Cataract is defined by the loss in transparency of the crystalline, the natural lens of the eye, causing visual impairment sometimes as severe as blindness. Crystalline naturally stiffens and becomes more opaque with age, yet apart from aging, there are other potential underlying causes. Cataract surgery, consisting of replacing the damaged crystalline with an artificial lens, is advised. Crystalline can be completely or partially removed surgically, and it is also possible to crack or chop it then vacuum the "fragments" out of the eye using ultrasonic waves, this is called phacoemulsification. In the majority of cases, an artificial crystalline lens is then implanted. Crystalline surgery is performed under local anesthetic, without putting the patient to sleep, and the patient can be discharged the day after the operation, or even on the day itself. This is the most frequently performed operation in France.