Images of Grating Through Toric Intraocular Lenses in Model Eye

A model eye was constructed based on Gullstrand’s model of the human eye. The axial length was 24.0 mm, and the diameter of pupil was 7.0 mm. The anterior surface of the cornea was aspherical, with a spherical aberration of +0.220 μm ( Figure 1 ). A 1951 United States Air Force (USAF) test target (Edmund Optics, Barrington, New Jersey, USA) was glued to the posterior surface of the model eye at the position of the retina. The target consisted of gratings of different orientations and spatial frequencies.

Cross-sectional drawing of the model eye. (Top left) Schematic drawing of the model eye based on the Gullstrand model eye. (Top right) The United States Air Force (USAF) grating target is glued to the posterior surface of the model eye. (Bottom left) The toric marks (arrowheads) of the flat meridian of the toric intraocular lens are positioned at the horizontal marks (arrows) of the model eye. (Bottom right) The horizontal and vertical lengths of the central target are measured.

The model eye was filled with balanced salt solution at room temperature, with care taken to remove all air bubbles. Toric aspheric IOLs, SN6AT5 (AcrySof IQ toric; Alcon Laboratories, Fort Worth, Texas, USA) and 311T5 (AF-1; HOYA Surgical Optics, Frankfurt, Germany) with a cylinder power of 3.0 diopters (D) or SN6AT9 (AcrySof IQ toric; Alcon Laboratories) and 311T9 (AF-1; HOYA Surgical Optics) with a cylinder power of 6.0 D were studied. The orientation of the toric IOL was adjusted so that the toric markings of the flat meridian were at the horizontal mark of the model eye ( Figure 1 ). Nontoric aspheric IOLs, SN60WF (AcrySof IQ; Alcon Laboratories) and NY-60 (AF-1 iMics 1; HOYA Surgical Optics) were used as controls. All of the IOLs had the same spherical power of +20.0 D and were centered on the optical axis of the eye.

Two 23-gauge chandelier light pipes (Alcon Chandelier lighting system; Alcon Laboratories) were inserted through 2 openings on opposite sides of the model eye at positions similar to the sclerotomy sites used during pars plana vitreous surgery ( Figure 2 ). A flat quartz contact lens (HOYA Corporation, Tokyo, Japan) or a wide-angle viewing contact lens (Mini Quad; Volk Optical Inc, Mentor, Ohio, USA) was placed on the cornea of the model eye to view the grating target. Viscoelastic material (Viscoat; Alcon Laboratories) was applied beneath the contact lens on the cornea.

Schematic diagram of the experiment set-up. The United States Air Force (USAF) grating target is glued to the posterior surface of the model eye. Following the placement of each type of intraocular lens (IOL), the target illuminated with chandelier light pipes is photographed with a digital camera.

The grating target was photographed with a digital camera (EOS KISS X3; Canon Inc, Tokyo, Japan) through a surgical microscope (VISU140; Carl Zeiss Meditec, Tokyo, Japan) with a magnification of 25.5×. To evaluate the differences in the quality of the images quantitatively, the contrasts of the gratings in the photographs were measured using Photoshop CS4 (Adobe System Inc, San Jose, California, USA). The intensity at the center of the black stripe was set as I _max and the intensity at the center of the white stripe was set at I _min . The contrast was calculated as (I _max – I _min )/(I _max + I _min ) for each spatial frequency (cycle/mm). The results were compared for the different IOLs and for the flat or wide-angle viewing contact lens. For the toric IOLs, the contrast of the target lines that were parallel (horizontal) to the flat meridian of the toric lenses or the lines (steep meridian) that were perpendicular (vertical) to the flat meridian of the toric lenses was calculated separately.

The lengths of the horizontal and vertical borders of the square targets that were aligned to the flat meridian of the toric IOL ( Figure 1 ) were measured through both the flat contact lens and wide-angle contact lens. The ratio of the vertical to horizontal lengths (V/H) was calculated for each toric and aspheric IOL.

Wavefront Analysis of Toric and Aspheric Intraocular Lens in Model Eye

The toric and aspheric IOLs were implanted in the model eye and the flat meridian was set at 0 degrees. A wavefront analyzer (KR-1 W; Topcon Medical System Inc, Tokyo, Japan) was used to measure the cylindrical aberration of the IOLs for an image of 5 mm because the diameter of the optics of all the IOLs was 6.0 mm, and the valid optics of the toric IOLs was at least 5 mm. The internal aberrations that represented the aberrations of the IOLs were measured with the wavefront analyzer. In the device, the internal aberration was calculated to subtract the corneal aberration of the model eye from the aberration of the whole eye.

Statistical Analyses

Statistical analyses were performed by Student t tests using Microsoft Office Excel 2007 (Microsoft, Redmond, Washington, USA). A P value <.05 was taken as statistically significant.

