most such cases can be successfully treated. preventative use of nonsteroidal anti-inflammatory drugs has been reported to reduce the risk of macular oedema to some extent. uveitis–glaucoma–hyphema syndrome is a complication caused by the mechanical irritation of a mis-positioned iol over the iris, ciliary body or iridocorneal angle. other possible complications include elevated intraocular pressure; swelling or oedema of the cornea, which is sometimes associated with transient or permanent cloudy vision (pseudophakic bullous keratopathy); displacement or dislocation of the iol implant (rare); unplanned high refractive error—either myopic or hypermetropic—due to error in the ultrasonic biometry (measurement of the eye length and calculation of the required intraocular lens power); cyanopsia, in which the patient's vision tinted blue and often occurs for a few days, weeks or months after removal of a cataract; and floaters, which commonly appear after surgery. it may be necessary to exchange, remove or reposition an iol after surgery, for any of several reasons: capsular block syndrome, which is hyper-distention of the lens capsular bag due to the iol blocking fluid from draining through the anterior capsulotomy. this may cause a myopic refractive error; chronic anterior uveitis, which is a persistent inflammation of the anterior segment; chronic loss of endothelial cells faster than the rate due to normal aging; iris pigment epithelium loss; pain; progressive elongation of the pupil in direction of the long axis of the iol; progressive closing of the anterior chamber angle due to propagation of anterior synechiae without apparent anterior uveitis; incorrect iol refractive power; incorrect positioning of the iol,including decentring, tilt, or rotation that reduces correct function; damage or deformation of the iol; unexpected optical results due to defects of the iol; undesirable optical phenomena reported by the patient due to any other cause. risk statistically, cataract surgery and iol implantation are procedures with the safest and highest success rates in eye care. as with any type of surgery, some level of risk remains. as of 2011, cataract surgery is the most-frequently performed surgical procedure in the united states, with 1.8 million medicare beneficiaries undergoing the procedure in 2004. this rate is expected to increase as the population ages. most complications of cataract surgery do not result in long-term visual impairment but some severe complications can result in irreversible blindness. a survey of adverse results affecting medicare patients recorded between 2004 and 2006 showed an average rate of 0.5% for one or more severe post-operative complications, with the rate decreasing by about 20% over the study period. the most important risk factors identified were diabetic retinopathy and a combination of cataract surgery with another intraocular procedure on the same day. in the study, 97% of the surgeries were not combined with other intraocular procedures; while 3% were combined with retinal, corneal or glaucoma surgery on the same day. recovery and rehabilitation a shield or patch may be needed for a few days, mainly to protect from physical impact and contamination following cataract surgery, side-effects such as grittiness, watering, blurred vision, double vision or a red or bloodshot eye may occur, and will usually clear after a few days. full recovery can take four-to-six weeks. patients are usually advised to avoid getting water in the eye during the first week after surgery, and to avoid swimming for two-to-three weeks as a conservative approach, to minimise risk of bacterial infection. most patients can return to normal activities the day after phacoemulsification surgery. depending on the procedure, patients should avoid driving for at least 24 hours after the surgery, largely due to effects from the anaesthesia, possible swelling affecting focus, and pupil dilation causing excessive glare. at the first post-operative check, the surgeon will usually assess whether vision is suitable for driving. with small-incision self-sealing wounds used with phacoemulsification, some of the post-operative restrictions common with intracapsular and extracapsular procedures are not relevant. restrictions against lifting and bending were intended to reduce the risk of the wound opening, because straining increases intraocular pressure. with a self-sealing tunnel incision, however, higher pressure closes the wound more tightly. routine use of a shield is not usually required because inadvertent finger pressure on the eye should not open a correctly structured incision, which should only open to point pressure. after surgery, to prevent contamination, the eyes should not be rubbed and the use of eye makeup, face cream or lotions should be avoided. excessive dust, wind, pollen or dirt should also be avoided. sunglasses should be worn on bright days because the eyes will be more sensitive to bright light for a while. topical anti-inflammatory drugs and antibiotics are commonly used in the form of eye-drops to reduce the risk of inflammation and infection. a shield or eye-patch may be prescribed to protect the eye while sleeping. the eye will be checked to ensure the iol remains in place, and once it has fully stabilised, after about six weeks, vision tests will be used to check whether prescription lenses are needed. where the focal length of the iol is optimised for distance vision, reading glasses will generally be needed for near focus. in some cases, the patient is dissatisfied with the optical correction provided by the initial implants, and removal and replacement is necessary. this can occur with more-complex iol designs when patient expectations do not match with the compromises inherent in these designs, or the patient cannot accommodate the difference in distance and near-focusing of monovision lenses. the patient should not participate in contact or extreme sports or similar activities until cleared to do so by the eye surgeon. outcomes