The benefits of aspheric intraocular lenses (IOLs) compared with conventional spherical IOLs are now widely understood and accepted. For many surgeons, they are now the lenses of choice because of their improved contrast sensitivity and functional vision. When assessing the results of cataract surgery, patients expect a quality of vision that goes beyond simply restoring 20/20 acuity: the range of currently available IOLs continues to expand to meet that need.

Wavefront technology allowed surgeons to measure higher order aberrations accurately for the first time. This, in turn, led to a greater understanding both of the eye as an optical system, and of how that system can be changed by an IOL. It is from this understanding that the latest aspheric lenses have been developed.

One of the newest aspheric lenses to be made available is the XL Stabi ZO (Carl Zeiss Meditec) (Figure 1).

Figure 1

I recently headed a study that compared the visual performance of the aspheric XL Stabi ZO lens with the conventional spherical XL Stabi Sky lens (Carl Zeiss Meditec). Dr Orang Seyeddain presented the preliminary findings from the study during the recent XXVI Congress of the European Society of Cataract and Refractive Surgeons in Berlin, Germany.

Different lens types explained

Before one can truly appreciate the advantages of aspheric IOLs, one must first understand the rationale behind the development of different types of aspheric IOL.

All conventional IOLs have biconvex spherical surfaces that create positive spherical aberrations, which increase the eye's total aberrations. This can impact negatively on postoperative quality of vision. Aspheric IOLs have been designed to decrease the induced total spherical aberrations utilizing a number of different strategies.

Aberration-neutral IOLs introduce no spherical aberration to the eye. One benefit of this approach is that there is no need to exclude patients with pre-existing corneal aberrations, keratoconus or previous LASIK treatment, nor those patients whose corneal parameters fall outside the average corneal model.

Figure 2

Asphericity-correcting IOLs are designed to offset the mean amount of positive spherical aberration of an average cornea so that the resulting total spherical aberration of the eye is zero. The Tecnis lens (AMO), for example, accomplishes this via its modified anterior prolate surface, while the AcrySof SN60WF (Alcon) has an aspheric posterior surface.

The XL Stabi ZO is based on a different concept and features a posterior aspheric optic designed to correct some of the positive spherical aberration of the cornea, up to 5 mm pupil size, while beyond that, it is aberration-neutral (Figure 2). Furthermore, the optic has been designed to take into account the physiological misalignment of the visual axis meaning that optical performance is negligibly affected by lens tilt or decentration. The advantages of this design include an improvement in patients' retinal image quality and enhanced contrast perception in mesopic conditions.

The hydrophilic acrylic XL Stabi ZO has an optic diameter of 6 mm. The mean diameter is 10.5 mm and the angulation of the haptics is 10°. The ZO lens is supplied with a mark on the haptics to assist correct positioning. It is available in powers ranging from 10 to 30 D, and is supplied in 0.5 D steps between 14.5 D and 24.5 D. Lenses are preloaded in an injector for implantation through a 2.8 mm incision.