The Aspheric Cornea, Spherical Aberration, and Intraocular Lenses: Considerations for Surgical Management

The cornea is the main refractive surface of the eye. The prolate shape of the cornea plays a role in maintaining the optimum total spherical aberration (SA) of the eye by neutralizing the negative SA of the lens. As we age the lenticular SA shifts in the positive direction while the corneal SA remains the same, making the total ocular SA more positive. SA of the eye is directly proportional to the Q coefficient that represents the corneal aspheric shape. In order to replace the natural lenticular SA after senile cataract surgery, aspheric lenses with different amounts of inherent SA have been developed. The evidence supporting the use and benefits of these lenses is controversial with many limitations. Despite limited evidence, most of the existing studies show that aspheric lenses give better contrast sensitivity and lower higherorder aberrations (HOAs) as well as SAs postoperatively. The outcomes are dependent on many factors such as the pre operative HOAs and SAs, decentration/tilt, pupil size and the surgically induced HOAs. Superior outcomes were demonstrated when the preoperative HOAs and SAs were measured when choosing an aspheric intraocular lens (IOL) to target zero postoperative SA. This issue of Ophthalmology Rounds will review the SA of the eye, its relation to the image quality and how it changes with aging, IOL implantation, and corneal surgery. Furthermore, the theory behind developing aspheric IOLs and the evidence available to demonstrate their benefits over spheric lenses will be summarized.