Monofocal IOL provides optimal spherical aberration compensation

January 1, 2006

The development of IOLs has come full circle. In 1949, Sir Harold Ridley attempted to emulate the curvature and size of the natural lens with the development of the Ridley IOL. In subsequent years, the focus was on IOL mechanical issues, materials, and optics. Today, with wavefront technology, researchers are going back to the beginning and are attempting to emulate some of the more subtle features of the natural lens.

Additionally, IOL power calculation accuracy has dramatically improved. In fact, since the early 1980s, IOL power calculation accuracy has doubled every 5 to 10 years. By the year 2001, with the advent of optical coherence biometry and newer-generation formulas, IOL power calculation accuracy for normal eyes can routinely be within ±0.25 D spherical equivalent.

However, IOL calculations don't end in the examination lane. They also carry over into the operating room. In fact, the construction of the capsulorrhexis can have as much impact on the refractive outcome as biometry. If a surgeon makes a capsulorrhexis much larger than the optic, as the forces of capsular bag contraction are brought to bear, the lens implant optic can shift forward. For example, if a +21-D lens shifts anteriorly by as little as 0.5 mm, its relative power would be the same as if it were a +22-D lens.

The Tecnis Z9000 lens (AMO) is a multi-piece lens made of second-generation silicone (with a recently approved acrylic version). It has a refractive index of 1.46 (1.47 for the acrylic), and it corrects aberrations in both the cornea and the lens. The anterior prolate aspheric optic design creates a thicker lens compared with the AcrySof IQ IOL, in both versions of this lens, and it filters only UV light.

The SofPort LI61AO lens (Bausch & Lomb) is also a multipiece lens made of second-generation silicone. It has a refractive index of 1.43, and it corrects aberrations in the lens only. The AO lens has a thicker profile compared with the other two products above and it only filters UV light.

High refractive index

The AcrySof IQ lens has several advantages over the other available lenses. It has a high refractive index with a thinner lens profile, and it is made of AcrySof Natural material using the single-piece SN60AT platform. Additionally, it has a blue-light-filtering chromophore, and it introduces negative spherical aberration into the human optical system, which mirrors the posterior surface of the cornea and the natural lens.

Additionally, aspheric optic IOLs have several advantages over traditional spherical IOLs. Positive spherical aberration occurs when marginal rays come into focus in front of paraxial rays. Pseudophakic eyes with spherical IOLs implanted have significantly higher spherical aberration compared with age-matched phakic eyes. An aspheric lens counteracts the positive spherical aberration of the cornea.