Cataracts are the leading cause of preventable blindness among adults in the United States. The incidence of cataracts in the United States is 1.2 to 6.0 cases per 10,000 people. A cataract is defined as opacity of the normally transparent lens that distorts the image projected on the retina. The lens opacity reduces visual acuity. As the eye ages, the lens loses water and increases in size and density, causing compression of lens fibers. A cataract then forms as oxygen uptake is reduced, water content decreases, calcium content increases, and soluble protein becomes insoluble. Over time, compression of lens fibers causes a painless, progressive loss of transparency that is often bilateral. The rate of cataract formation in each eye is seldom identical. Without surgery, a cataract can lead to blindness.