Contact lenses are in immediate contact with the cornea. Although they are foreign bodies, most patients adapt to properly fitted contact lenses. Contact lenses differ from eyeglasses in that they correct the refractive error closer to the location of its origin. For this reason, the quality of the optical image viewed through contact lenses is higher than that viewed through eyeglasses. Contact lenses have significantly less influence on the size of the retinal image than does correction with eyeglasses. Lenses do not cloud up in rainy weather or steam, and peripheral distortion is minimized. The cosmetic disadvantage of thick eyeglasses in severe ametropia is also eliminated. Severe anisometropia requires correction with contact lenses for optical reasons, i.e., to minimize aniseikonia.
Contact lenses are defined by the following characteristics:
❖ Diameter of the contact lens.
❖ Radius of curvature of the posterior surface.
❖ Geometry of the posterior surface, i.e., spherical, aspherical, complex curvature, or toric.
❖ Refractive power.
❖ Oxygen permeability of the material (Dk value).
The cornea requires oxygen from the precorneal tear film. To ensure this supply, contact lens materials must be oxygen-permeable. This becomes all the more important the less the contact lens moves and permits circulation of tear fluid. Contact lenses may be manufactured from rigid or flexible materials.
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