Geometry of contact lenses
The cornea of the eye is shaped like an ellipse. The curvature in the center is greater than the periphery. The geometry of a contact lens must be adjusted as closely as possible to the shape of the cornea in order to be comfortable.
The back surface of a contact lens must maintain a correct alignment with the cornea in order to favor a slight movement that allows the exchange of tears. Semi-rigid contact lens materials, such as those that are not very much flexible, must be adjusted with a curvature that is very tight in the cornea radius (+0.05-0.15 mm). In hydrogel lenses or silicone hydrogel, due to its low elasticity, there is a much higher tolerance. In this fact, manufacturers of disposable contact lenses or frequent replacement lenses are based. One or two radii of curvature are usually sufficient to adapt most people. Also, subsequent aspheric surfaces are common, achieving a better adaptation to the cornea.
The front surface of a contact lens is conditioned by the customer's visual need. Thus, in soft lenses or silicone hydrogel when there is myopia or hypermetropia, geometry is spherical (with equal power to all meridians). If there is astigmatism, with toric geometries (with different power in the main meridians). They are harder to manufacture because to avoid turning of eyelids, they must be designed with a balloon prism. As an exception, semi-rigid contact lenses with stigmatism can be spherical on both sides. This is because a semi-rigid contact lens, between it and the surface of the cornea, is filled with tears. The interest in this case is that tears have optical lens shape and that particularity is used to offset the stigmatism (as long as it is not very high). If there is presbyopia, the most common are with aspheric geometry (the curvature is changing and this allows different powers) or with concentric rings (a zone for distant vision and another for near vision). They give a simultaneous view of different images, whether they are for far o near vision. It is the brain that has to select the one that interests in each moment (it is the same that we do when in a conversation with several interlocutors we have to select the voice that we want). In the semi-rigid there is another modality, which is rarely used today, in which the vision is alternating (the eyelids raise the lens with the lower part of the lens with the power of close vision).
In people operated with myopia with Lasik, the geometry of the cornea is the opposite. Curvature on the periphery is higher than centrally. If over the years, these people make myopia again, normal contact lenses are displaced laterally. Then, reverse geometry contact lenses must be adjusted so that they remain well-centered.
Normal cornea. Centrally the radii are more curved than the periphery
Cornea operated by Lasik. Centrally radii are flatter than on the periphery
In people whose myopia is increasing, it has been seen that they have a greater peripheral hyperopic blur. There are now reverse geometry contact lenses that correct it. The effect sought is to slow down this progression. The results of recent clinical studies show that this is the case.
In recent years, semi-scleral semi-rigid contact lenses (diameter 12.50 to 16 mm) and scleral (16 to 24 mm) have appeared. The usual diameter of a semi-rigid is up to 10 mm. This larger size makes them very comfortable because it shows less movement and is very appropriate for certain pathologies: keratoconus, post-lasik, post-transplantation of the cornea, traumatisms, dry eye, ...