Several types of IOLs are used for cataract surgery. The most common is the monofocal IOL, which allows for focus at a fixed distance. A disadvantage of the monofocal IOL is the requirement of the patient to wear corrective lenses after cataract surgery. Monofocal lenses include both spheric and aspheric IOLs. The difference between the two is in the surface of the lens, whether it is spherical or aspherical. This curvature is related to the correction of spherical aberration, or the excessive refractive power of the cornea at its periphery. Aspheric lenses are generally considered to be superior. They have been shown to produce clearer vision and greater contrast sensitivity. In the Nordic region 90% of the market is aspheric. Southern Europe uses more spherical lenses while Germany uses more aspheric. However, the use of aspheric IOLs involves a trade-off, with conventional (spheric) IOLs having been shown to produce better depth of focus and near vision.
Refractive errors can be corrected by procedures such as Phakic IOL implantation and Laser Assisted In-Situ Keratomileusis (LASIK). LASIK modifies corneal thickness; thus it is limited by the amount of corneal tissue that can be safely removed. If too much is removed, the cornea becomes too thin and weak, and bulging of the cornea could occur, which is a condition known as ectasia. Treatment of ectasia involves exchanging the cornea in a procedure called keratoplasty, which leaves the patient with very poor vision. Phakic IOL implantation has the advantage of being able to correct a higher degree of refractive error than LASIK. Even with these disadvantages, LASIK is the leading refractive surgery procedure because phakic IOL implantation is more expensive, and implantation of these lenses is a more invasive procedure.