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The lens is a transparent structure that is located behind the iris and is attached to the so-called ciliary muscle through ligaments called the zonula. The shape of the lens resembles a transparent lentil and thanks to its elasticity it can vary in shape, such that a contraction of the ciliary muscle relaxes the zonular ligaments and the lens increases its curvature. This mechanism helps us to increase the dioptric power of the whole eye and focus on nearby objects. On the contrary, as the ciliary muscle relaxes, the zonular ligaments are tightened, which "stretch" the lens, reducing its curvature. We use this mechanism for distance vision that requires less diopter power.

The aging of the lens begins with the increase in rigidity of the lens, so that the effort to focus closely is difficult. This phenomenon is called Presbyopia or "eyestrain" and the treatment consists of increasing the positive diopters with a close-up glasses or adding positive diopters to the basic glasses prescription, using a bifocal or multifocal lens.

This phenomenon begins over 40 years of age, being progressive. A posteriori, in addition to stiffness, there is associated the loss of transparency that, finally, produces a decrease in visual acuity that cannot be compensated with glasses. This "cloudiness" of the lens associated with aging is called Senile Cataract.

There are many other causes that can produce a cataract. Any direct trauma to the lens can produce a Traumatic Cataract, inevitable if, for example, a perforating wound occurs.

Other causes of cataract production are metabolic, such as diabetes or hypothyroidism or numerous congenital metabolic disorders.

Chronic inflammation in the eye can also prematurely decompensate the lens, producing the so-called Secondary Cataracts.

Finally, there are external agents that can cause cataracts such as infrared radiation, ionizing radiation and some drugs such as corticosteroids, both topical and oral.

Cataract treatment is surgical. Currently, an aspiration of the cataract is performed inside the eye after fragmenting it with the help of ultrasound, a technique called Phacoemulsification. To replace the lens, an intraocular lens made of acrylic material is inserted, with a power calculated in advance to minimize residual graduation after surgery. Likewise, intraocular lenses can be multifocal and / or toric to correct near vision and astigmatism respectively.