Contrast of Grating Targets Through Flat Contact Lens

The contrasts of the gratings that were oriented vertical and horizontal to the flat meridian of the toric SN6AT9 IOL viewed through flat contact lens were significantly lower than those through the aspheric SN60WF at 16 cycle/mm ( P = .002, P = .023, respectively). However, the contrasts of the gratings oriented vertically and horizontally were not significantly different for 32 cycle/mm ( P = .097, P = .116, respectively) and 64 cycle/mm ( P = .554, P = .197, respectively; Table 1 , Figure 3 ). The contrasts of the grating in the vertical to the flat meridian through the toric SN6AT5 IOL were significantly lower at 16 cycle/mm ( P = .028) but not at other spatial frequencies, nor at other frequencies for the horizontal line.

The central target image through the toric SN6AT9 and SN6AT5 intraocular lenses (IOLs) in a model eye with flat and wide-angle contact lenses. The horizontal bars of the targets through the flat contact lens are shortened with the toric SN6AT9 IOL (Top left) and the toric SN6AT5 IOL (Middle left). In contrast, the vertical bars of the targets are elongated in the vertical direction through these toric IOLs compared to the image through the aspheric SN60WF IOL (Bottom left). These reductions and elongations are more apparent with the toric SN6AT9 IOL. The horizontal and vertical bars of the targets viewed through the toric SN6AT9 (Top right) and SN6AT5 (Middle right) IOLs through the wide-angle lens are not different from those with aspheric SN60WF (Bottom right).

The contrasts of the grating bars vertical and horizontal to the flat meridian viewed through the toric 311T9 IOL were significantly lower than through the aspheric NY-60 IOL with flat contact lenses at 16 cycle/mm ( P = .002, P = .005, respectively) and 32 cycle/mm ( P = .020, P = .039, respectively) but not at 64 cycle/mm ( P = .144, P = .324, respectively; Table 1 , Figure 4 ). The contrasts of grating vertical to the flat meridian through the toric 311T5 IOL were significantly lower at 16 cycle/mm ( P = .011) but not at 32 cycle/mm ( P = .055) and 64 cycle/mm ( P = .423). The contrasts of targets horizontal to the flat meridian through the toric 311T5 IOL were not significantly different at 16 cycle/mm ( P = .053), 32 cycle/mm ( P = .212), and 64 cycle/mm ( P = .423).

The central target image viewed thorough toric 311T9 and 311T5 intraocular lenses (IOLs) in a model eye with flat and wide-angle contact lenses. The horizontal bars of the targets viewed through the flat contact lens are shortened in the horizontal direction with the toric 311T9 IOL (Top left) and the toric 311T5 IOL (Middle left). On the other hand, the vertical bars of the targets are elongated in the vertical direction through the toric IOLs compared to the bars with the aspheric NY-60 IOL (Bottom left). These reductions and elongations are more apparent with the toric 311T9 IOL. The horizontal and vertical bars of the targets viewed through the toric 311T9 IOL (Top right) and 311T5 IOLs (Middle right) and wide-angle contact lens are not different from those with the aspheric NY-60 IOL (Bottom right).

The contrasts of the horizontal and vertical grating bars through the toric SN6AT9 and toric 311T9 IOLs and the flat contact lens were higher at lower frequencies than those of toric SN6AT5 and toric 311T5 IOLs ( Figures 4 and 5 , Table 1 ). The contrasts of the horizontal and vertical grating bars through toric SN6AT5 and toric 311T5 IOLs with the flat contact lens were higher at lower frequencies than those of the aspheric SN60WF and NY-60 IOLs. The contrast of the horizontal grating bars through toric SN6AT9, SN5AT5, 311T9, and 311T5 IOLs with the flat contact lens tended to be greater than that of the vertical grating bars.

Contrasts of grating viewed in a model eye through toric and aspheric intraocular lenses (IOLs) with a flat corneal contact lens. The contrasts through the toric SN6AT9 IOL (Left) and the toric 311T9 IOL (Right) in the vertical and horizontal directions are lower than those through the SN60WF IOL and NY-60 IOL. The contrasts through the toric SN6AT5 IOL (Left) and the toric 311T5 IOL (Right) are lower than that through the aspheric SN60WF IOL and NY-60 IOL at lower spatial frequencies but not at higher spatial frequencies.

Contrast of Grating Targets Through Wide-Angle Viewing Contact Lens

The contrasts of the gratings bars vertical and horizontal to the flat meridian viewed through the toric SN6AT9 and SN6AT5 IOLs with the wide-angle contact lenses were not significantly different from that viewed through aspheric SN60WF for any spatial frequency of the grating ( Table 1 , Figures 3 and 6 ). Similar findings were made with the toric 311T9 and 311T5 IOLs with the wide-angle contact lenses ( Table 1 , Figures 4 and 6 ).

Contrasts of grating viewed in a model eye through toric and aspheric intraocular lenses (IOLs) with a wide-angle viewing corneal contact lens. The contrasts through toric SN6AT9 (Left), SN6AT5 (Left), 311T9 (Right), and 311T5 (Right) IOLs in the vertical and horizontal directions were not lower than those through the aspheric SN60WF and NY-60 IOLs.

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