<unk> es have been used since 1999 for correcting larger errors in near-sighted, far-sighted, and astigmatic eyes. This type of<unk>  is also called phakic<unk>  (PIOL), as it is implanted without removing the patient's natural crystalline<unk> . Phakic<unk>  appear to be less dangerous than excimer laser surgery (LASIK) in those with significant nearsightedness. More commonly<unk> s are implanted via Clear<unk>  Extraction And Replacement (CLEAR) surgery. During CLEAR, the crystalline<unk>  is extracted and an<unk>  replaces it in a process that is very similar to cataract surgery: both involve<unk>  replacement, local anesthesia, last approximately 30 minutes, and require making a small incision in the eye for<unk>  insertion. People recover from CLEAR surgery 1–7 days after the operation. During this time, they should avoid strenuous exercise or anything else that significantly raises blood pressure. They should visit their ophthalmologists regularly for several weeks to monitor the<unk>  implants. CLEAR has a 90% success rate (risks include wound leakage, infection, inflammation, and astigmatism). CLEAR can be performed only on patients ages 40 and older. This is to ensure that eye growth, which disrupts<unk> <unk> es, will not occur post-surgery. Once implanted,<unk> <unk> es have three major benefits. First, they are an alternative to the excimer laser procedure (LASIK), a form of eye surgery that does not work for people with serious vision problems. Effective<unk>  implants also eliminate the need for glasses or contact<unk> es post-surgery for most patients. The cataract will not return, as the<unk>  has been removed. The disadvantage is that the eye's ability to change focus (accommodate) has generally been reduced or eliminated, depending on the kind of<unk>  implanted. Some of the risks that FDA have been found so far during a three-year study of the Artisan are: * a yearly loss of 1.8% of the endothelial cells, * 0.6% risk of retinal detachment, * 0.6% risk of cataract (other studies have shown a risk of 0.5–1.0%), and * 0.4% risk of corneal swelling. Other risks include: * 0.03–0.05% eye infection risk, which in worst case can lead to blindness. (This risk exists in all eye surgery procedures and is not unique to<unk> s.) * glaucoma, * astigmatism, * remaining near or far sightedness, * rotation of the<unk>  inside the eye one or two days after surgery. One of the causes of the risks above is that the<unk>  can rotate inside the eye 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<unk> s must be powered and aligned inside the eye on a meridian that corrects the patient's preexisting astigmatism. Again, these<unk> es 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<unk> s 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<unk> s 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<unk> es 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<unk> , 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.
Intraocular lens
Scleral lens
Corneal button
Eye dropper
Optic cup 
Intrastromal corneal ring segment
Lacrimal sac
Fundus