Current techniques and technology allow us to perform increasingly atraumatic cataract surgery and to prevent more effectively functional visual impairment from PCO, astigmatism and other causes related to crystalline lens removal. With these improvements, our goals for visual outcomes of our patients are increasing steadily.
Adequate vision under all light conditions is a necessary component of functional vision restoration after lensectomy. Intra-patient comparisons of visual performance of IOLs by contrast sensitivity and mesoptometer testing are uncovering significant differences.

Correlations of these results with wavefront, ray-tracing and modulation transfer function data from optical bench testing of IOLs are elucidating the functional impact of optical errors caused by IOL materials, indices of refraction, and designs.
Randomised intra-patient comparisons of Acrysof MA60 and CeeOn Edge 911A IOLs by Crandall et al demonstrated that statistically better contrast sensitivity at 3 years post-operatively in eyes with CeeOn Edge 911A IOLs than in fellow eyes with Acrysof MA60 IOLs. There were no significant differences in PCO. The investigators concluded that the IOL material and the significant differences in severity of glistenings contributed to the differences in contrast sensitivity.

Intrapatient comparisons of Acrysof MA60, CeeOn Edge 911A and SI-40 IOLs by Hettlich et al showed statistically worse mesoptometer results at 3-6 months after Acrysof MA60 IOL Implantation. There were no significant differences in mesoptometric values in eyes with CeeOn Edge and SI-40 IOLs.

All patients in both studies achieved excellent visual acuity. Nonetheless, the documented differences in contrast sensitivity and mesoptometric results would suggest varying impacts on patients depending on lifestyle and visual needs. For instance, driving ability in low light conditions and with increased glare would be affected adversely in some patients.

The results of these types of clinical studies of functional vision help to clarify for us the clinical significance of wavefront aberration differences noted in optical bench testing of these IOLs.

In wavefront studies by Terwee et al, hydrophobic acrylic Acrysof MA60, silicone CeeOn Edge 911A, silicone SI - 40, and hydrophobic acrylic Sensar IOLs were analysed with a Hartmann-Shack sensor in an ISO eye model. The IOLs were mounted in a ring with an internal diameter of 10mm, and soaked in pure water at 370 during 4 days to stabilise optic and haptic shape and position. Wavefront aberrations were then measured with fixed apertures in low-, medium- and high- powered IOLs .

Both the one-piece and three-piece Acrysof IOLs at all dioptric powers tested had higher root mean square (RMS) values than the other IOLs measured in the Terwee study. The RMS indicates the magnitude of the total wavefront error. The increases in wavefront aberrations were more pronounced in the optical zone beyond 4mm.

According to the investigators, the much higher index of refraction ( 1.55) of the Acrysof IOLs compared to those about 1.46 of the other hydrophobic acrylic and silicone lenses was an important factor by contributing to internal reflections and mechanical stability of the optic surface.

The RMS value is an indicator of the quality of the image that an IOL can project on to the retina. Before the application of wavefront sensors for IOL evaluation, standard testing methods for assessing IOL optical quality included modulation transfer function (MTF) measurements in an ISO eye Model. Modulation transfer describes how precisely a lens allows an image to pass through it. Modulation transfer function values can be calculated for different pupil sizes and spatial frequences. The higher the MTF value, the better the optical transfer. Measurements by Terwee et al. of the same IOLs demonstrated lower MTF values at 50 c/mm for the Acrysof IOLs, particularly when a 5mm aperture was used.

Whereas MTF values measure overall quality of the optical transfer and decrease with increased wavefront errors, wavefront sensors allow evaluation of the separate components.

Wavefront analysis will be used increasingly to help us understand more fully the impact of higher order aberrations, such as spherical aberration, coma and trefoil, on functional vision. IOLs designed to add compensatory aberrations without inducing undesirable errors will enhance the quality of functional vision after lensectomy.

If you would like to read previous "Reflections on Refractive Surgery", check out the archive.

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