The human eye is a very complex organ. It contains a number of smaller components that work in perfect tandem to help us see the world around us. However, at times, it so happens that some of these organs cease to work at their peak efficiency, so the quality of our vision becomes compromised.
In order to deal with such a situation, man has invented corrective glasses and contact lenses that help improve the vision of patients suffering from nearsightedness and farsightedness. If you know a person who wears contact lenses, you may have observed that, unlike corrective glasses, contact lenses must be changed quite frequently.
While spectacles need to be changed only when the wearer’s vision improves/deteriorates, or if there is structural damage to the glass, contact lenses must be changed much more regularly than that. There are contact lenses that have to be changed in a month, a fortnight, a week… in fact, some are good for only 24 hours!
What makes a contact lens ‘good’ for such a short time (compared to glasses)? And also, why do different contact lenses have different durabilities?
A contact lens is a very thin, curved lens that’s placed on the film of tears that covers the human eye. It acts as a corrective lens, i.e., it helps to correct the problem of nearsightedness or farsightedness in patients with less than perfect vision. Note: There are colored contact lenses too, but they are primarily used for cosmetic purposes, rather than correcting one’s vision.
Also referred to simply as contacts, they are designed to fit the exact shape of the wearer’s eyes (for obvious reasons).
A contact lens works pretty much as a normal lens; it’s designed to improve vision by correcting refractive error (a condition wherein light is not properly focused on the retina of the patient’s eye, resulting in poor vision). The lens focuses light rays in such a way that they are incident on a single point on the retina, which helps in forming a crisp image of an object.
The role of the cornea
The cornea is that transparent portion of the eye that covers a number of very important parts, including the pupil, iris and anterior chamber, all of which are critical for vision. In fact, the cornea is responsible for two-thirds of the eye’s total optical power.
The cornea has a plenty of nerve endings, which makes it one of the most sensitive tissues in the body; it’s sensitive to temperature, touch and chemicals. If you accidentally touch your cornea, an involuntary reflex is triggered that causes the eye to shut.
Cornea’s oxygen requirement
Cornea is transparent, and as such, it doesn’t have blood vessels in it. Furthermore, it doesn’t need them, since it gets its supply of oxygen from tears. More specifically, oxygen dissolves in tears, and subsequently gets diffused throughout the cornea and keeps it healthy.
However, if you wear contact lenses, this process (i.e., supplying oxygen to the cornea) could become hampered. Also, if the cornea is deprived of oxygen, it could lead to a number of nasty complications, including the growth of blood vessels in the cornea, and even the formation of ulcers (which is particularly painful). Therefore, the contact lenses should be such that they do not interfere with the corneal oxygen supply.
This is where the life of contact lenses comes into the picture.
Durability of contact lenses
Contact lenses have different durabilities depending on the material of which they are made. ‘Hard’ lenses are tougher and harder, and as such, do not allow much oxygen to reach the cornea. Therefore, they last only for as long as a day, and have to be disposed thereafter.
Then, there are lenses made of silicone hydrogel, which offer better oxygen permeability and can therefore be worn for more than a day. That’s why it’s so important to NOT wear contact lenses beyond their marketed life, as it could potentially lead to a number of nasty symptoms and even cause severe damage to the eyes.
Simply put, the more readily the contacts allow oxygen to permeate them, the longer they can be worn before being discarded.