How does the contact lens stay in the center of the eye and not go anywhere in the white part when already worn?
In: Biology
The optometrist who prescribes them performs a contact lens fit as part of the eye exam, to be sure they fit properly.
The eye bulges out a little in that spot due to the cornea, the clear part of the eye in front of the iris and pupil that does most of the focusing of the light that enters the eye. A contact lens is designed to fit over that shape.
Most contacts are made within a few variations in size and shape, so that one of the combinations of options is likely to fit most people. Think of it like having S, M, L, and XL shirts: one of the standard sizes is likely to fit most people.
A good fit is tight enough to stay in place well and not cause ulcers by moving around too much on the eye, and loose enough to let a little of your eyes’ tears underneath to lubricate and nourish the eye and to not damage it by digging in to the cornea. The doctor checks for these conditions during the fitting (as well as other factors like how likely you are to take care of them properly and what your use/lifestyle is, to recommend ones made of materials that are better for how you will use them).
Some people have corneas too oddly shaped to wear normal contacts. Keratoconus, for example, is a condition of the eye that causes the cornea to grow out into a cone shape instead of the normal sphere.
A nice normally rounded contact lens won’t stay put over a conical cornea; think of it like trying to balance a bowl on a traffic cone with highway traffic roaring by. So mild to moderate stage may wear special scleral contacts that are larger, covering some or most of the whites of the eye (called the sclera) as well as the cornea, in order to stay on and also to help constrict the abnormal growth of the cornea.
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