IOL.

For many years, lens research has centred around the development of new lens materials and designs in order to combat PCO, attaining the smallest possible incision for IOL implantation and sharpening the retinal image.

Dr Mester explained that refraction is not determined by the retinal image made by the lens alone. The cornea and lens make up an optical system which must be viewed as a unit.

He said that along with the sphere and cylinder, the spherical aberration of the optical system must be taken into account as it is a major factor influencing the sharpness of the images projected onto the retina.

The ideal substitute for the natural lens is not an IOL with the best-isolated optical performance, but one designed to compensate for the aberrations of the cornea - a design inspired by the crystalline lens of younger subjects.
One study found that the average retinal image quality of eyes with biconvex IOLs was paradoxically worse than in same-aged healthy subjects, despite the good optical quality of the IOLs.

Dr Mester said that the positive corneal spherical aberration of the young eye is balanced by the natural lens’ own negative spherical aberration, rendering a sharper retinal image. He pointed out that while there is optimal ray bending at a young age, even of peripheral rays, the internal compensatory mechanism of the lens becomes less effective with advancing age.

The spherical aberration of the natural lens increases in older eyes. In other words, it becomes less negative, causing poorer image quality. Aspherical IOLs were developed based on what has been learned about spherical aberrations to improve the retinal image by evening out the spherical aberration of the optical system through a modified, flatter anterior IOL surface.

In a study performed by Jack Holladay MD in 2002, the corneal topography of 71 patients was used to determine their wavefront aberrations. He averaged the corneal surface shapes and calculated an average spherical aberration. IOLs were designed which had a fixed amount of negative spherical aberration, based on this average, to partially compensate for the positive spherical aberration of the cornea.

Dr Holladay found that this modified prolate aspherical IOL revealed better optical performance than conventional spherical IOLs, if decentration did not exceed 0.4mm and tilting did not exceed 7°, without an apparent loss in depth of focus. A reduction in depth perception with aspherical IOL implantation has been shown in other studies, however.

Ulrich Mester MD
Augenklinik der Bundesknappschaft Sulzbach, Germany
augen@kksulzbach.de

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