if the PIOL is too short, if the eye was incorrectly measured, or because the sulcus has a slightly oval shape (the height is slightly smaller than the width). Toric IOLs must be powered and aligned inside the eye on a meridian that corrects the patient's preexisting astigmatism. Again, these lenses can rotate inside the eye postoperatively or be placed incorrectly by the operating surgeon. Either way, the patient's preexisting astigmatism may not be corrected completely or may even increase. When standard IOLs are implanted with a CLEAR procedure, in substitution of the patient's crystalline, astigmatism is typically not corrected, as astigmatism is mainly attributable to a deformation of the cornea. Toric IOLs may be used during the CLEAR procedure to correct astigmatism. The surgeon can ascertain the astigmatic, or steepest, meridian in a number of ways, including manifest refraction or corneal topography. Manifest refraction is the familiar test where the eye doctor rotates lenses in front of the eye, asking the patient, "Which is better (or clearer), this one or this one?" Corneal topography is considered a more quantitative test, and for purposes of aligning a toric IOL, most surgeons use a measurement called simulated keratometry (SimK), which is calculated by the internal programming of the corneal topography machine, to determine the astigmatic meridian on the surface of the cornea. The astigmatic meridian can also be identified using corneal wavefront technology or paraxial curvature matching. Type of surgery Implants with or without removal of natural crystalline Phakia is the presence of the natural crystalline lenses. Phakic IOL (PIOL) refers to an intraocular lens implanted without removal of the patient's original crystalline lens, and this is performed solely to correct refractive error in the presence of a clear crystalline lens. Aphakia is the absence of the natural crystalline lens. The aphakic state is usually due to surgery to remove a cataractous lens, but post-surgical aphakia is rare nowadays because of the ubiquity of intraocular lenses. Rarely, aphakia can be post-traumatic or congenital in nature. Aphakic IOL refers to a lens implanted secondarily in an eye already aphakic from previous surgery or trauma some time ago. Pseudophakia is the substitution of the natural crystalline lens with an IOL, as is often done after cataract extraction or to correct refractive error. Pseudophakic IOL refers to a lens implanted during cataract surgery, immediately after removal of the patient's crystalline lens. Many aphakic and