Contact lenses smooth the cornea as shown below…

		Keratoconic cornea without a contact lens When the cornea is distorted in keratoconus and related conditions, light rays passing through the cornea are scattered so that it is impossible to create a clear, focused image on the retina. This is why spectacles do not work, as they cannot correct this type of problem. A way has to be found of recreating a smooth regular surface so that light rays pass through properly to create a clear image.
		..with a properly fitted RGP lens A correctly fitting RGP lens (just brushing the apex of the cornea) creates this smooth surface to give better vision. However, the gap between the back of the lens and the front of the cornea is filled with tears which acts as a lens. The surface in contact with the back of the lens is obviously regular but the back surface, defined by the irregular cornea, is not and this can still cause poor vision.
		..with a flat fitting RGP lens A flat fitting RGP lens will eliminate a lot of this tear lens and often gives much better vision. This can often lead to conflict between a keratoconic wanting better vision and a professional wanting to fit a healthier lens – because a rigid lens bearing down on the cornea often causes damage through abrasion and rubbing.
		..with a small RGP lens To achieve a better fit, keratoconic RGPs tend to be smaller, so they can fit more closely to the cone. However, these can often be unstable and displace easily so that light passes through the pupil without passing through the lens, causing multiple images. They also tend to be more uncomfortable because the lid catches on them more easily than larger lenses that fit well under the top lid.
		..with a soft lens A soft lens works by being soft enough to be compressed so that it follows the contours of the front surface of the cornea quite closely while being stiff enough to maintain a regular front surface. This is achieved with a combination of material and design. Too soft and the lens is too distorted by the cornea ot give good vision. Too stiff and it acts more like a rigid lens with an irregular tear film under the lens.

Other Contact Lens Types

Hybrid lenses: Soft Perm/Synergeyes – these have are an RGP with a soft “skirt”. They generally improve comfort but in some cases
can cause oxygen transmission problems through the skirt area.

Scerals – these are rigid lenses that vault the whole cornea. They can be very comfortable and are often the only lens a patient can
wear if they have a very irregular cornea. They are generally more expensive and are not fitted by all hospital departments. Visually
they can be excellent but in some cases have the same vision issues as a large RGP.

Soclears – a relatively new name to the keratoconic contact lens field but based on the older principle of fitting very large RGP lenses.
In size they are similar to soft lenses and can be very comfortable. They can also be expensive and take a while to get a good fitting.

All the above rely on rigid materials to obtain good vision. They work well if fitted well and as long as that fitting remains optimal.